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Novel catalysts are expanding beyond the polyolefin markets that nurtured them
ALEXANDER H. TULLO, C&EN NORTHEAST NEWS BUREAU
Single-site catalysts long ago made it from the laboratory bench to the polymerization reactor. Indeed, they are being used around the world to make 2 billion lb of polyethylene a year.
Although polymer makers have succeeded in supplying their customers with large volumes of plastics never seen before, they are now eyeing new worlds for the catalysts to conquer.
||COMPLEX A researcher examines metallocene catalysts at Univation's Baytown, Texas, research center.
The next few years may see single-site catalysts based on entirely new concepts that give polymer producers unprecedented flexibility in process and polymer design. Moreover, technology developers are looking at polymerizing polar monomers in addition to the conventional olefin feedstocks currently used--marking an expansion beyond high-volume polymers and into specialty materials and other chemical applications.
The industry is also witnessing single-site catalyst technology blossoming outside of the elite group of players like Dow Chemical, ExxonMobil, Basell, and Chevron Phillips that originally sponsored the technology. New companies not associated with high-volume polyolefins are coming out with single-site catalyst technologies of their own.
One of these companies is Bayer, which is adding a new dimension to catalyst design. The company has developed a series of metallocene catalysts with donor and acceptor atoms in the ligands. As a result, the ligands can switch from being bridged to unbridged, depending on the temperature.
Normally, the ligands on a metallocene catalyst are either sandwich structures, with no covalent bonding between them, or bridged, with a covalent chain between the two ligands. The bridged structures are usually used to control the stereochemistry of the polymerization to make products like syndiotactic polypropylene. The unbridged structures are normally used for semicrystalline products like high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE).
Aleksander Ostoja-Starzewski, the scientist who developed the Bayer catalysts, says the advantage of the new donor-acceptor technology is flexibility. "When you look at the literature on metallocenes, you will find hundreds of derivatives that belong to either of two classes, but nothing in between," he says. "These catalysts are in between in that you can control the state of the system."
The Bayer system offers plastics makers a new variable they can control in polymerization. The switching property makes the catalysts especially suited for making copolymers and block copolymers. In fact, Starzewski says the catalysts can be used to synthesize styrene-ethylene copolymers, such as Dow's Index product line, as well as ethylene-norbornene copolymers--such as Ticona's Topas.
But Starzewski says Bayer is developing the catalysts to support its own synthetic rubber business and not to enter a new market. Bayer plans to license most of the technology for other applications.
Late-transition-metal catalysts are another single-site class that is coming closer to commercialization. DuPont and Eastman are reporting a number of capabilities of this catalyst class, including new polyethylenes, -olefins, and copolymers of ethylene and polar monomers.
Under DuPont's Versipol trademark are nickel-palladium -diimine catalysts and iron-cobalt tridentate catalysts. Eastman also has iron-cobalt tridentate and palladium-nickel catalysts under the Gavilan trademark.
DuPont's iron-cobalt tridentate catalysts can make very linear, high-density polyethylene--an area that BP is also focusing on with similar catalysts. To circumvent the kinds of intellectual property problems that slowed the development of earlier single-site catalyst technologies, the two companies cross-licensed the technologies in 1999.
THE TRIDENTATE CATALYSTS can also make -olefins--an area not covered under the cross-license. At the Metcon 2001 conference in Houston last May, DuPont reported a number of advantages over commercial -olefin technologies, including a higher yield of linear -olefins as well as operation under lower temperatures and pressures.
||FOREFRONT Eastman Chemical is working with its Gavilan catalyst (nickel is shown in green, nitrogen in blue, and sulfur in yellow) to copolymerize polar monomers.
The company is even more excited about the technology now than it was last spring. "The catalysts attain high selectivity in a much simpler process than other -olefin catalysts," says Rinaldo S. Schiffino, a DuPont researcher.
The -diimines, in contrast, have the potential to make a-olefins obsolete in LLDPE manufacturing because they can make branched polyethylene using ethylene only. In LLDPE production today, -olefins are used as comonomers to add branches to a mostly 1inear polymer chain, improving processibility.
The technology works because of the catalysts' "chain walking" mechanism. The -diimine catalysts initiate polymerization like other single-site catalysts: through a coordinating bond among the metal, olefin raw material, and polymer chain.
Schiffino explains that the difference between the new catalysts and other technologies lies in what happens next. "In the nickel catalyst, that coordination bond stays together for a longer time than in a metallocene and there's the possibility for readdition of the metal hydride after some rearrangement in the polymer chain," he says. "So that readdition forms a structure in which the metal has actually walked into the chain." The catalyst in the middle of the chain can start a branch.
