An orthogonal, one-pot, simultaneous UV-mediated route to thiol-ene/sol-gel film

We describe a novel combination of orthogonal reactions based on UV-driven thiol-ene and alkoxysilyl condensation reactions to form a single-step route toward thioether-bridged silsesquioxane films. Our chemical strategy consists of using two bifunctional (meth) acrylate (E) and propanethiol (T) trimethoxysilyl precursors containing two complementary functional moieties for thiol-ene coupling and sol-gel process. The reaction kinetics revealed that c.a. 85% of thiol and ene conversions were consumed concomitantly. Meanwhile, a complete hydrolysis was accomplished, affording ultimately a high degree of condensation (81%). Emphasis was placed on differences of mechanical properties between sol-gel hybrids resulting from thiol-ene reaction (E-T mixture) and ene homopolymerization (E only) using scratch test measurements. For the methacrylate system, the formation of thioether linkages within a vitreous silica network emerged as a useful strategy for the formation of a uniform, low-stress and flexible crosslinked hybrid structure. Enhanced mechanical properties were manifested by an expanded elastic domain, and better resistance to cracking. Moreover, there are clear indications that mechanical properties can be easily tuned upon varying the ratio of the two hybrid precursors.