Kinetic studies of two-step polymerization of propylene over a titanium-aluminum catalyst
Polimery 1997, No 10, 620
The influence was studied of the modification of an unsupported titanium-aluminum catalytic system with an internal Lewis base incorporated, via prepolymerization, on the activity of this system in propylene polymerization. In order to clarify the reasons for the activating effect of prepolymerization, comparative studies were carried out on the kinetics of polymerization catalyzed by a titanium system and its prepolymerized counter part. Based on the kinetic model suggested for the polymerization mechanism, the concentration of active sites and rate constants were determined for chain propagation, chain transfer and termination reactions. The following conclusions were deduced: (i) the initial rateof propylene polymerization is higher over a prepolymerized catalyst on account of the increased number of active sites formed, (ii) in the course of polymerization, the difference between the process rates offered by two catalytic systems becomes higher, on account of different rates of the chain termination reactions, the rate over the prepolymerized catalyst being slowerowing to advantageous changes in its morphology, (iii) prepolymerization has no influence on the reactivity of active sites as the propagation rates in either case are similar, just like the molecular characteristics of the polymers obtained.