A kinetic model for chain propagation that can account for reaction orders higher than one on monomer concentration, observed for several insertion polymerizations, is discussed. This model is based on the presence of a "single-center, two-state catalyst" system, where the slower state is of lower energy and the interconversion rate between the two states is intermediate between the fast and slow propagation rates. This kinetic model fits available experimental data on polymerization rates, as a function of propane concentration, in the presence of zirconocene catalysts. Different possibilities for the chemical nature of the two catalyst states are discussed.