Stereoblock-like Brush Copolymers Consisting of Poly(l-lactide) and

†Division of Biotechnology and Macromolecular Chemistry, Faculty of Engineering, and ‡Graduate School of Chemical Sciences and Engineering, Hokkai...
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Stereoblock-like Brush Copolymers Consisting of Poly(L‑lactide) and Poly(D‑lactide) Side Chains along Poly(norbornene) Backbone: Synthesis, Stereocomplex Formation, and Structure−Property Relationship Takuya Isono,† Yohei Kondo,‡ Shun Ozawa,§ Yougen Chen,† Ryosuke Sakai,§ Shin-ichiro Sato,† Kenji Tajima,† Toyoji Kakuchi,† and Toshifumi Satoh*,† †

Division of Biotechnology and Macromolecular Chemistry, Faculty of Engineering, and ‡Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-8628, Japan § Department of Materials Chemistry, Asahikawa National College of Technology, Asahikawa 071-8142, Japan S Supporting Information *

ABSTRACT: Random and block copolymerizations of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) macromonomers having an exo-norbornene group at the α- or ωchain end (D/L ratio = 1/1, Mn = ca. 5000 g mol−1) were performed via ring-opening metathesis polymerization to produce the brush random and block copolymers consisting of parallel or antiparallel aligned PLLA and PDLA side chains on a poly(norbornene) backbone. The molecular weight and polydispersity index of the brush copolymers were in the range of 40 300−458 000 g mol−1 and 1.03−1.14, respectively. Despite such high molecular weights, these brush copolymers formed a stereocomplex without homochiral crystallization. The melting temperature (Tm) and crystallinity (X) of the resulting stereocomplex varied depending on the backbone length, relative chain direction, and distribution of the PLLA/PDLA side chains. The parallel brush copolymers showed significantly higher Tm and X values than the antiparallel ones.

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INTRODUCTION

The significance of the macromolecular architecture, such as branched11−14 and cyclic shapes,15−18 in the block copolymer system has recently been recognized because of the capability of altering the self-assembly behaviors, leading to unusual physical properties that are unobtainable from the linear counterpart. Considerable efforts have been devoted to synthesizing linear diblock,7−9,19 triblock,20 and multiblock type stereoblock PLAs,10,21−23 while the synthesis of nonlinear stereoblock PLAs, such as star24,25 and cyclic shapes,26 has been a remarkably challenging task to be achieved because of the difficulty in integrating the two chemically identical PLLA and PDLA segments into one molecule. We recently reported the synthesis of 3-, 4-, 5-, and 6-armed miktoarm star-shaped stereoblock PLAs consisting of PLLA and PDLA arms by combining the living ring-opening polymerization and click reaction, which formed a stereocomplex without homochiral crystallization regardless of the arm number.27 The branched architecture strongly correlated to the Tm and crystallinity of the resultant stereocomplex, in which the arm number and symmetry of the star-shape were found to be the important

The stereocomplex type polylactide (PLA), which consists of alternating 31 helical poly(L-lactide) (PLLA) and poly(Dlactide) (PDLA) chains side-by-side in the crystallite, shows improved material properties, such as a 50 °C higher melting temperature (Tm), as compared to the pure PLLA or PDLA counterpart.1−5 Although the stereocomplex can be easily prepared from the mixture of enantiomeric PLAs, the simultaneous homochiral crystallization frequently occurred as a consequence of the phase separation between the PLLA and PDLA chains.6 The other important approach is the use of a block copolymer consisting of PLLA and PDLA segments, i.e., stereoblock PLA, in which an efficient stereocomplexation can be achieved through the enhanced interaction between the neighboring PLLA and PDLA segments.7,8 Such a property of stereoblock PLA has a critical advantage in the preparation of the stereocomplex from high molecular weight PLAs (>100 kg mol−1).9,10 However, there is less information available about the stereocomplex properties of the stereoblock PLAs as compared to those for the blend of enantiomeric PLAs. Thus, a fundamental study of the synthesis and properties of stereoblock PLAs is still required to understand how the molecular structures affect the resulting stereocomplex properties. © XXXX American Chemical Society

