Synthesis and Stereocomplex Formation of Star-Shaped

Article pubs.acs.org/Macromolecules

Synthesis and Stereocomplex Formation of Star-Shaped Stereoblock Polylactides Consisting of Poly(L‑lactide) and Poly(D‑lactide) Arms Takuya Isono,† Yohei Kondo,† Issei Otsuka,‡ Yoshiharu Nishiyama,‡ Redouane Borsali,‡ Toyoji Kakuchi,† and Toshifumi Satoh*,† †

Division of Biotechnology and Macromolecular Chemistry, Graduate School of Chemical Sciences and Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan ‡ Centre de Recherche sur les Macromolécules Végétales (CERMAV, UPR-CNRS 5301), affiliated with the Grenoble Alpes University and member of the Institute de Chimie Moléculaire de Grenoble (ICMG, FR-CNRS 2607), BP53, 38041 Grenoble Cedex 9, France S Supporting Information *

ABSTRACT: The synthesis of star-shaped polylactides (PLAs) having both poly(L-lactide) (PLLA) and poly(Dlactide) (PDLA) arms in one molecule, i.e., stereo-miktoarm star-shaped PLAs, is described. The azido-functionalized PDLAs and ethynyl-functionalized PLLA possessing linear and two- and three-branched structures were prepared by the ring-opening polymerization of D-lactide and L-lactide using azido- or ethynyl-functionalized initiators. The number-average molecular weights (Mn,NMRs) of the PLAs were ca. 5000 g mol−1 with narrow molecular weight distributions (Mw/Mns) of less than 1.18. The click reaction of the azido-functionalized PDLAs and the ethynyl-functionalized PLLAs using the copper(I) bromide/N,N,N′,N″,N″-pentamethyldiethylenetriamine catalyst in a mixed solvent of dichloromethane/1,1,1,3,3,3hexafluoro-2-propanol (14/1, v/v) gave a linear stereoblock PLA as well as 3-, 4-, 5-, and 6-armed stereo-miktoarm star-shaped PLAs with Mn,NMRs of ca. 10 000 g mol−1 and Mw/Mns of less than 1.16. The wide-angle X-ray scattering and differential scanning calorimetry measurements proved that these stereo-miktoarm star-shaped PLAs formed stereocomplex crystals without any trace of homochiral crystallization.

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PLLA and PDLA homopolymers.13,16−18 When PLLA and PDLA are linked to form the stereoblock PLA (sb-PLA), the formation of the stereocomplex was further enhanced.19 After the first synthesis of sb-PLA with a relatively low molecular weight, many sb-PLAs with varying molecular weights, PLLA/ PDLA sequences, PLLA/PDLA ratios, and macromolecular architectures have been synthesized and their stereocomplexation behaviors characterized. Kimura et al. reported the synthesis of sb-PLAs with molecular weights of >100 kg mol−1 by the two-step ROP of L-lactide and D-lactide.20 In addition, the two-step ROP system was utilized to prepare high molecular weight stereo triblock PLAs.21,22 They also demonstrated the synthesis of stereo multiblock PLAs by the melt/solid-state polycondensation23−25 as well as Diels−Alder coupling26 of a mixture of PLLA and PDLA prepolymers. Tezuka et al. recently reported the synthesis of macrocyclic sbPLAs with head-to-tail and head-to-head linkages of the PLLA and PDLA blocks via the combination of ROP, click reaction, and ring-closing metathesis.27

INTRODUCTION Polylactides (PLAs) have received much attention because of their broad range of potential applications from packaging materials to biomedical materials as well as their good biocompatibility, biodegradability, and nontoxicity.1−9 To exploit its full potential, precise tuning of their physical properties, such as thermal stability and mechanical properties, is required for each application.10−12 Modification of the chemical structure of the PLAs is one of the straightforward strategies to vary their properties. Lactides, including L-lactide, D-lactide, and rac-lactide, are commonly used as the monomers for the ring-opening polymerization (ROP) leading to PLAs with the following three different stereochemistries: poly(Llactide) (PLLA), poly(D-lactide) (PDLA), and poly(raclactide). These PLAs exhibit various chemical and physical properties depending on their stereochemistry along the polymer backbone. For example, PLLA and PDLA having isotactic configurations are semicrystalline with a melting temperature (Tm) range of 170−190 °C, while the atactic poly(rac-lactide) is amorphous.13,14 Ikada et al.15 reported that a blend of PLLA and PDLA formed a stereocomplex, and the stereocomplex-type PLA was found to show improved thermal (Tm = ca. 230 °C) and mechanical properties as compared to those of the original © 2013 American Chemical Society

