Trialkyl Phosphites and Diaryliodonium Salts as Co-initiators in a System for Radical-Promoted Visible-Light-Induced Cationic Polymerization
2013-04-19T00:00:00Z (GMT) by
Trialkyl phosphites ((RO)<sub>3</sub>P) 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<sup>+</sup>(OR)<sub>3</sub>). 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-<i>tert</i>-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 (<i>M</i><sub>W</sub>) of 25000. We also used competition experiments to determine approximate rate constants for reactions of phenyl radicals with CHO (<i>k</i> = 2 × 10<sup>6</sup> M<sup>–1</sup> s<sup>–1</sup>) and with TFP (<i>k</i> = 2 × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup>).