Received: August 10, 2014 Revised: October 2, 2014

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Scheme 1. Schematic Representation of Parallel/Antiparallel Brush Random/Block Copolymers Consisting of Poly(norbornene) Backbone Having PLLA and PDLA Side Chains

factors dictating the stereocomplex properties.27 We hypothesized that brush copolymers consisting of densely grafted PLLA and PDLA side chains, which may be termed the “stereoblocklike PLA brush copolymers”, could behave like stereoblock PLAs, allowing the effective formation of a stereocomplex in the confined space created along the backbone. Since high molecular weight brush copolymers are readily available via the “graft through” approach coupled with the ring-opening metathesis polymerization (ROMP),28−30 it would be beneficial for ensuring the mechanical strength as well as film formability. In addition, the structural diversity of the brush copolymer, such as backbone length, side chain length, and distribution of two different kinds of side chains, is attractive for the precise tuning of its physical properties. Although the synthesis of PLA brush copolymers has been extensively studied,31−37 there have been only a few reports on the stereocomplex formation in the brush copolymer system.38−41 For example, Grubbs et al. recently reported the blend of PLLA brush copolymer with a linear PDLA formed stereocomplex without homochiral crystallization, while the stereocomplex formation in the blend of the enantiomeric PLA brush copolymers was highly restricted.41 However, a detailed investigation into the precise synthesis and characterization of brush copolymers containing both the PLLA and PDLA side chains still remains challenging. The other interesting structural factor that influences the PLA stereocomplex formation is the relative direction of the PLLA and PDLA chains within the stereocomplex crystallite, i.e., a parallel or antiparallel orientation.42 Okihara et al.43 proposed that the PLLA and PDLA helices are ordered in the parallel orientation in the stereocomplex crystallite, while Brizzolara et al.44 pointed out that both parallel and antiparallel orientations are possible because of the similar interaction energy and packing structure. In terms of improving the material properties of the stereocomplex type PLAs, the antiparallel orientation should be excluded from the crystallite. Indeed, Tezuka et al. realized the preferred parallel orientation of the PLLA and PDLA chain in orientationally defined macrocyclic stereoblock PLAs, which revealed that the parallel orientation in the stereocomplex crystallite gives rise to a higher Tm value than for the antiparallel one.26 Further detailed investigations into the directional effect on the stereocomplex

properties would provide new insight into the molecular design strategy for high performance PLA materials. The brush copolymers containing both the PLLA and PDLA side chains are the ideal system for examining the directional effect. The brush copolymers obtained by the copolymerization of the PLLA and PDLA macromonomers having a polymerizable group at the α-chain end should form a stereocomplex with a parallel orientation of the PLLA/PDLA side chains, resulting in eliminating any undesirable antiparallel orientation from the crystallite. Meanwhile, the brush copolymers from PLLA with a polymerizable group at the α-chain end and PDLA with a polymerizable group at the ω-chain end should form a stereocomplex with an antiparallel orientation. In order to understand the relationship between the molecular structures and stereocomplex properties of the stereoblock-like PLA brush copolymers, we now report the efficient synthesis of a series of brush random/block copolymers consisting of a poly(norbornene) backbone having parallel/antiparallel PLLA and PDLA side chains. The stereocomplex properties of the brush random and block copolymers as well as the corresponding blended mixture of enantiomeric brush homopolymers were characterized by powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) measurements to clarify the effect of the various structural factors, i.e., chain growth direction (backbone to outside or outside to backbone), relative chain direction (parallel or antiparallel), distribution of PLLA/PDLA side chains (random or block), and the backbone length, on the resulting stereocomplex properties (Scheme 1).

