9-bromoanthracene photodimers as initiators in controlled radical polymerization: Silane radical atom abstraction coupled with nitroxide mediated polymerization

Slow initiation relative to propagation has previously prevented photodimers of 9-bromoanthracene or 9-chloroanthracene, formed by [4 + 4] photocyclization reactions of the analogous 9-haloanthracene, from being viable initiators in atom transfer radical polymerization (ATRP) reactions. The resulting polymers were found to possess high polydispersity index (PDI) values, much higher than expected number average molecular weight (M-n) values, with the reaction displaying a nonlinear relationship between monomer conversion and Ma. We report here the use of silane radical atom abstraction (SRAA) to create initiating bridgehead radicals in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to mediate the polymerization. When using SRAA coupled with nitroxide mediated polymerization, a dramatic decrease in PDI values was observed compared with analogous ATRP reactions, with M-n values much closer to those anticipated based on monomer-to-initiator ratios. Analysis using UV-Vis spectroscopy indicated only partial anthracene labeling (similar to 25%) on the polymers, consistent with thermolysis of the anthracene photodimer coupled with competition between initiation from the bridgehead photodimer radical and silane-based radical. (C) 2008 Wiley Periodicals, Inc.