Enantiomorphic Site-Assisted Chain End Control Stereospecific Alternating Copolymerization of Chiral Cyclic Diesters

Stereospecific alternating copolymerization of different chiral cyclic esters is one feasible approach to enrich the structural diversity of copolyesters and tailor their properties. However, dramatically different reactivities of different cyclic esters let a perfectly stereospecific alternating polymerization of these cyclic esters be a challenge, thus the catalyst is required to balance their reactivities. Herein, a remarkable enantiomorphic site effect on chain end control was discovered and successfully utilized to balance the reactivities of highly reactive S, S-lactide (S, S-LA) and low reactive R, R-ethylglycolide (R, R-EG)/R, R-propylglycolide (R, R-PG) during their heterospecific alternating copolymerization. The enantiomorphic site of R, R-SalenAl complex can increase the relative reactivity of R, R-EG/R, R-PG and suppress that of S, S-LA, then a perfectly alternating sequence of the copolymer of S, S-LA and R, R-EG/R, R-PG can be achieved (P_alt=0.96/0.91); inversely, using S, S-SalenAl complex, the significant enantiomorphic site effect enlarges the reactivity difference of two monomers, the alternating level was just 0.70/0.68 even to 0.61. Poly(S, S-LA-alt-R, R-EG) with a high alternating regularity exhibits lower glass transition temperatures and a dramatically higher elongation at break (ϵ_B=449±51 % (P_alt=0.96) vs ϵ_B=6±1% (P_alt=0.70)).