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EXPERIMENTAL SECTION

Materials. Grubbs third-generation catalyst (G3)45 and (±)-exo-5norbornene-2-methanol (1a)46 were prepared according to reported methods. N,N-Dimethyl-4-aminopyridine (DMAP; Wako Pure Chemical Industries, Ltd., >99.0%), 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC; Wako Pure Chemical Industries, Ltd., >99.0%), and (±)-exo-5-norbornenecarboxylic acid (1c; Aldrich, 97%) were used as received. L-Lactide (Musashino Chemical Laboratory, Ltd., >99%) and D-lactide (Musashino Chemical Laboratory, Ltd., >99%) were purified twice by recrystallization using dry toluene and stored in the glovebox. 1,8-Diazabicyclo[5.4.0]undec7-ene (DBU; Tokyo Chemical Industry Co., Ltd. (TCI), >98%), ethyl B

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Scheme 2. Synthesis of PLA Brush Copolymers by the Living Ring-Opening Polymerization and Ring-Opening Metathesis Polymerization

vinyl ether (TCI, >98%), and n-butanol (1b; Kanto Chemical Co., Inc., >99%) were purified by distillation over CaH2 under reduced pressure and stored in the glovebox. Instruments. All the polymerization experiments were carried out in an MBRAUN stainless steel glovebox equipped with a gas purification system (molecular sieves and copper catalyst) in a dry argon atmosphere (H2O, O2 99%. bDetermined by SEC with RI detector in CHCl3 as polystyrene standard. cDetermined by SEC-MALS in CHCl3.

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Table 3. Random Copolymerizations of 2L, 2D, 4L, and 4D via ROMP Using G3 as the Initiatora label

MML

MMD

[MML]0/[MMD]0/[G3]0

Mn,SECb (g mol−1)

Mw/Mnb

Mw,MALSc (g mol−1)

yield (%)

poly(2L-r-2D)48k poly(2L-r-2D)147k poly(2L-r-2D)262k poly(2L-r-4D)74k poly(2L-r-4D)166k poly(2L-r-4D)335k poly(4L-r-4D)40k poly(4L-r-4D)152k poly(4L-r-4D)458k

2L 2L 2L 2L 2L 2L 4L 4L 4L

2D 2D 2D 4D 4D 4D 4D 4D 4D

3/3/1 12.5/12.5/1 25/25/1 3/3/1 8.9/8.9/1 25/25/1 3/3/1 8.3/8.3/1 8.9/8.9/1

59 200 87 100 145 000 60 300 107 000 150 000 46 600 82 900 189 000

1.05 1.03 1.04 1.05 1.05 1.05 1.09 1.06 1.14

48 200 147 000 262 000 73 500 166 000 335 000 40 300 152 000 458 000

93.2 91.4 88.9 70.5 87.7 78.0 77.2 89.2 87.0

Polymerization condition: Ar atmosphere; solvent, CH2Cl2; temperature, room temperature; [MM]0 = 10 mmol L−1; conv, >99%. bDetermined by SEC with RI detector in CHCl3 as polystyrene standard. cDetermined by SEC with MALS detector in CHCl3.

