Vanadium(iii) phosphine complexes as versatile catalysts for the polymerization of 1,3-butadiene and olefins

The impact of vanadium based catalysts for olefins and 1,3-dienes polymerization has been relevant, and today vanadium-based Ziegler-Natta technology is very well established for the manufacture of syndiotactic poly(propylene), ethylene/propylene/diene terpolymers, cyclic olefin copolymers [1], and trans-1,4 poly(1,3-butadiene)s [2]. In this field, vanadium phosphine complexes have been known since the mid 80's, but they were less investigated compared to those with multidentate ligands having, in different combinations, N and O hard donor atoms [3]. Keeping this in mind, a series of V(III) chloride complexes bearing monodentate phosphine ligands of the type PRnPh3-n (n = 0, 1, 2, 3; R = Me, Et, iPr, Cy), PR3 (R = nPr, tBu, Cyp, Cy) and bidentate phosphineligandsofthetypeR2P(CH2)nPR2 (n=2,R=Me,Et,Ph;n=1,R=Ph;n=3,R=Ph)were synthesized and characterized with different techniques. The behaviour of all the above complexes in the polymerization of 1,3-butadiene and in the copolymerization of ethylene with norbornene was examined. Upon activation with methylalumoxane, the complexes exhibited a moderate activity in the polymerization of 1,3-butadiene, giving poly(1,3-butadiene)s with mixed 1,4/1,2 structure and properties close to those reported for elastomers. In combination with Et2AlCl, the complexes exhibited remarkable activities in the copolymerization of ethylene with norbornene, giving mainly alternating, amorphous copolymers with a high norbornene content (up to 39.3 mol%) and glass transition temperature up to 93 °C. We found that ligands properties (both electronic and steric) have a great influence on the catalytic performance and (co)polymers properties. Overall, vanadium complexes with aryl- phosphine (weaker ?-donor character) seem to be more active than those with alkyl-phosphines (stronger ?-donor character). Moreover, in the case of ethylene homo- and co-polymerization, we found that the use of chelating diphosphines allowed the synthesis of (co)polymers with higher molecular weight, because the resulting transition states are less prone to ?-hydride elimination, and copolymers with more homogenous composition, and hence higher glass transition temperatures. Finally, alternating copolymers with high comonomer incorporation (up to 30 mol%) were also obtained through the copolymerization of ethylene with cyclic dienes (i.e., 5-ethylidene-2- norbornene and dicyclopentadiene). This constitutes another interesting feature of these vanadium(III) phosphine complexes: indeed, the presence of pendant double bonds allowed further chemical modification (e.g., hydrogenation and epoxidation), leading to functionalized polymers with different properties compared to the starting material.