N-Heterocyclic Olefins as Organocatalysts for Polymerization: Preparation of Well-Defined Poly(propylene oxide)

Abstract

The metal-free polymerization of propylene oxide (PO) using a special class of alkene - N-heterocyclic olefins (NHOs) - as catalysts is described. Manipulation of the chemical structure of the NHO organocatalyst allows for the preparation of the poly(propylene oxide) in high yields with high turnover (TON>2000), which renders this the most active metal-free system for the polymerization of PO reported to date. The resulting polyether displays predictable end groups, molar mass, and a low dispersity (D strok sign M <1.09). NHOs with an unsaturated backbone are essential for polymerization to occur, while substitution at the exocyclic carbon atom has an impact on the reaction pathway and ensures the suppression of side reactions. One carbon makes a difference: The efficient and controlled formation of poly(propylene oxide) (PPO) at a very low catalyst loading of N-heterocyclic olefins showcases the use of this group of highly polarized alkenes as catalysts for organopolymerization. A strong structure-activity relationship is found, which is fundamentally different from the reactivity of N-heterocyclic carbenes (TON=turnover number, PDI=polydispersity index).