Bicyclic
isothioureas <b>1</b> and <b>2</b> mediate
controlled ring opening polymerizations (ROP) of lactides in the absence
of protic initiators to afford high molecular weight polylactides
(PLA) with narrow polydispersities. The cyclic structure of the resulting
PLA was determined by dilute solution viscosity measurement and MALDI-TOF
mass spectrometry. Compared to DBU initiator, isothioureas are more
selective for producing cyclic PLA without appreciable linear contaminants.
Mechanistic studies involving acyl amidinium support our hypothesis
that DBU-initiated ZROP generates linear chains from a ketene aminal
intermediate