The Relationship of Cycloaddition Reactions to Spontaneous Vinyl Polymerizations

The zwitterionic–biradical tetramethylene proposed by Huisgen as the key intermediate in stepwise [2+2] cycloaddition reactions has been shown to be the crucial intermediate in spontaneous vinyl polymerizations as well. Predominantly biradical tetramethylenes initiate free‐radical copolymerizations, while predominantly zwitterionic tetramethylenes initiate cationic or anionic homopolymerizations. Stepwise cycloaddition is viewed as a spontaneous polymerization lacking a propagation step. These tendencies could be correlated in the form of an ‘organic chemist's Periodic Table’, which has recently been put on a quantitative basis. Huisgen also showed experimentally that [4+2] Woodward–Hoffman‐allowed cycloadditions are completely concerted. Spontaneous copolymerizations accompanying these cycloadditions, therefore, were ascribed to the s‐trans diene form. This concept was given support by kinetics studies, as well as by exclusive cycloaddition from s‐cis cyclopentadiene, and exclusive copolymerization from s‐trans verbenene.