Received: July 2, 2013 Revised: September 16, 2013 Published: October 22, 2013 8509

dx.doi.org/10.1021/ma401375x | Macromolecules 2013, 46, 8509−8518

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Scheme 1. Synthetic Strategy for the Stereo-Miktoarm Star-Shaped PLAs and the Corresponding PLLA Counterparts

lactide (>98%) and D-lactide (>99%) were purchased from Tokyo Chemical Industry Co., Ltd. (TCI) and Musashino Chemical Laboratory, Ltd., respectively, and purified by recrystallization from dry toluene (twice). Tetra-n-butylammonium hydrogen sulfate, propargyl bromide, and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) were purchased from TCI and used as received. Copper(I) bromide (99.999%) was purchased from Sigma-Aldrich Chemicals Co. and used as received. 1,8-Diazabicyclo[5.4.0]undec-7ene (DBU) was purchased from TCI and distilled over CaH2 under reduced pressure. Propargyl alcohol (HCC−OH) was purchased from the Sigma-Aldrich Chemicals Co. and distilled over CaH2 under reduced pressure. Dry tetrahydrofuran (THF; >99.5%; water content, 99.5%; water content, 99 >99 98.2 97.6 98.8

4840 4850 5000 4910 5020 5020 4730 4800 4870

4940 4850 5040 4980 5170 5060 4950 4880 5000

1.11 1.10 1.18 1.15 1.11 1.11 1.11 1.11 1.11

−136.0 +137.8 −137.1 +138.4 −138.7 +137.7 −149.7 −145.6 −140.5

D-lactide L-lactide D-lactide L-lactide D-lactide L-lactide L-lactide L-lactide

Ar atmosphere; solvent, CH2Cl2; temperature, room temperature; [lactide]0 = 1.0 mol L−1; [lactide]0/[initiator]0/[DBU] = 33/1/0.33. bEstimated by the 1H NMR spectrum. cEstimated by SEC in CHCl3 using PSt standards. dSpecific rotation ([α]D) was measured in CHCl3 at a concentration of 1.0 g dL−1. a

time-of flight mass spectrometry (MALDI-TOF MS). As an example, the MALDI-TOF MS spectrum of N3-(PDLA)2 is shown in Figure 2. Two sets of peaks were observed in the spectrum, which have a regular interval of 144 Da for the molar mass corresponding to the lactide unit. The major set of peaks were assigned to the structures of N3-(PDLA)2, while the minor set of peaks were assigned to the structure of NH2-(PDLA)2 that were produced through the decomposition of the azido group during the MALDI-TOF MS measurement. For example, the peak at m/z = 5167.06 Da corresponded to the 34-mer of N3-(PDAL)2; (MW of D-lactide) × 34 + (MW of N3-(OH)2) + Na+ = 5167.61 Da, and the peak at 5140.99 Da corresponded to the 34-mer of NH2-(PDLA)2. Similarly, all the obtained PLAs were definitely identified to be the requisite structure based on the MALDI-TOF MS analysis (Figures S20−S27). These results based on the SEC, NMR, and MALDI-TOF MS

