Stereoselective ring-opening polymerization of racemic ethylglycolide: precisely regulated polyester properties via stereomicrostructure control

Abstract

The development of high-performance, renewable polyesters with tunable mechanical properties is critical for advancing sustainable materials to replace petroleum-based plastics. Herein, a conceptually simple but highly effective strategy was employed for the stereoselective ring-opening polymerization (ROP) of racemic ethylglycolide (rac-EtG), affording polyesters with diverse sequence structures and a record-high isotacticity (P_m up to 0.99) among ROP of glycolide-type monomers. Strikingly, a simple adjustment of the steric hindrance at the 3-position of the salicylaldehyde moiety enabled a tunable switch in the stereoselectivity exhibited by the catalyst from atactic to isotactic and further to heterotactic. By tuning the stereomicrostructure of the polymer, a remarkable transition in ductility from 2.8% to 2119% was achieved. Typically, isotactic-rich poly(ethylglycolide) (ir-PEtG) exhibited an elongation at break of 569.9 ± 36.5% while maintaining a tensile strength of 26.7 ± 0.9 MPa. Furthermore, we demonstrated that a balanced distribution of crystalline and amorphous domains in the polymer microstructure enables simultaneous tuning of both strength and toughness.

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Introduction

With the depletion of petroleum resources and the worsening of environmental pollution, polyester materials derived from renewable resources attracted extensive research interest over the past several decades.^1–4 Among them, polylactic acid (PLA), a biomass-derived aliphatic polyester, has emerged as one of the most promising alternatives to petroleum-based plastics due to its excellent biocompatibility and biodegradability.^5,6 However, its poor toughness and limited mechanical properties pose critical challenges to its broader application.^7,8 An effective strategy for enhancing the performance of PLA is the precise modulation of its properties through rational molecular design.^9–11 Recent studies conducted by Baker,^12,13 Wu,^14–16 Wang^17,18 and our group have demonstrated that the properties of polymers synthesized via ring-opening polymerization (ROP) of substituted glycolide monomers can be finely tuned across a broad range. In particular, polymers bearing ethyl side groups exhibit mechanical performance comparable to that of commercial isotactic polypropylene (it-PP) and high-density polyethylene (HDPE).¹⁹ Nonetheless, issues such as slow crystallization rates and low melting temperatures (T_m = 108 °C) remain significant barriers to practical applications.^20,21 Previous studies have shown that stereoselective polymerization of racemic monomers can afford isotactic multiblock polymers with significantly elevated melting points.²² However, the relationship between the polymer sequence structure and its performance-especially mechanical behavior-remains inadequately understood. Therefore, the development of stereoselective polymerization methods for racemic ethylglycolide (rac-EtG) to obtain stereoregular polyesters and elucidate the impact of the polymer microstructure on material properties holds considerable scientific importance and application potential.

The key to synthesizing polymers with distinct sequence structures via stereoselective polymerization of racemic monomers lies in the rational design of the catalyst architecture. Over the past thirty years, the scope of catalysts and racemic monomers utilized in stereoselective ROP has been extensively explored.^23,24 Organometallic catalysts, owing to their intrinsic high efficiency and selectivity, were widely favored by researchers.^25,26 Structurally, these catalysts could be classified as either chiral or achiral, and the stereoselectivity of the polymerization process was generally governed by one of two primary mechanisms: the chain-end control (CEC) mechanism or the enantiomorphic site control (ESC) mechanism.²⁷ In 1996, Spassky et al.²⁸ first reported the stereoselective ROP of rac-lactide (rac-LA) using a chiral binaphthyldiamine-based salen–Al complex, which yielded PLA stereocomplexes with T_m as high as 187 °C. Subsequently, Coates,^29–32 Feijen,^33,34 Chen,^35–39 Kol,^40–42 and many other excellent scientists^43–46 developed a series of chiral catalysts for the stereoselective ROP of rac-LA, racemic propylene oxide (rac-PO), and racemic cyclic diolide (rac-DL). More recently, Wang and co-workers achieved a significant breakthrough in the stereoselective ROP of glycolide-type monomers by employing an innovative dual-ligand strategy, which enabled the kinetically perfect resolution polymerization of racemic phenethylglycolide.¹⁷ It is worth noting, however, that such chiral catalysts, which functioned through the ESC mechanism, typically resulted in a maximum conversion of approximately 50%, leaving a substantial amount of unreacted monomer that required additional processing, thus increasing both production cost and process complexity.

