Trialkyl Phosphites and Diaryliodonium Salts as Co-initiators in a System for Radical-Promoted Visible-Light-Induced Cationic Polymerization

Trialkyl phosphites ((RO)3P) can act as co-initiators for the diaryliodonium-induced cationic polymerization of cyclohexene oxide (CHO) or THF. A radical initiation step is also required, consistent with the essential role of a radical chain reaction of the phosphite with the iodonium salt to form polymerization-starting aryltrialkoxyphosphonium salts (ArP+(OR)3). We used the visible photolysis of phenylazoisobutyronitrile (PAIBN) as the radical initiation step. The presence of multiple fluorine substituents on the phosphite, as in tris­(2,2,2-trifluoroethyl) phosphite (TFP), allows polymerization to proceed with a minimal amount of chain transfer from nucleophilic attack by the phosphite. In a typical experiment, a CHO solution of bis­(4-tert-butylphenyl)­iodonium hexafluorophosphate (0.05 M), TFP (0.1 M), and PAIBN (0.02 M) was illuminated with a 65-W compact fluorescent bulb for 1 h, resulting in a 78% conversion to poly­(cyclohexene oxide) with an average molecular weight (MW) of 25000. We also used competition experiments to determine approximate rate constants for reactions of phenyl radicals with CHO (k = 2 × 106 M–1 s–1) and with TFP (k = 2 × 108 M–1 s–1).