Cyclopolymerization of nonconjugated dienes produces poly(methylene-1,3-cycloalkanes) and provides a pathway to a number of stereochemically complex polymers. Activation of a diastereomeric mixture of a six-membered metallacycle complex (rac-1) in the presence of 1,5-hexadiene produced poly (methylene-1,3-cyclopentane) (PMCP) with >98% cyclization of the diene monomer. The catalyst was found to cyclopolymerize 1,5-hexadiene with relatively high activity. The microstructure of the PMCP furnished by rac-1 was found to contain a high proportion of cis-cyclopentane rings (sigma = 0.70-0.74) and a relatively high isotactic content (alpha = 0.93-0.96). These are the first cis-enriched isotactic cyclopolymers of 1,5-hexadiene. Cyclopolymerization of 1,6-heptadiene with rac-1/B(C6F5)(3) produced poly(methylene-1,3-cyclohexane) containing 97% cis-isotactic rings. This is the first report of this highly isotactic and diastereomerically-pure microstructure.