Cationic polymerization stop-flow studies

Although the concept of pseudo-cationic mechanism may be accepted by some, it is not accepted by many. An alternative mechanisms based on ion pairs were offered instead in the past [111-113]. A publication by Scwarc claims that all detailed re-examinations of the evidence for pseudo-cationic polymerizations shows that all the features of styrene polymerization initiated by acids are accounted for satisfactorily and convincingly by the orthodox ionic mechanism. To support his claim, he reported that optical absorption and electric conductance of the styrene solutions undergoing polymerization induced by perchloric acid in stopped-flow studies demonstrated protonations of the monomer by the acid and formation of ionic species [114]. 

Prof. Szwarc also contributed to the understanding of mechanisms of cationic polymerizations. In addition to the previously mentioned study of the cationic polymerization of styrene initiated by trifluoroacetic acid, he developed novel methods of initiation of cationic polymerization, e.g., the initiation of cationic polymerization by transfer of Cl " and N02 ions, and initiation by electron-transfer. In cooperation with deSorgo and David Pepper, he carried out the first stop-flow study of cationic polymerization that demonstrated the formation of the positive polystyryl cation and allowed its spectrum to be recorded. This work was published in J.C.S. Chem. Comm. 419, (1973). He was the first to point out that cationic polymerization induced by ionizing radiation is propagated by free cations [Makromol. Chem. 35a. 123 (I960)]. 

Protonation of vinyl monomers initiates their cationic polymerization. Hopefully, the presence of the resulting carbonium ions might be observed spectrophotometrically and then their concentration in the polymerizing system could be determined. In cooperation with Prof. David Pepper, who stayed with us for 6 months, we investigated by stop-flow technique polymerization of styrene initiated by perchloric acid. The formation of the -CH2 CH(Ph) was indeed observed and the propagation constant calculated. This study is continued by Pepper and the detailed features of the reaction require further clarification. 

Onium salt cationic photoinitiators present many unique and interesting opportunities for basic studies of cationic ring-opening polymerizations. Since they are latent photochemical sources of strong Bronsted adds, they can be dissolved in the subject monomers and then precisely tri ered on demand by the application of light. Mixing problems and the use of complex stopped-flow devices and other apparatuses required to overcome them are thus avoided. Only the rate of initiation is different in a photoinitiated cationic polymerization as compared to a conventional thermally initiated polymerization. The rate of initiation for an onium salt-photoinitiated cationic potymerization (eqn [68]) is... 

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