Copolymerization of L-lactide/trimethylene carbonate by organocatalysis: controlled synthesis of comb-like graft copolymers with side chains with different topologies

Abstract

A series of linear-comb poly(trimethylene carbonate)-block-poly(L-lactide) [lcP(TMC-b-LLA)] with polybutadiene (PB) as backbone and a P(TMC-b-LLA) diblock copolymer as side chain were obtained using controlled synthesis and evaluated as thermoplastic elastomers. The comb-like graft copolymer was achieved using sequential ring-opening polymerization of trimethylene carbonate (TMC) and L-lactide (L-LA) at room temperature (RT) using an organocatalyst, associated with the multi-hydroxyl PB (PB–OH) as a macroinitiator. The thermal and mechanical properties of these graft copolymers were investigated using differential scanning calorimetry, dynamic mechanical analysis and tensile testing. The Young's modulus (E) and the elongation at break (ε_b) followed predicted trends as the monomer composition changed. Furthermore, well defined linear-comb PLLA-gradient-PTMC [lcP(LLA-grad-TMC)] and linear-comb PLLA-random-PTMC [lcP(LLA-ran-TMC)] were also synthesized for comparison. Results from ¹H and ¹³C-NMR spectroscopy, and gel permeation chromatography confirmed the formation of the chain microstructures. It was found that the properties of the copolymers depended not only on the comonomer content but also on their topologies. The results showed that the gradient and random structure of the side chain could yield unusual properties: the lcP(LLA-grad-TMC) copolymer possessed a distinctive, broad glass transition temperature in comparison with the other two graft copolymers. The lcP(LLA-ran-TMC) random copolymer performed like a typical rubber with a significant high ε_b value (ε_b = 1800%) which was even larger than the pure linear-comb PTMC (ε_b = 865%). Furthermore, this simple “one-pot” method using an organocatalyst to synthesize comb-like graft copolymers with “block”, “gradient” and “random” side chains is demonstrated systematically in this work.

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Introduction

Biodegradable and biocompatible aliphatic polyesters and polycarbonates are important classes of polymers which are gaining increasing attention both in academia and industry for biomedical and pharmaceutical applications.¹ In addition, a large number of the corresponding monomers, namely cyclic esters and cyclic carbonates, are derived from renewable bioresources as substitutes for polymers derived from fossil feedstock. For example, poly(L-lactide) (PLLA) is a plastic, with a glass transition temperature (T_g) at 60 °C, derived from starch containing raw materials, and it is regarded as a tough and brittle commodity plastic for packaging, electronics or other biomedical applications.² Whereas, poly(trimethylene carbonate) (PTMC) can be derived from glycerol, and it is considered as a rather rubbery polymer, with a T_g of about −15 °C, and it can be used for biomedical implants or matrices for controlled drug release.³ In an effort to improve the mechanical properties and widen the field of applications of both homopolymers, L-lactide (L-LA) and trimethylene carbonate (TMC) were copolymerized together. The resulting aliphatic copolymer of L-LA and TMC, [P(LLA-co-TMC)] are indeed biocompatible/biodegradable and they offer various opportunities as specialty polymers.⁴

Copolymerization of L-LA and TMC by ring-opening polymerization (ROP) has attracted interest in recent years. Compared with the polycondensation of diol and diacid, or dialkyl carbonate, the ROP approach offers more attractive features for both the synthesis process and the products. The latter is more effective and well controlled.⁵ Also, with living ROP process well defined polymers are obtained in terms of molar mass predictability, narrow dispersity, chain-end fidelity and tunable microstructure and tacticity.^4d,6 To that end, a large array of active metal-based or organic catalytic systems have been developed to achieve P(LLA-co-TMC) copolymers.^4d–j Among these catalysts, the most common ROP catalyst is tin(II) octoate, which could lead to undesirable toxicity and inflammation in vivo. It is also difficult to remove the residual catalyst from the polymer completely. An organocatalyst is an excellent alternative to transition metal catalysts, because it avoids the use of metals and provides better control over the polymerization reaction.⁷ Recently, the commercially available guanidines [1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), N-methyl-1,5,7-triazabicyclododecene] and amidines[1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)] have been reported to be effective for the ROP of cyclic carbonate and lactone monomers.⁸ Guillaume's group studied copolymerization kinetics during the ROP of L-LA and TMC using TBD as a catalyst.^4g It was shown that the conversion rates of the comonomers were equal when both of them were associated with toluene (C₇H₈) under 110 °C. As a result, well defined random copolymers P(LLA-ran-TMC) were produced using TBD. A recent approach allowed the synthesis of gradient copolymers P(LLA-grad-TMC) from a L-LA/TMC eutectic melt using DBU as a catalyst.⁹ Coulembier et al. used DBU as a catalyst for ROP of L-LA from its eutectic melt ([LA]₀/[TMC]₀ = 41/59) at RT. They sequentially copolymerized TMC by increasing the temperature to 61 °C or quickly solubilized it in dichloromethane (CH₂Cl₂).⁹ This provided an opportunity to synthesize various P(LLA-co-TMC) copolymers using organocatalysts without a metallic residual.

