Photo-crosslinking of polymers containing pendant epoxy groups

The photo-crosslinking of polymers containing pendant epoxy groups was carried out at room temp. in the presence of a photosensitive sulfonium salt which is an initiator of cationic polymerization. The crosslinking reactivity of the epoxide-containing polymers was found to depend on the mobilities of both the backbone of the polymers and the pendant epoxy groups, and on the epoxide structure. The introduction of spacer chains between the polymer backbone and the epoxy groups increases the mobilities of the backbone and the epoxides, resulting in an increases crosslinking reactivity. The introduction of a methyl group at the epoxy ring of the polymers with glass transition temp. (T_g) lower than room temp. enhances the crosslinking reactivity, whereas it diminishes the reactivity for the polymers with T_g higher than room temp. This result can be explained by assuming that the presence of the methyl group results not only in an increase of the epoxide reactivity due to both the inductive effect of the methyl group and the increased ring strain, but also the decreased mobility of the epoxy groups. Thus, the reactivity of the rubbery polymers is principally controlled by the intrinsic reactivity of the epoxides, and the reactivity of the rigid polymers is mainly controlled by the mobility of the reactive sites.