Palladium(II)-Catalyzed Synthesis of a Vinyl-Addition Ultrahigh-Molecular-Weight Polynorbornene Copolymer with an Entanglement Network for Enhanced Fracture Resistance

The fracture resistance of a cyclic olefin copolymer (COC) was enhanced by incorporating an entanglement network. The vinyl-addition copolymerization of norbornene (NB) and 5-octyl-2-norbornene (OctNB) was achieved using a catalyst system composed of Pd­(MesNCHC6H4PPh2)­Cl2 (Mes = 2,4,6-Me3C6H2), triisobutylaluminum, and triphenylmethylium tetrakis­(pentafluorophenyl)­borate. Despite exhibiting limited initiation efficiency, this catalyst system successfully produced vinyl-addition ultrahigh-molecular-weight poly­(norbornene-random-5-octyl-2-norbornene) (VA-UHMWP­(NB-r-OctNB)) with a number-average molecular weight (M n) exceeding 103 kDa. To assess the impact of the molecular weight, four VA-P­(NB-r-OctNB)­s with M n values of 292–1120 kDa were synthesized by introducing 1-hexene as a chain transfer agent during polymerization. Tensile testing of the VA-P­(NB-r-OctNB) films revealed an increase in tensile toughness, accompanied by an increase in elongation at break, up to an M n value of 680 kDa. This discovery suggests that the entanglement network plays a crucial role in enhancing the fracture resistance of this COC, ultimately contributing to decoupling its stiffness and brittleness.