Disclosed is a method of making a cross-linked poly(dicyclopentadiene) with a metathesis-catalyst system, where the metathesis-catalyst system contains an organoaluminum or organoaluminum halide activator and a pentavalent tantalum catalyst. In a preferred embodiment, the cross-linked polymer is formed by combining, in a first place, a plurality of reactant streams, one reactant stream which contains the activator, a second reactant stream which contains the catalyst and at least one reactant stream which contains dicyclopentadiene; then injecting the combined reactant streams into a second place, such as a mold, where polymerization occurs. Use of the pentavalent tantalum catalyst results in the formation of a cross-linked polymer having less color and greater transparency than cross-linked poly(dicyclopentadiene) made without a pentavalent tantalum catalyst.

High stability composition of ring-opening polymerization product which is formed of (A) 100 parts by weight of at least one ring-opening polymerization product selected from the group consisting of (a) ring-opening polymerization homopolymers of norbornene derivatives containing at least one cyano group, (b) ring-opening polymerization homopolymers of norbornene derivatives containing at least one ester group, (c) ring-opening polymerization homopolymers of norbornene derivatives containing at least one ether group, (d) ring-opening polymerization homopolymers containing at least one N-substituted cyclic imide group, (e) ring-opening polymerization interpolymers of the above-mentioned types of norbornene derivatives, and (f) other ring-opening polymerization copolymers obtained by the ring-opening copolymerization of said various types of norbornene derivatives and 50 mol% at most of other unsaturated cyclic compounds; and (B) 0.01 to 10.0 parts by weight of specified phenolic compounds.