Monomer-Promoting Asymmetric Kinetic Resolution-Alternating Copolymerization To Afford AAB-Type Copolyesters
Chixian He, Ji Xian, Hongjun Fu, Feixiang Cheng, Xinning Han, Xiaobo Pan, Yu Tang, Jincai Wu
Abstract
Living copolymerization of mixed monomers can enrich the diversity of copolymer materials with well-defined performance via controlling both monomers and stereosequences. However, periodic sequence-controlled living copolymerization of same-type monomers with more than two components in synthetic polymer science remains a challenge. In this work, a new method of monomer-promoting asymmetric kinetic resolution-alternating copolymerization can let a tricomponent mixture of l -lactide ( S,S -LA or l -LA) and two enantiomeric isomers of racemic tropic acid cyclic esters (tropicolactone) be polymerized into sequence-controlled −(A S A S B S ) n – type biodegradable copolyesters (the subscript S presents the configuration and A and B present lactic acid units and tropic acid units, respectively), and diblock copolymers of −(A S A S B S ) n - b -(A R A R B R ) n – can further be obtained upon addition of R,R -LA ( d - LA). Compared to previous asymmetric kinetic resolutions of racemic chemicals via polymerization or organic reactions, no enantiopure catalyst/initiator is required in this system. After the resolution and alternating copolymerization of S,S -LA and rac -tropicolactone, the ee value of unreacted tropicolactone can reach 99.4%. The alternating probability between tropicolactone and lactide monomers is more than 96% in periodic sequence polymers of −(A S A S B S ) n –. The tetracomponent mixture of rac -lactide and rac -tropicolactone can be copolymerized into an alternating copolymer with a −((A S A S B S ) x - ran -(A R A R B R ) y ) n – structure, in which the stereoselective linkage probability of 95% after S, S -lactide ( R, R- lactide) followed by S -tropicolactone ( R -tropicolactone) keeps very high too.