POLYMERS OF METHYL-SUBSTITUTED N-PHENYLNORBORNENE-5,6-DICARBOXIMIDE -CHARACTERIZATION OF STRUCTURE AND DYNAMICS

Synthesis of N-phenylnorbornene-5,6-dicarboximide (NDI) with substitue nts at different positions in the aromatic ring and their polymerizati on by ring-opening metathesis polymerization (ROMP) leading to high-te mperature polymers has been previously described. It was shown that th e properties of these polymer are significantly influenced by the size and position of the substituent groups on the ring. This paper descri bes ROMP of NDI's with methyl substituents at ortho, meta, and para po sitions. These methyl-substituted polymers show similar trends in mech anical and thermal properties, with ortho substitution raising the tem perature of the transition in dynamic modulus and the glass transition temperature, T-g, (stiffening) and meta substitution lowering these t ransition temperatures (plasticizing effect) relative to unsubstituted PNDI. Solid-state NMR relaxation experiments provide insight into the se trends in mechanical and thermal properties. Experiments were perfo rmed to measure the carbon laboratory-frame relaxation, T-1(C), and bo th 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 perfor med dipolar rotational spin-echo (DRSE) NMR experiments to demonstrate the phenyl rings are flipping in PNDI and p-methyl PNDI, but not in e ither the ortho or the meta isomer. These relaxation results are inter preted in terms of a model which provides a molecular basis for the ob served trends in the thermal and mechanical properties of these polyme rs.