The desymmetrization copolymerization of meso-epoxides with CO2 using chiral catalysts or reagents is regarded as a valuable strategy for the synthesis of optically active polycarbonates with main-chain chirality. The present study demonstrates that the biphenol-linked dinuclear Co(III) complexes show unprecedented activity, enantioselectivity, and broad substrate scope for coupling CO2 with various meso-epoxides to afford the corresponding isotactic polycarbonates. This investigation also focuses on the mechanistic understanding on the origin of enantioselectivity and highly catalytic activity in the enantiopure dinuclear Co(III) complex mediated copolymerization process, using cyclohexene oxide as a model monomer of meso-epoxides. The kinetic study by in situ infrared spectroscopy revealed a first-order dependence on the catalyst concentration in the systems of biphenol-linked dinuclear Co(III) complex alone or in the presence of an ionic cocatalyst. An intramolecular bimetallic cooperation mechanism was proposed to be predominantly responsible for the copolymerization process, wherein alternating chain growth and dissociation take turns between two Co(III) ions from the inside cleft of the catalyst by the nucleophilic attack of the growing carboxylate species at one metal center toward the activated epoxide at the other. Density functional theory calculations suggested that in the two diastereoisomers of the biphenol-linked dinuclear Co(III) complexes the matched configuration was (S,S,S,S,S)- rather than (S,S,R,S,S)-conformer for CO2/meso-epoxide copolymerization to give the corresponding polycarbonate with S,S-configuration. The addition of an ionic cocatalyst with bulky cation significantly improves both the catalytic activity and enantioselectivity, while the presence of a coordination Lewis base caused dramatically a change in the chiral induction orientation.
Selective transformation of CO 2 into biodegradable polycarbonates (CO 2 -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 CO 2 -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 CO 2 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 CO 2 -based polycarbonates are typical semicrystalline polymers, possessing melting points (T m ) of 179−257 °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 CO 2 . The isotactic CO 2 /CXO copolymer (PCXC) with >99% enantioselectivity possesses a high T m of 242 °C and a decomposition temperature of 320 °C, while its atactic copolymer has a high T g of up to 140 °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 °C in thermal decomposition temperature. This hydroxyl-functionalized CO 2 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 ringopening polymerization to give fully degradable brush polymers with polycarbonate backbones and polylactide side chains. Furthermore, enantiopure dinuclear Co(III)-complex-mediated asymmetric terpolymerization of CO 2 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.
The development of efficient processes for CO 2 transformation into useful products is a long-standing goal for chemists, since CO 2 is an abundant, inexpensive and non-toxic renewable C1 resource. Here we describe the enantioselective copolymerization of 3,4-epoxytetrahydrofuran with CO 2 mediated by biphenol-linked dinuclear cobalt complex, affording the corresponding polycarbonate with 499% carbonate linkages and excellent enantioselectivity (up to 99% enantiomeric excess). Notably, the resultant isotactic polycarbonate is a typical semicrystalline polymer, possessing a melting point of 271°C. Furthermore, the enantioselective terpolymerization of 3,4-epoxytetrahydrofuran, cyclopentene oxide and CO 2 mediated by this dinuclear cobalt complex gives novel gradient polycarbonates, in which the decrement of one component and the increment of the other component occur sequentially from one chain end to the other end. The resultant terpolymers show perfectly isotactic structure and have unique crystalline-gradient nature, in which the crystallinity continuously varies along the main chain.
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