Synthesis of ABCBA Penta Stereoblock Polylactide Copolymers by

Jun 3, 2013 - BCB triblock copolymers consisting of poly-l-lactide (PLLA: B) and poly(3-methy-1,5-pentylene succinate) (SA/MPD: C) were first synthesi...
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Synthesis of ABCBA Penta Stereoblock Polylactide Copolymers by Two-Step Ring-Opening Polymerization of L- and D‑Lactides with Poly(3-methyl-1,5-pentylene succinate) as Macroinitiator (C): Development of Flexible Stereocomplexed Polylactide Materials Masayuki Hirata, Kazunari Masutani, and Yoshiharu Kimura* Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan ABSTRACT: BCB triblock copolymers consisting of poly-Llactide (PLLA: B) and poly(3-methy-1,5-pentylene succinate) (SA/MPD: C) were first synthesized by ring-opening polymerization (ROP) of L-lactide by using a dihydroxylterminated SA/MPD (Mn ≈ 20k) and tin octoate as the macroinitiator and catalyst, respectively. The telechelic dihydroxyl-terminated SA/MPD was readily synthesized by the controlled melt-polycondensation of succinic acid and 3methyl-1,5-pentandiol (MPD). The resultant triblock copolymers, dihydroxyl-terminated, were subsequently utilized as the macroinitiators in the second-step ROP of D-lactide to obtain ABCBA penta-block copolymers (penta-sb-PLA) consisting of poly-D-lactide (PDLA), PLLA, and SA/MPD as the A, B, and C blocks, respectively. The weight-average molecular weights of the resultant penta-sb-PLAs became higher than 150 kDa. The cast films of these penta-sb-PLAs exhibited flexible nature due to the presence of the SA/MPD soft block as well as excellent heatstability owing to the easy stereocomplex formation of the neighboring enantiomeric PLLA and PDLA blocks.

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most effective way.8−10 The presence of the soft segments can effectively weaken the inner stress to enhance the impact strength. This modification ought to be effective for improving the brittle nature of sb-PLA, and for this purpose, copolymers consisting of three blocks, i.e., enantiomeric PLLA and PDLA blocks and a soft polymer block, must be designed. In the previous reports, enantiomeric ABA triblock copolymers consisting of PLA (A: PLLA or PDLA) and a soft segment (B) were synthesized and mixed in 1:1 ratio. The enantiomeric copolymer blends obtained showed better heat and impact resistances. For example, Hillmeyer et al. succeeded in preparing enantiomeric copolymers of polylactide-b-polymenthide-b-polylactide and demonstrated that their enantiomeric blends show satisfactorily high elastomeric properties.11 However, the molecular weights of these block copolymers were less than 65 kDa, being deficient for securing durability and reliability in their real long-term application. For increasing the total molecular weight of these block copolymers by maintaining a constant block ratio, the macroinitiator used as the central block must have higher molecular weights. However, the increase in the molecular weight of macroinitiators is likely to break the telechelic nature (dihydroxyl functionalities) and make the content of the initiating hydroxyl groups too small, retarding the ring-opening polymerizability

INTRODUCTION Polylactides (PLAs) have been developed as biobased and biodegradable polymers that can replace a part of conventional oil-based polymers for temporary use as packaging and mulching. However, the thermal resistance and impact strength of PLA materials are inferior to those of the ordinary plastic materials, preventing their application in large scale. For improving the heat-deformation temperature of PLA materials, stereocomplex polylactides (sc-PLA) consisting of poly(Llactide) (PLLA) and poly(D-lactide) (PDLA) have been developed because their melting temperature (Tm = 230 °C) is 50 °C higher than that of PLLA or PDLA1 and comparable to that of engineering plastics such as poly(butylene terephthalate) (PBT) and polyamide 6. However, when high-molecularweight PLLA and PDLA are melt-blended for obtaining scPLA, the stereocomplex (sc) crystallization is likely accompanied by homochiral (hc) crystallization, deteriorating the properties of the resulting sc-PLA products.2 Hence, the stereoblock polylactides (sb-PLA) consisting of PDLA and PLLA sequences have been developed.3−7 Since the sb-PLA should have a mixing state of the enantiomeric chains in molecular level, the sc formation is much enhanced to afford a high-performance material. On the other hand, the highly crystalline sb-PLA can not escape the problems related with the brittle nature often shown by the ordinary PLLA materials. Various modifications have been proposed thus far to impart flexibility to PLLA. Among them, introduction of elastomeric soft segments by copolymerization has been known to be the © 2013 American Chemical Society

