Soluble mu-oxo-bimetallic trinuclear alkoxides are known to be very effective catalysts for the ring-opening polymerization of oxiranes. In the particular case of methyloxirane, oligomers are formed in addition to high molecular weight polyethers. An alkoxide-alcohol exchange reaction has proved to be an efficient way to form oligomers preferentially, when the monomer-alcohol molar ratio is small. Upon increasing this ratio, formation of high MW chains occurs increasingly. The use of Ti instead of Al in Al, Zn mu-oxo-n-butoxide in the presence for phenoxyethanol as a co-initiator inhibits formation of exceedingly long polyether chains and the chain length is basically controlled by the monomer to all potential initiating species molar ratio until MW of ca. 6000. Substitution of phenoxyethanol by polypropylene glycol has led to oligomers of a monomodal MW distribution and a MW of 13,000. This study has highlighted that conditions unfavorable to coordination of the polyether chains onto the catalyst improve the control of the oligomer formation.