Chemical & Engineering News,
September 11, 1995

Copyright © 1995 by the American Chemical Society.

Metallocene Catalysts Initiate New Era In Polymer Synthesis

Well-defined catalysts now allow producers to design polymers with exact properties and to create as yet unknown materials

Ann M. Thayer,

C&EN Northeast News Bureau

Metallocene catalysts are "just a rumble on the tracks," says one industry consultant, "but it's certainly a train that's coming." The plastics industry, by most accounts, is moving into an era based on a new generation of catalysts and entirely new polymeric materials.

To those most bullish on metallocene technology, the changes taking place in the plastics industry are among the most important in its history. In 1960, high-activity Ziegler-Natta catalysts catapulted the industry forward by making possible the inexpensive and easily controlled production of polyethylene and polypropylene. Today, more than 80 billion lb of polymers are produced worldwide with Ziegler-Natta technology.

Now, with metallocene catalysts discovered in the 1980s, producers can refine, even design, the structure of polymers. Although based on transition metals such as titanium and zirconium - as are Ziegler-Natta catalysts - metallocenes differ in that they have well-defined single catalytic sites and well-understood molecular structures. Typically, they consist of a transition-metal atom sandwiched between ring structures to form a sterically hindered site.

The sandwich structures have been known for decades, but were not considered practical as catalysts. Then, in the mid-1980s, German professors Walter Kaminsky of the University of Hamburg and Hans H. Brintzinger of the University of Konstanz showed that metallocenes had industrial potential.

Since then, research - by some estimates as much as $3 billion worth - has focused on modifying, improving, and extending this catalyst family. Stereoselective catalytic sites can polymerize almost any monomers - beyond the traditional C3 to C8 &agr;-olefins - in an exact manner. Polymer molecular weight and molecular weight distribution, comonomer distribution and content, and tacticity can be independently controlled. And with well-characterized molecular structures, catalyst composition and geometry can be varied systematically to produce extremely uniform homo- or copolymers "programmed" with the desired physical properties.

Metallocene-based polymers, ranging from crystalline to elastomeric materials, have been available commercially since about 1991. They tend to have features such as increased impact strength and toughness; better melt characteristics, because of the control over molecular structure; and improved clarity in films. Most early applications have been in specialty markets where value-added and higher priced polymers can compete. As the technology develops and catalyst costs decrease, metallocene-based polymers are expected to compete in the broader plastics market.

Exxon Chemical and Dow Plastics are leading the plastics industry into the metallocene era. "These two giants are battling it out and, without question, are going to drive the industry" believes David Highfield, executive vice president of Spring House, Pa.-based consulting firm Catalyst Group. Competition is coming from other plastics producers that are polishing technologies to increase productivity, reduce costs, and create intellectual property estates.

Metallocenes are being used to produce improved or entirely new types of polymers. Whereas Exxon will say only that it works with both mono- and bis-cyclopentadienyl metallocenes, Dow has focused on titanium monocyclopentadienyl metallocenes, which it calls "constrained geometry catalysts."

Global capacity to rise for metallocene-based polymers

Capacity(b)
(millions of
Company Location lb per year)
Polyethylenes
Dow Plastics U.S. 250
Dow Plastics Spain 125
Exxon Chemical U.S. 253
Mitsubishi Japan 220
Nippon Petrochemicals Japan 110
Ube Industries Japan 44
Total 1,002
Polypropylenes
BASF Germany 26
Chisso Japan 44
Exxon Chemical U.S. 220
Hoechst Germany 220
Mitsui Toatsu Japan 165
Total 675
EPDM(b)
Dow/DuPont U.S. 198
Cyclic Olefins
Dow Plastics U.S. Pilot
Hoechst Germany Pilot
Mitsui Petrochemical Japan 7
Polystyrene
Idemitsu Petrochemical Japan 22
TOTAL WORLDWIDE 1,904

a Forecasted by 1996. b Ethylene-propylene-diene monomer rubber. Source: Catalyst Group

Exxon first produced metallocene-based polymers with its Exxpol catalysts in 1991. It now markets about 30 grades of ethylene-butene and ethylene-hexene copolymers under the Exact trade name. Within the next year, the company plans to announce capacity expansions from its current 40 million lb per year.

Dow uses its Insite technology to make ethylene-octene copolymers, which the company launched in 1993. Copolymers with up to 20% (by weight) octene are sold as Affinity "plastomers" and compete with specialty polymers in packaging, medical devices, and other applications. Dow says its catalysts allow for the uniform introduction of comonomers and long-chain branches that improve processibility in otherwise essentially linear polymers.

