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Diffusion of deuterated poly(methylmethacrylate) (dPMMA) is sloweddown in a PMMA matrix filled with hydroxyl-capped spherical silicananoparticles, from 13 to 50 nm in diameter and at loadings up to40 vol %. At constant T– Tg= 75 K, the normalized diffusion coefficients (D/D0) collapse onto a mastercurve, when plotted against the confinement parameter, ID/2Rg, where IDis interparticledistance and 2Rgis probe size. This resultsuggests that the confinement parameter captures the effect of nanoparticlesize, size polydispersity, and volume fraction on polymer dynamicsfor the PMMA composite. For ID< 2Rg, the master curve exhibits a strongly confined regionwhere D/D0decreasesby up to 80%, whereas for ID> 2Rg, the curve falls in a weakly confined region where D/D0decreasesonly moderately by up to 15%. Surprisingly, D/D0is reduced even when IDis8 times larger than 2Rg. A comparisonbetween the master curves for PMMA and polystyrene nanocompositesindicates that attractive interactions in the PMMA system do not significantlyalter the center-of-mass diffusion of macromolecules in polymer nanocomposites. [ABSTRACT FROM AUTHOR]