Furthermore, by varying the loading of comonomers or the copolymer structure, the content of each segment and the sequencing of the comonomers should allow for tailoring the properties of the resulting copolymers. It is remarkable that the properties of copolymers, including thermal behavior, mechanical properties, solubility and biodegradability, are strongly dependent on chain microstructure, whereas the content of the comonomers are similar.¹⁰ Sarasua and co-workers investigated the effect of chain microstructure on crystallization and degradation of statistical poly(L-lactide-co-ε-caprolactone) copolymers.¹¹ Li and co-workers investigated poly(glycolide-co-ε-caprolactone) and P(LLA-co-TMC) copolymers, and demonstrated that the degree of randomness dramatically affects the final properties.¹² However, these studies only investigated random copolymers with different degrees of randomness and the blocky or gradient copolymers were not involved.

Besides the previously mentioned parameters, such as chemical composition, comonomer unit distribution, dispersity, and so on, chain architecture plays an important role in the design of specific properties of the macromolecule. Polylactides with well defined architecture such as star-shaped polymers, comb-shaped polymers, branched polymers and dendrimers are attracting increasing interest.¹³ In previous work, a series of comb-like graft PLLA with poly(butadiene) (PB) as a backbone have been synthesized and characterized.^13a The possibility of synthesizing narrow dispersed graft copolymers by using this “graft from” strategy led to attempting the synthesis of a comb-like graft copolymer with various microstructures (block, gradient, and random). A comparison of the properties of these graft lcP(LLA-co-TMC) copolymers will help to develop the structure–property relationship for comb-like copolymers. New polymers with complex architecture, including topology, functionality and composition, can lead to new functional materials with an advanced nanostructure.¹⁴

In this work, comb-like graft copolymers were synthesized by attaching polymeric side chains of P(TMC-co-LLA) to the PB backbone using a “graft from” strategy. The graft chains were prepared by ROP of L-LA and TMC using organocatalysts and multi-functional PB with a number of hydroxyl groups (PB–OHs) as a macroinitiator. For the first time, three different comb-like copolymers with well defined side chain microstructures, were prepared. These copolymers were lcP(TMC-b-LLA) as block copolymer, lcP(LLA-grad-TMC) as gradient copolymer and lcP(LLA-ran-TMC) random copolymer. Nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC) techniques were used to characterize the synthesized polymers, validating the formation of desired side chain structures. The thermal and mechanical properties of these comb-like graft copolymers were investigated by differential scanning calorimetry (DSC) and tensile tests.

Experimental section

Materials

Butadiene (Yanshan Petrochemical Co., polymerization grade) was treated with little n-butyllithium (n-BuLi, J&K Chemical, 2.5 M solution in n-hexane) to remove the moisture and inhibitor. Dichloromethane (CH₂Cl₂, Aladdin), toluene (Aladdin) and cyclohexane were distilled from calcium hydride (CaH₂) under nitrogen. DBU (Sigma-Aldrich, 98%) was dried over CaH₂, distilled under reduced pressure and stored in a glovebox. L-LA (Jinan Daigang Biomaterial Co., 99%) and TMC (Jinan Daigang Biomaterial Co., 99%) were triply recrystallized from ethyl acetate and then dried and stored in a glovebox. Trifluoromethanesulfonic acid (TfOH, Aladdin, 98%), formic acid (HCOOH, Aladdin, 88%), hydrogen peroxide (H₂O₂, 30%), TBD (J&K Chemical, 98%), and other reagents were used as-received without further purification.

Instrumentation

The average molar mass (M_n,_GPC) and polydispersity index (PDI) values of the polymers were determined using GPC with a Waters 1515 HPLC pump, a Waters 2414 refractive index detector, and poly(styrene) (PS) columns (one PL gel 5 μm 10E4A and one Shodex KF-805) in tetrahydrofuran (THF) as eluent at a flow rate of 0.6 mL min⁻¹ at 35 °C. The calibration was performed using PS standards (Shodex PS STD SM-105).

¹H-NMR and ¹³C-NMR spectra were record on a Bruker Avance 400 MHz spectrometer at 25–30 °C in deuterated chloroform with a concentration of 4% w/v. Chemical shifts (δ) are reported in ppm and were referenced internally relative to tetramethylsilane (δ 0 ppm) using the residual ¹H (δ 7.26 ppm) and ¹³C (δ 77.16 ppm) solvent resonances.

The thermal behavior of the polymers was measured on a TA Instruments Q20 differential scanning calorimeter. Each sample was heated to 200 °C at a heating rate of 10 °C min⁻¹ in aluminum pans under a nitrogen atmosphere. Thermal history was removed by keeping the samples at 200 °C for 3 min. Then the samples were cooled to −40 °C at 10 °C min⁻¹, followed by heating to 200 °C at 10 °C min⁻¹.

Tensile testing was performed on solvent cast films. Films (50–100 μm thick) were prepared by casting polymer solutions in chloroform (10–15% w/v) onto glass plates and cut to the correct dimensions (length × width = 40 mm × 8 mm). Tensile tests were then carried out on at least five samples at RT (25 °C) using an Instron 5567A tensile tester with a load cell of 100 N at a head speed of 10 mm min⁻¹. Elongation at break (ε_b) and ultimate tensile strength (σ_b) values were obtained from the dynamic tensile diagrams. The sample specimen deformation was measured from the grip-to-grip separation, for which the initial value was 20 mm.

