Frederick John Karol

Studies with High Activity Catalysts for Olefin Polymerization

Abstract Catalysis continues to play a vital role in polymerization of such olefins as ethylene and propylene. A voluminous patent and scientific literature describing transition metal catalysts for olefin polymerization has emerged since the original discoveries by Ziegler, Natta, and other workers [1–6], Significant progress in polymerization catalysis has been made in the last 15 years, particularly with the development of methods to increase the efficiency of transition metal catalysts in olefin polymerization. Success in this area has provided the basis of simplified, less costly plant operations which do not require removal of residual catalyst from the polymer [3–9].

Role of silanol groups in formation of supported chromocene catalysts

Abstract Deposition studies have established maximum values for the adsorption of chromocene on dehydroxylated silicas. The process of chemisorption of chromocene changes from reaction of predominantly two to one hydroxyl group as the temperature of silica increases from 100 to 800 °C. Chromocene, deposited on Cab-O-Sil type silicas heated at 200 and 400 °C, formed highly active catalysts for ethylene polymerization. Results from studies with this support are compatible with an active site model which involves reaction of chromocene with free, isolated silanol groups. Sterically hindered hydroxylic compounds such as triphenylsilanol and t-butanol react in solution with chromocene to form dimeric cyclopentadienyl chromium alkoxides. These chromium compounds do not show catalytic activity for ethylene polymerization under conditions typical for the ( C 5 H 5 ) 2 Cr SiO 2 catalyst. Furthermore, deposition of these new chromium compounds on silica did not provide, in most cases, a route to catalytic activity. However, the addition of alkylsilanes to these supported chromium compounds did lead to active catalysts. The polymerization behavior of these catalysts resembles the supported chromocene catalyst. These overall results lend support to an active site model previously described.

Developments with High-Activity Titanium, Vanadium, and Chromium Catalysts in Ethylene Polymerization

Recent developments with high-activity catalysts based on titanium, vanadium, and chromium -compositions continue to illustrate the unique behavior of each metal center in ethylene polymerization catalysis. Bimetallic complexes containing magnesium-titanium- or magnesiumaluminum-electron donor complexes have been identified and characterized. Interestingly, studies with TiCl3 reveal that complex formation with MgCl2 is not required for obtaining high-activity catalysts.

New Catalysis and Process for Ethylene Polymerization

Olefin polymerization catalysis continues to be a fertile area of research with worldwide participation in both industrial and academic laboratories.1–3 While much of this research has centered on methods to increase the productivity of catalysts, there has been and continues to be much active research on other features of olefin polymerization catalysis. The specific composition of the catalyst exerts an important effect on polymer molecular weight and molecular weight distribution (MWD), comonomer incorporation and copolymerization kinetics, and on the degree of stereoregularity. Moreover, the size, shape, and porosity (morphology) of the catalyst particle plays an important role in regulating the morphology of the resultant polymer. Development of low cost, reproducible processes for catalyst manufacture continues to be another important objective in catalyst research.4 The focus of industrial research in olefin polymerization catalysis centers on the chemistry and technology necessary to obtain simultaneously favorable catalyst responses in all of the areas described above.

Features of Cyclopentadienyl Metal Catalysts for Ethylene Copoly-Merization in Gas and Liquid Phase

Catalysis in gas phase reactions for the production of olefin polymers is well known in the polyolefin industry [1]. Developments in the UNIPOL® process for polyethylene, EPR/EPDM, and polypropylene continue to demonstrate the broad versatility of the process. The emergence of metallocene catalysis in many laboratories around the world has added yet another significant catalytic tool for manipulating and controlling the molecular framework of polyolefms. Selection of the appropriate metallocene catalyst, with control of ligand environment at the active site, continues to provide the basis of improved process operations and unique product opportunities.