a

copolymer showed a monomodal molecular weight distribution with the Mw/Mn value less than 1.14. Finally, the two types of brush block copolymers (see Scheme 1), i.e., the block copolymers prepared from the pair of 2L and 2D ((poly(2L-b-2D); parallel orientation) and 2L and 4D (poly(2L-b-4D); antiparallel orientation), were prepared for comparison with the random copolymer counterparts. The synthetic results are listed in Table 4. The ROMP of 2L using G3 was carried out with the perfect macromonomer conversion at the [2L]0/[G3]0 ratios of 3/1, 12.5/1, and 25/1. The subsequent addition of 3, 12.5, and 25 equiv of 2D allowed a further chain extension from the poly(2L)s generated by the first polymerization, which was confirmed by SEC measurement as shown in Figure 4. Every block copolymerization showed a complete macromonomer conversion, and the obtained poly(2L-b-2D)s and a poly(2L-b-4D) exhibited a monomodal molecular weight distribution with Mw,MALS and Mw/Mn values of 48 400−364 000 g mol−1 and 1.04−1.09, respectively. Stereocomplex Properties of Stereoblock-like PLA Brush Copolymers. We initially examined the stereocomplex formation of the brush random copolymers, i.e., poly(2L-r-2D)s, poly(2L-r-4D)s, and poly(4L-r-4D)s, on the basis of powder Xray diffraction (XRD) measurements. Stereocomplex crystals were prepared by solvent casting from the polymer solution in CH2Cl2 (concentration = 10 g L−1). The XRD profiles for the solvent cast samples of every brush random copolymer clearly showed three diffraction peaks at 2θ = 12°, 21°, and 24° due to the stereocomplexed PLA crystal2 (Figure 5). Surprisingly, no diffraction peak due to the homochiral crystal was observed in the XRD profiles, suggesting that the brush random copolymers rapidly formed the stereocomplex crystals during the solvent evaporation without homochiral crystallization despite their high molecular weights. Similar XRD profiles were also observed for the brush block copolymer specimens, i.e., solvent cast samples of poly(2L-b-2D)s and poly(2L-b-4D) from CH2Cl2 solution, irrespective of the molecular weight, demonstrating the predominant stereocomplex formation in the brush block copolymers as well as the brush random copolymers. The crystallite size estimated by Scherrer equation (Scherrer constant = 1.0) for the (110) diffraction peak was in the range of 10.2−14.1 nm (see Table S1), which decreased with the increasing Mw,MALS. The blend samples of the corresponding brush homopolymers, i.e., poly(2L)/poly(2D) and poly(2L)/poly(4D) blends prepared by solvent casting from a CH2Cl2 solution, showed two sets of diffraction peaks corresponding to the homochiral crystal and stereocomplex crystal (Figure 5e,f). There was a

used, respectively, for the ROMP of 2L and 2D with the macromonomer concentration of 10 mmol L−1. As an example, the SEC traces of poly(2L)s prepared using different macromonomer-to-initiator ratios are depicted in Figure 2 together with the corresponding macromonomer 2L, which clearly showed the absence of any residual macromonomer. The 1H NMR spectra of the products also confirmed the absence of a signal due to the vinyl proton of the norbornene end group at around 6.1 ppm (Figures S3−S9). The absolute weight-average molecular weight (Mw,MALS) of the obtained poly(2L), poly(2D), and poly(4D), which was determined by SEC equipped with a multiangle light scattering (MALS) detector (SEC-MALS), was found to be in the range of 45 000−330 000 g mol−1 (Table 2). The molecular weight distribution of every brush homopolymer was monomodal, and their Mw/Mn values were in the range of 1.03−1.14. A similar procedure was used for the ROMP of the 1:1 mixture of the L- and D-form macromonomers (MML and MMD, respectively) to produce brush random copolymers with three different PLLA/PDLA chain directions (see Scheme 1), i.e., the random copolymers from the pair of 2L and 2D (poly(2L-r-2D); parallel direction (A)), 2L and 4D (poly(2L-r4D); antiparallel direction), and 4L and 4D (poly(4L-r-4D); parallel direction (B)). Each brush random copolymer was synthesized by varying the macromonomer-to-initiator ratio to provide three different molecular weight series (Table 3). The complete macromonomer conversion was observed for all the random copolymerizations by SEC measurement, ensuring the ca. 1:1 incorporation of the MML and MMD into the final brush copolymers. As an example, the SEC trace of poly(2L-r-2D)s and the corresponding macromonomers are depicted in Figure 3. The Mw,MALS values of the random copolymers were in the range of 47 100−331 000 g mol−1, and every brush random

Figure 3. SEC traces of poly(2L-r-2D)s and the corresponding macromonomers, 2L and 2D (eluent, CHCl3; flow rate, 1.0 mL min−1; detector, RI). F

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Table 4. Block Copolymerization of 2L, 2D, and 4D via ROMP Using G3 as the Initiatora label

MML

MMD

[MML]0/[MMD]0/[G3]0

Mn,SECb (g mol−1)

