Synthesis of N-phenylnorbornene-5,6-dicarboximide (NDI) with substituents at different positions in the aromatic ring and their polymerization by ring-opening metathesis polymerization (ROMP) leading to high-temperature polymers has been previously described. It was shown that the properties of these polymer are significantly influenced by the size and position of the substituent groups on the ring. This paper describes ROMP of NDI＇s with methyl substituents at ortho, meta, and para positions. These methyl-substituted polymers show similar trends in mechanical and thermal properties, with ortho substitution raising the temperature of the transition in dynamic modulus and the glass transition temperature, T-g, (stiffening) and meta substitution lowering these transition temperatures (plasticizing effect) relative to unsubstituted PNDI. Solid-state NMR relaxation experiments provide insight into these trends in mechanical and thermal properties. Experiments were performed to measure the carbon laboratory-frame relaxation, T-1(C), and both proton and carbon rotating-frame relaxation, T(1)rho(H) and T(1)rho(C). The observed trends in these relaxation values are as follows : (a) T-1(C), o-CH3, m-CH3 > PNDI, p-CH3; (b) T(1)rho(H), o-CH3 > PNDI, p-CH3, m-CH3; (c) T(1)rho(C), o-CH3 > PNDI, p-CH3, m-CH3. We also performed dipolar rotational spin-echo (DRSE) NMR experiments to demonstrate the phenyl rings are flipping in PNDI and p-methyl PNDI, but not in either the ortho or the meta isomer. These relaxation results are interpreted in terms of a model which provides a molecular basis for the observed trends in the thermal and mechanical properties of these polymers.