PURPOSE: To obtain a heat resistant polymer having a norbornene dicarboximide unit shown by the specifying formula, a high molecular weight, high glass transition temperature, high strength and high elasticity as well as better processability.

CONSTITUTION: A polymer containing a norbornene dicarboximide unit of the formula (wherein R is (substituted)phenyl) with a number average molecular weight of > about 6,000 and glass transition temperature Tg of > about 200°C is prepared. The polymer, because of having the above physical properties, is a heat resistant polymer having desirable characteristics of high strength, high elasticity and better processability. The polymer is obtained by double decomposition ring-opening polymerization of N-phenyl-norbornene-dicarboximide having about 15% or more of exo-stereoisomer.