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When one looks inside his or her household, one notices the typical television, refrigerator, computer desktop, table, etc. Yet, how often do people realize the importance of plastics in their manufacture? Plastics, long chains of carbon monomers known as polymers, created the frame that holds the television together, contains the contents of one’s leftover dinner from spoiling in the refrigerator and safeguards the silicon chips in one’s CPU. It is evident that polymers are essential ingredients to one’s everyday life. To understand the impact of polymers in our present lives and the effect they will have in our futures, we must look towards the past when Nobel Prize winner Karl Ziegler discovered an “organometallic mixed catalyst” that catalyzed the polymerization process as well as catalyzing in-depth research into effective ways to create polymers. Before Ziegler explored the properties of organometallic compounds, he was captivated by the characteristics of alkali organic compounds. It was his research in alkali organic compounds that eventually led to his interest and discovery of the organometallic mixed catalyst. In 1932, Ziegler developed a method to prepare lithium butyl by reacting lithium and butyl chloride. He used lithium butyl and other low activity catalysts in order to study the polymerization of butadiene. Normally, the monomers underwent polymerization so rapidly it was difficult to study, yet his use of lithium alkyls and dilation of butadiene with ether allowed Ziegler to isolate small chains of the polymer. His studies showed that the catalyst also became part of the resulting polymer . The same process was studied except in this second scenario, lithium alkyls were replaced with sodium alkyls. The results were the same and in general, the catalyst was found to be an active anion or a metal that reacted with butadiene to give a negative ion . This research led Ziegler to the study of organo-aluminum compounds and the start of a new field in chemistry- organometallic chemistry. The first part of Ziegler’s discovery had to do with a new synthesis of aluminum trialkyls. The reaction between aluminum hydride and three molecules of olefins was the foundation for the direct synthesis of these aluminum trialkyls. This reaction, discovered in 1949 by Ziegler, was also found in addition to another reaction containing lithium aluminum hydrides . These two reactions can be written as: AlH3 + 3CnH2n = Al(CnH2n+1)3 The results of the second reaction can be reacted with aluminum chloride to give aluminum trialkyls as well. However, the starting materials aluminum hydride and lithium aluminum hydride were expensive starting materials. But Ziegler had found a relatively simple process to synthesize these aluminum hydrides, thereby increasing the effectiveness of these expensive materials. He found that one could not directly form these hydrides but rather one must synthesize aluminum hydride with aluminum alkyls. Aluminum alkyl is reacted with powdered aluminum and hydrogen under pressure and then stabilized with additional aluminum alkyl: Al + 3/2 H2 + 2Al(CnH2n+1)3 = 3Al(CnH2n+2)2H In another reaction, the resulting dialkyl aluminum hydride reacts with olefins to form aluminum trialkyls. Aluminum trialkyl, then, is synthesized from nothing other than hydrogen, aluminum and olefins . Before Ziegler made his discovery, ethylene was thought extremely hard to polymerize. The conditions in which polymerization occurred prior to the Ziegler catalyst made the polymerization of polyethylene extremely difficult: pressures between 1000 and 2000 atmospheres and in 200 Celsius. But in his experiment, Ziegler managed to create a low-pressure polymer (polyethylene) at 100, 50, 20 and 5 atmospheres. In addition, Ziegler’s new polyethylene held rather different properties than its high-pressure polyethylene sibling. Not only did it have a better resistance to elevated temperatures and a higher density, but was also more rigid . These astonishing properties were due to the fact that throughout polymerization, the monomers were joined together linearly, differing from the disturbed process in high-pressure polymerization. The catalyst that Ziegler used that also won him the Nobel Prize was prepared by mixing aluminum triethyl with titanium tetrachloride. This is, however, merely one example of the countless combinations of “organometallic mixed catalysts”. Aluminum, the most popular of the organometallic compounds to be used, was reacted with other heavy metals such as titanium, zirconium, chromium, and cobalt . As such, there are countless numbers of combinations of these Ziegler catalysts, a testament to the growth and development of the science in the many years following this discovery. The Ziegler catalysts quickly spread throughout the world and immediately, companies and laboratories set to work to improve upon Ziegler’s work. As others have noted, “the [Ziegler] catalysts had the same effect as the starting gun of a race in which the laboratories of the interested industries had been entered” . Immediately following the discovery, many more discoveries based on the Ziegler catalyst were found. The first major revelation was the polymerization of propylene to create polypropylene, discovered by Guilio Natta just days before Ziegler and his coworkers discovered the same process. The second discovery was made by Günther Wilke. During the polymerization of butadiene, a trimer of butadiene, 1, 5, 9-cyclododecatriene, was produced. Wilke found a way to push this reaction in an entirely new direction towards the dimerization of an eight-carbon ring, and in the reverse direction towards a ten-carbon ring with the addition of ethylene . These ring reactions proved useful in the polymerization of polycondensation polymers such as Nylon 8, 10 and 12. The third major realization is the most important of all discoveries pertaining to the new Ziegler catalysts. Ziegler realized that his catalysts could be used to regulate the structure and shape of the polymer. The structure of any compound, when altered, will give the compound entirely new properties. Therefore, it was important to understand how to manipulate the polymerization process in order to gain access to various structures of a long polymer chain. The two carbon atoms that are double bonded are the ones that actually react with other carbons that make up the polymer