Heptane-soluble homogeneous zirconocene catalyst: Synthesis of a single diastereomer,polymerization catalysis,and effect of silica supports

Bis(1-indenyl)-di[1′S, 2′R, 5′S)-methoxy]silane ( 1 ) was converted into a mixture of corresponding ansa-diastereomeric zirconocenes. Further purification afforded a single dia-stereomer, di[(1′S, 2′R, 5′S)-methoxy] silylene-bis[η⁵-1(R, R)-(+)-indenyl] dichlorozirconium ( 2 ), which is optically active and hydrocarbon soluble. Extremely rapid ethylene, propylene, and ethylene-hexene polymerizations were observed both in toluene and n-heptane solutions; for instance, at 50°C, activity for ethylene polymerization reaches ～ 1.5×10¹⁰ (g of PE/((mol of Zr) · [C₂H₄] · h). The “bare” zirconocenium ion generated from 2/TIBA/Ph₃CB(C₆F₅)₄ exhibits unusual polymerization behaviors; the polymerization activity increases monotonically with temperature of polymerization (T_p) up to a conventional polymerization condition (50–70°C), and the ¹³C NMR study shows that the isotactic poly-propylene obtained has fairly high [mmmm] methyl pentad distributions at high T_p (?25°C with [mmmm] ～ 0.93–0.75) and a perfect stereoregularity at low T_p (?0°C with [mmmm] > 0.99). The catalyst precursors 2 and Et(Ind)₂ZrCl₂ ( 3 ) supported on silica by different approaches produced poly(olefins) of different molecular weights and stereoregularities, and a methylaluminokane and Ph₃CB(C₆F₅)₄ free silica-supported zirconocene system was found to be activated by triisobutylaluminum. © 1995 John Wiley & Sons, Inc.     

Olefin terpolymerizations. II. 4-vinylcyclohexene and cyclooctadiene

4-Vinylcyclohexene (VCH) and cyclooctadiene (COD) were investigated as termonomers in EPDM (ethylene/propylene/diene) synthesis by using rac-ethylenebis (1-η⁵-indenyl) zir-conium dichloride ( 1 ) as a catalyst precursor. Homopolymerizations of VCH, vinylcycloh-exane and cyclohexene were compared. The parameter K_πκ_p, which is the apparent rate constant for Ziegler-Natta polymerization, is about the same for VCH and vinylcyclohexanebut is 10 times smaller for cyclohexene. Therefore, the linear olefinic double bond is more active than the cyclic internal double bond. VCH reduces ethylene polymerization rate but not propylene polymerization rate in copolymerizations. In terpolymerizations, VCH tends to suppress ethylene incorporation especially at elevated polymerization temperature and Lowers the polymer MW by about two-fold. COD has very low activity as a termonomer. © 1995 John Wiley & Sons, Inc.     

Molecular mechanics study of zirconocenium catalyzed isospecific polymerization of propylene

Stereocontrol energy (ΔE⁰) is investigated as a measure of enantioselectivity of ansa-zircoocenium catalyst in propylene polymerization; it was calculated with MM2 (molecular mechanics) force field using π complex (°C) and transition state (TS) geometries obtained by ab initio molecular orbital methods. Both rac-ethylenebis (1-η⁵-indenyl) - ( 1 ) and rac-ethylenebis (1-η⁵-4,5,7,8-tetrahydroindenyl) ( 2 ) zirconocenium species are isospecific in either the π-complexes or the transition states. The stereoselectivity is greater if there is α-agostic interaction; it is lowered in the cases of β and γ agostic interactions. The ¹³C-NMR steric pentad distribution indicates the poly(propylene) to be much less stereoregular than that predicted by ΔE⁰. Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavorable. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving different agostic interactions could be one explanation for the observed loss. © 1995 John Wiley & Sons, Inc.     

ansa-Zirconocenium catalysis of syndiospecific polymerization of propylene: Theory and experiment

