Zirconocene-catalyzed propene-ethene copolymer elastomers: Kinetic investigations at low ethene concentration and characterization of microstructure

摘要

The copolymerization of propene with ethene in feed concentrations up to 9 mol-% ethene was investigated in dependence on catalyst substitution, activation and process conditions. Polymerization reactions were performed using rac-[ethylene-bis(eta(5)-1-indenyl)]ZrCl2 (1a), [bis(eta(5)-1-indenyl)dimethylsilane]ZrCl2 (2), and [bis(eta(5)-1-(2-methylindenyl))dimethylsilane]ZrCl2 (3a) after MAO- and rac-[ethylene-bis(eta(5)-1-indenyl)]Zr(CH3)(2) (1b) as well as [bis-(eta(5)-1-(2-methylindenyl))dimethylsilane]Zr(CH3)(2) (3b) after borate-activation. A detailed study of the polymerization kinetics of the different metallocene catalysts was performed by using an autoclave system that allowed to follow the changes in concentration of propene and ethene in the gas and in the liquid phase, even for the first minutes of the polymerization experiments. It is known that 2-methyl substitution retards the rate of chain transfer to propene monomer, leading to increase molecular weights of the resulting homopolypropenes. However, the catalyst system 3a/MAO, bearing a CH3-substituent in 2,2'-position of the indenyl moieties, shows a significant decline of the molecular mass of propene-ethene copolymers by rising the ethene concentration, while catalysts 1a and 2, having no methyl substitution, give a distinct increase of molecular weight. Our investigations show that the observed molecular weight decline is independent of the process conditions, but it depends clearly on the catalyst, substitution and, in some cases, on the activation process. beta-Hydride elimination to the Zr(IV)-center is the dominating chain termination process for metallocene catalysts bearing methyl groups in the 2,2'-position. Chain termination via transfer to monomer and to aluminum, are the preferred termination mechanisms for complexes lacking the CH3 substituents. [References: 18]