title: Thiourea-based bifunctional organocatalysis: supramolecular recognition for living polymerization.
abstract: A versatile, metal-free, organocatalytic approach to the living ring-opening polymerization of lactide using a bifunctional thiourea-tertiary amine catalyst is described. Mild and highly selective polymerization conditions produced poly(lactides) with predictable molecular weights and extremely narrow polydispersities ( approximately 1.05), characteristic of a living polymerization. The extraordinary selectivity of this catalyst system for polymerization relative to transesterification is remarkably unusual. The low polydispersities and exceptional control observed are a consequence of selective transesterification of lactide relative to the open chain esters. Presumably, the ring strain of lactide provides both a driving force for the polymerization and a kinetic preference for polymerization relative to transesterification with catalyst. We postulate that the initiating/propagating alcohol is activated by acid-base interaction with the tertiary amine moiety and the carbonyl of the lactide monomer is simultaneously activated by hydrogen bonding to the thiourea moiety of the catalyst.
authors.affiliation: IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, USA.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Dove
authors.forename: Russell C
authors.initials: RC
authors.lastname: Pratt
authors.forename: Bas G G
authors.initials: BG
authors.lastname: Lohmeijer
authors.forename: Robert M
authors.initials: RM
authors.lastname: Waymouth
authors.forename: James L
authors.initials: JL
authors.lastname: Hedrick
mesh_terms: D000588:Amines / Q000737:chemistry; D002384:Catalysis; D019344:Lactic Acid / Q000138:chemical synthesis* / Q000737:chemistry; D015394:Molecular Structure; D011108:Polymers / Q000138:chemical synthesis*; D013890:Thiourea / Q000737:chemistry*
chemical_list: D000588:Amines; D011108:Polymers; D019344:Lactic Acid; D013890:Thiourea