Degree of polymerization
For a homopolymer, there is only one type of monomeric unit and the number-average degree of polymerization is given by , where Mn is the number-average molecular weight and M0 is the molecular weight of the monomer unit. For most industrial purposes, degrees of polymerization in the thousands or tens of thousands are desired.
Some authors, however, define DP as the number of repeat units, where for copolymers the repeat unit may not be identical to the monomeric unit. For example, in nylon-6,6, the repeat unit contains the two monomeric units —NH(CH2)6NH— and —OC(CH2)4CO—, so that a chain of 1000 monomeric units corresponds to 500 repeat units. The degree of polymerization or chain length is then 1000 by the first (IUPAC) definition, but 500 by the second.
In step-growth polymerization, in order to achieve a high degree of polymerization (and hence molecular weight), Xn, a high fractional monomer conversion, p, is required, as per Carothers' equation: Xn = 1/(1−p). A monomer conversion of p = 99% would be required to achieve Xn = 100. For chain-growth polymerization, however, this is not generally true and long chains are formed for much lower monomer conversions.
Correlation with physical properties
Polymers with identical composition but different total molecular weights may exhibit different physical properties. In general, increasing degree of polymerization correlates with higher melting temperature  and higher mechanical strength.
Kinds of degree of polymerization
Mainly, there are two types used to measure the degree of polymerization, number average degree of polymerization and weight average degree of polymerization. Number Average degree of polymerization is found by finding the Weighted mean of mole fraction o. While the weight average degree of polymerization is found by finding the weighted mean of weight fraction .
- IUPAC Definition in Compendium of Chemical Terminology (IUPAC Gold Book)
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