Sequence controlled copolymerization of lactide and a functional cyclic carbonate using stereoselective aluminum catalysts

Aluminum complexes 1 (L1AlMe, L1 = N,N′-dibenzyl-N,N′-bis[(3,5-dichloro-2-hydroxyphenyl)methylene]-1,2-diaminoethane) and 2 (L2AlEt, L2 = N,N′-(2,2-dimethylpropylene)-bis(3,5-di-tert-butylsalicylideneimine) were applied for the ring-opening copolymerization of lactide and a functional cyclic carbonate (MAC) with different alcohols as chain transfer agents. Modulated by catalyst 1 of high heteroselectivity towards rac-LA polymerization, the copolymerization of L-LA/MAC afforded a random poly(MAC-r-L-LA) while the copolymerization of rac-LA/MAC afforded a gradient poly(MAC-grad-rac-LA), which were determined by the comonomer reactivity ratios (rL-LA : rMAC = 0.217 : 0.578 vs. rrac-LA : rMAC = 0.584 : 2.478). By using catalyst 2 of high isoselectivity towards rac-LA polymerization, the copolymerizations of L-LA/MAC and rac-LA/MAC afforded a quasi-diblock poly(L-LA-b-MAC) (rL-LA : rMAC = 2.035 : 0.112) and a tapered poly(rac-LA-tapered-MAC) (rrac-LA : rMAC = 1.619 : 0.283), respectively. These results were further proved by the detailed kinetics investigation of the polymerization process, as well as NMR spectrum analysis and thermal characterization of the harvested copolymers. This strategy exhibits an advantage of expanding the applications of stereospecific catalysts to efficiently modulate sequence distributions and functionalization of aliphatic polyesters.