The calculation of the number‐average degree of polymerization starting from intrinsic viscosity and overall rate

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Abstract
The relation between the intrinsic viscosity [η] and the number‐average degree of polymerization P̄n 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 P̄n 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 P̄n in these cases, without necessity of osmotic (or related) measurements or fractionation. The viscosity‐average degree of polymerization P̄η, which can be obtained exactly by experiment, is related to the number average by P̄η = P̄n(1 + F). Here F is given as a function of experimental data: P̄η and the overall rate of polymerization. This function is represented in a graph which enables the exact determination of P̄n without knowledge of number and extent of additional reactions.The P̄n values calculated according to the proposed method are compared with measured values (osmosis, endgroup analysis) for several polystyrene samples. The agreement is very satisfactory.