The main chain dynamics of amorphous poly(ethyl methacrylate) (PEMA) and poly(methyl methacrylate) (PMMA) below and above their respective glass transition temperatures T-g are analyzed by two-dimensional solid-state exchange H-2 NMR spectroscopy. In both polymers, a restricted mobility of the polymer backbone is already present in the glassy state, as is directly demonstrated and quantified using samples deuterated at the methyl and methylene moieties of the polymer main chain. The unusual main chain mobility below T-g is coupled to the beta-relaxation process, which involves 180 degrees flips of the carboxyl side groups. At their respective glass transition temperatures, the coupling of the beta-process to the main chain motions manifests itself differently in both polymers; the smaller ester side group reorients comparatively fast in PMMA, whereas in PEMA, the reorientation of the bulkier side group remains anisotropic and the correlation times are slower by about 1 order of magnitude. Therefore, in PMMA, the beta-relaxation predominantly influences the time scale of the alpha-relaxation, leading to a particularly high mobility of the main chain itself. In contrast, in PEMA, a slow uniaxial diffusion of the main chain around its local axis sets in at T-g, the beta-process thus affecting mainly the geometry of backbone motions, as is further corroborated by comparing one-dimensional C-13 NMR spectra with two-dimensional exchange H-2 NMR spectra at higher temperatures. In summary, the coupling of the alpha- and beta-processes leads to longer mean correlation times for the alpha-relaxation in PEMA than in PMMA.