Polymers

Polymerisation

Polymerisation is the process of joining together a large number of small molecules to make a smaller number of very large molecules. The reactants (i.e. the small molecules from which the polymer is constructed) are called Monomers and products of the polymerisation process are called Polymers. There are significant differences between the chemical and physical properties of polymers and those of the monomers from which they are made. This polymerisation process can occur by two different mechanisms : by addition polymerisation and condensation polymerisation

Addition Polymerisation

Addition polymerisation is the joining together of two or more simple molecules, called Monomers, to form a new compound of the same empirical formula, called a polymer which has a very high molecular weight.

The addition polymerisation process can only occur when the monomer molecule is unsaturated (i.e. contain double bonds or triple bonds). Thus, addition polymerisation is characteristic of ethene and the other ethenes.

The polymers formed by addition polymerisation are thermoplastic. These include Polythene, Polypropylene and Polystyrene.

Polymers

Polymers are giant molecules, built up from small units called Monomers which combined together to form a repeating structure. Polymers are classified into :

Natural Polymers: which include proteins starch, cellulose and rubber.
Synthetic Polymers,: include bakelite, perspex, terylene (a polyester) and nylon (a polyamide). These are man-made polymers, which are also called plastics.

Polymerisation of ethylene to polythene

This is one of the most used plastics of the polymers. The double bonds in ethene is opened up and then joined together to form the polymer.
The conditions under which this happens is as follows.
Ethene is heated up to about 200 deg.C. under a pressure of about 1000 atmospheres in the presence of Oxygen which acts as a catalyst.

Another method was devised in which the high temperatures and pressures were not used. The catalyst used were known as Ziegler initiators or Ziegler catalyst called after the German chemist who discovered them in the 1950's. These catalyst were organic peroxides or organo-metallic compounds.

The organic peroxide, which has the following structure :


			R-O-O-R'

breaks at the single covalent bond, between the two oxygen atoms, to give two free radicals thus :-


	R-O-O-R'	==>	R-O*	+	*O-R'

The oxygen of each free radical contains a septet of electrons. The odd electron is shown by the dot (*) and hence it is very reactive chemically and only exist for a fraction of a second.
This free radical joins the ethylene molecule as can be seen from the following reaction :


	R-O*	+	CH2=CH2	===>	R-O-CH3-C*H2

The pi-bond of the C=C double bond is broken. The oxygen of the free radical is joined Covalently with a carbon atom, leaving the carbon at the other end short one electron for the octet. This end now becomes a radical, which is chemically reactive and adds another ethene molecule as follows :


	R-OCH2C*H2	+	CH2=CH2	==>	R-OCH2CH2CH2C*H2

This chain proceeds rapidly to form a giant molecule, the chain continues until it is stopped by the introduction of some inhibitor -O-R'


	R-O-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2- - -O-R'

Polythene is a tough translucent plastic of high molecular of about 20,000, it is very resistant to chemical change and is an excellent electrical insulator. It is moulded for the making of buckets, lunch boxes squeezy bottles.

It's main disadvantage is that the plastic will soften in boiling water.

Condensation polymerisation

This may be defined as the process in which the monomer molecules of different compounds combine with the loss of some simple molecules, like water, or HCl.

This process can produce both thermoplastics and thermosetting plastics.
e.g. Polyesters and nylon are formed by this process.

Polymerisation of adipic acid and hexamethylene diamine to nylon

Nylon is a synthetic fiber, composed of long molecules. It is the product formed by the condensation polymerisation of both the following monomers.


		HO.O=C-CH2-CH2-CH2-CH2-C=O.OH
			adipic acid

		H2N-CH2-CH2-CH2-CH2-CH2-CH2-NH2 
			hexamethylene diamine

Both adipic acid and hexamethylene diamine can be produced from phenol which is a by-product of coal.

A molecule of adipic acid can react with a molecule of hexamethylene diamine, to give a nylon salt. This salt, when heated under pressure, undergoes condensation polymerisation, water expelled). We then have a linear super polymer, which is nylon.


	[C==O-CH2-CH2-CH2-CH2-C=O-NH-CH2-CH2-CH2-CH2-CH2-CH2-NH2]n

The solid polymer is melted and forced through fine jets to make filaments, which are then collected in the form of yarn and can be woven into nylon fabric, ropes, cords, nets etc. It is also used as a plastic in the making of combs and brushes.

Nylon was the first synthetic fiber to be made comically.

Polymerisation of benzene-1,4-dicarboxylic acid and dihydric alcohol to polyesters.

Polyesters are composed of long molecules. It is the product formed by the condensation polymerisation of both the following monomers


		HO.O=C-C6H4-C=O.OH	
		benzene-1,4-dicarboxylic acid

HO-CH2-CH2-OH and dihydric alcohol ethane-1,2-diol or ethylene glycol

Both benzene-1,4-dicarboxylic acid and dihydric alcohol ethane-1,2-diol or ethylene glycol can be produced from petroleum.


HO.O=C-C6H4-C=O.OH	+	HO-CH2-CH2-OH	===> 
[O.O=CC6H4C=O.O.CH2CH2]n+	H2O

A molecule of benzene-1,4-dicarboxylic acid can react with a molecule of ethane-1,2-diol or ethylene glycol, to give a salt. This salt, when heated under pressure, undergoes condensation polymerisation, water is expelled.
The solid polymer is very strong, it is resistant to shrinkage, creasing, abrasion, mildew and sunlight.
It's main uses are in making cloths, tablecloths, sewing thread, yacht sails, photographic films etc..

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