Homo- and Copolymerization of Ethylene with Norbornene Catalyzed by Vanadium(III) Phosphine Complexes

Herein, we report the homo- and co-polymerization of ethylene (E) with norbornene (NB) catalyzed by vanadium(III) phosphine complexes of the type VCl(3)(PMe(n)Ph(3-n))(2) [n = 2 (1a), 1 (1b)] and VCl(3)(PR(3))(2) [R = phenyl (Ph, 1c), cyclohexyl (Cy, 1d), tert-butyl (tBu, 1e)]. In the presence of Et(2)AlCl and Cl(3)CCOOEt (ETA), 1a–1e exhibit good activities for the polymerization of ethylene, affording linear, semicrystalline PEs with a melting temperature of approximately 130 °C. Mainly alternating copolymers with high comonomer incorporation were obtained in the E/NB copolymerization. A relationship was found between the electronic and steric properties of the phosphine ligands and the catalytic performance. Overall, the presence of electron-withdrawing ligand substituents increases the productivity, complexes with aryl phosphine (weaker σ–donor character) exhibiting a higher (co)polymerization initiation rate than those with alkyl phosphines (stronger σ–donor character). Steric effects also seem to play a key role since 1d and 1e, having large size phosphines (PCy(3) θ = 170° and PtBu(3) θ = 182°, respectively) are more active than 1a (PMe(2)Ph θ = 122°). In this case, the larger size of PtBu(3) and PCy(3) likely compensates for their higher donor strength compared to PMe(2)Ph.