Dynamic mechanical analysis (DMA) was carried out on a TA Instruments DMA Q800, at a heating rate of 3 °C min⁻¹ in the tension film mode with a deformation amplitude of 10 mm and a frequency of 1 Hz.

Synthesis of macroinitiator PB–OH. This procedure is a modification of the synthesis, which was originally reported.^13a,15 PB was synthesized by anionic polymerization of butadiene with BuLi in cyclohexane. The PB polymer was then epoxidized ([1,4-butadiene]₀/[HCOOH]₀/[H₂O₂]₀ = 1/0.4/0.4) and then hydroxylated ([epoxy group]₀/[TfOH]₀/[H₂O]₀/[THF]₀ = 1/1.5/15/30). The detailed information about the synthesis and characterization of the PB backbone can be found in our previous work. Number average molar mass (M_n) = 2850 g mol⁻¹, PDI = 1.14 (determined by GPC analysis with PS standards); degree of hydroxylation = 16.4% (determined using ¹H-NMR); the number of hydroxyl groups on each backbone (No._OH) = 15.6 mol⁻¹.

Typical procedure for the synthesis of lcP(TMC-b-LLA) from a TBD/PB–OH system. All the copolymerizations were performed similarly according to the following typical procedure [lcP(TMC-b-LLA)1]. The only differences lie in the molar ratio of the comonomers. In a glove box, a two-necked round bottomed flask with macroinitiator PB–OH (0.68 g, 0.24 mmol, 1 equiv.) inside had previously been dried by three azeotropic distillations with toluene.¹⁶ A predetermined amount of TMC (2.44 g, 24 mmol, 100 equiv.) in CH₂Cl₂ (20 mL) was added first. After rapidly injecting TBD (0.066 g, 0.48 mmol, 2 equiv.) in CH₂Cl₂ (1 mL), the mixture was immediately stirred at RT, over the appropriate reaction time which allowed complete TMC consumption. The conversion of TMC (conv._TMC) was not systematically determined, but it can be ultimately observed within the recovered block copolymer. Then the solution of L-LA (10.37 g, 72 mmol, 300 equiv.) in CH₂Cl₂ (20 mL) was injected into the flask. The polymerization reaction proceeded at RT over the appropriate time and was stopped by the addition of an excess of benzoic acid solution (0.78 g, 6.43 mmol, 26.8 equiv.) in ethanol (4 mL). The copolymer was purified by precipitation with methanol twice and dried under vacuum to constant weight. The reaction steps involved in the synthesis of lcP(TMC-b-LLA) copolymer are depicted in Scheme 1.

Scheme 1 Synthesis of comb-like graft lcP(TMC-b-LLA) from a TBD/PB–OH system.

Typical procedure for the synthesis of lcP(LLA-grad-TMC) from the DBU/PB–OH system. In a glove box, a two-necked round bottomed flask with macroinitiator PB–OH (0.68 g, 0.24 mmol, 1 equiv.) inside had previously been dried by three azeotropic distillations with toluene.¹⁶ Predetermined TMC (4.88 g, 48 mmol, 200 equiv.) and L-LA (6.91 g, 48 mmol, 200 equiv.) were added into the flask. After 15 minutes of intensive shaking at RT, the corresponding liquid eutectic melt appeared (a 50/50 wt% mixture of L-LA and TMC yields a eutectic melt at 21.3 °C).⁹ DBU (0.14 g, 0.96 mmol, 4 equiv.) was then rapidly injected into the eutectic solution. The medium was immediately homogenized by stirring for about 20 s. After 40 s (1 min in total), the as-obtained PLLA started to nucleate and crystallize out of the eutectic solution. Before total crystallization occurred (ideally just after the nucleation), the temperature was increased quickly to 61 °C. When the temperature rose higher than the T_g of PLLA, the mixture became a melt and was kept under agitation for 20 hours before solubilization in CH₂Cl₂ (30 mL). The copolymer was purified by precipitation with methanol twice and dried under vacuum to constant weight.

Typical procedure for the synthesis of lcP(LLA-ran-TMC) from the TBD/PB–OH system. In a glove box, a two-necked round bottomed flask with macroinitiator PB–OH (0.68 g, 0.24 mmol, 1 equiv.) inside had previously been dried by three azeotropic distillations with toluene.¹⁶ A solution of TMC (4.88 g, 48 mmol, 200 equiv.) and L-LA (6.91 g, 48 mmol, 200 equiv.) in toluene (40 mL) were added into the flask. After 10 min stirring at 110 °C, TBD (0.066 g, 0.48 mmol, 2 equiv.) in CH₂Cl₂ (1 mL) was then rapidly injected into the solution. The mixture was stirred at 110 °C for 2 h. The reaction medium remained homogeneous over the entire course of the reaction and was quenched by the addition of an excess of benzoic acid solution (0.78 g, 6.43 mmol, 26.8 equiv.) in ethanol (4 mL). The copolymer was purified by precipitation with methanol twice and dried under vacuum to constant weight.