Received: February 11, 2013 Revised: May 28, 2013 Published: June 3, 2013 2154

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Figure 1. 1H NMR spectra of the (A) SA/MPD, (B) Tri 30k, and (C) Penta 40k together with their expanded regions A′, B′, and C′, respectively, at δ 3.35−4.45 ppm. monohydrate (TSA) was purchased from Kanto Chemical Co. (Tokyo). 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) was purchased from Central Glass Co. Ltd. (Yamaguchi, Japan). These chemicals were used as received unless otherwise noted. Measurements. 500 MHz 1H and 150 MHz 13C NMR spectra were measured on ARX and AV600 spectrometers (Bruker, Germany), respectively, with samples dissolved in deuterated chloroform (CDCl3) containing 0.03 or 1.0 vol-% tetramethylsilane as the internal reference. The number- (Mn) and weight-average (Mw) molecular weights were determined by size exclusion chromatography (SEC) with a Shimadzu (Kyoto) analyzer system comprising LC-10ADvp pump, a RID-10A refractive index detector, and a C-R7A Chromatopac data processor. HFIP containing 1 mol % sodium trifluoroacetate was eluted at 40 °C through a styrene-divinylbenzene copolymer gel column, Shodex HFIP-806 (8.0 mm in inner diameter and 300 mm in length). The molecular weights were calibrated as relative values to poly(methyl methacrylate) (PMMA) standards. Another SEC system using 1,3dioxolane as the eluent was also utilized for determination of Mn and Mw. The analyzer system was identical to the above, and 1,3-dioxolane was eluted at 45 °C through two Tosoh TSKgel GMHHR-M columns. The molecular weights were calibrated as relative values to polystyrene (PS) standards. Thermal properties of polymer samples were measured on a Perkin-Elmer (USA) Diamond DSC thermal analyzer with α-alumina as the reference under a nitrogen atmosphere. The samples were heated from −50 to 240 °C at a heating rate of 20 °C/ min (the first scan). Wide-angle X-ray diffraction (WAXD) was recorded on a Rigaku (Tokyo) 2000 FSL X-ray diffraction system with a Rigaku RINT 2000 X-ray generator operated at 40 kV and 50 mA using nickel-filtered Cu−Kα radiation (λ = 0.1542 nm) in a 2θ range of 5−40° at a scan rate of 2°min−1. Dynamic mechanical analysis (DMA) was performed on a DMS6100 analyzer (SEIKO, Ltd., Japan) manipulated on tension mode at a constant frequency of 1.0 Hz. The experiments were carried out in a temperature range from −100 to 240 °C at a heating rate of 3 °C/min. Mechanical properties of polymer films were evaluated on an STA-1150 tensile tester (ORIENTEC, Co., Ltd., Japan) at room temperature with a gauge length of 20 mm at a crosshead speed of 50 mm/min. The data obtained over five test specimens were averaged in each measurement.

(ROP) of lactides. We therefore lead to a macromolecular design in which both the enantiomeric PLLA and PDLA blocks are directly connected with a soft block to form a penta-block copolymer in order to achieve better sc formation and flexible nature of the copolymers with the total molecular weight remaining high enough. There were several trials to synthesize such copolymers having penta-block configuration in the past,12−15 but few have been known that involved a PLLA/ PDLA stereoblock structure and a high molecular weight. In this study, we synthesize a new ABCBA-type penta-block copolymer (penta-sb-PLA) consisting of PDLA (A), PLLA (B), and poly(3-methyl-1,5-pentylene succinate) (SA/MPD: C). The SA/MPD used as the soft C block shows excellent amorphous nature in spite of significantly simple chain structure. It can readily be prepared by melt-polycondensation of 3-methyl-1,5-pentanediol (MPD) and succinic acid (SA) in a form of dihydroxyl-terminated prepolymer. Here a SA/MPD prepolymer having an appropriate molecular weight is selected, and the two-step ROP of L- and D-lactides are conducted to finally obtain the ABCBA penta-block copolymers, PDLA-bPLLA-b-SA/MPD-b-PLLA-b-PDLA via BCB triblock copolymers, PLLA-b-SA/MPD-b-PLLA. The resultant sb-PLA copolymers are characterized by the flexible nature and better sc formation that are achieved by the connection of block segments with enough high molecular weight maintained.

