Chain Propagation and Termination Mechanisms for Polymerization of Conjugated Polar Alkenes by [Al]-Based Frustrated Lewis Pairs

A combined experimental and theoretical study on mechanistic aspects of polymerization of conjugated polar alkenes by frustrated Lewis pairs (FLPs) based on N-heterocyclic carbene (NHC) and Al(C6F5)(3) pairs is reported. This study consists of three key parts: structural characterization of active propagating intermediates, propagation kinetics, and chain-termination pathways. Zwitterionic intermediates that simulate the active propagating species in such polymerization have been generated or isolated from the FLP activation of monomers such as 2-vinylpyridine and 2-isopropenyl-2-oxazoline-one of which, IMes(+)-CH2C(Me)-(C3H2NO)Al(C6F5)(3)(-) (2), has been structurally characterized. Kinetics performed on the polymerization of 2-vinylpyridine by (IBu)-Bu-t/Al(C6F5)(3) revealed that the polymerization follows a zero-order dependence on monomer concentration and a first-order dependence on initiator ((IBu)-Bu-t) and activator [Al(C6F5)(3)] concentrations, indicating a bimolecular, activated monomer propagation mechanism. The Lewis pair polymerization of conjugate polar alkenes such as methacrylates is accompanied by competing chain-termination side reactions; between the two possible chain-termination pathways, the one that proceeds via intramolecular backbiting cyclization involving nucleophilic attack of the activated ester group of the growing polymer chain by the O-ester enolate active chain end to generate a six-membered lactone (D-valerolactone)-terminated polymer chain is kinetically favored, but thermodynamically disfavored, over the pathway leading to the beta-ketoester-terminated chain, as revealed by computational studies.