Nuclear magnetic resonance (NMR) studies on poly a-methyl-styrene prepared at high temperatures (>25°C) and having multimodal gel permeation chromatographic (GPC) molecular weight distributions built up of components D + A and B + C showed that their propagation reactions follow Bernoullian statistics, the Pmvalue being in the range of 0.34 to 0.45 for the former and 0.26 for the latter component. The NMR analyses of polymers prepared at lower temperatures (5 to 25° C) and having unimodal GPC molecular weight distributions also showed the presence of component D + A whose propagation followed Bernoullian statistics but with lower (0.25 to 0.35) Pmvalue. On further lowering the polymerization temperatures to -25 or -78°C, the reaction continued to yield component D + A; however, the propagation reaction did not correspond to Bernoullian but a first-order Markov statistics. The propagation reaction responsible for the formation of component B + C, on the other hand, always obeyed Bernoullian propagation with Pmof about 0.26. This suggests that the formation of component D + A results from a reaction mechanism which is different from that of B + C.