The molecular weights of natural and synthetic polyisoprenes have been measured by the light‐scattering method. The changes of intensity of scattered light caused by heat aging of these rubbers in air have been investigated. During aging of natural rubber considerable depolymerization takes place, with simultaneous formation of new macromolecules with a higher degree of branching, as confirmed by the unusual shapes of Zimm diagrams. The experimental values of the Pθ functions have been compared with theoretical ones calculated from the combined Debye and Mie theories.
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