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The variation of the real parts of the dynamic modulus E′, the dynamic strain‐optical coefficient K′, the mechanical loss tangent tan δ, and the optical loss tangent tan α have been measured as a function of temperature between -40° and +120°C at constant frequency of 1 cps for low‐density polyethylene, high‐density polyethylene, polypropylene, and nylon‐6. The results are interpreted in terms of both phenomenological theory and molecular mechanisms. The results for these polymers may be understood in terms of differing contributions by the processes of (a) spherulite or superstructure deformation, (b) delayed crystal reorientation within the deformed superstructure, and (c) relaxation of crystal orientation. For low‐density polyethylene, processes (b) and (c) are the principal contributors while for high‐density polyethylene, (a) becomes important at the lower temperatures. For polypropylene, (a) dominates and (b) begins to contribute at the higher temperatures, while for nylon‐6, (a) gives way to (c) without any appreciable region of important contribution of (b).