measurements strongly suggested that the DBU-catalyzed ROP of D-lactide and L-lactide using the azido- and ethynylfunctionalized initiators proceeded without any side reactions to afford the well-defined azido- and ethynyl-functionalized PLAs possessing the desirable degree of branching. The azido-functionalized PDLAs and the ethynyl-functionalized PLLAs were coupled by a click reaction to give the linear stereoblock and stereo-miktoarm star-shaped PLAs, PLLA-bPDLA, (PLLA)2-b-(PDLA)2, (PLLA)3-b-(PDLA)3, (PLLA)2-bPDLA, (PLLA)3-b-PDLA, and (PLLA)3-b-(PDLA)2, as shown in Scheme 3. The corresponding linear and star-shaped PLLA counterparts, PLLA-b-PLLA, (PLLA)2-b-(PLLA)2, (PLLA)3-b(PLLA)3, (PLLA)2-b-PLLA, (PLLA)3-b-PLLA, (PLLA)3-b(PLLA) 2 , PDLA-b-PDLA, (PDLA) 2 -b-(PDLA) 2 , and (PDLA)3-b-(PDLA)3, were also prepared by the click reaction of the azido- and ethynyl-functionalized PLLAs, as shown in 8512


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precipitates were insoluble in common organic solvents. We then explored a suitable reaction medium, and a 14/1 (v/v) mixture of CH2Cl2/1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was found to be suitable for the reactions without products precipitation. The progress of the click reactions was followed by checking the absorption band at ca. 2100 cm−1 due to the azido group using FT-IR spectroscopy, and the peak almost disappeared within 36 h, indicative of near-quantitative coupling yield. The crude reaction products were subjected to preparative SEC to completely remove the starting materials, which led to significant decrease in isolated yield of the products. Table 2 summarizes the molecular characteristics and isolated yields of the obtained products. The SEC traces of the products displayed unimodal peaks with the Mw/Mn values of 1.08−1.16, which clearly shifted toward the high molecular weight region as compared to both of starting polymers (Figure S28). As an example for the comparison of the SEC traces among N3-(PDLA)2, HCC-(PLLA)2, and (PLLA)2-b(PDLA)2 shown in Figure 3, an increase in the molecular weight is observed from the starting polymers to the resulting polymer, confirming that the adduct formation from N3(PDLA) 2 and HCC-(PLLA) 2 proceeded to produce (PLLA)2-b-(PDLA)2. The 1H NMR spectra of the obtained products showed a new signal at 7.5 ppm assignable to a triazole ring, which was formed through the click reaction, while the characteristic signals due to both starting polymers, such as the ethynyl proton and methylene protons adjacent to the azido group, had disappeared (Figures S29−S40). The Mn,NMR values of the products well agreed with the molecular weight summing up the Mn,NMRs of both starting polymers, confirming the PLLA/PDLA ratio of 1/1. For example, the Mn,NMR of 10 400 g mol−1 for (PLLA)2-b-(PDLA)2 matched the sum of the Mn,NMR values of HCC-(PLLA)2 (Mn,NMR = 4880 g mol−1) and N3-(PDLA)2 (Mn,NMR = 5040 g mol−1). These results based on IR, 1H NMR, and SEC analyses strongly indicated the successful formation of the targeted linear stereoblock and stereo-miktoarm star-shaped PLAs, i.e., (PLLA)x-b-(PDLA)y (x and y = 1, 2, and 3), as well as the corresponding PLLA counterparts, i.e., (PLLA)x-b-(PLLA)ys (x and y = 1, 2, and 3). General Aspect of the Solvent Cast Samples. The solvent cast samples of (PLLA)x-b-(PLLA)ys formed brittle but smooth films that could be manipulated with care. On the other hand, the (PLLA)x-b-(PDLA)ys resulted in powders and did not form films. The crystal precipitation preceded the film formation during the solvent evaporation in the case of the stereocomplex formation as was suggested by Tsuji et al.53 This powder could not be redissolved in CH2Cl2 under ambient conditions. Crystal Structure of Stereo-Miktoarm Star-Shaped PLAs. The crystal structures of the solvent cast samples of the (PLLA)x-b-(PDLA)ys as well as the (PLLA)x-b-(PLLA)ys were investigated based on their wide-angle X-ray scattering (WAXS). The WAXS profiles of the (PLLA)x-b-(PLLA)y samples are shown in Figure 4 together with the theoretical diffraction profile calculated from the published structure51 (solid line) assuming a crystal size of 20 nm perpendicular to the chain direction and 10 nm along the chain direction. The (PLLA)x-b-(PLLA)y samples showed peaks at 2θ = 15°, 17°, 19°, and 22.5°, corresponding to 011, 200, the composite of 014 and 203, and 211 of the α-form PLLA crystals, respectively. The profiles qualitatively matched the profile calculated from the latest crystal coordinates refined against the X-ray and

