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 acceptors, such as two cyano groups, at the carbanion center; diradical character is favored by acceptor groups, 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 o-bonds, such as halogens and peroxides. Radical-ion pairs, charge-transfer complexes and adventitious impurities are excluded as significant initiators.
The Tetramethylene Concept
IotroductionSpontaneous addition and polymerization reactions between alkenes of different electron densities lead to a wide variety of products, including small molecules and polymers. The small molecules include cyclobutanes, l-butenes, Diels-Alder adducts (from styrenes), ene-reaction products (from alkylethylenes), and pyrans (from unsaturated esters). The modes of formation of the small molecules have been interpreted in terms of 1,4-diradicals, 1,4-zwitterions, and concerted processes. The polymers can be either homopolymers of one or both alkenes, or copolymers. Radical-ion pairs, charge-transfer (CT) complexes, 1,4-diradicals and 1,4-zwitterions have been postulated as intermediates for the polymerizations. Occasionally, CT complexes have been regarded as precursors of free radicals. So far, there has been no concept which could equally interpret both addition as well as polymerization, and enable useful predictions to be made.In recent years, tetramethylene intermediates have been extensively studied on a theoretical basis. Tetramethylenes with terminal substituents are currently viewed as resonance hybrids of varying diradical and zwitterionic nature. In this paper we postulate that they frequently function as key intermediates in cycloadditions and polymerizations.This concept can be extended to reactions in which n-and o-bonds participate.
Unsubstituted TetramethyleneHoSfmann et al."] first studied theoretically the parent tetramethylene. Later studies showed the existence of ...
