Ring-Opening Polymerization of Cyclic Esters and Carbonates with (Thio)urea/Cyclopropenimine Organocatalytic Systems

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 pK a of the various ureas and cyclopropenimine bases revealed the critical importance of matching the pK 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.

Highly Selective O-Phenylene Bisurea Catalysts for ROP: Stabilization of Oxyanion Transition State by a Semiflexible Hydrogen Bond Pocket

“…(a) Previously reported (thio)urea anion and indolocarbazole anion catalysts tested (in parentheses are the relative k p for the polymerization of ε-CL initiated with 4-methylbenzyl alcohol (MBA) normalized with respect to U-1) *p K a of U-2 in DMSO reported by Schreiner is 16.1, our lab measured a similar value of 15.8. , (b) O -Phenylene bisurea (OPBU) catalysts for this study (in parentheses are the relative k p normalized to U-1 after single or double deprotonation). p K a1 and p K a2 are measured by UV–vis titration experiments in DMSO, corresponding to the 1st and 2nd deprotonation from neutral bisurea to form mono/dianionic potassium salts, respectively.…”