New heterogenized catalytic systems in norbornadiene allylation

New heterogenizated catalysts for the allylation of norbornadiene (NBD) were prepared by the adsorption immobilization and surface assembly methods. The obtained catalysts were screened in the allylation of NBD, and the main technological parameters of the process were determined.     

A new catalytic approach to germasiloxanes

A new ruthenium(0) complex catalyzed coupling reaction of silanols with vinylgermanes leading to SiO-Ge bond formation with the evolution of ethylene is described. Under optimum conditions the catalytic O-germylation of silanols gives exclusively germasiloxanes.     

A modular approach to main-chain organometallic polymers

A highly efficient route to a new class of organometallic polymers containing difunctional N-heterocyclic carbenes has been developed. Bis(imidazolium) halides and divalent group X metals were copolymerized to afford organometallic polymers in up to quantitative yields and with molecular weights up to 10(6) Da, depending on the structure of the N-heterocyclic carbene and the incorporated transition metal. Enhanced solubilities were demonstrated through post-polymerization ligation with phosphines. Finally, selective end-group functionalization and excellent molecular weight control was achieved through the inclusion of monofunctional chain transfer agents during the polymerization.     

A general approach to polysubstituted pyrroles

Exposure of a range of 3-hydroxy-2-sulfonylamino-4-alkynes to excess iodine delivers good yields of a series of iodopyrroles. Unexpectedly, the hydroxyl-dihydropyrroles, which were assumed to be the first-formed intermediates, turn out to be stable entities which have been isolated for the first time.     

A molecular approach to chain entanglement in linear polymers

The phenomenon of chain entanglement in undiluted linear amorphous polymers is treated by calculating the probability of forming closed intramolecular loops. Adoption of the rotational isomeric state model of polymer chains permits an appropriate accounting of the detailed molecular structure to be made through the configurational characteristics of the polymer. The second (〈r〉₀) and fourth (〈r〉₀) moments of the vector r_hk connecting groups h and k in the isolated polymer chain and averaged over all chain conformations are calculated and used to evaluate the probability W_x(0) that r_hk is 0, or that an intramolecular loop of x = k ? h bonds is formed. Several linear polymers with widely differing molecular structures are treated. An attempt is made to correlate the degree of chain entanglement they manifest in the bulk with their ability to fold back upon themselves to form closed intramolecular loops.     

A synergistic approach to the polydispersity of polymers

A scheme for calculating polydispersity coefficients of polymers during chain propagation in living polymerization is proposed on the basis of the synergistic approach (the master equation) with allowance for fluctuations in the number of free radicals at the stage of initiation. The molecular-mass-distribution function of living chains is calculated. The form of this function is shown to be dependent on the ratio between the constants of the elementary stages of chain initiation and propagation.     

A combinatorial approach to branched polymers’ statistics

At least ideally, a certain class of polymers presents itself as a collection (set) of connected components. Each of these components is a cycle of trees, that is branched polymers eventually rooted on a cycle. We derive (and study) an equilibrium statistical model that accounts for the main connectivity features of such structures, whose origin is to be found in combinatorial probability. Phase transition (gel–soltransition) is shown to occur when some internal control parameter crosses one (critical parameter). Various structural asymptotic results are shown to be available using singularity analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

A general approach to determining the evolution of molecular weight distribution curves of linear polymers undergoing chain scission

A numerical method is developed to compute the development of molecular weight distribution (MWD) curves of linear polymers undergoing chain scission. The method can be applied to complex chain scission kinetics and for arbitrarily complex initial MWD curves. Our method is based on the method of lines (MoL). Different from the existing numerical scheme, we propose the use of logarithmically spaced points. This development ensures the accuracy of the computed MWD curves at low molecular weights, and it does not require a very fine discretization to produce an accurate result.     

A general catalytic allylation using allyltrimethoxysilane

A general and mild catalytic allylation of carbonyl compounds, applicable to aldehydes, ketones, and imines is developed using allyltrimethoxysilane as the allylating reagent. The reaction proceeds smoothly with 1-10 mol % of CuCl and TBAT in THF at ambient temperature. Mechanism studies indicated that the copper alkoxide, allylfluorodimethoxysilane, and allyltrimethoxysilane are essential to promote the reaction efficiently. Preliminary extension of the reaction to the first catalytic enantioselective allylation of ketones using an allylsilane produced the product with 61% ee from acetophenone, using a CuCl-p-tol-BINAP-TBAT catalyst (15 mol %).     

A thermodynamic approach to the fragility of glass-forming polymers

We have connected the dynamic fragility, namely, the steepness of the relaxation-time variation upon temperature reduction, to the excess entropy and heat capacity of a large number of glass-forming polymers. The connection was obtained in a natural way from the Adam-Gibbs equation, relating the structural relaxation time to the configurational entropy. We find a clear correlation for a group of polymers. For another group of polymers, for which this correlation does not work, we emphasize the role of relaxation processes unrelated to the alpha process in affecting macroscopic thermodynamic properties. Once the residual excess entropy at the Vogel temperature is removed from the total excess entropy, the correlation between dynamic fragility and thermodynamic properties is reestablished.     

