Polyethylene (Molecule of the Month for April 2007)
Polyethylene is a thermoplastic commodity made by the chemical industry and heavily used in consumer products. Polyethylene is a polymer consisting of long chains of the monomer ethylene. Polyethylene is created through polymerization of ethene. It can be produced through radical polymerization, anionic addition polymerization, ion coordination polymerization or cationic addition polymerization. This is because ethene does not have any substituent groups which influence the stability of the propagation head of the polymer. Each of these methods results in a different type of polyethylene.
Polyethylene is classified into several different categories based mostly on its density and branching. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight. The following are types of polyethylene : UHMWPE (ultra high molecular weight PE) - because of its outstanding toughness, cut, wear and excellent chemical resistance, UHWMPE is used in a wide diversity of applications. These include can and bottle handling machine parts, moving parts on weaving machines, bearings, gears, artificial joints, edge protection on ice rinks, butchers' chopping boards., HMWPE (high molecular weight polyethylene), HDPE (high density PE) - is used in products and packaging such as milk jugs, detergent bottles, margarine tubs, garbage containers and water pipes, HDXLPE (high density cross-linked PE), PEX (cross-linked PE), MDPE (medium density PE) - is typically used in gas pipes and fittings, sacks, shrink film, packaging film, carrier bags, screw closures, LDPE (low density PE), LLDPE (linear low density PE) - is used predominantly in film applications due to its toughness, flexibility, and relative transparency., VLDPE (very low density PE) - used for hose and tubing, ice and frozen food bags, food packaging and stretch wrap.
Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while heating diazomethane. When his colleagues Eugen Bamberger and Friedrich Tschirner characterized the white, waxy substance he had created, they recognized that it contained long -CH2- chains and termed it polymethylene. The first industrially practical polyethylene synthesis was discovered (again by accident) in 1933 by Eric Fawcett and Reginald Gibson at the ICI works in Northwich, England. Upon applying extremely high pressure (several hundred atmospheres) to a mixture of ethylene and benzaldehyde, they again produced a white waxy material. Since the reaction had been initiated by trace oxygen contamination in their apparatus, the experiment was at first difficult to reproduce. It was not until 1935 that another ICI chemist, Michael Perrin, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial LDPE production beginning in 1939. Subsequent landmarks in polyethylene synthesis have revolved around the development of several types of catalyst that promote ethylene polymerization at more mild temperatures and pressures. The first of these was a chromium trioxide based catalyst discovered in 1951 by Robert Banks and John Hogan at Phillips Petroleum. In 1953, the German chemist Karl Ziegler developed a catalytic system based on titanium halides and organoaluminum compounds that worked at even milder conditions than the Phillips catalyst. The Phillips catalyst is less expensive and easier to work with, however, and both methods are used in industrial practice. By the end of the 1950s both the Phillips and Ziegler type catalysts were being used for HDPE production. Phillips' initially had difficulties producing a HDPE product of uniform quality, and filled warehouses with off-specification plastic. However, financial ruin was unexpectedly averted in 1957, when the hula hoop, a toy consisting of a circular polyethylene tube, became a fad among youth in the United States. A third type of catalytic system, one based on metallocenes, was discovered in 1976 in Germany by Walter Kaminsky and Hansjörg Sinn. The Ziegler and metallocene catalyst families have since proven to be very flexible at copolymerizing ethylene with other olefins and have become the basis for the wide range of polyethylene resins available today, including VLDPE, and LLDPE. Such resins, in the form of fibers like Dyneema, have (as of 2005) begun to replace aramids in many high-strength applications. Until recently, the metallocenes were the most active single-site catalysts for ethylene polymerisation known - new catalysts are typically compared to zirconocene dichloride. Much effort is currently being exerted on developing new single-site (so-called post-metallocene) catalysts, that may allow greater tuning of the polymer structure than is possible with metallocenes. Recently, work by Fujita at the Mitsui corporation (amongst others) has demonstrated that certain salicylaldimine complexes of Group 4 metals show substantially higher activity than the metallocenes.
Formal Chemical Name (IUPAC)
Update by Karl Harrison
(Molecule of the Month for April 2007 )