Copolymerization of Lactide and Cyclic Carbonate via Highly

7 mins ago - The controlled ring-opening polymerizations of enantiopure (L-LA or D-LA) or racemic lactide (rac-LA) with achiral functionalized cyclic ...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Copolymerization of Lactide and Cyclic Carbonate via Highly Stereoselective Catalysts To Modulate Copolymer Sequences Xinli Liu,†,# Xiufang Hua,†,‡,# and Dongmei Cui*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ University of Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China S Supporting Information *

ABSTRACT: The controlled ring-opening polymerizations of enantiopure (L-LA or D-LA) or racemic lactide (rac-LA) with achiral functionalized cyclic carbonates (TMCR) have been performed by using the aminobisphenolate yttrium and lutetium alkyl complexes or an organo-base catalyst DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), in combination with various alcohols as chain transfer agents. When using the highly heteroselective precatalyst 1Y, the copolymerization of rac-LA and TMCR afforded tapered copolymer poly(rac-LA-t-TMCR), while the copolymerization of enantiopure L-LA or D-LA, respectively, with TMCR gave pseudo-random product poly(L-LA-r-TMCR). When the low heteroselective rare-earth metal precatalysts or organic DBU were applied, the difference in the monomer distributions for the above two systems disappeared gradually. On the contrary, the monomer sequence distribution was not influenced by the types of chain transfer agents and TMCR. The kinetics study revealed that the competitive reaction ratios were rL‑LA = 0.58 and rTMCR1 = 0.70 for the copolymerization of L-LA and TMCR1 while rrac‑LA = 2.29 and rTMCR1 = 0.51 for the copolymerization of rac-LA and TMCR1, which were reasonably explained by the synergistic effect of high heteroselectivity of catalyst and better coordination capability of LA than other monomers.

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supported aluminum complex to accomplish the first controlled random copolymerization of CL with LA, in which the bulky ligand modified the coordination capability of both monomers to make their competitive reactivity ratios much closer (rCL = 1.09, rLA = 0.73).10 Polycarbonates that can be prepared by either ROP of corresponding cyclic carbonates or alternating ring-opening copolymerization of epoxides with anhydrides or carbon dioxide represent an important class of biodegradable polymers featuring good biocompatibility, low toxicity, and favorable thermomechanical properties.11−13 Moreover, they can be easily modified by functional groups.14−19 Carpentier and Guillaume extensively investigated the copolymerizations of cyclohexyl- or ethylene-carbonates with cyclic esters catalyzed by using the zinc diamoniphenolate, zinc βdiketiminate, tris[N,N-bis(trimethylsilyl)amide]yttrium, or 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) systems to obtain copolymers with block or random chain structures.20−23 By using syndioselective precatalyst of tetradentate phenoxyaminebased [N,N-bis(dimethylsilyl)amide]yttrium, Thomas and Coates established a smart approach for synthesizing

INTRODUCTION Over the past decades, polylactide (PLA), as a representative aliphatic polyester with intrinsic biodegradability and biocompatibility, has found growing applications such as packaging, drug delivery materials, textile fibers, etc.1 Ring-opening polymerization (ROP) of LA with other monomers like glycolide, ethylene oxide, ε-caprolactone (ε-CL), and cyclic carbonates is the most efficient method to modify the thermal, mechanical, or biomedical properties of PLA through introducing flexible units or functional groups into PLA backbones or varying the sequence lengths of PLA segments in the main chains of the copolymers.2−8 To achieve this target, researchers have focused on constructing organometallic catalysts by means of synergistic effects of Lewis acidity of the metal centers and sterics and electronics of the ligands. Duda demonstrated that altering the chiral configuration of the binaphthyl Schiff base ligands of the aluminum alkoxide initiators could change the reactivity ratios of CL and LA comonomers (rCL = 7.2, rLA = 112 for (S)-Al-OiPr vs rCL = 3.1, rLA = 4.6 for (R)-Al-OiPr) to gain a broad range of copolymers with microstructures varying from gradient to pseudo-random.9 Nomura employed a Schiff-base ligand (N,N′-bis[3(triisopropylsilyl)salicylidene]-2-dimethyl-1,3-propanediamine) © XXXX American Chemical Society

Received: December 19, 2017

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DOI: 10.1021/acs.macromol.7b02696 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Selected Initiators and Their Catalytic Performances for the Copolymerization of LA and TMCR

