A Proximal Bisnitroxide Initiator: Studies in Low-Temperature Nitroxide-Mediated Polymerizations

Nitroxide-mediated "living" radical polymerization with bisalkoxyamine 2,5,5,8,8,11-hexamethyl-4,9-(1-phenylethoxy)-3,10-diphenyl-4,9-diazadodecane produces polymers of controlled length and narrow molecular weight distributions at temperatures ranging from 70 to 110 °C. Polymerizations were run successfully with styrene (St), tert-butyl acrylate (tBA), and dimethylacrylamide (DMA). EPR measurements of the homolysis of this bisalkoxyamine and monoalkoxyamine 2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane at temperatures ranging from 85 to 105 °C give rate constants for the bisalkoxyamine that are approximately twice as large as those for the monoalkoxyamine. (1)H NMR investigations of the decomposition of both alkoxyamines at 125 °C show enhanced decomposition for the bisalkoxyamine. EPR decomposition studies at 120 °C on the corresponding bis- and mononitroxides 2,5,5,8,8,11-hexamethyl-3,10-diphenyl-4,9-diazadodecane-4,9-bisnitroxide and 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide also show accelerated bisnitroxide decomposition. Low-temperature EPR studies of the bisnitroxide reveal an unusually strong radical-radical interaction, suggesting enhanced stabilization of the intermediate mononitroxide formed during polymerization by interaction with the proximal N-alkoxyamine. The transient mononitroxide is postulated to be stabilized by delocalization of the unpaired electron density over five atoms.