Various solvents and cocatalysts were examined for the purpose of achieving high initiator efficiency in the living polymerization of substituted acetylenes catalyzed by MoOCl4-based catalysts. Eventually, anisole and n-Bu4Sn were found to be very useful as solvent and cocatalyst, respectively. The initiator efficiency of the MoOCl4-n-Bu4Sn-EtOH (1:1:2)/anisole system was as high as 40% in the polymerization of o-CF3-phenylacetylene, whereas the value for the corresponding toluene system was no more than 10%. o-Me3Si-phenylacetylene, 1-chloro-1-octyne, 2-octyne, and 1-chloro-2-phenylacetylene and a few other monomers were also polymerized in a living manner by the present catalyst/solvent system, the initiator efficiencies being 20-40% and larger than those of the previous systems.