Enantioselective Copolymerization Of Mew-epoxides With C02： Catalyst Design And Mechanism Study

The desymmetrization ring-opening of meso-epoxides via the enantioselective addition of nucleophiles can simultaneously construct two contiguous stereogenic centers, which has been found wide applications in total synthesis of natural products and the preparation of chiral drugs. When such a powerful synthetic strategy is applied to the alternating copolymerization of CO2with meso-epoxides, optically active with chiral units in the main chain polycarbonates (CO2polymers) can be produced. Although much progress has made in this field, the enantioselectivity of the copolymerization was unsatisfactory, moreover, the scope of epoxides used is very narrow. In terms of the above issue, we made an effort to design the highly active and enantioselective catalyst system for this transformation, and develop the crystalline and functional CO2-based copolymer.1) The dinuclear Co(III)-Salen complexes based on biphenyl linkers with two chiral diaminocyclohexane backbones were demonstrated to be beneficial for the desymmetrization copolymerization CO2with meso-epoxides, such as cyclopentene oxide (CPO), cyclohexene oxide (CHO), cis-2,3-epoxybutane (CDO),2,3-epoxy-1,2,3,4-tetrahydronaphthalene (CTO) in conjunction with PPNX (PPN=bis(triphenylphosphine)iminium, X=2,4-dinitrophenoxide). The effects of the substituents on the phenolate ortho positions, reaction temperature, CO2pressure and solvents were investigated. From the DSC study, the CO2polymers from CPO and CDO are amorphous with a Tg of85and71℃, respectively. However, for the CO2polymers from CHO, a quite sharp and high melting endothermic peak at272℃with△Hm=25J/g and a crystallizing exothermic peak at234℃with△Hc=-22J/g are found, which is confirmed that it is crystalline material. It is worthwhile noting here parenthetically that the CO2ploymers from CTO owns up to150℃Tg, which is similar to that of bisphenol-A polycarbonates.2) The dinuclear Co(III)-Salen complex and two chiral diaminocyclohexane backbones in conjunction with PPNX is a highly efficient for the desymmetrization alternating copolymerization of CO2with3,4-epoxytetrahydrofuran (COPO) to afford perfectly isotactic CO2polymers, which is a typical semicrystalline thermoplastic. From WAXD and DSC study, it possesses a high Tm of271℃and the strong diffraction peaks2θ at18.1°,19.8°,23.3°. Moreover, the blends of the polymers with two opposing configurations formed a stereocomplex with294℃Tm. The dinuclear cobalt complex/PPNX catalyst system is also proved to be effective in the terpolymerization of C02/COPO/CHO and CO2/COPO/CPO to afford the according random, stereogradient terpolymers. The random polymers is amorphous with a Tg of126℃, and the stereogradient polymers have the equimolar crystalline blocks and amorphous blocks, one Tm at241℃and a Tg at106℃are detected.3) The dinuclear Co(Ⅲ)-Salen complex in conjunction with PPNX is a highly efficient catalyst for the desymmetrization copolymerization of CO2with meso-3,5-dioxaepoxides to afford perfectly isotactic CO2polymers, which is a typical semicrystalline thermoplastic, possessing Tm of179-257℃, dependent on the substitute groups at4-position of the meso-epoxides. As a model monomer of4,4-dimethyl-3,5,8-trioxabicyclo[5.1.0]octane, the isotactic copolymer possesses a high Tm of242℃, while its atactic copolymer has a high Tg of up to140℃. Moreover, the hydrolysis was performed to provide stereoregular poly(1,2-bis(hydroxymethyl)ethylene carbonate)s with two hydroxyl groups in a carbonate unit. The hydroxyl groups appended in polycarbonate backbone serve as macro initiators and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) as catalyst, affording fully degradable brush polymers with polycarbonate backbones and polylactide side chains.4) The mechanistic study is carried out for the desymmetrization copolymerization of CO2with meso-epoxides mediated by dinuclear Co(Ⅲ)-Salen complex. The kinetic study and X-ray analysis have led us to propose an intramolecular bimetallic cooperation mechanism inside the cleft predominantly responsible for the copolymerization process, however, the monometallic mechanism is operated outside the cleft. Density functional theory (DFT) calculations is performed for studying the effects of the substituent groups on the ligand framework on the activity and enantioselectivity and the difference in the chiral induction orientation. The effect of ionic cocatalyst and solvents in the copolymerization process was studied. The binding of Lewis base significantly altered the coordination environments of epoxide to Co(III) ions, and realize the transformation from bimetallic mechanism to monometallic mechanism.