Figure 1. 1H NMR spectrum of N3-(PDLA)2 in CDCl3 (400 MHz). The asterisks indicate residual methanol.

Figure 2. MALDI-TOF MS spectrum, the expanded spectrum, and the assignment for N3-(PDLA)2.

Scheme 3. The click reactions between the azido- and ethynylfunctionalized PLLAs were carried out using the CuBr/ N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) catalyst system in CH2Cl2 at room temperature for 36 h, and the reactions homogeneously proceeded. On the other hand, the click reactions between the azido-functionalized PDLA and the ethynyl-functionalized PLLAs in CH2Cl2 resulted in precipitation of the products due to a stereocomplex formation, which led to a significant decrease in the product yields because the 8513


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Scheme 3. Synthesis of Linear Stereoblock and Stereo-Miktoarm Star-Shaped PLAs and the Corresponding PLLA Counterparts

Table 2. Synthesis of Linear Stereoblock and StereoMiktoarm Star-Shaped PLAs as Well as the Corresponding PLLA Counterparts sample name PLLA-b-PDLA PLLA-b-PLLA (PLLA)2-b-PDLA (PLLA)2-b-PLLA (PLLA)2-b(PDLA)2 (PLLA)2-b(PLLA)2 (PLLA)3-b-PDLA (PLLA)3-b-PLLA (PLLA)3-b(PDLA)2 (PLLA)3-b(PLLA)2 (PLLA)3-b(PDLA)3 (PLLA)3-b(PLLA)3

Mn,clcda (g mol−1)

Mn,NMRb (g mol−1)

Mw/Mnc

yieldd (%)

10 080 9 890 9 710 10 000 9 860

10 400 10 000 10 400 11 000 10 400

1.11 1.11 1.12 1.09 1.08

12.3 15.0 13.4 9.1 13.5

9 920

11 100

1.08

12.3

9 850 9 670 9 980

10 500 10 800 10 700

1.10 1.16 1.10

13.2 14.9 9.9

10 040

10 700

1.10

19.5

10 060

10 500

1.10

12.5

10 310

10 800

1.09

9.9

Figure 3. SEC traces of (PLLA)2-b-(PDLA)2, N3-(PDLA)2, and HC C-(PLLA)2.

a

Mn,clcd was the sum of Mn,NMR value of the two starting materials. Estimated by the 1H NMR spectrum. cEstimated by SEC in CHCl3 as PSt standard. dIsolated yield after the purification by preparative SEC. b

neutron fiber diffractions, except that the peak position of 200 was shifted to a higher angle, corresponding to the unit cell parameter a = 10.59 Å, compared to the value from the fiber diffraction of a = 10.683 Å at 25 °C. Fiber diffraction gives a = 10.535 Å at −150 °C. The crystallite sizes perpendicular to the chain direction estimated from the peak width were in the range between 17 and 25 nm but fluctuated among the different preparations, and we could not find a tendency with respect to the arm numbers. This is probably due to the fact that the

Figure 4. WAXS profiles of (a) PLLA-b-PLLA, (b) (PLLA)2-b-PLLA, (c) (PLLA)2-b-(PLLA)2, (d) (PLLA)3-b-PLLA, (e) (PLLA)3-b(PLLA)2, and (f) (PLLA)3-b-(PLLA)3. The sold line is the theoretical diffraction profile calculated from the published structure.51

