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论文类型：期刊论文

发表时间：2021-01-11

发表刊物：MACROMOLECULES

卷号：47

期号：4

页面范围：1269-1276

ISSN号：0024-9297

摘要：Selective transformation of CO2 into biodegradable polycarbonates (CO2-based copolymers) by the alternating copolymerization with epoxides represents a most promising green polymerization process. Despite the tremendous progress this field has made, most of the CO2-based polycarbonates are known to be amorphous, and their low thermal resistance makes them difficult to use as structural materials. Herein, we report the selective synthesis of highly isotactic CO2 copolymers from meso-3,5-dioxaepoxides in perfectly alternating nature by the enantiopure dinuclear Co(III)-complex-mediated desymmetrization copolymerization under mild conditions. These isotactic CO2-based polycarbonates are typical semicrystalline polymers, possessing melting points (T-m) of 179-257 degrees C, dependent on the substitute groups at 4-position of the meso-epoxides. As a model monomer of 3,5-dioxa-epoxides, 4,4-dimethyl-3,5,8-trioxabicyclo[5.1.0]octane (CXO) was studied in detail in the asymmetric copolymerization with CO2. The isotactic CO2/CXO copolymer (PCXC) with >99% enantioselectivity possesses a high T-m of 242 degrees C and a decomposition temperature of 320 degrees C, while its atactic copolymer has a high T-g of up to 140 degrees C. Moreover, the acid hydrolysis of highly isotactic PCXC was performed to provide stereoregular poly(1,2-bis(hydroxymethyl)ethylene carbonate)s (PCFC) with two hydroxyl groups in a carbonate unit, which showed a remarkable decrease of 80 degrees C in thermal decomposition temperature. This hydroxyl-functionalized CO2 copolymer accords with an unmet need for a readily degradable biocompatible polycarbonate and was further explored to prepare bush copolymers for biomedical and pharmaceutical applications. This approach was initially demonstrated by the hydroxyl groups appended in polycarbonate backbone of a hydroxyl-functionalized terpolymer serving as macroinitiators for direct graft polymerization via organocatalytic lactide ring-opening polymerization to give fully degradable brush polymers with polycarbonate backbones and polylactide side chains. Furthermore, enantiopure dinuclear Co(III)-complex-mediated asymmetric terpolymerization of CO2 with CXO and cyclohexene oxide (CHO) at various feed ratios was carried out in toluene solution, affording optically active terpolymers poly(CHC-co-CXC) with highly enantioselective ring-opening of the both meso-epoxides. These stereospecific terpolymers were found to be crystallizable and their crystallization capacity could be tuned by changing the feed ratio of the epoxides.