Copolymerization of ethylene with cyclic olefins and ?-olefins catalyzed by phosphine adducts of (imido)vanadium(IV) complexes

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] As a part of an ongoing study concerning vanadium-based catalysts for the synthesis of cyclic olefin copolymers,[4] a series of phosphine adducts of (imido)vanadium(IV) complexes of the type V(=NR)Cl2(PMe2Ph)2 (Fig. 1) was synthesized and characterized.[5] Complexes 1-3 were evaluated as catalysts precursors for the homopolymerization of ethylene and for the copolymerization of ethylene with different cyclic olefins (i.e., norbornene, dicyclopentadiene, 5-ethylidene-2- norbornene, and 5-vinyl-2-norbornene). In combination with Et2AlCl and Cl3CCO2Et (ETA), 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 compositions under mild conditions. The effect of the imido ligand substitution of the investigated complexes, polymerization temperature and time, and NB/E (NB = norbornene, E = ethylene) and ETA/V mole ratio on the catalytic activity, reactivity toward the target comonomers, and (co)polymer structure and properties (i.e., molecular weight, comonomer incorporation, microstructure, and thermal behavior) was investigated. The introduction of more electron-donating groups led to an increase in polymers molecular weight and provided increased stability over temperature to the catalysts. 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. Since vanadium catalysts are industrially renowed for the synthesis of propylene-based elastomers, complexes 2 and 3 were also tested in the copolymerization of ethylene with ?-olefins (i.e., propylene and 1-octene). Random copolymers with high molecular weight and narrow molecular weight distributions were obtained with high activities also at low ?-olefin feed concentrations.