A proximal bisnitroxide initiator: Studies in low-temperature nitroxide-mediated polymerizations

Nitroxide-mediated living radical polymerization with bisalkoxyamine2,5,5,8,8,11-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 degrees 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 ranaing from 8 5 to 105 degrees C give rate constants for the bisalkoxyamine that are approximately twice as large as those for the monoalkoxyamine. H-1 NMR investigations of the decomposition of both alkoxyamines at 125 degrees C show enhanced decomposition for the bisalkoxyamine. EPR decomposition studies at 120 degrees C on the corresponding bis- and mononitroxides 2,5,5,8,8,1 1-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. Lowtemperature 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-alkoxyarriine. The transient mononitroxide is postulated to be stabilized by delocalization of the unpaired electron density over five atoms.