Results and discussion

Synthesis and characterization of lcP(TMC-b-LLA) copolymers

The sequential ROP of TMC and L-LA was produced using a TBD organocatalyst, combined with a PB–OH macroinitiator (Scheme 1). The synthetic strategy involved the ROP of TMC in CH₂Cl₂ at RT for 1 h to achieve a precursor lcPTMC first. Subsequently, this catalyst, end-capped lcPTMC was used in situ as a macroinitiator for the ROP of L-LA at RT for another 1 h, ultimately giving the lcP(TMC-b-LLA) block graft copolymers in a one-pot reaction. Copolymer composition and molecular weight of all the samples are shown in Table 1.

Table 1 Sequential copolymerization of TMC and L-LA promoted by the TBD/PB–OH system in CH₂Cl₂ at 25 °C (no prior isolation of PB-g-PTMC)

Sample	[TMC]0/[L-LA]0/[I]0^a/[TBD]0	Conv.TMC^b (%)	Conv.L-LA^b (%)	TMC/L-LA^c (wt%)	Mn,theo^d (kg mol^−1)	Mn,GPC^e (kg mol^−1)	PDI^e
a Macroinitiator PB–OH: Mn = 2850 g mol^−1 degree of hydroxylation = 16.4%.b Determined by NMR analysis of the crude reaction mixture.c TMC/L-LA wt% in the recovered copolymer determined by ^1H-NMR.d Calculated from the relationship: ([TMC]0/[I]0) × MTMC × conv.TMC + ([L-LA]0/[I]0) × ML-LA × conv.L-LA + Minitiator with ML-LA = 144 g mol^−1, MTMC = 102 g mol^−1, and Minitiator = 2850 g mol^−1.e Number average molar mass and polydispersity values determined by GPC in THF versus polystyrene standards.
lcPLLA	0/400/1/2	—	99	0/100	59.9	51.2	1.4
lcP(TMC-b-LLA)1	100/300/1/2	100	96	20/80	54.5	57.6	1.3
lcP(TMC-b-LLA)2	150/250/1/2	100	97	30/70	53.1	53.7	1.3
lcP(TMC-b-LLA)3	180/220/1/2	98	93	38/62	50.3	48.6	1.2
lcP(TMC-b-LLA)4	200/200/1/2	96	96	41/59	50.1	48.2	1.4
lcP(TMC-b-LLA)5	230/170/1/2	98	95	50/50	49.1	46.7	1.3
lcP(TMC-b-LLA)6	270/130/1/2	98	89	62/38	46.5	44.4	1.4
lcP(TMC-b-LLA)7	350/50/1/2	98	91	84/16	44.4	43.8	1.2
lcPTMC	400/0/1/2	99	—	100/0	43.2	50.1	1.4

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The molecular weight of the macroinitiator PB–OH (M_n = 2850 g mol⁻¹), precursor lcPTMC (M_n = 22.7 kg mol⁻¹) and block copolymer lcP(TMC-b-LLA) (M_n = 44.4 kg mol⁻¹) were determined using GPC and the results are shown in Fig. 1. The GPC curves are all monomodal and clearly show the increase of the molar mass upon complete conversion of TMC and L-LA. The PDI values of lcP(TMC-b-LLA) copolymers are small, and range from 1.2 to 1.4. The experimental molar mass values determined using GPC (M_n,GPC) do not exactly reflect the real molar mass variations, because these values are calibrated against linear PS standards, for which the correction factor for the comb-like copolymers is unknown. The M_n,GPC values of TMC and LA copolymer should be higher than the actual value,¹⁷ whereas the M_n,GPC value of the graft polymer is usually lower than the actual value. Overall, the molecular weight determined by GPC is very close to the theoretical one.

Fig. 1 GPC traces of the macroinitiator (PB–OH, M_n = 2850 g mol⁻¹); the linear-comb PB-graft-PTMC copolymer (lcPTMC, M_n = 22.7 kg mol⁻¹) after ROP of TMC; and the linear-comb PB-graft-P(TMC-block-PLLA) copolymer (lcP(TMC-b-LLA) (Table 1), lcP(TMC-b-LLA)6, M_n = 44.4 kg mol⁻¹).

The chemical structure of the graft block copolymer lcP(TMC-b-LLA) was confirmed by the results of the ¹H and ¹³C-NMR analyses (Fig. 2 and 3). Fig. 2 displays the ¹H-NMR characteristic signals of both the prepolymer lcPTMC [Fig. 2(a)] and lcP(TMC-b-LLA) [Fig. 2(b)]. The residual monomer content and the copolymer composition were determined using ¹H-NMR analysis of the crude polymerization products. From the integration (Int) ratio Int_PTMC/[Int_PTMC + Int_TMC], using the α-methylene group of the carbonate (CH₂OC(O), δ_TMC = 4.45 ppm, δ_PTMC = 4.25 ppm), and from the integration ratio Int_PLLA/[Int_PLLA + Int_LLA], using the methine hydrogen in (OCHCH₃C(O), δ_LLA = 5.05 ppm, δ_PLLA = 5.15 ppm) for L-LA. Also, it is worth noting that the composition of the block side chain could be controlled by varying the comonomer-to-initiator ratios. The conversions of TMC monomer in all the samples were pretty high (>96%), while the conversions of the L-LA monomer were much lower (89–97%) because the longer lcPTMC side chains lower the mobility of the chain ends. The molar ratios of the TMC and L-LA monomers in the copolymers were determined from the characteristic signal of each segment at δ 4.25 ppm for PTMC and at δ 5.15 ppm for PLLA. The weight fraction of TMC in these copolymers varied between 20 wt% (Table 1, lcP(TMC-b-LLA)1) and 83.7 wt% (Table 1, lcP(TMC-b-LLA)7). The representative peak of the PB backbone was identified and marked at around δ 5.4 ppm.

