Carbon-13 spin-lattice relaxation times (T1) have been determined for pyridine and for CDCI3 solutions of both predominantly isotactic and predominantly syndiotactic poly(methyl methacrylate), PMMA, at 38 and 100 °C. The T1s of both backbone and side-chain carbons are longer in the isotactic chain configuration. The relative T1 data for both quaternary and methylene backbone carbons suggest the isotactic chain skeleton is more flexible than that of the syndiotactic polymer. In addition, spin-spin relaxation data (T2) and 13C[1H] nuclear Overhauser enhancements for the CH2 carbon indicate that backbone motion in the PMMA systems is characterized by a distribution of correlation times. Both Cole-Cole and log x2 distributions were found to provide reasonable fits of the experimental data; however, a broader distribution was required to fit the syndiotactic results than the isotactic results. Finally, the T1 results for the a-methyl carbons in the different stereoforms of the polymer suggest that the constraint to methyl internal rotation in the isotactic chain is less than in the syndiotactic chain. © 1977, American Chemical Society. All rights reserved.