Polyesters derived from bio-based eugenol and 10-undecenoic acid: synthesis, characterization, and structure–property relationships

Abstract

Renewable monomers derived from eugenol and 10-undecenoic acid were synthesized via thiol-ene click and nucleophilic substitution reactions. With these monomers in hand, a series of thermoplastic polyesters with tunable thermo-mechanical properties were prepared via two-step melt polycondensation. The prepared polyesters exhibit weight-average molecular weights in the range of 21 000–48 000 g mol⁻¹, together with polydispersity values between 1.8 and 2.1. Experiments involving thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were carried out in order to study the thermal and mechanical properties of the polymers. All prepared polyester species prove to be thermally stable at temperatures up to 300 °C. Furthermore, the aromatic and semi-aromatic polyesters exhibit fully amorphous behaviours with glass transition temperatures (T_gs) ranging between −34.13 and 6.97 °C. Herein, the density of the rigid aromatic rings along the main chains of the polyester has a distinct influence on the T_g values. Furthermore, the fully aliphatic polyesters prove to be semi-crystalline with inconspicuous T_g. The incorporation of aromatic eugenol into the polyester chains and the density of the benzene rings were also found to result in a significant improvement in the mechanical properties, including Young's modulus and elongation at break in the range of 11.6 to 44.2 MPa and 33.6–106.7%, respectively. The relatively high tan δ and the low storage modulus indicate that eugenol-based polyesters feature superior viscosity properties and may therefore find future applicability in a variety of high-tech materials.

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Introduction

As a result of the rapid depletion of fossil resources and continuously increasing environmental concerns, humanity is facing unprecedented challenges. Further emphasized by phenomena like global warming, environmental pollution and land desertification, it is urgent to find substitutes of fossil energy sources.^1–5 Polyesters, a type of multifunctional material, are receiving an increasing demand in a multitude of industrial applications. However, most of the polyesters are currently prepared from compounds extracted from fossil feedstock. Due to decreasingly low availability of fossil resources, bio-based resources represent interesting “green” alternatives to provide the precursors needed for polyester synthesis.^6–10

Amid a selection of renewable resources, plant oils,^11,12 including castor oil and palm oil, stand in a critically important position historically and currently due to their multifunctional transformation potential.^13,14 Classical and well-established chemical transformations may preferentially be done at the ester group of the native triglycerides. Examples include hydrolysis to form free fatty acids groups and glycerol, transesterification to yield corresponding fatty acid methyl esters or hydrogenation of both fatty acids and their corresponding dimethyl esters to result in the formation of fatty alcohols. Primary oleochemicals, such as fatty acids, their corresponding methyl esters, amines and alcohols, are generally used in the production of surfactants,¹⁵ lubricants¹⁶ and coatings.^17,18 The use of modern synthetic techniques, together with enzymatic and microbial methods, has led to an extraordinary expansion of synthetic routes for the production of novel fatty acid compounds,^19–22 generated from selectively modification in the alkyl chains. Due to the steric demand of the alkyl chains and the generally complex structural parameters of fatty acids, it proves to be rather tedious to obtain symmetrical diols or diesters via conventional organic synthesis. 10-Undecenoic acid, a synthetic unsaturated fatty acid derived from thermal cleavage of the naturally occurring ricinoleic acid, contains a terminal olefinic group as well as a terminal carboxylate.²³ No other functional groups or sterically demanding chains can be found, making 10-undecenoic acid an ideal candidate for the synthesis of high-purity symmetrical diesters or diols via one-step thiol-ene coupling reactions and subsequent simple recrystallization processes for purification.

Eugenol,^24,25 represents a natural phenolic allylbenzene compound extracted from lignocellulosic biomass, and presents particularly in clove oil. Clove is commercially cultivated in India, Madagascar, SriLanka, Indonesia and the southern region in China. Eugenol is responsible for the intense aroma of cloves. The component accounts for 72–90% of the essential oils extracted from cloves and is mainly used as an antibiotic and antihypertensive agent. Furthermore, the natural product can be used in the production of perfumes, cosmetics, soaps, and even food. In recent years, study on eugenol mainly focused on ruthenium-catalysed olefin metathesis reactions (e.g. self-metathesis,²⁶ cross-metathesis²⁷ with electron-deficient olefins) due to a terminal olefinic bond present that proves to be particularly suitable for transformations via olefin metathesis. Therefore, it may be possible to prepare different types of functionalized phenol derivatives to develop new routes for the production of different multifunctional products from renewable sources.²⁸ Noteworthy in this context is the fact that ruthenium-promoted isomerization reactions to form isoeugenol generally result in improved selectivity properties and product yields.^29–31 Hence, the isomerization of allylbenzenes offers an attractive synthetic approach that can be applied particularly to β-methylstyrenes, which represent common starting materials particularly in the flavour and fragrance industries, as well as valuable intermediates for the production of pharmaceutical compounds. Other than the applications listed above, Sarojadevi and Thirukumaran have also discovered a novel synthetic route towards the production of polybenzoxazines via a Mannich-like condensation reaction of eugenol, formaldehyde and an amine.³² However, to the best of our knowledge, the synthesis of polyesters starting from renewable eugenol has never been reported. The terminal olefinic bond renders eugenol substrate ideal for click reactions, by which ester or hydroxyl functional groups may be incorporated. Recently, we have developed a variety of polyesters based on renewable eugenol and α,ω-diols. The polymers obtained exhibited excellent ductility.³³ Inspired by this promising result, we explored the possibility of designing polyesters from entirely renewable plant-oil sources using 10-undecenoic acid and eugenol as starting materials.

