A Methylene-bridged salicylaldiminato tridentate [ONS] binuclear titanium complex for ethylene-norbornene copolymerization

A binuclear titanium complex Ti²L_a with methylene-bridged salicylaldiminato tridentate ONS] ligand bearing octylthio sidearm was synthesized and used for the copolymerization of ethylene and norbornene (E-NB). The complex exhibited activity over 10⁶ g/mol(Ti)^.h^.atm and high degree incorporation of co-monomer (up to 49.9 mol %), affording high-molecular-weight copolymers with narrow molecular weight distribution. The E-NB copolymers produced by Ti²L_a/MMAO contained NN dyad and NNN triad sequences even at low norbornene feeds, in contrast to the observation of such sequences only at high level of NB incorporation for most other catalyst systems. The monomer reactivity ratios were calculated to be r_E = 14.62 and r_N = 0.08, of which the r_N value was much larger than that from non-metallocene titanium catalyst systems. The catalytic performances of the binuclear complex Ti²L_a and its mononuclear analogue TiL_b were also compared, with the binuclear complex exhibiting higher catalytic activity and NB incorporation ratio due to the binuclear cooperative effect, and producing much higher molecular weight copolymer due to the increased steric hindrance caused by close proximity of two growing chains. To the best of our knowledge, this is one of the few examples of binuclear catalyst for E-NB copolymerization with high activity and efficient incorporation of norbornene.