Prop-2-yn-1-yl 2-Bromo-2-methylpropanoate: Identification and Suppression of Side Reactions of a Commonly Used Terminal Alkyne-Functional ATRP Initiator

The atom transfer radical polymerization (ATRP) of styrene was investigated using the popular alkyne-functional initiator prop-2-yn-1-yl 2-bromo-2-methylpropanoate (PBiB). The polymerization kinetics and evolution of molecular weight as a function of monomer conversion were systematically studied with PBiB and similar initiators with protecting groups at the reactive propargylic and terminal acetylenic sites. These studies were compared to control studies using the nonfunctional initiator ethyl 2-bromoisobutyrate. As confirmed by NMR analysis of a model reaction, the terminal alkynes undergo oxidative alkyne-alkyne Coupling under ATRP conditions, resulting in polymers with bimodal molecular weight distributions. This side reaction is significant because it diminishes the orthogonality of ATRP/copper-catalyzed azide-alkyne cycloaddition procedures as well as the control of ATRP.