Copolymerization of Propylene and Vinylcyclohexane with the Metallocene Catalyst: Effect of Side Cyclohexyl Groups on Oxidation and Properties of Polypropylene

摘要

Copolymerization of propylene with vinylcyclohexane in liquid propylene medium at 60℃ with effective isospecific homogeneous metallocene catalyst rac-Me_2Si(4-Ph-2-MeInd)_2ZrCl_2 activated by polymethylaluminoxane has been investigated. Random copolymers with comonomer content from 0.02 to 0.37 mol % were obtained. It was shown that introduction of comonomer lead to decreasing of catalytic activity and copolymer molecular weight. The effect of the comonomer on thermophysical and mechanical properties of copolymers obtained were studied. Polypropylene modification with low content of cyclohexyl groups allows to change these characteristics significantly (decrease of melting temperature and elongation at break and increase in crystallization temperature and Young's modulus). Characteristics of oxidation of copolymers obtained have been measured: the induction periods for the consumption of oxygen and the rate constants of termination of peroxyl macroradicals determined by chemiluminescence of copolymers. The complex dependence of kinetic parameters of oxidation of copolymers on content of side cyclohexyl groups has been found. The induction period of oxidation and the rate constant of radical termination go through a maximum with increasing content of side groups at low levels of cyclohexyl groups. Then the induction period of oxidation is increased and the rate constant of radical termination is decreased under the further increase in the comonomer content. These results indicate that effect of side cyclohexyl groups on copolymer oxidation can occur through two mechanisms: on the one hand they disturb the lengths of regular sequences in the main chain of crystallizing PP, leading to a decrease in ordering of molecules and an increase in their mobility, but on the other hand a structure-forming effect of side groups becomes important with increasing their concentration that leads to formation of physical network and to corresponding decrease in molecular mobility and oxidation rate.