Degree of polymerization

The degree of polymerization, or DP, is the number of repeat units in an average polymer chain at time t in a polymerization reaction ^[1]. The length is in monomer units. The degree of polymerization is a measure of molecular weight (MW). For most industrial purposes, lengths in the thousands or tens of thousands are desired.

For a homopolymer, the number-average degree of polymerization may be calculated as shown below

In polycondensation, in order to achieve a high degree of polymerization (and hence molecular weight), X_n, a high fractional monomer conversion, p, is required, as per Carothers' equation: X_n = 1/(1−p). A monomer conversion of p = 99% would be required to achieve X_n = 100.

Correlation with physical properties

Relationship between degree of polymerization and melting temperature for polyethylene. Data from Flory (1963).

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 ^[2] and higher mechanical strength.

See also

• Carothers equation
• Kinetic chain length

References