TECHNOLOGY
Zirconium Catalysts Polymerize Olefins Faster Hie new catalysts, unlike the Ziegler-Natta type, are extremely stable in solid form and remain active for days, yielding long polymer chains A new generation of Ziegler-Natta type catalysts holds considerable promise for olefin polymerizations. Developed by Walter Kaminsky and his associates at the University of Hamburg's applied chemistry department in West Germany, the catalysts consist of a zirconium compound complexed with an aluminoxane, a novel material comprised of five to 20 alternating aluminum and oxygen atoms either in ring form or in chains. The West German chemist detailed the current status of the work at a recent conference marking the golden jubilee of polyethylene's discovery. The new catalysts, he claims, have a number of attributes; for example, they: • Have an extremely high level of activity. • Promote rapid buildup of large polymer chains. • Generate high-molecularweight atactic polymers from propylene and higher alpha-olefins. • Yield homogeneous polyolefin composites with starch, cellulose, and other fillers. • Have a long active life under reaction conditions. • Can be stored indefinitely. A fortuitous accident led to the discovery of the new catalysts. The Hamburg chemists started out studying bis(cyclopentadienyl) derivatives of titanium and zirconium complexed with the more conventional aluminum alkyls and alumi-
Kaminsky: fortuitous accident num halides. At one stage, water inadvertently got into the flask during the preparation. The result was an unexpected increase in catalytic activity. The enhanced activity was traced to the presence of aluminoxane, formed when an aluminum alkyl reacts with water. Some 60% of the aluminoxane has a cyclic structure, about 40% exists as chains. Methylaluminoxane is associated with bis(cyclopentadienyl)zirconium dichloride in one of the catalysts. In another, it is associated with the derivative bis(cyclopentadienyl)dimethylzirconium. The current method of preparation of aluminoxane entails gently heating trimethylaluminum, for example, dissolved in hexane or toluene, with crystalline copper sulfate or aluminum sulfate, which provides the water needed to produce the aluminoxane. The degree of oligomerization is controlled by monitoring the reaction. After filtering and stripping
off the solvent, the aluminoxane is isolated as a stable white powder. Making the catalyst itself is equally simple. This is done by adding aluminoxane to an aromatic solution of the commercially available dichloro or dimethyl derivatives of bis(cyclopentadienyl)zirconium. Evaporation yields the powdered catalyst, which has a long storage life. The new catalysts, unlike ZieglerNatta catalysts, are extremely stable in solid form and soluble in a variety of organic solvents. Under reaction conditions, they remain active for several days instead of the more usual few hours for titanium-aluminum catalysts—an advantage when making block copolymers. More significant is their very high level of catalytic activity. "Using bis(cyclopentadienyl)zirctonium dichloride and methylaluminoxane, activities of up to 5 million g of high-density polyethylene per gram of zirconium per hour are found," Kaminsky notes. "This means that the insertion of an ethylene molecule takes only 5 X 10~5 second, assuming that every zirconium atom forms an active polymerization center. "Kinetic measurements show that about 70% of the zirconium atoms are active when polymerization is conducted at 20 °C. At higher temperatures, the number is closer to 100%," Kaminsky says. This is in sharp contrast to the 3% or even less of titanium atoms in Ziegler's original catalysts that are active, and also is an improvement over the new ZieglerNatta catalysts. One obvious advantage of the zirconium catalysts' higher activity is that far smaller amounts are needed to produce the same amount of polymer. Consequently, contamination of the final product with catalyst is negligible, and the need to remove it is avoided. July 4,1983 C&EN
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Technology A factor influencing the activity of Kaminsky's catalysts is the degree of oligomerization of the aluminoxane moiety. The catalytic effect is enhanced as the number of alternating aluminum and oxygen atoms increases. The highest productivity to date was measured at 70 °C using 5 X 10~ 10 mole of catalyst per L of ethylene. About 425 million g of highdensity polyethylene per gram of zirconium were formed in 120 hours. "Each zirconium atom inserts more than 1.3 billion ethylene molecules," Kaminsky observes, "and produces approximately 35,000 macromolecules with an average degree of polymerization of over 1 million." Chain length of the resulting polyethylene is influenced by the temperature at which polymerization takes place. Polymer with a molecular weight of 500,000 is formed at room temperature. Above 100 °C, the value is closer to 50,000. Because Kaminsky's catalysts