Abstract: MoCl₄in ethyl acetate is of a brown-red color and paramagnetism, which is the result of three absorption bands in the red and blue parts of the spectrum due to the d-d spin-allowed transtition. The change in the types of ligands attached to Mo(Ⅳ), such as C^-, C₈H₁₇O^-, PhO^-, Bd, C₈H₁₇OH, PhOH and CH₃COOC₂H₅, has no influence on the position of the absorption band. The peak-value of characteristic absorption band of Mo (Ⅳ) complex at 512 nm varies with the amount of cocatalyst added into Mo-Al system. The disappearance of the characteristic absorption band implies that Mo(Ⅳ) is reduced to a low-valency state Mo(Ⅲ). The reduction of Mo(Ⅳ) is improved and the catalytic activity is raised in the aging catalyst system MoCl₄-2(OC₈H₁₇)₂-Bd-Al(i-Bu)₂OPhCH₃.

Key words: Ziegler-Natta catalyst, Molybdenum tertrachloride, UV-Vis spectrum, Polybutadiene