POLYMERS AND POLYMERIZATION –
What do you suppose our DNA, a plastic bottle, and your wooden closet have in common? ¬ The fact that they are all Polymers!!!
What are polymers?
The word ‘polymer’ is coined from two Greek words: poly means many and mer means unit or part. The term polymer is defined as very large molecules having high molecular mass (103-107u). These are also referred to as macromolecules, which are formed by joining of repeating structural units on a large scale. The repeating structural units are derived from some simple and reactive molecules known as monomers and are linked to each other by covalent bonds.
What is polymerization?
This process of formation of polymers from respective monomers is called polymerisation.
What are the various types of polymers?
Based on source
1. Natural polymers These polymers are found in plants and animals. Examples are proteins, cellulose, starch, resins and rubber. 2. Semi-synthetic polymers Cellulose derivatives as cellulose acetate (rayon) and cellulose nitrate, etc. are the usual examples of this sub category. 3. Synthetic polymers A variety of synthetic polymers as plastic (polythene), synthetic fibres (nylon 6,6) and synthetic rubbers (Buna - S) are examples of manmade polymers extensively used in daily life as well as in industry.
Based on structure
1. Linear polymers These polymers consist of long and straight chains. The examples are high density polythene, polyvinyl chloride, etc. 2. Branched chain polymers These polymers contain linear chains having some branches, e.g., low density polythene. 3. 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, e.g. bakelite, melamine, etc
Based on mode of polymerization
1. Addition polymers The addition polymers are formed by the repeated addition of monomer molecules possessing double or triple bonds, e.g., the formation of polythene from ethene and polypropene from propene. However, the addition polymers formed by the polymerisation of a single monomeric species are known as homopolymers, e.g., polythene. The polymers made by addition polymerisation from two different monomers are termed as copolymers, e.g., Buna-S, Buna-N, etc. 2. Condensation polymers The condensation polymers are formed by repeated condensation reaction between two different bi-functional or tri-functional monomeric units. In these polymerisation reactions, the elimination of small molecules such as water, alcohol, hydrogen chloride, etc. take place. The examples are terylene (dacron), nylon 6, 6, nylon 6, etc. For example, nylon 6, 6 is formed by the condensation of hexamethylene diamine with adipic acid.
Types of Polymerization Reactions
1. Free radical mechanism- A variety of alkenes or dienes and their derivatives are polymerised in the presence of a free radical generating initiator(catalyst) like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc.
2. Condensation Polymerization - This type of polymerisation generally involves a repetitive condensation reaction between two bi-functional monomers. These polycondensation reactions may result in the loss of some simple molecules as water, alcohol, etc., and lead to the formation of high molecular mass condensation polymers.
3. Copolymerisation -its a polymerisation reaction in which a mixture of more than one monomeric species is allowed to polymerise and form a copolymer. The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also. It contains multiple units of each monomer used in the same polymeric chain. For example, a mixture of 1, 3 – butadiene and styrene can form a copolymer.