To overcome this limitation, considerable efforts were devoted to investigating the catalytic performance of achiral catalysts in stereoselective ROP.^47,48 These catalysts are capable of simultaneously polymerizing both enantiomers of the monomer and typically operate by a CEC mechanism, leading to the formation of polymers with diverse stereochemical microstructures. However, the correlation between catalyst architecture, polymer sequence structure, and resulting material properties remains poorly understood (Scheme 1A). Furthermore, although highly isotactic polymers often exhibit elevated melting points due to their regular backbone packing,⁴⁹ effective catalyst systems capable of inducing high isotacticity in the ROP of glycolide-type monomers remain rare.

Scheme 1 Stereoselective ring-opening polymerization of lactones for sequence-controlled polyester synthesis.

In this work, structurally simple mononuclear salen–Al complexes were employed to catalyze the stereoselective ROP of rac-EtG, affording polyesters with varied and well-defined chain microstructures. In addition to systematically investigating the influence of the catalyst backbone and substituents on the stereoselectivity of rac-EtG polymerization, another motivation for this study is to improve the thermal properties of PEtG by synthesizing highly isotactic PEtG (it-PEtG). Finally, by synthesizing PEtG samples with distinct sequence structures, we provide an in-depth discussion of the relationship between the stereochemical microstructure of the polymers and their resulting physical properties (Scheme 1B).

Results and discussion

Stereoselective ROP of rac-EtG

To achieve a highly isotactic selectivity system for the ROP of rac-EtG, we synthesized 17 salen–Al complexes, in which the length and nature of the phenoxy substituents and the diimino-linking units are varied (Scheme 2 and Fig. S1–S34). Subsequently, the ROP of rac-EtG was analyzed at 60 °C under conditions where the ratio of [rac-EtG]₀/[salen–Al]₀/[BnOH]₀ was 50/1/1. The corresponding stereoselectivity ROP of rac-EtG was investigated as shown in Table 1. Considering the prominent stereoselectivity of chiral catalysts in the ROP of cyclic lactone monomers, we initially evaluated the catalytic performance of classical chiral catalysts. At the outset, chiral cyclohexanediamine-based complexes were employed for the stereoselective ROP of rac-EtG (Table 1, entries 1 and 2). The 1a/BnOH system exhibited low reactivity in the ROP of rac-EtG in toluene at 60 °C, achieving only 13.4% conversion after 192 h. When the substituent on the complex was changed to Br, a significant enhancement in catalytic activity was observed, with over 90% conversion attained within 68 h, affording PEtG with a high melting point and a slightly broad dispersity (T_m = 166 °C and Ð = 1.28). The stereoregularity of the resulting PEtG was analyzed by quantitative ¹³C NMR spectroscopy (Fig. 1A), and the P_m was calculated based on the ESC statistics (Fig. 1B and Table S1).⁵⁰ The pronounced [mmm] tetrad peak indicated an isotactic-rich architecture (ir-PEtG) derived from 1b, with a P_m value of 0.88 (Fig. S35). We further explored chiral binaphthyl-based catalysts, which have been reported as effective for the ROP of cyclic lactones.⁵¹ The result showed that 1c exhibited lower catalytic activity than 1b, but yielded PEtG with slightly higher isotacticity (Table 1, entry 3 and Fig. S36), suggesting that a relatively slower polymerization rate may be beneficial for improving polymer stereoregularity.

Scheme 2 Salen–Al mediated stereoselective ROP of rac-EtG.

