Homo‐ and Copolymerization of Styrene and Alkylstyrenes with the Highly Active Catalyst System CpTiF3/MAO

Syndiotactic copolymers with p‐methylstyrene, p‐tert‐butylstyrene and α‐methylstyrene were obtained using fluorinated half‐sandwich catalyst systems. In the presence of p‐alkylstyrenes the activities were high. The incorporation of p‐alkylstyrenes is nearly as good as it is for styrene. Molar masses increase using p‐alkylstyrene comonomers. The steric hindrance of the α‐methyl group in α‐methylstyrene results in low activities and poor incorporation rates. Since the melting points of the copolymers are lower than the melting point of the styrene homopolymer the products are easier to process. Crystalline p‐tert‐butylstyrene‐rich copolymers were obtained if appropriate crystallization procedures were carried out.     

A non-invasive method for the assessment of hemostasis in vivo by using dynamic light scattering

We present a new non-invasive method for assessing hemostasis in vivo. This method is based on the analysis of the movement characteristics of red blood cells (RBCs) during blood stasis condition. Stasis is intermittently induced by occlusion of arterial blood flow at the finger root. We assumed that under zero flow conditions, RBC movement is driven mostly by Brownian motion, and we characterized the RBC movement by utilizing the dynamic light scattering (DLS) technique in vivo. We found that during the stasis the RBCs diffusion coefficient in plasma decreases. We speculate that the RBC diffusion coefficient is most strongly related to endothelial and hemostatic activity. This assumption is supported by our findings that RBC movement, being expressed through the characteristics of the measured DLS signal, is correlative to the biological age and also is related to the coagulation factors. This new method can serve as a new diagnostic and research tool for the assessment of hemostasis and vascular function.     

A mesomechanical fracture model for an orthotropic material with different tensile and compressive strengths

The paper addresses a fracture problem for an orthotropic cracked plate made of a material with different tensile and compressive strengths and subjected to biaxial loading. The problem is solved using a micromechanical fracture model proposed earlier by the authors. It is assumed that the fracture of the material in the fracture process zones at the crack front is described by the Gol’denblat–Kopnov failure criterion. Strength curves for an orthotropic cracked plate with different strength and fracture-toughness parameters are plotted     

On stability of the equilibrium of a cottrell crack

The Wiener–Hopf method is used to find the exact solution to the static symmetric plane problem of elasticity for a homogeneous isotropic plate with a finite-length crack emerging from the point of intersection of two semi-infinite straight slip (dislocation) lines. An expression for the crack-tip stress intensity factor is derived. Crack initiation is described by the Cottrell mechanism. The equilibrium of the crack is analyzed for stability     

Syndiospecific polymerization of styrene with BzCpTiCl3 and methylaluminoxane as cocatalysts

Benzyl cyclopentadienyl titanium trichloride (BzCpTiCl₃) was synthesized from benzyl bromide, cyclopentadienyl lithium, and titanium tetrachloride and used in combination with methylaluminoxane (MAO) for the syndiospecific polymerization of styrene. Kinetic measurements of the polymerization were carried out at different temperatures. The polymerization with BzCpTiCl₃/MAO differs from the polymerization with cyclopentadienyl titanium trichloride in its behavior toward the Al/Ti ratio. In addition, high activities are observed at high Al/Ti ratios. By analyzing the polymerization runs and the physical properties of the polymers with differential scanning calorimetry, ¹³C NMR spectroscopy, wide‐angle X‐ray scattering measurements, and gel permeation chromatography, we found that the phenyl ring coordinates to the titanium atom during polymerization. Other known substitutions of the cyclopentadienyl ring (V. Scholz, Dissertation, University of Hamburg, 1998) in principle influence the polymerization activity. The physical properties of the polymers produced by the catalysts already known are nearly identical. BzCpTiCl₃ is the first catalyst that leads to polystyrene obviously different from the polystyrene produced by other highly active catalysts. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2805–2812, 2001     

Ethylene propylene diene terpolymers produced with a homogeneous and highly active zirconium catalyst