In this study, bio-based monomers derived from eugenol and 10-undecenoic acid were synthesized. The precursors P1 and P2 were prepared using eugenol and methyl thioglycolate or 2-mercaptoethanol via thiol-ene click reactions,^34–37 followed by Williamson ether synthesis³⁸ using methyl chloroacetate or 1,4-dibromobutane to obtain the four aromatic monomers M1-M4 were obtain. Next, methyl 10-undecenoate and 10-undecen-1-ol, derivatives of 10-undecenoic acid, were coupled with methyl thioglycolate or 2-mercaptoethanol via thiol-ene click reactions to afford the three aliphatic monomers, M5-M7. Using these monomers, fully aromatic polyesters, semi-aromatic polyesters and fully aliphatic polyesters were obtained via a two-step melt-polycondensation method. The influence of the aromatic eugenol incorporated into the polyester chains on the thermal and mechanical properties of the obtained polyesters was studied and compared with the conventional fully synthetic polymers PET and PBT.

Experimental section

Materials

Eugenol (99%), methyl thioglycolate (99%), 2-mercaptoethanol (98%), methyl chloroacetate (98%), methyl 10-undecenoate (98%), 10-undecen-1-ol (98%), tetrabutyl titanate (TBT) (≥99%), benzoin dimethyl ether (DMPA, 99%), and ethylene carbonate (98%) were purchased from Sigma-Aldrich. 1,4-Dibromobutane (98%) and other commonly used chemical reagents were purchased from Tianjin Chemical Reagent Co. (Tianjin, China) and used without further purification. Acetonitrile, N,N-dimethylformamide (DMF) and other solvents were dried according to the standard methods in the literature if necessary.

Instruments and methods

¹H NMR and ¹³C NMR spectra were recorded in CDCl₃ at room temperature using a Bruker AVANCE III 400 MHz NMR spectrometer. Tetramethylsilane was used as the internal standard. Fourier transform infrared spectra (FTIR) were measured at room temperature using a Bio-Rad FTS6000 spectrophotometer. Polymer samples were prepared by grinding the polymers with KBr powder, followed by compressing the mixture to form a pellet. Fourier transform high resolution mass spectra (FTMS) were measured on a Varian 7.0T FTMS with positive electrospray ionization (+ESI). The molecular weight and polydispersity of the obtained polyesters were determined via size exclusion chromatography (SEC, Waters 1525 dual pump, Waters 2414 differential refraction detector, Waters 2487 dual UV detector and Waters 717 plus automatic sampler) at 35 °C. THF was used as the eluent at a flow rate of 1.0 mL min⁻¹. The average molecular weights were calibrated using polystyrene samples as standards. Thermogravimetric analysis (TGA) was performed using a NETZSCH TG209 analyzer. In a general method, the polymer sample was heated from 25 to 800 °C under a nitrogen atmosphere at a rate of 10 °C min⁻¹. The temperature leading to 5% weight loss, the temperature for maximum degradation rate, and residue weight (%) at 800 °C were recorded. Differential scanning calorimetric (DSC) thermograms were measured using a DSC Q100 apparatus from TA instruments. Polymer samples were first heated from 25 to 150 °C and then cooled to −80 °C. The glass transition (T_g), melting (T_m) and crystallization temperatures (T_c) and their respective enthalpies ΔH_m and ΔH_c were calculated from the second heating run. All runs were performed at a rate of 10 °C min⁻¹. The tensile assays were performed in triplicates on dog-bone-shaped sample bars (12 mm × 2 mm × 0.5 mm) at 25 °C at a tensile rate of 50 mm min⁻¹. Young's modulus, tensile stress at break and tensile strain at break were acquired by averaging the data from the three paralleled test specimens. Dynamic mechanical analysis (DMA) were carried out using a NETZSCH DMA242 analyzer in the controlled force–tension film mode with a preload force of 0.1 N, a deformation of 0.1%, and an amplitude of 10 mm, at a fixed frequency of 1 Hz in the −60 to 60 °C range and a heating rate of 3 °C min⁻¹. The measurements were performed in triplicates using rectangular specimens (length × width × thickness = 25 × 5 × 1 mm³) obtained from casting chloroform solutions at a concentration of 0.1 g mL⁻¹.

Synthesis of precursors and monomers

The synthetic routes of precursors, monomers are presented in Schemes 1–3. The detailed synthetic routes, characterization data and spectra are presented in the ESI (Experimental Section and Characterization Section, Fig. S1–S13†).

Scheme 1 Synthetic routes of precursors P1 and P2 via thiol-ene click reactions from eugenol and mercapto-containing compounds.

Scheme 2 Synthetic routes of monomers M1-M4 via nucleophilic substitution reactions from precursors P1 and P2.

Scheme 3 Synthetic routes of monomers M5-M7 via thiol-ene click reactions from methyl 10-undecenoate and 10-undecen-1-ol.

General procedure for the synthesis of polyesters

The polymerization reactions were uniformly performed in a 50 mL Schleck flask equipped with a magnetic stirrer, a nitrogen inlet and a vacuum distillation outlet. The individual reactions in this synthetic route are formulated in Scheme 4. A 1 : 1.05 molar ratio of diesters to diols was adopted to guarantee hydroxyl terminated. Tetrabutyl titanate (TBT, 0.6% mmol per mol of diester) was used as the catalyst. Before transesterification, the apparatus was vented with nitrogen for 10 minutes to ensure no residual oxygen was present in the system. Transesterification reactions were carried out at 140–160 °C for about 4 hours at a low nitrogen flow. Polycondensation reactions were then conducted at 180 to 220 °C for 3–5 h under a 0.03–0.06 mbar vacuum until the viscosity of the mixture resulted in the stirrer to get stuck, indicating completeness of the reaction. The mixtures were cooled to room temperature and the atmospheric pressure was recovered with nitrogen to prevent degradation of the products. The obtained polymers were dissolved in a minimum amount of chloroform and precipitated in excess methanol, resulting in removal of any remaining oligomers and impurities formed. The product was collected by filtration and was dried in vacuo. PM57 and PM66 prove to be semi-crystalline materials, and all other polymers were found to be yellow viscous amorphous materials. The characterization data of PM14 and PM57 is presented below. The other obtained polymers have similar data and their corresponding spectra can be seen in Fig. 1 and S16–S32.†

Scheme 4 Synthetic strategies of the polyesters (PM13, PM14, PM15, PM23, PM24, PM25, PM37, PM47, PM57 and PM66).

