The relation between the intrinsic viscosity [η] and the number-average degree of polymerization n is a function of the molecular weight distribution of the polymer. In a polymer in which the termination of polymer radicals occurs partly by combination of two growing chains, this molecular weight distribution depends on number and extent of additional reactions such as chain transfer; i.e., it is variable. Therefore n of such polymers cannot be obtained from intrinsic viscosity measurements by means of an equation of the type [η] = KP̄na.

A new method is proposed which allows the evaluation of n in these cases, without necessity of osmotic (or related) measurements or fractionation. The viscosity-average degree of polymerization η, which can be obtained exactly by experiment, is related to the number average by η = n(1 + F). Here F is given as a function of experimental data: η and the overall rate of polymerization. This function is represented in a graph which enables the exact determination of n without knowledge of number and extent of additional reactions.

The n values calculated according to the proposed method are compared with measured values (osmosis, endgroup analysis) for several polystyrene samples. The agreement is very satisfactory.