"They are very different both in molecular design and in density," explains Chris Pappas, director of marketing for specialty olefins and elastomers at Dow Plastics. "There are no polyethylenes in the [same density] ranges, so they compete in an entirely different world than traditional polyethylene." At more than 20% octene, the copolymers fall into the elastomers area and have been sold under the Engage name since early 1994.

In Freeport, Texas, Dow has converted 250 million lb per year of solution process capacity, which previously produced its Dowlex polyethylene, to produce metallocene-based polymers. And a 125 million-lb-per-year plant will be converted in Tarragona, Spain, by April 1996. According to Dow, it takes about nine to 12 months to convert to the new catalysts.

To Sidebar Economics is key to adoption of metallocene catalysts

Dow currently is supplying its own catalyst needs, says Bruce A. Story, intellectual asset manager at Dow Plastics, but the company is working to develop relationships with catalyst suppliers. "Lots of people," he adds, "want to get involved and supply catalysts, and the cost of metallocenes is going down as suppliers look for more efficient synthetic routes."

Polymerization catalysts have been supplied by producers such as Akzo Nobel, Albemarle, and Witco. These firms also manufacture aluminum alkyls, which play significant roles in catalyst activators. Witco, for example, was awarded a U.S. patent this spring on a process for preparing aluminoxanes. Roy Simmons, group leader for metallocene technology at Witco, says the company has secrecy agreements with most of the major metallocene developers and offers commercial metallocene products in small quantities "without infringing any composition of matter patents."

Dow and DuPont anticipate launching a $1 billion joint elastomers venture by early 1996 to which Dow will contribute its Engage elastomers business (C&EN, Feb. 6, page 6). The venture also plans on starting up a 198 million-lb-per-year ethylene-propylene-diene monomer (EPDM) plant in Plaquemine, La., by the end of 1996. "It will be the world's first EPDM plant based on single-site catalyst technology," comments Pappas. He predicts that more than half of the venture's anticipated growth to $2 billion in sales by 2002 will come from metallocene technology.

For the future, Dow is developing what it calls ethylene-styrene "interpolymers" in collaboration with Idemitsu Petrochemical of Japan. Syndiotactic polystyrene is to be produced by the end of 1996 at a 22 million-lb-per-year plant under construction in Chiba, Japan. These polymers are a new composition of matter with unique and very different properties - ranging from engineered plastics to elastomers - which have yet to be fully understood, according to Pappas.

"In the specialty polymer and elastomer areas, Dow anticipates that by the year 2000 it will be selling 500 million lb per year globally," says Pappas. "But in polyethylene, the company expects to be to be selling over 1 billion lb per year of metallocene-based material in the year 2000." Dow currently produces about 5 billion lb of polyethylene annually.

The initial specialty polymers from metallocenes are extending the performance range of traditional polyethylenes. Eventually, many in the plastics industry expect metallocene-derived polymers to compete with commodity-grade materials. "The next frontier of metallocene chemistry is the ability to make materials at the right costs to compete in polyethylene markets," says Pappas.

Other firms less keen on metallocenes except as specialties, such as Union Carbide and Quantum Chemical, are focusing on cutting the production costs of existing technologies and developing second-generation, non-metallocene-based, polymers. However, Carbide's new Unipol II plant is reportedly "metallocene-ready." Comments Highfield, "I would worry if companies don't have metallocene systems."

Cyclopentadienyl ring also may form part of indenyl ring structure. The two cyclopentadienyl ring structures need not be idetical. Monocyclopentadienyl structures also exist in which one ring has been replaced by a heteroatom (often N) attached to a bridging atom. M = transition metal, usually goup 4b (Zr, Ti, Hf); A = optional bridging atom, generally Si or C atom with R=CH3; R = H, alkyl, or other hydrocarbon groups, which need not all be identical; X=halogen atom (generally CI) or alkyl group.

Metallocene-based polymers "are definitely superior, but they are going to be sold into very cost-sensitive markets," admits Pappas, such as those for low-density and linear low-density polyethylene (LLDPE). Catalyst costs and production rates in the plants must be addressed, he adds, but Dow is confident it will be "in the [commodity] polyethylene game with metallocenes pretty aggressively in 1996."

