Organocatalytic Regulation from Salicylic O-Carboxyanhydrides and <sc>l</sc>-Lactide Mixtures to Sequence-Defined Copolymers

For the same monomer combination, the properties of copolymers are closely related to the copolymerization sequence. One-pot production of sequence-defined copolymers from mixed monomers is a crucial yet challenging task. Using suitable catalysts comprising organobase and urea, we have accomplished sequence-controlled polymerization from a mixture of salicylic acid O-carboxyanhydrides (SAOCA) and l-lactides (l-LA), developing a simple and effective strategy of regulating composition sequence. In the catalysis of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,3-bis[3,5-bis(trifluoromethyl)phenyl]urea (U1), SAOCA exhibits exceedingly high polymerizability, and the sequential conversion of SAOCA and l-LA into the polymer chain occurs via a smooth transformation of the chain end from carbonate to alkoxide, forming diblock or even multiblock polyesters. Conversely, when 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) is paired with 1-[3,5-bis(trifluoromethyl)phenyl]-3-phenylurea (U2) for ring-opening copolymerization (ROCOP), SAOCA and l-LA show similar polymerization activity, resulting in a random copolymerization sequence. Experimental results suggest a probable chain end-determining mechanism: the carbonate chain end of poly(salicylic acid) (PSA) generated in the presence of DBU and U1 shows preferential selectivity of SAOCA over l-LA for ring-opening polymerization (ROP). In contrast, in the catalysis of DBN and U2, PSA is terminated with a phenoxyl anion at the omega-end, which exhibits identical nucleophilic attack ability toward both monomers. The sequence-defined copolymerization of SAOCA and l-LA paves the way for functional polymers in biomedical and plastic applications.