Mw/Mnb

Mw,MALSc (g mol−1)

yield (%)

poly(2L-b-2D)48k poly(2L-b-2D)188k poly(2L-b-2D)364k poly(2L-b-4D)285k

2L 2L 2L 2L

2D 2D 2D 4D

3/3/1 12.5/12.5/1 25/25/1 25/25/1

57 800 126 000 166 000 152 000

1.06 1.04 1.05 1.09

48 400 188 000 364 000 285 000

97.7 76.7 92.4 91.6

Polymerization condition: Ar atmosphere; solvent, CH2Cl2; temperature, room temperature; [MM]0 = 10 mmol L−1; conv, >99%. bDetermined by SEC with RI detector in CHCl3 as polystyrene standard. cDetermined by SEC with MALS detector in CHCl3.

a

blend of enantiomeric brush homopolymers, based on a thermal analysis.41 A comparison between the brush copolymers and corresponding enantiomeric blends indicated that the enhanced stereocomplex formation in the stereoblock-like PLA brush copolymers was due to the intramolecular interaction between the neighboring PLLA and PDLA side chains along the poly(norbornene) backbone. Therefore, the brush copolymers containing the PLLA and PDLA side chains had a truly stereoblock nature. Assuming an unfavorable stereocomplex formation in the blended brush homopolymers, the stereocomplexes formed in brush random and block copolymers should predominantly consist of an intramolecular interaction of the PLLA and PDLA side chains. The melting temperature and crystallinity of the solvent cast samples were then evaluated using differential scanning calorimetry (DSC). Table 5 summarizes the DSC results for poly(2L-r-2D)s, poly(2L-r-4D)s, poly(4L-r-4D)s, poly(2L-b-2D)s, and poly(2L-b-4D). As shown in Figure 6a−c, the DSC trace of every type of brush random copolymers showed only one endothermic peak at 198−211 °C corresponding to the melting temperature of the stereocomplex (Tm,sc), which was apparently higher than the melting temperature of the homochiral crystal (Tm,hc) for the corresponding brush homopolymers (Tm,hc = 135−139 °C, see Table S2 and Figure S10) as well as the

Figure 4. SEC traces of poly(2L-b-2D)s and the corresponding macromonomers, 2L and 2D (eluent, CHCl3; flow rate, 1.0 mL min−1; detector, RI). The dashed lines indicate the SEC traces for the poly(2L)s obtained by the first polymerization.

general tendency that the higher molecular weight of the blended brush homopolymers resulted in a decreased population of the stereocomplex crystal relative to the homochiral one. This result reflects that the increased degree of polymerization of the backbone caused the steric repulsion between the side chains, resulting in the predominant homochiral crystallization as a consequence of the reduced intermolecular entanglement. A previous report by Grubbs et al. also revealed a predominant homochiral crystallization in the

Figure 5. Powder X-ray diffraction profiles of (a) poly(2L-r-2D)s, (b) poly(2L-r-4D)s, (c) poly(4L-r-4D)s, (d) poly(2L-b-2D)s and poly(2L-b-4D), (e) blend of poly(2L)/poly(2D), and (f) blend of poly(2L)/poly(4D). The blue and red lines indicate the diffraction peaks corresponding to stereocomplex crystal and homochiral crystal, respectively. G