Fig. 1 ¹H NMR spectra of PM15, PM23 and PM57.

PM14. ¹H NMR (CDCl₃, 400 MHz): δ 1.84–1.93 (m, 6H, –SCH₂–CH₂–CH₂–), 1.96–2.08 (m, 4H, ArOCH₂–CH₂–), 2.51–2.60 (m, 4H, –S–CH₂–CH₂CH₂–), 2.60–2.70 (m, 8H, –S–CH₂–CH₂CH₂– and –SCH₂CH₂–CH₂–), 2.71–2.80 (m, 4H, –COOCH₂–CH₂–), 3.21 (s, 2H, –S–CH₂–CO–), 3.83 (s, 6H, ArO–CH₃), 3.86 (s, 3H, ArO–CH₃), 4.02–4.12 (m, 4H, ArO–CH₂–CH₂–), 4.23–4.34 (dt, 4H, J₁ = 26.8 Hz, J₂ = 6.8 Hz, –COO–CH₂–), 4.66 (s, 3H, ArO–CH₂–CO–), 6.63–6.85 (m, 9H, Ar–H) ppm; ¹³C NMR (CDCl₃, 100.6 MHz): δ 26.06 (ArOCH₂–CH₂–), 30.29 (–SCH₂–CH₂–CH₂–), 30.35 (–SCH₂–CH₂–CH₂–), 30.53 (–SCH₂–CH₂–CH₂–), 31.27 (–S–CH₂–CH₂CH₂–), 31.62 (–S–CH₂–CH₂CH₂–), 31.68 (–S–CH₂–CH₂CH₂–), 32.05 (–SCH₂CH₂–CH₂–), 33.52 (–S–CH₂–CO–), 34.25 (–S–CH₂–CH₂O–), 55.91 (ArO–CH₃), 55.98 (ArO–CH₃), 63.95 (–SCH₂–CH₂–O–), 64.23 (–SCH₂–CH₂–O–), 66.69 (ArO–CH₂–CO–), 68.83 (ArO–CH₂–CH₂–), 112.37 (Ar–C), 112.59 (Ar–C), 113.41 (Ar–C), 114.86 (Ar–C), 120.30 (Ar–C), 120.34 (Ar–C), 134.11 (Ar–C), 135.88 (Ar–C), 145.54 (Ar–C), 146.79 (Ar–C), 149.42 (Ar–C), 149.61 (Ar–C), 168.96 (ArOCH₂–CO–), 170.25 (–SCH₂–CO–) ppm; FTIR: 2929, 1757, 1734, 1513, 1264, 1232, 1144, 1036, 807 cm⁻¹.

PM57. ¹H NMR (CDCl₃, 400 MHz): δ 1.21–1.43 (m, 26H, –CH₂– aliphatic), 1.52–1.69 (m, 8H, –SCH₂–CH₂–CH₂–, –COOCH₂–CH₂–, and –CH₂–CH₂CO–), 2.24–2.35 (q, J = 8.0 Hz, 2H, –CH₂–CH₂–CO–), 2.51–2.79 (m, 6H, –S–CH₂–CH₂CH₂– and –S–CH₂–CH₂O–), 3.18–3.25 (d, 2H, –S–CH₂–CO–), 4.01–4.16 (dt, J₁ = 6.4 Hz, J₂ = 26.4 Hz, 2H, –COO–CH₂–CH₂CH₂–), 4.17–4.32 (dt, J₁ = 6.8 Hz, J₂ = 23.2 Hz, 2H, –COO–CH₂–CH₂S–), ppm; ¹³C NMR (CDCl₃, 100.6 MHz): δ 24.94–25.04 (–CH₂– aliphatic), 25.88–25.96 (–CH₂– aliphatic), 28.61 (–CH₂– aliphatic), 28.70, 28.79 (–CH₂– aliphatic), 28.87 (–CH₂– aliphatic), 29.02–29.04 (–CH₂– aliphatic), 29.16 (–CH₂– aliphatic), 29.21 (–CH₂– aliphatic), 29.26 (–CH₂– aliphatic), 29.42 (–CH₂– aliphatic), 29.47 (–CH₂– aliphatic), 29.53 (–CH₂– aliphatic), 29.72 (–CH₂– aliphatic), 30.39 (–CH₂–CH₂O–), 30.56 (–CH₂–SCH₂CO–), 32.43 (–COOCH₂CH₂S–CH₂–), 32.75–32.78 (–CH₂–CH₂–CO–), 33.63–33.76 (–S–CH₂–CO–), 34.26–34.41 (–S–CH₂–CH₂O–), 63.37 (–CH₂–O–), 64.29 (–CH₂–O–), 64.41 (–SCH₂–CH₂–O–), 65.46 (–SCH₂–CH₂–O–).

170.42 (–SCH₂–CO–), 170.72 (–SCH₂–CO–), 173.63 (–CH₂CH₂–CO–), 173.97 (–CH₂CH₂–CO–) ppm; FTIR: 2918, 1732, 1470, 1310, 1201, 1178, 992, 719 cm⁻¹.

