SG1 Nitroxide-Mediated Polymerization of Isoprene: Alkoxyamine Structure/Control Relationship and α,ω–Chain-End Functionalization

Abstract

Nitroxide-mediated polymerization of isoprene was carried out at 115 °C using a small library of SG1-based alkoxyamine initiators. Well-defined polymers were obtained, with no need to add free SG1. Efficient postfunctionalization reactions in α- (reaction of ethylene diamine with a succinimidyl-ester-functional initiator) and ω-positions (replacement of SG1 by benzyl trithiocarbonate) were demonstrated. Polymerizations mediated by alkoxyamines with tertiary initiating groups were observed to follow first order kinetics with respect to monomer, with number-average molecular weights (Mn) approximately 77% of theoretical Mn (Mn,theo). Use of alkoxyamines with primary and secondary initiating groups gave 2/3 order kinetics with respect to monomer and slightly lower molecular weights (Mn,exp ∼ 0.61 × Mn,theo). At higher conversions, the polydispersity of all polyisoprenes approached 1.1. At low conversions, however, significant differences in polydispersity were observed, indicating differences in the rate of consumption of the initiators. Significantly, the alkoxyamines with propionic acid and isobutyric acid initiating groups were consumed substantially more slowly than their ester analogues or even cyanomethyl-SG1. This may be a result of intramolecular hydrogen bonding between the acid and phosphonate groups of the initiators.