Trialkyl phosphites and onium salts as co-initiators in an efficient system for visible-light-induced cationic polymerization

ORGN 125

Thomas W. Nalli,, Lee G. Stanek, Rebekka H. Steidler, Krysia L. Weidell, Timothy T. Steckler, Jay Wm. Wackerly, and Missy Jo Studler, Chemistry Department, Winona State University, Winona, MN 55987
Trialkyl phosphites ((RO)3P) and diaryliodonium salts (Ar2I+) function as co-initiators of the cationic polymerization of cyclic ethers such as cyclohexene oxide (CHO). A radical initiator is also required, consistent with the central role of a radical chain reaction of the phosphite with the iodonium salt to form polymerization-starting aryltrialkoxyphosphonium salts (ArP+(OR)3). Visible photolysis of phenylazoisobutyronitrile (PAIBN) (PhN=NCMe2CN) provides a radical initiation step that makes it possible to use visible light to start the cationic polymerization. Nucleophilic attack by unreacted phosphite on the growing polymer can lead to chain transfer and lower the average molecular weight of the polymer. However, the use of a halogen-substituted phosphite, such as tris(2,2,2-trifluoroethyl) phosphite (TFP)((CF3CH2O)3P), ameliorates this problem. 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 one hour resulting in a 78% conversion to poly(cyclohexene oxide) with an average molecular weight (MW) of 25,000. In addition, we report that TFP is also an effective co-initiator for triarylsulfonium-induced cationic polymerizations.