Hydrogen bond vs cationic activation: Exploring next generation design motifs for ROP organocatalysts

Abstract

Organocatalytic ring-opening polymerization (ROP) is a powerful method for generating polyesters and polycarbonates with superior control over molecular weight and polydispersity. Organocatalysts employing hydrogen bond activation strategies such as guanidine and (thio)urea (anions) have been developed over the past decades and have proven to be highly efficient. Recently, we reported that mono-deprotonated 2,2’-bisindole achieves high reactivity and selectivity by a different mechanism, aided by its alkali metal counterion. In this system the cationic counterion provides the electrophilic activation of the monomer rather than a N-H hydrogen bonding interaction, hinting at a new design strategy for ROP catalysts. This presentation will discuss a library of N-H containing heterocyclic compounds that we have now examined as potential ROP organocatalysts upon deprotonation. These catalysts span a wide ($~10^8$) range of reactivity and show excellent selectivity, yielding polymers with narrow molecular weight distributions (PDi = 1.01~ 1.1). These catalysts’ reactivity shows different extents of counterion dependence, and the cation roles are elucidated by mechanistic studies. Their catalytic performances are evaluated and compared to existing hydrogen bond organocatalysts, giving insights into what next generation organocatalysts should look like.