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structure of stereocomplex crystals is not known, the relative peak intensities we measured agreed with those measured for highly oriented specimens. The scattering intensity profile was fitted using the structure factors measured by Okihara49 with the crystallite size as the only parameter and shown as the solid line in Figure 5. The crystallite size estimated by the Scherrer equation in directions perpendicular to the chain axis was in the range of 10−13 nm. There was a general tendency that the shorter arm in the (PLLA)x-b-(PDLA)ys resulted in slightly sharper peak widths, i.e., larger crystallite sizes. The formation of a crystal seems to precede complete evaporation of the solvent, and thus the crystal structure is governed more by the macromolecular architecture rather than the solvent evaporation condition, contrary to the case of the (PLLA)x-b(PLLA)ys. Thermal Properties of Stereo-Miktoarm Star-Shaped PLAs. The thermal properties of the solvent cast polymer samples were studied using differential scanning calorimetry (DSC) in a nitrogen atmosphere. Figure 6 shows the DSC thermograms of the (PLLA)x-b-(PLLA)y and (PLLA)x-b(PDLA)y samples during the first heating. Table 3 summarizes the results of the DSC measurements. The (PLLA)x-b-(PLLA)y samples showed one or two endothermic peaks at 120−140 °C corresponding to the melting temperature (Tm,hc) of the PLLA homochiral crystal. In the case of two melting peaks observed in the DSC thermogram, we assumed a lower melting peak as a real melting peak and the other peak as the melting of the crystals formed or thickened during the DSC scanning.54 The

(PLLA)x-b-(PLLA)ys formed solvent swollen gels during the solvent evaporation of the cast leading to different crystallite growths dependent on the evaporation rate. Figure 5 shows the WAXS profiles of the (PLLA)x-b(PDLA)y samples. The peaks at 2θ = 12°, 21°, and 24°

Figure 5. WAXS profiles of (a) PLLA-b-PDLA, (b) (PLLA)2-b-PDLA, (c) (PLLA)2-b-(PDLA)2, (d) (PLLA)3-b-PDLA, (e) (PLLA)3-b(PDLA)2, and (f) (PLLA)3-b-(PDLA)3. The scattering intensity profiles were fitted using the structure factors measured by Okihara49 with the crystallite size as the only parameter and shown as solid lines.

correspond to 100, the composite of 110 and −201, and 200 for the stereocomplex-type PLA crystals, respectively, using the unit cell proposed by Okihara et al.49 Although the exact

Figure 6. DSC thermograms of linear and star-shaped PLAs: (a) linear PLAs of PLLA-b-PLLA and PLLA-b-PDLA, (b) 3-arm PLAs of (PLLA)2-bPLLA and (PLLA)2-b-PDLA, (c) 4-arm PLAs of (PLLA)2-b-(PLLA)2 and (PLLA)2-b-(PDLA)2, (d) 4-arm PLAs of (PLLA)3-b-PLLA and (PLLA)3b-PDLA, (e) 5-arm PLAs of (PLLA)3-b-(PLLA)2 and (PLLA)3-b-(PDLA)2, and (f) 6-arm PLAs of (PLLA)3-b-(PLLA)3 and (PLLA)3-b-(PDLA)3. 8515


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Table 3. Thermal Properties of Linear PLAs (PLLA-b-PLLA and PLLA-b-PDLA), 3-Arm PLAs ((PLLA)2-b-PLLA and (PLLA)2b-PDLA), 4-Arm PLAs ((PLLA)3-b-PLLA, (PLLA)3-b-PDLA, (PLLA)2-b-(PLLA)2, and (PLLA)2-b-(PDLA)2), 5-Arm PLAs ((PLLA)3-b-(PLLA)2 and (PLLA)3-b-(PDLA)2), and 6-Arm PLAs ((PLLA)3-b-(PLLA)3 and (PLLA)3-b-(PDLA)3) sample name

Tm,sca (°C)

ΔHm,sca (J g−1)

Xscb (%)

sample name

Tm,hca (°C)

ΔHm,hca (J g−1)

Xhcb (%)