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EXPERIMENTAL SECTION

Materials. Both L- (optical purity ≥99.8%ee: L/D = 99.9/0.1) and D-lactides (optical purity ≥99.4%ee: L/D = 99.7/0.3) were supplied by Musashino Chemical Laboratory Ltd. (Tokyo, Japan). Tin octoate (Sn(Oct)2) and tin(II) chloride dihydrate (SnCl2·2H2O) were purchased from Nacalai Tesque Co. (Kyoto, Japan). Sn(Oct)2 was distilled in high vacuum and dissolved in dry toluene in a concentration of 0.05 g/mL. SA and MPD were purchased from Tokyo Chemical Industry Co. (Tokyo). p-Toluenesulfonic acid 2155

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Scheme 1. Synthesis of Penta-sb-PLAs (PDLA-b-PLLA-b-SA/MPD-b-PLLA-b-PDLA) via Triblock Copolymers (PLLA-b-SA/ MPD-b-PLLA)

Synthesis of SA/MPD Prepolymers. Both SA (59.1 g) and MPD (70.9 g) (1.0: 1.2 in molar ratio) were mixed and dehydrated in melt state at 150 °C and 30 Torr for 3 h to obtain an SA/MPD oligomer having a degree of polymerization (DP) of 2−4 in a yield of about 83%. This SA/MPD oligomer (50.0 g) was mixed with SnCl2·2H2O (0.15 g) and TSA (0.15 g), and the mixture was heated at 190 °C by reducing the pressure gradually to 5 Torr. On reaching 5 Torr, the heating was further continued for 36 h at 190 °C. Finally, an SA/MPD polymer with an Mn of 20 kDa was obtained in a yield of 85%. 1H NMR (CDCl3, Figure 1a): δ = 0.95 (−CH2CH(CH3)CH2−, 3H), 1.49 (−OCH2CH2CH(CH3)−, 4H), 1.68 (−CH2CH(CH3)CH2−, 1H), 2.61 (OOCCH2CH2COO, 4H), 3.43 (CH2OH, small), 3.68 (−OCH2CH2CH(CH3)CH2CH2OH, small), 4.13 (−COOCH2CH2CH(CH3)−, 4H) ppm. 13C NMR (CDCl3): δ = 19.2 (OCH2CH2CH(CH3)CH2CH2O), 19.5 (OCH2CH2CH(CH3)CH 2 CH 2 OH), 26.1 (OCH 2 CH 2 CH(CH 3 )CH 2 CH 2 OH), 26.5 (OCH2CH2CH(CH3)CH2CH2OH), 27.1 (OCH2CH2CH(CH3)CH 2 CH 2 O), 29.1 (OCH 2 CH 2 CH(CH 3 )CH 2 CH 2 O), 29.6 (COOCH 2 CH 2 COOCH 2 CH 2 CH(CH 3 )CH 2 CH 2 OH), 35.3 (OCH 2 CH 2 CH(CH 3 )CH 2 CH 2 O), 39.6 (OCH 2 CH 2 CH(CH 3 )CH 2 CH 2 OH), 60.6 (OCH 2 CH 2 CH(CH 3 )CH 2 CH 2 OH), 62.5 (OCH2CH2CH(CH3)CH2CH2OH), 62.8 (OCH2CH2CH(CH3)CH 2 CH 2 O), 172.0 (COOCH 2 CH 2 COOCH 2 CH 2 CH(CH 3 )CH2CH2OH), 172.2 (COOCH2CH2COO) ppm. Synthesis of BCB and ABCBA Block Copolymers. Typical example was as follows. A portion (5.0 g) of the SA/MPD with an Mn of 20 kDa was charged into a glass tube, desiccated under vacuum for an hour, and purged with nitrogen. Then, a predetermined amount of L-lactide (2.5 g in the case of Tri 30k, vide infra) and a portion of the aforementioned catalyst solution (1.78 × 10−5 mol % of Sn(Oct)2 relative to the L-lactide) were added to it under a nitrogen atmosphere. The tube was desiccated again in high vacuum for 30 min at 90 °C and purged with nitrogen. This desiccation and nitrogen purge was repeated twice. The glass tube was then sealed and heated in an oil bath at 180 °C for 1.0 h. The polymer produced was dissolved in a dichloromethane and reprecipitated into an excess methanol. The precipitates were separated by filtration and dried in vacuum at 60 and 80 °C for 6 h in each temperature to obtain a BCB triblock copolymer in a yield of 83%. 1H NMR (CDCl3, Figure 1b): δ = 0.95 (CH3 of the MPD units), 1.49 (CH2 of the MPD units), 1.58 (CH3 of the lactate units), 1.69 (CH of the MPD units), 2.62 (CH2 of the SA units), 5.17 (CH of the lactate units) ppm). 13C NMR (CDCl3): δ = 15.8 (COCH(CH3)OH), 16.6 (COCH(CH3)O), 19.2 (OCH2CH2CH(CH3)CH2CH2O), 20.5 (OCH2CH2CH(CH3)CH2CH2OCOCH(CH3)OH), 27.1 (OCH2CH2CH(CH3)CH2CH2O), (OCH2CH2CH(CH3)CH2CH2OCOCH(CH3)OH), 29.1 (OCH2CH2CH(CH3)CH2CH2O), (OCH2CH2CH(CH3)CH2CH2OCOCH(CH3)O), 35.3 (OCH 2 CH 2 CH(CH 3 )CH 2 CH 2 O), 62.8 (OCH 2 CH 2 CH(CH 3 )CH2CH2O), 63.7 (OCH2CH2CH(CH3)CH2CH2OCOCH(CH3)OH), 66.7 (OCH2CH2CH(CH3)CH2CH2OCOCH(CH3)OH), 69.0

