Synthesis, Characterization and Cleavage of Surface-Bound Linear Polymers Formed Using Thiol-Ene Photopolymerizations

The formation of linear polymers films using thiol-ene photopolymerizations was investigated by grafting thiol-ene films on the surface with subsequent cleavage and polymer analysis. Silica nanoparticles were first modified with an acrylated silane molecule, and then a dithiol was reacted to this surface using a base catalyzed thiol-acrylate conjugate addition reaction to prepare a uniform surface presenting thiol groups attached to the surface with a cleavable ester linkage. Photopolymerization of difunctional thiol and ene monomers present in various stoichiometric ratios was carried out in the presence of the nanoparticles, and a thiol-ene film was attached to the surface. The polymer film was cleaved from the surface using acid-catalyzed hydrolysis and separated from the particles. It was found that the particle presence does not affect the polymerization of thiol-ene polymers in bulk; however, the presence of surface thiols changes the relative stoichiometry of the thiol and ene monomers at the surface and hence strongly affects the molecular weight of the attached polymer. The number average and the weight average molecular weight of the unattached polymers ranged from 900 to 12,600 g/mol and 1500 to 25,700 g/ml, respectively, as the thiol:ene ratio in the bulk increased from 0.6 to 1. The highest molecular weight of the grafted polymers (M-n = 1600 g/mol, M-w = 2700 g/mol) was achieved when the thiol:ene ratio was close to 0.77:1 in the bulk and decreased monotonically for both higher and lower stoichiometric ratios. When the polymerization rate was decreased and the time scales for reaction increased, the grafted polymer molecular weight increased for stoichiometric thiol-ene monomer mixtures, because the relative effect of surface thiols was decreased.