Study On The Preparation Of Novel Ziegler-Natta Catalysts With Low Ti-loading Amount Using One-pot Ball Milling Method And Their Application In Olefin Polymerization

Ziegler-Natta (Z-N) catalysts are an important class of commercial catalysts for the preparation of polyolefins. However, some problems such as how to improve the method of preparing the Z-N catalysts to control the type and distribution of active centers, propagating mechanism of the polymer chain and polymerization kinetic of some special catalyst systems still are unclear completely. It is of great theoretical significance and practical value to clear above metioned problems for development of new polymer material with special structures and properties.In this work, a new route for preparing MgCl₂-supported and low Ti-loading Z-N catalysts using a new one-pot ball milling method was developed, and a series of MgCl₂-supported and low Ti-loading Z-N catalysts, such as TiCl₄(0.4%)/MgCl₂ and TiCl₄(0.8%)/MgCl₂, were prepared successfully. This route is easy to be industrialized and the resultant catalysts can be employed in a study of propylene polymerization with AlEt₃ as cocatalyst. It was found that the resultant polymer had low isotactic and block microstructure, and can be widely used in many fields, for example, as additive for building top grade high way.The influence of polymerization temperature, catalyst concentration and [Al]/[Ti] molar ratio on catalytic activities and polymerization kinetics has been investigated. The results showed that the better polymerization conditions were: Ti-loading amount=0.8(wt)%, polymerization temperature =40°C, [Al]/[Ti]=30. Changing polymerization conditions, a series of novel low isotactic polypropylene and block polypropylene were prepared successfully. Using the resultant novel low isotactic polypropylene and block polypropylene as modifier for asphalt, it was found that it had evident effects to improve the properties of asphalt.In this work, the complex GPC data of polypropylene synthesized by MgCl₂-supported and low Ti-loading Z-N catalyst were analyzed using the method of fitting the molecular weight distribution (MWD) curves with multiple Flory-Schulz functions. It was found that these MWD curves could be actually fitted with 6 Flory-Schulz most-probable distributions with different position and area, which showed that it was a reasonable suggestion that there exist six types propagating way of the polymer chain in the polymerization system.Based on microstructure character of obtained polypropylene, we developed a new two-site propagating model: the polymer chain propagates at two-site which can intertransform. Site-1 obeys the enantiomorphic-site model while site-2 follows the chained-end growing dependent model, and the propagation of the polymer chain are controlled by the site-1 and site-2 alternately. The theoretical expressions of the pentad intensities of the polypropylene based on this new propagating model were deduced. A novel multi-active-centers propagating model in the propylene polymerization catalyzed by MgCl₂-supported and low Ti-loading Z-N catalysts prepared using a new one-pot ball milling method was also proposed. After simulating the pentadintensities of the resultant polypropylene using multi-active-centers model, it was found that the calculated results from this new model could satisfactorily account for the experimental data, which showed the proposed model is plausible and reasonable.Based on six types propagating way of the polymer chain in the MgCl2-supported and low Ti-loading Z-N catalysts prepared using one-pot ball milling method, a polymerization kinetics model was proposed as following:When tto: RM = *;M(Ai^('-'"> + pl2e*<^ + Pu) + k]M^e^ + p^'^ + pj-* + Pl9) + klM(p2Oe^ + p2]e?'-'-> + p22)Applying this kinetics model, the good agreement between the experimental data and the fitting profiles was achieved, which indicated the proposed model is reasonable.Molecular weight and its distribution of the polyethylene have extensively effects to the product properties. And so it is of great significance to adjust the molecular weight distribution of the polyethylene for developing novel kinds of polyethylene with high properties. In this work, MgCb/SiC^—bi-supported Z-N catalysts were prepared using the one-pot ball milling method and were employed in ethylene polymerization with AlEt3 as cocatalyst. The results showed that by changing the molar ratios of two component of supports in the catalysts and polymerization temperature, the molecular weight and molecular weight distribution of the resultant polyethylene could be widely adjusted.The copolymerizations of ethylene/ct-olefin catalyzed by the catalysts prepared using the one-pot ball milling method were studied. A series of copolymer with various content of hexane and dodecene were synthesized successfully. It was found that these catalyst systems had stable kinetic curves and the content of co-monomer in the resultant copolymer could be widely adjusted, which showed that these catalysts might be useful to commercial preparation of polyolefins.