AbstractThe anionic preparation of monodisperse polymers of predictable molecular weight, by simple techniques involving conveniently purified starting materials, has been investigated using styrene as a model. Active impurities, predominantly water, oxygen and carbon dioxide present in the solvent, the monomer and in the reaction system which would cause deactivation of propagation centers, are rendered inactive prior to polymerization by gradual addition of (colored) initiator at a temperature low enough to suppress chain growth. Upon completion of the purging step, indicated by the persistence of colored propagation centers, additional initiator appropriate for the molecular weight of the sample desired\documentclass{article}\pagestyle{empty}\begin{document}$${\rm \bar M}_{{\rm nk}} = \frac{{ {\rm monomer} \left( {\rm g.} \right) }} {{ {\rm initiator} \left( {\rm moles} \right) }}$$\end{document}is added. The system is then warmed to the polymerization temperature at which the reaction is allowed to go to completion. The predictability of the molecular weights was found to be within 10% for two polymers prepared. The M̄w/M̄nratio, calculated from fractionation data, was 1.06 in both case