Monomer-Promoting Asymmetric Kinetic Resolution-Alternating Copolymerization To Afford AAB-Type Copolyesters

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 (<i>S,S</i>-LA or l-LA) and two enantiomeric isomers of racemic tropic acid cyclic esters (tropicolactone) be polymerized into sequence-controlled -(A_SA_SB_S)_<i>n</i>- 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_SA_SB_S)_<i>n</i>-<i>b</i>-(A_RA_RB_R)_<i>n</i>- can further be obtained upon addition of <i>R,R</i>-LA (d<i>-</i>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 <i>S,S</i>-LA and <i>rac</i>-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_SA_SB_S)_<i>n</i>-. The tetracomponent mixture of <i>rac</i>-lactide and <i>rac</i>-tropicolactone can be copolymerized into an alternating copolymer with a -((A_SA_SB_S)_<i>x</i>-<i>ran</i>-(A_RA_RB_R)_<i>y</i>)_<i>n</i>- structure, in which the stereoselective linkage probability of 95% after <i>S</i>,<i>S</i>-lactide (<i>R</i>,<i>R-</i>lactide) followed by <i>S</i>-tropicolactone (<i>R</i>-tropicolactone) keeps very high too.