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Becky Kriger

What are Ziegler-Natta Catalysts? - 0 views

www.akzonobel-polymerchemicals.com/...re+Ziegler-Natta+Catalysts.htm

Ziegler-Natta Catalysts Ziegler-Natta Catalysts polymerization

shared by Becky Kriger on 08 Dec 08 - Cached
  • It was discovered that Group IV metals, especially titanium, were effective polymerization catalysts for ethylene. Following Ziegler’s successful preparation of linear polyethylene in 1953, Giulio Natta prepared and isolated isotactic (crystalline) polypropylene at the Milan Polytechnic Institute. This was immediately recognized for its practical importance. Ziegler and Natta shared the Nobel Prize in Chemistry in 1963.
  • A Ziegler-Natta catalyst is composed of at least two parts: a transition metal component and a main group metal alkyl compound. The transition metal component is usually either titanium or vanadium. The main group metal alkyl compound is usually an aluminum alkyl. In common practice, the titanium component is called "the catalyst’ and the aluminum alkyl is called "the co-catalyst".
  • In some instances, especially for catalyzing the polymerization of propylene, a third component is used. This component is used to control stereoregularity
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  • Today, Ziegler-Natta catalysts are used worldwide to produce the following classes of polymers from alpha olefins: High density polyethylene (HDPE) Linear low density polyethylene (LLDPE) Ultra-high molecular weight polyethylene (UHMWPE) Polypropylene (PP)--homopolymer, random copolymer and high impact copolymers Thermoplastic polyolefins (TPO’s) Ethylene propylene diene monomer polymers (EPDM) Polybutene (PB)
Becky Kriger

Polymerization catalysts past, present, and future - 0 views

scientific.thomsonreuters.com/...8254437

Polymers catalysts polymerization

shared by Becky Kriger on 08 Dec 08 - Cached
  • The most common polymers are polyolefins, especially polyethylene (better known as Polythene, although this is a trade name owned by DuPont) and polypropylene. However, efficient ways of producing these vital materials are only the result of recent discoveries and have been dependant on the chemistry of catalysts.
  • Since the 1950s, the production of polyolefins has depended on the use of Ziegler-Natta catalysts.
  • Ziegler-Natta catalysts are based on a mixture of a transition metal, commonly a titanium compound, and an alkali metal, most commonly aluminium oxide.
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  • their products have variable physical properties. To this day, the systems are little understood, but the monomers (polymer starting materials) react through a number of reaction sites on the catalyst. Unfortunately, this means the polymer can grow from many sites and at different rates, leading to a very wide distribution in the molecular weight, based on the polymer chain length.
  • Modern life has demanded more of the humble polymer. We want polymers that are stronger than steel, lighter than aluminium, and can be dyed any colour imaginable
  • Metallocenes are positively charged metal ions, most commonly Titanium or Zirconium, sandwiched between two negatively charged cyclopentadienyl rings (see fig). Their big advantage over the Ziegler-Natta systems is that they catalyse the reaction of olefins through only one reactive site. Due to this “single site” reaction, the polymerization continues in a far more controllable fashion, leading to polymers with narrow ranges of molecular weight and, more importantly, predictable and desirable properties.
  • it has been found that changing the ligands (functional groups attached to the metal) upon the metallocene molecule can controllably affect the properties of the polymer.
  • The drawback of metallocene catalysts is that they are unable to polymerize polar molecules, such as common acrylics or vinyl chloride. This is due to the metallocenes’ oxophilicity – their propensity for binding to oxygen.
  • Catalysts using late transition metals – those metals from groups 6 and higher in the Periodic Table – have become increasingly utilized. These compounds have good polymerization activity, although slightly less than metallocenes. However, crucially they can polymerize reactions with polar monomers.
  • The most commercially advanced of this type of catalysts are the Brookhart catalysts [6], which are diimine complexes of palladium or nickel (see fig).
Becky Kriger

