PROCESS FOR THE RING OPENING POLYMERISATION OF CYCLOMONOOLEFINS BY CONTACTING THE CYCLOMONOOLEFINS WITH A CATALYST CONSISTTING OF (A) A REACTION PRODUCT OF A TUNGSTEN OR TANTALUM HALIDE WITH AN ACETAL AND (B) AN ORGANO COMPOUND OF A METAL OF GROUPS IA TO IVA OF THE PERIODIC TABLE THE MOLAR RATIO OF TUNGSTEN:METAL OF GROUPS IA TO IVA BEING FROM 1:0.3 TO 1:10 AND THE MOLAR RATIO OF TANTALUM: METAL OF GROUPS IA TO IVA BEING FROM 1:1 TO 1:10.