21.2:
Characteristics and Nomenclature of Copolymers
Copolymers are obtained by the polymerization of two or more different monomers together.
Consider the copolymerization of styrene and acrylonitrile monomers to make polystyrene-co-acrylonitrile. Here, the polymer name contains both monomer units, the infix 'co', and the prefix 'poly'.
Copolymers are usually represented by placing separate parentheses or brackets and degrees of polymerization around each type of repeating unit.*
Alternatively, a simplified expression places the parentheses around the bonded comonomers with a common degree of polymerization.
If each monomer preferentially binds to the comonomer rather than the same type, the polymerization will result in an alternating copolymer. On the other hand, a lack of any preference between homo- and copolymerization leads to a random copolymer.
Blocks of homopolymers from a unique monomer can be synthesized by switching between polymerization conditions favorable for each monomer.
If the homopolymer blocks are alternating, it is a block copolymer.
If one homopolymer is grafted onto the other type of homopolymer chain, it is a graft copolymer.
21.2:
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
Suggested Reading
- Bruice, P. Y. (2004). Organic Chemistry. Upper Saddle River: Pearson, 088.
- Wade Jr, L. G. (2013). Organic Chemistry. Upper Saddle River: Pearson, 1232.
- Oullette, R. J., & Rawn, J. D. (2014). Organic Chemistry: Structure, Mechanism, and Synthesis. Elsevier, 1001-1002.
- https://old.iupac.org/publications/books/pbook/PurpleBook-C7.pdf, IUPAC (1985). Source Based Nomenclature for Copolymers 131.