Table 1 Aluminum-catalyzed ring-opening polymerization of rac-EtG^a

Entry	Catalyst	[M]0/[Cat.]0/[I]0	Temp. (°C)	t (h)	Conv.^b (%)	Mn,theo.^c (kDa)	Mn,GPC^d (kDa)	Ð^d	Tm^e (°C)	Pm^f
a Polymerization conditions: [M]0 = 1.0 M, solvent = Tol.b Determined by ^1H NMR analysis of the reaction mixture.c Mn,theo. = ([M]0/[I]0) × Conv.% × Mw (EtG) + Mw (BnOH).d Measured by SEC (THF, 40 °C) and PS calibration.e Tm determined by DSC.f Pm represents the probability of mid-chain linkages between monomeric units, as determined by ^13C NMR analysis in the methylene region of PEtG, based on the ESC statistics.g [M]0 = 2.0 M.h Calculations based on the CEC statistics.
1	1a	50 : 1 : 1	60	192	13.4	1.1	—	—	—	—
2	1b	50 : 1 : 1	60	68	92.5	8.0	12.0	1.28	162	0.88
3	1c	50 : 1 : 1	70	125	93.1	8.0	11.7	1.23	168	0.90
4	2a	50 : 1 : 1	60	33	91.7	7.9	11.5	1.07	191	0.94
5^g	2a	500 : 1 : 1	60	128	88.9	76.4	88.7	1.10	183	0.92
6	2b	50 : 1 : 1	60	33	91.8	7.9	11.7	1.06	184	0.92
7	2c	50 : 1 : 1	60	33	92.3	7.9	11.5	1.12	178	0.91
8	2d	50 : 1 : 1	60	1	96.3	8.3	12.2	1.08	111	0.79
9	2e	50 : 1 : 1	60	4.5	93.3	8.0	11.6	1.09	—	0.66
10	2f	50 : 1 : 1	60	45	93.1	8.0	11.8	1.08	194	0.96
11	2g	50 : 1 : 1	60	120	91.6	7.9	11.5	1.07	92	0.73
12	2h	50 : 1 : 1	60	168	83.9	7.2	9.5	1.12	—	0.27^h
13	2h	50 : 1 : 1	80	96	91.8	7.9	11.5	1.07	—	0.30^h
14	2h	50 : 1 : 1	50	288	86.2	7.4	10.1	1.10	—	0.23^h
15	3d	50 : 1 : 1	60	33	90.2	7.8	11.1	1.10	192	0.95
16	3e	50 : 1 : 1	60	72	87.2	7.5	10.0	1.08	165	0.89
17	4a	50 : 1 : 1	60	45	92.3	7.9	11.6	1.07	194	0.96
18	4a	50 : 1 : 1	40	144	91.1	7.8	11.0	1.08	205	0.99
19^g	4a	500 : 1 : 1	60	192	90.5	77.8	90.4	1.08	189	0.94
20^g	2d	500 : 1 : 1	60	12	96.5	83.0	100.2	1.12	—	0.76

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Fig. 1 (A) Quantitative ¹³C NMR spectrum of PEtG in the methylene region: (i) hr-PEtG obtained using 2h (Table 1, entry 12), (ii) at-PEtG obtained using Sn(Oct)₂ (Table S2, entry 22), (iii) L-PEtG obtained using Sn(Oct)₂ (Table S2, entry 23) and (iv) it-PEtG obtained using 4a at 40 °C (Table 1, entry 18). (B) Methylene region of the quantitative ¹³C NMR spectrum of atactic PEtG obtained by Sn(Oct)₂ catalyzed ROP of rac-EtG. (C) Kinetic study of EtG polymerization mediated by 1b and (D) 1c. Conditions: [EtG]₀/[Cat.]₀/[BnOH]₀ = 50/1/1, 60 °C, [EtG]₀ = 1.0 M in toluene. (e) ee values of the unreacted monomer. Black line: 49.1% conversion and ee = 12.6%; red line: 48.7% conversion and ee = 43.0%.

To further investigate the mechanism that chiral catalysts may follow in the ROP of rac-EtG, kinetic studies were conducted using 1b and 1c to catalyze the polymerization of different enantiomers of EtG. As shown in Fig. 1C and D, the chiral catalysts exhibited significantly different polymerization rates for the different enantiomers. The first-order kinetic results of the polymerization process indicated that (R)-salen–Al consistently showed a pronounced preference for D-EtG, resulting in faster polymerization rates, while L-EtG underwent slower polymerization, particularly with the chiral binaphthyl-based catalyst 1c. The selectivity factor s (s = k_L/k_D) for 1c was as high as 17.9. Moreover, the ROP of rac-EtG revealed a first-order kinetic profile with two approximately linear phases, where a reduction in the polymerization rate was observed after approximately 50% monomer conversion. This deceleration was attributed to the preferential consumption of a given enantiomer by the chiral salen–Al catalyst, which also aligned with the polymerization kinetics observed for pure enantiomers. These experimental results suggested that the chiral salen–Al catalysts may follow the ESC mechanism during the ROP of rac-EtG.

According to a strict ESC mechanism, one enantiomer was preferentially consumed, while the other remained largely unreacted until the favored enantiomer was nearly exhausted, ultimately yielding a polymer with high stereoregularity. To further validate this mechanism, we used chiral HPLC to determine the enantiomeric excess (ee) of the remaining monomer at 50% monomer conversion, and the enantiomeric selectivity of the catalysts was evaluated via the kinetic resolution coefficient (k_rel), which was calculated using Kagan's equation: k_rel = ln[(1 − c)/(1 − ee)]/ln[(1 − c)/(1 + ee)],⁵² where c is the monomer conversion. As shown in Fig. 1E, the k_rel values for complexes 1b and 1c were 1.5 and 4.0, respectively. The relatively low k_rel values suggested that the synthesis of highly isotactic polyesters via the structurally simple mononuclear chiral salen–Al catalysts was quite challenging. It was worth noting that the k_rel values obtained from the ee calculations differed from the s determined through kinetic experiments, which indicated that these chiral catalysts in the ROP of rac-EtG were co-controlled by both ESC and CEC mechanisms.^18,40 This conclusion was also consistent with the relatively low melting points observed for the resulting polymers.

