Sustainable bioplastics build on D-xylose cores: from backup to the center stage

Abstract

The widespread use of petroleum-based plastics has led to severe environmental pollution due to their poor biodegradability and the accumulation of plastic waste. As a promising alternative, bioplastics derived from renewable and biodegradable polysaccharides have attracted growing attention. In recent years, more researchers have begun to explore the development of high-performance bioplastics while preserving the sugar ring structure. This review aims to provide recent progress in the preparation and application of bioplastics that build on D-xylose cores. Modification strategies of xylan, such as esterification, etherification, oxidization, graft polymerization, and chemical crosslinking, and synthetic routes of xylose-core polymers, like ring-opening polymerization, polycondensation, acyclic diene metathesis (ADMET) polymerization, and click polymerization, have been emphasized. The potential applications of these bioplastics in agriculture, packaging, 2D/3D printing, solid polymer electrolytes, and luminescence materials are also presented. Finally, the challenges and future directions of xylose-derived bioplastics are presented, stimulating further efforts in utilizing natural and synthetic biopolymers based on biomass, ultimately contributing to realising a more sustainable and eco-friendly society.

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Xiang Hao

Dr Xiang Hao is currently an associate professor at Beijing Forestry University. He completed his BS and PhD in polymer chemistry from the University of Science and Technology of China and Fudan University, respectively. In 2019, he joined the Department of Materials Science and Technology at Beijing Forestry University, working with professor Feng Peng. His research focuses on developing low-carbon and high-performance resins, adhesives and chemicals that are derived from polysaccharides.

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Green foundation

1. We discuss various green chemical modifications or advanced polymerization methods to fabricate xylose-core bioplastics. The distinctive properties and applications of xylose-based plastics, including their uses in packaging, 3D printing, and solid polymer electrolytes are highlighted.

2. Xylose and its derived polysaccharides are widely present in plants, and their unique rigid sugar ring structure and excellent biocompatibility have rendered them a favored feedstock for the development of novel bioplastics in recent years. Notably, the abundant chemical modifiability of xylose has enabled it to be utilized as a diverse range of new monomers with potential applications across various domains.

3. Apart from focusing on the green aspects of polymerization synthesis, a comprehensive evaluation of the environmental impact across the entire life cycle of xylose-derived bioplastics is required. At the same time, the economic viability of the materials should also be considered.

1. Introduction

Plastics are among the most ubiquitous materials in daily human life, industry, commerce, and agriculture, with characteristics such as light weight, good processability, high durability, and low cost.¹ However, conventional petroleum-based plastics are difficult to naturally degrade and have high recycling costs, and their landfilling and incineration disposal methods have led to severe pollution. It is estimated that plastic production causes 400 million tons of CO₂ emissions annually, and if usage continues to increase, this will rise to 650 million tons of CO₂ equivalent by 2050.² Meanwhile, the accumulated waste plastics in the global environment are estimated to be 51–88 million tons per year, and this quantity is increasing at an astounding rate. The Organisation for Economic Co-operation and Development (OECD) in Paris projects that by 2060, the increasing plastic production will result in a threefold increase in annual waste to over one billion tons.³ Besides this, the primary means of handling these waste plastics is currently through landfilling (79%). Their degradation typically requires over 450 years, and the generated microplastics also invisibly threaten ecosystems and human health.⁴ Consequently, the pursuit of renewable and environmentally friendly alternative materials to replace conventional petroleum-based plastics represents the most critical trend both presently and in the future.

Polysaccharides are widely occurring natural macromolecular compounds in plants and animals, including cellulose, starch, hemicellulose, chitosan, etc. These polysaccharides possess excellent biodegradability and renewability, and are capable of sequestering CO₂ into plant cell wall polymers, while also yielding diverse monomers and polymers through chemical transformation, rendering them crucial feedstocks for sustainable bio-based plastics.^5,6 Compared with conventional petroleum-based plastics, they can be effectively treated through composting or recycling, significantly reducing pollution and microplastic hazards. The most widely well-known is cellulose, with extensive research dating back to the last century on chemical modification or physical blending.^7–9 Additionally, by enzymatic or acid hydrolysis, cellulose can be depolymerized and further converted to produce diverse bio-platform compounds, thereby furnishing a rich feedstock for the manufacture of bioplastics.¹⁰ Bio-plastics derived from glucose, including polylactic acid (PLA),¹¹ polyhydroxyalkanoates (PHA),¹² and polycaprolactone (PCL),¹³ have been widely applied in medical and agricultural applications, food packaging, disposable tableware, agricultural films, etc.

As an alternative to cellulose, hemicellulose is a class of polysaccharides encompassing xylans, glucuronoxylans, galactomannans, glucomannans, and β-1,3/1,4-glucans.¹⁴ Hemicelluloses are ubiquitous in the plant cell walls, ranking second to cellulose in natural abundance, with a global annual production reaching 60 billion tons.¹⁴ Xylans are the most common and representative hemicelluloses, consisting of a branched polymer of D-xylose or arabinose units linked by β-1,4-glycosidic bonds, and are widely present in the cell walls of woody plants, rice husks, wheat straw, and others. Compared with the ordered arrangement and high crystallinity of cellulose molecules, the molecular structure of xylans is more disordered, resulting in inferior mechanical properties.¹⁵ On the one hand, in pulping and biorefining processes, xylans from lignocellulosic biomass are primarily broken down into oligosaccharides or monosaccharides, which then dissolve alongside lignin in black liquor and are subsequently incinerated as fuel, leading to inefficient utilization. On the other hand, xylans contain abundant hydroxyl functional groups in their side chains, which can undergo various chemical modifications or graft polymerizations, further imparting thermoplasticity and processability.¹⁶ Combined with the exceptional biocompatibility and biodegradability of xylans, they are highly promising for developing renewable bio-based plastics.

D-Xylose is the predominant monosaccharide constituent in hemicellulose, accounting for 60–90% of its composition. Analogous to glucose, D-xylose can also produce various platform compounds, including furfural, furfuryl alcohol, 2,3-butanedione, acetic acid, and furan-2,5-dicarboxylic acid.^17,18 Compared with the derived platform compounds, the inherent beneficial properties of D-xylose itself should not be overlooked, such as its non-toxic and non-harmful nature to the human body, its moderate anti-cariogenic effects, and its ability to serve as a partial sugar substitute as a sweetener.¹⁹ The D-xylose units can be further metabolized by microorganisms and ultimately decomposed into carbon dioxide and water. Thus, D-xylose is an attractive option for polymer building blocks because they are widely available, environmentally benign, generally nontoxic, and structurally diverse.

This review aims to provide a tutorial overview of the recent advancements in the rational modulation of xylan and its hydrolysis product, D-xylose, for bioplastic (Scheme 1). Previous approaches that viewed xylan as a plasticizer,^20,21 or breaking down D-xylose to obtain non-sugar-based platform compounds for bioplastic production,²² are not the focus of this review. This review pays more attention to the application and development of natural or synthetic biobased polymers with xylose as the main repeating structural unit in the field of low-carbon materials, encompassing the fundamental structures and characteristics of D-xylose derived biopolymers, emphasizing the key properties that contribute to their suitability for bioplastics. The review further explores the structural variations of xylans during chemical modification and discusses the changes in their corresponding properties. Throughout the review, particular emphasis will be put on the green and advanced strategies for the preparation of D-xylose-derived synthetic polymers. The applications of these D-xylose-core sustainable plastics will also be further discussed. By providing an overview that encompasses the current state-of-the-art research findings and future development prospects, this review aims to drive the high-valued utilization of D-xylose-core polymers in the field of sustainable plastics and point the way for their further improvement and advancement.

Scheme 1 Schematic illustration of the preparation methods and applications of sustainable plastics build on D-xylose cores.

2. Structures and characteristics of xylan and xylose

The content and structure of xylan exhibit diversity depending on the raw material source, with the content accounting for 25–30 wt% of the dry weight of hardwoods and grasses, and even reaching as high as 50 wt% in some herbs and cereals.¹⁴ Unlike the linear and uniform structure of cellulose, xylan possesses a branched side-chain structure, and is composed of various monosaccharide units connected through diverse linkages, forming a heterogeneous polysaccharide. The main chain of xylan is composed of D-xylose units connected by β-(1,4)-glycosidic bonds, with the side chains primarily consisting of arabinose, 4-O-methylglucuronic acid, O-acetyl, and ferulic acid groups. Based on the differences in side chains, xylans can be categorized into O-acetyl-4-O-methylglucuronoxylan, arabinoxylan-4-O-methylglucuronoxylan, arabinoxylans, and homogeneous xylans (Scheme 2a).²³ The degree of polymerization (DP) of xylan is generally lower compared with cellulose, ranging between 50–200. Besides, xylans extracted by dilute alkali solutions typically exhibit diverse branched structures with relatively low molecular weights. In contrast, xylans derived from high-quality dissolving pulp using concentrated alkali solutions possess linear chain structures, higher crystallinity, and higher molecular weights.^24,25 Due to the lack of a –CH₂OH group at the C5 position, the crystallization of hemicellulose requires the participation of water molecules to stabilize the molecular chains, resulting in loosely hexagonal crystals.^26,27

Scheme 2 (a) Schematic representation of typical xylan structure that is presented in hardwood, softwood, and grain, and extracted by concentrated alkali, respectively;²³ (b) schematic structural representation of D-xylose and L-xylose, respectively.

The glass transition temperature (T_g) of xylan is between 160–200 °C; due to the abundance of hydroxyl groups like cellulose and other polysaccharides, xylans lack thermoplasticity. To enhance the processability of xylan, it is often necessary to modify its hydroxyl groups.²⁸ The multi-branched structure of xylan allows for a wide selection of reactive solvents, such as DMSO, dilute alkaline solutions, dimethylformamide (DMF), water, and quaternary ammonium hydroxide compounds.²⁹ The abundance of glycosidic bonds in xylans leads to hydrolysis in acidic conditions, peeling reactions in alkaline environments, and alkaline hydrolysis in high-temperature alkaline environments, necessitating a consideration of xylan's inherent degradation when designing reactions.³⁰ Overall, xylans share some structural similarities with cellulose, such as both having linear structures and the ability to form crystalline structures, as well as the presence of numerous modifiable hydroxyl groups on the side chains. However, there are also some differences, such as the water-dependent crystallinity, and the reduced reactivity, which warrant further exploration of the hemicellulose structure in the future.