The syndiospecific propylene polymerizations catalyzed by isopropylidene(cyclopentadienyl)(fluorenyl)- and (2,2-dimethylpropylidene)(cyclopentadienyl)(fluorenyl)-zirconocenium ( 1 ⁺ and 2 ⁺) have been investigated theoretically and compared with experimental observations. With the ab initio calculated structures for the transition state (TS) of 1 ⁺(M)P and 2 ⁺(M)P (M = propylene, P = 2-methylpentyl), their steric energies (E°) have been computed using MM2 force-field. The difference between steric energies E°(m) and E°(r) for the meso and racemic enchainment of propylene, respectively, is defined as the stereocontrol energy [δE°(m ? r)] for syndiotactic propagation. The δE°(m ? r) for the TS of 1 ⁺ (M)P is about 2.1 kcal/mol, the value is 1 kcal/mol greater for 2 ⁺(M)P. The observed steric pentad distributions of the syndiotactic poly(propylene) obtained by these catalysts are consistent with smaller effective stereocontrol energy, which is about two-third as large as δE°(m ? r) values calculated for the MM2 optimized structure. Syndiotactic enchainment is favored over isotactic enchainment for all combinations of site configurations in the catalyst. α-Agostic interaction seems to enhance syndioselectivity, whereas γ-agostic interaction changes the stereoselectivity to meso enchainment. The mirror plane symmetry of the syndiotactic propagating species renders the stereoselectivity of the polymerization insensitive to reaction conditions. These catalysts are also highly regiospecific. © 1995 John Wiley & Sons, Inc.     

Electro-disclinic effect in tilted smectic phases of banana-shaped liquid crystal materials

We give the first evidence that the director tilt angle can be reduced by electric fields in the tilted smectic phase of banana-shaped molecules. In these phases the value of polarization is determined by the molecular packing and no electro-clinic effect is expected. Our studies show that high electric fields eventually induce a meta-stable phase with zero director tilt. The tilted phase slowly recovers at low fields. We propose that the field-induced quenching of the layer fluctuations is responsible for the observed effects.     

Synthesis,Crystal Structure,Photophysical and Electrochemical Properties of rac‐6,13‐Dihydro‐6,13‐methanopentacene

The title compound, rac‐6,13‐dihydro‐6,13‐methanopentacene ( 1 ), has been synthesized and characterized by elemental analysis, FT‐IR, ¹H NMR, UV‐Vis, HRMS spectra, cyclic voltammetry and single‐crystal X‐ray diffraction. The crystal belongs to orthorhombic, space group P2₁2₁2₁, with Z = 4 and cell dimensions a = 6.0185(4), b = 8.1914(6), c = 31.4080(19) Å. In the crystal structure, two types of intermolecular C–H···π hydrogen bonds are observed, and further stabilize the crystal structure. Its photophysical and electrochemical properties and complementary density functional theory (DFT) calculations are reported.     

Euphorbiane: A Novel Triterpenoid with an Unprecedented Skeleton from Euphorbia tirucalli

Euphorbiane ( 1 ), a novel triterpenoid exhibiting a unique skeleton, together with seven known compounds were isolated from the 95% EtOH extract of the fresh stems of Euphorbia tirucalli. The structure of 1 and relative stereochemistry were elucidated by extensive NMR and a single‐crystal X‐ray crystallographic analysis.     

Fingerprints of Phalaenopsis Tissues in Growth and Spike Induction Periods—A Solid‐state 13C NMR Approach

Solid‐state Nuclear Magnetic Resonance (ss‐NMR) ¹³C single‐pulse excitation spectroscopy in combination with the magic‐angle spinning (MAS) technique was applied to a series of Phalaenopsis tissues, including the leaf, sheath, stem, and root, at different growth and spiking periods. Compared with{¹H}/¹³C cross‐polarization MAS spectra, the ¹³C single‐pulse excitation MAS spectra displayed very distinct spectral patterns, recognizable as fingerprints of the tissues studied. ¹Here, we demonstrate that solid‐state ¹³C single‐pulse excitation NMR spectroscopy provides a direct and robust analytical tool for studying the various tissues of Phalaenopsis in different growth and spiking induction periods.