Formation of absorbance anisotropy under linearly polarized irradiation at 405 nm in films of poly(ethyl methacrylate) (PEMA), poly(n-butyl methacrylate) (PnBMA), and poly(n-hexyl methacrylate) (PnHexMA) doped with azo compound has been studied in a wide temperature range below Tg. The anisotropy formation kinetics has been well described under the assumption that there are two types of environments in the polymer matrixes: mobile and immobile. The mobile environment allows the dopant to reorient in the course of cis-trans-cis isomerization cycle and the immobile environment does not. The fraction of mobile environments was estimated to be 0.12 in PEMA, 0.65 in PnBMA, and 1.0 in PnHexMA. An increase in the fraction of mobile environments within the series PEMA-PnBMA-PnHexMA have been associated with an increase in the size of alkyl nanodomains. The average angle of transition dipole rotation upon isomerization and the amplitude of molecules librations have been estimated in the temperature range studied.