Because DuPont isn't a polyolefin producer, it plans on licensing most of the Versipol platform; however, the company is saving one capability for itself: the ability to copolymerize ethylene and polar monomers. The company plans to use these catalysts in its packaging and industrial polymers business, which makes polymers for a range of applications from asphalt modification to paper sizing.
EASTMAN HAS also been experimenting with copolymerizing polar monomers such as methyl methacrylate and vinyl acetate with its nickel catalysts. "It really has been a difficult part of the technology," says Christopher M. Killian, group leader of polymer synthesis R&D at Eastman. "It has been a dream to copolymerize ethylene and other functional monomers."
Neither DuPont nor Eastman will discuss its polar monomer work in detail, but Killian observes that companies want to extend to copolymers the properties that make single-site catalysts useful in commodity polyolefins. For example, ethyl acrylate polymers are already available in highly branched versions; Eastman is looking at making them more linear. "History teaches us that the more control you have over a system, the more probability there is to tailor a material to fit a certain application," Killian says.
DuPont says there have been commercial tests on the entire Versipol platform, especially on the tridentate side. "The diimines are coming along a bit slower," says James E. Smith, DuPont's Verispol business manager.
However, Smith notes that DuPont has signed agreements with W.R. Grace and Akzo Nobel to supply commercial quantities of catalyst to potential licensees. "There are a number of end users who are actively working with the catalysts in each of the product areas--developing the technology to fit their platforms and facilities," he says.
Killian says Eastman is also looking at licensing its platform. "This technology is more than a novelty," he states. "It will make inroads in the marketplace." He notes that the ability to use the catalysts under high temperatures and to use hydrogen as a chain transfer agent are two of its advantages.
Japan Polychem is also breaking new catalyst ground with its clay-mineral-based catalysts. These are metallocene complexes on clay-mineral support activators that do the job of both the catalyst support and the boron or methylaluminoxane activators.
The catalysts are already commercial on one of Japan Polychem's random copolymer polypropylene lines, and the company is considering licensing the catalyst, which it says is compatible with a number of commercial polyolefin technologies. "Cost is one of the advantages that clay-mineral has demonstrated," saysYoshiyuki Ishihama, a research scientist for Japan Polychem's process and catalyst R&D center.
Cost is also a key feature of a new catalyst family from Borealis, in which siloxy groups are substituted on the catalyst ligand structure.
Borealis, which was one of the first companies to commercialize metallocenes when it launched its Borecene line in 1995, notes some advantages of siloxy-substituted single-site catalysts over conventional metallocenes. "It has been found that siloxy groups give you special advantages in the catalyst system, like higher molecular weight capability and easier activation than a metallocene catalyst system," says Erik Van Praet, the firm's research manager for single-site catalyst technology.
Van Praet claims these traits translate into cost savings and could make for a catalyst system that is useful for bimodal polyethylenes--resins with a broad distribution of molecular weight that are easy to process and have good mechanical properties. "One of the problems with standard single-site polymers is that they are difficult to process," he notes.
THE COMPANY plans on commercializing the catalysts with its Borstar two-stage polymerization process that combines slurry and gas phases. Borealis is still deciding what type of resin it will make, but the firm says it will most likely produce an LLDPE film with the catalysts in the second quarter of 2002.
Univation, the Dow-ExxonMobil polyethylene technology venture, is also targeting bimodal HDPE resins with a new catalyst system that can make them in a single gas-phase reactor instead of a multistaged plant. It claims technology can lead to huge capital cost savings for licensees that want to make the resins.
F. Gregory Stakem, vice president of R&D for Univation, says the venture had to overcome a number of obstacles in developing the technology. "The first question is whether you have a set of organometallic compounds that will function together to create the desired molecular architecture," he says.
Second, Stakem says, Univation had to balance the activity of the catalysts with their propensity to interact with each other. "This is considerably more difficult than it sounds, as many catalysts are cross-poisoned by each other or by various types of activators," he says.
Difficulties not withstanding, Stakem says only a small retrofit is usually necessary for installation of the catalysts. Univation will run them on commercial reactors later this year and plans commercialization in 2002.
If successful, polyethylene processes like those from Univation and Borealis could create yet another huge market for metallocene catalysts. Add niche applications from firms like Bayer, DuPont, Japan Polychem, and Eastman, and it's clear that metallocenes still have polymer worlds to conquer.
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