Table 1. Copolymerization of LA and TMCR1 with Complexes 1Y, 1Lu, 2Y, 3Y, and DBU entrya 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

cat. Y

1 1Y 1Y 1Y 1Y 1Y 1Y 1Y 1Y 1Y 1Y 1Y 1Lu 1Lu 2Y 2Y 3Y 3Y DBU DBU

LA

CTA

[LA]0:[TMCR1]0:[CTA]0:[cat.]0

Mn,NMR (kDa)b

Mn,calcd (kDa)c

LLAd

LTd

Đe

L-LA D-LA L-LA L-LA L-LA L-LA L-LA L-LA rac-LA rac-LA rac-LA rac-LA L-LA rac-LA L-LA rac-LA L-LA rac-LA L-LA rac-LA

Ph2CHOH Ph2CHOH BnOHf PEG2kg PEG2k PEG2kh PEG2ki PEG2kj Ph2CHOH BnOH PEG2k PEG2k PEG2k PEG2k PEG2k PEG2k PEG2k PEG2k PEG2k PEG2k

100/100/5/1 100/100/5/1 100/100/5/1 100/100/5/1 100/100/10/1 200/200/5/1 50/50/5/1 50/50/5/1 + 50/50 100/100/5/1 100/100/5/1 100/100/5/1 200/200/5/1 100/100/5/1 100/100/5/1 100/100/5/1 100/100/5/1 100/100/5/1 100/100/5/1 50/50/5/1 50/50/5/1

6.68 6.77 6.75 7.86 5.39 14.94 5.38 8.12 6.57 6.57 8.57 15.32 8.91 8.54 9.12 8.69 8.99 8.83 5.63 5.55

7.02 7.02 6.94 8.84 5.42 15.68 5.42 8.84 7.02 6.94 8.84 15.68 8.84 8.84 8.84 8.84 8.84 8.84 5.42 5.42

2.8 2.8 2.7 2.8 2.8 2.9 2.7 2.8 6.6 6.5 6.5 6.7 2.8 5.1 3.0 5.7 3.1 4.5 4.6 4.4

1.6 1.5 1.6 1.5 1.6 1.6 1.5 1.5 3.1 3.2 3.1 3.2 1.5 2.7 1.4 2.9 3.0 3.3 2.6 2.4

1.26 1.19 1.17 1.15 1.17 1.17 1.11 1.12 1.42 1.21 1.23 1.24 1.25 1.27 1.19 1.28 1.13 1.09 1.28 1.63

Polymerization conditions: precatalyst = 10 μmol; CTA = 50 μmol; T = 25 °C; time = 3 h; THF = 0.5 mol/L; conv = 100%.36 bExperimental molar mass value of related polymer determined by 1H NMR from the integral value ratio of the signals of end-group hydrogens to internal methane hydrogens (refer to the Experimental Section). cCalculated from the equation (molar mass of LA × [LA]0/[CTA]0 × conversion % + molar mass of TMCR × [TMCR]0/[CTA]0 × conversion % + the molar mass of CTA). dAverage LA sequence lengths (LLA) and TMCR sequence lengths (LT) in the related copolymers. eMolecular weight dispersity values (Đ) determined by SEC in THF at 30 °C using polystyrene standards. fBnOH = benzyl alcohol. gPEG2k = m-PEG (Mn = 2000 g mol−1, Đ = 1.06). hPolymerization time = 6 h. iPolymerization time = 2 h. jExtra amount of [L-LA]0 and [TMCR1]0 with 50/50 mole ratio (compared with [Cat]0) was added after 2 h. Copolymerization was quenched after another 2 h with 100% conversion. a

alternating polyesters composed of racemic β-lactones through epoxide carbonylation.29 Carpentier skillfully employed the

syndioselectivity of the o,p-dichloroalkoxyaminobis(phennolate)yttrium amido precatalyst to access the alternating B

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Figure 1. (a) 1H NMR and (b) 13C NMR spectroscopy in CDCl3: (a) poly(L-LA); (b) poly(rac-LA); (c) poly(L-LA-co-TMCR1); (d) poly(rac-LAco-TMCR1); (e) poly(TMCR1).

