Light induced polymer and polymerization reactions. 9—Initiation of photopolymerization by the initiator systems trisoxalatoferrate/arene onium salts

Abstract

Trisoxalatoferrate [Fe(ox) 3] 3− is suited for the effective initiation of radical photopolymerization of acrylamide only in combination with a co-initiator. As co-initiators aryldiazonium, diaryliodonium, and triarylsulfonium salts were used, which react with the formiate radical formed in the [Fe(ox) 3] 3− photolysis and, depending on their reduction potential, also with Fe(II) species. In this electron transfer reaction, aryl radicals are formed which initiate the polymerization. The efficiency of initiation increases in the order: triarylsulfonium salts ⪡ diaryliodonium salts<aryldiazonium salts. Quantum yields of polymer formation are linearly dependent on the acrylamide concentration and the square root of the quantum yields of initiator decomposition. The molecular weights of the polymer decrease with increasing [Fe(ox) 3] 3− concentration because an increasing proportion of the polymer radicals is oxidized by the initiator (k′ ox =2052 mol litre −1 s −1). The efficiency of the initiator system can be compared with that of benzoin isopropylether; its sensitivity region, however, extends up to 500 nm.