Dimethylsubstituted pyrroles: A new approach to protective polymers

The development of sacrificial or protective films based on dimethylpyrrole analogues is described. The design considerations required for successful electropolymerisation of the monomer and the subsequent, selective dissolution of the film is described. A general synthetic strategy is presented for the formation of a diverse range of bifunctional monomers which, when polymerised, can be subsequently removed after exposure to hydroxylamine under neutral aqueous conditions at room temperature. Film formation and removal has been studied using cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) studies.     

New approach to filled polymers

Mechanopolymerization of pentabromobenzyl (mono)acrylate (PBB-MA) on the surface of inorganic fillers Mg(OH)₂ and CaCO₃ was studied. The role of activated surface of fillers was investigated using DSC and FTIR. The influence of milling time and of the filler chemical content on polymerization starting temperature and polymerization enthalpy was studied using DSC. It was shown that an increase of the filler concentration leads to a higher maximum conversion degree. The correlation between conversion kinetics and polymerization enthalpy of the material notpolymerized during milling was shown. This paper was presented on the Second Conference of The Israel Group of Mechanochemistry.     

A general strategy for creating "inactive-conformation" abl inhibitors

Kinase inhibitors that bind to the ATP cleft can be broadly classified into two groups: those that bind exclusively to the ATP site with the kinase assuming a conformation otherwise conducive to phosphotransfer (type I), and those that exploit a hydrophobic site immediately adjacent to the ATP pocket made accessible by a conformational rearrangement of the activation loop (type II). To date, all type II inhibitors were discovered by using structure-activity-guided optimization strategies. Here, we describe a general pharmacophore model of type II inhibition that enables a rational "hybrid-design" approach whereby a 3-trifluoromethylbenzamide functionality is appended to four distinct type I scaffolds in order to convert them into their corresponding type II counterparts. We demonstrate that the designed compounds function as type II inhibitors by using biochemical and cellular kinase assays and by cocrystallography with Abl.     

An approach to phase behaviour in polymers

Aspects of phase transition will be surveyed such as are direct consequences of straightforward thermodynamic considerations yet are not normally taken count of in traditional treatments of phase behaviour even if, as is here shown, they can assume special significance in polymers. The central theme will be the common experience that in most materials, but especially in polymers, the state of ultimate thermodynamic stability is hardly ever attained, consequently that we are dealing with metastable states, the issue of metastability thus becoming the main theme of the present treatise. This metastability can manifest itself either through the phase transformation, even in the thermodynamically stable phase, not reaching completion and/or that phases other than those of ultimate stability appear first often taking on a dominant role. The above two strands are being followed through first separately and finally, with the example of polymer crystallisation, in combination. In the course of it all a variety of phase transformations and numerous potentially intriguing consequences are being touched upon. These are encompassing aspects of liquid-liquid phase separation, liquid-crystal formation and crystallisation, the intervention of glass transition, the significance of supercooling, the effect on and influence of the morphology and the connection and interrelation between thermodynamic stability (including metastability) and the rates of the phase transformation, with passing comments on gels and also on some aspects of chain conformation in solution. In the case of the most extensively covered item of crystallisation attention is being drawn to the possibility of stability inversion with phase (here crystal) size. The latter, in terms of ‘Phase Stability Diagrams’, offers a new approach to crystal growth, which in the case of polymers, creates both new perspectives and opens up the possibility of treating so far largely disconnected aspects of the subject within a unifying framework. The survey ends with reference to recent work on amorphisation through pressure pointing to new aspects of material behaviour and to some new and possibly surprising variants of otherwise familiar phase diagrams.     

One-pot multi-component asymmetric cascade reactions catalyzed by soluble star polymers with highly branched non-interpenetrating catalytic cores

Non-interpenetrating star polymer catalysts designed to mimic the site isolation characteristics of enzymes enable the one-pot combination of multiple otherwise incompatible catalysts for asymmetric cascade reactions that involve iminium, enamine, and H-bonding catalysis. Control experiments replacing star polymer catalysts with the corresponding small molecule or linear polymer analogues lead to little or no cascade reaction. Our strategy also allows straightforward access to all possible stereoisomers of the cascade product individually by proper choice of catalyst chirality. To our knowledge, this work represents the most sophisticated study of soluble polymers for site isolation, enzyme-like catalysis that generates cascade products with multiple chiral centers.     

A novel layer-by-layer approach to immobilization of polymers and nanoclusters

This paper reports a simple method for the multilayer immobilization of conjugated polymers, gold nanoparticles on solid supports. Poly(phenyenevinylene) functionalized with aldehyde and aminooxy groups was chemoselectively immobilized onto both glass and gold substrates via layer-by-layer deposition. The physical properties of the thin films were characterized by grazing angle IR, TM-AFM, fluorescence, and UV-visible spectroscopy. This methodology was also successfully applied to prepare polymer/gold nanocluster alternating multilayers. The results show that this methodology provides a general route for preparing robust and functionalizable multilayer films on solid substrates with molecular-level thickness control.     

A first approach to the catalytic properties of porous glass

The stopped-flow technique was used to study the catalytic properties of porous glass on a number of model reactions. It was found that porous glass is an excellent catalyst for the dehydrohalogenation of alkyl halides, dehydration of aliphatic alcohols, and deamination of primary aliphatic amines.

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