Anionic polymerization is more likely to proceed when there are electron-withdrawing substituents present in the monomer (e.g. –CN, –NO2 and phenyl). In principle initiation may take place either by addition of an anion to the monomer, viz:

In the absence of impurities there is frequently no termination step in anionic polymerizations. Hence the monomer will continue to grow until all the monomer is consumed. Under certain conditions addition of further monomer, even after an interval of several weeks, will cause the dormant polymerization process to proceed. The process is known as living polymerization and the products as living polymers. Of particular interest is the fact that the follow-up mono mer may be of a different species and this enables block copolymers to be produced. This technique is important with certain types of thermoplastic elastomer and some rather specialised styrene-based plastics.

polyethylene, in addition to its low price, are excellent electrical insulation

Since 1900 other methods have been devised for producing “polymethylene”, including the use of boron trifluoride-diethyl ether catalysts at 0°C. Some of these methods give unbranched linear polyme rs, often of very high molecular weight, which are useful for comparing commercial polyethylene which have molecules that are branched to varying extents.

(1) Modified Fischer-Tropsch reduction of carbon monoxide with hydrogen.

(2) Reduction of poly(vinyl chloride) with lithium aluminium hydride.

(3) Hydrogenation of polybutadiene.

Commercially, polyethylene is produced from ethylene, the polymer being produced by this route in March 1933 and reported verbally by Fawcett in 1935. The basic patent relating to the polymerization of ethylene was applied for by ICI on 4th February 1936 and accepted on 6th September 1937.

During the 1990s there was enormous activity in the development of a further type of polyethylene based on metallocene catalysis methods. Commercial production commenced in the late 1990s and it is estimated that in 2000 metallocene-catalyzed polyethylene will comprise about 2% of the total polyethylene market. This is somewhat less spectacular than achieved by LLDPE and reflects the fact that al though these materi als may have many superior properties in the finished product they are more expensive than the traditional materials and in some respects more difficult to process. Whereas the metallocene polymers can be of LDPE, LLDPE and HDPE types it is anticipated that LLDPE types (referred to as mLLDPE) will take over 50% of the market; mainly for film application.

At one time ethylene for polymerization was obtained largely from molasses, a by-product of the sugar industry. From molasses may be obtained ethyl alcohol and this may be dehydrated to yield ethylene. Today the bulk of ethylene is obtained from petroleum sources. When supplies of natural or petroleum gas are available the monomer is produced in high yield b y high-temperature cracking of ethane and propane. Good yields of ethylene may also be obtained if the gasoline (“petrol”) fraction from primary distillation of oil is “cracked”. The gaseous products of the reaction include a number of lower alkanes and olefins and the mixture may be separated by l ow-temper a t u r e fr a ct i o n al distillation and by selective absorption. Olefins, in lower yield, are also obtained by cracking gas oil. At normal pressures (760 mmHg) ethylene is a gas boiling at -103.71°C and it has a very high heat of polymerization (3350-4185 J/g). In pol ymerization reactions the heat of polymerization must be carefully controlled, particularly since decomposition reactions that take place at elevated temperatures are also exothermic and explosion can occur if the reaction gets out of control.

Since impurities can affect both the polymerization reaction and the properties of the finished product (particularly electrical insulation properties and resistance to heat aging) they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low level. A number of patents require that the carbon monoxide content be less than 0.02%.

Although there are a number of publications dealing with the basic chemistry of ethylene polymerization under high pressure, little information has been made publicly available concerning details of current commercial processes. It may however be said that commercial high polymers are

generally produced under conditions of high pressure (1000-3000 at m) and at temperatures of 80-300°C. A free-radical initiator such as benzoyl peroxide, azodi-isobutyronitrile or oxygen is commonly used. The process may be operated continuously by passing the reactants through narrow-bore tubes or through stirred reactors or by a batch process in an autoclave. Because of the high heat of polymerization care must be taken to prevent runaway reaction. This can be done by having a high cooling surface­volume ratio in the appropriate part of a continuous reactor and in addition by running water or a somewhat inert liquid such as benzene (which also helps to prevent tube blockage) through the tubes to dilute the exotherm. Local runaway reactions may be prevented by operating at a high flow velocity. In a typical process 10-30% of the monomer is converted to polymer. After a polymer-gas separation the polymer is extruded into a ribbon and then granulated. Film grades are subjected to a homogenization process in an internal mixer or a continuous compounder machine to break up high molecular weight species present.

Although in principle the high -pressure polymerization of ethylene follows the free-radical-type mechanism the reaction has two particular characteristics, the high exothermic reaction and a critical dependence on the monomer concentration.