Tris(2-phenylphenoxo)aluminum ($(2-PhC_6H_4O)_3Al$) exists as a dimeric form in toluene. When toluene-insoluble anhydrous cobalt acetate is treated with tris(2-phenylphenoxo)aluminum in toluene, the toluene-soluble adduct $(2-PhC_6H_4O)_3Al{cdot}Co(OAc)_2$ is formed. The 2-phenylphenoxo ligand in the adduct can be replaced with another aryloxo ligand to give (aryloxo)$(2-PhC_6H_4O)_2Al{cdot}Co(OAc)_2$ (aryloxo = 2-methylphenoxo, 2-isopropylphenoxo, 4-methylphenoxo, 4-isopropylphenoxo, or 4-tert-butylphenoxo). These complexes are active for butadiene polymerization without gel formation when activated with an equivalent amount of $(2-PhC_6H_4O)AlEt_2$ for 2 h. The highest activity, 175 kg/mol-Co (turnover number, 3200) was achieved with $(2-PhC_6H_4O)_3Al{cdot}Co(OAc)_2$ at $65^{circ}C$ for 2 h. The microstructure of the polymer chains is mostly trans-1,4-configuration (70-75%) with the remaining being 1,2-vinyl. The cis-1,4-configuration observed by IR is minimal (1-5%). By replacing the 2-phenylpheoxo with a 4-alkylphenoxo ligand, the amount of 1,4-configuration slightly increases, resulting in increase in the endothermic melting signal at $-30{sim}50^{circ}C$ in the DSC curve. The molecular weights of the polymers are high ($M_n$: 300000~800000) with a fairly narrow molecular weight distribution ($M_w/M_n$, 2.0-2.7).