PLLA-b-PDLA (PLLA)2-b-PDLA (PLLA)2-b-(PDLA)2 (PLLA)3-b-PDLA (PLLA)3-b-(PDLA)2 (PLLA)3-b-(PDLA)3

218 202 201 191 189 187

67 49 60 44 36 47

47 34 42 31 26 33

PLLA-b-PLLA (PLLA)2-b-PLLA (PLLA)2-b-(PLLA)2 (PLLA)3-b-PLLA (PLLA)3-b-(PLLA)2 (PLLA)3-b-(PLLA)3

140 132 125 122 129 120

35 25 29 18 18 14

38 27 31 20 20 15

Tm,sc and Tm,hc as well as ΔHm,sc and ΔHm,hc were determined by DSC measurement during the first heating. bXsc and Xhc were calculated as Xsc% = (ΔHm,sc/142 J g−1) × 100% and Xhc% = (ΔHm,hc/93 J g−1) × 100%, respectively. a

Figure 7. Dependence of (a) Tm,sc and (b) Xsc on the arm number of the linear stereoblock and stereo-miktoarm star-shaped PLAs. The open circle (○) and filled circle (●) indicate the symmetric and asymmetric star-shaped architectures, respectively.

Tm,hcs are lower than the reported values of 170−190 °C for the linear PLLAs,13,14 which is likely due to the lower molecular weights and/or imperfect structures related to the branching points of the polymers. The (PLLA)x-b-(PDLA)y samples showed one or two endothermic peaks due to the melting temperature of the stereocomplex (Tm,sc) above 190 °C, which were apparently higher than the T m,hc values of the corresponding PLLA samples. For example, the Tm,sc of 201 °C for (PLLA)2-b-(PDLA)2 were 76 °C higher than Tm,hc of 125 °C for (PLLA)2-b-(PLLA)2, as shown in Figure 6c. The increase in the melting temperatures for the (PLLA)x-b(PDLA)y samples should be attributable to the stereocomplex formation. Importantly, no Tm,hc was observed in the DSC thermograms for the (PLLA)x-b-(PDLA)y samples, suggesting that the (PLLA)x-b-(PDLA)ys preferentially form stereocomplex crystals. These results are consistent with the WAXS results. To clarify the structure−property relationship, the DSC data for the (PLLA)x-b-(PDLA)y samples were plotted versus the arm number, as shown in Figure 7. It is apparent that the Tm,sc and the crystallinity of the stereocomplex (Xsc) for the (PLLA)x-b-(PDLA)y samples decreased with the increasing arm numbers. This can be interpreted that the increase in chain ends and branching points in the polymers leads to increase in crystalline imperfections. It is noteworthy that the stereomiktoarm star-shaped PLAs with symmetric star-shaped architectures (PLLA-b-PDLA, (PLLA) 2 -b-(PDLA) 2 , and (PLLA)3-b-(PDLA)3) exhibited higher Tm,sc and Xsc values as compared to those of the stereo-miktoarm star-shaped PLAs with asymmetric star-shaped architectures ((PLLA)2-b-PDLA, (PLLA)3-b-PDLA, and (PLLA)3-b-(PDLA)2). For example, (PLLA)2-b-(PDLA)2 showed higher Tm,sc and Xsc values

