Poly(phenylenevinylene)s as sensitizers for visible light induced cationic polymerization

A novel visible light sensitive photoinitiating system for the cationic polymerization of oxiranes such as cyclohexene oxide (CHO) and vinly monomers such as isobutyl vinylether (IBVE) and N-vinyl carbazole (NVC) using poly(phenylenevinylene) (PPV) derivatives as photosensitizers is described. The cationic polymerization of these monomers was initiated at room temperature upon irradiation in the visible region (λ_inc > 400 nm) in bulk or methylene chloride solutions with poly(phenylenevinylene)-g-polystyrene (PPV-g-PSt) or poly(phenylenevinylene)-g-poly(ε-caprolactone) (PPV-g-PCL), in the presence of oxidizing salts such as diphenyliodonium hexafluorophosphate (DPI), and N-ethoxy-2-methylpyridinium (EMP). The initiation mechanism, based on the optical absorption measurements, free energy changes (ΔG), and proton scavenging studies, involves formation of exciplex by the absorption of light in the first step. Subsequently, an electron transfer from the excited PPV-g-PSt or PPV-g-PCL to oxidizing salt occurs to yield radical cations of the PPV backbone. Thus, formed radical cations abstract hydrogen from the surrounding monomer or the solvent to release a Bronsted acid, which initiates the polymerization. Triphenylsulphonium hexafluoroarsenate (TPS) was found to be inefficient in the photoinduced electron transfer process due to the unfavorable thermodynamic conditions.