The purpose of this work was to obtain a polymer based on a mixture of exo, exo- and endo,endo-dimethyl esters of bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid (DME). The effect of concentration of a trifunctional comonomer based on norbornene-dicarboxyimide—tris-exo,exo-(5-norbornene-2,3-dicarboximidoethyl)amine (exo, exo-N3C2T) — on the physicomechanical properties of polydimethyl ether bicyclo[2.2 .1]hept-5-ene-2,3-dicarboxylic acid (PDME), obtained by ring-opening metathesis polymerization, was also determined. It was found that an increase in the initial concentration of exo,exo-N3C2T leads to an increase in the physico-mechanical properties of bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid polydimethyl ether copolymers.  When 1 mol.% exo,exo-N3C2T was introduced into a mixture of dimethyl esters of bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, the flexural modulus of the resulting copolymer increased to 1958 MPa, the tensile modulus of elasticity was 2175 MPa, and the glass transition temperature increased to 145 °C. The Izod impact strength for the copolymer containing 1 mol.% exo,exo-N3C2T decreased by 2 times and amounted to 2.8 KJ/m².