21.3: Characteristics and Nomenclature of Copolymers
Copolymers are the products obtained from the polymerization of multiple monomer species. So, in a polymer chain itself, there can be multiple repeating units that come from different monomers. The process of synthesizing a polymer from different monomer species is called copolymerization. When two monomers are involved, the polymer is known as a bipolymer. Polymers with three and four monomers are termed terpolymers and quaterpolymers, respectively. Figure 1 depicts the copolymerization of styrene and acrylonitrile to yield the bipolymer poly(styrene-co-acrylonitrile) (styrene–acrylonitrile resin, or SAN). Acrylonitrile butadiene styrene (ABS) is a popular example of the terpolymer, widely used for 3D printing.
Figure 1: Copolymerization of styrene and acrylonitrile
In copolymers, since more than one type of repeating unit exists, the chemical formula of a copolymer is represented by placing separate parentheses and the corresponding degree of polymerization for each unique repeating unit.
Depending on the distribution of each comonomer in the polymer chain, the copolymer can be classified as an alternating, random, block, or graft copolymer.
If the monomer at the end of the growing polymer chain prefers to react with the alternate monomer in the mixture and vice versa, systematic incorporation of different monomers yields alternating copolymers.
Figure 2: Alternating copolymer from the polymerization of two monomers
When one of these monomers at the end of the growing polymer chain has no preference towards any comonomer, random incorporation of the monomer occurs.
Figure 3: Random copolymer from the polymerization of two monomers
Blocks of homopolymers can be made from a mixture of monomers with suitable reaction conditions for the homopolymerization of each monomer. These homopolymer blocks can combine at different locations. If the homopolymers join at the edges, blocks of different homopolymers alternate in the polymer chain.
Figure 4: Block copolymer from the polymerization of two monomers
Another possibility is the grafting of homopolymer chains from one monomer onto the internal atoms in a homopolymer chain of the other monomer.
Figure 5: Graft copolymer from the polymerization of two monomers