On the basis of analysis of key engineering factors predominating in cationic polymerization, butyl rubber (IIR) as an example was synthesized by cationic polymerization in the high-gravity environment generated by a rotating packed bed (RPB) reactor. The influence of the rotating speed. packing thickness, and polymerization temperature on the number average molecular weight (M-n) of IIR was studied. The optimum experimental conditions were determined as rotating speed of 1200 r min(-1), packing thickness of 40 mm and polymerization temperature of 173 K, where IIR with M-n of 289,000 and unimodal molecular weight distribution of 1.99 was obtained. According to the experimental results and elementary reactions, a model for the prediction of M was developed, and the validity of the model was confirmed by the fact that most of the predicted M(n)s agreed well with the experimental data with a deviation within 10%. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 1053-1062, 2010