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 : See Table 1.
Polymers, which are formed by more than one type of monomers are known as copolymers. Some examples are given below in the table : See Table 2.
`text(Types of copolymers :)`
Depending upon the distribution of monomer units, the following types of copolymers are possible.
(i) `text(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
`nA + nB -> undersettext(segment of random copolymer)(- A- A- B- A- B- B- A- B-A- A- A- )`
(ii) `text(Alternating Copolymer :)` If the two monomer units occur alternatively throughout the polymer chain, it is said to be alternating copolymer. For example,
`nA + nB -> undersettext(segment of alternating copolymer)(- A- B - A- B - A- B -)`
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.
(iii) `text(Block copolymer :)` Polymers in which different blocks of identical monomer units alternate with each other are called block copolymers. For example,
`undersettext(segment of a block polymer)(- B- B-A-A-A-A-A-A- B- B-B- B- B- B-A-)`
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.
(iv) `text(Graft copolymer :)` Polymers in which homopolymer branches of one monomer unit are grafted onto a homopolymer chain of another monomer unit are called graft co-polymers. For example : See fig.
Graft copolymers are prepared by `gamma`-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 polymerisarion of the second monomer.
Polymers, which are formed by more than one type of monomers are known as copolymers. Some examples are given below in the table : See Table 2.
`text(Types of copolymers :)`
Depending upon the distribution of monomer units, the following types of copolymers are possible.
(i) `text(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
`nA + nB -> undersettext(segment of random copolymer)(- A- A- B- A- B- B- A- B-A- A- A- )`
(ii) `text(Alternating Copolymer :)` If the two monomer units occur alternatively throughout the polymer chain, it is said to be alternating copolymer. For example,
`nA + nB -> undersettext(segment of alternating copolymer)(- A- B - A- B - A- B -)`
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.
(iii) `text(Block copolymer :)` Polymers in which different blocks of identical monomer units alternate with each other are called block copolymers. For example,
`undersettext(segment of a block polymer)(- B- B-A-A-A-A-A-A- B- B-B- B- B- B-A-)`
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.
(iv) `text(Graft copolymer :)` Polymers in which homopolymer branches of one monomer unit are grafted onto a homopolymer chain of another monomer unit are called graft co-polymers. For example : See fig.
Graft copolymers are prepared by `gamma`-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 polymerisarion of the second monomer.