Since it was challenging to obtain highly it-PEtG when the polymerization mediated by chiral catalysts was simultaneously governed by both ESC and CEC mechanisms, we shifted our focus to achiral mononuclear salen–Al catalysts, which are more likely to operate under CEC control in the polymerization of racemic monomers. It is worth noting that these catalysts can be synthesized in high yields (61–85%, see the SI for details) through a straightforward, established procedure using commercially available diamine compounds and salicylaldehyde derivatives.

Initially, the activity and stereoselectivity of the complex 2a were investigated for the ROP of rac-EtG. Excitingly, high conversion of 50 equivalents of rac-EtG was achieved within a short time. As the [rac-EtG]/[2a] ratio was increased from 50/1 to 500/1, the molecular weight of the resulting PEtG increased from M_n = 11.5 kDa (Ð = 1.07) to M_n = 88.7 kDa (Ð = 1.10). More importantly, according to DSC and ¹³C NMR analyses, when the [rac-EtG]/[2a] ratio was 50/1, 2a produced it-PEtG with a T_m of up to 191 °C and a P_m of 0.94. Even for high molecular weight PEtG, a high stereoregularity (P_m = 0.92) was maintained (Table 1, entries 4–5 and Fig. S37). This was likely because the achiral salen–Al complexes predominantly followed the CEC mechanism for the stereoselectivity of the rac-EtG polymerization (Fig. S51). The larger steric hindrance of rac-EtG, combined with the bulky substituents of 2a, synergistically enhanced the catalyst selectivity for the same type of chiral monomer, thereby leading to the formation of PEtG with high isotacticity.

To verify this hypothesis, we thoroughly investigated the effects of the electronic effects, steric effects, and the geometry of the skeleton diamine linkers in the salen ligand framework on the polymerization activity and stereoselectivity of rac-EtG. Regarding the electronic effects, electron-withdrawing substituents (Br and NO₂) at the 5-position of the salicylaldehyde moiety slightly decreased the stereoregularity of the resulting polymers (Table 1, entries 6–7), while the polymerization rates of 2b and 2c were slightly higher than that of 2a (Fig. 2A). Notably, complex 2d, bearing 3,5-dibromo substituents, exhibited a dramatically increased polymerization rate, reaching over 96% conversion within just 1 h. Kinetic studies showed that the polymerization rate of 2d was approximately 44 times faster than that of 2a. However, unfortunately, the polymer chain regularity obtained from 2d was relatively poor (P_m = 0.79, Table 1, entry 8). These results clearly highlight the important influence of electron-withdrawing groups on polymerization activity in rac-EtG stereoselective ROP.

Fig. 2 (A) Kinetic study of rac-EtG polymerization mediated by salen–Al catalysts with different substituents. (B) Kinetic study of rac-EtG polymerization mediated by salen–Al catalysts with different main chain backbone linkages. Conditions: [rac-EtG]₀/[Cat.]₀/[BnOH]₀ = 50/1/1, 60 °C, [rac-EtG]₀ = 1.0 M in toluene. (C) Isotactic selectivity of various salen–Al catalysts. (D) 4a-mediated plots of polymer T_m and P_m versus polymerization temperature. From left to right, the samples correspond to Table S2, entry 7; Table S2, entry 8; Table 1, entry 18; Table S2, entry 9; Table 1, entry 17 and Table S2, entry 10.

After establishing the influence of electronic effects on the ROP process, we shifted our focus to the steric effects of the catalyst. By simply replacing the ortho-substituent of the phenoxy group with a hydrogen atom, complex 2e exhibited a significant increase in polymerization activity compared to 2a, but with a notable decrease in selectivity (Table 1, entry 9). The resulting sequence structure exhibited characteristics typical of a random polymer (Fig. S41). Notably, although the higher polymerization activity of 2d is generally unfavourable for isotactic selectivity, its larger steric hindrance from the ortho-positioned Br atoms leads to a higher isotactic selectivity compared to 2e (Fig. S52), suggesting that the steric hindrance of the ortho-substituent on the phenoxy group is crucial for isotactic selectivity. To further enhance the stereoregularity of the resulting polymers, we attempted to increase the steric bulk of the ortho-substituents on the phenoxy group. Initially, we synthesized catalyst 2f bearing a bulky CPhMe₂ group and investigated its catalytic performance. Notably, 2f exhibited excellent isotactic selectivity, affording it-PEtG with a P_m up to 0.96 under conditions at 60 °C (Table 1, entry 10). When the steric hindrance was further increased to obtain catalyst 2g, a decrease in polymerization activity was observed compared to 2f. Contrary to our expectations, the isotactic selectivity of 2g did not improve but instead declined significantly (Table 1, entry 11), despite this catalyst having previously demonstrated excellent isotactic control in the stereoselective ROP of rac-LA.⁴⁷ These results suggested that, in the stereoselective ROP of rac-EtG, the isotacticity of the resulting polymer does not increase monotonically with the steric bulk of the catalyst ortho-substituents.

