What are polymers and how do they form?

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Chemistry Open Book Exam

Firstly a polymer is a long chained molecule which is made up of many hundreds or thousands on monomer units. Addition polymerisation is the joining of these monomers to produce the polymer molecule. In most cases there are C=C double bonds, which are found in alkenes. These double bonds are then broken in order to form a long chain of monomer units.

        The mechanism of polymerisation of ethene to form low density polyethene requires a catalyst, which can be dioxygen or an organic peroxide. The reaction involves radicals and can be shown in three steps.

The first step is the initiation which involves the creation of the radical:

R* + H2C=CH2                        R-CH2-CH2*                Where R is the alkyl group

                                                                From the Peroxide and

                                                                * Indicates the Radical

The radical formed now reacts with another ethene molecule, during the propagation reaction, which then happens again and again to form a long chain polymer

R-CH2-CH2* + H2C=CH2                R-CH2-CH2-CH2-CH2*

R-CH2-CH2-CH2-CH2* + H2C=CH2         R-CH2-CH2-CH2-CH2-CH2-CH2* etc

During this reaction the radical combines with one of the electrons from the double bond of the ethene. One of the electrons from the double bonds is put into joining the CH2* part from the radical with one of the CH2 parts from the ethene molecule.  The other electron comes from the CH2* part. The other electron from the double bond remains with the second carbon atom and so the product formed is another radical.

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When ldpe forms, it is branched. This occurs when the growing chain attacks itself and is known as ‘back-biting.’

                        |                                                        |

                        CH2                                                        CH2        

                        |                                                        |

-CH2-CH2-CH2* +          CH2                -CH2-CH2-CH3 +     *CH

                        |                                                        |

                        CH2                                                        CH2

                        |

Attack by another radical can then take place at that site which causes the chain to grow out of that radical. This makes the polymer branched. This is shown in the diagram below.

                        |                                                |

                        CH2                                                CH2        

                        |                                                |

-CH2-CH2-CH2* +    *CH                                               -CH2-CH2-CH2-CH

                        |                                                |

                        CH2                                                CH2

                        |

...

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