degree of polymerisation (rus. полимеризации, степень) — number of monomeric units in a macromolecule.

Description

The degree of polymerisation uniquely determines the weight of a macromolecule of known composition. As a rule, synthetic polymers comprise a set of macromolecules of varying degrees of polymerisation and, accordingly, molecular weight.

The physical and chemical properties of an oligomer change with increasing molecular weight. There is a certain critical value beyond which further increase in the molecular weight no longer significantly affects these properties. This value characterises the transition from the oligomer to the polymer, and it differs for different properties.

Polydisperse polymers are described by different molecular weight averages, and a complete characterisation is given by the molecular weight distribution. The average molecular weight Mn is equal to the arithmetic mean of molecular weight of a polymer. This value is determined experimentally by osmometry (by measuring osmotic pressure), ebullioscopy (by measuring the boiling point) and cryoscopy (by measuring the freezing temperature) of polymer solutions, as well as data on the number of terminal groups of macromolecules obtained by spectroscopic and chemical methods.

Molecular weight distribution can be characterised as weight-average Mw and z-average molecular weights that correspond to other ways of averaging over a set of macromolecules. Weight-average molecular weight is determined experimentally by elastic light-scattering data; z-average molecular weight and approximate form of molecular weight distribution can be determined by sedimentation equilibrium, for example, using an ultracentrifuge.

Another quick way to find the molecular weight is to calculate it by the Mark – Kuhn – Houwink equation from the value of viscosity defined by viscometry methods. The obtained value is called the viscosity average molecular weight, it corresponds to a different type of averaging, but gives values close to the weight-average molecular weight.

By comparing the values of the average molecular weight found by various methods, we can describe the form of molecular weight distribution. Polydispersity is an important characteristic Kn, which is the ratio of weight-average molecular weight to number average molecular weight: Kn = Mw/Mn. For monodisperse polymers, Kn = 1, for linear polymers prepared by polycondensation, Kn = 2, the industrial polymers may have Kn = 3-10.

Authors

  • Khokhlov Alexey R.
  • Govorun Elena N.
  • Nazarov Victor G.

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