Polymers Obtained by Addition Polymerization

Carothers of DuPont company also invented a synthetic rubber, chloroprene. This polymer is produced by addition-polymerization unlike the polymers mentioned so far. Let us see how it is done with an example of polyethylene. Polyethylene is used in everyday life as a wrap, the thin plastic film. It is a polymer of ethylene, ethene in technical terms. Ethylene has the chemical formula of CHj=CHj. Addition polymerization can be conducted with a compound having one or two double bonds such a compound is often called olefin, Ethylene is the simplest olefin. One of the two bonds in a double bond is a T-bond and the other is a n-bond, which is weaker and more readily splits than the T-bond. Alternatively, the electron in the n-bond easily binds to an entity called a free radical. A free radical has an unpaired (single) electron, which seeks another electron to pair up. The electrons in the n-bond provide such an electron. So what happens is this  

In this reaction, the entity R is artificially produced or added and is called an initiator (of polymerization). The resulting entity RCH -CH now has an unpaired electron, which can react with another molecule of ethylene  

This process can go on and on, making a long chain of -CH -CH - units hence, the product of such a polymerization is called polyethylene. Polyethylene is essentially the same as the molecules in gasoline or wax only the chain length is enormous, often tens of thonsands of ethylene units. [You might have noticed that you smell wax when you bum polyethylene plastic wrap]. In this polymerization process, you will note, no small molecule like water is removed as in condensation polymerization, and a monomer is simply added to the growing end of a polymer. Hence, polymerization of this type is called addition polymerization. 

Derivatives of ethylene in which one of the hydrogen atoms is replaced by another entity are called vinyls. For example, vinyl chloride is H2C=CHC1, and vinyl acetate is H2C=CH(OCOCH3). The polymer of vinyl chloride is poly(vinylchloride), PVC, which has been widely used for plastic cover and water pipe. 

Other compounds of vinyl type that are used for polymers include acrylonitrile (vinyl cyanide), H2C=CH(CN), and styrene, H2C=CH(C H3). Vinyl compounds are all polymerized by addition process. Tetrafluoroethylene, F C=CF, in which all the hydrogen atoms of ethylene are replaced by fluorine atoms, polymerizes similarly to form a polymer of unique character, teflon. 

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Polymers of other chemical compositions provide still greater variety in physical and chemical properties. TABLE 12.5 lists. several other common polymers obtained by addition polymerization. 

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