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to be 27−45%. The Tm,sc and Xsc values of the brush random copolymers were slightly lower than the reported values for a linear stereoblock PLA (Mn of PLLA/PDLA blocks = ca. 5000 g mol−1; Tm,sc = 218 °C; Xsc = 47%27). Note that Grubbs et al.41 recently reported only one example of a brush random copolymer with the Mn value of 487 kg mol−1 (Mn of side chains = ca. 6000 g mol−1), whose Tm,sc and ΔHsc values were determined to be 175.8 °C and 12 J g−1, respectively, which were quite lower than those of the present brush random copolymer system. To gain insight into the effect of the parallel and antiparallel directions of the PLLA/PDLA chains on the stereocomplex properties, the Tm,sc, Xsc, and half-peak width of the melting transition for the brush random copolymers were plotted versus the Mw,MALS (Figure 7). The Tm,sc values for the random copolymers with the parallel PLLA/PDLA direction, i.e., poly(2L-r-2D) and poly(4L-r-4D) series, were in the range of 208−211 °C, which were higher than for the random copolymers with the antiparallel PLLA/PDLA direction, i.e., poly(2L-r-4D) series (Tm,sc = 198−205 °C). Similarly, higher Xsc values for poly(2L-r-2D) and poly(4L-r-4D) (Xsc = 36−45%) than for poly(2L-r-4D) (Xsc = 27−29%) were observed. We found that the Tm,sc, Xsc, and half-peak width for the poly(4L-r4D) series were very close to those of the poly(2L-r-2D) series, which indicates that the chain growth direction of the PLLA/ PDLA side chains had little impact on the stereocomplex formation. It should be noted that the Tm,sc values of the poly(2L-r-2D) and poly(4L-r-4D) series are independent of the Mw,MALS values. This observation implies that poly(2L-r-2D) and poly(4L-r-4D) formed stable stereocomplex crystallites by the intramolecular interaction between the neighboring PLLA and PDLA chains with a parallel direction. On the other hand, the Tm,sc value of the poly(2L-r-4D) series apparently decreased with the increasing Mw,MALS (Figure 7a,b). In addition, the half-peak width of the melting peaks for the poly(2L-r-4D) series (7.4−

Table 5. Thermal Properties of the Solvent Cast Samples of Poly(2L-r-2D)s, Poly(2L-r-4D)s, Poly(4L-r-4D)s, Poly(2L-b2D)s, and Poly(2L-b-4D) sample

Tm,sca (°C)

ΔHsca (J g−1)

Xscb (%)

half-peak widtha (°C)

brush random copolymer/parallel direction (A) 210 57 40 poly(2L-r-2D)48K poly(2L-r-2D)147K 209 58 41 poly(2L-r-2D)262K 210 57 40 brush random copolymer/antiparallel direction poly(2L-r-4D)74K 205 39 27 poly(2L-r-4D)166K 202 40 28 poly(2L-r-4D)335K 198 42 29 brush random copolymer/parallel direction (B) poly(4L-r-4D)40K 211 64 45 poly(4L-r-4D)152K 209 64 45 poly(4L-r-4D)458K 208 52 36 brush block copolymer/parallel direction poly(2L-b-2D)48K 211 57 40 poly(2L-b-2D)188K 210 57 40 poly(2L-b-2D)364K 207 43 30 brush block copolymer/antiparallel direction poly(2L-b-4D)285K 202 38 27

6.9 6.4 5.9 7.4 9.8 14.7 6.9 6.9 7.8 5.4 6.5 6.4 11.8

Tm,sc, ΔHsc, and half-peak width were determined by DSC measurement during first heating (heating rate, 10 °C min−1). bXsc was calculated as Xsc% = (ΔHhc/142 J g−1) × 100%. a

corresponding macromonomers (Tm,hc = 136−139 °C, see Table S2 and Figure S10). This observation indicated the exclusive stereocomplex formation without homochiral crystallization, which is in agreement with the XRD results. The enthalpy of melting (ΔHsc) for the stereocomplex of the brush random copolymers was in the range of 39−64 J g−1, from which the crystallinity of the stereocomplex (Xsc) was calculated

Figure 6. DSC traces for the solvent cast samples of (a) poly(2L-r-2D)s, (b) poly(2L-r-4D)s, (c) poly(4L-r-4D)s, (d) poly(2L-b-2D)s, and poly(2L-b4D) during first heating under a nitrogen atmosphere (heating rate, 10 °C min−1). H

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Figure 7. Dependence of (a) Tm,sc, (b) Xsc, and (c) half-melting-peak width for poly(2L-r-2D)s (blue open circle), poly(2L-r-4D)s (blue closed circle), poly(4L-r-4D)s (blue open triangle), poly(2L-b-2D)s (red open square), and poly(2L-b-4D) (red closed square) on the weight-average molecular weight determined by multiangle light scattering (Mw,MALS).

