Thermal and spectroscopic characterization of polypropylene-carbon nanotube composites

Thermogravimetric (TG) and varied temperature Raman spectroscopic measurements of melt-blended polypropylene composites (PP) with double wall (DWNT) and multi-wall carbon nanotubes (MWNT) revealed that the incorporation of carbon nanotubes into polymer matrix increased the thermal stability comparing to the virgin polypropylene. The characterization of reference nanotubes was also done by Raman microscopy and TG measurements. Varied temperature rheological analysis provided further information about the thermal decomposition of the composites indicating the formation of high strength char in case of MWNT and limited applicability of DWNT at high temperature. The residue of the decomposition of PP-MWNT nanocomposites consists of nanotubes of spectroscopically higher purity comparing to the original one indicating the thermally induced chemical changes in the solid phase.     

Thermal degradation of phosphonated polystyrenes: Part 1—Chain end condensation

Polystyrenes substituted with chloromethoxyphosphonated groups at chain ends undergo a condensation reaction at a relatively low temperature (ca. 200°C) prior to extensive degradation of the polymeric chain (280–430°C). The condensation leads to an increase in the molecular weight through the formation ofPOPbonds between phosphonated chain ends with elimination mainly of CH₃Cl. Trace amounts of CH₃OH are also evolved in this step. Mechanisms for the condensation reaction are proposed and its relevance to the fire retardant activity of these polymers is discussed.     

Li/MgO catalysts, I. Effect of precursor salts on their structural and surface properties

Li/MgO solids have been prepared from MgO, by thermal decomposition of Mg₅(OH)₂(CO₃)₄·4H₂O or by precipitation from aqueous solutions of magnesium nitrate, and then impregnating the solid thus obtained with aqueous solutions of lithium carbonate or nitrate. Non-porous solids are obtained in all cases. Specific surface area development depends on the nature of the precursors, the MgOMg(OH)₂MgO process being topotactic.

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Li/MgO MgO Mg₅(OH)₂(CO₃)₄·4H₂O . . , MgOMg(OH)₂MgO .

     

Intercalation ofn-alkylamines into α-titanium phosphate from aqueous solutions

The intercalation reactions betweenn-alkylamines and -titanium phosphate in aqueous media have been investigated. The compounds with the maximum intercalation have the formula -Ti(HOPO₃)₂ · 2 C _n H _2n+1 NH₂ · H₂O (n=1–10). Defined crystalline phases with lower amine content are described, the general formula being -Ti(HOPO₃)₂ · _m C _n H _2n+1 NH₂ ·pH₂O (m=1.0 1.3, 1.5, 1.7). Whenm=1.0 then-alkylamines form a monomolecular layer. Whenm>1.0 the layer is bimolecular. The inclination angle and the packing density of then-alkylamines in the interlayer space is determined.     

Thermal degradation of polymer-fire retardant mixtures: Part VI—Mechanism of interaction in polystyrene-chloroparaffin mixtures

In mixtures of polystyrene with a fire retardant chloroparaffin, the rate of volatilisation of the polymer is initially increased during the thermal dehydrochlorination of the additive. In the residue which results, the volatilisation of polystyrene occurs at a lower rate than when it is heated alone. The mechanisms of the reactions which occur in the mixture are discussed and related to the fire retardant action of the chloroparaffin.     