are soluble, they lead to atactic polymers of propylene and higher alpha-olefins. These are polymers in which the pendant groups are randomly dispersed, spatially, along the chains. In contrast, Ziegler-Natta catalysts, being insoluble, are noted for the high degree of stereoregularity of the polymers they generate. Atactic polymers obtained with Kaminsky's catalysts have high molecular weight (ranging from 50,000 to 100,000) and haven't been made before. Because they are amorphous rather than crystalline, their rubberlike properties could open the way to new applications. They are completely saturated, and so aren't prone to degradation by ultraviolet light-induced oxidation, as occurs in elastomers containing double bonds. The Hamburg workers are testing their new catalyst system in a variety of copolymerizations. For instance, they can make linear low-density polyethylene, a polymer with considerable commercial potential, by incorporating small amounts of 1-butene, 1-hexene, or 1-octene into the chain. "The combination of alpha-olefins with diolefins to form copolymers and block polymers on the one hand, or cross-linked polyolefins on the 30
July 4, 1983 C&EN
other, is at the beginning of development," Kaminsky remarks. Terpolymers involving ethylene and propylene units together with styrene, for example, also are being examined. Yet another area of study centers on developing composites of polyolefins with starch or cellulose. The research points to "new avenues for the variation of polymer properties, and additional fields of application," Kaminsky observes. The present method for making such composites is to physically blend polymer with 15% or so of starch or cellulose filler. The resulting product, however, isn't homogeneous. Properties can vary from one batch to another depending on how the blending operation is carried out. The presence of starch clusters significantly decreases the tensile strength of an extruded sheet. Kaminsky's route to the composites overcomes such problems. He first adds trimethylaluminum to the finely powdered starch or cel-
lulose. This reacts with water entrained in the particle to form methylaluminoxane, which yields catalyst when the zirconium component is added. Consequently, active catalyst is adsorbed onto the surface of the particles. Subsequent exposure to olefin results in polymerization taking place rapidly. The polymer thereby incorporates filler particles into its structure, producing a homogeneous product. Sheets extruded from such homogeneous polyolefin composites can be printed in the same way as ordinary paper using conventional inks and machines. "This overcomes the current problem where you need to activate the plastic surface before you can print on it," Kaminsky remarks. "Now, you can print normally on the sheets." Additionally, film drawn from the composites have liquid and vapor barrier properties that likely will make them of potential use in packaging fresh fruits, vegetables, and the like. Dermot O'Sullivan, London
EDUCATION
Computer use in chemical education promoted Materials to help bring microcomputers into use in chemical education are now available from a National Science Foundation education development project called Project Seraphim. The project, which has existed for a little more than a year, aims to develop and disseminate modular instructional materials for chemistry, particularly computerbased materials; to distribute basic information about the use of microcomputers in chemical education; and to promote the writing and distribution of programs that make writing computer software for chemistry easier. "We have found that chemists' interest in microcomputers currently is very high and that Project Seraphim has become a clearinghouse for materials and information," says project director John W. Moore, a professor of chemistry at Eastern Michigan University. The program is now offering a listing of the computer programs that are available in chemical education.
"We have compiled a list of about 120 entries with all of the instructional computer programs for chemistry about which we have been able to gather information," explains Moore. The list gives information on the type of computer, subdiscipline of chemistry, specific hardware requirements, author's name, supplier, cost, and a brief description of the program. Both commercial and noncommercial offerings are included. Moore says the project intends to update this list of available material quarterly. The listing has been sent out to subscribers of the Computers in Chemical Education Newsletter and is also available from Moore at Eastern Michigan University, Department of Chemistry, Ypsilanti, Mich. 48197. Moore also can supply a catalog of the other materials available from Project Seraphim. These include information on other sources of information, writing computer software, user reviews of instructional programs developed under the project, and software systems available. D
