No abstract

The polymerization of ethylene with late transition metal complexes typically provides highly branched amourphous poly(ethylene)s through a chain-walking mechanism, while the chain-straightening polymerization of α-olefins gives semicrystalline polymers. Herein, we report the polymerization of 1-hexene, 1-octene, and 1-dodecene catalyzed by an α-diimine Ni(II) complex in combination with diethylaluminum chloride. The effect of monomer chain length and monomer concentration on the productivity, polymerization “livingness”, selectivity of monomer insertion, and polymer structure/properties is investigated. The polymerization results indicate the possibility of precise microstructure control, depending on the monomer employed and monomer feedstock concentration, which in turn strongly affects the physical polymer properties. Additionally, di- and triblock copolymers were successfully fabricated in an easy, high-yielding route, at room temperature. The synthesis was accomplished through the sequential monomer addition from 1-dodecene and ethylene, without requiring any intermediate separation step. Each block exhibited distinct features from semicrystalline to amorphous, taking advantage of the different mechanism involved in the polymerization of 1-dodecene and ethylene. Investigation on mechanical behavior by uniaxial stretching until failure and step-cycle tensile tests showed that the block copolymers behave as thermoplastic elastomers with different performances depending on the composition, length of the blocks, and crystallinity. We demonstrate that this approach is a route to fabricate thermoplastic elastomers from readily accessible starting materials.

The synthesis and the characterization of a series of phosphine adducts of (imido)vanadium(IV) dichloride complexes of the type V(NR)Cl 2 (PMe 2 Ph) 2 [R = 2,6-Cl 2 -Ph (1), 2,6-i Pr 2 -Ph (2), and t Bu (3)] and V( N t Bu)Cl 2 (PMe 3 ) 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 Et 2 AlCl (500 equiv to V) and Cl 3 CCO 2 Et (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.

With Brookhart type α-diimine Ni(II) based catalysts, it is highly challenging to tune polymers branching level and branch-type distribution, which in turn strongly affects thermal and mechanical properties, through the aryl ortho-positions modification, while maintaining high turnover frequencies (TOFs). Herein, we are interested in performing a systematic investigation on the polymerization of 1-octene, 1-decene, and 1-octadecene catalyzed by a series of α-diimine nickel(II) complexes with methyl ligand backbone and different substituents in aryl positions (Ni1−Ni6). In addition to bulky isopropyl and tert-butyl substituents described in the original Brookhart's work, complexes with different aryl ortho-and para-substituted α-diimine ligands, including the less sterically demanding methyl and ethyl substituents, are investigated. The 13 C NMR spectra of the polymers have been assigned in detail, and some unique features have been identified and related to the chain-walking coordination/insertion mechanism. Changes in the ligand structure and monomer size have important effects on the numerous combinations of insertion and chain-walking paths from which different branches are installed. We have also carried out a comprehensive investigation of the mechanical behavior of the polymers by means of uniaxial stretching until failure, step-cycle, and creep tensile tests. Overall, the resulting polymers exhibited a broad spectrum of tensile properties, depending on their microstructure and crystallinity which in turn are strongly affected by monomer length and type of α-diimine ligand. 1-Octene and 1-decene polymers behave as elastomers with excellent mechanical properties, i.e., high elongation at break (up to 2000%) and good strain recovery, while 1-octadecene polymers behave as plastomers.