Tetramethylene polymerization intermediate

In recent years, more and more photochemists consider that many photocycloaddition reactions take place stepwise, that is, one bond is first formed. The formed tetramethylene intermediate may close to a cyclobutane or initiate the polymerization. We will leave this discussion to Sect. 4. 

Thus, Hall has proven that tetramethylene intermediates, arising from bond-formation between the P-carbons of reacting olefins, are the key to small molecule formation and the thermal copolymerization. This unifying concept of bondforming initiation has been also extended to spontaneous addition and polymerization reaction of heteroatom acceptor molecules and 7,7,8,8-tetrasubstituted quinomethanes. 

The zwitterionic tetramethylene intermediate has been trapped with methanol as a linear 1-methoxybutane, indicating that the addition of tetracyanoethylene to NVK occurs in a stepwise manner. Once the zwitterions are formed, the course of the reaction is determined largely by experimental conditions. With excess NVK, cationic polymerization occurs. If the concentrations of NVK and tetracyanoethylene are comparable, collapse of the intermediate to the corresponding cyclobutane is favored. The cyclobutane formation is reversible, and the isolated cyclobutane alone is capable of initiating the cationic polymerization of NVK. The polymerization of NVK by the stable radical cation salts phenothiazin hexafluoroantimonate [464] and tris-p-bromophenylamminium hexachloro-antimonate [465] were shown to occur via charge transfer initiation too. 

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