The cationic polymerizations of isobutylene (IB) initiated by MeOH／BF3 system in CH2Cl2 were carried out at various temperatures (Tp) from -100℃ to -50℃ in the presence of different concentrations of water ([H2O]) from 1.5×10-3 mol／L to 4.6×10-3 mol／L in the polymerization systems．The effects of [H2O] and Tp on the conversion

molecular weight and molecular weight distribution (MWD) of the product polyisobutylene (PIB) were investigated in order to make use of the trace of water which is impossible to remove from the polymerization system．The equations based on molecular weight with Tp were obtained and the corresponding active energy of degree of polymerization (Ep) were also calculated at [H2O] of 1.5×10-3 mol／L

2.2×10-3 mol／L

2.9×10-3 mol／L

3.5×10-3 mol／L and 4.6×10-3 mol／L respectively．The results showed that both [H2O] and Tp did affect the cationic polymerization of IB and the molecular parameters of PIB．In the cases of different [H2O] of 1.5×10-3 mol／L

2.2×10-3 mol／L and 3.5×10-3 mol／L

the conversions increased with Tp from -lOO℃ to -80℃

reached the maximum around -70℃～-80℃

and then decreased with Tp from -70℃ to -50℃．It was shown that Tp affect on molecular weight more obviously at low [H2O] than that at high [H2O]．Molecular weight increased and MWD narrowed with decreasing Tp and／or [H2O]．On the other hand

traces of water in the polymerization systems did the disadvantageous effects on the IB cationic polymerization．All Ep was minus at different [H2O] and it increased from -40kJ／mol to -17 kJ／mol when [H2O] increased from 1.5×10-3 mol／L to 4.6×10-3 mol／L．It indicates that the disadvantageous influences of H2O on the polymerization processes

the molecular weight and its distribution would become more serious when [H2O] and/or Tp increased．H2O restrained the propagation of polymer chain and accelerated the side reactions of active centers

such as chain transfer reaction and termination

which leading to low molecular weight and broad molecular weight distribution．