compared to those of (PLLA)3-b-PDLA though both polymers have almost the same PLLA/PDLA ratios and molecular weights with four-armed star architectures; Tm,sc = 201 °C and Xsc = 42% for (PLLA)2-b-(PDLA)2 and Tm,sc = 191 °C and Xsc = 31% for (PLLA)3-b-PDLA. This result implied that the symmetry of the star-shaped architectures have significantly affected the stereocomplex formation of the stereo-miktoarm star-shaped PLAs. In order to elucidate the effect of the stereoblock nature on the stereocomplex properties, the thermal properties of (PLLA)x-b-(PDLA)y were compared with that of relevant blend samples of (PLLA)x-b-(PLLA)y/(PDLA)x-b-(PDLA)y55 (x = y = 1, 2, and 3). The blend samples were prepared by solvent casting from 1:1 mixture of (PLLA)x-b-(PLLA)y and (PDLA)x-b-(PDLA)y in CH2Cl2. DSC thermograms of PLLAb-PDLA/PLLA-b-PDLA, (PLLA)2 -b-(PLLA) 2/(PDLA)2 -b(PDLA) 2 , and (PLLA) 3 -b-(PLLA) 3 /(PDLA) 3 -b-(PDLA) 3 blends showed a endothermic peak due to Tm,sc without a peak due to Tm,hc (Figure S53 and Table S1). The Tm,sc of the blend samples were higher than that of the Tm,hc of the corresponding PLLA or PDLA alone, indicating the stereocomplex formation in the blend samples. For example, the Tm,sc of 196 °C for the (PLLA)2-b-(PLLA)2/(PDLA)2-b-(PDLA)2 blend were 71 °C higher than Tm,hc of 125 °C for (PLLA)2-b(PLLA)2 and (PDLA)2-b-(PDLA)2 alone. Tm,SC of the (PLLA)x-b-(PLLA)y/(PDLA)x-b-(PDLA)y blends were found to decrease with the increasing arm number, and such a tendency observed for the blend samples agreed with that of (PLLA)x-b-(PDLA)y samples. More importantly, we found that Tm,sc of (PLLA)x-b-(PDLA)y samples were 5−7 °C higher than that of the corresponding (PLLA)x-b-(PLLA)y/(PDLA)x-b(PDLA)y blend samples, for example, 196 °C for (PLLA)2-b8516


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(PLLA)2/(PDLA)2-b-(PDLA)2 and 201 °C for (PLLA)2-b(PDLA)2. Thus, connecting the branched PLLA and PDLA blocks to produce (PLLA)x-b-(PDLA)y, rather than mixing (PLLA)x-b-(PLLA)y and (PDLA)x-b-(PDLA)y, results in the formation of stereocomplex with higher Tm,sc.

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CONCLUSION We have successfully demonstrated the synthesis of star-shaped PLAs having both PLLA and PDLA arms in one molecule, i.e., the stereo-miktoarm star-shaped PLAs. The preparation of the azido-functionalized PDLAs and the ethynyl-functionalized PLLAs followed by their click coupling yielded the linear stereoblock and stereo-miktoarm star-shaped PLAs, i.e., PLLAb-PDLA, (PLLA)2-b-(PDLA)2, (PLLA)3-b-(PDLA)3, (PLLA)2b-PDLA, (PLLA)3-b-PDLA, and (PLLA)3-b-(PDLA)2, having a PLLA/PDLA ratio of 1/1. WAXS and DSC studies revealed that the solvent cast samples of the linear stereoblock and stereo-miktoarm star-shaped PLAs preferentially formed a stereocomplex crystal without any homochiral crystallization. We found that the arm number in the stereo-miktoarm starshaped PLAs strongly affected their Tm and crystallinity; the increase in arm number caused a decrease in the Tm and crystallinity of the stereocomplex crystal. Moreover, the stereomiktoarm star-shaped PLAs with symmetric star-shaped architectures, PLLA-b-PDLA, (PLLA) 2 -b-(PDLA) 2 , and (PLLA)3-b-(PDLA)3, were found to exhibit a higher Tm and crystallinity than those with asymmetric star-shaped architectures, (PLLA)2-b-PDLA, (PLLA)3-b-PDLA, and (PLLA)3-b(PDLA)2. The present synthetic strategy provides a versatile way to produce stereoblock PLAs having a variety of branched architectures, giving stereocomplex-type PLA-based materials with tunable physical properties.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental procedures, 1H NMR, MALDI-TOF MS, IR, SEC, and DSC data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the MEXT (Japan) program “Strategic Molecular and Materials Chemistry through Innovative Coupling Reactions” of Hokkaido University and the MEXT Grant-in-Aid Project FY2011-2015 “Advanced Molecular Transformation by Organocatalysts”. R.B. thanks “Institut Carnot PolyNat” and Labex Arcane ANR-11-LABX0003-01 for financial support. T.I. was funded by the JSPS Fellowship for Young Scientists.

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REFERENCES

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Synthesis and Stereocomplex Formation of Star-Shaped

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