Some aspects of the reactivity of photo‐dimerizable esters grafted onto silicone main chain polymers

The reactivity under UV light and at room temperature of two series of photo-crosslinkable silicone polymers containing pendant cinnamic or α-cyano-β-styrylacrylic1 (CSA) esters was examined by means of UV spectroscopy. Two types of behaviour were evidenced on the basis of qualitative spectrum observation and quantitative measurements of reaction rates. Polymers with trans-cinnamic pendant groups were found to undergo during the first moments of 280 nm irradiation a fast photo-isomerization, preceding the dimerization process which takes place with a smaller rate. The mobility of the reactive cinnamic chromophores attached to the highly flexible silicone main chain leads to a collision-controlled process contrasting with the behaviour of the polymers containing more polar CSA esters. In this case, a clean dimerization with high reversibility suggests a matrix-controlled process. This is supported by kinetic data indicating a levelled reactivity for several polymers containing different amounts of CSA esters.