Homo- and co-polymerization of ethylene with norbornene catalyzed by phosphine Vanadium(III) complexes

The study of phosphine complexes with transition metal halides is an important topic and includes different compounds depending on the metal oxidation state and its position in the periodic table.[1] The preparation of vanadium-phosphine complexes has been the subject of research,[2] even if only for VCl3(PMePh2)2 has been determined the structure,[3] and none of the published complexes has ever been tested as pre-catalyst in processes of interest to the polymer and organic chemistry communities. In this study, we have synthesized and fully characterized a series of V(III) complexes with alkyl and aryl phosphine ligands such as PMexPh3-x (x = 0, 1, 2) and PR3 (R = tBu, Cyp, Cy). The complexes were readily prepared in one simple, high-yielding step from commercially available reagents. All the complexes were used, in combination with AlEt2Cl as cocatalyst, in the homo- and co-polymerization of ethylene (E) with norbornene (NB). The results have been carefully compared with those from the precursor VCl3(THF)3 and V(acac)3. The polymerization of ethylene gives linear poly(ethylene)s with Tms of about 130 °C, while the E/NB copolymerization gives mainly alternating, atactic, amourphous copolymers with a NB content ranging from 35.5 to 39.3 mol%, and Tgs from 81 to 93 °C. The catalysts based on V(III)-phosphine complexes exhibit excellent activities in the E/NB copolymerization, from 10080 to 26424 kgpol/(molV×h). A relationship exists between the phosphine electronic and steric properties and the rate of catalyst initiation (phosphine dissociation), thus indicating that initiation can be tuned by changing the substituents on the phosphorus atom. Overall, catalysts based on V(III) complexes with aryl-phosphine (weaker ?-donor character) are more active than those with alkyl-phosphines (stronger ?-donor character), thus suggesting that the activity increases with decreasing phosphine donor strength. However, steric properties too play an important role on the activity: catalysts based on V-complex with a large size ligand such as PtBu3 (? = 182°, strong ?-donor) are more active than those with PMe2Ph (? = 122°, weak ?-donor). In this case the larger size of PtBu3 likely compensates for its higher donor strength. References 1.Burt J., Levason W., Reid G. Coord. Chem. Rev. 2014, 260, 65. 2.(a) Issleib K., Bohn G. Z. Anorg. Allg. Chem. 1959, 301, 188; (b) Holt D. G. L., Larkworthy L. F., Povey D. C., Smith G. W., Leigh G. J. Inorg. Chim. Acta 1993, 207. 3.Leone G., Pierro I., Zanchin G., Forni A., Bertini F., Rapallo A., Ricci G. J. Mol. Catal. A: Chem. 2016, 424, 220.