A polymer is a high-molecular-mass substance formed by the repeated joining of a large number of small molecules called monomers. The monomer units combine through chemical bonds to form long chains or three-dimensional networks. The number of repeating monomer units present in a polymer chain is expressed by a quantity known as the degree of polymerisation. The degree of polymerisation, commonly abbreviated as DP rather than DM, is defined as the average number of repeating structural units or monomer units present in a polymer molecule or chain. It is an important parameter in the chemistry of materials because it gives an indication of the size of the polymer molecule and is closely related to its molecular mass and physical properties. For a simple polymer formed from identical monomer units, the degree of polymerisation can be expressed mathematically as: [ DP = \frac{\text{Molecular mass of the polymer}}{\text{Molecular mass of the repeating unit}} ] For example, consider polyethylene, which is produced by the polymerisation of ethene: [ nCH_2=CH_2 \rightarrow (-CH_2-CH_2-)_n ] The repeating unit of polyethylene is (-CH_2-CH_2-), having a relative molecular mass of 28. If a polyethylene molecule has an average molecular mass of 28,000, its degree of polymerisation will be: [ DP = \frac{28,000}{28}=1000 ] Thus, the polymer chain contains, on average, 1000 repeating ethylene units. The degree of polymerisation may be expressed as a number-average degree of polymerisation or a weight-average degree of polymerisation, depending upon the method used to describe the polymer molecular-weight distribution. The number-average degree of polymerisation, (DP_n), is related to the number-average