Charge transfer complexes as dual thermal/photo initiators for free-radical frontal polymerization

Frontal polymerization is a process in which a localized reaction zone propagates from the coupling of thermal transport and the Arrhenius rate dependence of an exothermic polymerization; monomer is converted into polymer as the front passes through an unstirred medium. Herein we report the first study of charge transfer complexes (CTCs) as photo/thermal initiators for free-radical frontal polymerization. Front velocity was studied as a function of mole ratio between an aromatic amine, such as dimethyl-p-toluidine or dimethylaniline, and an iodonium salt. It was found that the front velocity reached a maximum at a certain mole ratio of amine to iodonium salt. The velocity remained constant upon increasing the ratio of amine to iodonium salt past this critical ratio. Fronts were also studied using N-phenyl glycine as an electron donor, but its utility was limited by low solubility. Lastly, the steric and electronic effects of the iodonium salt and counter anion were explored. It was found that CTCs using iodonium salts with less nucleophilic anions gave higher front velocities. In terms of intrinsic reactivity, the CTC composed of N,N-dimethyl-p-toluidine and bis4-(tert-butyl)phenyl]iodonium tetra(nonafluoro-tert-butoxy)aluminate gave the highest front velocity per molal of iodonium salt.