Electron donors (EDs) effectively mediate living carbocationic polymerizations (LC(+)Pzn), for example, that of isobutylene (IB). The purpose of this research was to investigate the effect of various new EDs, specifically esters and ketones on the LC(+)Pzn of IB, and to determine the effect of these EDs on the molecular weight distribution, (MWD) of polyisobutylene (PIB) under various experimental conditions. Thus the LC(+)Pzn of IB was effected by the dicumyl chloride (DCC)/BCl3 initiating system in the presence of various EDs, such as dimethyl phthalate, di-t-octyl phthalate, methyl benzoate, ethyl isobutyrate, dimethyl 3,3-dimethylglutarate, dimethyl terephthalate, dimethyl 5-t-butylisophthalate, trimethyl 1,3,5-benzenetricarboxylate; and acetophenone and benzophenone, in CH3Cl at -80 degrees C, and the ($) over bar M(w)/($) over bar M(n) of the PIB was determined. It was found that the nature and the concentration of the ED strongly influence MWD. Under well-defined conditions in the presence of dimethyl phthalate or di-t-octyl phthalate, polymers with close to Poisson distribution (($) over bar M(w)/($) over bar M(n)=1.03) could be obtained.