Bond‐Forming Initiation in Spontaneous Addition and Polymerization Reactions of Alkenes

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Abstract
Spontaneous addition and polymerization reactions of alkenes of different electron‐densities result in a wide variety of small organic molecules and high polymers. Tetramethylenes are proposed as key intermediates, i.e. resonance hybrids of 1,4‐diradical and zwitterionic limiting structures. Their character is determined by the substituents at the terminals: Zwitterionic character is favored by strong donors, such as alkoxy and dialkylamino groups, at the carbenium center and strong acceptor group such as diesters and cyano‐esters, at the acceptor end, and aryl and vinyl groups as donors. Zwitterionic tetramethylenes initiate ionic homopolymerization, while diradical tetramethylenes initiate alternating copolymerization, thus providing an extremely sensitive technique for the detection and characterization of these intermediates. The effects of the donor and acceptor substituents can be arranged as an “Organic Chemist's Periodic Table”, wherein the areas of mechanistic change clearly emerge and which provides predictive capability.—This unifying concept of bondforming initiation is extended to spontaneous addition and polymerization reactions of heteroatom acceptor molecules and 7,7,8,8‐tetrasubstituted quinodimethanes and of compounds possessing labile σ‐bonds, such as halogens and peroxides. Radical‐ion pairs, charge‐transfer complexes and adventitious impurities are excluded as significant initiators.