Mechanism of the Polymerisation of Ethene

Thousands of ethene molecules join up to form a type of poly(ethene) known as low density polythene.

[CH2=CH2]n [ CH2-CH2 ]n number of joining molecules can vary from 200-2000

The mechanism is known as a free radical reaction as it is the radicals which initiate the three stage process of polymerisation.

Stage 1: Initiation by free radicals

R l + H2C=CH2 R CH2 CH2 l where r is the radical

Each time a radical hits an ethene molecule a new, longer radical is formed.

R CH2 CH2 l + H2C=CH2 R CH2 CH2 CH2 CH2 l

R H2 CH2 CH2 CH2 l + H2C=CH2 R CH2 CH2 CH2 CH2 CH2 CH2 l

Eventually two radicals combine and the process stops as no new radicals are formed (1)

As the chains grow by radical mechanism, it is possible for a radial exchange

(1) http://www.chemguide.co.uk/mechanisms/freerad/polym.html

The Zeigler-Natta catalysts allowed greater control however they were easily poisoned or damaged. The transitional metal part also makes the catalyst strongly polar, allowing electron interactions of the covalent type. This can lead to the formation of temporary bonds which can wrap around the catalyst which can prevent chain growth. The metal sites can also then trigger the growth of several side chains. Small particles of catalysts could also had the same effect. Its tendency to branch uncontrollably means the polymers produced are suitable only for a limited range of refined uses. The catalyst only works with a restricted range of monomers so types of polymers available are limited.

Chance enabled chemists to better control polymerisation reactions. The discover of metallocene such as titanocene means that reliance on serendipity is now at an end.

Some common words found in the essay are:
Kaminsky Brintzinger, Density Polyethene, Structure Polyethene, Solving Zeigler-Natta, Isotactic Syndotatic, Fawcett Gibson's, Termination Eventually, CH2 CH2, H2C5 C2H5, CH2=CH2n CH2-CH2, ch2 ch2, ch3 ch3, ch2 ch2 ch2, ch3 ch3 ch3, low density, polymerisation ethene, + h2c=ch2, intermolecular forces, h2c=ch2 ch2 ch2, density polyethene, h2c=ch2 ch2, c ch3 ch3, c ch3, c c, + h2c=ch2 ch2,
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