Fig. 2 ¹H-NMR spectrum of: (a) lcPTMC copolymer; and (b) lcP(TMC-b-LLA) copolymer (Table 1, lcP(TMC-b-LLA)1) prepared from the TBD/PB–OH system.

Fig. 3 ¹³C-NMR spectrum of lcP(TMC-b-LLA) copolymer (Table 1, lcP(TMC-b-LLA)1) prepared from the TBD/PB–OH system.

Correspondingly, the ¹³C-NMR spectrum (Fig. 3) showed the expected signals of each segment of the copolymer including the distinctive backbone carbon atoms (δ 154.88 ppm (OCO), 64.27 ppm (OCH₂CH₂), 28.03 ppm (CH₂CH₂CH₂) for PTMC; δ 169.56 ppm (OC(O)–CH), 69.23 ppm (C(O)CH(CH₃)O), 16.61 ppm (COCH(CH₃)) for PLLA). Close examination of the ¹³C-NMR spectra of the lcP(TMC-b-LLA) copolymers allowed unambiguous identification of the chain-end and also of the junction resonances in the carbonyl and methylene regions. Clear observation of the TMC-L-LA junction units (signals corresponding to hydrogen atoms c, d, e, f and g, Fig. 3) provided further evidence of the blocky nature of the copolymer. The absence of the characteristic ether signals (δ ¹H = 3.3–3.1 ppm, δ ¹³C = 66.5–67.7 ppm) in the spectra of lcP(TMC-b-LLA) copolymers revealed the absence of undesired decarboxylation during the copolymerization, a phenomenon often observed in the ROP of a cyclic carbonate.^4e,18 Together, these observations were consistent with the successful synthesis of the desired comb-like lcP(TMC-b-LLA) block copolymer. They highlight the good control of the ROP of TMC and L-LA promoted by the TBD organocatalysis system.

Synthesis and characterization of lcP(LLA-co-TMC) copolymers

Gradient structure constitutes a relatively new class of polymers with a molecular structure that differs from those of block and random structures. In particular, gradient copolymers show a gradient in repeat unit composition along much or all of the copolymer chain. In contrast, random copolymers show no gradient in average composition along the chain, while diblock copolymers show a constant composition along the chain except at one position where there is a step change in composition. The simultaneous copolymerization of L-LA and TMC with same molar ratio ([TMC]₀/[L-LA]₀/[I]₀ = 200/200/1) was performed with the organocatalyst/PB–OH system to synthesize well defined lcP(LLA-grad-TMC) and lcP(LLA-ran-TMC) copolymers. Representative results are shown in Table 2.

Table 2 lcP(LLA-co-TMC) copolymers prepared with different organocatalyst/PB–OH systems

Sample	Solvent	Cat.	[TMC]0/[L-LA]0/[I]0^a/[Cat.]0	Temp. (°C)	Time (h)	^1H-NMR (TMC/L-LA)	Mn,GPC^c (kg mol^−1)	PDI^c
a “One pot, two step” TMC and L-LA sequential copolymerization (TMC for 1 h, L-LA for 1 h, total 2 h) were produced to obtain the block copolymer.b Copolymerization of L-LA/TMC at 25 °C for 1 min and heating up to 61 °C for 20 h to get the gradient copolymer.c Number average molar mass and PDI values determined by GPC in THF versus PS standards.
lcP(TMC-b-LLA)	CH2Cl2	TBD	200/200/1/2	25	2^a	1/1.03	48.2	1.4
lcP(LLA-grad-TMC)	—	DBU	200/200/1/4	25–60	20^b	1/1.09	46.7	2.1
lcP(LLA-ran-TMC)	C7H8	TBD	200/200/1/2	110	2	1/1.02	44.7	1.4

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Recently, Coulembier et al. reported that a gradient copolymer of L-LA and TMC could easily be synthesized from the eutectic melt system.⁹ According to that method, the synthesis of lcP(LLA-grad-TMC) gradient copolymer was performed from a eutectic melt of a L-LA/TMC mixture with the DBU/PB–OH system (Table 2, lcP(LLA-grad-TMC)). Because reactions were carried out in bulk at RT, PLLA chains started to nucleate into the eutectic melt (at a time depending on the targeted molecular weight) and to crystallize out of the melt, limiting the growing polymer to a pure lcPLLA as expected. After 1 min, the mixture was heated to 61 °C (off set of the PLLA T_g) to make the lcPLLA active chain ends more accessible to the TMC units. The relatively high PDIs obtained were attributed to the diffusion limitation during the very fast ROP process.

The synthesis of lcP(LLA-ran-TMC) random copolymer was performed in toluene at 110 °C with the TBD/PB–OH system [Table 2, lcP(LLA-ran-TMC)]. In copolymerization promoted by TBD, L-LA and TMC were converted as fast as in their respective, corresponding homopolymerization.^4g The organocatalyst proved to be quite effective in terms of activity but required optimized conditions for good control – prolonged reaction times would result in extensive transesterification side reactions.

