The term polymer is used to describe a very large molecule that is made up of many repeating small molecular units. These small molecular units from which the polymer is formed are called monomers. The chemical reaction that joins the monomers together is called polymerisation. Starting from n molecular of a compound M, linking in a linear manner will form polymer x–M–(M)n–2–M–y. The nature of linkages at the terminal units i.e. M–x and M–y depends upon the mode of reaction used in making the polymers. Homopolymers and Copolymers: Polymers which are formed by only one type of monomer are called Homopolymers. Some examples of homopolymers and their monomers are given below :
Homopolymers and Copolymers:
Polymers which are formed by only one type of monomer are called Homopolymers. Some examples of homopolymers and their monomers are given below :
Polymers, which are formed by more than one type of monomers are known as copolymers. Some examples are given below in the table:
Vinyl chloride and vinylidene chloride
Styrene and acrylonitrile
Acrylonitrile, butadiene and styrene
Isobutylene and Isoprene
Styrene and Butadiene
Acrylonitrile and Butadiene
Hexamethylen diamine and Adipic acid
Terephthalic acid and ethylene glycol
Types of copolymers:
Depending upon the distribution of monomer units, the following types of copolymers are possible.
(1) Random Copolymer:
If the monomer units have random distribution throughout the chain, it is called random copolymer. For example, if the monomer A and monomer B undergo copolymerisation, then the structure of the random copolymer is:
(2) Alternating Copolymer:
If the two monomer units occur alternatively throughout the polymer chain, it is said to be alternating copolymer. For example,
The exact distribution depends upon the proportion of the two reactant monomers and their relative reactivities. In practice, neither perfectly random nor perfectly alternating copolymers are usually formed. However, most copolymers tend more towards alternating type but have many random imperfections.
(3) Block Copolymer:
Polymers in which different blocks of identical monomer units alternate with each other are called block copolymers.
Block copolymer can be prepared by initiating the radical polymerisation of one monomer to grow homopolymer chains followed by addition of an excess of the second monomer.
(4) Graft Copolymer:
Polymers in which homopolymer branches of one monomer unit are grafted onto a homopolymer chain of another monomer unit are called graft copolymers. For example:
Graft copolymers are prepared by g-irradiation of a homopolymer chain in the presence of a second monomer. The high energy radiation knock out H-atoms from the homopolymer chain at random points thus generating radical sites that can initiate polymerization of the second monomer.
CLASSIFICATION OF POLYMERS:
Polymers are classified in following ways:
(I) CLASSIFICATION BASED UPON SOURCE:
(1) Natural polymers-
Polymers which are obtained from animals and plants are known as natural polymers, Examples of natural polymers are given below:
3. Nucleic acid
|5. Natural Rubber (cis polyisoprene) Isoprene||(2-Methyl-1, 3-butadiene)|
6. Gutta percha (trans polyisoprene)
Natural polymers which take part in metabolic processes are known as biopolymers. Examples are polysaccharides, proteins, RNA and DNA.
(2) Semi-synthetic polymers-
Polymers which are prepared from natural polymers are known as semi-synthetic polymers. Most of the semi-synthetic polymers are prepared from cellulose.
Examples are: cellulose acetate, cellulose nitrate, cellulose xanthate and Rayon.
(3) Synthetic polymers-
Man-made polymers, i.e. polymers prepared in laboratory are known as synthetic polymers.
Example are : PVC, polyethylene, polystyrene, nylon-6, nylon-66, nylon-610, terylene, synthetic rubbers etc.
(II) CLASSIFICATION BASED UPON SHAPE:
(1) Linear polymers-
Polymer whose structure is linear is known as linear polymer. The various linear polymeric chains are stacked over one another to give a well packed structure.
The chains are highly ordered with respect to one another. The structure is close packed in nature, due to which they have high densities, high melting point and high tensile (pulling) strength. Linear polymers can be converted into fibres.
(i) All fibers are linear polymers. Examples are cellulose, silk, nylon, terylene etc.
(ii) Linear polymers may be condensation as well as addition polymers. Examples are cellulose, Polypeptide, nucleic acid, nylon, terylene etc.
(2) Branched chain polymers-
Branched chain polymers are those in which the monomeric units constitute a branched chain. Due to the presence of branches, these polymers do not pack well. As a result, branched chain polymers have lower melting points, low densities and tensile strength as compared to linear polymers.
(3) Cross-linked or Three Dimensional network polymers-
In these polymers the initially formed linear polymeric chains are joined together to form a three dimensional network structure. These polymers are hard, rigid and brittle. Cross-linked polymers are always condensation polymers. Resins are cross linked polymers.
CLASSIFICATION BASED UPON SYNTHESIS:
(1) Condensation polymerization-
(i) They are formed due to condensation reactions.
(ii) Condensation polymerisation is also known as step growth polymerisation.
(iii) For condensation polymerisation, monomers should have at least two functional groups. Both functional groups may be same or different.
(iv) Monomers having only two functional group always give linear polymer.
(v) Condensation polymers do not contain all atoms initially present in the monomers.
Some atoms are lost in the form of small molecules.
(vi) Monomers having three functional groups always give cross-linked polymer.
Examples are : Urea-formaldehyde resin, phenol-formaldehyde resin.