Results and discussion

Synthesis of precursors and monomers

Precursors P1 and P2 derived from eugenol. As a naturally occurring compound, eugenol features distinct structural characteristics. The terminal olefinic group and the phenolic hydroxyl group provide functional groups that render eugenol rather easy to derivatize. Click chemistry proposed by Sharpless in 2001,³⁴ has attracted widespread applications in organic chemistry and materials science. As the reaction procedures are usually simple to execute, highly efficient and highly selective, click chemistry provides an appealing route to heteroatom-linked molecular systems. The radical-mediated thiol-ene reaction represents one type of click reaction that is typically photo-initiated and conducted under rather mild conditions. As a consequence, sulfur-linked ester, hydroxyl and other functional groups can be introduced to the terminal olefin of eugenol in a feasible fashion. In this study, eugenol and methyl thioglycolate were reacted using DMPA and UV radiation according to a standard β-addition (Scheme 1). In the ¹H NMR spectrum of P1 (Fig. S1†), the appearance of two singlets at 3.2 and 3.7 ppm, one quintet at 1.8 to 1.9 ppm and the disappearance of olefinic characteristic peaks at 4.9 and 5.8 ppm, indicate the successful formation of a thioether linkage. By means of a similar protocol, 2-mercaptoethanol was unquestionably attached to the terminal olefin of eugenol (Scheme 1). The four triplets at 3.7, 2.7, 2.6 and 2.5 ppm and one quintet at 1.8 to 1.9 ppm in the ¹H NMR spectrum of compound P2 (Fig. S1†) can be clearly identified, indicating the successful formation of a thioether bond, established in a similar fashion as the β-addition in P1. The structures of compounds P1 and P2 were also confirmed by ¹³C NMR (Fig. S2 and S3†) and HRMS.

Monomers M1-M4 synthesized from precursor P1 and P2. Precursor P1 was reacted with methyl chloroacetate to afford the asymmetrical diester monomer M1. As reported previously, the incorporation of phenoxy ether linkages typically increases the solubility, processability, and toughness of aromatic polymers, without compromising the thermal properties.^39–41 In order to evaluate the impact on the thermal and mechanical properties, a 1,4-dibromobutane bridged symmetrical monomer M2 was also prepared. Monomer M3 was prepared from P2 and ethylene carbonate (Scheme 2). Following the same protocol of M2, M4 could also be produced. The structures of M1-M4 were all confirmed by ¹H NMR, ¹³C NMR (Fig. S4–S9†), FTIR and HRMS.

Monomers M5, M6 and M7 prepared from methyl 10-undecenoate and 10-undecen-1-ol. The terminal olefinic bonds render methyl 10-undecenoate and 10-undecen-1-ol ideal natural substrates for thiol-ene click chemistry. A solution of methyl 10-undecenoate and methyl thioglycolate was exposed to UV-light (λ = 365 nm) and reacted similarly as the β-addition described above (Scheme 3). The two singlets at 3.2 and 3.7 ppm in the ¹H NMR spectrum of M7 (Fig. S10†) is a clear indication for a successful reaction of methyl thioglycolate with methyl 10-undecenoate. Following a similar protocol, M5 and M6 could be prepared. The ¹H NMR spectra of M5 and M6 (Fig. S10†) provide clear evidences for the successful synthesis. Structural parameters of M5, M6 and M7 were also obtained by ¹³C NMR (Fig. S11–S13†), FTIR and HRMS.

Synthesis of polyesters

The bio-based monomers were polymerized mutually via a two-step melt-polycondensation technique to afford a series of thermoplastic polyesters as depicted in Scheme 4. TBT was used as the catalyst. The transesterification stage was carried out at 140–160 °C for approximately 4 hours at a low nitrogen flow. The polycondensation reaction was then continued at temperatures ranging between 180 and 220 °C for 3–5 hours under dynamic vacuum to facilitate the removal of methanol. Upon increasing viscosity, the stirrer was found to get stuck, indicating that the polymerization reaction proceeded to completion. Noteworthy in this context is the fact that the polymerization temperature should not exceed 220 °C, since titanium alkoxides are found to decompose above this temperature, resulting in the formation of stained products. After purification, the polyesters were obtained at yields ranging between 78% and 90%. PM66 was prepared from the self-condensation-type monomer M6. The stoichiometric ratio of ester and hydroxyl groups was strictly controlled to be 1 : 1. The molecular weights as well as polydispersities of the resulting polyesters were estimated by SEC (Fig. S14 and S15†). The polyesters were obtained with M_w ranging between 21 600 and 47 600 g mol⁻¹ and polydispersities ranging between 1.8 and 2.1 (Table 1).

Table 1 Molecular weights, polydispersities and isolated yields of the synthesized polyesters

Polyester	Mn (g mol^−1)	Mw (g mol^−1)	Polydispersity (PDI)	Isolated yield
PM13	13 000	23 600	1.8	82%
PM14	11 300	21 600	1.9	83%
PM15	21 100	41 600	1.9	85%
PM23	15 600	32 500	2.0	83%
PM24	15 900	30 800	1.9	81%
PM25	23 100	47 600	2.0	82%
PM37	14 600	28 300	1.9	78%
PM47	12 900	23 500	1.8	80%
PM57	20 700	44 400	2.1	88%
PM66	21 800	40 900	1.8	90%

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The chemical compositions of the obtained polyesters could be confirmed by ¹H NMR (Fig. 1, S16–S18†), ¹³C NMR (Fig. S19–S28†) and FTIR (Fig. S29–S32†). From the representative ¹H NMR spectra of polyesters in Fig. 1, the most obvious characteristic peaks were found between 4.1 and 4.4 ppm, corresponding to the signals for the two methylene groups adjacent to the hydroxyl group of the substrate diols being split into four triplets (PM15 and PM57). However, the ¹H NMR spectrum of PM23 features only two triplets between 4.1 and 4.4 ppm, corresponding to the two methylene groups adjacent to hydroxyl group in M3. This phenomenon can be ascribed to the asymmetrical nature of M1 and M7, which results in the formation of two types of ester groups with different chemical environments. Meanwhile, M2 proves to be a symmetrical diester and M3 is determined to be an asymmetrical diol. Therefore, in the ¹H NMR spectrum of PM23 the two methylene groups adjacent to the ester groups are split into two singlets between 3.2 to 3.3 ppm due to the asymmetrical nature of M3. The same feature can also be observed in the ¹H NMR spectra of PM15 and PM57, where the signals for the two methylene groups adjacent to ester groups of M1 and M7 are uniformly split into the corresponding singlets or multiplets as a result of the asymmetrical nature of M5. The other polyesters exhibit similar splitting modes in their respective ¹H NMR spectrum as mentioned above. A detailed discussion on the chemical microstructures is presented in the subsequent section. This distinct difference of the ester groups in the polyesters could also be observed in the FTIR spectra. In the FTIR spectrum of PM15 (Fig. S29†), two stretching vibrations of the ester carbonyl groups were observed at approximately 1734 and 1758 cm⁻¹. Meanwhile, in the FTIR spectrum of PM23 only one stretching vibration can be found at approximately 1734 cm⁻¹.