Catalysis and the Unipol Process

Production of low-density polyethylene (LDPE) is undergoing the kind of revolution not seen in the field since the discoveries by Ziegler and Natta. Union Carbide has developed a unique and versatile low-pressure, fluid-bed process (UNIPOL) that yields vastly improved polyethylene resins, linear low-density polyethylenes (LLDPE), at greatly reduced costs. Proprietary catalysts are key to success of the UNIPOL process. Catalysts have an important effect on productivity, polymer molecular weight, polymer molecular weight distribution, copolymerization kinetics, and degree of stereoregu-larity. Moreover the size, shape, and porosity (morphology) of the catalyst particle play an important role in regulating the morphology of the resultant polymer.

Catalysis and the Polyethylene Revolution

The world of polyolefins is enormous (Table I).[1a] Indeed the world of polyethylene itself is very large and is undergoing a major revolution (Table II).[1b-4]. Developments in catalysis for polyethylene in the last decade have had a major impact on the polyethylene industry. After nearly a half century of producing low-density polyethylenes (LDPE) at 20,000–50,000 psi and 300°C (Figure 1), new technology capable of operating at less than 300 psi and near 100°C has emerged. Union Carbide Corporation has developed a unique and versatile low-pressure, gas-phase process that does away with the extremely high pressures and temperatures characteristic of the conventional processes for making low-density polyethylenes. The scientific community and industrial organizations around the world have recognized Union Carbide’s gas-phase, UNIPOL process, as a major technological accomplishment.[2]

37. Ligand Effects at Transition Metal Centers for Olefin Polymerization

Abstract Ethylene polymerization catalysts based on titanium, vanadium, chromium, and zirconium display dramatic changes when the ligand environment at the active site is altered. Catalysts with a ligand environment involving chloride, alkoxide, electron-donor compounds, alkyl, cyclopentadienyl, or indenyl groups show distinctive polymerization characteristics. These characteristics include polymer molecular weight and molecular weight distribution. Catalyst response parameters such as comonomer incorporation, hydrogen response, and catalytic activity are influenced by ligand environment at the active site. Changes in the level of unsaturation at active metal centers can also have a profound effect on polymerization kinetics and the nature of the final reaction products. Alpha-olefins play a subtle role in altering the structure of the active centers. Copolymerization studies provide a powerful tool for probing the steric and electronic features of olefin polymerization catalysts.

SUPPORTED CATALYSTS FOR ETHYLENE POLYMERIZATION

Supported catalysts for ethylene polymerization may be divided into three classes: (1) metal oxide, particularly CrO 3 /SiO 2 ; (2) Ziegler, particularly R 3 Al+TiCl 4 /Mg(OH)Cl; (3) organotransition metal compounds, particularly (C 5 H 5 ) 2 Cr/SiO 2 . A significant improvement in polymerization activity probably represents the single most important advantage of supported catalysts. These supported catalyst systems show high efficiency presumably because the active transition metal compound resides only on the support surface, permitting availability of a larger concentration of active sites for polymerization. Reaction of a transition metal compound with a support surface provides an anchoring device, preventing destruction of potential sites by mutual interaction. Studies with the CrO 3 /SiO 2 catalysts have shown that formation of a surface chromate takes place by reaction of CrO 3 and surface silanol groups on silica. Polymer chain growth is believed to occur by a coordinated anionic mechanism. With the Ziegler catalysts, R 3 Al+TiCl 4 supported on magnesium hydroxy chloride, chemisorption can be represented by: TiCl 4 +Mg(OH)Cl→Cl 3 TiO-MgCl+ECl. After reduction by aluminum alkyl, formation of a highly active catalyst occurs. Chromocene deposited on silica supports forms a highly active catalyst for ethylene polymerization. The catalyst formation step liberates cyclopentadiene and leads to a new, chemically-anchored chromium species attached to one cyclopentadienyl ligand. This catalyst shows a high response to hydrogen. Exchange of ligands at the active sites of these organotransition metal catalysts represents a potentially powerful tool for regulating catalytic behavior.