Oh the mechanism of stereoregulation in the allylnickel complex catalyzed butadiene polymerization

After a brief account of the present knowledge on the mechanism of the allylnickel complex catalyzed stereospecific butadiene polymerization, synthesis and catalytic properties of the cationic allylnickel(II) complexes, [C₃H₅NiL₂]PF₆ (L = P(OAlk)₃, P(OAr)₃, PPh₃, AsPh₃, SbPh₃, MeCN, Me₃CNC, 1,5–COD), are described. From kinetic and ³¹P-NMR spectroscopic investigations, especially with [C₃H₅Ni(P(OPh)₃)₂]PF₆ as a typical trans-catalyst, the definite course of the catalytic stereoregulation has been elucidated.     

Electronic excitation in anionic polymerization of butadiene: Nonempirical calculations of reaction complexes

A new mechanism of anionic polymerization of butadiene is proposed. In the elementary chemical act, the “living” polymer–monomer complex is excited into the low‐lying triplet state. This state has the character of charge (electron) and cation (Li⁺ or Na⁺) transfer from the terminal unit of the active center to the monomer molecule. In the framework of this concept, the probability of chemical bond formation is determined by spin density on radical centers of reagent molecules. Semiempirical and ab initio 6‐31G** quantum‐chemical calculations showed stable interaction between components of the complex in the ground electronic state (9–11 kcal/mol) and low energy levels of triplet excited states (<14 kcal/mol). This new approach is shown to be useful in the analysis of polymerization kinetics and the microstructure of polybutadiene depending on the cation type and the ion pair state. The mechanism of cis‐trans isomerization in the terminal unit of the living polymer consists in concerted rotation about the C_β? C_γ bond and the migration of Li between C_α and C_γ atoms. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Cobalt-mediated radical polymerization of acrylonitrile: kinetics investigations and DFT calculations

The successful controlled homopolymerization of acrylonitrile (AN) by cobalt-mediated radical polymerization (CMRP) is reported for the first time. As a rule, initiation of the polymerization was carried out starting from a conventional azo-initiator (V-70) in the presence of bis(acetylacetonato)cobalt(II) ([Co(acac)(2)]) but also by using organocobalt(III) adducts. Molar concentration ratios of the reactants, the temperature, and the solvent were tuned, and the effect of these parameters on the course of the polymerization is discussed in detail. The best level of control was observed when the AN polymerization was initiated by an organocobalt(III) adduct at 0 degrees C in dimethyl sulfoxide. Under these conditions, poly(acrylonitrile) with a predictable molar mass and molar mass distribution as low as 1.1 was prepared. A combination of kinetic data, X-ray analyses, and DFT calculations were used to rationalize the results and to draw conclusions on the key role played by the solvent molecules in the process. These important mechanistic insights also permit an explanation of the unexpected "solvent effect" that allows the preparation of well-defined poly(vinyl acetate)-b-poly(acrylonitrile) by CMRP.     

The nature of active sites in butadiene polymerization catalyzed by organotitanium(III) derivatives

The stereospecificity of benzyl derivatives of trivalent titanium (R_n TiX_3–n, where X = Cl, I, n = 1–3) in butadiene polymerization was studied. It was found that dibenzyltitaniumiodide is an efficient catalyst of the 1,4-cis-polymerization of butadiene and that tribenzyltitanium forms 1,2-units. In both cases all the titanium-benzyl bonds participated in the initiation reaction and the active sites were polymeric analogues of crotyl derivatives of Ti(III); namely, bis-π-oligobutadienyltitaniumiodide and tris-π-oligobutadienyltitanium. These sites are stable at room temperature. The nature of the active sites in the polymerization of butadiene with Ziegler's 1,4-stereo-specific systems Til₄ (or Til₂Cl₂) + AIR₃ are described.     

Electroinitiated polymerization of butadiene sulfone

Electroinitiated polymerization of butadiene sulfone was achieved by direct electron transfer in acetonitrile—tetrabutylammonium fluoroborate system by controlled potential electrolysis technique. High conversions were obtained at reasonable temperatures and polymerization times. The polymer was found to be composed of linear segments along with some cyclic units. The effect of monomer concentration, temperature, and polymerization potential on the rate of polymerization was investigated. Temperature and polymerization potential have positive effects on the rate of polymerization. The effect of ultrasonic vibration was also investigated by conducting electrolyses at different monomer concentrations in the presence and absence of ultrasonic vibration. It was observed that the rate of polymerization increases significantly in the presence of ultrasonic vibration. The inverse relationship between the rate of polymerization and monomer concentration was observed in presence and absence of ultrasonic vibration.     

Non-empirical calculations on the electronic spectrum of butadiene

Ab initio SCF and CI calculations of the electronic spectrum of butadiene are reported which employ a gaussian basis of double zeta quality augmented by diffuse 3s and 3p functions. Good agreement is obtained with experimental details of this spectrum, both for π→π* and certain Rydberg transitions, and it is concluded that the important NV₁ and NV₂ absorption systems both involve diffuse upper states of ¹B_u symmetry.     