As the second most abundant monosaccharide in lignocellulose after glucose, xylose accounts for approximately 5–20 wt% of the dry weight of lignocellulosic biomass, and can comprise up to 31 wt% of the dry weight in some herbaceous angiosperms. Xylose typically appears as fine needle-like crystals, and due to its abundant hydroxyl groups, it readily forms hydrogen bonds with water molecules, making it soluble in water but insoluble in ethanol. As a C5 sugar with a particular chiral structure, xylose exists as stereoisomers, namely D-xylose and L-xylose, which gives rise to its optical activity (Scheme 2b). The configuration of xylose is predominantly present in the form of D-xylose. The traditional methods for D-xylose production mainly employ the two process routes of dilute acid thermal hydrolysis and biological enzymatic hydrolysis. During the hydrolysis process, hydrophilic and low-molecular-weight xylans readily undergo dissolution and hydrolysis, ultimately transforming into xylose. However, with increasing acid concentration, temperature, and reaction time, D-xylose can further dehydrate and convert to furfural, leading to a reduction in yield.³¹ The biological enzymatic hydrolysis method primarily utilizes hemicellulose enzymes (exo-type xylanases) to hydrolyze raw material into xylose. This method significantly reduces the consumption of acids and alkalis during the production process, and eliminates the neutralization and acid removal steps required in dilute acid thermal hydrolysis, allowing the xylose purity after enzymatic hydrolysis to reach over 99.5%.³² It should be noted that the biological enzymatic hydrolysis method has high requirements for the enzymes, and the specialized hemicellulose enzymes indicated high production costs. In addition, process technologies such as self-hydrolysis, steam explosion, and ionic liquids have also been applied to the production of xylose.^33–37 As a compound with extremely low natural abundance, L-xylose requires complex chemical and biosynthetic conversion from D-xylose, resulting in higher costs.^38,39

The metabolic utilization efficiency of D-xylose by microorganisms is limited by the specific strains and related enzymes.⁴⁰ Since the human body lacks xylose-degrading enzymes, xylose intake is not digested or absorbed, and does not increase blood glucose levels. It also has special functions such as regulating the intestinal microbiome, which allows it to be used as a zero-calorie sweetener in the food industry.⁴¹ Moreover, D-xylose has a vast potential for conversion into high-value products, and it can be transformed into various products through chemical catalysis or biological fermentation. D-Xylose can be hydrogenated to produce xylitol, or it can be dehydrated in the presence of catalysts to produce furfural.¹⁷ Other value-added chemicals such as furan-2,5-dicarboxylic acid, furfuryl alcohol, 2-methylfuran, levulinic acid, and levulinate esters can also be produced from D-xylose.⁴² Furthermore, during biomass degradation and fermentation, D-xylose can be converted into fuel ethanol by microorganisms such as yeasts. Compared with xylan, D-xylose has already achieved a variety of industrial applications, particularly in the fields of food, pharmaceuticals, and fuels.⁴³ Nevertheless, the literature on developing synthetic D-xylose-core polymer materials is extremely limited, particularly within the resin material field, which has only recently begun to garner attention.

3. Xylan bioplastic

As early as the 1940s, researchers prepared xylan acetate into film plastics through extrusion casting, initiating the research on xylan derivatives for bioplastic production. Xylans inherently possess a large number of hydroxyl groups, alongside the rigid cyclic structure, leading to challenges in shaping xylan-based plastics. Over a substantial period, xylans were predominantly regarded as plasticizers or additives for other polysaccharide plastics, rarely being used as the primary material for bioplastic production. In recent years, researchers have started focusing on the development of plastics based on xylans, introducing functional groups or polymers through esterification, etherification, oxidation, graft polymerization, etc., to impart certain thermoplastic properties, and have further expanded properties such as hydrophobic, antibacterial, and high extension.

3.1. Esterification

Esterification is one of the most commonly used methods for xylan derivatization (Fig. 1). The introduction of alkyl chains or benzene rings through esterification can weaken the intermolecular interaction forces, and increase the mobility of the polymer chains, further enhancing the thermoplasticity and processability.⁴⁴ In 2014, Gordobil et al. applied the acetic anhydride/pyridine catalytic method to acetylate corn cob xylan in formamide with a controllable degree of substitution (DS).⁴⁵ With the introduction of acetyl groups, the thermal decomposition temperature (T_d) of xylan increased from 239 to 334 °C, indicating that the acetylated xylan exhibited higher thermal stability. Then, they used glycerol as a plasticizer and cast-coated films of xylan/acetylated cellulose hydrophilic composites and acetylated xylan/acetylated cellulose hydrophobic composites. While the addition of acetylated cellulose improved the performance of both types of film, the latter hydrophobic films (with Young's modulus of 2300 MPa, strength of 44.1 MPa, and elongation at break of 5.7%) exhibited significantly stronger mechanical properties compared with the hydrophilic one (with Young's modulus of 3.3 MPa, strength of 3.3 MPa, and elongation at break of 5.3%).

Fig. 1 (a) Synthetic routes to xylan esterification and etherification;^44–51 (b) appearance of acetylated xylan plastics and the UV response of cinnamate esters;^49,52 (c) appearance of arabinoxylan ether heat-compacted and soluble dyed carboxymethyl xylan films.^53,54

To avoid the potential environmental risks of formamide and pyridine, Martins et al. carried out acetylation using acetic acid as the solvent, sulfuric acid as the catalyst, and acetic anhydride as the esterification agent.⁴⁶ Prolonging the reaction time helped to increase the DS, but overly long reaction times would result in a decrease in yield due to hydrolysis of the polymer chains. The acetylated xylan film could reach a Young's modulus of 300 MPa, exhibiting an increase of nearly 100 times that of non-acetylated ones. As the transesterification reaction with vinyl esters is a mild acetylation process without acidic byproducts, Zhang et al. employed the [Emim]OAc ionic liquid as a dual solvent and activating agent to conduct the transesterification between xylan and vinyl acetate (VA), introducing acetyl groups to enhance its solubility and film-forming capabilities.⁴⁷¹H nuclear magnetic resonance (¹H NMR) analysis indicated that the xylan was partially substituted with acetyl groups (DS = 0.38), and this lower DS enabled good solubility in water and improved film-forming properties, providing a new method for the green manufacturing of hemicellulose-based film plastics, avoid using toxic catalysts while reducing the carbon footprint.

Recently, Chen et al. used cinnamoyl chloride as the esterification agent, anhydrous DMAC/acetonitrile as the solvent, and pyridine as the catalyst to introduce cinnamate ester substituents onto the xylan chains.⁴⁸ Disruption of hydrogen bonding interactions effectively enhanced the thermal mobility of hemicellulose chains, resulting in thermoplastic cinnamate-esterified xylan with a T_g ranging from 146.5 to 175.0 °C. ¹H NMR results demonstrated that the C3–OH of the xylose unit was more accessible to cinnamoylation. Incorporating numerous aromatic rings enabled xylan to engage in π–π interactions with commercial PBAT resin, ensuring excellent molecular-level compatibility. However, when the degree of substitution is too high, π–π stacking tends to aggregate, which needs to be considered in batch production. At a 1 : 1 mixing ratio, the bio-based composite exhibited a maximum tensile strength of 19.4 MPa and an elongation at a break of 330%, surpassing the mechanical properties of PBAT alone and its blends with lignin, starch, cellulose, etc.

To reduce reliance on toxic reagents and catalysts, our group employed ionic liquids (ILs, AmimCl) as reaction solvents, yielding cinnamate-esterified xylans with T_g ranging from 64 to 148.0 °C. These can be reused multiple times through thermal cycling, with a mechanical property retention rate of >80%.⁴⁹ AmimCl solvent was applied to dissolve crystalline xylan, and can be recycled (recovery rate >90%) to reduce economic and ecological issues in industrial-scale production. The T_g did not exhibit a monotonic decrease with increasing DS. When the DS was between 0 and 1, the disruption of inter- and intramolecular hydrogen bonds of hemicellulose and the softening effect of the cinnamate ester groups could increase the free volume and mobility of the chains, reducing the T_g from 71.5 to 64.2 °C. However, when DS exceeded 1.00, the increased aromatic rings of the cinnamate ester substituents could cause π–π stacking interactions, which enhanced the rigidity of the chains, leading to an increase in T_g from 128.2 to 148.7 °C. In addition, the introduction of cinnamate esters imparted enhanced photoresponsive properties on the material, where its mechanical performance continuously improved upon light exposure, and also provided the film material with UV resistance (Fig. 1). This cinnamate-esterified hemicellulose had thermoplastic processability and also exhibited excellent mechanical and biodegradable properties, making it suitable for use as a seed coating material.

3.2. Etherification

Etherification is also an important method for preparing xylan-based thermoplastics (Fig. 2a). The hydroxyl groups on the xylan chains were deprotonated in a strong alkali solution, and then reacted with etherification reagents to introduce alkyl chains or ionic groups to improve the thermoplastic properties.⁵⁰ This process can be carried out in water, and the costs are lower than esterification reactions, making it more suitable for industrial-scale utilization. However, the strong alkali system may cause a certain degree of degradation. Thus, controlling the etherification conditions and optimizing the reaction conditions are crucial to minimizing the impact of this degradation on the etherification reaction product performance. The groups of Jain conducted a homogeneous etherification reaction between xylan and propylene oxide in NaOH solution, where the introduction of hydroxypropyl side chain groups disrupted the hydrogen bonds, and produced water-soluble hydroxypropylated xylan (HPX) film plastics with low T_g (70 °C).⁵⁵ The prepared HPX films have a tensile strength of around 45 MPa, which is 6–8 times that of hydroxypropyl cellulose (HPC). Furthermore, the DS of the hydroxypropyl groups is a critical factor affecting the film-forming properties. When the DS is relatively low between 0.2 and 0.5, the HPX exhibits good water solubility and film-forming properties; however, if the DS is excessively high at 1.6, the high content of the hydroxypropyl groups restricts the mobility of the polymer chains, leading to a decrease in water solubility and poor film-forming ability.

Fig. 2 (a) Synthetic routes to xylan oxidization and graft polymerization;^62–69 (b) comparison of the flexibility of side-chain oxidised xylans;⁶⁴ (c) appearance and enhancement mechanism of aldehyde xylan.⁶³

Considering epoxides often have a certain level of toxicity and potential explosive characteristics, the reaction process carries inherent risks. Akil et al. developed a new green method for xylan etherification, using 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) as the catalyst and non-toxic propylene carbonate instead of the harmful propylene oxide. This process generates fewer byproducts (such as CO₂), avoiding persistent organic pollutants that may be generated when using epoxide reagents. The catalyst DBU can be recycled, reducing the consumption of reagents.⁵⁶ They successfully performed hydroxypropylation of oat xylan and beech xylan in DMSO, followed by decarboxylation to prepare hydroxypropylated xylan with a DS range of 0.10–1.51. Compared with beech xylan, the oat xylan rich in arabinose had more hydroxyl groups, and exhibited higher reactivity during the derivatization process. The selection of green reaction feedstock provided a new approach for xylan-based thermoplastics.

