Polymerization behaviors of isoprene under different polymerization conditions with 2-(1-methyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl) pyridine cobalt(II) dichloride and ethylaluminum sesquichloride or diethylaluminum chloride catalyst system were evaluated. The effects of temperature, solvents, [Al]/[Co] molar ratio, and the structure of cocatalysts on the catalyst activity and the characteristics of polyisoprenes (PIs) were investigated and optimum conditions for synthesizing stereoselective PIs were obtained. The results showed that pre-catalyst activity was strongly influenced by polymerization temperature, [Al]/[Co] molar ratio and solvents, and the kinds of solvents and cocatalysts affected the microstructures of PIs greatly. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39703.