Thermal behaviour of bromine-metal fire retardant systems

Chemical reactions which occur on heating fire retardant mixtures of decabromodiphenyloxide (DBDPO) with antimony trioxide (AO) or bismuth carbonate (BC) have been studied by means of weight loss, elemental analysis, X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive system (EDS) microanalysis.

It has been shown that on rapid heating to 700°C, DBDPO volatilisation competes with debromination by AO or BC leading to volatile SbBr₃ or BiBr₃ which are formed by different mechanisms. BC is rapidly transformed into BiOBr which gives BiBr₃ by further debromination of DBDPO. At high conversion (high temperature) this process is in competition with reduction of BiOBr to metallic bismuth. In the case of AO slower formation of progressively bromine-richer antimony oxybromides takes place which liberates SbBr₃ by thermal degradation.

The formation of a rod-shaped Br-Sb containing phase is shown to relate to the progression of chemical reactions which occur on heating the AO-DBDPO mixture.     

Thermal degradation of polymer-fire retardant mixtures—Part I: Polypropylene-chlorinated paraffin. Evidence of interactions

Evidence is given that the thermal behaviour of a mixture of polypropylene and a fire retardant chlorinated paraffin (Cl 70%) is not the sum of their individual behaviours. An interaction is shown to take place in the degrading mixtures which is related to the low temperature dehydrochlorination step of the chloroparaffin. This occurs at temperatures at which pure polypropylene is stable and leads to an increase in the overall rate of volatilisation of the mixtures compared with that of the separate components. A partial overlapping of volatilisation of polymer and additive, which otherwise degrade in two well separated temperature ranges when heated alone, is observed.The relevance of these observations to the mechanism of fire retardance is discussed.     

Structural and reactivity properties of finite length cap-ended single-wall carbon nanotubes

The reaction of C2 with growing single-wall carbon nanotubes of different chiralities is investigated using density functional theory. It is found that the energy of the frontier orbitals for (5,5) and (6,6) armchair carbon nanotubes exhibits periodic behavior with an increasing number of carbon atoms in the nanotube. Such periodic behavior induces oscillations in the reaction energy released by adsorption of C2 to the nanotube open edge. In contrast, the energy of the frontier orbitals of the (6,5) chiral tube remains constant as the number of C atoms increases, and the same stability is observed in the adsorption energy. It is suggested that this may be one of the reasons for the low percent of armchair single-wall carbon nanotubes found in the experimental synthesis.     

Adjacency matrices of polarity graphs and of other C 4-free graphs of large size

In this paper we give a method for obtaining the adjacency matrix of a simple polarity graph G _q from a projective plane PG(2, q), where q is a prime power. Denote by ex(n; C ₄) the maximum number of edges of a graph on n vertices and free of squares C ₄. We use the constructed graphs G _q to obtain lower bounds on the extremal function ex(n; C ₄), for some n < q ² + q + 1. In particular, we construct a C ₄-free graph on ${n=q^2 -\sqrt{q}}$ vertices and ${\frac{1}{2} q(q^2-1)-\frac{1}{2}\sqrt{q} (q-1) }$ edges, for a square prime power q.     

On Superconnectivity of (4, g)-Cages

A (k, g)-cage is a graph that has the least number of vertices among all k-regular graphs with girth g. It has been conjectured (Fu et?al. in J. Graph Theory, 24:187?C191, 1997) that all (k, g)-cages are k-connected for every k??? 3. A k-connected graph G is called superconnected if every k-cutset S is the neighborhood of some vertex. Moreover, if G?S has precisely two components, then G is called tightly superconnected. In this paper, we prove that every (4, g)-cage is tightly superconnected when g ???11 is odd.     

RhIAr/AuIAr′ Transmetalation: A Case of Group Exchange Pivoting on the Formation of M−M′ Bonds through Oxidative Insertion

By combining kinetic experiments, theoretical calculations, and microkinetic modeling, we show that Pf/Rf (C₆F₅/C₆Cl₂F₃) exchange between [AuPf(AsPh₃)] and trans‐[RhRf(CO)(AsPh₃)₂] does not occur by typical concerted Pf/Rf transmetalation via electron‐deficient double bridges. Instead, it involves asymmetric oxidative insertion of the Rh^I complex into the (Ph₃As)Au?Pf bond to produce a [(Ph₃As)Au?RhPfRf(CO)(AsPh₃)₂] intermediate, followed by isomerization and reductive elimination of [AuRf(AsPh₃)]. Interesting differences were found between the LAu?Ar asymmetric oxidative insertion and the classical oxidative addition process of H₂ to Vaska complexes.     

Addition reactions of alkyl and carboxyl radicals to vinylidene fluoride

The addition reactions of alkyl radicals CF3* and CH3* and carboxyl radicals C2H5O*, C2H5OCOO*, CF3COO*, and CH3COO* to a vinylidene fluoride (VDF) molecule are studied using ab initio calculations. These radicals were selected because they are intermediate or final products of diacyl peroxides decomposition in the initiation reactions of VDF polymerization. Two combinations of methods for energetics and structure optimization are applied: QCISD/6-311G(d,p)//HF/6-31G(d) and B3LYP/6-311G+(3df, 2p)//B3LYP/6-31G(d). It is found that the formed bond length of the product, the forming bond length of the transition state, and the attack angle of the product structures are not sensitive to the level of theory even though the attack angle of the transition state structures is. Early transition states are obtained upon attack at both high-substituted and nonsubstituted carbon atom VDF ends. Kinetic and thermodynamic control rules play different roles on governing the reactivity of the addition with the studied radicals. Both theoretical methods yield the same trends for the preferential attack site in terms of regioselectivity, barrier energies, and reaction enthalpies. It is shown that the addition reactions of the intermediate radicals C2H5OCOO*, CF3COO*, and CH3COO* of the decomposition of diethyl peroxydicarbonate, trifluoroacetyl peroxide, and diacetyl peroxide initiators yield smaller energy barriers than the additions of the corresponding final radicals, C2H5O*, CF3*, and CH3*; therefore, the reactions of the intermediate radicals should not be ignored when analyzing the initiation process of the VDF polymerization using those initiators.