Search results for "Homogeneous"

POSS nanostructures in catalysis

2020

Polyhedral oligomeric silsesquioxanes (POSS) are organic-inorganic hybrid molecules piquing the interest of researchers thanks to their synergistic features. The great versatility of POSS nanostructures arises from the easy tunability of peripheral organic moieties combined with the high thermal and chemical stability of the inner inorganic core. In this review, we highlight the use of POSS nanostructures as molecular precursors for the synthesis of homogeneous and heterogeneous catalysts able to promote many processes including alkene epoxidation, C-C bond formation, CO2 conversion, "click reactions", hydrogenation, and ethylene polymerisation, among others. In this scenario, POSS units fo…

Hemicellulose hydrolysis catalysed by solid acids

2013

Depolymerising hemicellulose into platform sugar molecules is a key step in developing the concept of an integrated biorefinery. This reaction is traditionally catalysed by either enzymes or homogeneous mineral acids. We compared various solid catalysts for hemicellulose hydrolysis, running reactions in water, under neutral pH and relatively mild temperature and pressure (120°C and 10 bar) conditions. Sulphonated resins are highly active, but they leach out sulphonic groups. Sulphonated silicas are less active, but more stable. They have weakly and strongly bound sites and the strongly bound ones do not leach. Zeolites are moderately active and stable. Among them, H-ferrierite especially, d…

Homogeneous ethylene-polymerization catalysts based on alkyl cations of the rare-earth metals: are dicationic mono(alkyl) complexes the active specie…

2003

CPF : Continuous polymer fractionation

1992

A method is presented by means of which it is possible to fractionate polymers according to their solubility on a technical scale. The CPF consists of a continuous counter-current extraction process in which a homogeneous mixture of a molecularly or/and chemically non-uniform high molecular weight product is divided into two portions of considerably lower non-uniformity. The principle of the CPF is described in general terms and examples are given for the fractionation of homopolymers.

1994

Polymer blends can undergo, during processing, degradation phenomena because of the presence of both temperature and mechanical stresses. With respect to pure homogeneous polymers, the degradation of polymer blends shows distinct features because of the possible interaction between the different chemical species. These interaction can give rise to both a faster degradation or to the formation of copolymers which act as stabilizing agents. This latter phenomena is particularly important in the processing of condensation polymers

Effective pair potential between charged nanoparticles at high volume fractions

2017

Simulations of charged colloidal dispersions are technically challenging. One possible workaround consists in reducing the system to the colloids only, whose interactions are described through an effective pair potential, wf. Still, the determination of wf is difficult mainly because it depends on the colloidal density, ϕ. Here we propose to calculate wf from simulations of a pair of colloids placed in a cubic box with periodic boundary conditions. The variation in ϕ is mimicked by an appropriate change in the concentration of counterions neutralized by an homogeneous background charge. The method is tested at the level of the primitive model. A good description of the structure of the coll…

Immobilized Chiral ortho-Metalated Dirhodium(II) Compounds as Catalysts in the Asymmetric Cyclopropanation of Styrene with Ethyl Diazoacetate

2007

Immobilization of ortho-metalated dirhodium(II) compounds has been achieved by a carboxylate interchange reaction between (M)-Rh2(l-protos)2[(p-XC6H3)P(p-XC6H4)2]2 diastereoisomers and carboxyethylpolystyrene polymer (PS-C6H4(CH2)2CO2H). The immobilized chiral catalysts have been tested in the standard reaction of asymmetric cyclopropanation of styrene with ethyl diazoacetate, giving higher yields than homogeneous chiral trifluoroacetate derivatives, but their diastereo- and enantioselectivities were lower. Some of the immobilized catalysts have proved to be very robust. The catalytic behavior of (M)-Rh2(O2C(CH2)2C6H5)2[(p-XC6H3)P(p-XC6H5)2]2 compounds has been studied as a model for the im…

Heterogeneously catalyzed Suzuki-Miyaura conversion of broad scope

2012

The reaction tolerates a broad range of functional groups in the coupling partners and is usually performed in solution under homogeneous conditions at T ¢ 60 uC using 2–3 mol% catalytic amounts. The catalyst is often a Pd(0) complex with triarylphosphane ligands. 2 The catalytic cycle (Scheme 2) begins with the oxidative addition of an aryl halide to a Pd(0) species formed in situ to form an arylpalladium(II) halide intermediate. 3 Chloroarenes, especially nonactivated aryl chlorides, are notoriously less reactive due to the stability of the C–Cl bond (the relative reactivity of Ar–X is correlated to the respective bond dissociation energy: Ph–Cl: 96 kcal mol 21 ,P h–Br: 81 kcal mol 21 ,P …

Iron-Catalyzed C(sp2)-C(sp3) Cross-Coupling of Alkyl Grignard Reagents with Polyaromatic Tosylates

2017

Molekulargewichtsverteilungen einiger emulsionspolymerisate von styrol

1965

An einigen unter normalen Bedingungen hergestellten Emulsionspolymerisaten werden die Verteilungen durch Fallungschromatographie bestimmt. Die Uneinheitlichkeit ist erheblich hoher als bei unter homogenen Bedingungen hergestellten Polymerisaten, und die Verteilungskurven weisen einige Besonderheiten auf. Einige Schlusse uber die Kinetik der Emulsionspolymerisation konnen daraus gezogen werden. Vergleichende Viskositats- und Lichtstreuungsmessungen im Bereich der Molekulargewichte 106 bis 107 ergeben, das die [η]-M-Beziehung bei Emulsionspolymerisaten und homogen polymerisierten Polystyrolen innerhalb der Fehlergrenzen ubereinstimmt. The molecular-weight distributions of several polymers pre…