DFT and ONIOM Simulation of 1,3-Butadiene Polymerization Catalyzed by Neodymium-Based Ziegler-Natta System

Using modern methods of quantum chemistry, a theoretical substantiation of the high cis-stereospecificity of 1,3-butadiene polymerization catalyzed by the neodymium-based Ziegler-Natta system was carried out. For DFT and ONIOM simulation, the most cis-stereospecific active site of the catalytic system was used. By analyzing the total energy, as well as the enthalpy and Gibbs free energy of the simulated catalytically active centers, it was found that the coordination of 1,3-butadiene in the trans-form was more favorable than in the cis-form by 11 kJ/mol. However, as a result of π-allylic insertion mechanism modeling, it was found that the activation energy of cis-1,3-butadiene insertion into the π-allylic neodymium-carbon bond of the terminal group on the reactive growing chain was 10-15 kJ/mol lower than the activation energy of trans-1,3-butadiene insertion. The activation energies did not change when both trans-1,4-butadiene and cis-1,4-butadiene were used for modeling. That is, 1,4-cis-regulation was due not to the primary coordination of 1,3-butadiene in its cis-configuration, but to its lower energy of attachment to the active site. The obtained results allowed us to clarify the mechanism of the high cis-stereospecificity of 1,3-butadiene polymerization by the neodymium-based Ziegler-Natta system.

Side-Arm Assisted Anilido-Imine Based Rare-Earth Metal Complexes for Isoprene Stereoselective Polymerization

Anilido-imine ligands o-C₆H₄(NHAr₁)(CH=NAr₂), in which Ar₁ is 2,6-diisopropylbenzyl group and Ar₂ contains fluorine (HL₁) or methoxyl (HL₂) group on ortho-position of phenyl substituent, were synthesized for constructing rare-earth metals based complexes of 1a–1c (HL₁ based Sc, Lu, Y) and 2a–2c (HL₂ based Sc, Lu, Y). Based on their NMR spectra and X-ray single-crystal structures, the side-arm group of -F and -OMe is identified to chelate to the corresponding central metal. The twisted angles between two planes formed by chelated heteroatoms (N, N, F for HL₁ and N, N, O for HL₂) are observed, in which the largest dihedral angle (53.3°) for HL₁-Y and the smallest dihedral angle (44.32°) for HL₂-Sc are detected. After being activated by AlⁱBu₃ and [Ph₃C][B(C₆F₅)₄], these catalysts showed great activity for isoprene polymerization. Bearing the same ligand HL₁, smaller scandium based complex 1a and middle size of lutetium based 1b provided lower cis-1,4-selectivity (57.3% and 64.2%), larger yttrium complex 1c displayed high cis-1,4-selectivity (84%). Chelating by crowded HL₂, small size of scandium complex 2a provided impressive trans-1,4-selectivity (93.0%), middle lutetium based 2b displayed non-selectivity and larger yttrium complex 2c showed clear cis-1,4-selectivity (83.3%). Moreover, 2a/AlⁱBu₃ system showed the quasi-living chain transfer capability.

Highly Active Iminopyridyl Iron-Based Catalysts for the Polymerization of Isoprene

A series of iminopyridyl-based ligands, 6-[(Ar)N=C(R)]-2-C₆H₅N [(Ar = 2,6-Me₂-C₆H₃, R = Me (L1); Ar = 2,6-ⁱPr₂-C₆H₃, R = Me (L2); Ar = 2,6-Me₂-C₆H₃, R = H (L3); Ar = 2,6-ⁱPr₂-C₆H₃, R = H (L4); Ar = 3,5-(CF₃)₂-C₆H₃, R = Me (L5); Ar = C₆F₅, R = Me (L6)], and their corresponding iron (II) complexes were developed to investigate their application in the controlled coordinative polymerization of isoprene. The modulation of steric and electronic properties within this family of ligands/pre-catalysts has shown to influence the stereo-selectivity and activity of the polymerization of isoprene after activation. Upon activation with various co-catalysts such as AlⁱBu₃/[Ph₃C][B(C₆F₅)₄], AlEt₃/[Ph₃C][B(C₆F₅)₄] or MAO, the resulting catalysts produced polyisoprenes with an excellent conversion (>99% of 500–5000 equiv.) within less than 1 h (TOF > 500 h⁻¹) and having a variety of stereo-/regio-regularities. The presence of electron-donating and withdrawing groups drastically impacted the activity and the stereoselectivity of the catalysts during the course of the polymerization of isoprene. When activated with AlⁱBu₃/[Ph₃C][B(C₆F₅)₄], the complexes {6-[(2,6-Me₂-C₆H₃)N=C(Me)]-2-C₆H₅N}FeCl₂ (C1) and {6-[(2,6-ⁱPr₂-C₆H₃)N=C(Me)]-2-C₆H₅N}FeCl₂ (C2) exhibited moderate trans-1,4 selectivity (>67%) while the iron-based systems bearing related aldiminopyridyl ligands {6-[(2,6-Me₂-C₆H₃)N=C(H)]-2-C₆H₅N}FeCl₂ (C3) and {6-[(2,6-ⁱPr₂-C₆H₃)N=C(H)]-2-C₆H₅N}FeCl₂ (C4) were found to afford significant cis-1,4 selectivity at low temperature (>86% at −40 °C). On the other hand, the ternary {6-[(3,5-(CF₃)₂-C₆H₃)N=C(Me)]-2-C₆H₅N}FeCl₂ (C5) or {6-[(C₆F₅)N=C(Me)]-2-C₆H₅N}FeCl₂ (C6)/AlⁱBu₃/[Ph₃C][B(C₆F₅)₄] catalytic combinations showed exceptional activity for the polymerization of isoprene (TOF > 1,000,000 h⁻¹), albeit providing less stereoselectivity.