To further investigate the effect of the side chain content on the thermoplastic properties of the arabinoxylan, Janewithayapun et al. fractionally separated the arabinoxylans (AX) from wheat bran, and carried out etherification reactions of arabinoxylan fractions with butyl glycidyl ether (BGE) in a 25 wt% NaOH solution (Fig. 2b).⁵⁷ The results showed that as the arabinose proportion increased, the modified xylan plastics transformed from brittle to ductile, ultimately exhibiting elastic characteristics. When the arabinose content in AX was low (arabinose/xylose ratio of 0.3), there were ordered regions between the polymer chains, leading to increased crystallinity and thus higher hardness and brittleness, exhibiting a Young's modulus of 670 MPa but a low elongation at break of only 15%. As the arabinose side chain content increased, the intermolecular bonding between the AX chains was disrupted, making it less likely that crystalline domains would be formed. When the arabinose/xylose ratio was 0.5, Young's modulus increased to 740 MPa, the elongation at break improved to 68%, and the material exhibited ductile fracture behavior. If the arabinose content was further increased, the flexibility imparted by the grafted BGE chains far exceeded the crystalline domains at the nanoscale, resulting in a significant reduction in the Young's modulus to 110 MPa and an increase in the elongation at break to 117%.

In addition to preparing neutral xylan ether materials, ionic groups can also be introduced through etherification reactions to improve the thermoplastic properties. Alekhina et al. synthesized carboxymethylated xylan (CMX) with different DS (0.36, 0.58, 1.13) in a 25% NaOH/isopropanol system.⁵⁸ The mechanical strength of the carboxymethylated hemicellulose films decreased with increasing DS, which was attributed to two factors: the reduction of hydroxyl groups on the polymer chains weakened the intermolecular hydrogen bonding; and the increase in carboxyl content caused ionization during water casting, generating electrostatic repulsion and weakening the interactions between the polymer chains. To overcome this drawback, our research group incorporated graphene oxide (GO) and chitosan (CS) into the carboxymethylated xylan films.⁵⁹ Since the polymer chains have electronegativity and a large number of free hydroxyl groups, the CMX can bond with chitosan through electrostatic and hydrogen bonding interactions.⁵¹ The addition of GO improved the mechanical properties of the composite films through strong electrostatic interactions, van der Waals forces, and hydrogen bonding interactions, thus acting as a crosslinking agent and reinforcing filler in the composite films. When the mass fraction of GO is 0.5 wt%, the composite films exhibit the best mechanical properties, with a tensile stress of 50.81 MPa, elongation at break of 47.61%, and Young's modulus of 1.39 GPa.

Etherification modification (green reagents) can significantly enhance environmental friendliness and material performance, while also utilizing renewable resources to promote the circular economy. However, there is still a need to develop low-toxicity solvents (such as ionic liquids), optimize decarboxylation pathways to utilize CO₂, develop efficient catalyst recovery systems, and promote water-based processes to reduce energy consumption and pollution.

3.3. Oxidization

The xylan chain comprises xylose units connected by β-1,4-glycosidic bonds, with secondary hydroxyl groups at the C2 and C3 positions, lacking the primary hydroxyl group at the C6 position that is present in cellulose. Thus, the commonly used TEMPO (2,2,6,6-tetramethylpiperidine N-oxyl) oxidation system effective for cellulose⁵⁹ cannot selectively oxidize xylan. Periodate (such as sodium periodate, NaIO₄) is a highly selective oxidizing agent that can specifically target the vicinal diol structure in the sugar units, converting the vicinal diols at the C2 and C3 positions into dialdehyde structures.^60,61 This reaction process does not disrupt the glycosidic bond connections in hemicellulose, so the main chain structure remains intact after oxidation. As the oxidized xylose units undergo ring-opening, an overall increase in the flexibility of the molecular chains would be observed. The dialdehyde groups formed during oxidation have high chemical reactivity and can further participate in other chemical reactions, such as forming Schiff bases with amino groups or being reduced to primary alcohols.⁶²

In 2021, our research team first reported on aldehyde xylan-derived film plastics with excellent mechanical performance and enhanced extensibility.⁶³ The original xylan was a near-linear structure (containing up to 90% xylose) that was extracted through concentrated alkali treatment, which possessed a relatively high crystallinity and was therefore difficult to form into films (Fig. 2c). By using NaIO₄ as the oxidizing agent to partially ring-open and oxidise the xylan, the inherent rigidity and crystalline structure were disrupted, significantly improving its water solubility and film-forming properties. During the film formation process, the aldehyde xylan formed acetal or hemiacetal structures upon water evaporation, ultimately producing a dense and uniform transparent film. This process does not require the addition of plasticizers or other chemical additives, reducing the use of potentially toxic substances. As the aldehyde content increased, the water contact angle of the films gradually increased from 93.26 ± 1.87° to 103.47 ± 0.95°, indicating the formation of intermolecular hemiacetals and imparting a certain degree of hydrophobicity to the films. The film exhibited excellent mechanical performance with a tensile strength of 132.6 ± 14.5 MPa and Young's modulus of 4.84 ± 0.42 GPa, surpassing the existing xylan-derived film plastic.

The highly reactive aldehyde groups can be reduced to form flexible diols, which can improve the thermoplasticity of the xylan chains.⁶⁴ Deralia et al. simultaneously oxidized the rings of the arabinose xylan main chain/side chains, reduced them to diol xylans by NaHB₄, and further leveraged n-butyl glycidyl ether (BuGE) for etherification to prepare thermoplastic xylan. The newly generated large number of primary hydroxyl groups, as well as alkyl chains, enhance the flexibility of the xylan.⁶⁵ The experimental results showed that when the modification degree was 8%, the T_g decreased from 189 °C in the unmodified xylan to 80 °C; and when the oxidation degree reached more than 20%, the modified xylan exhibited two obvious T_g values (T_g1 = −42 to −73 °C, T_g2 = 148–172 °C), which was caused by the relatively long and more numerous BuGE alkoxy side chains. As the oxidation degree increased to 31%, more flexible side chains weakened the hydrogen bonding interactions between xylan chains, leading to a significant increase in the elongation at break to 309%, which was several orders of magnitude higher than the unmodified xylan film.

3.4. Graft polymerization

By introducing polymer side chains onto the xylan main chain, the hydrogen bonding interactions between them can be effectively reduced, improving the thermoplasticity and rheological properties.⁶⁶ Besides, the introduction of hydrophobic side chains can improve the water resistance and moisture resistance of the material, which is particularly important for food packaging materials, and the introduction of responsive polymer side chains can impart intelligent properties such as pH-responsiveness and temperature-responsiveness on the material.⁶⁷

Due to the abundance of hydroxyl groups present along the xylan chain, ring-opening polymerization (ROP) is the primary approach for performing graft polymerization. In the common ROP systems, the use of stannous octoate or triethylamine catalytic systems in the grafting of polylactic acid (PLA) often produces large amounts of PLA homopolymer byproducts or results in relatively low yields (<50%). To solve this problem, Persson et al. used 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) as the catalyst for the graft copolymerization of birch xylan and PLA, which resulted in a low content of homopolymer byproducts (<10%) and a high overall yield exceeding 90%.⁶⁸ Furthermore, the entire reaction conditions were mild (40 °C, 5 to 120 minutes), and the PLA branch length and molecular weight exhibited great controllability. Short PLA side chains led to the grafted polymer being a completely amorphous material (T_g: 48 to 55 °C), while the xylan with longer PLA side chains, due to the inherent crystallinity of PLA, transformed from an amorphous material to a semi-crystalline material with a T_m of around 130 °C. Xylan-g-PLA exhibited superior film-forming properties, with a tensile strength of up to 7.9 MPa and an elongation at break as high as 190%. Zhang et al. used the ionic liquid AmimCl/DBU as the catalyst system to prepare beech xylan-g-poly(p-dioxanone) copolymers (XGP) through ROP. In this process, the emission of volatile organic compounds (VOCs) is reduced, and the grafting yield of PDO reaches 431% with a high raw material utilization rate.⁶⁹ The grafted PPDO side chains could disrupt the intermolecular hydrogen bonding in xylan, acting as an internal plasticizer. After blending with polyvinyl alcohol (PVA), the formation of new hydrogen bonds between PVA and XGP helped increase the compatibility. Compared with the original beech xylan/PVA composite, the XGP/PVA film had an increased tensile strength from 1.5 ± 0.2 MPa to 2.9 ± 0.6 MPa and an increased elongation at break from 10.5 ± 1.3% to 133.5 ± 18.4%, exhibiting good flexibility.

3.5. Chemical crosslinking

Xylan often exhibits very high hydrophilicity, resulting in poor solvent resistance, and chemical crosslinking can significantly improve the solvent resistance and enhance the mechanical performance. Common crosslinking agents for xylan include citric acid, oxalic acid, maleic acid, epoxides, and isocyanates.⁷⁰ Shao et al. used citric acid (CA) for a high-temperature (110 °C) crosslinking reaction with xylan extracted from poplar, which improved the mechanical strength and moisture resistance of the xylan material (Fig. 3a). Meanwhile, the yield of the cross-linking reaction is high (conversion rate >90%), and there are few byproducts. The amount of CA should be controlled, as the excess small-molecule CA acted as a plasticizer that reduced the overall mechanical strength. The dense crosslinked structure reduced the inter-molecular spacing, thereby improving the oxygen barrier performance, with an oxygen permeation rate of 1.8 (cm³ μm) (m² d kPa)⁻¹, which was much lower than that of low-density polyethylene (LDPE) and polylactic acid (PLA). The cross-linked xylan-based plastic can completely degrade within 60 days.⁷¹

Fig. 3 (a) Crosslinking reaction between CA and xylan;⁷¹ (b) schematic demonstrating the preparation of XP (GTE crosslinking as an example).⁷²

Recently, our team has performed etherification crosslinking of xylan under a tetrabutylammonium hydroxide/urea (TBAH/urea) system using diglycidyl ether.⁷² After solvent exchange and hot-pressing, a xylan-derived plastic film with high mechanical performance and excellent biodegradability was achieved (Fig. 3b). The chemical crosslinking network and the physical crosslinking network formed by hydrogen bonding constructed an energy dissipation system, which allowed the bio-based plastic to have high strength (63 MPa) while also exhibiting high toughness (50%). Furthermore, this xylan plastic exhibited significant low cost, processability, and thermal recoverability, and could be completely biodegraded within a few months, making its overall performance superior to existing commercial petroleum-based plastics such as polyethylene (PE), polypropylene (PP), or polytetrafluoroethylene (PTFE).

