AbstractSpontaneous addition and polymerization reactions between olefins of different electron densities result in a wide variety of small molecules and polymers. The small molecules include cyclobutanes, 1‐butenes, and pyrans; the polymers can be homopolymers and/or alternating copolymers. Tetramethylenes are proposed as key intermediates in most of these reactions. The tetramethylenes, generated by bond‐forming initiation, are hybrids of 1,4‐biradical and zwitterionic forms. The character is mostly determined by the substituents at the terminals. Zwitterionic character is favored by strong donors such as alkoxy and dialkylamino groups at the carbenium ion end, and strong acceptors such as two cyano groups at the carbanion end. Biradical character is favored by acceptor groups such as diester and cyanoester at the acceptor end, and aryl and vinyl as donor groups. Zwitterionic tetramethylenes initiate ionic homopolymerization, while biradical tetramethylenes initiate alternating copolymerization.This unifying concept ofbond‐forming initiationis extended to spontaneous addition and polymerization reactions of hetero‐atom acceptor molecules and 7,7,8,8‐tetrasubstituted quinodimethanes. Finally, compounds possessing labile σ‐bonds (mainly halogens and peroxides) fit the general scheme. In a few cases, coupling of tetramethylenes may contribute to polymer formation. Ion‐radical pairs, charge‐transfer complexes, and adventitious impurities are excluded as significant initiators.The bond‐forming initiation has been thoroughly discussed in a rece