Effects of tris(pentafluorophenyl)borane on the activation of the Ni(II)-based catalyst in the addition polymerization of norbornene

The activity of the transition metal complex, such as Ni(2-ethyl hexanoate)2 (1), Co(2-ethyl hexanoate)2 (2), TiCl4 (3), or CpTiCl3 (4) (Cp = cyclopentadiene), in combination with MAO (methylaluminoxane), was investigated in the polymerization of norbornene. The Ni(II) complex 1 with MAO showed moderate activity to give 20.8 kgpolymer/molNi h, while the other three complexes 2–4 with MAO just showed trivial activity. Effects of the Lewis acids on the activation of the catalyst of 1/MAO were examined. The employment of B(C6F5)3 with 1/MAO significantly enhanced the activity to give up to around 133 kgpolymer/molNi h. The use of other borane compounds, such as B(C6H5)3 and BEt3, or the stronger electron acceptor BF3 · OBu2, with 1/MAO in place of B(C6F5)3 clearly showed the main functions of B(C6F5)3. The high Lewis acidity of B(C6F5)3 enabled it to develop matured active complexes, thus enhancing the activity. Several Ni(II) complexes were employed to determine whether their activity was comparable to that of complex 1 in norbornene polymerization. The study of the 1H and 13C NMR spectra of the polynorbornene produced with 1/B(C6F5)3/MAO showed that the initiation of addition polymerization occurred through the insertion of the exo face of the norbornene into the active complex. Effects of the variation in the polymerization variables, such as the levels of B(C6F5)3 and MAO, temperature, and solvent, on the polymerization were discussed.