(COCH(CH3)O), 72.4 (COCH(CH3)OH), 169.5 (COCH(CH3)O), 170.0 (COOCH2CH2COOCH2CH2CH(CH3)CH2CH2OCOCH(CH3)O), 172.0 (COOCH2CH2COOCH2CH2CH(CH3)CH2CH2O), 172.2 (COOCH2CH2COO), 175.0 (COCH(CH3)OH) ppm. In the following second step polymerization with D-lactide, a predetermined amount of the triblock copolymer obtained above (5.0 g in the case of Tri 30k) was charged into a glass tube. It was first dried in vacuum at 90 °C for more than 2.0 h and mixed with predetermined amounts of D-lactide (1.67 g to prepare Penta 40k, vide infra) and the Sn(Oct)2 catalyst solution (1.78 × 10−5 mol % relative to the Dlactide). Then, the tube was desiccated at 90 °C for an hour, sealed, and heated at 190 °C for 1 h. The copolymer finally obtained was dissolved in a dichloromethane/HFIP (90/10 in vol%) mixture and precipitated into excess methanol. The precipitates were filtered and dried in vacuum at 60 and 80 °C for 6.0 h in each temperature to obtain an ABCBA penta-block copolymer in a yield of 85%. The 1H (Figure 1c) and 13C NMR spectra of this copolymer were intrinsically identical with those of the starting triblock copolymer. Preparation of Copolymer Films. Each of the copolymer samples prepared was dissolved in a dichloromethane or dichloromethane/HFIP (90/10 in vol.%) mixture at a concentration of 5 g/dL and cast into a Petri dish that had previously been surface-treated with Sigmacoat (Aldrich) to prevent adhesion of the cast film. After airdrying of the solution, the solidified film was peeled off and thoroughly dried in vacuum at 40, 60, and 80 °C for 4 h in each temperature. The complete evaporation of the solvent from the films was confirmed by 1 H NMR spectroscopy.

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RESULTS AND DISCUSSION The synthetic scheme for the novel stereo ABCBA-type pentasb-PLA is shown in Scheme 1. In the first step, a dihydroxylterminated SA/MPD prepolymer was first prepared by meltpolycondensation and used as the macroinitiator in the firststep ROP of L-lactide to synthesize BCB triblock copolymers. The resultant triblock copolymers, also dihydroxyl-terminated, were reacted with the enantiomeric D-lactide monomer to finally obtain the ABCBA penta-sb-PLAs, for which characterization was conducted in detail. Preparation of an SA-MPD Prepolymer. In the molecular design of the ABCBA penta-block copolymers, it is important to control the Mn of the soft C block or the SA-MPD polymer because it ought to determine the total Mn and soft block content of the final products. Since the total Mn ought to be around 100 kDa for real application at a soft block content above 20 wt %, the Mn of the SA/MPD polymer must be higher than 20 kDa. Taking this feature into consideration, we carefully examined the melt-polycondensation of SA and MPD 2156

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Table 1. Syntheses of BCB Triblock and ABCBA Penta-block Copolymersa run no.

[M]/[I]

lactide

initiator

copolymer (average sequence lengths)

SA/MPD 1 2 3 4 5 6 7

69.5 174 278 69.5 174 347 278

L L L D D D D

SA/MPD 20k SA/MPD 20k SA/MPD 20k Tri 30k Tri 45k Tri 45k Tri 60k

SA/MPD 20k Tri 30k (5k:20k:5k) Tri 45k(12.5k:20k:12.5k) Tri 60k (20k:20k:20k) Penta 40k (5k:5k:20k:5k:5k) Penta 70k (12.5k:12.5k:20k:12.5k:12.5k) Penta 95k (25k:12.5k:20k:12.5k:25k) Penta 100k (20k:20k:20k:20k:20k)

