Addition Polymerization: PVC, Poly(ethene), Polystyrene, Perspex and Polyacrylonitrile

Addition polymerization is a process whereby two or more monomers of the same chemical compound are linked to form form a polymer without the elimination of any by-product.

Monomers for addition polymerization are mostly derived from petrochemicals. Ethene is the most important petrochemical used in addition polymerization. It is produced, like a host of other unsaturated hydrocarbons, by the process of cracking petroleum. Polymers made from ethene are called vinyl polymers and include: poly(chloroethene), poly(ethene), poly(phenylethene), perspex and poly(propenonitrile).

Poly(chloroethene), also known as polyvinyl chloride or simply PVC; is a polymer used in the harden form to making everyday materials like cable coverings, leather cloths, plastic pipes, and in soft and flexible plasticised form to make raincoats, shower curtains, raincoats, hand gloves, e.t.c.

Either ethene or ethyne is used to make the chloroethene (vinyl chloride) monomer. In the case of using ethene, the ethene is initially chlorinated to chloroethene (vinyl chloride). Then, the chloroethene (nCH₂=CHCl) is made to undergo polymerization in the presence of hydrogen peroxide catalyst at 55°C and under pressure to form the poly(chloroethene) or polyvinyl chloride.

nCH₂=CHCl → -CH₂-CHCl-[CH₂-CHCl]_n-CH₂-CHCl-

Poly(ethene) is the polymer of ethene. Poly(ethene) exist as either low density poly(ethene) or high density poly(ethene). Ethene, in the presence of high temperature (250°C), pressure (over 1500 atmosphere) and traces of oxygen initiator; polymerizes to low density poly(ethene) - [CH₂-CH₂]_n. Where, n could be 1000 to 10,000. The polymerization of high density poly(ethene) on the other hand, is carried out in the presence of catalyst, lower temperature, and also lower pressure.

Crates, dustbins, bottles, boxes and hard bags are made from high density poly(ethene). Low density poly(ethene) on the other hand, is used for making bags and bottles, and a wide range of other products like kitchen wares.

Poly(phenylethene) is also known as polystyrene. You can visualise the nature of polystyrene in everyday materials like CD case, shaving stick and ice cream container.It is made from phenylethene (styrene) monomer by refluxing in the presence of benzoyl peroxide catalyst.

Expanded poly(styrene), or styrofoam is made when air is blown with a foaming agent into heated poly(styrene). Styrofoam is a whitish and opaque solid that floats on water that is used in making lamp shade, ceiling tiles, containers for fast food packaging, safety devices as shock absorbers, e.t.c.

Poly(styrene) in general, is a transparent, hard and brittle material so that is used in making transparent materials for lighting and other purposes.

Perspex is unique for its glass-like appearance, small wonder it is usually used to replace glass in so many materials. Perspex is commonly known as PMMA - poly(methyl 2-methylpropenoate). It is made by polymerization when methyl 2-methylpropenoate is heated in the presence of lauryl peroxide catalyst.

Although perpex is used for making window-panes, contact lenses, car rear light, wind-shields, street lamp, and fish-tanks; there is an attention on the ease with which it is scratched.

Poly(propenonitrile) is used in textile industry to make woolly garments. It is usually spun into thread that are eventually used to make clothes. An example of textile material made from poly(propenonitrile) is the popular Orlon.

Poly(propenonitrile) polymer, also known as polyacrylonitrile, is made from the polymerization of propenonitrile CH₂=CHCN monomer which is obtained from the substitution of one hydrogen atom with a cyano group (-CN group).

nCH₂=CHCN → ···CH₂-[CHCN- CH₂]-_nCHCN···

The process of polymerization is carried out in the presence of organic peroxide catalyst.