β-malolactonate copolymer through the copolymerization of enantio-opposite β-malolactonates with different functionality.30,31 Recently, a highly syndioselective ROP of O-carboxyanhydrides was first achieved by Wu with [nitrilotris(methylene)]tris[4,6-ditert-butylphenol]hafnium isopropoxides.32 These high syndio- (or hetero-) selective catalysts copolymerizing enantiomers showed a great potential in building alternating sequence controlled copolymers.24−28 In contrast, the copolymerization of LA with cyclic carbonates, particularly in sequence-controlled manner, has been known to be difficult and thus less explored, since the two types of monomers have rather different competitive reactivity ratios. Obviously, the copolymerization of LA with cyclic carbonates is important, anticipated to combine the advantages of both polymers. For instance, PLAs are high modulate but brittle and poor in blow molding while polycarbonates are flexible and easy in blow molding. Therefore, investigating the influence of catalytic heteroselectivity on the monomer sequence distributions in the copolymerization of LA and cyclic carbonates is a challenging but promising project. Herein, we report the efficient copolymerization of LA with functional trimethylene carbonates TMCR (R = ethynyl, ethenyl, ethyl, phenyl, or cyanophenyl) by using different heteroselective precatalysts to achieve sequence modulated copolymers with functionalized side arms and tailored chainends (Scheme 1).33−35 The effects of the heteroselectivity of the employed precatalysts on the sequence distributions of LA and TMCR were investigated via analyzing the monomer sequence lengths, measuring competive reactivity ratios. Furthermore, the resultant poly(L-LA-co-TMCR) were employed as building blocks for constructing functional materials.

[precatalyst]0:[CTA]0 = 1:5), all the copolymerizations went on smoothly to achieve full conversion in 3 h. The representative results for the copolymerizations of TMCR1 with L-LA and rac-LA are collected in Table 1, and those with other TMCR are provided in Table S1 and shown as Figures S15−S18. In particular, the copolymerization of L-LA/ TMCR1 with the catalytic system 1Y performed in the “immortal” mode, since the obtained copolymers have very close molecular weights to their theoretical values calculated according to each CTA growing one polymer chain, despite the variation of the type and the loading of CTA. Moreover, the copolymers have a Ph2CHO or BnO fragment at one end when Ph2CHOH or BnOH was used as the CTA and a HO group at the other end, the characteristic chain ends for the polymers isolated from “immortal” polymerizations (Figures S24−S26). Noted that when PEG2k was used, amphipathic copolymers were also achieved.35 The stereoregularity and the monomer sequence distribution of these copolymers were determined by 1H and 13C NMR spectroscopy in chloroform-d (CDCl3), and the assignments were based on those homopolymers. For the isotactic poly(LLA), the typical quartet at δ 5.18 ppm is assigned to the methine protons on L-LA units (Figure 1a, (a)). For the highly heterotactic poly(rac-LA) (Pr ≥ 99%), the methine protons arising from the alternating sequence of L-LA and D-LA units show two symmetrical quartets within the range of δ 5.10−5.30 ppm (Figure 1a, (b)). Poly(TMCR1) gives a doublet at δ 4.74 ppm and a quartet at δ 4.32 ppm attributed to the protons on side methylenes and on main chain methylenes, respectively (Figure 1a, (e)). Therefore, for the copolymer poly(L-LA-coTMCR1) (Table 1, entry 1), given that L represents LA and T represents TMCR unit, the multiple resonances around 5.25 are arising from the overlapped LL and TL diads while the multiple peaks centered at 5.06 ppm are assignable to LT diad (Figure 1a (c)). These assignments are in accordance with those reported in the literature.15 Whereas, the LL sequence in poly(rac-LA-co-TMCR1) (Table 1, entry 9) shows two major symmetric quartets around δ 5.25 ppm, indicating a highly heterotactic distribution of LA units along the copolymer main chains. Note that the integration intensities (I) of the two symmetric quartets are slightly different because the right quartet incorporates the TL diad (which cannot be separated by fitting these peaks, as depicted in Figure S12). Thus, the integration intensity centered at δ 5.25 (LL) and 5.06 ppm

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RESULTS AND DISCUSSION Copolymerizations of Lactide and Cyclic Carbonates. Typical copolymerizations of L-LA (or rac-LA) with TMCR (Scheme 1) were performed in THF by using bicomponent catalytic systems composed of aminobisphenolate yttrium or lutetium complexes 1Y ([(ONO-NMe2)Y{CH2SiMe3}THF]), 1Lu ([(ONO-NMe2)Lu{CH2SiMe3}THF]), 2Y ([(ONOpyridyl)Y{CH2SiMe3}THF]), 3Y ([(ONNO-)Y{CH2SiMe3}THF]), and the organo-base catalyst DBU, in combination with different alcohols as the chain transfer agents (CTA). These precatalysts were selected from our own laboratory for their different stereoselectivity toward rac-LA polymerization.34 In the presence of an excess amount of CTA (typically C

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Figure 2. Averaging sequence lengths vs different precatalysts.

