For anionic polymn. with a slow initiation and a rapid propagation step, anal. expressions are presented to calc. av. mol. wts. and mol. wt. distributions (MWD) for a semi-batch process in which a monomer soln. contaminated with impurities is fed to an initiator soln. in the reactor. The validity of the model was checked for the case of polymn. of isoprene by n-BuLi in n-heptane. The differences obsd. between theor. and exptl. values are explained by the changes in the rate consts. of the initiation and the propagation step caused by the change in the polarity of the reaction soln. by the continuously fed impurities. Polymers with bimodal MWD are formed if the semi-batch procedure is followed by a batch one to increase the monomer conversion.