Synthesis and Thiol-ene Photopolymerization of (meth)allyl-terminated Polysulfides
Thiol-terminated polysulfides (PS) are cured by mixing with an oxidant, resulting in limited shelf- and/or pot-life, depending on whether formulated as a one- or two-component system. Mixtures of thiol- and alkene-terminated polysulfides offer the potential for an on-demand curing process through thiol-ene photopolymerization. Thiol end groups of commercial polysulfides, PS-1 (1000 g/mol) and PS-2 (3000 g/mol), were converted to alkene by reaction with (meth)allyl bromide. Photopolymerizations were performed by irradiating films of equimolar thiol:ene mixtures at 320-500 nm (30 mW/cm(2)) in the presence of 5 wt % 2,2-dimethoxy-2-phenyl-acetophenone (DMPA). Reaction kinetics were measured using real-time FTIR by monitoring absorbances at 3075 cm(-1) (alkene) or 2550 cm(-1) (thiol). In the absence of any reactive diluent, mixtures of thiol and alkene polysulfides failed to gel notwithstanding high reaction conversion (>90%). Partial or total replacement of the thiol polysulfide component with pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) yielded solid elastomeric films and ultimate reaction conversions of 80-96% after 5 min irradiation. Crosshatch adhesion measured on glass, aluminum, and steel was very poor (0B) for (meth)allyl PS-1/PETMP and poor (2B) for (meth)allyl PS-2/PETMP without adhesion promoters. (3-Mercaptopropyl)trimethoxysilane (1 wt %) significantly improved adhesion of (meth)allyl PS-2/PETMP on all substrates (4B) but yielded no improvement for (meth)allyl-terminated PS-1/PETMP. (c) 2017 Wiley Periodicals, Inc.