Limitations in the synthesis of high molecular weight polymers via nitroxide-mediated controlled radical polymerization: Experimental studies

Limitations associated with preparing high molecular weight polystyrene (PS) by nitroxide-mediated controlled radical polymerization have been tested by considering the role of unimolecular initiator concentration on active polymer radical concentration and thus degree of polymerization. Recent theories ignoring autopolymerization effects lead to the conclusion that, at low monomer conversion, the number-average molecular weight, M_n, scales with the -2/3 power of unimolecular initiator concentration. Bulk polymerizations were done using either α-methylstyryl di-tert-butyl nitroxide (A-T) as unimolecular initiator or PS macroinitiator made from A-T. These initiators allow relatively low reaction temperature (77, 87, or 97 °C) and moderate, but not eliminate, the contribution of autopolymerization or thermal initiation of polymerization. By varying unimolecular initiator concentration over nearly 4 orders of magnitude, well-controlled PS, with polydispersity index ≤ 1.4, can be made with M_n values in the range 114 000-238 000 g/mol using either A-T as initiator or a PS macroinitiator. For conditions yielding controlled PS, in general the experimental M_n-initiator concentration data afforded good agreement with the -2/3 power-law expression and allowed estimation of the equilibrium constant for the capping-uncapping reaction. However, attempts to make controlled, higher molecular weight PS by further reducing initiator concentration resulted in loss of control due to autopolymerization effects. The impact of autopolymerization in producing well-controlled PS was evident from studies yielding a nearly constant conversion as a function of macroinitiator concentration.