Organocatalytic\nRegulation from Salicylic O‑Carboxyanhydrides\nand l‑Lactide Mixtures to Sequence-Defined Copolymers

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