Chemical microstructures

The chemical microstructures of the obtained polyesters were studied using quantitative ¹³C NMR spectroscopy. The signals of all magnetically different carbons in the polyesters can be found well resolved in the ¹³C NMR spectra (Fig. 2, S33–S35†). Apart from the non-protonated aromatic carbons discussed in previous reports,⁴² the carbonyl and the methylene carbons close to the asymmetrical functionalities appear to be sensitive to sequence distributions at the dyad level. As shown in Fig. 2(A) and (B), corresponding to the splitting modes of the ester carbonyls and the methylene carbon adjacent to hydroxyls of diols along the main chains of PM15, respectively, the resonances of all magnetically different carbon atoms present in the backbone are well resolved. Due to the unequivalence of the two ester groups in M1 and the two methylene groups adjacent to the hydroxyl groups in M5, as well as the interaction between the uncoordinated esters and hydroxyl groups, four sets of dyads(RL-RR, RL–RL, LL-RR and LL-RL) with different sequence arrangements along the polyester chains were generated in PM15. Four methylene splitting peaks and four carbonyl splitting peaks appear in the 64–66 ppm and 168–172 ppm chemical shift regions, respectively. By integration of the four peaks, the dyad contents are found to be almost equal, which confirms the same reactivity of the participating functional groups in M1 and M5. Nevertheless, M2 features two of the same ester groups. Therefore, no splitting of the methylene carbon signals in the asymmetrical diols is observed. On the contrary, the asymmetrical methylene groups in the diols will split the ester carbonyl carbon in M2 into two peaks (Fig. S33†). Furthermore, M4 exhibits two equal hydroxyl groups equal to M2. It is expected that the ester carbonyls and the methylene groups adjacent to the hydroxyl groups will not split uniformly in PM24. According to the characteristics noted above, the chemical microstructures of the obtained polyesters can be divided into four types, i.e. I (PM13, PM15, PM37 and PM57), II (PM14, PM23, PM25 and PM47), III (PM24) and IV (PM66). Congeneric polyesters feature similar splitting modes. Therefore, the asymmetrical nature of the monomers plays a crucial role in determining the microstructures of the resulting polyesters. The ¹³C NMR signals used for the microstructure analysis of PM47 and PM66 are shown in Fig. S34 and S35,† respectively.

Fig. 2 ¹³C NMR signals used for the microstructure analysis of PM15 with indication of the dyads to which they are assigned.

Thermal stability

The thermal stabilities of the polyesters have been comparatively investigated by TGA. The TGA and TGA derivative curves are shown in Fig. 3, S36–S38,† respectively, and the thermal parameters are given in Table 2. The synthesized polyesters exhibit a 5% weight losses in the range of 307–351 °C. Maximum degradation rates were found at temperatures ranging between 374 and 391 °C. The latter finding indicates a significant thermal stability, similar to commercially available polyesters. The fully aliphatic polyesters PM57 and PM66 were found to be more stable than the fully aromatic polyesters and the semi-aromatic polyesters. Similarly, the fully aromatic polyesters, PM12, PM14, PM23, and PM34, show comparable thermal stability as the semi-aromatic polyesters, PM15, PM27, PM35, and PM47. The relatively low thermal stability of the eugenol-based fully aromatic polyesters and the semi-aromatic polyesters is most likely attributed to the inferior structural symmetry of the monomers derived from eugenol and the presence of sulphur atoms along the polymer chains, susceptible to thermal oxidation degradation. It is well known that linear aliphatic sections can decompose almost entirely compared to aromatic sections under the same TGA conditions.⁴³ As a consequence, the fully aliphatic polyesters PM57 and PM66 show negligible remaining weights lower than 2% at 800 °C. Simultaneously, the eugenol-based fully aromatic polyesters and semi-aromatic polyesters feature remaining weights in the range of 6.3–17.5% at 800 °C. The increase of remaining weight may be attributed to the incorporation of aromatic benzene rings into polymer chains and the resulting reduction of aliphatic sections. All the polyester species exhibit a single-step degradation behaviour, indicating that the ester groups display comparable thermal stabilities both in the aliphatic and aromatic segments.

Fig. 3 TGA curves of representative polyesters recorded from 25–800 °C at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere.

Table 2 Thermal properties of the synthesized polyesters

Polymer	T5%^a (°C)	Td^b (°C)	W^c (%)
a Temperature at which 5% weight loss was observed.b Temperature for maximum degradation rate.c Remaining weight at 800 °C.
PM13	330	374	10.4
PM14	332	378	10.2
PM15	338	374	6.3
PM23	348	389	10.9
PM24	338	382	17.5
PM25	339	388	8.3
PM37	307	381	12.0
PM47	327	375	8.8
PM57	351	388	1.8
PM66	348	391	1.4

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Thermo-mechanical properties