To further elucidate the relationship between ortho-position steric hindrance and the isotacticity of the resulting polymer, an additional steric bulk was introduced by synthesizing a salen–Al complex 2h, bearing a bulky triphenylmethyl (Ph₃C) substituent at the 3-position. Polymerization kinetics revealed that the polymerization rate of 2h was significantly lower than that of 2a (k_2h = 0.015 h⁻¹ and k_2a = 0.076 h⁻¹). Interestingly, the polymer produced using complex 2h at 60 °C exhibited a markedly different stereochemical structure. Quantitative ¹³C NMR spectroscopy showed a significant increase in the tetrad signals corresponding to [mrm] and [rmr] (Fig. S44). The calculated probability of racemic linkages (P_r) equal to 0.73 based on the CEC statistics indicated that 2h, with its bulky Ph₃C substituent, exhibited a certain degree of hetero-enrichment stereoselectivity (Table 1, entry 12). On one hand, the large ortho-substituent near the chain end created excessive steric congestion around the active site, which likely interfered with pre-coordination between the monomer and the metal center during polymerization.⁵³ On the other hand, the notably slow polymerization rate of 2h may have hindered the continuous insertion of monomers with the same chirality, as this would increase the relative concentration of monomers with opposite configurations in the remaining monomers. As a result, insertion of monomers with opposite chirality at the chain end became more favorable, leading to a shift in selectivity and the formation of heterotactic-rich PEtG (hr-PEtG) (Fig. 3A). When the polymerization rate of 2h was increased, its heterotactic selectivity decreases, whereas lowering the rate enhanced heterotacticity-further supporting our hypothesis (Table 1, entries 13–14). Additionally, similar switchable polymerization behavior induced by steric hindrance was recently reported by Wang,¹⁸ Rieger, and co-workers.⁵⁴ In summary, these findings demonstrated that the size of the ortho-substituent on the phenoxy group had a significant impact on stereoselectivity. To further quantify the steric effect, we performed buried volume (% V_Bur) analysis using the SambVca 2.1 program (r = 4.5 Å),^55,56 which provides a reliable measure of ligand steric hindrance (Fig. 3B). The results revealed that 2h exhibited a much larger buried volume (% V_Bur = 71.3%) compared with the other catalysts, confirming its higher steric hindrance that caused the observed selectivity inversion. Substituents of moderate steric bulk were found to be particularly favorable for enhancing the isotactic selectivity of salen–Al complexes. More importantly, a systematic variation in steric hindrance enabled a tunable transition in the polymer microstructure from atactic to isotactic to heterotactic sequences (Fig. 3C).

Fig. 3 (A) Schematic diagram of the possible mechanism of rac-EtG ROP catalyzed by cat 2h to form heterotactic-enriched polymers. (B) Buried-volume analysis. The percentage of buried volume (% V_Bur) was calculated using the SambVca 2.1 web-based program with a sphere radius of 4.5 angstroms. (C) Stereoselective ROP of rac-EtG using salen–Al complexes yielding different sequences depending on the phenoxy ortho-substituent.

With the optimal substituent on the salicylaldehyde framework determined, we finally explored the possible impact of the backbone linker on the polymerization process. Shortening the backbone to C₂ linkers, such as 1,2-ethylenediyl (3a) and 2-methylpropane-1,2-diyl (3b), caused a significant reduction in the polymerization activity, with less than 10% monomer conversion achieved after over 120 h of polymerization (Table S2, entries 1–2). For the sample catalyzed by the phenyl-based backbone catalyst (3c), a similar polymerization rate to 3a and 3b was observed (Table S2, entry 3). When the backbone was extended to a C₃ linker, 1,3-propanediyl (3d) yielded it-PEtG with a P_m of 0.95 at 60 °C (Table 1, entry 15). On keeping the C₃ backbone but introducing a phenyl substituent, catalyst 3e exhibited a decrease in polymerization activity (Fig. 2B), and the isotacticity of the resulting polymer also significantly decreased (Table 1, entry 16). Overall, the polymer chain regularity generally increased with the flexibility of the backbone linker (Fig. 2C),⁴⁸ which was likely attributed to the enhanced conformational adaptability of the metal complex, facilitating the geometric requirements of key transition states involved in the ROP process.