chain. The substituents, then, do not react and stay on the outside of the chain. The chains can be joined in a purely random fashion; in this case, the polymer chain is called “atactic”. When the substituents are all on one side of the chain, it is called “isotactic”. When they alternate in a right-left sequence, the polymer is called “syndiotactic”. In addition, because of the double bond, the two adjacent carbon atoms can either stay on one side or stay on alternating sides. The first sequence is called a cis configuration, and the second sequence, trans . Many synthesized polymers have a random assortment of cis and trans and are never uniformly one structure. But the introduction of the Ziegler catalyst changed all of this. Ziegler catalysts allowed scientists to lead the polymerization process towards a certain structure, both structure-specific and stereo-specific. One example, 1,2-polybutadiene, is polymerized using the catalyst mixed from titanium acid ester and aluminum triethyl. Using titanium chloride and diethyl aluminum chloride, trans-1,4-polybutadiene is made. Altering the catalyst slightly will result in cis-1,4-polybutadiene. And increasing the ratio of aluminum to titanium to 5:1 will lead to cyclododecatriene . A catalyst’s structure, therefore, is highly selective of the polymerization process it will push the monomers into, but the study of the various catalysts will allow scientists to create the polymers of their choice in the most efficient manner. Today, the study of polymerization and Ziegler catalysts has become a field of its own in chemistry. Since the time of Ziegler’s first discovery, low pressure polyethylene and later, other improved polymers have been manufactured by the thousand tons and used around the world. The old Ziegler catalysts have been replaced by newly discovered catalysts and processes that place superior control over polymer growth. One such recent process, the Spheripol process of Himont in Italy, begins polymerization within the porous catalyst grains. This results in the fragmentation of the grains and a spherical structure for polymer granules. The tensile strength of the polymer can be improved by two orders of magnitude by orienting the crystalline phase of the polyolefin. Catalysts that generate a statistically comonomer distribution best achieve improved tensile strength; one such example is the nascent metallocene dichloride catalyst. These catalysts result in a narrower molecular weight distribution, balanced comonomer distribution and higher stereo-regularity of the polymer . Metallocenes, first discovered in 1953, have been known to catalyze the polymerization of polyethylene, but the reaction was too slow to be used by manufacturers. However, Hansberg Sinn and Walter Kaminsky made an important discovery in 1976: adding controlled amounts of water to the reaction miraculously sped up the process tremendously . Thus, metallocenes attracted much attention as a potential catalyst. In addition, the basic metallocene was composed of two five-carbon rings, yielding ten hydrogen atoms in the side chains. Replacing these hydrogen atoms with other elements would alter the properties of the metallocene. Therefore, metallocene compounds were systematically studied by one John A. Ewen. John A. Ewen found that zirconium metallocenes yielded hundreds of grams of plastic in a high pressure reactor in an hour. But the reaction needed to be faster to be of practical use. Ewen believed the key lay in the ten free hydrogen atoms. He started experimenting with replacing hydrogens with a simple hydrocarbon group. Experimenting with polyethylene, he came to two conclusions: adding a group that made the net charge of the ring negative sped up the reaction but overcrowding of the rings slowed down the reaction. Substituting each of the hydrogens with a methyl group, Ewen found that the new catalyst had a much higher propensity to react ethylene than it did for propylene. So great was this attraction that in a mixed container of both ethylene and propylene, the catalyst would always react with the ethylene, forming long chains of crystalline polyethylene. This had many practical applications- manufacturers could start different reactions for different polymers all mixed together in a single container . The next step was to create a catalyst that polymerized polypropylene. Ewen needed a catalyst that would regulate the shape of the polymer- either isotactic or syndiotactic. In order to create an isotactic polypropylene, Ewen needed to make sure the monomers were linked in one orientation. In order to accomplish this, he had to find a catalyst that had a “king of diamonds” symmetry in which the monomer looked the same from either viewpoint of the bond. Hans Brintzinger, a chemist from Konstanz University, synthesized a catalyst made up of two rings linked by a double carbon bond with side groups in the proper symmetry. But the results were inadequate; the catalyst still left too much room for erroneous insertions . Thus, a silicon atom was used to bridge the two carbon rings, thereby reducing the gaps and reducing the space for error. A more useful form of polypropylene, syndiotactically formed, was even more difficult to polymerize. Ewen believed that the key to creating this form of polypropylene was again in the structure of the catalyst. Except this time, the catalyst needed a “mirror-image” symmetry. This catalyst was created with two six-carbon rings attached to a five-carbon ring, which was then bonded to another five-carbon ring. This shape allowed monomers to be polymerized in an alternating fashion, resulting in the desired syndiotactic configuration . These catalysts, however, are not perfect. There is always room for improvement. The chemists of today, therefore, have studied and continue to study new metallocene catalysts in an effort to create more efficient and more controlled agents in the polymerization process. And therefore, the chemists of tomorrow will strive to create lightweight but durable plastics that will create the frame of our more massive inventions. Or plastics that are highly resistant to radiation and contamination to bottle food and medicines. Or even plastics that are resistant to tear and insulate heat to clothe humans and keep us warm. Plastics are the world of tomorrow, and it was Karl Ziegler and his research that catalyzed it all. References |
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