This paper describes the synthesis of N-phenylnorbornene-5,6-dicarboximides (NDIs) with substituents at different positions in the aromatic ring and their polymerization by ring-opening metathesis polymerization (ROMP) leading to high-temperature polymers. A structure-property relationship study showed that the polymer properties are significantly influenced by the size and position of the substituent groups on the ring. Substitution at the ortho position leads to an increase in the glass transition temperature (T(g)) of the polymer, where as metasubstitution has the opposite effect. An increase in the size of the substituent for ortho-substituted polymers is found to further increase the T(g). Polymers with T(g)s up to 270-degrees-C are prepared by choosing an appropriate substituent at the ortho position. It is proposed that changes in the polymer properties, observed when the size and/or position of the substituent is changed, are due to the differences in the relative ease of segmental motion of the polymer backbone. Molecular modeling studies suggest that the barrier to rotation of the phenyl group is at a maximum when the substituent is at the ortho position. In addition, resistance to rotation increases with the increase in the size of the substituent. Phenyl rotation is not significantly affected when the substituent is at the meta or the para position. Blends of poly(N-phenylnorbornenedicarboximide) with ortho and meta substituents are found to immiscible, giving two distinct T(g)s corresponding to the T(g)s of the homopolymers. Copolymerization of the ortho- and meta-substituted NDIs results in a homogeneous random copolymer with one glass transition temperature. The glass transition temperature increases linearly as the amount of ortho-substituted phenyl content in the copolymer increases.