Based on the combined evaluation of salicylaldehyde substituents and amine-bridged backbones in the stereoselective ROP of rac-EtG, complex 4a was ultimately synthesized and its catalytic behavior was investigated. As anticipated, under the same conditions, 4a exhibited the highest isotactic selectivity, affording highly it-PEtG with a P_m of up to 0.96 within a relatively short reaction time, while maintaining a narrow molecular weight distribution (Table 1, entry 17, Fig. S46). The catalyst also demonstrated excellent control over molecular weight. By varying the feed ratio of [rac-EtG]₀/[4a]₀/[I]₀ from 50/1/1 to 500/1/1, the M_n of the resulting polymers increased linearly from M_n = 11.1 kDa (Đ = 1.10) to M_n = 90.4 kDa (Đ = 1.08) (Fig. S53B), and GPC traces showed unimodal distributions. Remarkably, all PEtG samples obtained under these conditions exhibited high stereoregularity (P_m = 0.94–0.95) and elevated melting points (T_m = 189–193 °C), indicating that 4a maintained excellent stereocontrol throughout the polymerization. The controlled nature of the 4a-mediated ROP was further confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) (Fig. S53C). Together, these results demonstrate that the ROP of rac-EtG mediated by 4a exhibits characteristics of controlled polymerization.

Finally, we further investigated the influence of various reaction parameters on the polymerization behavior mediated by 4a. Among the examined factors-including the monomer concentration, solvent, and temperature–temperature was found to have a pronounced impact on the isotactic selectivity of the catalyst (Table S2, entries 4–17). As shown in Fig. 2D, within the temperature range of 40 to 80 °C, P_m values increased with decreasing temperature. At 40 °C, a maximum P_m of 0.99 was achieved (Fig. S50), and the corresponding ¹³C NMR spectrum displayed peak patterns closely resembling those of L-PEtG (Fig. 1A), confirming the high isotacticity of the polymer produced by 4a under these conditions. This improvement in isotacticity was attributed to the reduced reactivity of the growing chain end at lower temperatures, which stabilized the chain conformations and thus favored isotactic propagation. Further lowering of the polymerization temperature did not result in a continued increase in isotacticity; instead, a slight decrease was observed. This was presumably due to the significantly reduced polymerization rate at lower temperatures, which prolonged the reaction time and slightly diminished the isotacticity.⁵⁷

Thermal and mechanical properties

The correlation between the polymer sequence structure and material properties has long been a subject of considerable interest.⁵⁸ Given that the catalytic system developed in this study enabled the controlled synthesis of PEtG with varying sequence structures, the thermal, crystallization, and mechanical properties of the resulting polymers were systematically investigated (Tables S3 and S4). Differential Scanning Calorimetry (DSC) was first employed to determine the T_m of the polymers, as T_m provides a practical reference for polymer processing conditions. As shown in Fig. 4A and B, a progressive and orderly increase in T_m-from amorphous to 205 °C-was observed with increasing content of the highly crystalline [mmm] triad sequences. In particular, the highly it-PEtG synthesized using 4a exhibited a T_m of 205 °C, representing a remarkable 97 °C enhancement compared to L-PEtG obtained via direct ROP of L-EtG. This substantial increase in T_m was likely due to the formation of isotactic stereoblock copolymer structures through the stereoselective ROP of rac-EtG catalyzed by 4a, which promoted the development of stereocomplex crystallites with enhanced thermal stability. To validate this hypothesis, L-PEtG and D-PEtG were individually synthesized and subsequently blended in a 1 : 1 mass ratio to form stereocomplex PEtG (sc-PEtG), which was then compared with other PEtG samples in terms of thermal behavior. The DSC thermogram of sc-PEtG revealed a melting temperature of 208 °C, only 3 °C higher than that of the polymer obtained using 4a (Fig. 4C), thereby confirming the stereoblock architecture and high stereoregularity of the 4a produced PEtG.

