Factors affecting the nascent structure and morphology of polyethylene obtained by heterogeneous Ziegler-Natta catalysts: 1. Polymerization kinetics

Kinetic measurements were carried out to study the influence of polymerization conditions on the morphology and macroconformation of nascent polyethylene obtained with a heterogeneous $\text{TiCl}{}_3\text{Al}\text{(}\text{C}{}_2\text{H}{}_5\text{)}{}_3$ Ziegler-Natta catalyst. The polymerization of ethylene in a n-paraffin viscous medium was studied for very different conditions, such as polymerization temperature from 0° to 120°C, pressures of 1 and 10 atm and stirring speed of 10 and 1000 r.p.m. It was established by electron microscopy that new polymer chains originate due to formation of new surfaces by break-up of catalyst aggregates and also through crystal scission. The average size of polymer globules growing on the catalyst surface was of the order of 10⁸cm^?2. The same kinetic relationships are approximately obeyed when the polymerization is controlled by mass transfer of the monomer. The overall activation energies for the maximum polymerization rate, decay period and the stationary polymerization rate were: 8.4, 3.4 and 5.0 kcal/mol, respectively. Molecular weight measurements combined with polymerization rate determinations lead to the conclusion that the number of growing polymer chains increases with the polymerization temperature, being of the order of 10¹²cm^?2 at low temperature. This corresponds to a concentration of active sites of about 10⁴ per polymerization locus.