The microstructure of L-LA and TMC copolymers was compared using ¹H and ¹³C-NMR spectroscopy (Fig. 4 and 5). By comparing the differences between the block side chain and the other two gradient and random side chain copolymers, ¹H-NMR analysis highlighted the appearance of chain reshuffling reactions (Fig. 4). Both gradient and random copolymer arms are characterized by a peak at 5.05 ppm, while the random structure performs a higher level of polymer segment redistribution. The differences can also be discerned in the corresponding ¹³C-NMR spectra (Fig. 5). The relative intensity of the block junction resonances in the carbonyl region [δ 154.32 and 169.90 ppm, Fig. 5(a)], as well as in the methine/methylene regions [δ 61.81, 64.74, 69.24 and 71.36 ppm, Fig. 5(b)], were considered and they were compared with the main resonances assigned to the blocks in the carbonyl region (δ 154.88 and 169.56 ppm) and in the methine/methylene regions (δ 64.27 and 69.23 ppm), respectively. Very specific signal assignment in the carbonyl and methylene carbons regions of the ¹³C-NMR spectra to the appropriate, detailed comonomer sequences of L-LA and TMC-based copolymers were achieved by Dobrzynski and Kasperczyk in 2006.^4e

Fig. 4 ¹H-NMR spectrum (the methylene and methine region) of lcP(LLA-co-TMC) copolymers (Table 2) prepared using the organocatalyst/PB–OH system under different conditions.

Fig. 5 ¹³C-NMR spectrum of lcP(LLA-co-TMC) copolymers (Table 2) prepared from the organocatalyst/PB–OH system under different conditions: (a) details of the carbonyl region; (b) details of the methine and methylene region.

Thermal and mechanical properties of lcP(TMC-b-LLA) copolymers

The thermal properties (Fig. 6), and consequently the mechanical properties (Fig. 7), of the L-LA and TMC copolymers can be varied to a great extent, by adjusting the comonomer composition. Because of the lesser content of PB macroinitiator in each graft copolymer, it was assumed that the effect of the PB backbone on the thermal and mechanical properties was limited and thus, it will not be considered in the following discussion.

Fig. 6 (a) DSC curves (second heating run). (b) DMA curves of a lcP(TMC-b-LLA) copolymer (Table 3, lcP(TMC-b-LLA)4).

Fig. 7 Representative stress–strain curves of lcPLLA; lcP(TMC-b-LLA); and lcPTMC. Experiments were conducted with a constant crosshead speed of 10 mm min⁻¹ at room temperature. The point of break is indicated by “×”.

The thermal transitions of the graft block copolymers were determined using DSC. The DSC curves of lcPLLA, lcP(TMC-b-LLA), and lcPTMC samples are illustrated in Fig. 6(a), and the data are summarized in Table 3. The samples of lcP(TMC-b-LLA)1–4 are richer in L-LA units (>60 wt%), according to the results from the ¹H-NMR analyses. All of these four graft block copolymers were able to crystallize with obvious melting temperature (T_m) and ΔH, which slightly decreased as the content of the L-LA units decreased, demonstrating the decreasing crystallization ability of the copolymer. Furthermore, these crystalline domains were supposed to hinder the mobility of amorphous chains, leading an increasing in T_g. For the samples of lcP(TMC-b-LLA)5–7, no T_m was observed, because the PLLA sequence lengths were too short to initiate the formation of a crystalline phase. As expected, the T_g values of all the block samples, lcP(TMC-b-LLA)1–7, decreased as the TMC units increased, which agreed with the trend obtained by Silvino et al.¹⁹ and Dobrzynski and Kasperczyk.^4e Two T_g values were observed in the sample lcP(TMC-b-LLA)4, with a 50/50 molar ratio of TMC/L-LA, these values were −7.9 °C and 45.3 °C, respectively. DMA measurements [Fig. 6(b)] were used to confirm this result. Two separated T_g values of 1.6 °C and 60.7 °C indicated the separated morphologies of the block copolymer.

Table 3 Weight fraction, thermal properties and stress–strain behavior of lcP(TMC-b-LLA) tensile bar samples

Sample	^1H-NMR (TMC, wt%)	Tg1^a (°C)	Tg2^a (°C)	Tm^a (°C)	ΔH^a (J g^−1)	E^b (MPa)	σb^c (MPa)	εb^d (%)
a Glass transition temperatures and melting point values were determined using DSC on the second heating.b Young's modulus.c Ultimate tensile strength.d Elongation at break.
lcPLLA	0	—	53.5	164.8	34.2	1302 ± 79	13 ± 2	42 ± 18
lcP(TMC-b-LLA)1	20.0	—	44.6	164.2	29.2	1022 ± 41	18 ± 6	210 ± 41
lcP(TMC-b-LLA)2	30.2	—	34.9	162.5	26.1	565 ± 19	15 ± 1	270 ± 23
lcP(TMC-b-LLA)3	37.6	—	21.7	160.2	25.1	464 ± 39	27 ± 1	342 ± 16
lcP(TMC-b-LLA)4	40.7	−7.9	45.3	156.7	6.2	382 ± 35	28 ± 1	454 ± 63
lcP(TMC-b-LLA)5	50.2	9.9	—	—	—	212 ± 37	10 ± 1	525 ± 47
lcP(TMC-b-LLA)6	62.0	−1.6	—	—	—	53 ± 21	0.3 ± 0.5	629 ± 7
lcP(TMC-b-LLA)7	83.7	−10.7	—	—	—	17 ± 2	0.6 ± 0.3	708 ± 28
lcPTMC	100	−15.6	—	—	—	3 ± 1	0.7 ± 0.5	865 ± 37

