Additive effects of donors on the initial polymerization of 1,5-hexadiene with the MgCl2-supported Ziegler catalysts were investigated by using the stopped-flow method, temperature rising elution fractionation (TREF) analysis, and kinetic study. The cyclopolymerization of 1,5-hexadiene proceeded within an extremely short period (less than or equal to 0.2 s) and yielded a unique poly(methylene-1,3-cyclopentane), cis ring content and cis-cis unit in meso dyad of the resulting polymer were increased by the addition of electron donors. The influence of internal and external donors was examined by the estimation of kinetic parameters and TREF analysis. Because the addition of the internal donor caused an obvious change in one of the kinetic parameters and the microstructure, an isospecific active site was considered to be formed by the addition of the internal donor. In the case of the external donor, the additive effects on the stereospecificity were weaker than those of the internal donor. It was expected from TREF measurements that the external donor modified an aspecific active site into a lower isospecific active site.