Photocrosslinking of Silicones. Part XV. Cationic Photocrosslinking of α, ω, 4-Terminated Disiloxanes

The photoinduced cationic crosslinking of α, ω-terminated disiloxanes (epoxy, vinyl ether, propenyl ether) has been investigated by means of Real-Time IR spectroscopy. A lipophilic iodonium salt and three lipophilic sulfonium salts were used as photoinitiator. The crosslinking rate is influenced by the type of α, ω-terminated disiloxane used and differed by a factor of more than 100 from the aliphatic epoxy to the vinyl ether derivatives. Moreover, the sulfonium salts were found to have a lower initiation efficiency than the lipophilic iodonium salt in the various systems studied. These results are in good agreement with the quantum yield of proton formation in a hexamethyldisiloxane/dimethoxyethane mixture. The final degree of conversion is larger with the ene derivatives than with the epoxy derivatives. The application of a kinetic method allows us to estimate the rate constant of the termination step (kt and for the propenyl derivative the rate constant of the propagation step kp. The termination step can be described by means of a first order reaction. kt was found to depend on the light intensity and the type of initiators used, whereas kp is independent of the initiator used.