Important thermal properties, i.e. T_g, T_m, and the crystallization behaviour, were also determined by DSC. The second heating DSC curves of the polyesters without being subjected to any thermal treatments are shown in Fig. 4, and the corresponding analytical data are summarized in Table 3. As can be seen from inspection of the data in Table 3, the T_g values range from −34.13 to 6.97 °C, which prove to be rather low for engineered polymers and therefore result in a rubbery material. The relatively low T_g values are most likely due to the presence of heteroatoms S and O in the scaffold, which support internal rotation characteristics of the polyester chains. Aromatic rings are known to exhibit unfavourable effects on the internal rotation and therefore lead to an enhancement of the corresponding T_g. The T_g values of the polymers in this study are closely related to the densities of aromatic rings along the polyester chains. As shown in Fig. 5, the T_g values gradually increase in the range of −35 to 7 °C due to improving aromatic ring densities. Polyesters with the same aromatic ring densities, e.g. PM15 and PM37; PM25 and PM47; PM14 and PM23, exhibit similar T_g values. The T_g of PM24 was found to be −8.36 °C, similar to the T_g value found for PM25 (−6.35 °C) and PM47 (−4.05 °C). PM24, prepared from 1,4-dibromobutane bridged M2 and M4, contains a large number of phenolic oxygens along the main chains and the rotation of C–O proceeds easier than C–C. Consequently, PM24 exhibits a T_g close to those found for PM25 and PM47. The latter finding proves to be surprising since PM25 and PM47 contain more aliphatic methylene groups along their corresponding chains.

Fig. 4 DSC curves of polyesters 2nd scan carried out from −80 to 150 °C at a heating/cooling rate of 10 °C min⁻¹.

Table 3 Melting and crystallization values obtained from 2nd DSC heating scans at a heating rate of 10 °C min⁻¹

Polymer	Tg^a (°C)	Tm^b (°C)	ΔHm^b (J g^−1)	Tc^b (°C)	ΔHc^b (J g^−1)
a Glass transition temperature measured by DSC during the 2nd heating scan at 10 °C min^−1.b Melting (Tm) and crystallization (Tc) temperature and their respective enthalpies (ΔHm, ΔHc) measured by DSC at a heating/cooling rate of 10 °C min^−1.
PM13	6.97	—	—	—	—
PM14	−1.61	—	—	—	—
PM15	−34.13	—	—	—	—
PM23	0.23	—	—	—	—
PM24	−8.36	—	—	—	—
PM25	−6.35	—	—	—	—
PM37	−32.39	—	—	—	—
PM47	−4.05	—	—	—	—
PM57	—	45.35	80.06	30.13	76.82
PM66	—	Tm1 = 41.66	ΔHm1 = 24.82	28.56	98.74
		Tm2 = 54.83	ΔHm2 = 81.98

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Fig. 5 Stress–strain curves of polyesters at 20 °C, 50 mm min⁻¹.

From the second heating traces of DSC, it can also be observed that the fully aliphatic polyesters PM57 and PM66 exhibit semi-crystalline behaviours with inconspicuous T_g. All other polymer species are found to be amorphous viscous materials without fusion and crystallization characteristics, this finding is most likely due to the asymmetrical nature of M1 and M2. The presence of methoxy groups adjacent to the phenyl rings in M1 and M2 increases the structural asymmetry further, and the incorporation of long-chain linear aliphatic diols into the polyester chains cannot compensate for these structural defects. Therefore, random polyesters derived from renewable eugenol seem to be unfavourable for the regular packing of crystalline chains.

Tensile properties

To evaluate the influence of the eugenol-based monomers on the mechanical properties, tensile assays were carried out using films that were prepared via casting from chloroform solutions. Due to the relative brittleness exhibited by PM57 and PM66, films of these polymers could not be prepared. The majority of the other polymer species were found to be in an amorphous state caused by the lack of symmetry and the low T_g values (−34.13 for PM15 to 6.97 °C). As a consequence, we have only been able to produce fashioned films of polymers PM13, PM14, PM23 and PM47 and the tensile assays were carried out thereafter. The stress–strain curves recorded from essayed polyesters are shown in Fig. 5 and the mechanical parameters measured are illustrated in Table 4. The stress–strain curves demonstrate that the mechanical properties of eugenol-based fully aromatic polyesters prove to be far superior compared to the semi-aromatic polyesters. The Young's modulus ranging from 11.61 to 44.23 MPa were found to increase with the density of rigid phenyl rings incorporated into the polyester backbones. PM13, prepared from single aromatic M1 and M3, features the highest aromatic rings density compared to other polyesters and therefore exhibits the maximal Young's modulus. PM47, prepared from 1,4-dibromobutane bridged M4 and aliphatic M7, features the smallest number of aromatic rings and the most methylene units along the polyester chains. As a consequence, the smallest Young's modulus is observed. Unfortunately, the values of Young's modulus observed are most likely not high enough for potential application as structural materials compared to conventional PET (M_w: 32 100, PDI: 2.5)⁴⁴ and PBT (M_w: 41 300, PDI: 2.4).⁴⁵ The presence of methoxy groups adjacent to the phenoxy groups reduces the structural symmetry and crystallinity so that the mechanical strength of the polyesters is reduced accordingly. The elongation at break ranging from 33.64–106.67% displays an opposite effect. The semi-aromatic polyester PM47 features a maximal elongation at break of 106.67%, while the fully aromatic polyester PM13 exhibits an elongation at break of 33.64%. The higher elongations at break indicate that eugenol-based polyesters exhibit excellent ductility characteristics. Although PET and PBT feature higher modulus and strength properties, the elongations at break found are significantly lower (i.e. 14–23%). It has already been demonstrated that PET or PBT materials without any additives prove to be rather brittle. Plasticizers are usually added to increase structural integrity of PET or PBT materials in an effort to meet strict engineering standards set by the polymer industry in the production of synthetic polymers. Copolymerization with monomers derived from renewable eugenol may therefore offer an innovative approach to increase the structural toughness of PET and PBT.