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It is always difficult to combine biodegradability and elastomeric characteristics in biomaterials. Generally, thermoplastic elastomers (TPEs) are in the form of ABA triblock copolymers with B as the soft segment and A as the hard segment. For example, linear PLLA-b-PTMC-b-PLLA triblock copolymer is composed with an amorphous PTMC middle block and two crystalline PLLA end a blocks. The lcP(TMC-b-LLA) copolymers with a diblock side chain can be assumed to be several ABA triblock copolymers, which are connected at the middle of the polymer chains by the PB backbone. Therefore, these materials can exhibit TPE behavior just like other conventional TPEs.

The tensile properties of lcP(TMC-b-LLA) copolymers with varied TMC content are compared in Fig. 7, and summarized in Table 3. In order to investigate the behavior of the thermoplastic elastomer lcP(TMC-b-LLA), the tensile tests were obtained and compared with those obtained using lcPTMC and lcPLLA graft polymers, which were synthesized using the same method. All the samples were tested to their breaking points, and the failures occurred at the middle of the tensile bar and not in the grips. Representative stress-strain curves of lcPLLA, lcP(TMC-b-LLA), and lcPTMC copolymers pulled to ultimate tensile failure are given in Fig. 7. For the graft block copolymers lcP(TMC-b-LLA)1–4 with less PTMC content (TMC wt% < 41%): at low strains, a linear response was observed in the stress-strain curve for all of the samples, however, beyond the low strain elastic region, the copolymers showed a yielding behavior which is common for TPEs. For copolymers lcP(TMC-b-LLA)5–7 with a higher TMC content and a lower T_g (<25 °C, the testing temperature), typical rubber behavior was obtained. Over the range of compositions investigated, both Young's modulus (E) and ultimate tensile strength (σ_b) decreased with PTMC content whereas the elongation at break (ε_b) increased.

The influence of the weight fraction of TMC on the E and ε_b parameters was evaluated (Fig. 8). A very swift increase of the ductility for a PTMC fraction less than 20 wt% in PTMC-b-PLLA diblock copolymers were reported by Guerin et al.^4f However, the PTMC fractions larger than 20 wt% were tested with only two samples in that report. In this research, the objective was to study PTMC contents larger than 20 wt%, and seven samples, lcP(TMC-b-LLA)1–7, with similar molecular weights (M_n = 43.8–53.7 kg mol⁻¹) and diffused content of PTMC were prepared and tested. In contrast, graft block copolymers with a 20 wt% PTMC fraction showed a large elongation at break (ε_b = approximately 210%), but still with a high Young's modulus (E = approximately 1022 MPa) compared with the graft polymer lcPLLA (E = approximately 1302 MPa), which is indicative of a TPE behavior.²⁰ The decrease in Young's modulus (E) was expected with the decreasing content of PLLA in the samples, because of the reduced crystallinity. The elongation at break of the samples showed an linear increase with the increasing PTMC content and were still lower than those obtained for the pure lcPTMC. Therefore, the copolymers obtained ranged from plastics to rubbers as the amount of PTMC increased.

Fig. 8 Variation of Young's modulus (E) and elongation at break (ε_b) as a function of the PTMC weight fraction in lcP(TMC-b-LLA) copolymers.

Thermal and mechanical properties of lcP(LLA-co-TMC) copolymers

The DSC curves of representative lcP(LLA-co-TMC) copolymers with three microstructures are illustrated in Fig. 9, and the results are summarized in Table 4. The lcP(TMC-b-LLA) copolymer possessed two T_gs (−7.9 °C and 45.3 °C), and a T_m (156.7 °C), which indicated a separated microphase of PTMC and PLLA. The lcP(LLA-grad-TMC) and lcP(LLA-ran-TMC) copolymers with a similar content of TMC (about 40 wt%) both displayed an unique glass T_g at 17.5 °C and 7.2 °C, respectively. Interestingly, the graft random copolymer possesses a narrow T_g (−2 to 10 °C), whereas the graft gradient copolymer possesses a much broader T_g (−4 to 30 °C). The broadness of the T_g area is marked by the “×” signals in Fig. 9. As a result, the graft block copolymer exhibits two narrow T_gs originating from the nanophase separation into an ordered domain with nearly pure PTMC or nearly pure PLLA. For copolymers with a similar composition, the graft gradient copolymer exhibits a T_g response with a 34 °C breadth. These features can be easily understood from the nature of the composition of the lcP(LLA-co-TMC) copolymers. In these gradient side chain copolymers of L-LA/TMC, the composition of the chain gradually changes from almost PLLA to almost PTMC. On the basis of self-consistent field theory calculations, Lefebvre et al. predicted that lamellar ordered gradient copolymers exhibited sinusoidal composition profiles across the ordered lamellae.²¹ As a result, the T_g of the lcP(LLA-grad-TMC) copolymer were expected to be broad, reflect the range of T_g associated with the local compositions varying smoothly across the lamellae. Wong et al. have also demonstrated experimentally, that the A–B gradient copolymers [poly(styrene)-grad-acrylic acid] could exhibit an unusually broad T_g.²²

Fig. 9 DSC curves (second heating run) of lcP(LLA-co-TMC) copolymers (Table 4) with various topologies prepared using an organocatalyst/PB–OH system under different conditions. The gap between the “×” indicate the broadness of T_g for respective samples.