3.6. Physical blending

3.6.1. Blended for plasticization. Adding molecular plasticizers, including glycerol, xylitol, and sorbitol, to xylan films is a common method to modify the thermoplastic properties.^73,74 The addition of plasticizers often reduces the T_g of xylans because they can decrease the aggregation of polymer chains and weaken the hydrogen bonding interactions between chains, thereby enhancing the processability of the material. Salmén et al. investigated the influence of different plasticizer types, such as urea, low-molecular-weight polyethylene glycol (PEG600), glycerol, and sorbitol, on the ductility of xylan-based films.⁷⁵ Compared with the relatively high T_d (300 °C) of original xylan, all the plasticized samples exhibited a lower T_d (150–250 °C), which was due to the weakening of hydrogen bonding interactions. Among these plasticizers, the addition of sorbitol can reduce the T_g of xylan from 242 to 123 °C, while PEG600 had no significant effect on T_g. Although both urea and glycerol can reduce the T_g, phase separation occurs at higher contents. Therefore, the films prepared with 20–30 wt% sorbitol content exhibited the best mechanical properties, with a tensile strength of 43–50 MPa, an elongation at break of 27–50%, and relatively high transparency.

The addition of molecular plasticizers not only modifies the mechanical properties but also tends to have a certain impact on water vapour permeability and oxygen barrier performance. Wang et al. prepared xylan/PVA composite films using citric acid as a plasticizer, and the results showed that citric acid acted as a crosslinking agent at high temperatures (>100 °C) under dry conditions, exhibiting high mechanical properties.⁷⁶ At low temperatures (75 °C), citric acid partially formed ester bonds with PVA or xylan, showing lower water vapour permeability due to the hydrophobic effect of the ester bonds. As the citric acid content increased from 10% to 50%, the tensile strength decreased from 35.1 to 11.6 MPa, while the elongation at break increased sharply from 15.1% to 249.5%, and the water vapour transmission rate decreased from 2.95 to 2.35 × 10⁻⁷ g (mm² h)⁻¹.

3.6.2. Blended for reinforcement. Blending enhancement is a method that involves mixing xylan with other polymers or inorganic compounds, such as polyvinyl alcohol, chitosan, polylactic acid, montmorillonite, and bentonite. To improve the mechanical strength of xylan films, Schnell et al. prepared xylan/chitosan composite films at room temperature using solvent casting with chitosan as the reinforcing agent.⁷⁷ As the chitosan content increased from 15 to 30 wt%, the tensile strength of the composite films increased from 7 to 37 MPa, and the elongation at break reached 12.2%, which can be ascribed to the formation of new hydrogen bonds. The tensile strength values were similar to those of LDPE films (7–25 MPa) and had a higher strain at break than polystyrene films (2–3%). Moreover, after adding chitosan, the composite films exhibited excellent oxygen barrier properties (oxygen permeability (OP) < 0.3 (cm³ μm) (m² d kPa)⁻¹).

The issue of microscopic phase separation is an important consideration when co-blending. Fialho and co-workers used CA as a compatibilizer and successfully prepared xylan/PLA composite films by solvent casting.⁷⁸ During the co-blending process, CA underwent an esterification reaction with xylans and further formed strong hydrogen bonding interactions with PLA, which prevented microscopic phase separation in the blend and improved the mechanical properties of the modified composite films. Compared with the directly co-blended films, the CA/xylan/PLA films exhibited approximately 6.5 times higher maximum tensile strength, 6 times higher elongation at break, and a 142.1 times higher modulus of elasticity.

From the above literature, it can be found that physical blending is an environmentally friendly and cost-effective method. The flexible process can adapt to different requirements and is easy to scale up for production, which is suitable for rapid industrialization. The key to preparing xylan-derived bioplastics lies in disrupting the hydrogen bonds between polymer chains and enhancing their mobility, thereby endowing them with film-forming properties or processability. Compared with cellulose, the intermolecular hydrogen bond interactions within xylan chains are relatively weaker, which enables a lower degree of substitution (such as acetylation, etherification) to impart water solubility and film-forming properties. Increasing the degree of substitution (DS) can significantly reduce its T_g, making it more thermoplastic. Due to the relatively low molecular weight of xylan, its inherent mechanical properties are often insufficient, so blending it with other biopolymers such as PCL or PLA can produce bio-based plastics with improved mechanical performance. In this process, it is crucial to select appropriate biobased compatibilizers to enhance interfacial wettability, and the biodegradability of the composites also needs attention. In addition, residual additives may affect the ecological environment, the energy consumption during the preparation process is relatively high, and there are certain bottlenecks in terms of mechanical properties.

4. D-Xylose polymerized bioplastic

Among the bioplastics derived from sugars, polymers with cyclic sugar structures can retain the original functions of sugars and maximize biodegradability and biocompatibility.⁷⁹ Furthermore, the rigid D-xylose core in the polymer backbone can impart a relatively high T_g on bio-based polyesters, enabling the facile regulation of thermodynamic properties.⁸⁰ Glycopolymers with cyclic sugar structures have been extensively studied, particularly in lectin recognition, bacterial adhesion, hydrogels, bioimaging, and drug and gene delivery.^80,81 Here, we will focus on reviewing the main synthetic approaches for D-xylose-based bioplastic, including ring-opening polymerization (ROP), acyclic diene metathesis (ADMET) polymerization, polycondensation, and click polymerization, as well as their material performance.

4.1. ROP

ROP has proved effective in the synthesis of versatile sugar-based polymers that maintain their cyclic sugar core, resulting in elevated T_g.⁸² ROP of anhydrosugar derivatives offers access to carbohydrate polyethers, and enables controlled polymerization of anhydrosugar derivatives, which serve as polysaccharide mimics with desired properties and structures.^83,84 Compared with their carbonyl-containing counterparts, polyethers are generally more stable and can exhibit biodegradability. In 2021, the groups of Buchard have reported the controlled anionic ROP of D- or L-xylose-derived oxetane monomers (Fig. 4a) to prepare enantiomerically pure, regioregular isotactic polyethers.⁸⁵ These oxetane monomers can be purified simply by distillation drying over CaH₂, without the need for column chromatography. A 20 g scale in the laboratory can be achieved within hours, with an E factor of 2.31 and an atom economy of 30%. The homochiral polymers are semi-crystalline and display high glass transition and melting temperatures. In contrast, the atactic copolymer obtained from a racemic mixture of the monomers is amorphous. The hydroxyl groups of the polyethers can be deprotected or further functionalized, providing opportunities for post-polymerization modifications. These renewable monomers derived from D-xylose may offer a new platform for the development of stereoselective catalysts, and the polyether with high glass transition and melting temperatures may find applications as hard segments in thermoplastic materials.

Fig. 4 (a) Illustration of the preparation of the D-xylose-derived oxetane;⁸⁵ (b) ROP reaction routes of D-xylose-derived oxetane with various comonomers.^86–91

Furthermore, they described the ring-opening copolymerization (ROCOP) of the D-xylose-derived oxetane monomer with various cyclic anhydrides to form a family of novel sugar-based polyesters.⁸⁶ The ROCOP reactions, catalyzed by a chromium salen complex, proceed with high alternating selectivity to produce AB-type copolymers. The polyesters exhibit a broad range of thermal properties, with T_g ranging from 60–145 °C and high thermal stability (T_d, onset >212 °C). The living nature of the ROCOP allows for control over molar masses and the synthesis of block copolymers through sequential monomer addition. The pendant hydroxyl group on the xylose unit can be further functionalized, for example by phosphorylation. The internal alkenes present in some of the anhydride units can also undergo thiolene reactions. This orthogonal post-polymerization modification strategy allows for the incorporation of diverse functional groups along the polyester backbone.

Copolymerizing the xylose-derived monomers with waste petrochemical resources such as carbon dioxide (CO₂),⁸⁷ carbon disulfide (CS₂),⁸⁸ or carbonyl sulfde (COS)^89,90 to prepare renewable polycarbonates or polythiocarbonates is an intriguing approach (Fig. 4b). Introducing CO₂ into xylose-core polymers not only effectively achieve carbon fixation, but also enables the more efficient construction of carbon neutral polymers. The groups of Wooley have described the sustainable synthesis of CO₂-derived polycarbonates from the natural product D-xylose.⁸⁷ The approach involves two pathways – a one-step ROCOP of an oxetane derived from D-xylose with CO₂ using a (salen)CrCl/onium salt catalyst system, or a two-step process where the oxetane is first converted to a six-membered cyclic carbonate followed by ROP. The ROCOP approach produced an alternating polycarbonate, with some regioirregularity, while the two-step pathway via the cyclic carbonate intermediate yielded a more regular polycarbonate structure. Both methods successfully incorporated CO₂ into the polymer backbone, demonstrating the ability to synthesize sustainable, CO₂-derived polycarbonates from a renewable carbohydrate feedstock.

CS₂ is typically generated in the petrochemical industry, and copolymerizing it with xylose not only has great significance for waste valorisation but also endows the polymer with special properties, such as a high refractive index. Buchard et al. described the ROCOP of a D-xylose-derived oxetane with CS₂ to form sugar-based polythiocarbonates.⁸⁸ The ROCOP proceeds with high regioselectivity towards alternating thiono- and trithiocarbonate linkages, with up to 95% head–head/tail–tail configuration. Kinetic studies suggest the ROCOP occurs at least partially through direct copolymerization, without necessarily going through the ROP of the cyclic xanthate monomer formed as a side-product. The polythiocarbonate exhibits partial chemical recyclability, and can undergo rapid degradation under UV radiation, especially in the presence of a radical source. The ROCOP using a CrSalen/onium salt catalyst system is effective even at mild temperatures like 25 °C, in contrast to the high temperatures required for ROCOP with cyclic anhydrides. Increasing the reaction temperature leads to decreased regioselectivity, with more thioether linkages formed at higher temperatures.

COS is also industrially discharged as a waste gas from fossil, which has been widely applied as a fumigant in grain storage. Recently, Wooley et al. highlighted the complex, in situ structural metamorphoses that can occur during ROCOP of D-xylose oxetane with COS (Fig. 5).⁹⁰ At 40 °C, the ROCOP selectively formed poly(1,2-O-isopropylidene-α-D-xylofuranose monothiocarbonate) with no oxygen/sulfur exchange reactions (O/S ERs). However, as the temperature increased from 60–140 °C, O/S ERs occurred, transforming the monothiocarbonate units into carbonate and thioether dimeric repeating units. This resulted in the formation of intermediate poly[(monothiocarbonate)-co-(carbonate-co-thioether)] copolymers and ultimately, at ≥120 °C, the selective formation of poly(1,2-O-isopropylidene-α-D-xylofuranose carbonate-co-thioether). The extent of O/S ERs could be tuned by varying the temperature and the catalyst/cocatalyst stoichiometry. Increasing the cocatalyst loading favored the formation of the carbonate–thioether copolymer at lower temperatures. In the ROP, using 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD) s as the cocatalyst also led to O/S ERs, but required higher temperatures (≥130 °C) for the selective production of the carbonate–thioether copolymer.