Fig. 4 Comparison of the performance of different types of PEtG. (A) Stacked quantitative ¹³C NMR spectra of PEtG in CDCl₃ highlighting signals from the carbonyl, methine, and methylene regions. (B) DSC thermograms of hr-, at-, ir-, and it-PEtG. For Fig. A and B, the samples from top to bottom correspond to Table 1, entry 18; entry 4; entry 16; entry 8; entry 11; Table S2, entry 22 and Table 1, entry 12. (C) DSC thermograms of L-PEtG (ΔH_m = 35.6 J g⁻¹, Table S2, entry 23), sc-PEtG (ΔH_m = 60.2 J g⁻¹), and it-PEtG (ΔH_m = 51.6 J g⁻¹, Table 1, entry 18) obtained using 4a at 40 °C. (D) Powder XRD profile of L-, at-, ir-, it- and sc-PEtG. From bottom to top, the samples correspond to Table S2, entry 23; Table S2, entry 22; Table 1, entry 8; Table 1, entry 18 and the L-PEtG/D-PEtG (1 : 1 w/w) blend. (E) TGA of L-PEtG (M_n,GPC = 94.3 kDa, Đ = 1.22, Table S2, entry 25), at-PEtG (M_n,GPC = 90.6 kDa, Đ = 1.25, Table S2, entry 26), ir-PEtG (P_m = 0.76, M_n,GPC = 100.2 kDa, Đ = 1.12, Table 1, entry 20) and it-PEtG (P_m = 0.94, M_n,GPC = 90.4 kDa, Đ = 1.08, Table 1, entry 19). (F) Stress–strain curves of it-PEtG (σ_b = 26.7 ± 1.1 MPa, ε_b = 2.8 ± 0.6%, M_n = 90.4 kDa, Table 1, entry 19), ir-PEtG (σ_b = 26.7 ± 0.9 MPa, ε_b = 569.9 ± 36.5%, M_n = 100.2 kDa, Table 1, entry 20), and at-PEtG (σ_b = 1.1 ± 0.1 MPa, ε_b = 2119 ± 220%, M_n = 90.6 kDa, Table S2, entry 26).

Wide-angle X-ray diffraction (WAXD) patterns of different types of PEtG provided valuable insights into the polymer microstructures. As shown in Fig. 4D, L-PEtG exhibited crystalline diffraction peaks at 2θ = 14.7°, 17.2°, 23.2°, and 25.6°. Sc-PEtG displayed diffraction peaks at 2θ = 10.7°, 18.5° [19.3°(subpeak)], and 21.5°. Consistent with expectations, it-PEtG and ir-PEtG displayed diffraction peaks at positions identical to those of sc-PEtG, with no observable peaks corresponding to L-PEtG. Additionally, ir-PEtG exhibited significantly reduced peak intensities compared to sc-PEtG and it-PEtG, indicating a decrease in both crystalline regions and overall crystallinity. Notably, the formation of the stereocomplex structure substantially accelerated the crystallization rate of the polymer. Both low and high molecular weight it-PEtG crystallized at faster cooling rates, while L-PEtG did not show crystallization peaks at a cooling rate of 10 °C min⁻¹ (Fig. S54).

The thermal stability of high molecular weight PEtG exhibited a trend similar to that of the melting point, with the thermal decomposition temperature at 5% weight loss (T_d,5%) gradually increasing as the [mmm] content increased. Notably, it-PEtG (P_m = 0.94) showed a T_d,5% of approximately 304 °C (Fig. 4E), which represented an increase of about 40 °C compared to that of L-PEtG. This enhancement was likely attributed to the higher crystallinity of it-PEtG relative to L-PEtG, resulting in improved thermal stability.

The relationship between the stereoregularity of PEtG and its mechanical properties was a central focus of this investigation. The resulting high molecular weight PEtG samples were processed into dumbbell-shaped specimens for tensile testing. As shown in Fig. 4F, it-PEtG exhibited strong yet brittle mechanical behavior (ultimate tensile strength σ_b = 27.3 ± 1.1 MPa; elongation at break ε_b = 2.8 ± 0.7%) due to its highly regular polymer backbone and high crystallinity. In contrast, at-PEtG, with lower backbone regularity and reduced crystallinity, demonstrated high ductility (ε_b = 2119 ± 220%) but a low tensile strength (σ_b = 1.1 ± 0.1 MPa). The ir-PEtG sample (P_m = 0.77), which exhibited an intermediate level of stereoregularity, showed a combination of high strength (σ_b = 26.7 ± 0.9 MPa) and excellent toughness (ε_b = 569.9 ± 36.5%). Unfortunately, mechanical properties of hr-PEtG were not accurately determined due to its low molecular weight and viscous solid form. These findings clearly demonstrated that the stereomicrostructures of PEtG had a substantial impact on their mechanical properties and that the mechanical properties could be precisely tuned by controlling the microstructural regularity of the polymer.

