Benzyl alcohols as accelerators in the photoinitiated cationic polymerization of epoxide monomers

Benzyl alcohols display marked accelerating effects on the onium salt photoinitiated cationic ring-opening polymerizations of epoxide monomers. The participation of radicals in these polymerizations was demonstrated, and a pronounced accelerating effect on the polymerization rate was observed depending on the character of the substituent on the benzene ring of the alcohol. 4-Methoxybenzyl alcohol was considerably more reactive than benzyl alcohol, whereas the analogous chloro- and nitrosubstituted compounds were less reactive. Piperonyl alcohol was even more reactive as an accelerator in these polymerizations. The UV irradiation of an onium salt cationic photoinitiator generates Bronsted acids that initiate the cationic ring-opening polymerization of the epoxide monomer. In the presence of benzyl alcohol, the usual polymerization mechanism is diverted and proceeds by a more-rapid activated monomer mechanism. As a result, benzyl ether groups are incorporated at the termini of the polymer chains. Aryl radicals that are also generated by photolysis of the onium salt abstract labile benzyl ether protons, and the benzyl ether radicals that are produced are subsequently oxidized by the onium salt to afford the corresponding benzyl carbocations. This takes place by a chain process. The polymer bound benzyl cations serve as additional sites for the initiation of cationic polymerization. It is proposed that these two mechanisms acting in synergy with one another account for the observed accelerating effect on the polymerization rate. (C) 2002 Wiley Periodicals, Inc.