Figure 3. Plots of composition vs time for the copolymerizations of (a) L-LA/TMCR1 and (b) rac-LA/TMCR1.

(LT) and eq 18,15 are feasible to calculate the sequence length of LA units (LLA) in these two kinds of copolymers. L LA =

I5.25 ppm I5.06 ppm

The averaging sequence lengths of LLA = 2.8 and LT = 1.6 for poly(L-LA-co-TMCR1) and LLA = 6.6, LT = 3.1 for poly(rac-LAco-TMCR1) were figured out. It is known that in an ideal random copolymer (r1 = r2 = 1, f1 = f 2 = 0.5) the averaging sequence lengths for both monomers are equal to 2.16 According to the above calculation results, the copolymer arising from the copolymerization of L-LA/TMCR1 with the highly heteroselective 1Y as the catalyst displays a random distribution, poly(L-LA-r-TMCR), while the copolymer isolated from the copolymerization of rac-LA/TMCR1 with the same catalyst 1Y possesses a tapered distribution (poly(rac-LAtapered-TMCR) (these sequence distributions were further proved by the comonomer reactivity ratios, vide inf ra). Using other TMCR monomers to replace TMCR1, and/or switching to complex 1Lu bearing the similar heteroselectivity (Pr = 0.99), seemed to have no effects on the sequence distribution of the resultant copolymers poly(L-LA-r-TMCR) and (poly(rac-LAtapered-TMCR). However, when employing the catalysts with low heteroselectivities, interestingly, the sequence lengths of LLA and LT changed obviously in the poly(L-LA-r-TMCR), which became longer gradually with the drop of the heteroselectivity of the applied precatalyst (Figure 2). For example, for the poly(L-LA-co-TMCR1) obtained by using complex 3 (Pr = 0.65), the averaging sequence lengths were LLA = 3.1 and LT = 3.0. When DBU (Pr = 0.50) was employed, the longer sequence lengths of LLA = 4.6 and LT = 2.6 were

+1 (1)

In 13C NMR spectra, the carbonyl carbon resonances on LA units in the isotactic poly(L-LA) show a singlet at δ 169.6 ppm (LLL), which give a doublet at δ 169.3 and 169.1 ppm (LLL) in poly(rac-LA) (Figure 1b, (a) and (b)). Referring to the above chemical shift values, the resonances at δ 169.7, 169.6, and 169.4 ppm are identified respectively to the triads of LLT, LLL, and TLL in poly(L-LA-co-TMCR1) (Figure 1b (c) and Figure S13). Correspondingly, in the poly(rac-LA-co-TMCR1), except the strong doublets at δ 169.3 and 169.1 ppm (heterotactic LLL), the small doublets of doublet within δ 169.6−169.4 ppm could be assigned to the LLT and TLL sequences (Figure 1b (d)). Also, one of the TLL resonances is covered by LLL resonances, which is observed on the fitting curves of related LLL resonances (Figure S14). Interestingly, the unambiguous diads of TT and TL at δ 154.3 and 154.0 ppm are easily assigned and applied to calculate the sequence length of TMCR1 (LT) according to eq 2.8,15 LT =

