Catalytic Lewis Pair Polymerization of Renewable Methyl Crotonate to High-Molecular-Weight Polymers

Abstract

Crotonates, unlike their constitutional isomers of methacrylates, are not readily polymerized by conventional radical or anionic polymerization methods. A recent effort in polymerizing biorenewable methyl crotonate (MC) using organic N-heterocyclic carbene (NHC) initiators led to only dimerization. This contribution reports an effective polymerization of MC using Lewis pairs consisting of an NHC or N-heterocyclic olefin (NHO) Lewis base and a group 13 Lewis acid, in particular sterically encumbered methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD), producing high-molecular-weight poly(MC) (PMC) with Mn up to 161 kg/mol under ambient temperature and solvent free conditions. Depending on the nature of the Lewis pair and reaction conditions, the polymerization proceeds either catalytically producing lower molecular weight PMC or noncatalytically leading to high-molecular-weight PMC. Investigations into initiation mechanisms have revealed both nucleophilic and basic pathways. The observation of the susceptibility of MC to simple deprotonation and subsequent propagation in the presence of MAD and a strongly basic NHC—the basic pathway—has led to a facile approach to high-molecular-weight vinyl-functionalized PMC (Mn = 97.1 kg/mol) using simple KOtBu and MAD.