Modern synthetic methods have revolutionized polymer chemistry through the development of new and powerful strategies for the controlled synthesis of complex polymer architectures. Catalysis has proven an enabling science for chemical synthesis, and the development of new classes of well-defined catalysts have demonstrated the enabling science for catalysis. In collaboration with Stanford University (Prof. Robert Waymouth) we have developed a family of organocatalysts for the controlled polymerization of strained heterocyclics to well-defined polymers of novel architectures and macromolecular topologies. N-heterocyclic carbenes, thio-ureas, and super bases such as guanidines and amidines are potent catalysts for the ring-opening polymerization of lactones.

These organocatalysts are both selective and highly active, exhibiting characteristics of living polymerization with turnover frequencies as high as 18 per second with turnover numbers up to 1000. The primary focus of this work is the ring-opening polymerization of lactones and other strained cyclic monomers such as lactide (LA), β-butyrolactone (BL), δ-valerolactone (VL), ε-caprolactone (CL), morpholine-2,6-dione (MDO), trimethylene carbonate (TMC), and 2,2,5,5-tetramethyl-1-oxa-2,5-disilacyclopentane (TMOSC), and hexamethylcyclotrisiloxane (D3).

A special emphasis is placed on mechanistic features of novel organocatalysts that enable high reactivity and selectivity for the construction of complex polymer architectures. For example, in the presence of alcohols, N-heterocyclic carbenes are potent catalysts for the ring-opening polymerization of lactide to generate linear polylactides( pathway A). In the absence of alcohols, N-heterocyclic carbenes mediate the polymerization of lactide to cyclic polylactides of high molecular weight and narrow molecular weight distribution (pathway B).

Polymerization catalysis, in addition to the general issues of turnover frequency, turnover number, molecular weight and molecular weight distribution of the macromolecules, poses additional challenges such as the need to control the selectivity (chemo-, regio- and stereoselectivity). The extraordinarily high activity of the carbenes for ring-opening polymerization enables the stereoselective polymerization of rac- and meso- lactide at low temperatures. For example, polymerization of rac-Lactide with the sterically-encumbered carbene Ph₂IMes in CH₂Cl₂ at -70°C for 2 hr (91% conv) yielded a crystalline polylactide (likely a stereoblock structure of L- and D-lactide) with a melting point of 153.3°C (DHf = 13 J/g). This result was interpreted in terms of a chain-end control mechanism where the stereogenic terminal alkoxide of the growing chain selectively attacks the acyl imidazolium of the same relative stereochemistry, leading to preferential isotactic enchainments (probability of isotactic placement Pi = 0.90).