I154.3 ppm I154.0 ppm

+1 (2) D

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Macromolecules obtained, which were very close to LLA = 4.4 and LT = 2.4 in poly(rac-LA-co-TMCR1) (Table 1, entries 17−20). Therefore, the strong dependence of monomer distributions of the LA/ TMCR1 copolymers on the heteroselectivity of the precatalyst is crucial for rationalizing the sequence-controlled mechanism in our system. The variation of the composition with polymerization time during the copolymerization of L-LA/TMCR1 and rac-LA/ TMCR1 catalyzed by 1Y was investigated. The contents of both L-LA and TMCR1 segments in poly(L-LA-r-TMCR1) maintained basically equal throughout the polymerization procedure, implying that the chain growing for both L-LA and TMCR1 monomers was considerably synchronous (Figure 3a). In addition, the linear relationship of Mn vs conversion was observed as depicted in Figure S20. Contrarily, the content of LA segments in poly(rac-LA-tapered-TMCR1) reached a high level at the initial stage of polymerization, and then TMCR1 was incorporated into the copolymer and continually increased until their content levels were equal (Figure 3b). These results revealed clearly the different monomer propagation modes during the copolymerization procedures, in agreement with the monomer sequence distributions of poly(L-LA-r-TMCR1) and poly(rac-LA-tapered-TMCR1). To rationalize the different behaviors of highly stereoselective precatalyst 1Y toward the copolymerizations of enantiopure LLA with TMCR1 and racemic LA with TMCR1, their reactivity ratios were measured according to the Fineman−Ross equation (Tables S4 and S5).10 For the copolymerization of L-LA/ TMCR1, rL‑LA = 0.58 and rTMCR1 = 0.70 were obtained. Both were rather close and smaller than 1, suggesting either L-LA or TMCR1 preferred cross-polymerization rather than homopolymerization, in agreement with the random sequence distribution of poly(L-LA-r-TMCR1). For the copolymerization of racLA/TMCR1, rrac‑LA = 2.29 and rTMCR1 = 0.51 were obtained, indicating that rac-LA tended to homopolymerization rather than copolymerization with TMCR1, to generate a tapered microstructure poly(rac-LA-tapered-TMC R1 ). Obviously, switching from the L-LA to rac-LA aroused different reactivity ratios and consequently gave different monomer sequence distributions when using the highly heteroselective precatalyst (vide supra). Thus, a reasonable mechanism was deduced as depicted in Scheme 2. The active species of Y-OCH(CH3)COR is formed after one enantiopure LA monomer such as L-

LA (or D-LA) insertion, which prefers to propagate the opposite D-LA (or L-LA) monomer owing to the highly heteroselective nature of the precatalyst 1Y, generating the alternating polymer chain. In the rac-LA and TMC R1 copolymerization system, the alternating propagation of L-LA and D-LA has the overwhelming privilege than crosspropagating TMCR1 (krac‑L > kLT). As a result, the longer heterotactic LA sequences are generated at the initial stage of the copolymerization, and then TMCR1 starts to incorporate into the macromolecular chains, generating the tapered copolymer. In contrast, for the copolymerization of the single enantiopure L-LA and TMCR1, the highly heteroselective property of 1Y makes the consecutive propagation of L-LA difficult, and then TMCR is forced to play the role of D-LA, the opposite enantiomer of L-LA, to insert into the Y-OCH(CH3)COR active species. Alternatively, the generated active species Y-OCH2CCH2OCOR favors L-LA rather than TMCR to insert next (kTL ≈ kLT > kTT), since the LA monomer has higher coordinating ability than TMCR that interrupts the fast TMCR homopolymerization (kapp(TMCR1) = 0.3467; kapp(rac-LA) = 0.0482; kapp(L-LA) = 0.0096, Figure S19).36 As a result, approximate reactivity ratios of rL‑LA and rTMCR1 with the time value of 0.41 are obtained, resulting in somewhat alternating microstructured copolymer poly(L-LA-r-TMCR1).29 Thermal Characterization of Homo- and Copolymerization of LA and TMCR. The thermal properties of these copolymers were examined by differential scanning calorimetry (DSC) analysis. The glass transition temperature (Tg) and melting temperature (Tm) of related polymers (obtained from the second heating scan curve) are given in Table 2 and Figure S22. Except for the crystalline poly(L-LA) giving a high Tm value at 177.1 °C, all other samples are amorphous. Poly(L-LAr-TMCR1) does not show any crystallinity because the LA units are very short. Meanwhile, these two kinds of copolymers display a single Tg values between those of homo-PLLA and PTMCR1, which fluctuate with the polymer chain structures. As depicted in Figure S23 and collected in Table S2, the Tg value of poly(L-LA-r-TMCR1) increases with the increase of the monomer LA content, which implies the thermal properties of these materials can be further modified by an appropriate monomer feed ratio. Functionalization of Poly(L-LA-r-TMCR1). With the benefit of pendant units, these copolymers have the advantage of performing readily chemical transformation under mild conditions to incorporate diverse functional groups.12 Depicted as Scheme 3, tetraphenylethylene (TPE) groups and furan groups have been successfully grafted onto the obtained poly(L-LA-r-TMCR1) by “click” reaction of azido compound with pendant alkyne units (Table S3). A quantitative conversion of these reactions was supported by 1H NMR spectroscopy (Figures S27−S32), which shows that the proton signals of the alkynyl groups completely disappear. Furthermore, IR (infrared) absorption spectroscopy (Figure S33) also indicates that the peaks corresponding to the alkynyl groups are absent while those arising from the newly generated triazole groups can be observed. The thermal properties of these copolymers were examined by TGA (thermogravimetric analysis) and DSC analysis. The TGA curve of the TPEgrafted copolymer shows a higher Td value of 219.0 °C, similar to Td = 222.9 °C in poly(L-LA-r-TMCR1) (Figure S34). Contrarily, a lower Td1 at 90.1 °C and a higher Td2 at 234.7 °C are observed in furan-grafted copolymers. The weight loss of about 20% between Td1 and Td2 in furan-grafted copolymer was