Table 4 Tensile properties of the synthesized polyesters

Polymer	Young's modulus (MPa)	Ultimate strength (MPa)	Elongation at break (%)
PM13	44.2 ± 4.8	0.3 ± 0.1	33.6 ± 4.2
PM14	24.7 ± 1.5	0.3 ± 0.1	40.5 ± 2.8
PM23	31.0 ± 2.4	2.2 ± 0.5	68.8 ± 3.6
PM47	11.6 ± 4.8	0.3 ± 0.1	106.7 ± 4.2
PET	1032 ± 52	45 ± 7	23 ± 5
PBT	841 ± 15	42 ± 5	14 ± 3

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Dynamic mechanical analysis

In order to evaluate the viscoelasticity of the synthesized polyesters and to further understand the structure–property relationships, dynamic mechanical analyses (DMA) of the polyesters were carried out. The T_g values are adopted as the inflection temperature points of the maximum in loss modulus (E′′).⁴⁶ As noted above, all polyesters exhibit common glass transitions temperatures, except for the fully aliphatic species PM57 and PM66. However, DMA provided more conclusive results. As for PM23, T_g was determined from the loss modulus versus temperature trace (Fig. S39†) to be approximately 0.06 °C. The latter value is almost comparable to the value obtained by DSC (i.e. 0.23 °C). The other three polyester species features similar results, with the T_g values being in close agreement with the DSC results obtained. Regarding the hysteresis loss tan δ, the value is closely related to the molecular structures (i.e. the density of aromatic rings). As mentioned above, the fully aromatic PM13, PM14 and PM23 feature higher aromatic ring densities compared the semi-aromatic species PM47. When the polyesters are subjected to alternative forces or strains, the internal friction resistances observed are found to be in the following order: PM13 > PM14 ≈ PM23 > PM47. The tan δ value gradually increases in a similar fashion as the internal friction resistance (Fig. 6), as well as the loss modulus. On the contrary, the storage modulus values adopt an adverse trend of tan δ (Fig. 7). According to the data listed in Table 5, the tan δ_max values are found to be higher than the one observed for vanillic acid-polyesters previously reported by our group.⁴⁷ On the other hand, the storage moduli are found to be rather lower, indicating that the eugenol-based polyesters presented here feature superior viscosity properties. Therefore, such materials may be able to absorb more impact energy and may potentially be utilized as high impact materials or toughening additives of conventional engineering plastics, e.g. PET and PBT, to increase structural integrity.

Fig. 6 tan δ as a function of temperature for PM13, PM14, PM23 and PM47.

Fig. 7 Storage modulus (E′) as a function of temperature for PM13, PM14, PM23 and PM47.

Table 5 Dynamic mechanical properties of PM13, PM14, PM23 and PM47

Polymer	Tg (°C)	E′ (20 °C) (MPa)	tan δmax
PM13	0.63	124.70	0.67
PM14	−0.33	110.77	0.55
PM23	0.06	52.54	0.57
PM47	−1.16	69.17	0.48

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Conclusions

In an effort to fully or partially replace petroleum-based resources using renewable biomass in the industrial production of polyesters, a feasible method for monomer synthesis from renewable eugenol and 10-undecenoic acid has been developed. Furthermore, a series of thermoplastic polyesters were prepared from these renewable monomers via two-step melt polycondensation. The chemical structures and compositions were confirmed by ¹H NMR, ¹³C NMR and FTIR spectroscopy. Results from thermogravimetric analyses show that all the prepared polyester species exhibit superior thermal stabilities at temperature up to 300 °C, comparable with the commercially available structural materials PET and PBT. Differential scanning calorimetry results demonstrate that the fully aromatic and semi-aromatic polyesters feature amorphous behaviours, with T_g values ranging from −34.13 to 6.97 °C. This finding indicates that the methoxy group adjacent to the phenolic hydroxyl group significantly impedes the structural symmetry. The fully aliphatic polyesters exhibit semi-crystalline behaviours, with discreet T_g values. The density of the rigid benzene rings along the polyester chains was also found to influence T_g. The stress–strain and dynamic mechanical analysis results reveal that the Young's modulus and ultimate strength are not particularly high. However, the ductility of the polyesters was found to be superior. The elongations at break reach values up to 106.7%, and the relatively high tan δ and the low storage modulus provide a clear indication for eugenol-based polyesters featuring superior viscosity properties. In addition, the tensile properties (Young's modulus, ultimate strength and elongations at break) and dynamic mechanical properties (storage modulus and tan δ) are also closely related to the density of the aromatic rings.

While some properties of the polymers obtained remain to be improved and further studies addressing these concerns are currently underway, implementation of such improvements may lead to materials incorporating eugenol-based polymers with potential applications in high impact rubbery-type materials or as additives in conventional engineered polymers designed to increase structural integrity.

Acknowledgements

This work was funded by NSFC (51203079), the Natural Science Foundation of Tianjin (14JCYBJC18100), and PCSIRT (IRT1257).

