Ethene homopolymerization and copolymerization with 1-hexene were catalyzed by methyl-substituted cyclopentadienyl (Cp) zirconium dichlorides, (RnC5- H5-n)(2)ZrCl2 (R-n = H, Me, 1,2-Me-2, 1,3-Me-2, 1,2,3-Me-3, 1,2,4-Me-3, Me-4, or Me-5), and methylaluminoxane. The polymers were characterized with Fourier transform infrared, nuclear magnetic resonance, gel permeation chromatography, and differential scanning calorimetry techniques. Generally, an increasing number of methyl substituents on the Cp ligand results in lower l-hexene incorporation in the copolymer. The two catalysts with split methyl substitution (R-n = 1,3-Me-2 and R-n = 1,2,4-Me-3) show a higher comonomer response than their disubstituted and trisubstituted counterparts (R-n = 1,2-Me-2 and R-n = 1,2,3-Me-3). They even incorporate more 1-hexene than R-n = H and R-n = Me. These findings are qualitatively in agreement with the results of a theoretical study based on density functional calculations. The presence of comonomer does not influence the termination reactions after the insertion of ethene. There is more frequent termination after each hexene insertion with increasing comonomer incorporation except for the two catalysts with split methyl substituents. The termination probability per inserted comonomer is highest for the less substituted catalysts.