Scheme 2. Diagram of Reactivities in Different Active Species

E

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Macromolecules Table 2. Homo- and Copolymerization of LA and TMCR1: Thermal Properties entry 1 2 3 4 5

cat. Y

1 3Y 1Y 1Y 1Y

[L-LA]0/[D-LA]0/ [TMCR1]0/[CTA]0/[cat.]0a

Mn,NMR (kDa)b

Mn,calcd (kDa)c

Đd

Tg (°C)e

Tm (°C)e

100/100/0/5/1 100/0/0/5/1 0/0/100/5/1 50/50/100/5/1 100/0/100/5/1

5.81 2.79 3.99 6.95 6.57

5.94 3.06 4.14 7.02 7.02

1.08 1.10 1.06 1.23 1.17

44.9 57.0 14.9 28.7 33.6

n.o. 177.1 n.o. n.o. n.o.

Polymerization conditions: precatalyst = 10 μmol; Ph2CHOH (as CTA) = 50 μmol; T = 25 °C; time = 3 h; THF = 0.5 mol/L; conv = 100%. Experimental molar mass value of related polymer determined by 1H NMR from the integral value ratio of the signals of end-group hydrogens to internal methane hydrogens. cCalculated from the equation (molar mass of LA × [LA]0/[CTA]0 × conversion % + molar mass of TMCR × [TMCR]0/[CTA]0 × conversion % + the molar mass of CTA). dMolecular weight dispersity values (Đ) determined by SEC in THF at 30 °C using polystyrene standards. eTg and Tm values were calculated by the STARe method with heating/cooling rate of 10 °C/min and using second cycle from 0 to 200 °C; n.o. = not observed. a b

Scheme 3. Functionalization of Poly(L-LA-r-TMCR1)

oven at 120 °C, and solvents were refluxed over a drying agent and distilled under nitrogen: THF solvent was dried over by distilling with Na/benzophenone. Monomers were purified prior to use: L-LA, DLA, and rac-LA were recrystallized from toluene three times and dried over before use. m-PEG (methoxypoly(ethylene glycol)s, Mn = 2000 g mol−1, Đ = 1.06) was purchased from Sigma-Aldrich and dried by an azeotropic distillation in toluene. Complexes 1−3 were prepared according to published procedures.34,35 TMCR monomer was synthesized according to the related papers,37,38 except for the recrystallization procedure of raw product with ethyl ether. All other reagents and solvents were commercially available and used without further purification. Instruments and Measurements. NMR spectra of complexes and polymers were performed in CDCl3 or C6D6 at 25 °C on a Bruker Avance 400 spectrometer. Molecular weight dispersities (Đ) were measured by size exclusion chromatography (SEC) on a TOSOH HLC-8220 SEC instrument (column: Super HZM-H×3) at 40 °C using THF as eluent with a flowing rate of 0.35 mL/min; the values were relative to polystyrene standards. Glass transition temperatures (Tg) and melting points (Tm) of related polymers were measured by differential scanning calorimetry (DSC) using a METTLER TOPEM DSC instrument under nitrogen flow. Any thermal history difference in the polymers was eliminated by first heating the specimen to above 200 °C, cooling at 10 °C/min to room temperature, and then recording the second DSC scan from 0 to 200 °C at 10 °C/min. TGA for polymers was done using a thermal analysis instrument (SDTQ 600, TG Instruments) from room temperature to 800 °C under a nitrogen atmosphere with a heating rate of 10 °C/min. General Catalytic Procedure for the (Co)polymerization. The following operations were performed in a golvebox. 50 μmol of CTA in THF (2 mL) together with a predetermined amount of LA (144 mg, 1 mmol) and equimolar quantities of another monomer were combined in a round-bottomed flask containing a magnetic stir bar. Then an amount of precatalyst (10 μmol) was added. After a small sample of the crude material was removed with a pipet for characterization by 1H NMR, the reaction was quenched at a certain time by acidified ethanol (0.5 mL of a 1.0 M HCl solution in EtOH). The polymers were dried under a vacuum to a constant weight.