Fig. 5 ROP of 3,5-anhydro-1,2-O-isopropylidene-α-D-xylofuranose and COS at 40, 60–100, and 120–140 °C, respectively.⁹⁰

The aforementioned ROP methods often rely on low-activity organometallic catalysts, which also result in the formation of a brown color and leave behind unpleasant metal residues in the final poly(thiocarbonate) product. Recently, Zhang et al. used alkyl borane to replace (salen)CrCl as an organic Lewis acid to copolymerize D-xylose oxetane with COS to yield metal-free poly(thiocarbonate).⁸⁹ The resulting poly(thiocarbonate) has a well-defined structure, complete alternating xylose oxetane/COS repeat units, and over 99% chemoselectivity. Density functional theory calculations suggest the Lewis acidic borane activates the xylose oxetane and stabilizes the growing anionic chain end. Terpolymerization of xylose oxetane, propylene oxide, and COS produces a series of poly(thiocarbonate) copolymers with tunable glass transition temperatures ranging from 26 to 138 °C. Compared with previous metal-catalyzed systems, the organocatalytic approach yields colorless, metal-free poly(thiocarbonate) polymers.

4.2. Acyclic diene metathesis (ADMET) polymerization

Acyclic diene metathesis (ADMET) polymerization is a powerful synthetic technique that offers several advantages for preparing bio-based polymers.⁹¹ It typically occurs under relatively mild, solvent-free conditions using transition metal catalysts, making them environmentally benign. Besides, the remaining carbon–carbon double bonds in ADMET polymers serve as convenient sites for further chemical modifications, expanding the range of accessible polymer properties and functionalities. In 2020, the groups of Buchard have synthesized the α,ω-unsaturated glycolipids that derived from the isopropylidene-protected D-xylose (O-isopropylidene-α-D-xylofuranose (IPXF)) and 10-undecenoic acid from castor oil, which were then polymerized with a Grubbs second-generation catalyst, generating polyesters with high molecular weights (up to 71 kg mol⁻¹).⁹² The entire reaction is a solvent-free bulk polymerization, which not only achieves high conversion rates, but also maximizes the molecular weight of the polymer. These non-crystalline polymers demonstrated remarkable thermal stability (up to 355 °C) and hydrolytic stability (pH 7, 0, 14), with relatively low glass transition temperatures (−28 to −8 °C) attributed to the flexible fatty acid chains (Fig. 6a). Subsequent ketal deprotection on the sugar moieties resulted in semicrystalline polymers (T_m: 48–49 °C) that enabled the casting of transparent polymer films. The resultant films exhibited low Young's moduli (31–64 MPa) resembling rubbery materials, low elongation at break (7–12%) and low tensile strength values (2–3 MPa).

Fig. 6 (a) Synthetic route of xylose-derived α,ω-diene monomers and their corresponding ADMET products.⁹² (b) The T_g difference of polyesters and polyethers with varying lengths of the carbon chain;⁹³ (c) the appearance of the ADMET product and its mechanical performance.⁹³

Furthermore, they employed ADMET polymerization to synthesize a series of unsaturated polyesters and polyethers, containing biobased rigid O-isopropylidene-α-D-xylofuranose cores and ω-unsaturated fatty acids and alcohols of varying chain lengths (C3, C5, C11).⁹³ This afforded polymers with number-average molar masses up to 63.0 kg mol⁻¹. The T_g of the polymers decreased linearly with increasing aliphatic chain length. Notably, the T_g for polyethers (−32 to 14 °C) was much lower than that of polyesters (−14 to 45 °C), which can be ascribed to the higher backbone flexibility of the ether linkages (Fig. 6b). Postpolymerization modifications could be carried out by hydrogenation and thiol–ene reactions. Hydrogenation of the internal alkenes converted the polyesters and polyethers with C20 hydrocarbon linkers into semicrystalline polymers with melting temperatures around 50 °C. The semicrystalline polyether displayed remarkable polyethylene-like mechanical and gas barrier properties. Hot-pressed films showed mechanical properties (Young's modulus 60–110 MPa, elongation at break 670–1000%, ultimate tensile strength 8–10 MPa) comparable to high-density polyethylene (Fig. 6c), representing a promising bioplastic for sustainable packaging applications.

4.3. Polycondensation

Polycondensation is a classic reaction for the preparation of bio-based polymers. Bio-based monomer molecules such as amino acids, alcohols, and plant oils can be effectively converted into various bioplastic through polycondensation. Furthermore, many polycondensation reactions have been successfully scaled and industrialized. For example, commercialized polylactic acid (PLA), poly(butylene adipate-co-terephthalate) (PBAT), and polyhydroxyalkanoates (PHA) are all produced through polycondensation reactions. However, reports on the preparation of bio-based plastics with sugar cores through polycondensation are not common, mainly because it is difficult to introduce reactive functional groups on both sides of the sugar units. In 2022, groups of Luterbache have proposed a facile approach using glyoxylic acid to directly produce the diacid precursor diglyoxylic acid xylose (DGAX) and the corresponding dimethylglyoxylate xylose (DMGX) from xylose and lignocellulosic biomass (Fig. 7a).⁹⁴ DMGX can be directly produced in 83% theoretical yield through a one-pot process from commercial xylose as well as from raw corn cobs containing 28 wt% hemicellulose (without drying or pre-extraction, Fig. 7b). Melt polycondensation of the obtained diester with a series of aliphatic diols yielded amorphous polyesters (M_n = 30–60 kDa) with high glass transition temperatures (72–100 °C), tough mechanical properties (ultimate tensile strength of 63–77 MPa, tensile modulus of 2000–2500 MPa, and elongation at break of 50–80%), and strong gas barrier performance (oxygen transmission rate (100 μm) of 11–24 cc m⁻² day⁻¹ bar⁻¹), and water vapor transmission rate (100 μm) of 25–36 g m⁻² day⁻¹, which can be processed by injection molding, thermoforming, twin-screw extrusion, and 3D printing. Most importantly, the presence of acetal linkages renders the polymer resin relatively stable under neutral conditions, while allowing for rapid degradation in acidic solutions, and enabling the recovery of monomers in methanol–acidic solutions, demonstrating excellent sustainability. The simple and high-yielding route to DMGX directly from hemicellulose, as well as the attractive material properties of the PAX polyesters, highlights the potential of this approach to produce sustainable and functional plastics from renewable resources.

Fig. 7 (a) Reaction route of the DMGX and its corresponding polycondensation product.^94,95 (b) Sankey diagram of the fractionation of birch wood with glyoxylic acid and the subsequent depolymerization and upgrading of the polysaccharide. Photo images representing the DMGX monomer (top) and its corresponding polymer product (bottom).⁹⁵

Furthermore, they demonstrated the synthesis of DMGX-based high-performance polyamides with various aliphatic diamines, without needing catalysts or specialized equipment (Fig. 7a).⁹⁵ The resulting polyamides exhibited high molecular weights and could be effectively recycled through both mechanical and chemical routes. Thermal and mechanical characterization revealed that these bio-based polyamides have properties comparable to semi-aromatic polyamides, including glass transition temperatures up to 151 °C and tensile strengths up to 75 MPa. This was significantly improved over typical aliphatic polyamides derived from bio-based C10 and C11 feedstocks. Techno-economic and life-cycle analyses suggest these DMGX-based polyamides could be cost-competitive with nylon 66, while offering a 56–75% reduction in global warming potential. The ability to incorporate an intact xylose core into the polymer backbone, combined with the efficient synthetic approach, enables high biomass utilization efficiencies of up to 80%.

Based on this, the groups of Scholten synthesized poly(butylene xylosediglyoxylate) (PBX) and explored compounding PBX with polyesters like PBAT to improve flexibility without compromising its thermomechanical and barrier properties.⁹⁶ The researchers explored two main approaches for enhancing PBX's flexibility – the addition of plasticizers like PEG and triethyl citrate (TEC) and the blending of PBX with more ductile commercial polyesters like poly(butylene succinate)(PBS) and PBAT. The plasticizer approach failed to significantly improve elongation at the break without greatly compromising T_g. However, blending PBX with PBAT was successful in improving flexibility. Adding 20 wt% PBAT to PBX increased the elongation at break from 9.8% to 148%, while still maintaining a high tensile modulus of 1.6 GPa, a T_g of 89 °C, and only moderately impacted barrier properties. Uyama et al. proposed a two-step condensation process to produce biodegradable poly(alkylene xylosediglyoxylates)-co-poly(lactic acid) by copolymerizing DMGX, 1,6-hexanediol, and oligo(lactic acid).⁹⁷ The thermal and mechanical properties of the copolymers could be tuned by adjusting the composition. The toughest copolymer, PHX-co-PLA, exhibited a high glass transition temperature of 106 °C and an elongation at a break of 106%. Both neat PHX and PHX-co-PLA showed significant disintegration in compost. They also exhibited partial biodegradability in seawater with 14% and 13% biodegradation, respectively. PHX-co-PLA retained its shape in water for two weeks and had improved water resistance compared with neat PHX.

4.4. Click polymerization

Thiol–ene/yne “click” reactions are a class of highly efficient and selective chemical transformations. These reactions involve the nucleophilic addition of thiol (–SH) groups to electron-deficient alkenes, generating stable thioether linkages. The process is mild, with minimal side reactions and high yields. The groups of Zhou have synthesized three novel hydroxyl-functionalized propiolate monomers derived from D/L-xylose and 2-deoxy-D-ribose, and then polymerized them via hydroxyl–yne click polymerization catalyzed by commercially available organic amines, particularly 1,4-diaza-bicyclo [2.2.2] octane (DABCO) (Fig. 8b).⁹⁸ The resulting poly(vinyl ether ester) (PVES) exhibited high thermal stability (decomposition temperatures up to 340 °C) and high glass transition temperatures (up to 143 °C), due to the incorporation of rigid cyclic structures and specific stereochemistry into the polymer backbone. Treatment with 1-butanethiol in the presence of an N-heterocyclic carbene catalyst selectively converted the PVES into thioacetal-functionalized monosaccharide derivatives in good to excellent yields. This depolymerization process is proposed to involve thio-Michael and thiol-monothioacetal exchange steps. The groups of Buchard prepared a series of xylose-derived monomers containing 1,3-oxazolidine-2-thione (OZT) functional groups, and polymerized them with dithiols via photoinitiated thiol–ene reactions, yielding a series of poly(ester-thioethers) that retained the OZT functionality (Fig. 8a).⁹⁹ The thermal properties of these polymers could be tuned by varying the dithiol comonomer and introducing different pendant functionalities. Crucially, the presence of unreacted OZT groups in some polymers allowed for facile post-polymerization modifications, such as alkylation reactions.

