Metallocene catalysts copolymer production

Commercial production of PE resias with densities of 0.925 and 0.935 g/cm was started ia 1968 ia the United States by Phillips Petroleum Co. Over time, these resias, particularly LLDPE, became large volume commodity products. Their combiaed worldwide productioa ia 1994 reached 13 X 10 metric t/yr, accouatiag for some 30% market share of all PE resias ia the year 2000, LLDPE productioa is expected to iacrease by 50%. A aew type of LLDPE, compositioaaHy uniform ethylene—a-olefin copolymers produced with metallocene catalysts, was first introduced by Exxon Chemical Company in 1990. The initial production volume was 13,500 t/yr but its growth has been rapid indeed, in 1995 its combiaed production by several companies exceeded 800,000 tons. 

A. Akimoto and A. Yano, Production of ethylene copolymers with metallocene catalysts at high pressure and its properties, MetCon 94 Proceedings, USA, May 1994. 

Some of the drawbacks of the metallocene catalysts are their limited temperature stability and the production of lower-molecular-weight materials under commercial application conditions. It follows that they have a limited possibility for comonomer incorporation due to termination and chain-transfer reactions prohibiting the synthesis of block copolymers by sequential addition of monomers. This led to the development of half-sandwich or constrained geometry complexes, such as ansa-monocyclopentadienylamido Group IV complexes (67) 575,576... 

It should be mentioned that many of the requirements necessary for the economic production of polyethylene and polypropylene have been achieved. However, catalysts of greater activity and of greater selectivity in the production of polymers and copolymers can be anticipated. This is of prime concern to alkene polymerisation processes in the presence of single-site metallocene catalysts. Such catalysts, undoubtedly of great scientific and commercial importance, have been developed on a large scale within recent years [29,30],... 

The rate of polymerization was determined from the amount of polymer obtained per unit of time. The productivity of the metallocene catalyst was calculated from the quantity of polymer and the catalyst metal fed into the reactor. The resulting polymers were investigated by gel permeation chromatography (GPC) to determine the molecular weight distribution together with the average molecular weights. The density was measured on pressed films by means of the suspension method in a mixture of water and isopropanol. 13C-NMR-spectroscopy was applied to analyze the composition of copolymers and to evaluate their structure. 

Of great industrial interest are the copolymers of ethene and propene with a molar ratio of 1/0.5, up to 1/2. These EP-polymers show elastic properties and, together with 2-5 wt% of dienes as third monomers, they are used as elastomers (EPDM). Since they have no double bonds in the backbone of the polymer, they are less sensitive to oxidation reactions. As dienes, ethylidenenorbomene, 1,4-hexadiene, and dicyclopentadiene are used. In most technical processes for the production of EP and EPDM rubber in the past, soluble or highly disposed vanadium components are used [69]. Similar elastomers can be obtained with metallocene/MAO catalysts by a much higher activity which are less colored [70-72]. The regiospecificity of the metallocene catalysts toward propene leads exclusively to the formation of head-to-tail enchainments. The ethylidenenor-bornene polymerizes via vinyl polymerization of the cyclic double bond and the tendency to branching is low. The molecular weight distribution of about 2 is narrow [73]. 

Metallocenes are very versatile catalysts for the production of polyolefins, polystyrene and copolymers. Some polymers such as syndiotaetic polypropene, syndiotactic polystyrene, cycloolefin copolymers, optically active oligomers, and polymethylenecycloalkenes can be produced only by metallocene catalysts. It is possible to tailor the microstructure of polymers by changing the ligand structure of the metallocene. The effect and influence of the ligands can more and more be predicted and understood by molecular modeling and other calculations. 

Nowadays homogeneous metallocene catalysts activated with oligomeric methylalumoxanes or other co-catalysts [16, 20, 46-54] open new prospects. These systems have an excellent activity, they have the ability to form random copolymers in combination with a narrow molecular mass and comonomer distribution. Further important advantages are that a broad variety of structures can be synthesized to obtain tailor-made catalysts [49, 53], and that zirconium compounds are scarcely reduced with the co-catalyst [54]. It is further reported that metallocenes have been used in combination with methylalumoxanes for EPDM production at temperatures below 100 °C in liquid propylene [55]. 

Within the past five years, commercial interest in metallocene catalyst components for the polymerization of olefins has increased enormously. Commercial production of a rising number of polyolefin types from different companies is creating a burgeoning and highly diversified demand for metallocenes. New brand names (e. g., Metocene (Basell), Elite (Dow Chemical), Engage (DuPont), Exact (ExxonMobil), Luflexen (Basell), Apel (Mitsui Chemicals), Borecene (Borealis), Finathene (TotalFinaElf), Topas (Ticona), just to name a few) characterize polyolefins such as PE, elastomers, PP, cycloolefin copolymers (COCs) and PS from metallocene-type catalysts [1-3]. 

Kashiwa, N. Fukui, K. Process for production of 4-methyl-1-pentene polymer or copolymer. US Patent 4,659,792, December 23, 1985 Kaminsky, W. The discovery of metallocene catalysts and their present state of the art. J. Polym. Sci. Part A Polym. Chem. 2004, 42, 3911-3921. Hlatky, G.G. Heterogeneous single-site catalysts for olefin polymerization. Chem. Rev. 2000, 100, 1347-1376. 

Metallocene catalysts were commercialized in 1991 by Exxon Chemical for the industrial production of ethylene-propylene (EP) elastomers in solution polymerization using zirconocene catalysts [37], As a result of extensive research of different metallocenes applied to the stereoregular control of polymeric materials, these systems were able to produce novel polymers such as syndiotactic polystyrene and ethylene-styrene copolymers, which were not possible to produce with traditional Ziegler-Natta catalysts. 

Kinetic data on olefin polymerization by polymer-immobilized zirconocene are scarce. It is generally accepted that homogeneous metallocene catalysts contain uniform active sites however, if they are immobilized on a polymer support, the MWD polymer production becomes broader compared with a homogeneous catalyst [103]. Kinetic analysis of gas-phase ethylene polymerization catalyzed by (CH3)2[Ind]2ZrCl2 bound at a hydroxylated copolymer of styrene with divinylbenzene and previously activated with MAO (0.17 wt.% Zr) has been carried out [104]. The influence of temperature (333 to 353 K), ethylene partial pressure (2 to 6 atm) and MAO level (molar ratio of MAO to zirconium from 2600 to 10,700) were studied. The activity of the catalyst in the gas-phase process changed from 5 to 32 kg PE (g of Zr atm h)It is possible that there are two types of active site. They are stable to temperature and deactivated by the same mechanism. A first-order reaction takes place. The propagation rate constants of two active sites show a similar dependence on temperature. 

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