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.