Fig. 8 (a)–(c) The preparation of D-xylose derived monomers and their corresponding click polymerization route.^98,99

Our research group has successfully synthesized diallyl glyoxylate xylose (DAGX) monomer at high yields (>80%), and then employed light-induced thiol–ene reactions to create a xylan-derived polyester (XP) with favorable mechanical (elongation at break 670–1000%, ultimate tensile strength 8–10 MPa), thermal (T_d > 200 °C), and degradation properties.¹⁰⁰ Due to the dynamic nature of acetal bonds, the fragmented XP film can be reshaped through a hot pressing process (Fig. 8c). The degradability of the XP-crosslinked network was significantly enhanced by the introduction of acetal and ester bonds, and rapidly decomposed within a few hours (1–6 h) under alkaline, acidic, and hot water conditions. Interestingly, DAXG can further impregnate cellulose pulp boards and undergo in situ polymerization under light irradiation, forming XP–cellulose biocomposites. Similar to the intertwined structure of hemicellulose and cellulose in plant cell walls, the XP resin and cellulose molecular chains are interwoven at the microscopic level, which leads to a significant increase in the macroscopic mechanical properties of the material (tensile strength: 43 MPa), far exceeding that of the pulp board itself and commercially available biodegradable plastics.

4.5. Method, performance, and techno-economic aspects

Xylose-core bioplastic derived from xylan often requires the involvement of large amounts of non-volatile organic solvents or strongly alkaline solvents, which not only increases the cost but also makes the reaction inconsistent with the green chemistry principles. To achieve a more low-carbon reaction route, various green and recyclable solvents such as deep eutectic solvents (DES) and ionic liquids have begun to receive increasing attention for xylan-based modifications.^101–105 Given that xylans generally possess relatively low molecular weights, chemical cross-linking was also applied to improve their mechanical properties, while bringing challenges to reprocessing and recycling. The balance between stability and processability can be achieved by introducing dynamic covalent cross-linking structures or constructing reversible cross-linking networks through solvent replacement. In addition to cross-linking, large amounts of non-degradable synthetic polymers were introduced to previous xylan-based composites, which made these xylan-derived plastics challenging to consider as truly bio-based and biodegradable.^106–108 Fortunately, in recent reported studies, we can find that the introduced polymers are starting to trend towards more biodegradable and bio-based macromolecules, which has made xylan-derived composite plastics even more green and low-carbon.

For preparing xylose-core bioplastics, click polymerization and polycondensation demonstrate better performance in green chemistry. The click polymerization employs a solvent-free reaction system with low-toxicity byproducts (thiol conversion >95%), while polycondensation utilizes demethylated dimethylglyoxylate xylose (DMGX) derived from corn cobs, achieving a biobased content exceeding 90%. Moreover, polycondensation exhibits the highest industrial potential due to its mature continuous process (analogous to PET production) and compatibility with high-temperature/pressure equipment. In contrast, ROP relies on metal complex catalysts (e.g., Cr residues up to 50 ppm),⁸⁸ necessitating enzymatic or organic base catalysts (e.g., TBD) to mitigate ecotoxicity. Although ADMET incorporates recyclable Grubbs catalysts (recovery rate >80%), post-treatment via ion-exchange resins remains critical to address metal leaching risks.

Among the two routes derived from xylan and xylose for the development of xylose-based plastics, only some research on the latter has provided a comprehensive assessment of the techno-economic and carbon emission aspects. Taking dimethylglyoxylate xylose (DMGX) in the polymerization section as an example,⁹⁴ technoeconomic analysis estimated a minimum selling price of $1543 per ton for purified crystalline DMGX produced from commercial D-xylose, representing an upper bound of production costs, as sugars from biorefining or agricultural residues are projected to be significantly less expensive than current commercial xylose. In this case, the price of DMGX is almost within the market price range of terephthalic acid ($800–1500 per ton) and lower than the market price range of PLA-grade lactic acid ($1900–2300 per ton) (Fig. 9a). A life cycle assessment revealed a global warming potential (GWP) of 2.33 kg CO₂ per kg of DMGX, which is 20% lower than that of terephthalic acid. If glyoxylic acid is produced from CO₂, the GWP of DMGX can be reduced to 0.8 kg CO₂, and this value can be further lowered through the process.

Fig. 9 (a) Minimum selling price of DMGX produced from commercial xylose under various economic scenarios;⁹⁴ (b)TEA and LCA of PA-8, DGX production;⁹⁵ (c) results of the contribution analysis per kg of XPCF.

Based on this, without the addition of a polymerization catalyst, crystalline DMGX was directly melted polymerized with stoichiometric amounts of various aliphatic diamines for 3 hours. High molecular weight polyamides were obtained in 98–100% yield.⁹⁵ The process combines this mostly unmodified natural carbohydrate ring into the main chain of the polyamide, possessing the properties of fossil-based polyamides while conferring economic, sustainable, and recyclable properties on the polyamides, which were confirmed by techno-economic analysis (TEA) and life cycle analysis (LCA) (Fig. 9b). Calculating the minimum selling price (MSP) and cost composition of poly(octamethylene xylosediglyoxylamide) (PA-8, DGX) produced from DMGX (derived from commercial xylose), and 1,8-diaminooctane under various economic scenarios, it is evident that in all scenarios, the MSP of PA-8, DGX is in the market price range of nylon 66 in 2022, well below the market price of unreinforced high-performance polyphthalamide (PPA), highlighting its profitability potential. Meanwhile, an LCA compared the global warming potential (GWP) of PA-8 and DGX under three different production scenarios. Even when using fossil glyoxylic acid and purified xylose to produce DMGX, the GWP of PA-8 and DGX was estimated to be 56% lower than that of nylon 66 and lower than other bio-based polyamides (Table 1).

Table 1 Systematic comparison of modification strategies and polymerization techniques

Strategies	Feedstock	Solvent	Catalyst	Temperature	Stress (MPa)	Strain (%)	Modulus (GPa)	Ref.
Esterification	Xylan/acetic anhydride	Acetylate	Pyridine	25 °C	3.3–41.1	5.3–5.7	0.003–2.3	45
	Xylan/acetic anhydride	Acetic acid	H2SO4	50 °C	1.56–7.83	2.48–52	0.003–0.29	46
	Xylan/vinyl acetate	[Emim]OAc	n.a.	50 °C	7.4	43.6	n.a.	47
	Xylan/cinnamoyl chloride	DMAC/acetonitrile	Pyridine	120 °C	19.4	330.9	n.a.	48
	Xylan/cinnamoyl chloride	AmimCl	DMAP/TEA	40 °C	29–50	6–12	1.2–1.8	49
Etherification	Xylan/propylene oxide	NaOH	n.a.	25 °C	35–45	7–10	n.a.	55
	Xylan/propylene carbonate	DMSO	DBU	160 °C	35–45	7–10	n.a.	56
	Arabinoxylan/BGE	25% NaOH	n.a.	45 °C	10–25	15–117	0.11–0.74	57
	Xylan/SMCA	25% NaOH	Isopropanol	55 °C	5–58	3–8	0.2–3.8	58
Oxidization	Non-branched xylan	NaIO4	n.a.	30 °C	105–132	3–5	2.89–4.84	63
	Arabinose xylan/BuGE	NaOH/NaIO4/NaHB4	n.a.	25 °C	42.6	309	n.a.	65
Graft polymerization	Beech xylan/PDO	AmimCl	DBU	50 °C	1.5–2.9	10.5–133.5	0.2–0.25	69
	Birch xylan/PLA	DMSO	TBD	40 °C	2.5–7.9	0.9–190	0.05–0.64	68
Chemical crosslinking	Poplar xylan	Citric acid	n.a.	110 °C	2–10	5.7–44.4	n.a.	71
	Xylan/diglycidyl ether	TBAH/urea	n.a.	25 °C	15–55	15–50	0.98–1.7	72
Physical blending	Xylan/sorbitol	H2O	n.a.	35 °C	43–67.7	2.3–50	0.65–4.1	75
	Xylan/citric acid	PVA	n.a.	75 °C	7.6–49.3	5.8–165.4	n.a.	76
	Xylan/chitosan	H2O	n.a.	75 °C	7–35	2–12.2	n.a.	77
ROP	D-/L-Xylose oxetane	CHCl3	KO^tBu/18-crown-6	100 °C	n.a.	n.a.	n.a.	85
	D-Xylose oxetane/PA	σ-Cl2Ph	CrSalen/PPNCl	100 °C	n.a.	n.a.	n.a.	86
	D-Xylose oxetane/CO2	Toluene/xylenes	(Salen)CrCl/PPNCl	110–140 °C	n.a.	n.a.	n.a.	87
	D-Xylose oxetane/CS2	σ-Cl2Ph	CrSalen/PPNCl	25–140 °C	n.a.	n.a.	n.a.	88
	D-Xylose oxetane/COS	n.a.	TEB/PPNCl	25–140 °C	11.9	11	n.a.	89
	D-Xylose oxetane/COS	Xylenes	(Salen)CrCl/PPNCl/TBD	40–140 °C	n.a.	n.a.	n.a.	90
ADMET	D-Xylose-derived diols/10-undecenoic acid	DCM/toluene	Grubbs G-II	60–90 °C	2–3	7–12	0.031–0.064	91
	D-Xylose-derived diols/ω-unsaturated fatty acids	DCM	Grubbs G-II	90 °C	8–10	670–1000	0.06–0.11	93
Polycondensation	Xylose/glyoxylic acid	Methanol	Sb2O3/Sn(Oct)2	190–200 °C	70	9.8–81	n.a.	94
	DMGX/aliphatic diamines	n.a.	n.a.	140–260 °C	54–75	140–165	n.a.	95
	DMGX/1,4-butanediol	n.a.	Dibutyltin oxide	170–200 °C	70	9.8–148	1.6–2.5	96
	DMGX/OLA/1,6-hexanediol	n.a.	Sn(Oct)2/Ti(OBu)4	150–190 °C	5.4–62	81–106	1.5–2.8	97
Click polymerization	D-/L-Xylose	THF/DMSO	DABCO	25 °C	18.2–43.4	2.4–2.7	0.06–0.11	98
	D-/L-Xylose/α,ω-diene monomer	CHCl3	IG819 (photoinitiator)	25 °C	n.a.	n.a.	n.a.	99

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Recently, our team has utilized xylose-core bioplastics to reinforce the mechanical properties of cellulose paperboards, and conducted a comprehensive techno-economic analysis and LCA of the entire low-carbon composites.¹⁰⁰ The results showed that the lowest price of xylan-based polyester reinforced cellulose fiber (XPCF) biodegradable plastics is about $ 10 000 per ton, which is lower than the market price range of polyvinyl fluoride (PVF) and polyhydroxybutyrate (PHB) ($11 800–12 800 per ton), and has considerable market promotion potential (Fig. 9c). Meanwhile, the LCA showed that the global warming potential (GWP) of XPCF composite plastics is 1.826 kg CO₂ eq. kg⁻¹, which is well below the net carbon intensity range of PHB and PVF (1.96–18 kg CO₂ eq. kg⁻¹). Furthermore, after taking into account functional unit conversion based on the density and tensile strength of the material, XPCF bioplastics were more environmentally friendly compared with PVF and PHB in most environmental impact categories.

