Covalent Adaptable Networks with rapid UV Response Based on Reversible Thiol-ene Reactions in Silicone Elastomers*

PhotoCAN silicone elastomers, based on thiol-ene reaction, exhibit rapid and reversible changes in dynamic modulus at room temperature when illuminated by UV. By combining results from magic angle spinning solid state NMR as well as EPR and rheometry measurements, both made under UV, it is concluded that the mechanical response can be attributed to a combination of dissociative, associative and oxidation reactions. The cleavage of the C-S bonds under UV in the presence of an excess of thiyl radicals is identified as the reversible dissociative reaction responsible for abrupt drops in storage modulus. A slower but concurrent reaction is a termination process, involving thiyl radicals, to form disulfide bonds.  A kinetic model is developed that successfully relates the rates of the underlying reaction mechanisms to changes in the storage modulus. The results provide a basis for designing new, ambient temperature photo-responsive covalently adaptive network materials.