Copolymers : A polymer formed from two or more different monomers is called copolymer or mixed polymer. As, nylon-66 is a polymer of two types of monomers : hexamethylenediamine and adipic acid.
Copolymer are usually more important.
As shown copolymerisation of vinyl chloride with vinylidene chloride (1, 1 dichloroethane) in a 1 : 4 ratio forms a copolymer known as saran.
Copolymerisation of monomer mixtures often leads to the formation of polymers which have quite different properties than those of either related to homopolymer. For example, a group of styrene and methyl methacrylate can form a copolymer.
The composition of the copolymer depends on the proportion of the monomers and their reactivity. It can be noticed that some monomers do not polymerise at all but copolymerize. As given, maleic anhydride does not polymerise as such. It copolymerises with respect to styrene in a highly symmetrical manner to form styrene maleic anhydride copolymer.
It may be noted that many types of copolymers can be obtained depending upon the distribution of monomer units in the chain. Two monomers may grouped in either regular fashion (although this is rare) or random fashion. As given, if monomer A is copolymerised with monomer B, the given product can have a random distribution of the two units throughout the chain or it might have alternating distribution.
(- A - B - A - B - A - B - A - B -) Alternating copolymer
(- A - A - A - B - A - B - B - A - B -) Random copolymer
The exact distribution of monomer units depends on the initial proportion of the two-reactant monomers and their reactivities. Most copolymers have changeable distributions. Two other kinds of copolymers that may be prepared under certain conditions are called block copolymers and graft copolymers.
(a) Block copolymers in which distinct blocks of identical monomer units alternate with each other as
(-A-A-A-A-B-B-B-B-A-A-A-A-B-B-B-B-)n
These are created by initiating the polymerisation of one monomer as if growing a homopolymer and then adding an excess of second monomer to the active reaction mixture.
(b) Graft polymers in which homopolymer branches of one monomer units are grafted on the homopolymer chains of another monomer units as :
These are prepared by radiation of γ-rays on a completed homopolymer chain in the presence of the second monomer. The high power radiation blows hydrogen atoms of the homopolymer chain at random points resulting radical sites for initiation of the included monomer. By watchful control of the polymerisation reaction, we can generate copolymers of desired properties by combination of different monomers in various ratios and geometric arrangements.
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