EPDM terpolymers with ethylidene norbornene as diene monomer could be prepared by means of a soluble Ziegler catalyst formed from biscyclopentadienyl zirconium dimethyl and methylaluminoxane. The overall activities lie between 100 and 1000 kg EPDM/(molZr h bar), obtainable at zirconium concentrations as low as 5 × 10^?7 mol/L. After an induction period (0.5–5 h) the polymerization rates increased and then leveled to a value which was constant for several days. From copolymerization kinetics reactivity ratios r₁₂ = 31.5, r₂₁ = 5 × 10^?3, and r₁₃ = 3.1 could be derived, and by ¹³C-NMR spectroscopy r₁₂ · r₂₁ = 0.3 was found (1: ethylene, 2: propylene and 3: ethylidene norbornene). The regiospecifity of the catalyst toward propylene leads exclusively to the formation of head-to-tail enchainments. The diene polymerizes via vinyl polymerization of the cyclic double bond, and the tendency to branching is low. Molecular weights were estimated between 40,000 and 160,000. The average molecular weight distribution of 1.7 is remarkably narrow. Glass transition temperatures of ?60 to ?50°C could be observed. The cure behavior and the physical properties of cured samples were also tested.     

Osmium phosphiniminato complexes: synthesis,protonation, structure,and redox-coupled hydrolytic scission of N-p bonds

The osmium(VI) nitrido complex TpOs(N)Cl(2) [1, Tp = hydrotris(1-pyrazolyl)borate] reacts with triarylphosphines to afford the Os(IV) phosphiniminato complexes TpOs(NPAr(3))Cl(2) [Ar = p-tolyl (tol) (2a), phenyl (2b), p-CF(3)C(6)H(4) (2c)] in nearly quantitative yield. Protonation of 2a-c with 1 equiv of HOTf in MeCN occurs at the phosphiniminato nitrogen to give [TpOs(IV)(NHPAr(3))Cl(2)]OTf (3a-c) in 68-80% yield. Solutions of 2a-c in CH(2)Cl(2) react with excess H(2)O over 1 week to form the disproportionation products 1 (28%), TpOs(III)(NHPAr(3))Cl(2) (4a-c) (60%), and OPAr(3) (35%). Treatment of solutions of 3a-c with H(2)O also affords 1, 4a-c, and OPAr(3). X-ray structures of 2b, 3b, and 4b are presented. Cyclic voltammograms of compounds 2a-c exhibit Os(V)/Os(IV) and Os(IV)/Os(III) couples at approximately 0.3 and -1 V versus Cp(2)Fe(+/0). Protonation to give 3 makes reduction easier by approximately 1.2 V, so that these compounds show Os(IV)/Os(III) and Os(III)/Os(II) couples. In the hydrolytic disproportionation of 2a-c, labeling studies using (18)O-enriched O(2) and H(2)O establish water as the source of the oxygen atom in the OPAr(3) product. The conversions are accelerated by HOTf and inhibited by NaOD. The relative rates of hydrolytic disproportionation of 2a-c vary in the order tol > Ph > p-CF(3)C(6)H(4). The data indicate that protonation of the phosphiniminato nitrogen is required for hydrolysis. The mechanism of the hydrolytic disproportionation is compared to that of the related reaction of the osmium(IV) acetonitrile complex [TpOs(NCMe)Cl(2)](+).     

The discovery of metallocene catalysts and their present state of the art

Metallocene and other transition metal complexes activated by methylaluminoxane are highly active catalysts for the polymerization of olefins, diolefins, and styrene, which was discovered at the University of Hamburg about 25 years ago. These catalysts allow the synthesis of polymers with a highly defined microstructure, tacticity, and stereoregularity, as well as new copolymers with superior properties such as film clarity, tensile strength, and lower extractables. A better understanding of the mechanism of olefin polymerization leads to findings of other new single site catalysts. The development of the metallocene/MAO‐catalysts from their discovery to their present state of the art is presented. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3911–3921, 2004