Figure 8. Schematic representation of parallel and antiparallel orientations in the stereocomplex crystallite formed from poly(2L-r-2D)s, poly(2L-r4D)s, poly(4L-r-4D)s, and blend of enantiomeric brush homopolymers.

14.7 °C) were significantly broader than those of poly(2L-r-2D) and poly(4L-r-4D) (5.9−7.8 °C), indicating the lower crystal uniformity of poly(2L-r-4D). Since the half-peak width becomes broader with the increasing Mw,MALS, the molecular weight of poly(2L-r-4D) also contributes to decreasing the crystal uniformity (Figure 7c). This information clearly demonstrated that the parallel direction of the PLLA/PDLA chains in the brush random copolymers has a critical importance for improving the Tm,sc and Xsc values and crystal uniformity of the resulting stereocomplex. The DSC traces of the brush block copolymers, i.e., poly(2Lb-2D) and poly(2L-b-4D), are depicted in Figure 6d. All the brush block copolymers also showed only one endothermic peak due to the melting transition of the stereocomplex crystal. The Tm,sc and Xsc values of poly(2L-b-2D)s were in the range of 207−211 °C and 30−40%, respectively (Table 5). On the basis of the DSC result for a poly(2L-b-4D)285K, we found that the antiparallel PLLA/PDLA chain direction contributed to decreased Tm,sc and Xsc values even in the brush block copolymer system. Indeed, the Tm,sc and Xsc values for poly(2Lb-4D)285K were determined to be 202 °C and 27%, respectively, which were apparently lower than those of the poly(2L-b-2D)s and were very close to those of the poly(2L-r-4D)s (Figure 7). In addition, the half-peak width for the melting transition of poly(2L-b-4D)285k (11.8 °C) was broader than for poly(2L-b2D)364k (6.4 °C). This again confirms the impact of the parallel

direction of the PLLA/PDLA chains on the stereocomplex formation. Finally, we elucidated the thermal properties of the enantiomeric blend of the brush homopolymers, i.e., poly(2L)/poly(2D) and poly(2L)/poly(4D), to clarify the advantage of the stereoblock-like brush copolymers. Figure S11 shows the DSC traces of the blended samples during first heating, and Table S3 summarizes the Tm and X values. The DSC traces of every blended sample exhibited two endothermic melting peaks, in which lower (138−144 °C) and higher transition peaks (209−213 °C) correspond to the melting temperature of the homochiral crystal (Tm,hc) and Tm,sc, respectively. Except for the poly(2L)56k/poly(2D)47K case, the melting endothermic peak due to the homochiral crystal is more prominent than for the stereocomplex, indicating the predominant homochiral crystallization in the blend system. The ΔHsc value for the stereocomplex portion was dependent on the molecular weight, which decreased with the increasing Mw,MALS. This result is consistent with the XRD analysis. Thus, we confirmed that the stereocomplexation in the enantiomeric blend of the brush homopolymers is strongly disturbed by the suppressed intermolecular interaction. Structure−Property Relationship. The results presented here suggested that various structural factors of the stereoblocklike PLA brush copolymers affected the resulting stereocomplex properties, including the Tm,sc, Xsc, and crystalline uniformity. The most remarkable structural factor was the relative direction I

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between the neighboring PLLA and PDLA chains along the poly(norbornene) backbone. More importantly, the brush random and block copolymers with a parallel PLLA/PDLA chain direction showed a higher Tm and crystallinity as well as a higher crystal uniformity than those of the corresponding antiparallel one, leading to the conclusion that the resulting stereocomplex properties can be improved by controlling the orientation of the PLLA and PDLA chains in the stereocomplex crystallite. However, the sterecomplex properties were only slightly influenced by the backbone length and distribution of the PLLA/PDLA side chains. Further investigation into the isothermal crystallization behaviors as well as mechanical properties and biodegradability should be necessary to fully understand the effect of the structural factors of the stereoblock-like PLA brush copolymers on their chemical and physical properties. With the simple preparation method and a wide array of molecular design diversities, the presented stereoblock-like PLA brush copolymer system is expected to be of interest for the design of high performance PLA-based materials.