Plastics - 0 views

nobelprize.org/...readmore.html

shared by Becky Kriger on 08 Dec 08 - Cached
  • lastics are synthetic materials, which means that they are artificial, or manufactured.
  • he building blocks for making plastics are small organic molecules - molecules that contain carbon along with other substances. They generally come from oil (petroleum) or natural gas, but they can also come from other organic materials such as wood fibers, corn, or banana peels! Each of these small molecules is known as a monomer ("one part") because it's capable of joining with other monomers to form very long molecule chains called polymers ("many parts")
  • 1. Crude oil, the unprocessed oil that comes out of the ground, contains hundreds of different hydrocarbons, as well as small amounts of other materials. The job of an oil refinery is to separate these materials and also to break down (or "crack) large hydrocarbons into smaller ones. 2. A petrochemical plant receives refined oil containing the small monomers they need and creates polymers through chemical reactions. 3. A plastics factory buys the end products of a petrochemical plant - polymers in the form of resins - introduces additives to modify or obtain desirable properties, then molds or otherwise forms the final plastic products.
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  • One such molecule is the ethylene monomer, the starting point for a variety of plastics. Ethylene is a small hydrocarbon consisting of four hydrogen atoms and two carbon atoms.
  • Polymerization is often started by combining the monomers through the use of a catalyst - a substance that aids a chemical reaction without undergoing any permanent chemical change itself. During the chemical reaction, hundreds or thousands of monomers combine to form a polymer chain, and millions of polymer chains are formed at the same time. The mass of polymers that results is known as a resin.
  • Polyethylene is made from just ethylene monomers - but it's also possible to create polymers from two or more different monomers. You can make hundreds of different polymers depending on which monomers and catalysts you use.
  • Cellulose, the basic component of plant cell walls is a polymer, and so are all the proteins produced in your body and the proteins you eat. Another famous example of a polymer is DNA - the long molecule in the nuclei of your cells that carries all the genetic information about you.
  • lastics are classified into two categories according to what happens to them when they're heated to high temperatures. Thermoplastics keep their plastic properties: They melt when heated, then harden again when cooled. Thermosets, on the other hand, are permanently "set" once they're initially formed and can't be melted. If they're exposed to enough heat, they'll crack or become charred.
  • Thermoplastics have long, linear polymer chains that are only weakly chemically bonded, or connected, to each other. When a thermoplastic object is heated, these bonds are easily broken, which makes the polymers able to glide past each other like strands of freshly cooked spaghetti. That's why thermoplastics can readily be remolded. The weak bonds between the polymers reform when the plastic object is cooled, which enable it to keep its new shape.
  • The linear chains are crosslinked - strongly chemically bonded. This prevents a thermoplastic object from being melted and reformed.
  • The most common method for making plastics is molding. To make a thermoplastic object, plastic granules known as resin are forced into a mold under high heat and pressure. When the material has cooled down, the mold is opened and the plastic object is complete. When making plastic fibers, the molten resin is sprayed through a strainer with tiny holes.
  • Thermosets are produced in two steps: 1. Linear polymers are formed. 2. The linear polymers are forced into a mold where "curing" takes place. This may involve heating, pressure, and the addition of catalysts. During this process, a cross-linked or networked structure forms, creating a permanently hard object that is no longer meltable or moldable.
  • For most applications, the ideal polymer is a long, straight chain with a highly regular molecular structure. Early synthetic polymers, however, often exhibited odd little branches and other irregularities. In the 1950s, German chemist Karl Ziegler (1898–1973) discovered that an entirely different type of catalyst - a combination of aluminum compounds with other metallic compounds - could solve some of these annoying problems and increase the length of a polymer chain, producing superior plastics.
  • olymers often have short side chains, which can occur on either side of the main chain. If side branches occur randomly to the left or right, the polymer has an irregular structure. Italian chemist Giulio Natta (1903–1979) discovered that some Ziegler catalysts led to a uniform structure in which all the side branches are on the same side.
  • Firstly, there is an environmental impact from plastics production; however the plastics industry has worked hard to reduce energy and water use, as well as waste generation during the manufacturing processes.
  • Secondly, during their lives, plastic products can save energy and reduce carbon dioxide emissions in a variety of ways. For example, they're lightweight, so transporting them is energy efficient. And plastic parts in cars and airplanes reduce the weight of those vehicles and therefore less energy is needed to operate them and lower emissions are created.
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