Polymers are compounds of very high molecular masses formed by the combination of a large number of simple molecules. Poly means many and meros means units or parts. The simple molecules which combine to give polymer are called monomers. The process by which the simple molecules (i.e. monomers) are converted into polymers is called polymerisation.
For example, polyethylene or commonly known as polythene is a polymer which is obtained from its monomers ethylene by the process of Polymerisation.
n CH2 = CH2 ———-> ― [CH2―CH2]n—
Ethylene Polyethylene
(monomers) (polymer)
For example, Terylene is a polymer of two types of monomers; ethylene glycol and terephthalic acid.
n HO-CH2-CH2-OH + n HOOC-C6H4-COOH ——-> ― [O-CH2CH2-OOC-C6H4-CO]n―
Ethylene glycol Terephthalic acid Terylene or Dacron
Polymers and macromolecules: – The polymers are also called macromolecules because of their big size. The terms polymers and macromolecules are often used without any distinction. But, strictly speaking, a polymer always consists of thousands of repeating monomer units. However, a macromolecule is a giant molecule which may or may not contain monomer units. For example, chlorophyll is a macromolecule but it is not regarded as a polymer because it does not contain monomers. However, polythene may be regarded as a polymer as well as a macromolecule because it contains a large number of repeating monomers. Thus, all polymers are macromolecules but all macromolecule are not polymers.
Classification of polymers:-
Polymers are classified in a number of ways as described below:
1) Classification of Polymers on the Basis of Origin-
The polymers can be broadly classified as—
(i) Natural Polymers
(ii) Synthetic polymers
(i) Natural Polymers: The polymers ‘Obtained from nature (plants and animals) are called natural polymers. For example, starch, cellulose, natural rubber, proteins, etc.
(a) Starch– It is a polymer of glucose. It is a chief food reserve of plants.
(b) Cellulose– It is also a polymer of glucose. It is a chief structural material of the plants. Both starch and cellulose are made by plants from glucose produced during photosynthesis
(c) Proteins– These are polymers of α-amino acids. They have generally 20 to 1000 α-amino acids joined together in a highly organized arrangement. These are building blocks of animals and constitute an essential part of our food.
(d) Nucleic acids– These are polymers of various nucleotides. For example, RNA and DNA are common nucleotides.
(e) Natural rubber– It is a polymer of unsaturated hydrocarbon, 2-methyl-1, 3-butadiene also called isoprene. It is obtained from latex of rubber trees.
(ii) Synthetic polymers: – The polymers which are prepared in the laboratories are called synthetic polymers. These are also called man-made polymers. For example, polyethylene, PVC, nylon, teflon, bakelite, terylene, synthetic rubber, etc.
2) Classification of Polymers on the Basis of Structure-
On the basis of structure of polymers, these can be classified as-
(i) Linear chain polymers
(ii) Branched chain polymers
(iii) Cross-linked polymers.
(i) Linear Chain Polymers– These are polymers in which monomeric units are linked together to form linear chains. These linear polymers are well packed and therefore have high densities, high tensile (pulling) strength and high melting points. For example, polyethylene, nylons and polyesters are linear chain polymers.
(ii) Branched Chain Polymers-These are polymers in which the monomers are joined to form long chains with side chains or branches of different lengths. These branched chain polymers are irregularly packed and therefore, they have low tensile strength and melting points than linear polymers. For example, low density polythene, glycogen, starch, etc.
(iii) Cross-linked polymers– These are polymers in which monomer units are cross linked together to form a three dimensional network. These polymers are hard, rigid and brittle because of network structure. For example, bakelite, melamine formaldehyde resin.
3) Classification of Polymers on the Basis of Mode of Synthesis–
On the basis of mode of synthesis, the polymers may be classified as-
(i) Addition polymers
(ii) Condensation polymers.
(i) Addition polymers:
A polymer formed by direct addition of repeated monomers without the elimination of any by product molecules is called addition polymer. In this type, the monomers are unsaturated compounds and are generally derivatives of ethene. The addition polymers have the same empirical formula as their monomers. For example, the addition polymers polythene or polypropylene are obtained as:
nCH2 = CH2 ——-> ― [CH2―CH2]n—
Ethylene Polyethylene
Other examples are polyvinyl chloride (PVC), Teflon, Orlon, etc.
(ii) Condensation polymers:
A polymer formed by the condensation of two or more than two monomers with the elimination of simple molecules like water, ammonia, hydrogen chloride, alcohol, etc. is called condensation polymer. In this type, each monomer generally contains two functional groups. For example, nylon-66 is obtained by the condensation of two monomers; hexamethylenediamine and adipic acid with the loss of water molecules.
n H2N-(CH2)6-NH2 + n HOOC-(CH2)4-COOH — [—NH-(CH2)6-NH-CO-(CH2)4-CO—]n— Hexamethylenediamine adipic acid Nylon–66 + nH2O
Some other examples of condensation polymers are Terylene, bakelite, alkyl resins, etc.