Copolymerization of Lactide and Cyclic Carbonate via Highly Stereoselective Catalysts To Modulate Copolymer Sequences

The controlled ring-opening polymerizations of enantiopure (L-LA or D-LA) or racemic lactide (rac-LA) with achiral functionalized cyclic carbonates (TMCR) have been performed by using the aminobisphenolate yttrium and lutetium alkyl complexes or an organo-base catalyst DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), in combination with various alcohols as chain transfer agents. When using the highly heteroselective precatalyst 1(Y), the copolymerization of rac-LA and TMCR afforded tapered copolymer poly(rac-LA-t-TMCR), while the copolymerization of enantiopure L-LA or D-LA, respectively, with TMCR gave pseudo-random product poly(L-LA-r-TMCR). When the low heteroselective rare-earth metal precatalysts or organic DBU were applied, the difference in the monomer distributions for the above two systems disappeared gradually. On the contrary, the monomer sequence distribution was not influenced by the types of chain transfer agents and TMCR. The kinetics study revealed that the competitive reaction ratios were r(L-LA) = 0.58 and r(TMCR1) = 0.70 for the copolymerization of L-LA and TMCR1 while r(rac-LA) = 2.29 and r(TMCR1) = 0.51 for the copolymerization of rac-LA and TMCR1, which were reasonably explained by the synergistic effect of high heteroselectivity of catalyst and better coordination capability of LA than other monomers.