The viscoelastic behavior of an alternating copolymer styrene-methyl methacrylate (S-alt-M) of composition 50:50 mol% has been investigated over a wide temperature range covering the glassy state, the glass transition region (governed by the mechanical relaxation a), and the rubbery plateau. Data were discussed by comparison with the styrene-methyl methacrylate random copolymer (S-r-M) of same composition. These studies based on tensile and shear DMTA measurements, were carried out at various temperatures and frequencies. In addition, stress-strain curves in compression mode were recorded at various temperatures and strain rates in the glassy state. The activation enthalpy and activation volume of the plastic deformation process with temperature was measured for both S-alt-M and S-r-M. Data analysis was performed on a molecular scale, by putting emphasis on the correlation existing between the amplitude of relaxation and the p-relaxation character associated to the sequence length distribution of the copolymers. From the orientation measurements of SM copolymers, carried out by infra-red dichroism, on films stretched at various gaps (T - T-alpha) from the main relaxation temperature and at various strain rates, it turned out that the M segments stayed even more oriented than the S segments. Two distinct orientation relaxation curves were obtained for each type of segments, irrespective of the type of copolymer. It was shown that they could be reduced to unique ones after normalization by the zero time or zero (T - T-alpha) orientation value. All these results were interpreted in relation to the role of polar-polar intermolecular interactions between M-M units, which are likely to occur in the S-r-M copolymer, but were quite lacking in the S-alt-M copolymer. However, the influence of the triad MSM on the mechanical properties was shown to progressively decrease as long as the testing temperature approaches or exceeds T-alpha. (C) 2005 Springer Science + Business Media, Inc.