The significant differences in toughness observed among PEtGs with different microstructures suggest that variations in crystallinity, induced by polymer stereoregularity may have a crucial impact on their mechanical performance (Fig. S57). This prompted us to conduct an in-depth investigation into how the crystallinity of PEtG-based materials influences their mechanical properties. Due to the poor crystallization ability of L-PEtG, we prepared L-PEtG* by incorporating a nucleating agent to enhance its crystallization behavior. The thermomechanical properties of L-PEtG, L-PEtG*, and it-PEtG were then compared under different crystallization conditions, including quenched and annealed crystallization (denoted as Q and C, respectively) (Tables S4 and S5). As shown in Fig. 5A and B, the quenched amorphous L-PEtG (L-PEtG-Q) exhibited the highest elongation at break, albeit with reduced yield strength. Upon annealing, L-PEtG-C displayed increased tensile strength but decreased elongation and toughness due to the growth of crystalline domains. Further enhancement in crystallinity in L-PEtG*-C led to the highest yield strength among all samples. As expected, the highly crystalline it-PEtG displayed brittle fracture behavior (Fig. S55). These results confirm that a proper balance between crystalline and amorphous domains within PEtG can enable simultaneous tuning of material hardness and toughness.

Fig. 5 (A) Stress–strain curves of L-PEtG, L-PEtG*, and it-PEtG after quenching and annealing crystallization treatment. (B) Comparison of mechanical properties of L-PEtG, L-PEtG*, and it-PEtG after quenching and annealing crystallization treatment. (C) Stress–strain curves of at-PEtG/it-PEtG blends (inserted picture: mechanical property comparison). (D) Overall mechanical property comparison of PEtG-based polymers.

The improvement in toughness resulting from reduced stereoregularity was often challenged by property trade-off in the form of a decreased T_m. Encouraged by the above results, we explored a simple blending strategy using at-PEtG to toughen the brittle it-PEtG without compromising its superior thermomechanical properties. This approach also adheres to the principle of single-material product design. As shown in Fig. 5C, with increasing content of highly crystalline it-PEtG in the at-PEtG matrix, the tensile strength of the blends gradually increased, while the elongation at break and toughness decreased. This trend demonstrates that the mechanical properties of the blends can be finely tuned over a wide range by adjusting the blending ratio. Notably, owing to the high crystallizability of it-PEtG, all resulting blends exhibited high melting points (T_m = 176–182 °C) and rapid crystallization rates (Table S6). Similar trends in mechanical behavior were observed for blends of L-PEtG and at-PEtG (Fig. S56); however, these systems showed relatively poor crystallinity (Table S7). Overall, these findings highlight that by precisely tuning the microstructure and crystallinity of PEtG, it is possible to develop a new class of polyester materials with high melting points, excellent thermal stability, and broadly tunable mechanical performance (Fig. 5D and S57).

Conclusions

By rational catalyst design, we achieved the stereoselective ROP of rac-EtG, leading to PEtG-based polymers with high stereoregularity and tunable mechanical properties. Our results demonstrate that chiral catalysts governed by cooperative ESC and CEC mechanisms can produce polyesters with moderate isotacticity (P_m = 0.90). In contrast, achiral catalysts primarily governed by the CEC mechanism can yield highly isotactic polymers (P_m up to 0.99 and T_m = 205 °C) through judicious modification of substituents and polymerization conditions. The catalyst structure plays a pivotal role in influencing both polymerization activity and stereoselectivity. In particular, tuning the steric hindrance at the 3-position of the salicylaldehyde framework enables selective modulation of the polymer microstructure from atactic to isotactic to heterotactic configurations. Moreover, these catalysts exhibit excellent control over the polymerization process, allowing for the synthesis of high molecular weight polymers for detailed performance evaluation. An increase in [mmm] triad content correlates with enhanced melting points and thermal stability. A balanced crystalline-amorphous domain in PEtG allows for simultaneous tuning of both strength and toughness, resulting in PEtG-based polyesters with widely adjustable mechanical performance. In summary, this efficient catalytic strategy affords bio-based polymers that not only reduce dependence on petroleum-derived resources but also introduce a generalizable approach to sequence-programmable polymer design through steric-hindrance-guided stereocontrol, offering a new paradigm for sustainable, high-performance plastic materials.

Author contributions

J. H.: conceptualization, data curation, methodology, writing – original draft, writing – review & editing, formal analysis; X. L.: data curation, formal analysis; W. W.: investigation; Y. D.: software; H. Z.: methodology, project administration; C. W.: funding acquisition, supervision; R. D.: validation, writing – review & editing; X. B.: supervision, validation, visualization; X. C.: supervision, resources.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI.

Supplementary information includes detailed experimental procedures, materials, and methods; additional polymerization and characterization data (NMR, GPC, DSC and Stress-strain curves). See DOI: https://doi.org/10.1039/d5sc05510e.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (No. 2021YFB3801901) and Tianjin Science and Technology Plan Project (No. 23YFYSHZ00060).

Notes and references

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