Synthesis, structures and DFT calculations on alkaline-Earth metal azide-crown ether complexes

The first examples of azide complexes of calcium, strontium or barium with crown ethers have been prepared and fully characterised, notably [Ba([18]crown-6)(N3)2(MeOH)], [Sr([15]crown-5)(N3)2(H2O)], [Ca([15]crown-5)(N3)2(H2O)] and [Sr([15]crown-5)(N3(NO3)]. Crystal structures reveal the presence of a variety of coordination modes for the azide groups including kappa 1-, mu-1,3- and linkages via H-bonded water molecules, in addition to azide ions. The [Ba([18]crown-6)(N3)2(MeOH)].1/3 MeOH contains dinuclear cations with three mu-1,3-NNN bridges, the first example of this type in main group chemistry. The structures obtained have been compared with molecular structures computed by density functional theory (DFT). This has allowed the effects of the crystal lattice to be investigated. A study of the M--N terminal metal-azide bond length and charge densities on the metal (M) and terminal nitrogen centre (N terminal) in these complexes has allowed the nature of the metal-azide bond to be investigated in each case. As in our earlier work on alkali metal azide-crown ether complexes, the bonding in the alkaline-earth complexes is believed to be predominantly ionic or ion-dipole in character, with the differences in geometries reflecting the balance between maximising the coordination number of the metal centre, and minimising ligand-ligand repulsions.     

Stereospecific polymerization of butadiene in the presence of isobutylene

The polymerization of isobutylene–butadiene monomer mixtures by a cobalt-containing catalytic system (cobaltous acetylacetonate–diethylaluminum chloride–water) yields a homopolymer mixture consisting of polyisobutylene and polybutadiene. It is assumed that the polymerization of both monomers proceeds according essentially identical reaction mechanisms, i.e., isobutylene polymerizes cationically and butadiene follows a coordination-cationic mechanism.     

CO oxidation catalyzed by a single gold atom: benchmark calculations and the performance of DFT methods

Quantum chemical calculations were carried out on CO oxidation catalyzed by a single gold atom. To investigate the performance of density functional theory (DFT) methods, 42 DFT functionals have been evaluated and compared with high-level wavefunction based methods. It was found that in order to obtain accurate results the functionals used must treat long range interaction well. The double-hybrid mPW2PLYP and B2PLYP functionals are the two functionals with best overall performance. CAM-B3LYP, a long range corrected hybrid GGA functional, also performs well. On the other hand, the popular B3LYP, PW91, and PBE functionals do not show good performance and the performance of the latter two are even at the bottom of the 42 functionals. Our accurate results calculated at the CCSD(T)/aug-cc-pVTZ//mPW2PLYP/aug-cc-pVTZ level of theory indicate that Au atom is a good catalysis for CO oxidation. The reaction follows the following mechanism where CO and O(2) adsorb on Au atom forming an Au(OCOO) intermediate and subsequently O(2) transfer one oxygen atom to CO to form CO(2) and AuO. Then AuO reacts with CO to form another CO(2) to complete the catalytic cycle. The overall energy barrier at 0 K for the first CO oxidation step (Au + CO + O(2)→ AuO + CO(2)) is just 4.8 kcal mol(-1), and that for the second CO oxidation step (AuO + CO → Au + CO(2)) is just 1.6 kcal mol(-1).     

Anionic polymerization of a fluorinated butadiene

1,1-Bis(trifluoromethyl)-1,3-butadiene (I) is cleanly prepared in three steps. I produces an amorphous polymer by free-radical catalysis. Crystalline poly-I is produced by butyllithium catalysis in tetrahydrofuran at ?78°C. Qualitative kinetic experiments indicate that the anionic polymerization proceeds by a “living polymer” process. An AB block copolymer may be formed by adding I to anionically propagating butadiene; however, the reverse does not occur.     

Host-guest adaptability within oxothiomolybdenum wheels: structures, studies in solution and DFT calculations

The formation of host-guest cyclic architectures, built up through the self-condensation process of [Mo(2)O(2)S(2)](2+) oxothiocations around linear dicarboxylate ions such as adipate (Adip(2-)), suberate (Sub(2-)) and azelaate (Azel(2-)) anions is reported. The complexes [Mo(12)Adip](2-), [Mo(12)Sub](2-) and [Mo(14)Azel](2-) have been characterized in the solid state by X-ray diffraction and in solution by (1)H NMR in different solvents (D(2)O, DMF, DMSO and CD(3)CN). The host-guest dynamics appear to be dependent on the nature of the system and are mainly governed by mutual adaptability between the host and the guest. (1)H NMR DOSY experiments show systematic differences, either positive or negative between the experimental and calculated molecular weights which appear to be correlated with the charge of the anion. The relative stabilities of the twelve-membered rings containing the Adip(2-), Pim(2-) (pimelate) or Sub(2-) anions were determined experimentally and decrease according to the order [Mo(12)Adip](2-) > [Mo(12)Pim](2-) > [Mo(12)Sub](2-). The host-guest adaptability depends on the length of the carbon chain and gives rise to selective encapsulation processes. Finally, theoretical DFT investigations in the gas phase yielded conformations whose symmetry and geometrical parameters proved consistent with X-ray structures and (1)H NMR spectra recorded in DMSO or DMF. Energy calculation highlights the high flexibility of the ring showing that only 3.1 kJ mol(-1) accompanies the conformational change from circular to elliptical. The host-guest bond energy (Delta E) calculated for the Mo(12)-based clusters is consistent with the experimental stability scale, major variations being due to some constraints undergone by the central alkyl chain.     

