Classification of Polymers

A polymer is a very large molecule (a macromolecule) built by covalently joining together a large number of small, repeating structural units called monomers. The number of monomer units joined in one polymer chain is called the degree of polymerisation, and it commonly runs into the hundreds or thousands. The WBCHSE Semester III syllabus asks for polymers to be classified along two independent lines -- by where they come from, and by how their monomers are chemically linked together (taken up in the sections that follow).

Classification by source. Natural polymers occur ready-made in living systems, produced by plants and animals. Familiar examples include starch and cellulose (both polymers of the sugar glucose, differing only in how the glucose units are linked -- starch's $\alpha$-linkage makes it digestible and a stored food reserve in plants, while cellulose's $\beta$-linkage makes it structural and largely indigestible to humans, forming the cell walls of plants and the fibre of cotton), natural rubber (a polymer of isoprene, produced as latex by the rubber tree, Hevea brasiliensis), and the fibrous proteins silk and wool (polymers of amino acids, linked by peptide/amide bonds). Synthetic polymers, by contrast, are manufactured entirely in the laboratory or factory from small organic monomers -- polythene (from ethene), PVC (from vinyl chloride), nylon-6,6 (from a diamine and a diacid), terylene (from a diol and a diacid) and bakelite (from phenol and formaldehyde) are all synthetic, and none of them is found in nature.

This natural/synthetic split does not by itself say anything about how a polymer's monomers are joined -- a natural polymer like natural rubber is joined by successive addition of isoprene units (much like a synthetic addition polymer), while a natural polymer like silk is joined by condensation (loss of water at each peptide bond, much like nylon). The two classification schemes -- by source, and by linking mechanism -- are therefore genuinely independent, and a full description of any polymer states both: for example, nylon-6,6 is correctly described as a synthetic, condensation polymer, and natural rubber as a natural, addition polymer. The next sections take up the linking-mechanism classification -- addition and condensation polymerisation -- in detail, including the important synthetic polymers this chapter requires by name.