The free-radical copolymerization of m-isopropenyl-alpha,alpha'-dimethylbenzyI isocyanate (TMI) and styrene was studied with H-1 NMR kinetic experiments at 70 degreesC. Monomer conversion vs time data were used to determine the ratio k(p) x k(t) (0.5) for various comonomer mixture compositions (where k(p) is the propagation rate coefficient and k(t) is the termination rate coefficient). The ratio k(p) X k(t) (0.5) varied from 25.9 X 10 (3) L-0.5 mol(-0.5) s(-0.5) for pure styrene to 2.03 X 10 (-3) L-0.5 mol (0.5) s (0.5) for 73 mol % TMI, indicating a significant decrease in the rate of polymerization with increasing TMI content in the reaction mixture. Traces of the individual monomer conversion versus time were used to map out the comonomer mixture composition drift up to overall monomer conversions of 35%. Within this conversion range, a slight but significant depletion of styrene in the monomer feed was observed. This depletion became more pronounced at higher levels of TMI in the initial comonomer mixture.