End-brominated poly(methyl methacrylate)
(PMMABr) was prepared
by atom transfer radical polymerization (ATRP) and employed in a series
of atom transfer radical coupling (ATRC) and radical trap-assisted
ATRC (RTA–ATRC) reactions. When coupling reactions were performed
in the absence of a nitroso radical traptraditional ATRC conditionvery
little coupling of the PMMA chains was observed, consistent with disproportionation
as the major termination pathway for two PMMA chain-end radicals in
our reactions. When 2-methyl-2-nitrosopropane (MNP) was used as the
radical trap, coupling of the PMMA chains in this attempted RTA–ATRC
reaction was again unsuccessful, owing to capping of the PMMA chains
with a bulky nitroxide and preventing further coupling. Analogous
reactions performed using nitrosobenzene (NBz) as the radical trap
showed significant dimerization, as observed by gel permeation chromatography
(GPC) by a shift in the apparent molecular weight compared to the
PMMABr precursors. The extent of coupling was found to depend on the
concentrion of NBz compared to the PMMABr chain ends, as well as the
temperature and time of the coupling reaction. To a lesser extent,
the concentrations of copper(I) bromide (CuBr), nitrogen ligand (N,N,N′,N′,N″-pentamethyldiethylenetriamine
= PMDETA), and elemental copper (Cu) were also found to play a role
in the success of the RTA–ATRC reaction. The highest levels
of dimerization were observed when the coupling reaction was carried
out at 80 °C for 0.5h, with ratio of 1:4:2.5:8:1 equiv of NBz:
CuBr:Cu:PMDETA:PMMABr.
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