Kinetic study of azidopentaammine cobalt(III) complex as photoinitiator and termination agent of N-vinylpyrrolidone radical polymerization

N-Vinylpyrrolidone polymerization photoinitiated at 365 and 546 nm by azido pentaammine cobalt(III) {[Co(NH3)(5)N-3](2+)} was investigated at room temp erature in an argon atmosphere. By excitation into the ligand to metal char ge transfer (LMCT), the cobalt complex showed an efficient photoredox proce ss leading to the formation of a cobalt(II) and an azide radical (N-3, Phi( photoredox) = 0.24). The same process was found to occur by excitation into the ligand field band with a low but not negligible quantum yield (Phi(pho toredox) = 0.016). Two different domains were clearly present when the plot of the rate of polymerization as a function of the cobalt(III) complex was studied; for [Co(III)] < 2.0 x 10(-4) M, the termination step mainly invol ved a mutual annihilation of growing radicals whereas an oxidative terminat ion was present in the range of 2.0 x 10(-4) M < [Co(III)] < 1.0 x 10(-3) M . Within the former domain the rate of polymerization (R-p) varied with the first power of the monomer concentration and with the square root of the a bsorbed light intensity while for the latter domain the R-p was proportiona l to the monomer concentration and absorbed light intensity. Further invest igations using the viscosity-average molecular weight data allowed us to co rroborate the proposed polymerization mechanism. (C) 2000 John Wiley & Sons , Inc.