Mn (th) kDa

yieldb %

Mn kDa

Mw/Mn

30 45 60 40 70 95 100

83 83 88 85 84 83 48

24.1c 38.2c 46.6c 79.8c 53.7d 70.7d 90.4d 56.4d

1.84c 1.77c 1.85c 1.81c 2.79d 3.04d 3.56d 3.60d

Polymerization temperature: 180 or 190 °C, time: 1.0 h, catalyst: Sn(Oct)2, in 1.78 × 10−5 mol % relative to the lactide monomer. bAfter reprecipitation. cDetermined by SEC relative to PSt standards in 1,3-dioxolane at 40 °C. dDetermined by SEC relative to PMMA standards in HFIP at 40 °C a

considerable amount of SA-MPD as macroinitiator, making the ring−chain equilibrium of the ROP system decline to the monomer side. The Mn values of the triblock copolymers measured by SEC became much higher than the Mn value of the starting SA-MPD prepolymer, suggesting the chain extension from the prepolymer. The Mn values recorded by the SEC method using 1,3-dioxolane as the eluent were slightly higher than the theoretical Mn values estimated from the monomer/macroinitiator ratios in the feed in all runs, because of the different hydrodynamic volume of the solutes. The universal calibration method indicated that the molecular weight of PLA samples determined in chloroform (M (CHCl3)) is 4 times larger than the molecular weight of PS standards determined in chloroform (M (PS-CHCl3)). Therefore, we used 1,3-dioxolane as the eluent for determination of Mn of the triblock copolymers for minimizing the difference in the hydrodynamic volume between the copolymers and PS standards. It was however evident that the increasing order of Mn well corresponded with the increased amount of L-lactide in the feed, supporting the copolymerization. Each of the BCB triblock copolymers (Tri Xk) obtained was used as the macroinitiator of the next ROP of D-lactide to synthesize the ABCBA penta-sb-PLA. The ROP was conducted with a minimum amount of the catalyst (Sn(Oct)2) at 190 °C where the triblock copolymers were homogeneously mixed with D-lactide monomer. Typical results of the second-step ROP are also involved in Table 1 (Run No. 4−7). Here, the resultant penta-sb-PLA products are named “Penta Xk” where X denotes the theoretical Mn value in kD estimated from the monomer to macroinitiator ratio. The copolymer yields were as high as 83−85% after the reprecipitation. Their Mn values were determined by SEC using HFIP containing 1 mol % of sodium trifluoroacetate as the eluent, because the copolymer samples were not well soluble in ordinary solvents because of their easy stereocomplex formation. The universal calibration method indicated that the molecular weights of PLA determined in chloroform (M (CHCl3)) and HFIP (M (HFIP)) are almost comparable with each other (M (CHCl3)/ (M (HFIP) = 1.02). It means that the (M (HFIP) ought to be also 4 times larger than the real molecular weight. Therefore, we used PMMA having an isomeric structure with PLA as the standard for determination of the Mn of penta-block copolymers for minimizing the difference in the hydrodynamic volume. Here, the Mn values of the obtained penta-block copolymers measured by SEC were almost comparable to the theoretical Mn values estimated from the monomer-to-triblock copolymer ratios in the feed in all runs except Run No. 7. In Run No. 7, the ROP of D-lactide was failed because of the too high

to prepare a dihydroxyl-terminated SA/MPD polymer. We found out that this polycondensation can be well catalyzed by the binary catalyst of SnCl2/TSA, which we efficiently utilized in the polycondensation of L-lactic acid.14 The degree of polymerization (DP) of the resultant SA-MPD polymer proportionally increased with increasing the reaction time, and the Mn of the SA-MPD polymer was successfully controlled around 20 kDa by setting the reaction time about 30 h. Figure 1a shows the 1H NMR spectrum of the obtained SAMPD polymer. As summarized in the Experimental Section, the SA-MPD polymer showed small signals (Figure 1a′ due to the hydroxyl and methylene protons of the terminal MPD units (HO−CH2−) at δ 3.43 (c) and 3.69 ppm (b), respectively, supporting the bis-hydroxyl-terminated structure. From the integral ratios of the main-chain and terminal HO−CH2 signals, the DP value was determined. The obtained DP value was almost comparable to that estimated from the Mn value determined by SEC (1,3-dioxolane as the eluent). In addition, multiple carbon signals due to the terminal groups were clearly detected in the 13C NMR spectrum (Experimental section), while no signal assigned to carboxyl carbon was detected even by expansion of the signal region. These data well supported the dihydroxyl-terminated structure of the SA-MPD prepolymer. When this SA-MPD polymer having an Mn of 20 kDa is used for the following two-step ROP, the Mn of the final penta-block copolymers should be around 60 and 100 kDa, for example, at PDLA/PLLA/SA-MPD compositions of 33/33/33 and 40/40/ 20 (wt/wt/wt), respectively. In fact, when the Mn of SA-MPD was higher than 20 kDa, the Mn of the BCB triblock copolymers became too large, and the second-step ROP of Dlactide did not proceed well. We therefore decided to start with the above SA-MPD polymer having an Mn of 20 kDa in the following ROP. Synthesis of ABCBA Penta-block Copolymers via BCB Triblock Copolymers. The SA-MPD sample (Mn = 20 kDa) obtained above was subjected to the first-step ROP of L-lactide as macroinitiator with a minimum amount of Sn(Oct)2 as the catalyst at 180 °C where the SA-MPD prepolymer was homogeneously mixed with L-lactide. Typical results of copolymerization are summarized in Table 1 (Run No. 1−3) where the resultant BCB copolymers are named “Tri Xk” where X denotes the theoretical Mn value in kDa estimated from the monomer to prepolymer ratio. The yields of the BCB copolymers were rather lower (81−87%) even though they were measured after the reprecipitation procedure for purification. This fact may be because the initial monomer concentration ought to be lower in the presence of a 2157

