PHOTO-CROSS-LINKING OF SILICONES .15. CATIONIC PHOTO-CROSS-LINKING OFALPHA,OMEGA-4-TERMINATED DISILOXANES

The photoinduced cationic crosslinking of alpha,omega-terminated disil oxanes (epoxy, vinyl ether, propenyl ether) has been investigated by m eans of Real-Time IR spectroscopy. A lipophilic iodonium salt and thre e lipophilic sulfonium salts were used as photoinitiator. The crosslin king rate is influenced by the type of alpha,omega-terminated disiloxa ne used and differed by a factor of more than 100 from the aliphatic e poxy to the vinyl ether derivatives. Moreover, the sulfonium salts wer e found to have a lower initiation efficiency than the lipophilic iodo nium salt in the various systems studied. These results are in good ag reement with the quantum yield of proton formation in a hexamethyldisi loxane/dimethoxyethane mixture. The final degree of conversion is larg er with the ene derivatives than with the epoxy derivatives. The appli cation of a kinetic method allows us to estimate the rate constant of the termination step (k(t) and for the propenyl derivative the rate co nstant of the propagation step k(P). The termination step can be descr ibed by means of a first order reaction. k(t) was found to depend on t he light intensity and the type of initiators used, whereas k(P) is in dependent of the initiator used.