Liquid copolymer formed by ring-opening copolymerization of cyclopentene, crosslinkable composition, and crosslinked rubber object

1. A liquid cyclopentene ring-opened copolymer comprising a monomer unit derived from cyclopentene and a monomer unit derived from a norbornene compound,

the monomer unit derived from cyclopentene being present in an amount of 60 to 95 wt %,

the liquid cyclopentene ring-opened copolymer having a hydroxyl group-terminated copolymer chain and having a melt viscosity at 25° C. of less than 500 P·s, which is measured using a B-type viscometer at a shear rate of within the range of 1.2 to 10 sec −1 .

2. The liquid cyclopentene ring-opened copolymer according to claim 1 ,

which has a number average molecular weight (Mn) of 500 to 20,000.

3. The liquid cyclopentene ring-opened copolymer according to claim 1 ,

wherein the cis/trans ratio of double bonds in repeating units is 10/90 to 40/60.

4. The liquid cyclopentene ring-opened copolymer according to claim 1 ,

which has a glass transition temperature (Tg) of −50° C. or lower.

5. A method for producing the liquid cyclopentene ring-opened copolymer according to claim 1 , comprising:

performing ring-opening polymerization of monomers including cyclopentene and the norbornene compound in an ether- or ketone-based solvent as a polymerization solvent using a polymerization catalyst containing a ruthenium compound and a hydroxyl group-containing chain transfer agent.

6. A cross-linkable composition comprising a polyfunctional isocyanate compound in an amount of 1 to 100 parts by weight with respect to 100 parts by weight of the liquid cyclopentene ring-opened copolymer according to claim 1 .

7. A cross-linked product obtained by cross-linking the cross-linkable composition according to claim 6 .