Ring-Opening Polymerization of Cyclic Esters and Carbonates with (Thio)urea/Cyclopropenimine Organocatalytic Systems
Romain Morodo, David M. Dumas, Jia Zhang, Kai Hin Lui, Paul J. Hurst, Riccardo Bosio, Luis M. Campos, Nathaniel H. Park, Robert M. Waymouth, James L. Hedrick
Abstract
Organocatalyzed ring-opening polymerization is a powerful tool for the synthesis of a variety of functional, readily degradable polyesters and polycarbonates. We report the use of (thio)ureas in combination with cyclopropenimine bases as a unique catalyst for the polymerization of cyclic esters and carbonates with a large span of reactivities. Methodologies of exceptionally effective and selective cocatalyst combinations were devised to produce polyesters and polycarbonates with narrow dispersities ( Đ = 1.01–1.10). Correlations of the p K a of the various ureas and cyclopropenimine bases revealed the critical importance of matching the p K a of the two cocatalysts to achieve the most efficient polymerization conditions. It was found that promoting strong H-bonding interactions with a noncompetitive organic solvent, such as CH 2 Cl 2, enabled greatly increased polymerization rates. The stereoselective polymerization of rac -lactide afforded stereoblock poly(lactides) that crystallize as stereocomplexes, as confirmed by wide-angle X-ray scattering.