Thiourea-based bifunctional organocatalysis: supramolecular recognition for living polymerization.
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.
Dove
Andrew P
AP
IBM Almaden Research Center, 650 Harry Road, San Jose, California 95120, USA.
Pratt
Russell C
RC
Lohmeijer
Bas G G
BG
Waymouth
Robert M
RM
Hedrick
James L
JL
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*
D000588:Amines; D011108:Polymers; D019344:Lactic Acid; D013890:Thiourea