title: Fast and selective ring-opening polymerizations by alkoxides and thioureas.
abstract: Ring-opening polymerization of lactones is a versatile approach to generate well-defined functional polyesters. Typical ring-opening catalysts are subject to a trade-off between rate and selectivity. Here we describe an effective catalytic system combining alkoxides with thioureas that catalyses rapid and selective ring-opening polymerizations. Deprotonation of thioureas by sodium, potassium or imidazolium alkoxides generates a hydrogen-bonded alcohol adduct of the thiourea anion (thioimidate). The ring-opening polymerization of L-lactide mediated by these alcohol-bonded thioimidates yields highly isotactic polylactide with fast kinetics and living polymerization behaviour, as evidenced by narrow molecular weight distributions (Mw/Mn< 1.1), chain extension experiments and minimal transesterifications. Computational studies indicate a bifunctional catalytic mechanism whereby the thioimidate activates the carbonyl of the monomer and the alcohol initiator/chain end to effect the selective ring-opening of lactones and carbonates. The high selectivity of the catalyst towards monomer propagation over transesterification is attributed to a selective activation of monomer over polymer chains.
authors: Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA.
authors: Xiangyi
authors: X
authors: Zhang
authors: IBM Research - Almaden, 650 Harry Road, San Jose, California 95120, USA.
authors: Gavin O
authors: GO
authors: Jones
authors: IBM Research - Almaden, 650 Harry Road, San Jose, California 95120, USA.
authors: James L
authors: JL
authors: Hedrick
authors: Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA.
authors: Robert M
authors: RM
authors: Waymouth