In April, Exxon received a patent and conducted a successful test run of a new LLDPE production process (C&EN, May 1, page 7). According to company claims, its supercondensed mode (SCM) process allows for the operation of gas-phase reactors with higher levels of liquids, increasing plant productivity by 60 to 200% at half the cost of building a plant.

The company believes the combination of its Exxpol metallocene catalysts and SCM process can produce cost-effective volume products. A 1 billion-lb-per-year reactor at Exxon's Mont Belvieu, Texas, plant has been retrofitted with the technologies. And Exxon intends to use metallocene catalysts in all of its gas-phase polyethylene facilities worldwide.

Exxon plans to introduce its metallocene-derived LLDPE in the second half of 1995. "We're close to launching a slate of LLDPE products made in that plant," says Gregory L. McPike, vice president of Exxon Chemical's Exxpol venture. "They will be premium products over today's LLDPE offerings because [metallocene-based LLDPE is] a much better polymer with much better properties, but it's going to be priced to compete with and displace conventional LLDPE."

Initial production at Mont Belvieu is expected to be about 220 million lb per year. Whether the plant's entire capacity is devoted to metallocene polymers will depend on market acceptance, McPike notes. "We've got the capacity and are quite optimistic that the plant ultimately will be running metallocenes solely."

Exxon and Mitsui Petrochemical of Japan have been collaborating on optimizing metallocene-based gas-phase processes and now are in a position to offer licenses, says McPike. And Mitsui and Ube Industries are retrofitting an LLDPE production line - which uses BP Chemicals' fluid-bed process - with metallocene catalysts for production later this year.

Combining metallocenes and gas-phase processes was considered a major milestone because the gas phase is the low-cost, industry standard for large-volume manufacturing and would move metallocenes out of niche markets. Just prior to Exxon's SCM process announcement, Mobil Chemical reported that, with minimal modifications, it had retrofitted a commercial-scale gas-phase reactor to use metallocenes. Mobil still is evaluating its material before deciding how or when to commercially introduce it.

Other firms such as BASF and Phillips Petroleum report the successful use of metallocenes as "drop-in" catalysts in slurry processes. BP Chemicals also says it has tested single-component, drop-in metallocene catalysts in its fluid-bed process at a "semicommercial" scale. The company expects to offer its process for license and to demonstrate it commercially within the next two to three years.

And BP Chemicals and Quantum Chemical are looking at combinations of metallocenes, Ziegler-Natta, and other catalysts in production processes. The mixed-catalyst approach is aimed at adjusting catalyst activities and ratios to improve polymer properties while maintaining processibility of standard LLDPE resins. Practical transitioning between catalyst systems, says BP, is expected to be an important factor in the next few years.

Exxon currently supplies its own catalysts, producing them on site in Texas, and says it has enough catalyst and cocatalyst production capabilities to match expansion plans. McPike says it is the company's strategy to continue to supply its own catalysts because of the "know-how" involved and the proprietary nature of its catalysts systems. The company also expects, McPike adds, to sell the catalyst systems when it licenses its technology to others.

Licensing of the "commodity-grade," but not specialty-grade, technology has become an important focus for both Dow and Exxon. However, no licensing deals have been signed. Both companies say they are looking to offer catalysts as drop-in or retrofit technology in existing plants or to provide both the catalyst and process.

To Enlarge Chart

Although many patents have been issued to both companies, patent rights to catalysts, processes, and products still are being sorted out. Several metallocene catalyst patents are in interference proceedings at the U.S. Patent & Trademark Office, but litigation at this stage has been minimal.

"In the polyethylene area, once cost and other issues are solved, the technology pickup around the world is going to be very fast," says Dow's Pappas. "We are not going to be capable of building an asset base fast enough on our own to drive that, and so we need licensing partners to leverage it.

"How quickly the technology pickup is going to happen is anyone's guess." The plastics fabrication industries have been built around "impure" polymers, he explains. "Now you don't have to make compromises ... but it will take new fabrication technology, and time to develop that, to take advantage of [new materials]."

In addition to producers developing new ethylene-based polymers, more than 15 companies have been identified by Houston-based consulting firm Phillip Townsend Associates as working toward producing metallocene-based polypropylene. Some are advancing catalyst and process technologies, whereas others are working only on catalyst development.

Polypropylene traditionally is produced as a mixture of forms - about 95% isotactic, a few percent of undesirable atactic, and an even smaller amount of syndiotactic polymer. In addition to dictating molecular-weight distribution, metallocenes allow the tacticity - or stereoregularity - of a polymer to be controlled by changes in the catalyst stereochemistry. In metallocene-based isotactic polypropylene, the melting point and other aspects can be customized by adjusting the isotactic sequence length.