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molecular weight of the Tri 60k used as the macroinitiator. The polydispersity, on the other hand, became significantly wider (PDI = 2.8−3.6) in all runs, suggesting that the initiation with the triblock copolymers became significantly slower, competing with the propagation. The wide polydispersity may have also been caused by the transesterification during the second ROP of D-lactide, particularly, in the later stage of ROP where the monomer concentration became low. However, we observed little change in the dispersities of the penta-block copolymers even when the reaction time became longer, and we believed that the main reason for the wide polydispersity is attributed to the relatively slow initiation using the macroinitiators having a wide dispersity by themselves. The slowed initiation may be because the hydroxyl group of the terminal lactyls is secondary and less reactive than the primary one of the SA-MPD polymer. In addition, the terminal concentration of the macroinitiators having higher molecular weight is too low to form the initiating species between the triblock copolymers and the catalyst. Figure 1 compares typical 1H NMR spectra of the SA-MPD prepolymer, triblock copolymer (sample: Tri 30k), and pentablock copolymer (sample: Penta 40k). The triblock and pentablock copolymers showed signals (h and i) due to the PLA units in addition to the signals due to SA-MPD units (a, d, e, f, and g). As mentioned earlier, the SA/MPD prepolymer showed small signals due to the methylene (b) and hydroxyl (c) protons of the terminal MPD units. The former signal (b) was completely lost and replaced by the signals (j) and (k) attributable to the lactate terminals in the spectra of the tri- and penta-block copolymers, respectively. It was therefore supported that the SA/MPD had been involved in the copolymers in the initiation of the first-step ROP of L-lactide to form the central C block. Comparing the expanded spectra, the relative intensities of the terminal signals became weaker in Figure 1C' (signals (k) and (c)) than in Figure 1B' (signals (j) and (c)), supporting the further ROP occurring in the second-step ROP of D-lactide. The 13C NMR spectra (Experimental section) also supported the propagation of the PLLA blocks from the hydroxyl terminals of SA-MPD prepolymer by the loss of the terminal signals of the SA-MPD and the appearance of the signals of connecting units (δ = 27.1, 29.1, 170.0, 172.0 ppm) and the signals of terminal lactyl groups (δ = 15.8, 20.5, 72.4, 175.0 ppm) and their penultimate units (δ = 63.7 66.7 ppm). In the penta-block copolymer, the latter signals became weaker, suggesting the chain elongation from the terminal groups of the triblock copolymer. We already reported that the PLLA−PDLA block nature of sb-PLA can be confirmed by the tacticity of the macromolecular chains that can be analyzed by carbonyl signals of the lactate units.16 Figure 2 compares the carbonyl signals of the tri(sample: Tri 30k) and penta-block copolymers (sample: Penta 40k) in the region of δ 169 ppm. The large signal at δ 169.6 ppm (a) is assigned to meso sequences (mmmmm, mmmmr, rmmmm, rmmmr in hexads) corresponding to the isotactic stereoblocks, while the weak signal at δ 169.32 ppm is assigned to the racemi sequences (mmrmm, rmrmm, mmrmr, rmrmr in hexads), which involve both racemized units and PLLA-PDLA hetero links. Although the latter signal is almost undetectable in the triblock copolymer (Tri 30k), it is slightly increased in the penta-block copolymer (Penta 40k). Since the integral ratio of the racemized-unit signal relative to the isotactic unit signal is about 2.2%, the long blocky nature of the copolymer is supported. The average meso block sequences of Penta 40k can be calculated to be over 100. Since this value is reasonably

