The role of components in MoOCl4-based living polymerization catalysts was studied. The MALDI TOF MS spectra of the poly(o-[(trifluoromethyl)phenyl]acetylene)s prepared with n-Bu4Sn and Et3Al as cocatalysts suggested a difference of 28(C2H4) in polymer molecular weight. This indicates that the formed polymer possesses an alkyl group of cocatalyst in the initiating chain end, which implies the transalkylation between MoOCl4 and a cocatalyst and the subsequent formation of a Mo carbene. Sterically uncrowded aliphatic alcohols were useful as the third catalyst components. Addition of EtOH to the catalyst system led to the increase of activation enthalpy and the change of activation entropy from a negative to positive value in the propagation reaction, which suggests that EtOH coordinates to the propagating end competitively with the monomer.