Thiocarbonylthio End Group Removal from RAFT-Synthesized Polymers by Radical-Induced Reduction

The removal of thiocarbonylthio groups from polymers synthesized by radical polymerization with reversible addition−fragmentation chain transfer (RAFT) by radical-induced reduction has been studied. The efficiency of reduction is strongly dependent on the H-donor and the polymer. The effectiveness of the H atom donors studied increases in the series toluene ≪ 2-propanol < triethylsilane < triphenylsilane ≪ tris(trimethylsilyl)silane ∼ N-ethylpiperidine hypophosphite < tributylstannane. The end groups of the (meth)acrylic polymers, e.g., poly(butyl acrylate) and poly(methyl methacrylate), are more readily reduced than those of polystyrene. With poor H-donors such as toluene or 2-propanol radical−radical reaction between propagating radicals competes with reduction even when they are used in vast excess as a solvent for the process. Thiocarbonylthio groups of polymers prepared with dithiobenzoate or trithiocarbonate RAFT agents can be replaced by hydrogen by radical-induced reduction with hypophosphite salts, in particular, N-ethylpiperidine hypophosphite. Their use overcomes issues of reagent toxicity and ease of workup associated with the use of stannane and silane H-donors.