Modification of ziegler–natta catalysts for propylene polymerization with organic and inorganic compounds

Abstract

Propylene polymerizations were carried out with the direct addition of CaO in a semibatch type reactor. A certain amount of CaO enhanced the activity of Ziegler-Natta catalyst, but decreased isotacticity. The results indicated that CaO accelerated the initial polymerization rates and improved catalyst against deactivation without effects on melting point, crystallinity and morphology of the polypropylene obtained. The electron spin resonance (ESR) results revealed that CaO facilitated the reduction of Ti4+ species to lower valence active Ti species. In a similar way, the addition of the SiO2-supported MAOs moderately increased the activity of Ziegler-Natta catalyst, but unchanged the properties of the polypropylene produced. Catalyst activity was improved in the order of dried-modified methylaluminoxane (dMMAO)/SiO2 > methylaluminoxane (MAO/SiO2) > modified methylaluminoxane (MMAO/SiO2) > SiO2-free > SiO2. However, the stronger interaction between the MAOs and SiO2 support caused a decrease in polymerization activity. The observed polymer microstructure reflected an increase of active site concentrations rather than the increased propagation rate constant. The effect of the chemically different poisoning materials on the activity and isospecificity of the catalyst has also been investigated. The variations in deactivation power were listed as follows: methanol > acetone > ethylacetate. Addition of the poisoning materials caused decrease in the activity through the reduction of the number of active centers which was confirmed by a kinetic analysis with the stopped-flow method, whereas the catalyst isospecificity was hardly affected by these materials.