In summary, the low carbon emissions and environmental impact of xylose-based bioplastics are consistent with global efforts to reduce greenhouse gas emissions and mitigate climate change. Additionally, from an economic perspective, the competitive pricing of xylose-based bioplastics and the potential to reduce costs using agricultural waste make them a viable alternative to petroleum-based polymers.

5. Applications

5.1. Agricultural

In the agricultural field, film mulch can prevent weed growth and soil structure, thereby increasing crop yields. Currently, most agricultural mulches are made from petroleum-based polymers such as LDPE, but the accumulation of non-biodegradable plastics leads to the deterioration of soil structure and a reduction in soil porosity and permeability. Therefore, biobased mulches as biodegradable agricultural covering materials are gradually replacing traditional petroleum-based mulches. Zhang et al. synthesized hydrophobic acetylated xylans with improved film-forming ability through a transesterification reaction with vinyl acetate, and developed a spray-coated mulch with growth-promoting capacity by incorporating the biopesticide avermectin (ABA) encapsulated in β-cyclodextrin (β-CD).⁴⁷ Unlike the traditional laying of a preformed film restricted by terrain, developing sprayable mulch film can carry different pesticides, offering maneuverability and economy. The germination and growth performance of cabbage seed experiments revealed that the mulch significantly improved the seed germination rate. After three days of sowing, the cabbage seeds started to germinate, and showed a seed germination rate under the xylan-derived mulch of 37.5 ± 0.5% (blank control: 8.0 ± 0.5%), reaching an optimal value of 98.6 ± 6.4% (LDPE: 97%) after 10 days. In addition, compared with non-degradable LDPE, the degradation rate of the xylan-based film reached 69.3% after 60 days. This indicates that the xylan-based film mulch has excellent water-retaining and insulating properties, which can effectively meet the basic requirements for plant growth, and has good biodegradability.

Our team synthesized thermoplastic xylan cinnamate esters (XC), and obtained transparent, foldable, and flexible bioplastic bags through hot pressing (Fig. 10a).⁴⁹ This xylan-based plastic can be mechanically recycled by reprocessing the shredded fragments through thermal pressing, and exhibits excellent mechanical properties (tensile strength: 25–50 MPa; PE: 8–30 MPa), water resistance (5.78 × 10⁻¹¹ g m⁻¹ s⁻¹ Pa⁻¹; PE: 4.28 × 10⁻¹¹ g m⁻¹ s⁻¹ Pa⁻¹), ultraviolet resistance (full blockage of UVC/UVB), cost-effectiveness, as well as biodegradability (completely degraded within 40 days) and biocompatibility. The seed storage results showed that after 30 days, the unpackaged seeds cracked due to dehydration; the PE-packaged seeds germinated but had some mold growth; while the seeds packaged with XC plastic remained intact and healthy. When these stored seeds were subjected to hydroponic cultivation, after 7 days the XC plastic-packaged seeds had a survival rate exceeding 90% and exhibited healthy roots and leaves. The unpackaged seeds experienced excessive moisture loss, while the PE-packaged seeds molded or rotted due to intense respiration, and neither could grow. Therefore, the XC plastic, with its excellent water vapor permeability and oxygen barrier properties, demonstrates great potential for applications in seed packaging.

Fig. 10 The typical application of xylan- and xylose-derived bioplastic for (a) seed storage;^47,49 (b) food preservation;¹¹³ (c) 3D printing;^94,95 (d) microplastic monitoring application.¹⁰⁰

To mitigate the detrimental effects of oxidant pretreatment on the structure of nanofiltration membranes for agricultural water purification, it is necessary to improve the chlorine resistance of the nanofiltration membranes while maintaining their separation performance and simplifying the treatment process. Xie et al. proposed a method to prepare nanofiltration membranes derived from xylose-based polyester with excellent separation performance and chlorine resistance.¹⁰⁹ The water-soluble monomer and 1,3,5-benzenetricarbonyl chloride (TMC) were subjected to interfacial polymerization, generating membrane-exhibited hydrophilicity and a thin separation layer, thereby achieving high water permeability with an optimal value of 28.7 L m⁻² h⁻¹ bar⁻¹. Traditional polyamide nanofiltration membranes generally have a low water flux, typically ranging from 10–20 L m⁻² h⁻¹ bar⁻¹. Meanwhile, due to its highly cross-linked structure and negatively charged surface, the membrane exhibited an excellent Na₂SO₄ rejection rate, reaching as high as 95.4% (polyamide nanofiltration membrane: 80–95%). Compared with the commercially available polyamide membranes, the xylose-based polyester nanofiltration membrane exhibited better chlorine resistance and structural integrity, and stable separation performance, laying the foundation for the utilization of xylose-based plastics in the field of agricultural irrigation water resources.

5.2. Food package

Traditional food packaging materials are predominantly made of petroleum-based plastics, which not only cause environmental pollution but also have poor oxygen barrier properties, posing limitations in extending the shelf life of foods. Shao et al. prepared a high oxygen-barrier xylan-based hydrophobic film using a one-step etherification method with epichlorohydrin, with an oxygen transmission rate as low as 1.9 (cm³ μm) (m² d kPa)⁻¹, much lower than that of the common LDPE with an oxygen transmission rate of 7900 (cm³ μm) (m² d kPa)⁻¹.¹¹⁰ Considering that antimicrobial properties are crucial in food preservation, Yang et al. incorporated an antimicrobial agent, sodium benzoate, assembled with β-cyclodextrin into the xylan-based composite package film, which enhanced the loading and release capacity of the antimicrobial agent.¹¹¹ It exhibited a water vapor transmission rate of 1.3 (g mm) (m² d kPa)⁻¹ and an oxygen transmission rate of 1.0 (cm³ μm) (m² d kPa)⁻¹, and significantly improved the film's ability to inhibit the growth of Staphylococcus aureus, while maintaining excellent barrier properties. Liu et al. prepared a composite film composed of xylan/polyvinyl alcohol (PVA) doped with nano-ZnO and nano-SiO₂.¹¹² Hydrogen bonding interactions exist between the nano-ZnO and nano-SiO₂ and the PVA and xylan, which can effectively enhance the mechanical strength, moisture and oxygen barrier properties, and surface hydrophobicity of the composite film. FTIR and X-ray diffraction results indicate that when the content of nano-ZnO and nano-SiO₂ is 3%, the tensile strength of the composite film increases from 16 to 22.5 MPa, the water vapour permeability (3.03 × 10⁻¹¹ g m⁻¹ s⁻¹ Pa⁻¹) and oxygen permeability (5.003 cm³ m⁻² d⁻¹ 0.1 MPa⁻¹; PVA: 10–20 cm³ m⁻² d⁻¹ 0.1 MPa⁻¹) reach minimum values, and the ultraviolet transmittance decreases, showing potential applications in the packaging field due to its anti-ultraviolet and anti-oxidation properties.

Recently, Zhang et al. utilized dodecyl-modified hemicellulose to prepare food packaging materials, which exhibited an ordered honeycomb-like surface structure, high transparency, and antioxidant activity (Fig. 10b).¹¹³ The transparent packaging film was observed to have excellent hydrophobicity and enhanced mechanical properties, with a water vapour transmission rate of 2.23 ± 0.11 (10⁻¹⁰ g m⁻¹ s⁻¹ Pa⁻¹) and an oxygen transmission rate of 4.24 ± 0.30 (cm³ μm) (m² d kPa)⁻¹. When applied for the packaging and storage of green peppers, the bioplastic was able to significantly extend the storage period to 15 days.

Therefore, xylan-based plastics derived from renewable resources exhibit safety, non-toxicity, and bio-degradability, making them environmentally friendly, and the packaging materials prepared from them also possess adequate oxygen barrier properties and excellent mechanical performance, which gives them great potential for application in the food packaging industry.

5.3. 2D and 3D printing

The increasing demand for more sustainable and environmentally friendly materials has driven significant research and development into bio-based polymers and resins for various applications, including 2D and 3D printing. The groups of Buchard have bio-derived polyesters via the ROP of the D-xylose oxetane with phthalic anhydride (PA) and glutaric anhydride (GA), respectively, followed by deprotection.¹¹⁴ The polymers were evaluated as potential amorphous solid dispersion (ASD) carriers for the BCS class II drugs nifedipine and mefenamic acid (MFA). Initial screening was performed using high-throughput 2D inkjet printing, which showed that the deprotected polymers could stabilize amorphous dispersions of the drugs. Upscaled ASD formulations were then prepared and characterized, revealing interactions between the drugs and polymers. Dissolution studies demonstrated the polymers’ ability to improve the apparent aqueous solubility of the drugs, with nifedipine solubility enhanced by up to 918% and MFA by 142%. The tunability of the polymer properties, such as glass transition temperature, through the choice of cyclic anhydride comonomer was highlighted as an advantage in tailoring the polymers for specific drug applications.

The amorphous, rigid nature of the DMGX monomer in the PAX polymers imparted high glass transition temperatures from 72 to 137 °C, larger than commercial bioplastics (PLA: 55–65 °C; PET: 80–85 °C). The PAX polymers, specifically poly(butylene xylosediglyoxylate) (PBX), poly(pentylene xylosediglyoxylate) (PPTX), and poly(hexylene xylosediglyoxylate) (PHX), were shown to be processable by common industrial techniques like compression-molding, vacuum-forming, injection-molding, and even 3D printing (Fig. 10c).⁹⁴ The PAX polymers demonstrated high thermal stability, with decomposition onset temperatures of 319–344 °C (PLA: 180 °C), allowing processing at temperatures as low as 140 °C and up to 200 °C, featuring a wide processing window with low energy consumption. In contrast to PLA being biobased but relying on food crops for raw materials and PET being non-degradable, the raw materials of PAX polymers are renewable xylose (derived from agricultural waste), which can be chemically recycled and biodegraded. In addition, PAX polymers display high tensile strength (30–70 MPa) and high ductility (elongation at break ∼100%), while PLA and PET are brittle (elongation at break ∼6%), thus showing feasibility in the field of 3D printing. The great ductility allowed the compression-molding, vacuum-forming, injection-molding, and 3D printing of parts from PHX, PPTX, and PBX without the polymers becoming soft and losing their form, even when exposed to boiling water.

The dried DMGX-based high-performance polyamides also exhibited good processability through piston injection molding at 260 °C and 1000 bar injection pressure, producing tensile specimens and rheology discs with only a 5% decrease in molecular weight.⁹⁵ The material was also successfully processed by high-shear twin-screw extrusion at 250 °C, 125 RPM, 7.8 Nm torque, and 30–50 bar die pressure, yielding both filaments and pellets. As a proof of concept, the polyamide was blended with a polyethylene carbon-black masterbatch, pelletized, and re-extruded at 250 °C into a 1.75 mm diameter filament with excellent dimensional accuracy, within ±0.1 mm. The fully amorphous structure avoids shrinkage and warping during the 3D printing process, making it suitable for complex geometries (such as thin-walled structures), while PLA and PET are semi-crystalline or partially crystalline materials. Furthermore, the polyamide was utilized in additive manufacturing via fused-filament fabrication to produce an iPhone 11 Pro case, employing a nozzle temperature of 275 °C and a bed temperature of 110 °C, demonstrating the material's ease of processability.

