Ethene was copolymerized with 1-dodecene and 1-octadecene to study the influence of the comonomer chain length on the rate enhancement effect where the catalyst system was stereorigid iPr-[FluCp]ZrCl2 (I) or Me2Si[Ind]2ZrCl2 (II) combined with methylaluminoxane (MAO). With catalyst II and both of the comonomers the polymerization rate of ethene increased 2.5-fold before starting to decrease above the [comonomer]/[ethene] ratio of 1. With catalyst I the situation was different With 1-octadecene the maximum in the curve of the polymerization rate of ethene was again at [comonomer]/[ethene] = 1. With 1-dodecene, on the other hand, the consumption of ethene, instead of decreasing, continued to increase above the [1-dodecene]/ [ethene] ratio of 1. The different behavior with the two catalysts and comonomers perhaps is explained by the gap aperture between the π-ligands of the two catalysts and the steric conditions at the catalytic center: the longer the comonomer, and the narrower the gap aperture, the more difficult it is for the ethene monomer unit to insert itself and the ethene propagation reaction becomes slower. The microstructure of the copolymers was investigated with Markovian first- and second-order statistics. The experimental triad distributions can satisfactorily be described with the second-order statistics. The r22 and r12 parameters obtained show that catalyst I copolymerizes α-olefins better and that, for the same catalyst, there is no marked difference in the values obtained with 1-dodecene and 1-octadecene. Furthermore, the r22 parameters were considerably greater with 1-dodecene and 1-octadecene than with 1-hexene. Evidently, a block of the long chain α-olefin opens the gap aperture between the π-ligands, facilitating insertion of an α-olefin. The addition of 1-butene decreased the reactivity of the higher α-olefin: the comonomer content decreased to half with catalyst I and to two-thirds with catalyst II when the 1-butene concentration was increased from 0 to 1 mol/dm3. Apparently, the synergistic effect works only with heterogeneous catalyst systems and not at all with the homogeneous metallocenes.