Configurational stereoselectivity in the nucleophilic substitution of poly(vinyl chloride) with sodium thiophenate: NMR study and Monte-Carlo simulation of the reaction

The nucleophilic substitution of the chlorine atoms of poly(vinyl chloride) with sodium thiophenate is assumed to obey an SN2 mechanism with inversion of configuration and a steric control according to which reactivities R of the triads follow the order R_mm > R_mr ? R_rr. Such a reaction cannot be described by simple differential equations, but a Monte Carlo simulation, first generating a random Bernouillian chain and then simulating random attack of the chain by the reactant according to the triad reactivities, allows a good agreement with the experimental results of both the kinetics of a reaction carried out in cyclohexanone solution at 40°C and the evolution of the ¹³C and ¹H NMR spectra of the tertiary carbon atoms, assuming the following parameters: Bernouillian probability for a meso placement in the initial chain: 0.431; relative reactivities of the mm, mr and rm, rr triads: 1, 0.5, and 0., respectively. The agreement is excellent until the reaction is about three-quarters complete; this shows a limiting level of about 60% substitution. Better agreement can be obtained up to the limit, if a secondary process of steric control by the substituant in a partially substituted mr triad is assumed. Other possibilities for that secondary process are discussed.     

On the scaling of three-dimensional homogeneous and isotropic turbulence

In this paper we investigate the scaling properties of three-dimensional isotropic and homogeneous turbulence. We analyze a new form of scaling (extended self-similarity) recently introduced in the literature. We found that anomalous scaling of the velocity structure functions is clearly detectable even at a moderate and low Reynolds number and it extends over a much wider range of scales with respect to the inertial range.     

Ion-Exchange equilibria in cationic polyelectrolytes

A method which allows the evaluation of phenomenological ion exchange constants K_ex is proposed and tested in a cationic polyelectrolyte system. Results for nitrate/chloride exchange are nearly constant over the entire concentration range studied. However, for nitrate/bromide exchange, the results depend on the polyelectrolyte concentration, with K_ex following the same trend as the mean activity coefficient for the polyion bromide. For polyion fluoride, a conformational change is probably induced as nitrate is added to the solution and the result deviates from the linear relationship predicted by the method. Benzene sulfonate behaves like a hydrophilic anion, whereas tosylate and ethylbenzene sulfonate also induce conformational changes by hydrophobic interactions. These results are supported by viscosity measurements and agree well with those recently reported by Satoh, who considered changes in apparent molar volume. © 1994 John Wiley & Sons, Inc.     

Ferromagnetic alloys: A Cluster-Bethe-lattice approach

We apply the Cluster-Bethe-lattice method to calculate the local density of one-magnon states for a binary substitutional ferromagnetic alloy. Short range order effects for the alloy are included. Simple clusters of one atom and one atom plus its nearest neighbors are studied. We show that, for the latter type of cluster, there are localized excitations the energies of which fall outside the continuum of excitations.     

Nucleophilic addition to C,C?double bonds. Part IX. Kinetics and mechanism of the base-catalyzed intramolecular cyclization of olefinic alcohols

The base-catalyzed intramolecular cyclization of polycyclic olefinic alcohols of type a (10-endo-hydroxy-anti^9,10-tricyclo [4.2.1.1^2,5]dec-7-en-9-ones (type h ), anti^9,10-tricyclo[4.2.1.1^2,5] dec-3-en-9-endo-ols (type j ), and anti^10,11-tricyclo[4.3.1.1^2,5]undec-3-en-10-endo-ols (type 1 )) to the ethers d and f , resp., has been studied. A mechanism for the nucleophilic addition of the corresponding alkoxide anion b to the isolated C,C? double bond is discussed. It is proposed that b is formed (fast acid/base equilibrium) in the first step. For the subsequent reaction sequence, there are two well distinguishable pathways: (a) Compounds with an additional carbonyl group ( h ) cyclize via a homoenolate-like intermediate c , which is protonated stereoselectively on the exo-side by the hydroxylic solvent. (b) Compounds without a carbonyl group ( j and l ) cyclize 10²-10⁴ times slower, and the reaction proceeds via a carbanion-like transition state e . The proton transfer from the hydroxylic solvent is clearly coupled with the C,O? bond formation. Steric compression in the olefinic alcohols a influences the cyclization rate: (a) Alcohols with a smaller ring ( h , X = CH₂CH₂) cyclize 70–200 times faster than the ones with a larger ring ( 1 , X = CH₂CH₂CH₂). (b) Replacement of the H-atom at the carbinol C-atom by a CH₃ group enhances the rate of ether formation by a factor of 50–100. Due to through-bond interactions between the C,C-double bonds, olefinic alcohols with an additional endocyclic C,C-double bond ( h and j , X = CH?CH) cyclize 20–300 times faster than the corresponding monoolefinic ones ( h and j , X = CH₂CH₂).     

Carbon nanotubes as reinforcement of styrene-butadiene rubber

This study reports an easy technique to produce cured styrene-butadiene rubber (SBR)/multi-walled carbon nanotubes (MWCNT) composites with a sulphur/accelerator system at 150 °C. Significant improvement in Young's modulus and tensile strength were achieved by incorporating 0.66 wt% of filler without sacrificing SBR elastomer high elongation at break. A comparison with carbon black filled SBR was also made. Field emission scanning electron microscopy was used to investigate dispersion and fracture surfaces. Results indicated that the homogeneous dispersion of MWCNT throughout SBR matrix and strong interfacial adhesion between oxidized MWCNT and the matrix are responsible for the considerable enhancement of mechanical properties of the composite.     

Three-body interactions in sociophysics and their role in coalition forming

An study of the effects of three-body interactions in the process of coalition formation is presented. In particular, we modify a spin glass model of bimodal propensities and also a Potts model in order to include a particular three-body Hamiltonian that reproduces the main features of the required interactions. The model can be used to study conflicts, political struggles, political parties, social networks, wars and organizational structures. As an application, we analyze a simplified model of the Iraq war.     

Study on the Influence of Chirality in the Threading of Calix[6]arene Hosts with Dialkylammonium Axles

The influence of chirality in calixarene threading has been studied by exploiting the “superweak anion approach”. In particular, the formation of chiral pseudo[2]rotaxanes bearing a classical stereogenic center in their axle and/or wheel components has been considered. Two kind of pseudo[2]rotaxane stereoadducts, the “endo-chiral” and “exo-chiral” ones, having the stereogenic center of a cationic axle inside or outside, respectively, the calix-cavity of a chiral calixarene were preferentially formed with specifically designed chiral axles by a fine exploitation of the so-called “endo-alkyl rule” and a newly defined “endo-α-methyl-benzyl rule” (threading of a hexaalkoxycalix[6]arene with a directional (α-methyl-benzyl)benzylammonium axle occurs with an endo-α-methyl-benzyl preference). The obtained pseudorotaxanes were studied in solution by 1D and 2D NMR, and in the gas-phase by means of the enantiomer-labeled (EL) mass spectrometry method, by combining enantiopure hosts with pseudoracemates of one deuterated and one unlabeled chiral axle enantiomer. In both instances, there was not a clear enantiodiscrimination in the threading process with the studied host/guest systems. Possible rationales are given to explain the scarce reciprocal influence between the guest and host chiral centers.