Table 4 Weight fraction, thermal properties and stress–strain behavior of lcP(LLA-co-TMC) copolymers (Table 2) prepared using an organocatalyst/PB–OH system under different conditions

Sample	^1H-NMR (TMC, %wt)	Tg1^a (°C)	Tg2^a (°C)	Tm^a (°C)	ΔH^a (J g^−1)	E^b (MPa)	σb^c (MPa)	εb^d (%)
a Glass transition temperatures and melting point values were determined using DSC on the second heating.b Young's modulus.c Ultimate tensile strength.d Elongation at break.
lcP(TMC-b-LLA)4	40.7	−7.9	45.3	156.7	6.2	382 ± 35	28 ± 1	454 ± 63
lcP(LLA-grad-TMC)	38.9	—	17.5	—	—	172 ± 24	12 ± 1	486 ± 38
lcP(LLA-ran-TMC)	41.0	—	14.3	—	—	3.7 ± 0.9	0.14 ± 0	1797 ± 73

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The tensile properties of lcP(LLA-co-TMC) copolymers with a similar TMC content were evaluated and the results are shown in Fig. 10, and the relevant data are summarized in Table 4. The comb-like block copolymer lcP(TMC-b-LLA)4, as discussed previously, behaved like a TPE, whereas the other two comb-like gradient and random copolymers, with T_gs below room temperature, behaved more like rubbers because of the flexibility observed during the whole process. lcP(LLA-grad-TMC) possessed a Young's modulus (172 MPa) less than half that of lcP(TMC-b-LLA)4 (E = 382 MPa), and a slightly higher value of elongation at break (ε_b = 486%) than lcP(TMC-b-LLA)4 (ε_b = 454%).This is in accordance with the result reported by Matyjaszewski et al., in which the gradient structure copolymer showed obvious lower E and higher ε_b than the block one.²³ Luo and coworkers reported both block and gradient copolymers showed bicontinuous nanomophology under AFM.²⁴ Instead of the clear nanophase boundary of block copolymer, the gradient copolymer exhibited a vague and transitional boundary. Due to the enhanced miscibility, the gradient transitions could help to dissipate the energy at the interfaces. The random structure The lcP(LLA-ran-TMC) uniform copolymer behaved more like a typical rubber with a very low Young’s modulus (E = 3.7 MPa) and a significant high ε_b value (ε_b = 1800%) even larger than the pure lcPTMC (ε_b = 865%). It has been shown that the microstructure of the comonomer distribution in the side chain could affect the mechanical properties. Understanding the deformation mechanisms associated with these materials, such as molar ratio, and alignment of the microstructure, could potentially guide future efforts to maximize the toughness of these L-LA and TMC copolymers.²⁵

Fig. 10 Representative stress-strain curves of lcP(TMC-b-LLA) graft block copolymer, lcP(LLA-grad-TMC) graft gradient copolymer, and lcP(LLA-ran-TMC) graft random copolymer. Experiments were conducted at RT. The point of break is indicated by “×”.

Conclusion

The comb-like copolymers with block, gradient and random side chains were synthesized successively upon sequential or random ROP of L-LA and TMC, using a TBD or DBU organocatalyst, associated with a multi-functional PB–OH as a macroinitiator. The polymeric side chains were grown from the PB backbone using a “graft from” strategy. NMR and GPC techniques were employed to characterize the copolymers, validating the formation of the desired structures with controlled molecular weights and narrow dispersity values. By increasing the content of TMC in the block side chain, the lcP(TMC-b-LLA) copolymer ranged from a semi-crystalline tough plastic material (with TMC < 20 wt%), to TPEs (with TMC = 20–41 wt%), to typical rubbers (with TMC > 50 wt%). The properties of these three topology lcP(LLA-co-TMC) copolymers with the same molar composition were investigated to compare their properties. Besides a block side chain structure, the gradient and random structure copolymers yield unique properties: the lcP(LLA-grad-TMC) copolymer can possess a distinctly broad T_g in comparison to that of the comb-like ordered block and random copolymers, and the lcP(LLA-ran-TMC) uniform copolymer behaves like typical rubber with a significantly high (ε_b value 1800%) which is even higher than that of the pure lcPTMC (ε_b = 865%). These studies show that there can be attractive applications for graft polymers with different topology structures of the side chains, which can yield unique properties. For example, the gradient copolymers may have use in damping applications, where broad T_g behavior is a highly desired feature. There is a great potential for the lcP(LLA-co-TMC) copolymer in biomaterials applications, and detailed investigations of its degradation are in progress to determine the effects of side chain molar composition and chain microstructure.

Acknowledgements

This work was financially supported by the National Program on Key Basic Research Program of China (973 Program No. 2015CB654700 ( 2015CB654701)), the National Science Foundation of China (No. U1508204) and the Fundamental Research Funds for the Central Universities (No. DUT16QY38).

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