References

1. A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411–2502.
2. B. Kamm, Angew. Chem., Int. Ed., 2007, 46, 5056–5058.
3. P. N. R. Vennestrom, C. M. Osmundsen, C. H. Christensen and E. Taarning, Angew. Chem., Int. Ed., 2011, 50, 10502–10509.
4. C. H. Christensen, J. Rass-Hansen, C. C. Marsden, E. Taarning and K. Egeblad, ChemSusChem, 2008, 1, 283–289.
5. A. L. Marshall and P. J. Alaimo, Chem.–Eur. J., 2010, 16, 4970–4980.
6. G. Q. Chen and M. K. Patel, Chem. Rev., 2012, 112, 2082–2099.
7. R. A. Sheldon, Green Chem., 2014, 16, 950–963.
8. A. Gandini, Green Chem., 2011, 13, 1061–1083.
9. A. Gandini, Macromolecules, 2008, 41, 9491–9504.
10. C. Vilela, A. F. Sousa, A. C. Fonseca, A. C. Serra, J. F. J. Coelho, C. S. R. Freire and A. J. D. Silvestre, Polym. Chem., 2014, 5, 3119–3141.
11. M. A. R. Meier, J. O. Metzger and U. S. Schubert, Chem. Soc. Rev., 2007, 36, 1788–1802.
12. U. Biermann, U. Bornscheuer, M. A. R. Meier, J. O. Metzger and H. J. Schafer, Angew. Chem., Int. Ed., 2011, 50, 3854–3871.
13. U. Biermann, W. Friedt, S. Lang, W. Luhs, G. Machmuller, J. O. Metzger, M. R. Klaas, H. J. Schafer and M. P. Schneider, Angew. Chem., Int. Ed., 2000, 39, 2206–2224.
14. P. Gallezot, Chem. Soc. Rev., 2012, 41, 1538–1558.
15. P. Foley, A. K. Pour, E. S. Beach and J. B. Zimmerman, Chem. Soc. Rev., 2012, 41, 1499–1518.
16. R. Garces, E. Martinez-Force and J. J. Sales, Grasas Aceites, 2011, 62, 21–28.
17. B. A. J. Noordover, A. Heise, P. Malanowski, D. Senatore, M. Mak, L. Molhoek, R. Duchateau, C. E. Koning and R. A. T. M. van Benthem, Prog. Org. Coat., 2009, 65, 187–196.
18. C. Gioia, M. Vannini, P. Marchese, A. Minesso, R. Cavalieri, M. Colonna and A. Celli, Green Chem., 2014, 16, 1807–1815.
19. G. Q. Chen, Chem. Soc. Rev., 2009, 38, 2434–2446.
20. W. H. Lu, J. E. Ness, W. C. Xie, X. Y. Zhang, J. Minshull and R. A. Gross, J. Am. Chem. Soc., 2010, 132, 15451–15455.
21. J. W. Song, E. Y. Jeon, D. H. Song, H. Y. Jang, U. T. Bornscheuer, D. K. Oh and J. B. Park, Angew. Chem., Int. Ed., 2013, 52, 2534–2537.
22. C. Aouf, E. Durand, J. Lecomte, M. C. Figueroa-Espinoza, E. Dubreucq, H. Fulcrand and P. Villeneuve, Green Chem., 2014, 16, 1740–1754.
23. M. van der Steen and C. V. Stevens, ChemSusChem, 2009, 2, 692–713.
24. J. Zakzeski, P. C. A. Bruijnincx, A. L. Jongerius and B. M. Weckhuysen, Chem. Rev., 2010, 110, 3552–3599.
25. J. A. M. Lummiss, K. C. Oliveira, A. M. T. Pranckevicius, A. G. Santos, E. N. dos Santos and D. E. Fogg, J. Am. Chem. Soc., 2012, 134, 18889–18891.
26. D. F. Taber and K. J. Frankowski, J. Org. Chem., 2003, 68, 6047–6048.
27. J. Moise, S. Arseniyadis and J. Cossy, Org. Lett., 2007, 9, 1695–1698.
28. H. Bilel, N. Hamdi, F. Zagrouba, C. Fischmeister and C. Bruneau, RSC Adv., 2012, 2, 9584–9589.
29. C. R. Larsen and D. B. Grotjahn, J. Am. Chem. Soc., 2012, 134, 10357–10360.
30. B. Lastra-Barreira and P. Crochet, Green Chem., 2010, 12, 1311–1314.
31. B. Lastra-Barreira, A. E. Diaz-Alvarez, L. Menendez-Rodriguez and P. Crochet, RSC Adv., 2013, 3, 19985–19990.
32. P. Thirukumaran, A. Shakila and S. Muthusamy, RSC Adv., 2014, 4, 7959–7966.
33. K. L. Hu, D. P. Zhao, G. L. Wu and J. B. Ma, Polym. Chem., 2015, 6, 7138–7148.
34. H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed., 2001, 40, 2004–2021.
35. C. E. Hoyle and C. N. Bowman, Angew. Chem., Int. Ed., 2010, 49, 1540–1573.
36. C. E. Hoyle, A. B. Lowe and C. N. Bowman, Chem. Soc. Rev., 2010, 39, 1355–1387.
37. A. B. Lowe, Polym. Chem., 2014, 5, 4820–4870.
38. E. Fuhrmann and J. Talbiersky, Org. Process Res. Dev., 2005, 9, 206–211.
39. G. P. Yu, C. Liu, J. Y. Wang, T. S. Gu and X. G. Jian, Polym. Degrad. Stab., 2009, 94, 1053–1060.
40. M. Zaheer and R. Kempe, ACS Catal., 2015, 5, 1675–1684.
41. M. R. Sturgeon, S. Kim, K. Lawrence, R. S. Paton, S. C. Chmely, M. Nimlos, T. D. Foust and G. T. Beckham, ACS Sustainable Chem. Eng., 2014, 2, 472–485.
42. C. Lavilla, E. Gubbels, A. Alla, A. M. de Ilarduya, B. A. J. Noordover, C. E. Koning and S. Munoz-Guerra, Green Chem., 2014, 16, 1789–1798.
43. J. Wu, P. Eduard, L. Jasinska-Walc, A. Rozanski, B. A. J. Noordover, D. S. van Es and C. E. Koning, Macromolecules, 2013, 46, 384–394.
44. C. Japu, A. M. de Ilarduya, A. Alla, M. G. Garcia-Martin, J. A. Galbis and S. Munoz-Guerra, Polym. Chem., 2013, 4, 3524–3536.
45. C. Lavilla, A. M. de Ilarduya, A. Alla, M. G. Garcia-Martin, J. A. Galbis and S. Munoz-Guerra, Macromolecules, 2012, 45, 8257–8266.
46. F. Stempfle, B. S. Ritter, R. Mulhaupt and S. Mecking, Green Chem., 2014, 16, 2008–2014.
47. C. C. Pang, J. Zhang, G. L. Wu, Y. N. Wang, H. Gao and J. B. Ma, Polym. Chem., 2014, 5, 2843–2853.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17457k

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