The rotationnal diffusion of Disperse Red One dopant in PMMA is studied by electrooptical polarimetry. The temperature is varied from Tg to far below the secondary transition temperature Tβ. Above Tβ, disordering but also ordering of the dopant are possible. This rotationnal diffusion cannot be described by a diffusion coefficient and is strongly related to the free volume relaxation inside the polymer. The mobility is almost completly frozen under Tβ.