Synthesis of Cyclic (Co)polymers by Atom Transfer Radical Cross-Coupling and Ring Expansion by Nitroxide-Mediated Polymerization

Abstract

A novel approach to prepare cyclic polymers via a combination of atom transfer radical polymerization (ATRP) and atom transfer radical cross-coupling (ATRC) is presented. A functional ATRP initiator possessing an alkoxyamine group was synthesized and used to prepare well-defined linear telechelic α-nitroxy,ω-bromo homo- and block copolymers via ATRP and subsequent thermal deprotection of the α-terminal nitroxide function. Cyclization reactions were achieved by intramolecular ATRC under high dilution. Ring expansion of a cyclic polystyrene alkoxyamine prepared by ATRC was conducted via nitroxide-mediated polymerization (NMP) of styrene and yielded a mixture of linear and cyclic polycondensates. This result is discussed in detail and explained by the persistent radical effect (PRE), thermal initiation, and the occurrence of radical crossover reactions during chain extension via NMP.