DFT calculations of the defect structures,electronic structures,and EPR parameters for three Rh2+ centers in AgCl

The local structures for various Rh²⁺ centers in AgCl are theoretically studied using density functional theory (DFT) with periodic CP2K program. Through geometry optimizing, the stable ground states with minimal energies and electronic structures are obtained for the tetragonally elongated (T_E), orthorhombically elongated (O_E), and tetragonally compressed (T_C) centers, and the corresponding g and hyperfine coupling tensors are calculated in ORCA level. The calculations reveal obvious Jahn–Teller elongation distortions of about 0.109 and 0.110 Å along [001] axis for T_E and O_E centers without and with 1 next nearest neighbor (nnn) cation vacancy V_Ag in [100] axis, respectively. Whereas T_C center with 1 nnn V_Ag along [001] axis exhibits moderate axial compression of about 0.066 Å due to the Jahn–Teller effect. For O_E and T_C centers with 1 nnn V_Ag, the ligand intervening in the central Rh²⁺ and the V_Ag is found to displace away from the V_Ag by about 0.028 and 0.024 Å, respectively. The present results are discussed and compared with those of the previous calculations based on the perturbation formulas by using the improved ligand field theory.     

The anionic polymerization of butadiene in the presence of dipiperidinoethane

The polymerization of butadiene by lithium-based initiators in aliphatic hydrocarbon solvents in the presence of dipiperidinoethane gives a highly 1,2 structure. This system was studied to determine the factors that govern the structure. The effect of this complexing agent on the kinetics of propagation, the microstructure of the polymer, and the ultraviolet spectrum of the active chain end was measured. The results suggest that more than one solvated species form, all giving highly 1,2 polymer structures, but that a solvate containing one chain end with one dipiperidinoethane molecule is the principal reacting species which gives close to 100% 1,2 structure. This reacting center has a high proportion of trans-structure in the allyllic ion, únlike most complexed active chain ends, which are often highly cis. It is this transstructure in the ion that is thought to be responsible for the polymer structure.     

Chemical structure-stereospecificity relationship of internal donor in heterogeneous Ziegler-Natta catalyst for propylene polymerization by DFT and MM calculations

The effect of chemical structure of 2,2′-disubstituted 1,3-dimethoxypropane (so-called 1,3-diether) on the performance of Ziegler-Natta (ZN) catalyst was investigated by using density functional theory and molecular mechanics. Calculation of the energy barrier during insertion of propylene reveals that the isospecific active site created on the (1 0 0) surface of MgCl₂ is more active than the aspecific active site created on the (1 1 0) surface of MgCl₂ for propylene polymerization. When the adsorption energies of various 1,3-diethers are calculated and analyzed in terms of isotacticity, it is found that the isotacticity of polypropylene increases as 1,3-diether is adsorbed more preferentially on the (1 1 0) surface. Since analysis of energetics for insertion of propylene into the active site created on the (1 1 0) surface with 1,3-diether coordinated to Mg atom in the vicinity of the active site reveals that the coordination of 1,3-diether does not transform the aspecific active site on the (1 1 0) surface into isospecific one, it is concluded that the primary function of 1,3-diether is to prevent the formation of aspecific site on the (1 1 0) surface, without significant decrease in the number of the isospecific active site created on the (1 0 0) surface. A systematic analysis of various model compounds for 1,3-diether suggests that the substitution of highly branched hydrocarbon at the C₂ position of 1,3-diether results in better performance of ZN catalyst.     

Total energy calculations in the DFT on binary compounds

Calculations are carried out using first-principles self-consistent local-density and nonlocal density theory of the electronic structure, the total energy, and the charge density of a variety of semiconducting and insulating compounds under hydrostatic and uniaxial pressure. For several cases, the transition pressure from one structure to another is determined as well as the pressure coefficients of the main band gaps. It is shown that several properties are calculated with adequate accuracy to be compared with experiment, so that values which have not yet been measured are trustworthy predictions. © 1995 John Wiley & Sons, Inc.     

Asymmetric radical polymerization of butadiene 1-carboxylic acid

Optically active S(?)-α-phenethylammonium butadiene 1-carboxylate was prepared and polymerized in methanol, using azobisisobutyronitrile as initiator. The optical rotation, optical rotatory dispersion and circular dichroism spectra of the polymers, before and after removal of chiral amine, have demonstrated that the asymmetric induction occurred in the main chain. An asymmetric inductive polymerization mechanism is discussed.