The word ‘polymer is coined from two Greek words ‘poly’ means ‘many’ and ‘mer’ means ‘unit or part’. Polymer is defined as very large molecules having high molecular mass. It is also known as macro molecules. The simple compounds from which polymers are made, are called monomers. The process of formation of polymers from their respective monomers, is called polymerization.
Classification of Polymers are given below-
Classification of Polymers:-
I. Based on Source
On the basis of origin of polymers, these are classified as
(i) Natural Polymers These are found in plants and animals, e.g., cellulose, starch (polymer of glucose), protein (polymer of α-amino acids), rubber (polymer of isoprene, 2-methyl-1,
3-butadiene). Enzymes are bio catalysts which are proteins and thus, these are also polymers.
(ii) Semi-Synthetic Polymers These are obtained from natural polymers. e.g., cellulose derivatives as cellulose nitrate and cellulose acetate (rayon). Cellulose acetate is used in making threads, films and sun glasses, etc.
(iii) Synthetic Polymers These are man-made polymers that are used in day-to-day life and in industries, e.g., plastic (polythene), synthetic fibres (nylon-6,6) and synthetic rubbers (Buna-S), etc.
II. Based on Structure of Polymers Classification of Polymers
On the basis of structures of polymers, these are classified as
(i) Linear Polymers These consist of long and straight chains. They have high densities, high melting points and high tensile strength. Thus, these are well packed structures, e.g., high density polythene, polyvinyl chloride, etc.
(ii) Branched Chain Polymers These contain linear chains having some branches.Due to their irregularly packed structure they have low tensile strength, low densities and lower melting point as compared to linear polymer, e.g.,low density polythene, glycogen,etc.
(iii) Cross-linked or Network Polymers These are usually formed from bi-functional and tri-functional monomers and contain strong covalent bonds between various linear polymer chains. They are hard and rigid, e.g., Bakelite, melamine,etc.
III. Based on Mode of Polymerisation
On the basis of mode of polymerisation, these are classified as
(i)Addition Polymers These are formed by the repeated addition of monomer molecules possessing double or triple bonds, e.g., polythene is formed from ethene.
These are further classified as
(a) Homopolymers These polymers are formed by the polymerisation of single monomeric species, e.g., formation of polythene from ethene.
(b) Copolymers These polymers are formed by the polymerisation of two different monomers, e.g., formation of buna-S from styrene and buta-1, 3-dienes. Some common addition polymers are polythene, polystyrene, teflon, buna-S, PVC, etc.
(ii) Condensation Polymers These are formed by the repeated condensation reaction between two different bi-functional or tri-functional monomeric units, with the elimination of small molecules such as water, alcohol, ammonia, hydrogen chloride, etc
e.g., nylon-6,6 is formed from hexamethylene diamine and adipic acid. Some common condensation polymers are terylene or dacron, nylon-6, nylon-6,6, bakelite, etc.
IV. Based on Molecular Forces
On the basis of molecular forces, the Classification of Polymers are
(i) Elastomers These are rubber-like solids with elastic properties. They have the weakest inter-molecular forces. e.g., buna-S, buna-N, neoprene, etc
(ii) Fibres These are thread forming solids possessing high tensile strength and high modulus. They have strong inter-molecular forces like H-bonding, e.g., polyamides (nylon-6,6), polyesters (terylene), etc.
(iii) Thermoplastic Polymers These are linear or slightly branched long chain molecules capable of repeatedly softening on heating and hardening on cooling. These have inter-molecular forces of attraction intermediate between elastomers and fibres, e.g., polythene, polystyrene, etc.
(iv) Thermosetting Polymers These are cross-linked or heavily branched molecules which when heated in a mould, undergo a permanent change in their chemical composition to
give a hard mass. They cannot be reused, e.g., bakelite, urea-formaldehyde resins, etc.