While photo-polymerising a (mercaptopropyl methyl siloxane-dimethyl siloxane copolymer with vinyl methyl siloxanes) a thin unpolymerized layer on the top surface is observed. Does anybody suggest to get rid of this problem please ?
This sounds for me as kind of inhibition. I assume you are trying to polymerize the two components in a thiol-ene reaction, which should not be inhibited by oxygen (http://onlinelibrary.wiley.com/doi/10.1002/anie.200903924/abstract). You can run a test under inert atmosphere conditions to see if the unpolymerized layer will still be observed.
In most publication it is claimed that thiol-ene click chemistry is free of oxygen inhibition like Christian Schmitz explained. Perhaps you have used a not ideal photoinitiator. I made best experiences with Irgacure 819 for this purpose, but also camphorquinone should be suitable. About 2 weight-% are sufficient for the reaction. Perhaps the photoinitiator is not soluble in the pure silicon matrix. In this case you must add a bit of compatibilizing solvent.
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