DFT Studies on cis-1,4-Polymerization of Dienes Catalyzed by a Cationic Rare-Earth Metal Complex Bearing an Ancillary PNP Ligand

Dnsity functional theory (DFT) calculations have been carried out for the highly selective cis-1,4-polymerization of butadiene catalyzed by a cationic rare-earth metal complex bearing an ancillary PNP ligand. It has been found that the chain initiation and propagation of butadiene polymerization occurs via the favorable cis-1,4-insertion route. The trans-1,4 and 1,2-insertion are unfavorable both kinetically and thermodynamically. The chain growth follows the π-allyl-insertion mechanism. The analyses of energy decomposition of transition states indicate that the likelihood of rival insertion pathways is predominantly controlled by the interaction energy of butadiene with a metal center and the deformation energy of butadiene moiety. The electronic factor of the central metal has a decisive influence on the cis- vs. trans-insertion and the regioselectivity (cis-1,4- vs. cis-1,2-insertion) is mainly determined by steric hindrance. Tetrahydrofuran (THF) coordination made monomer insertion less favorable compared with THF-free case and had more noticeable impact on the trans-monomer insertion compared with the cis case. During the chain propagation, cis-insertion of monomer facilitates THF de-coordination and the THF molecule could therefore dissociate from the central metal.

Bidentate forms of β-triketimines: syntheses, characterization and outstanding performance of enamine–diimine cobalt complexes in isoprene polymerization

When a normally tridentate β-triketimine ligand binds to Co(ii) in a bidentate enamine–diimine form, is partnered with the weakly co-ordinating BArF anion and activated with ethylaluminiumsesquichoride, the most active catalyst yet found forcis-1,4-selective polymerization of isoprene results.

Alpha and beta diimine cobalt complexes in isoprene polymerization: a comparative study

Isoprene was polymerized by diimine cobalt catalyst in the presence of DEAC to produce cis-1,4 and 3,4 polyisoprene.

The behaviour of β-triketimine cobalt complexes in the polymerization of isoprene

Cobalt complexes of β-triketimines activated by Et₂AlCl polymerize isoprene to give up to 80%cis-1,4 with 20% 3,4 polymer, at 35 °C.

Thio-Pybox and Thio-Phebox complexes of chromium, iron, cobalt and nickel and their application in ethylene and butadiene polymerisation catalysis

A series of bis(thiazolinyl)- and bis(thiazolyl)pyridine Thio-Pybox ligands and their metal complexes of chromium(III), iron(II), cobalt(II) and nickel(II) has been prepared, as well as a nickel(II) complex containing a monoanionic bis(thiazolinyl)phenyl Thio-Phebox ligand. These new metal complexes have been characterised and used as catalysts, in combination with the co-catalyst MAO, for the polymerisation of ethylene and for the polymerisation of butadiene. In the case of ethylene polymerisation, the Thio-Pybox and Thio-Phebox metal complexes have shown relatively low polymerisation activities, much lower compared to the related bis(imino)pyridine complexes of the same metals. In the polymerisation of butadiene, several Thio-Pybox cobalt(II) complexes show very high activities, significantly higher than the other metal complexes with the same ligand. It is the metal, rather than the ligand, that appears to have the most profound effect on the catalytic activity in butadiene polymerisation, unlike in the polymerisation of ethylene, where bis(imino)pyridine ligands provide highly active catalysts for a range of 1st row transition metals.