Nitroxide-mediated radical polymerization/increase of steric demand in nitroxides. How much is too much?

The influence of increasing steric demand in the nitroxide moiety of cyclic alkoxyamines derived from side chain modified TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) derivatives on nitroxide-mediated polymerization (NMP) of n-butyl acrylate and styrene is described. Polymerizations at low temperatures (as low as 70 degrees C) with good control over the molecular weight distribution (polydispersity index, PDI < 1.2) can be achieved using sterically highly demanding nitroxides. Polymerization efficiency increases with the size of the nitroxide substituents. It is shown that in these sterically highly hindered alkoxyamines it is not the rate of homolysis (k(d)) but the rate of recombination (k(c)) of the persistent nitroxide with the transient macroradical that is mainly responsible for the enhanced polymerization efficiency (lowering of the reaction time). However, for too bulky nitroxides, the trapping reaction of the nitroxide with the macroradical becomes too slow and uncontrolled NMP is obtained.