of the PLLA/PDLA side chains, i.e., parallel or antiparallel direction. The parallel brush copolymers, i.e., the poly(2L-r-2D), poly(4L-r-4D), and poly(2L-b-2D) series, exhibited higher Tm,sc and Xsc values than for the antiparallel ones, i.e., poly(2L-r-4D) and poly(2L-b-4D) series, demonstrating that the PLLA/PDLA side chains aligned parallel on the poly(norbornene) backbone facilitated the formation of the thermodynamically stable stereocomplex crystallites predominantly consisting of a parallel PLLA/PDLA chain orientation, as illustrated in Figure 8. It is worth noting that the Tm,sc values for the stereocomplex portion of the poly(2L)/poly(4D) blended samples (Tm,sc = 211−213 °C) were slightly higher than that for poly(2L)/poly(2D) blend samples (Tm,sc = 209−210 °C). This difference should also be attributed to the intermolecular parallel PLLA/PDLA chain orientation in the stereocomplex crystallite of poly(2L)/ poly(4D). When the PLLA and PDLA side chains of poly(2L) and poly(4D) interact with each other, they form a stereocomplex crystal with an intermolecular parallel PLLA/PDLA orientation, as shown in Figure 8, which might allow for forming a stable stereocomplex crystallite with a higher Tm,sc. Based on our investigation, the stereocomplex properties of the parallel brush random copolymers, i.e., poly(2L-r-2D) and poly(4L-r-4D) series, were insensitive to the backbone length (degree of polymerization of backbone (DPB) was calculated to be ca. 8−92 on the basis of the Mn,MALS value of the brush polymer and Mn,NMR value of the macromonomer). However, there was a significant negative effect of the backbone length on the Tm,sc, Xsc, and crystalline uniformity of the antiparallel random copolymers, i.e., poly(2L-r-4D) series. The difference in the backbone length dependence is possibly due to the slower crystallization kinetics for the poly(2L-r-4D) series than for the poly(2L-r-2D) and poly(4L-r-4D) series.53−55 We initially expected that the distribution (random or block sequence) of the PLLA/PDLA side chains should affect their stereocomplex properties due to the significant difference in the relative distance between the PLLA and PDLA side chains. However, the comparison between the parallel brush random and block copolymers, i.e., poly(2L-r-2D) (DPB = ca. 12−52) and poly(2L-b-2D) (DPB = ca. 10−73) series, revealed little effect on their stereocomplex properties in this molecular weight range.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, MALDI-TOF MS, SEC, 1H NMR, XRD, 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 Grant-inAid for Scientific Research on Innovative Areas “Advanced Molecular Transformation by Organocatalysts”, MEXT Grantin-Aid for Scientific Research (B) (25288093), Grant-in-Aid for JSPS Fellows (12J02090), and Grant-in-Aid for Regional R&D Proposal-Based Program from Northern Advancement Center for Science & Technology of Hokkaido Japan.

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CONCLUSION We have successfully synthesized a series of brush random and block copolymers bearing parallel or antiparallel PLLA and PDLA side chains along the poly(norbornene) backbone via the Ru-catalyzed ROMP of well-defined PLA macromonomers. Grubbs third generation Ru catalyst enabled the controlled/ living ROMP of the macromonomers with a quantitative conversion, allowing for obtaining structurally and compositionally well-defined brush copolymers with different molecular weights, i.e., poly(2L-r-2D), poly(2L-r-4D), poly(4L-r-4D), poly(2L-b-2D), and poly(2L-b-4D). On the basis of the XRD and DSC studies for the solvent cast samples of brush random and block copolymers, we found that the brush copolymers rapidly formed a stereocomplex without homochiral crystallization during the solvent evaporation. On the contrary, blended mixtures of enantiomeric brush homopolymers predominantly formed a homochiral crystal, implying that the intermolecular interaction between the brush homopolymers was an unfavorable process. This observation let us to conclude that the enhanced stereocomplex formation in the brush copolymers originated from the preferred intramolecular interaction

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REFERENCES

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