Enantioselective Copolymerization Of Epoxides And Cyclic Anhydrides

As important alternatives to petroleum-based materials,polyesters gradually become the center of attention and are used in a bulk of applications in both industrial and academic fields owing to exceptionally outstanding chemical and physical properties,and biocompatible and biodegradable nature.The alternating copolymerization of epoxides and cyclic anhydrides is of great interest as a promising method to produce polyesters.This process allows for polymer functionality to be easily adjusted due to the wide availability of cyclic anhydrides and epoxides.Additionally,the stereochemistry inherent to the epoxide monomer provides the possibility to synthesize stereoregular polyesters by establishing chiral centers in the main chain of the resultant polymers with control of the absolute configuration.Although a variety of metal complexes have been reported to catalyze this copolymerization process,several of which are capable of giving highly alternating copolymers,no literature demonstrated the formation of stereoregular polyesters with main-chain chirality.In terms of the above issue,in this dissertation,we made an effort to design highly active and enantioselective catalyst systems for the copolymerization of epoxides and cyclic anhydrides,and to develop the crystalline and functional polyesters.(1)In conjunction with PPNCl,enantiopure dinuclear Al(Ⅲ)complexes bearing a hydrogenated-binaphthol linker efficiently catalyze asymmetric copolymerization of meso-epoxides and cyclic anhydrides to provide stereoregular polyesters,in a high activity with the enantioselectivity up to 99%under mild conditions.Copolymers’ enantioselectivity and reaction activity were revealed to be strongly affected by the axial linker,chiral diamine structure,and phenolate ortho-substituents of dinuclear catalysts.A series of meso-epoxides(cyclopentene oxide,cyclohexene oxide,cis-2,3-epoxybuane,3,4-epoxytetraliydrofuran,2,3-epoxy-4-cyclohexene and 2,3-epoxy-1,2,3,4-tetrahydronaphthalene)and cyclic anhydrides(phthalic anhydride,succinic anhydride,3,6-dinethylphthalic anhydride and 4,5-dimethylphalic anhydride)were applied in the copolymerization to produce stereoregular polyesters.Density functional theory(DFT)calculations suggested that the reactions corresponding to ring-opening at(R)-C-O and(S)C-O bonds of the Al-coordinated meso-epoxide during copolymerization exhibited significantly different Gibbs free energies of activation(ΔC(?)).Notably,most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures between 121℃ and 239℃.Additionally,the mixture of selected isotactic(R)-and(S)-polyesters in a 1:1 mass ratio afforded two crystalline stereocomplexes,which exhibited an enhanced thennal stability and a new crystallization behavior.The thermal stability and crystallization behavior of stereocomplexes were significantly different from that of parent enantiopure polymers.(2)Enantiopure bimetallic complexes in conjunction with nucleophilic cocatalysts efficiently catalyze enantioselective resolution copolymerization of racemic terminal epoxides with anhydrides for preparing isotactic polyesters and chiral epoxides.The chirality of both the axial linker and the diamine backbones of the ligand is responsible for the chiral induction of this kinetic resolution copolymerization process.The catalyst systems exhibited exceptional levels of enantioselectivity with kinetic resolution coefficients exceeding 300 for various racemic epoxides,involving various aliphatic epoxides and glycidyl ethers,affording highly isotactic copolymers(selectivity factors of more than 300)with a completely alternating structure and low polydispersity index.Most of the produced isotactic polyesters are typical semicrystalline materials with melting temperatures in the range from 77 to 160℃.(3)Enantioselective terpolymerization of rac-epoxides,meso-epoxides and phthalic anhydride is achieved mediated by the enantiopure dinuclear aluminum(Ⅲ)complex in conjunction with a nucleophilic cocatalyst to provide chiral random and gradient terpolymers.The chiral gradient polymers from terpolymerization of rac-TBGE/CPO/PA exhibited a semi-crystalline nature with a Tg of 52℃ and Tms of 92 and 117℃.Meanwhile,the chiral random terpolymer resulting from terpolymerization of rac-TBGE/CHO/PA is amorphous with a Tg of 89℃.(4)Enantiopure dinuclear cobalt(Ⅲ)complex in conjunction with a nucleophilic cocatalyst,efficiently catalyzes asymmetric copolymerization of 3,4-epoxytetrahydrofuran and maleic anhydride to provide stereoregular unsaturated polyester,exhibiting high enantioselectivity up to 99%under mild conditions.Quantitative isomerization of the cis-maleate form of all polymers affords the trans-fumarate analogue with the addition of diethylamine.The isotactic trans-analogue exhibited a high melting temperature of 232℃ and diffraction peaks at 2θ=13.7°,18.7°,21.5°,24.60 and 26.7°,which was confirmed to be a semi-crystalline material.Additionally,the mixture of selected isotactic trans-(R)-and polyesters in a 1:1 mass ratio can afford crystalline stereocomplexes,which exhibited an enhanced thermal stability and new crystallization behavior.It is worth to be mentioned that chiral polyester-block-polycarbonate materials were obtained via enantioselective terpolymerization of 3,4-epoxytetrahydrofuran,maleic anhydride and CO2.