( iii) Semisynthetic polymers: Polymers obtained by making some modification in natural polymers by artificial means, are known as semi synthetic polymers, e. g., cellulose acetate( rayon), vulcanised rubber etc.
Classification of Polymers Based on Structure
( i) Linear polymers These are the polymers in which the monomer units are linked to one another to form long linear chains. These linear chains are closely packed in space. The close packing results in high densities, tensile strength and high melting and boiling points. e. g., high density polyethene, nylon and polyesters are linear polymers.
( ii) Branched chain polymers In such polymers, the monomer units are linked to form long chains with some branched chains of different lengths with source. As a result of branching, these polymers are not closely packed in space. Thus, they have low densities, low tensile strength as well as low melting and boiling points. Some common examples of such polymers are low density polyethene, starch, glycogen etc.
( iii) Cross-linked polymers or network polymers In such polymers, the monomer units are linked together to form three dimensional network. These are expected to be quite hard, rigid and brittle. Examples of cross linked polymers are bakelite, glyptal, melamine-formaldehyde polymer etc.