Role of Bimolecular Homolytic Substitution (SH2) Reactions in Free Radical Photopolymerization Under Air

A set of multivalent aloin-containing amines (e.g., tetrakis(dimethylamido)silane, tetrakis(dimethylamido)tin(W), tetrakis(dimethylamido)tin (IV), tetrakis(diethylamido)titanium(IV), and 1,3,5-triaza-7-phosphaadamantane) are proposed as new coinitiators ill Type 11 photoinitiating systems (based, for example, oil benzophenone) and additives for Type I photoinitiators (Such as phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide and 2-hydroxy-2-methyl-1-phenyl-1-propanone) for free radical photopolymerization under air. The polymerization rates (Rp's) were determined tinder experimental conditions where a strong detrimental oxygen effect is expected, that is, both tinder low light intensity and in thin low viscosity monomer samples. The remarkable performance achieved under air (Lip to a 100- and a 5-fold increase in Rp and final conversion (%), respectively) lies on the presence of a bimolecular homolytic substitution (S(H)2) reaction that allows Lis to convert peroxyls into new initiating radicals efficiently. The mechanisms are investigated by laser flash photolysis (LFP) and electron spin resonance (ESR) experiments.