Polymerization by Carbocations

Carbocation Intermediates.—Though the nature of the active centres in polymerizations propagated by carboanions has been well known for some time, and reasonably adequately characterized, the corresponding species in cationic propagations has proved difficult to identify with anything like the same certainty. However, the use of super acid media, e.g. FSOaH/SbFj, has now enabled Olah to obtain a n.m.r. spectrum of the elusive styryl carbocation, following experiments such as that in which a racemic mixture of products was obtained from the reaction of (—)-l-phenylethyl chloride with EtaAl, which provided powerful circumstantial evidence for the existence of the ion. More recently a particular definitive paper on and n.m.r. spectra of styryl species has appeared. 

The characteristic ultraviolet absorption spectra of styryl and related cations also now seem to have been established. Pepper s elaborate stopped-flow study of the styrene perchloric acid system has identified a transient absorption band with a maximum at 340 nm, and similar bands attributed to cationic species derived from styrene, a-methylstyrene, / -methylstyrene, and indene 

The active cation (1) derived from A -ethyl-3-vinyl carbazole would be expected to be somewhat more stable than simple styryl species and this appears to be so. 

Ledwith and his co-workers have tentatively assigned an absorption band at 468 nm to this cation. 

Hayashi, M. Irie, and K. Hayashi, Preprints, European Symposium on Electric Phenomena in Polymer Science, Pisa, Italy, 1978, p. 39. 

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Although styrene polymerized by ionic mechanism is not utilized commercially, much research was devoted to both cationic and anionic polymerizations. An investigation of cationic polymerization of styrene with an A1(C2H5)2C1/RC1 (R = alkyl or aryl) catalyst/cocatalyst system was reported by Kennedy.The efficiency (polymerization initiation) is determined by the relative stability and/or concentration of the initiating carbocations that are provided by the cocatalyst RCl. A/-butyl, isopropyl, and j c-butyl chlorides exhibit low cocatalytic efficiencies because of a low tendency for ion formation. Triphenylmethyl chloride is also a poor cocatalyst, because the triphenylmethyl ion that forms is more stable than the propagating styryl ion. Initiation of styrene polymerizations by carbocations is now well established. 

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