deduced as the decomposition of furan units. TPE-grafted poly(L-LA-r-TMCR1) shows a high Tg value of 94.6 °C (Figure S35) than the pure poly(L-LA-r-TMCR1) (33.6 °C), which would be attributed mainly to the rigid TPE grafted groups, greatly restricting the movement of polymer main chains (Figure S34).

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CONCLUSION In summary, a new strategy to modulate copolymer sequences has been established by the aid of highly heteroselective catalysts matching with different monomer combinations, enantiopure LA and TMCR vs racemic LA and TMCR. In the presence of the highly stereoselective catalyst, the single enantiopure LA copolymerizes with achiral TMCR to give the random copolymer poly(L-LA-r-TMCR1); by contrast, for the rac-LA/TMCR copolymerization, the priority of propagating alternatingly the enantio-opposite LA monomers rather than cross-propagating with TMCR or TMCR homopropagating deduces the tapered copolymer poly(rac-LA-tapered-TMCR1). The sequence distribution of the resultant copolymers is influenced by the different heteroselectivities of precatalysts, but nearly not affected by the types of CTAs and the substituents on TMCR. Based on the successfully controlled sequences of LA-TMCR1 copolymers, the functional groups are grafted on the copolymer main chains in the identical arrangement to the TMCR segments; thus, TPE and furangrafted copolymers have been obtained. This strategy exhibits an advantage on expanding the stereospecific catalyst applications coupled with effective utilization of chiral/achiral monomers to modulate copolymers sequence distributions.

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EXPERIMENTAL SECTION

General Procedures. Moisture- and air-sensitive materials were manipulated under a nitrogen atmosphere using Schlenk techniques or a MBraun glovebox. Before use, glassware was dried overnight in an F

