Thiol/ene photocurable polymers

Abstract

The photosensitized crosslinking of polyenes with polythiols has been demonstrated to be a free‐radical chain reaction whose apparent quantum yield is in the range of 2 × 10 2–4 × 10 2 mole/einstein. Initiation occurs by abstraction of a hydrogen atom from the thiol group by the excited n,π* triplet of the ketonic photosensitizer. Kinetic studies of model systems have shown that electron‐donating substituents attached or close to the double bond accelerate the thiol addition while electron‐withdrawing groups decrease the rate. With a given olefin, slight differences in reaction rates are observed depending on the structure of the thiol. Mercaptopropionate esters are more reactive than mercaptoacetates which, in turn, are more reactive than alkane thiols. Cure rates for polyene–polythiol systems are in accord with results found for the model systems. Aromatic carbonyl compounds are the most effective sensitizers for this reaction. To a first approximation, the effectiveness of the sensitizer depends only upon the absorbance. No correlation was found between photoactivity and the triplet energies or lifetimes of the sensitizers.