Syndiotactic polypropylene could not be produced as a pure polymer before the discovery of metallocenes. The material is significantly different from isotactic polypropylene in its physical properties - much softer, but also much tougher and much clearer. Joe Schardl, manager of market development for Fina's chemical division, expects it will compete, not as a replacement for isotactic polypropylene, but with other polymers in film, medical applications (because of its stability to gamma radiation), some adhesive and extrusion applications, or in blends.

The development of syndiotactic polypropylene has advanced further than anticipated, according to Phillip Townsend. Fina (an affiliate of the Belgian company Petrofina) has been working with specialty chemical producer Mitsui Toatsu Chemicals of Japan to develop syndiotactic polypropylene. The companies have been producing test quantities in an existing Fina facility and are preparing for a third run; the last run produced several hundred thousand pounds, says Schardl. Demand has not yet developed to warrant producing the polymer on a continuing basis, he adds.

The new polymer currently is more expensive to produce than traditional polypropylene. That's because, in part, production has not yet been optimized, explains Schardl, but the company is looking at making a capital investment. Prices should come down, he adds, as production and other costs are addressed. And Fina is working with fabricators to get their feedback on handling the new material.

Germany's Hoechst also has demonstrated metallocene catalysts for producing isotactic or syndiotactic polypropylene. Other firms working in the metallocene-based polypropylene area include Exxon Chemical, BASF, and Chisso of Japan.

In a paper presented at MetCon '95 in May, Hoechst said the optimum metallocene structure it has found to date to produce high molecular weight, highly isotactic polypropylene is a 2-ethyl-4-phenyl-substituted bisindenyl zirconocene. Key structural elements of high-performance bisindenyl metallocenes - which are 40 times more active than conventional systems - are a silicon bridge, an alkyl substituent in the position neighboring the bridge, and aryl substituents in the 4,4'-positions on the indenyl rings.

Hoechst has been a leader in developing the tools to create structural variations and new catalytic properties of metallocenes, targeting a desired polymer structure and performance. The company has a contract with Kaminsky of the University of Hamburg and files his patent applications. Hoechst says it has developed very efficient synthetic procedures for the preparation, and optimized the synthetic handling, of different metallocenes. These, say Hoechst researchers, "are key to the production of metallocenes" on a technical scale.

In 1994, Hoechst began operating a pilot plant for metallocene synthesis as a first step to larger scale use, production, and sale of the catalysts. It also conducted an initial polypropylene production run in August. "However, the speed of implementation of metallocene technology will also depend on the response of the market for new polypropylene grades, the licensing policy of the technology owners, and the costwise competitiveness of metallocene catalysts in existing technologies," says the company.

Hoechst and Exxon have been collaborating on developing isotactic polypropylene since mid-1994. Initial products may compete in fibers or engineering resin markets. The companies expect to make an announcement about production trials before the end of the year, and commercial production is expected within the next two years. The added value in metallocene-based polypropylene is expected to be even higher than for polyethylenes, notes Exxon's McPike. Exxon's SCM process also can be used to produce polypropylenes.

This year, Hoechst expanded another 1993 R&D agreement with Mitsui Petrochemical to include worldwide market evaluation of new polymers. The two companies will jointly produce cycloolefin copolymers (COC) using metallocene catalysts developed by Hoechst on a pilot scale. Mitsui is modifying a 7 million-lb-per-year facility in Iwakuni, Japan. Their first product will be an ethylene-norbornene copolymer called Topas.

Cycloolefins such as cyclobutene or cyclopentene make very rigid polymer chains with very high melting points. Metallocene catalysts can efficiently polymerize cycloolefins with linear olefins to yield amorphous thermoplastics with lower melting points, high transparency, better optical characteristics (because of the absence of double bonds or aromatic structures), and good heat stability and chemical resistance.They are expected to compete with polycarbonates in the optical, data storage, and medical device fields.

Other new materials in the R&D stage, according to John J. Murphy of the Catalyst Group, include terpolymers and block and graft copolymers. "Metallocene technology is becoming an important tool that allows polyolefin producers to generate added value at a time when forecasted industry returns require attention," he says. Application development has become important, he adds, as evidenced by both patent filings and close cooperation between material suppliers and product fabricators.

And next year is expected to be a significant year for the developers of metallocene systems as new production comes on-line.


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