Figure 2. Carbonyl spectra of the lactate units of (A) Tri 30k and (B) Penta 40k; signal a: mmmmm, mmmmr, rmmmm, and rmmmr in hexads, and signal b: mmrmm, rmrmm, mmrmr, and rmrmr in hexads.

related to those derived from the optical purities of both enantiomeric monomers, the transesterification to cause the chain scrambling is considered to have little occurred between the copolymer chains in the second-step ROP. This fact, combined with the molecular weight increase after the secondstep ROP of D-lactide, strongly indicated that the penta-block structure is involved in the final copolymers. Characterization of Polymer Films of ABCBA Pentablock Copolymers. The ABCBA penta-block copolymers obtained were fabricated into polymer films by solution casting. Figure 3 shows the WAXD profiles of the resultant semi or fully transparent films of Penta 40k, 70k, and 95k. The films of Penta 40k and 70k exhibited the diffraction peaks solely assigned to sc crystals (2θ = 12, 21, and 24°), whereas the film of Penta 95k entailed a weak diffraction of hc crystals (2θ = 16.5°). It was therefore evident that the exclusive sc crystallization had been promoted in the former films prepared by the solution casting. The similar crystalline nature was also confirmed by DSC. Table 2 summarizes the DSC data of the Tri Xk and Penta Xk samples (1st heating scan). In the former samples, a glass transition temperature of SA/MPD (Tg1) was detedcted at significantly higher temperature than that of SA/MPD prepolymer. No glass transition temperature of PLLA (Tg2) was shown because of the excellent compatibility of the PLLA and SA/MPD in amorphous domain. The melting temperatures (Tm) of hc crystals and the heat of fusion (ΔHm) increased with increasing the PLLA content. In the penta-block copolymers, in return, Tm was shown at much higher temperature region (194−200 °C), although the crystallinities of Penta Xk calculated from the ΔHm values (23−40 J/g) were lower than those of Tri Xk. No melting peak of hc crystals was detected even for the Penta 95k sample for which slight hc crystallization was detected by WAXD. The lower crystallinity of Penta Xk may be due to the difficulty in arranging enantiomeric helices in crystallization. The Tg1 values were not so different among the three samples. The Penta 95k, having a large PLA content, showed both Tg1 and Tg2, probably because the large PLA domains separated. Thermo-mechanical and Tensile Properties of the Copolymer Films. Figure 4 shows the temperature dependences of storage modulus (E′) and tan δ for the films of Penta Xk. The film of Penta 40k showed clear β relaxation around 2158

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Figure 3. WAXD profiles of the films of penta-block copolymers: (a) Penta 40k, (b) Penta 70k, (c) Penta 95k.

Tri Xk exhibited a similar level of tensile elongations in spite of showing an increase in modulus with increasing PLA content. This unusual relation is due to the decreased molecular weight in the polymers having increased soft segment ratios. On the other hand, the clearly opposite relation between the modulus and elongation was exhibited, because the molecular weights of the copolymers (particularly Mw) exceeded the threshold for resisting the inner stress. In fact, the film of Penta 40k showed much higher tensile strength and elongation than the films of Tri 30k or Tri 44k, though showing a relatively lower modulus that was comparable to that of thermo-plastic elastomers. Figure 5 shows typical stress−strain curves of the films of Penta Xk. It is evident that Penta 40k showed elastomeric nature while both Penta 70k and 90k, having a low soft block ratios, exhibited plastic nature with improved flexibility. In conclusion, Penta 70k should have improved impact strength and higher thermal resistance because of the balanced sc-PLA hard/SA/MPD soft block ratio and will be used as elastomeric materials, whereas Penta 95k may also have an excellent thermal property for use as heat-resistant materials. Here, we demonstrated the properties only of the synthesized copolymers having rather limited hard/soft block ratios and soft block lengths. Since the molecular weight of the SA/MPD prepolymer was rather limited in conducting the copolymerization, the properties of the final block copolymers could not be controlled in wide range.

Table 2. DSC Data for the Triblock (PLLA-Based) and Penta-block Copolymers (First Scan: Heating Rate = 20° C/ min) copolymer

soft segment (wt %)

Tg1a (°C)

Tg2b (°C)

Tmc (°C)

ΔHmd (J/g)

χce (%)

SA/MPD Tri 10k Tri 25k Tri 40k Penta 40k Penta 70k Penta 95k

100 67 56 33 30.0 13.5 6.2

−24 −17.6 −10.2 −16.5 −10.3 −16.7

-

156 160 164 194.0 200.1 196.6

21.3 29.1 29.8 23.5 40.7 24.8

22.7 31.0 31.7 16.5 28.7 17.5

37.2

a Tg1 shows the glass transition temperatures of SA/MPD. bTg2 shows the glass transition temperatures of PLA. cTm shows the melting temperatures of hc or sc crystals. dΔHm shows the heat of fusion of hc or sc crystals. eχc was calculated by assuming the ideal heats of fusions of hc and sc crystals as 93 and 142 J/g, respectively.

