Ring-Opening Polymerization with Lewis Pairs and Subsequent Nucleophilic Substitution: A Promising Strategy to Well-Defined Polyethylene-like Polyesters without Transesterification

Ring-opening polymerization (ROP) of omega-pentadecalactone (PDL) catalyzed by Lewis pairs was thoroughly explored, and a novel approach to well-defined aliphatic long chain polyester with high molecular weight (MW) was developed in the present work. The Zn(C6F5)(2)/1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) Lewis pair was proved to be a promising catalytic system for ROP of PDL, producing cyclic PPDL with high MW (M-w > 100 kg/mol) and relatively low polydispersity index (M-w/M-n = 1.6-1.9). Strikingly, no transesterification occurred in the ROP of PDL by Zn(C6F5)(2)/DBU. The cyclic topology of the polyester could be switched to linear structure in the presence of alcohol. The feeding mode and the structure of alcohol significantly influence the ROP. Compared with mixing alcohol with Zn(C6F5)(2)/DBU at first, adding Ph2CHOH with low nucleophilicity after full monomer conversion could afford linear PPDL without transesterification. It was noted that random chain scission or chain extension was not detected after adding Ph2CHOH. Well-defined block copolymer containing polyethylene-like segment can be easily prepared by sequential addition of PDL and lactide (LA) or caprolactone (CL). Cyclic block copolyesters c-poly(PDL-b-CL) and c-poly(PDL-b-LA) were obtained in the absence of alcohol. The blocky structures can be maintained even when prolonging reaction time after full monomer conversion. Similarly, introducing Ph2CHOH before quenching the polymerization led to well-defined linear block copolyesters l-poly(PDL-b-CL) and l-poly(PDL-b-LA).