Propene polymerization with Ziegler-type catalysts formed from Me2Si(Ind)2ZrMe2 and cation-generating reagents

Kinetic and polymer analytical results obtained with the systems Me₂Si(Ind)₂ZrMe₂/[B(C₆F₄Si(i-Pr)₃)₄]⁻ [(C₆H₅)₃C]⁺ (2) and Me₂Si(Ind)₂ZrMe₂/[B(C₆F₅)₄]⁻ [(C₆H₅)₃C]⁺ (3) in olefin polymerizations are compared (Me: methyl; Ind: indenyl). Both systems show that the polymerization rate increases with increasing metallocene concentration in a surplus due to the formation of binuclear species. The expected influence of increasing cation-anion distance on the stereo-errors of the poly(propylene)s when changing from system 3 into system 2 could not be detected.     

Threading of cyclodextrins onto a polyester of octanedicarboxylic acid and polyethylene glycol

A water-soluble polyester was synthesized from dimethyl octane-1,8-dicarboxylate and polyethylene glycol (M̄_n = 900). This polymer forms water-soluble inclusion compounds with α-, β- and hydroxypropyl-α-cyclodextrin as proven by surface tension measurements, ¹H NMR spectroscopy and titration microcalorimetry. Mainly the octamethylene segments were covered by the cyclodextrins. This inclusion can be used to increase the solubility and the critical micelle concentration of the polyester in water.     

The incorporation of side reactions into pseudostationary polymerization, 3. The closed solution of a kinetic scheme for pulsed-laser polymerization comprising concomitant continuous initiation

A procedure is developed that allows the calculation of chain-length distributions of polymers prepared by periodic modulation of the initiation process considering concomitant continuous initiation. For the case of a (pseudostationary) laser-pulse initiated polymerization process a closed solution could be derived for the pseudostationary radical concentration and for the chain-length distribution of dead polymer terminated by disproportionation or stabilized by chain-transfer to monomer or solvent. The analysability of the characteristic peaks appearing in the chain-length distributions of laser-pulse initiated polymers (which is the key for determining the rate constant k_p) is only moderately influenced by continuous thermal radical formation if the extent of this side reaction is not pathologically large, i.e. as long as the amount of primary radicals created by the laser-pulse appreciably exceeds that produced in the dark reaction.     

Lattice Monte Carlo investigations on copolymer systems, 1. Diblock copolymers

Symmetric diblock copolymers in dilute solution were examined by means of Monte Carlo simulations on a cubic lattice with respect to chain- and block dimensions, shape, local structure and number of contacts. The solvent was either a common good one, a common θ-solvent or a selective one for the two blocks. In all cases, repulsive interactions are operative between the blocks. In addition, the underlying homopolymers (athermal and θ) were divided into two parts (and treated as a block copolymer) for comparison. Chain-length was varied from 40 to 1280 segments leading to the expected values for the critical exponent 2v ≈ 1.2 for good solvent quality and 2v ≈ 1.0 for θ-solvent. Copolymers in a selective solvent scale with an intermediate exponent, 2v ≈ 1.13. The deviation of the mean squared dimensions of the copolymers from the sum of those of two homopolymers of the same length and for the same solvent quality as the blocks is largest for block copolymers in a common θ-solvent (where it exceeds 20%), while the blocks themselves have mostly the same dimensions as their underlying homopolymers of equal length. The shape of the copolymers, expressed by the parameter δ (asphericity) becomes more rod-like with increasing chain-length if there are (compact) θ-blocks in the molecule which are subject to mutual repulsive interaction. In these cases, θ exceeds the value of the homopolymers in the limit of infinite chain-length. The number of contacts per segment approaches a limiting value with increasing chain-length which is ≈0.20 for athermal chains and athermal blocks. For θ-chains and θ-blocks, a limiting value is not yet reached within the range of chainlengths investigated. The number of contacts per segment between two different blocks quickly tends to zero with increasing chain-length.     

Influence of the Particle Morphology on the Activity of Nanometer Platinum Aerosol Catalysts

For nanoparticle agglomerates, the catalytic activity may depend strongly on their structure. The influence of different parameters such as agglomerate structure, primary particle temperature history and surface preconditioning on the catalytic activity of nanoparticles was investigated. The fraction of agglomerate surface contributing to the reaction depends on the agglomerate structure and on the velocity of the reaction under investigation. For extremely fast reactions such as the oxidation of hydrogen on Pt nanoparticles, only the outermost surface (exposed surface) contributes substantially to the formation of water. For the system investigated here, the inner surface not substantially contributing to the reaction accounted for at least 70% of the total particle surface as determined from oxygen presaturation experiments of the agglomerate surface. A considerable activity loss of the platinum particles was observed on preheating the nanoparticle agglomerates. The preheating leads to an increase in the nanoparticle size by an order of magnitude due to sintering. It is unclear if this activity reduction is due to changes in the particle surface state or to a real size effect of the nanoparticles.     

N,N′‐Diorganylsubstituted Hydrazinium Azides

1,2‐Diorganylsubstituted derivatives of hydrazinium azide were examined in order to investigate their higher volatility and higher sensitivity to initiation compared to 1,1‐diorganylsubstituted hydrazinium azide derivatives. The compounds were synthesized from the respective hydrazines by reaction with HN₃ and characterized by elemental analysis, vibrational (IR, Raman) and multinuclear NMR spectroscopy (¹H, ¹³C, ¹⁴N). Their sensitivity to friction, shock, electrostatic impact and heat was examined and the explosion products were investigated. The crystal structure of pyrazolidinium azide was determined.     

Unusual ethylene polymerization results with metallocene catalysts supported on silica

With the development of methods to support metallocenes and methylaluminoxane cocatalysts on suitable carriers, it became possible to combine the specific advantages of homogeneous metallocene catalysis with those of heterogeneous Ziegler catalysts in olefin polymerization. By means of ethylene polymerization it could be shown that the method of supporting methylaluminoxane and metallocene on porous silica has a substantial influence on the progress of polymerization. In particular, fragmentation of catalyst particles during polymerization can be circumvented, maintaining the catalyst activity, if active catalyst sites are being generated on the particle surface only. A method of preparation for such newly designed supported metallocene catalysts is presented, where the active catalyst sites are located exclusively on the particle surface. Furthermore, the kinetics of ethylene polymerization and morphology properties prior to and after polymerization are discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 677–682, 1999