Homo- and Co-Polymerization of Ethylene with Cyclic Olefins Catalyzed by Phosphine Adducts of (Imido)vanadium(IV) Complexes

The synthesis and the characterization of a
series of phosphine adducts of (imido)vanadium(IV)
dichloride complexes of the type V(?NR)Cl2(PMe2Ph)2 [R
= 2,6-Cl2-Ph (1), 2,6-iPr2-Ph (2), and tBu (3)] and V(?
NtBu)Cl2(PMe3)2 (3?) are reported. The solid-state structures
of 1 and 3? were determined by X-ray crystallography. The
complexes present a geometry around the metal center
between a distorted trigonal-bipyramid and a square pyramid,
with an almost linear N-V-C bond. Complexes 1-3 were
evaluated as catalyst precursors for the polymerization of
ethylene and ethylene copolymerization with various cyclic
olefins (i.e., norbornene, dicyclopentadiene, 5-ethylidene-2-norbornene, and 5-vinyl-2-norbornene). In combination with Et2AlCl (500 equiv to V) and Cl3CCO2Et (ETA, 10 equiv to V), 1-3 are versatile and promising catalysts for the synthesis of high molecular weight linear poly(ethylene)s and alternating copolymers with efficient comonomer incorporation, unimodal molecular weight distributions, and uniform composition under mild conditions. Differences in the homo- and copolymerization of ethylene regarding the activity, stability over temperature, reactivity toward the target comonomers, and (co)polymer chain growth were investigated to probe the effects of imido ligand substitution. The introduction of more electron-donating groups led to an increase in polymers molecular weight and provided increased stability over temperature to the catalysts, particularly for 3. Both of these effects are likely because the tert-butyl imido moiety in 3 strengthens the V-N bond, thus improving the stability of the active intermediate. The steric shielding of the tert-butyl group may also contribute to inhibit the associative chain transfer. Control over the molecular weight of the resultant copolymers proved to be possible also by varying the ETA loading. ETA acts as a reoxidant, restarting the catalytic cycle, but it behaves also like a chain transfer agent and to a different extent strongly depending on the type of imido ligand.