Progress in advanced catalyst and their use in ethylene/propylene copolymerization

Ethylene-propylene rubber (EPR) and ethylene-propylene-diene rubber (EPDM) have important market shares of the industry due to their excellent resistance toward heat, oxidation, weather, and other chemicals. EPR and EPDM have been synthesized by ethylene/propylene copolymerization and ethylene/propylene/nonconjugated diene copolymerization, respectively, employing Ziegler-Natta, metallocene and metal complex catalysts. Catalyst plays important roles in the copolymerization of ethylene and propylene. The progress of Ziegler-Natta, metallocene and metal complex catalysts and their use in ethylene/propylene copolymerization were reviewed. The characteristics and application in the industry of these advanced catalysts were introduced. The effects of metals and their valence in catalysts, chemical structure of ligands, cocatalysts and additives on the catalytic activity, composition, molecular weight, molecular weight distribution and sequence of the resulting ethylene-propylene copolymer were discussed. Vanadium based Ziegler-Natta catalyst is still promising in ethylene/propylene copolymerization. The open active site of vanadium catalyst results in random ethylene-propylene copolymers, which prevent the formation of long crystallized ethylene sequences. Meanwhile, the good solubility in alkanes and low cost of vanadium-based Ziegler-Natta catalyst are benefit for the application for the production of ethylene-propylene rubber in industry. About 70% of the commercial E/P copolymers are conducted by using vanadium based Ziegler-Natta catalysts. However, the unstable active centers brought low activity and the catalyst residues should be removed from the polymer product after deactivation. In order to improve the catalytic activity of vanadium-based Ziegler-Natta catalyst, the additives including electron donor and promoter are usually introduced to the catalytic system. The reactivity ratio of the monomer and the microstructure of the resulting ethylene-propylene copolymer could be adjusted by the additives. Metallocene catalyst, as a single-site catalyst, becomes a promising choice for ethylene/propylene copolymerization because of its high activity, high thermal stability and excellent copolymerization ability. The microstructure of resulting ethylene-propylene copolymer could be controlled by modification of chemical structure of metallocene catalyst. The alternating ethylene-propylene copolymer could be prepared by ethylene/propylene copolymerization using bridged zirconocenes. The half-titanocenes containing monodentate anionic nitrogen ligand exhibit high catalytic activity for ethylene-propylene copolymerization in high reaction temperature. Some of metallocene catalyst such as constrained geometry catalyst and half-titanocene catalyst have been applied in the production of ethylene-propylene rubber. The metal complex catalyst for olefin coordination polymerization and copolymerization have attracted considerable attention. The titanium, zirconium and vanadium complexes exhibit high activity for ethylene/propylene copolymerization and ethylene-propylene copolymers with high and ultra-high molecular weight could be prepared by using these complex catalysts. Living copolymerization of ethylene and propylene is achieved by using titanium complex containing phenoxy-imine ligands. The application in industry for the preparation of ethylene-propylene rubber using complex catalyst is still on the way. In conclusion, the high catalytic activity and propylene incorporation in ethylene/propylene copolymerization could be obtained by using advanced catalysts. In addition, the molecular weight, molecular weight distribution, sequence and topological structure could be controlled by advanced catalysts. Therefore, ethylene-propylene rubber with high performance could be prepared by ethylene-propylene copolymerization using advanced catalysts.