Cylindrical samples of thermally polymerized methylmethacrylate (MMA) were investigated and the time dependent temperatures at certain positions within the samples were measured during this process. Owing to the polymerization heat evolved, the maximal observed temperature difference within this probe was Delta T = 3.5 degrees C, e.g. the MMA polymerization occurs under a thermal gradient. As a consequence, the maximal difference at the glass transition temperature of the solid PMMA is Delta T-g = 1.5 degrees C, and for samples prepared at a higher temperature gradient (arranged by means of a central "hot wire"), Delta T-g = 6 degrees C. For this cross sectional inhomogeneity, a difference in the molecular weight distribution for the annealed samples of Delta M = 3 x 10(4) is proposed.