5.4. Other applications

Monomers derived from xylose can not only be polymerized into resins, but also serve as recyclable and biodegradable solvents for the synthesis of new polyesters. The groups of Luterbacher have developed a new class of sustainable solvents derived from the xylose component of lignocellulosic biomass.^115,116 These xylose acetal solvents, including diformylxylose (DFX), dipropylyxose (DPX), dibutylxylose (DBX), and diisobutylxylose (DIBX), are produced in high yields of up to 83 mol% from agricultural waste like corn cobs. Compared with other bio-based solvents, the xylose acetals have excellent biomass utilization efficiency, preserving 97% of the xylose carbon atoms. The xylose acetals also exhibit desirable properties like low melting points, low flammability, and no peroxide formation, making them attractive alternatives to hazardous aprotic solvents. They were successfully applied in enzymatic polycondensation reactions catalyzed by Candida antarctica lipase B (CaLB) to synthesize both aliphatic and aromatic bio-based polyesters. The xylose acetal solvents, especially DBX, outperformed the traditional hazardous solvent diphenyl ether in facilitating the CaLB-catalyzed polycondensation reactions. Furthermore, up to 98 wt% of the xylose acetals could be recovered and recycled, enhancing the overall sustainability of the process.

The incorporation of bio-based polymers has improved the ionic transport performance and stability of solid polymer electrolytes (SPE), reducing the reliance on limited resources like rare metals. Bio-polymers can also enhance the interfacial contact between the electrolyte and electrode materials, improving battery cycle life and energy density. The groups of Buchard described the development of a novel solid polymer electrolyte (SPE) based on a crosslinked polyester made from D-xylose and 10-undecenoic acid, which is then combined with the lithium salt LiTFSI.¹¹⁷ The polyester was synthesized via acyclic diene metathesis (ADMET) polymerization, resulting in polymers with varying molecular weights. To transform the sticky, viscous polyester into a free-standing SPE film, the polymer was crosslinked using the PEO-like crosslinker 2,2′-(ethylenedioxy)diethanethiol (EDDET) via a UV-initiated thiol–ene reaction. Conductivity decreased by increasing polymer molecular weight, but could be optimized by tuning the crosslinking density and salt concentration, avoiding the room temperature crystallization problem of PEO-based SPEs. The best-performing SPE exhibited an ionic conductivity of 1.0 × 10⁻⁵ S cm⁻¹ at 60 °C and a high lithium transference number of 0.84 (traditional PEO-based SPEs: ∼0.2), which helps to suppress battery polarization and improve battery cycle performance.

Room-temperature phosphorescent (RTP) polymer materials exhibit high flexibility and scalable manufacturability, making them promising for organic electronics applications. When exposed to ultraviolet light, pure xylan powder inherently produces persistent and bright phosphorescence, with the emitted color varying depending on the wavelength of the incident light. Our team has recently succeeded in transforming dialdehyde-modified xylan into flexible, transparent, and high-strength phosphorescent thin-film materials.²⁷ The highly crystalline linear xylan displays a phosphorescence lifetime of 588.8 ms at room temperature, while the phosphorescence lifetime of microcrystalline cellulose is only 20.4 ms. Interestingly, upon deactivation of the excitation source, the material emits varying persistent luminescence colors, ranging from blue to yellow to green, showing a dual dependence on excitation wavelength and time. In contrast, traditional organic phosphorescent materials require the synthesis of different luminophores to achieve color changes, a process that is more cumbersome. This tunable persistent luminescence property of the natural hemicellulose-based UOP thin-film material holds promise for future applications in anti-counterfeiting and data encryption. Besides, the bioplastics we developed that derived from diallyl glyoxylate xylose (DAGX) exhibit typical fluorescence properties, which were primarily due to the retention of the rigid structure of natural xylose rings within the polyester structure (Fig. 10d).¹⁰⁰ The unique fluorescent properties of XP allow for effective tracking of the microplastics formed during its degradation in aqueous environments using fluorescence spectroscopy.

6. Conclusion and perspective

Over the past decade, both the modification of natural xylan and the synthetic xylose-derived resin have achieved significant progress. Benefiting from new and more advanced green modification methods, xylan and xylose have evolved from traditional biomass waste or intermediates into feedstocks for high-value-added bio-based plastics, which has also broadened the variety of sugar-based bioplastics. Some xylose-based functional plastics have also shown potential for scaled-up application, and more and more scholars have begun to focus on developing hemicellulose -and xylose-based functional plastics.

For xylan, although its structure is relatively complex due to the presence of side chains, the side chains significantly increase its water solubility, and the existence of side-chain sugar units can compensate for the low reactivity caused by the absence of the –CH₂OH group. It can also avoid the problem of poor extensibility and toughness of membrane plastics caused by their crystallization. On the other hand, recent studies have shown that most xylans will have a low side-chain content after concentrated alkali extraction, resulting in linear xylans with xylose content nearly above 90%, which makes the properties of the prepared materials more uniform. Furthermore, both chemical modification and graft polymerization methods can significantly improve the inherent lack of thermoplasticity of xylan. Due to its relatively low molecular weight, the glass transition temperature of its derivatives is much lower than that of cellulose derivatives, making it easier to process. Furthermore, as a polysaccharide, the inherent excellent biocompatibility and biodegradability make it particularly promising for applications in packaging and agriculture.

D-Xylose, as a degradation product of hemicellulose, has seen a growing interest in its utilization as a monomer for synthesising functional biodegradable polymers in recent years. Traditionally, if functional plastics were to be derived from xylose, it would require ring-opening transformations, such as the preparation of diols and furandicarboxylic acid. In recent years, the utilization of xylose as a main-chain monomer for polyesters or polyamides has exhibited some distinctive properties, such as a relatively high T_g, unique fluorescent/phosphorescent characteristics, and excellent processability. Among various polymerization approaches, ring-opening polymerization and condensation polymerization hold greater potential for scaled-up applications. Particularly commendable is the fact that the direct separation of components from woody fiber feedstocks (such as corn stover), coupled with the simultaneous catalytic conversion to obtain xylose-based monomers for polymerization, represents a green approach for the production of bioplastics. This approach not only has high production efficiency, low production cost, and low carbon emissions, but it also enables the high-value and sustainable utilization of biomass components.

For the future development of xylan and xylose-based bioplastics, our opinions are as follows:

Xylan from different plant sources has structural variations, and the targeted selection of suitable feedstock can be very helpful in improving bioplastic performance. For example, selecting low-branched xylans extracted through concentrated alkali treatment can greatly mitigate the heterogeneity in material properties caused by the diversity of raw material structures. Besides, give priority to using agricultural residues and non-food biomass sources (such as pulping waste, corn stover, wheat straw, and bagasse, etc.) to ensure sustainability and reduce competition with food production. Other natural macromolecules, such as starch and cellulose, can be introduced to construct xylan-based composite materials, synergistically leveraging the advantageous properties of each component to further expand the range of applications. The compatibility issue needs to be a key focus, and methods such as covalent coupling and physical crosslinking should be adopted to optimize interfacial bonding and prevent phase separation.

More methods are needed to obtain xylose-based monomers during the separation and extraction process of biomass, which can significantly shorten the synthesis pathway and improve atom utilization and energy efficiency. The “one-pot” integrated process not only can reduce costs, but also aligns with the principles of Green Chemistry. Given the high susceptibility of xylan to hydrolysis under acidic conditions, the introduction of suitable catalysis such as aldehydic acid to enable one-step hemiacetal monomer formation, or the incorporation of a small amount of sulphuric acid to facilitate concurrent acidic cleavage and esterification modification, holds promise for improving atom economy and significantly reducing economic costs. Furthermore, if these catalysts are applied in the form of deep eutectic solvents, it would be possible to optimize the environmental friendliness of monomer synthesis under milder conditions.

The polymerization synthesis pathways starting from xylose monomers, such as condensation, ring-opening, and click polymerization, can all be optimized for reaction conditions to achieve a more environmentally friendly preparation process. For example, by utilizing clean energy sources such as microwaves or ultrasound to drive the reactions, the use of organic solvents can be avoided. Alternatively, using enzymatic catalysis or low-boiling co-solvents as green media can significantly reduce energy consumption and chemical reagent input. Additionally, the ability to recycle the costly catalysts employed in ROP or ADMET polymerization is a priority for future research on the green polymerization of xylose monomers. Exploration can also focus on emerging technologies such as electropolymerization or catalyst-free polymerization, which enables these polymerization techniques to be scaled up at lower cost and with a reduced carbon footprint.

Apart from focusing on the green aspects of polymerization synthesis, a comprehensive evaluation of the environmental impact across the entire life cycle of xylose-derived bioplastics is also required. At the same time, the economic viability of the materials should also be considered, with improvements in raw material utilization efficiency and energy recovery, to reduce production costs and lay the foundation for industrial-scale applications. A comprehensive assessment of the environmental impact of the biodegradation products is required. It is necessary to conduct in-depth analyses of the intermediates and final products generated during the degradation process, and to assess their potential impact on the ecological environment in terms of toxicity and bioaccumulation. This not only helps to guide the molecular design of the materials but also provides important environmental safety evidence for future large-scale applications.

The application prospects of xylose-derived plastics are broad, not only limited to traditional areas such as packaging and agriculture, but also with the opportunity to enter the high-performance domains of engineering plastics and electronic devices. Through molecular design and composite modification, xylose-derived materials can be endowed with special functions such as heat resistance, flame retardancy, and conductivity, meeting the demanding use conditions in the engineering field. Meanwhile, their excellent biocompatibility and biodegradability also provide a sustainable solution for managing electronic waste. The focus in the future is to further enhance the mechanical and thermal stability properties of xylose-based materials to meet the demands of engineering applications.

Author contributions

Yuanting Dai: methodology, visualization, writing – original draft. Qiang Xia: review and editing. Zijun Mao: review and editing. Junjie Mu: review and editing. Feng Peng: review and editing. Xiang Hao: supervision, conceptualization, visualization, writing, review and editing, funding acquisition.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was financially supported by National Key Research and Development Program of China (2023YFD2201904), National Natural Science Foundation of China (22208022), National Science Fund for Distinguished Young Scholars of China (32225034), Fundamental Research Funds for the Central Universities (QNTD202302), and Ministry of Education, China-111 Project (BP0820033).

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Footnote

† These authors contribute equally.

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