Degree of Polymerization on Temperature

If the degree of polymerization is controlled principally by chain termination so that Xn is proportional to the kinetic chain length, the temperature coefficient of the average molecular weight will depend 

The activation energy Ed for the decomposition of the initiators commonly used is of the order of 30 kcal., while (Ep — Etl2) assumes values from 4 to 7 kcal. Hence, insofar as it is termination-controlled, the temperature coefBcient of the degree of polymerization is negative. Similarly for thermal polymerizations, the corresponding quantity 

Photochemically induced polymerizations in which the molecular weight is termination-controlled are exceptions to the rule that the average degree of polymerization diminishes with temperature. The rate of initiation at fixed intensity of illumination is virtually independent of temperature, hence din Xn/dT = Ep — Et/2)/RT j which is a positive quantity. 

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The dependence of the polymerization rate and the number-average degree of polymerization on temperature can be demonstrated if the respective relation is expressed in the form of the Arrhenius equation. 

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