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Fluorescence redistribution after pattern photobleaching has been used to measure the translational diffusion coefficient of labeled poly (propylene oxide)(PPO) chains in molten PPO at 25 C. Labeled PPO wav synthesized with exactly one fluorophore per molecule, starting from the dye 4-diethanolamino-7-nitrobenzofurazan. A narrow molecular weight distribution fraction with M „= 33600 (Mw/Mn= 1.1) was prepared by gel permeation chromatography. The labeled PPO was dissolved in mixtures of high (Afw= 32 000) and low (Aíw= 1000) molecular weight PPO. Diffusion coefficients were from 4.8 X 10'9 cm2 s'1 in 100% Mw= 1000 to 1.1 X 10~ 10 cm2 s'1 in 100% Mw= 32000. The dependence of the diffusion coefficient upon concentration of the higher molecular weight PPO is discussed in terms of scaling plusreptation theories. It is concluded that the reptation model does not provide an accurate prediction of the …