−50 °C and a constant decrease in E′ up to 190 °C, whereas the film of Penta 70k retained their E′ over 108 Pa up to the Tm of sc crystals because the presence of sc crystals can efficiently restrict the chain mobility of the soft SA/MPD segments and the PLA chains in amorphous domain. The film of Penta 95k, in return, retained E′ in a level of 109 Pa up to its α relaxation temperature because of the high sc crystallinity, but showed a sharp drop of E′ above Tg where the E′ reached that shown by the Penta 40k. This feature also depends on the presence of sc crystals that can effectively prevent the softening by the rubbery SA/MPD segments. We previously showed that the E′ of diblock sb-PLA films dropped down in one or two orders around 60−90 °C.16,17 The present retention of E′ shown by Penta 95k and 70k ought to be preferable in terms of the heat-resistivity of the films. Table 3 compares the tensile properties of the films of PDLA (control), Tri Xk, and Penta Xk. While the film of Penta 95k having a low soft block ratio showed a slightly higher modulus than a crystalline PDLA film, the other films having soft block ratios higher than 10 wt % showed much lower modulus irrespectively of triblock or penta-block structure. The films of

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CONCLUSION A novel ABCBA stereoblock copolymer consisting of PDLA (A), PLLA (B), and SA/MPD (C: an aliphatic polyester) was successfully prepared by three-step polymerization. In the first step, a partly biobased amorphous polymer, SA/MPD, having a relatively high molecular weight of Mn = 20 kDa was synthesized by the ordinary melt-polycondensation of SA (a biobased monomer) and MPD (an oil-based monomer). In the second step, this SA/MPD, dihydroxyl-terminated, was used as the macroinitiator of the first ROP of L-lactide to prepare a BCB triblock copolymer having PLLA (B) chains on both sides of SA/MPD (C). In the third step, the triblock copolymer was 2159

dx.doi.org/10.1021/bm400228x | Biomacromolecules 2013, 14, 2154−2161

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Figure 4. Temperature dependences of storage modulus and tan δ for the films of penta-block copolymers: (a) Penta 40k, (b) Penta 95k, (c) Penta 70k (from 30 to 210 °C at 3 °C/min).

PLA) having a high Mn reaching 100 kDa. The penta-sb-PLA samples thus prepared showed preferential stereocomplexation because of the molecular mixing state of the enantiomeric PLLA and PDLA segments while exhibiting elastomeric nature owing to the soft block of SA/MPD. Accordingly, their cast films were found to have balanced tensile properties with enhanced elongation. TMA analysis of these films revealed that their thermal resistivity reaches as high as 200 °C. It was therefore evident that the properties of the penta-sb-PLAs can be controlled by changing the block ratios from thermoplastic elastomers (as Penta 40k having higher SA/MPD contents) to flexible plastics having improved thermo-resistivity (as Penta 70k and 90k having higher PLA contents). Although their biobased contents are around 90%, it may be higher if the

Table 3. Mechanical Properties of the Films of Triblock and Penta-block Copolymers film (% soft segment) PDLA (100: control) Tri 30k (67) Tri 45k (56) Tri 60k (33) Penta 40k (50) Penta 70k (29) Penta 95k (21)

tensile modulus (MPa)

tensile strength (MPa)

elongation at break (%)

1980 ± 160

58 ± 1.6

5 ± 1.3

64 ± 2.5 400 ± 21 980 ± 160 280 ± 40 970 ± 41 2000 ± 140

4.5 ± 0.14 18 ± 1.1 40 ± 1.4 25 ± 2.2 36 ± 3.1 67 ± 1.9

54 ± 1.2 47 ± 1.2 62 ± 1.7 470 ± 10 42 ± 2.0 5.6 ± 0.5

used as the macroinitiator of the second ROP of D-lactide to finally obtain an ABCBA penta-block copolymer (penta-sb-

Figure 5. Typical stress−strain curves of penta-sb-PLA films: (a) Penta 40k, (b) Penta 70k, (c) Penta 95k. 2160

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constituent monomer MPD of the SA/MPD prepolymer is derived from biomass-based feedstock in the future.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-75-724-7804; Fax: +81-75-724-7804; E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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