The degree of polymerization (DP) is usually defined as the number of monomeric units in a macromolecule or polymer or oligomer molecule.

For a homopolymer, there is only one type of monomeric unit and the number-average degree of polymerization is given by

$DP_n\equiv{}X_n=\dfrac{M_n}{M_o}$

where

• Mn is the number-average molecular weight and
• Mo is the molecular weight of the monomer unit.

For most industrial purposes, degrees of polymerization in the thousands or tens of thousands are desired.

DP is also defined as the number of repeat units, where for co-polymers the repeat unit may not be identical to the monomeric unit.

For example, in , 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.

Significance of Degree of polymerization

The degree of polymerization has a dramatic effect on the mechanical properties of a polymer. As chain length increases, mechanical properties such as ductility, tensile strength, and hardness rise sharply and eventually level off. This is schematically illustrated by the 1st curve in the figure below.

However, in polymer melts, for example, the flow viscosity at a given temperature rises rapidly with increasing DP for all polymers, as shown by the 2nd curve in the diagram.

Calculating Degree of Polymerisation

Mol.wt. (Mn) = 56,000.

Monomer mol.wt (Mo) = 28

So, degree of polymerization is calculated as

$DP_n\equiv{}X_n=\dfrac{M_n}{M_o}$

$=\dfrac{56000}{28}$

$DPn=2000$