Cationic (fluoromesityl)palladium(II) complexes

Halide abstraction from [Pd(μ-Cl)(Fmes)(NCMe)]₂ (Fmes = 2,4,6-tris(trifluoromethyl)phenyl or nonafluoromesityl) with TlBF₄ in CH₂Cl₂/MeCN gives [Pd(Fmes)(NCMe)₃]BF₄, which reacts with monodentate ligands to give the monosubstituted products trans-[Pd(Fmes)L(NCMe)₂]BF₄ (L = PPh₃, P(o-Tol)₃, 3,5-lut, 2,4-lut, 2,6-lut; lut = dimethylpyridine), the disubstituted products trans-[Pd(Fmes)(NCMe)(PPh₃)₂]BF₄, cis-[Pd(Fmes)(3,5-lut)₂(NCMe)]BF₄, or the trisubstituted products [Pd(Fmes)L₃]BF₄ (L = CN^tBu, PHPh₂, 3,5-lut, 2,4-lut). Similar reactions using bidentate chelating ligands give [Pd(Fmes)(L-L)(NCMe)]BF₄ (L-L = bipy, tmeda, dppe, OPPhPy₂-N,N′, (OH)(CH₃)CPy₂-N,N′). The complexes trans-[Pd(Fmes)L₂(NCMe)]BF₄ (L = PPh₃, tht) (tht = tetrahydrothiophene) and [Pd(Fmes)(L-L)(NCMe)]BF₄ (L-L = bipy, tmeda) were obtained by halide extraction with TlBF₄ in CH₂Cl₂/MeCN from the corresponding neutral halogeno complexes trans-[Pd(Fmes)ClL₂] or [Pd(Fmes)Cl(L-L)]. The aqua complex trans-[Pd(Fmes)(OH₂)(tht)₂]BF₄ was isolated from the corresponding acetonitrile complex. Overall, the experimental results on these substitution reactions involving bulky ligands suggest that thermodynamic and kinetic steric effects can prevail affording products or intermediates different from those expected on purely electronic considerations. Thus,water, whether added on purpose or adventitious in the solvent, frequently replaces in part other better donor ligands, suggesting that the smaller congestion with water compensates for the smaller M-OH₂ bond energy.     

Study of flame retardance in brominated liquid polybutadienes

Bromination, carried out with either bromine or carbon tetrabromide, imparts good fire retardant characteristics to low molecular weight polybutadienes, independently of the method of bromination. The fire retardant action is due mainly to a condensed phase mechanism at a bromine content below 7%, whereas a gas phase mechanism is involved at higher bromine content. Synergism with Sb₂O₃ occurs only with polybutadienes brominated with carbon tetrabromide, apparently by a gas phase mechanism.     

Pt(II) uptake by dendrimer outer pockets: 1. Solventless ligand exchange reaction

Density functional theory is used to elucidate molecular-level details of the complexation of Pt(II) metal compounds with PAMAM dendrimers. Particular attention is given to the ligand exchange reaction (LER). Binding of Pt(II) complexes to one dendrimer atom site (monodentate binding) is found to be thermodynamically feasible. Tertiary amine nitrogen (N3) is found to be the most favorable binding site in agreement with previous experimental work. Comparing the binding of Pt(II) species to atom sites in simple molecules with those to similar sites in dendrimer outer pockets allowed us to assess the impact of dendrimer branches on the binding. The impact of branches is manifested in more complex reaction profiles for complexation of Pt(II) species, because of the numerous ways in which a single molecule could be hosted by an outer dendrimer pocket. It is found that branches slightly improve the binding strength to all sites, particularly to N3. However, they could also be responsible for the increase of the activation energy for direct LER of PtCl(4)(2-) and PtCl(3)(H(2)O)- at the N3 site. Considering the thermodynamics of both complexation steps, namely noncovalent binding (NCB) and LER, it is found that to have a PtCl(3)(-) moiety bound to N3, as a result of NCB + LER operating on PtCl(4)(2-), is more likely than to have any other ion hosted in the outer pockets. However, the activation energy for direct LER of PtCl(4)(2-) at the N3 site is found to be the largest among all Pt(II) metal complexes and even larger than the barrier to its own aquation yielding PtCl(3)(H(2)O)(-).     

Catalytic activity tuning of a biomimetic HO-FeV=O oxidant for methane hydroxylation by substituents on aromatic rings: theoretical study

HO-(TPA)FeV=O (TPA = tris(2-pyridylmethyl)amine) has been proposed in the literature as the key high-valent iron-oxo intermediate involved in alkane hydroxylation. Here the structure of this species is investigated theoretically in the framework of density functional theory (DFT). A detailed electronic structure analysis leads to the presumption that the properties of the FeV=O bond can be modified by introducing substituents to the aromatic rings of TPA and thus the reactivity of HO-(TPA)FeV=O for the hydrogen atom abstraction of methane hydroxylation can be tuned on the quartet potential energy surface. The validity of our presumption is verified by DFT calculations. According to the rebound mechanism, the H-abstraction step is examined by using five complexes with TPA and TPA-derivative ligands and the corresponding reaction energies and energy barriers are obtained and compared with each other. The results are fully in agreement with our qualitative model, showing that electron-withdrawing groups are able to lower the barrier and facilitate the reaction, whereas the electron-donating groups increase the barrier and reduce the reactivity.