The structural relaxation of poly(methyl-methaerylate) (PMMA)-based copolymers with different chain flexibility has been studied by DSC with the classical procedure of the isothermal and dynamical approach. Modified PMMA with different chain flexibility have been prepared by free radical polymerization in solution using a mixture of monomers containing 10 mol % of alkyl methacrylate (i.e., ethyl, buthyl, and hexyl methacrylate). The molecular characteristics of all the prepared copolymers have been performed by a multiangle laser light scattering (MALS) photometer on-line to a size exclusion chromatography (SEC) system (SEC-MALS) after and before the thermal treatments, NMR (H-1 and C-13) and MALDI-TOF mass spectrometry. A comparison of the apparent relaxation rate (R-H) was appraised from the enthalpy loss by annealing the different samples at the same level of undercooling (T-a = T-g - 18 degrees C). It was found an increase of R-H increasing the chain flexibility in the copolymers. Dynamical tests, performed at different cooling rates, have been used to estimate the apparent activation energy of the relaxation process. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 596-607, 2009