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Synthesis of Homopoly(L-LA), Poly(rac-LA), and Poly(TMCR1). The polymerization procedures were similar as depicted above. Poly(rac-LA) and poly(TMCR1) were accomplished with complex 1Ywith 100% yield; poly(L-LA) was synthesized by complex 3 with 100% yield. Poly(L-LA): 1H NMR (400 MHz, CDCl3): δ 5.19−5.13 (q, 2H, J = 8 Hz, CH(CH3)C(O)O), 1.59−1.57 (d, 6H, J = 8 Hz, CH(CH3)C(O)O)). 13C NMR (100 MHz, CDCl3): δ 169.6, 69.0, 16.6. Poly(rac-LA): 1H NMR (400 MHz, CDCl3): δ 5.25−5.19 (q, 1H, J = 8 Hz, CH(CH3)C(O)O), 5.18−5.11 (q, 1H, J = 8 Hz, CH(CH3)C(O)O), 1.57−1.54 (q, 6H, J = 4 Hz, CH(CH3)C(O)O)). 13 C NMR (100 MHz, CDCl3): δ 169.3, 169.1, 69.2, 69.0, 16.7, 16.6. Poly(TMCR1): 1H NMR (400 MHz, CDCl3): δ 4.73 (d, 2H, J = 4 Hz, CH2CCH), 4.35−4.28 (q, 4H, J = 12 Hz, CH2CCH2), 2.54 (m, 1H, CCH), 1.29 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ 171.3, 154.2, 75.6, 68.4, 52.8, 46.5, 17.3. Poly(L-LA-r-TMCR1): 1H NMR (400 MHz, CDCl3): δ 5.22−5.01 (m, 2H, CH(CH3)C(O)O), 4.72 (s, 2H, CH2CCH), 4.40−4.18 (m, 4H, CH2CCH2), 2.50 (m, 1H, CCH), 1.60−1.49 (m, 6H, CH(CH3)C(O)O), 1.29 (br, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ 171.3, 169.7, 169.6, 169.4, 154.3, 154.0, 75.5, 71.6, 69.1, 69.0, 68.9, 68.7, 68.6, 68.5, 67.9, 66.0, 65.9, 65.8, 58.4, 52.8, 46.5, 25.6, 18.4, 17.7, 16.9, 16.5. Poly(rac-LA-tapered-TMCR1): 1H NMR (400 MHz, CDCl3): δ 5.22−5.02 (m, 2H, CH(CH3)C(O)O), 4.73 (s, 2H, CH2CCH), 4.35− 4.20 (m, 4H, CH2CCH2), 2.54 (m, 1H, CH2CCH), 1.60−1.50 (m, 6H, CH(CH3)C(O)O), 1.29 (br, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ 171.3, 169.3, 169.3, 169.2, 169.1, 154.3, 154.0, 75.5, 71.6, 69.1, 69.0, 68.5, 67.9, 65.9, 52.8, 46.5, 25.6, 18.4, 17.3, 16.7, 16.6. General Procedure for the Functionalization of Poly(L-LA-rTMCR1). 2-(Azidomethyl)furan and (2-(3-(azidomethyl)phenyl)ethene-1,1,2-triyl)tribenzene were synthesized according to the literature procedures.38,39 In a 10 mL THF solution, 0.05 mmol of CuBr was reacted with 0.05 mmol of PMDETA (N,N,N′,N′,N″pentamethyldiethylenetriamine) for 4 h until the solution turned bright green. Then, copolymers (containing 0.1 mmol of alkyne group) and 0.1 mmol of azide-compound were added and stirred at 40 °C for 17 h. The product was purified by aluminum oxide chromatography using THF as eluent. TPE-grafted poly(L-LA-r-TMCR1) was obtained as a white powder with 85% yield: 1H NMR (400 MHz, CDCl3): δ 7.50 (s, 1H, triazole), 7.15−6.91 (m, 19H, TPE), 5.30 (s, 2H, CH2OC(O)), 5.21 (m, 2H, CH2CCH2), 5.15−4.95 (m, 2H, CH(CH3)C(O)O), 4.40−4.15 (m, 4H, CH2CCH2, CH2Ph), 1.60−1.48 (m, 6H, CH(CH3)C(O)O), 1.30 (br, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ 171.9, 171.8, 169.6, 169.5, 169.3, 153.9, 147.2, 143.6, 142.2, 123.7, 110.8, 110.27, 71.5, 69.1, 68.9, 68.8, 68.7, 68.5, 68.4, 65.9, 65.8, 58.5, 46.6, 46.4, 31.6, 31.1, 29.5, 17.3, 16.7, 16.6, 16.5. Furan-grafted poly(L-LA-r-TMCR1) was obtained as a pale yellow elastomer with 76% yield: 1H NMR (400 MHz, CDCl3): δ 7.67 (s, 1H, furan), 7.41 (s, 1H, triazole), 6.44 (d, 1H, J = 4 Hz, furan), 6.36 (d, 1H, J = 4 Hz, furan), 5.53 (m, 2H, CH2−furan), 5.24 (s, 2H, CH2OC(O)), 5.10−4.95 (m, 2H, CH(CH3)C(O)O), 4.35−4.17 (m, 4H, CH2OC(O)), 1.62−1.51 (m, 6H, CH(CH3)C(O)O), 1.29 (br, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ 171.8, 169.6, 153.9, 144.3, 143.2, 142.4, 141.7, 139.9, 132.4, 131.9, 131.4, 131.2, 127.7, 127.6, 127.4, 126.6, 126.5, 123.8, 71.5, 69.0, 68.9, 68.5, 65.9, 65.8, 59.5, 58.5, 53.7, 46.5, 38.1, 31.1, 29.6, 17.3, 17.2, 16.7, 16.6.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.C.). ORCID

Dongmei Cui: 0000-0001-8372-5987 Author Contributions #

X.L. and X.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank financial support from the National Natural Science Foundation of China for project nos. 21774119 and 21361140371.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02696. 1 H NMR spectra for different TMCR monomers; NMR spectroscopy of poly(L-LA-r-TMCR) and poly(rac-LAtapered-TMCR);TGA and DSC analysis of corresponding copolymers, NMR and infrared absorption spectroscopy for grafted-poly(L-LA-r-TMCR1) (PDF) G

DOI: 10.1021/acs.macromol.7b02696 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.7b02696 Macromolecules XXXX, XXX, XXX−XXX