Précisions apportées au mécanisme de la polymérisation du chlorure de vinyle par le chlorure de tertiobutyle-magnésium en milieu tétrahydrofurannique. Etude par résonance magnétique nucléaire

NMR analysis of the low molecular weight poly(vinyl chloride) obtained from polymerizations initiated by t-BuMgCl in THF solution suggests that all the polymer chains have the structure: Enhanced signal/noise ratio by NMR spectrum accumulation confirms this structure in products of higher molecular weights. Examination of the NMR spectrum of vinyl chloride monomer in THF solution reveals the formation of a monomer–THF complex; it seems that the initiation step of the polymerization is governed by this complexation phenomenon.     

Détermination de constantes de stabilité de chélates par résonance magnétique nucléaire du proton

In a solution containing an excess of two metal ions in regard to a ligand (e.g. NTA) a competitive equilibrium is established. The measure, by proton NMR. spectroscopy, of the individual concentrations of both chelates, and eventually of the free ligand, allows to determine the ratio of the stability constants of the two complexes. As an illustration the ratios of the stability constant of the 1:1 nitrilotriacetate chelates of Pb and Zn, respectively Mo and W, were determined.     

Résonance magnétique nucléaire du noyau 14N par Transformée de Fourier

¹⁴N-NMR. measurements using the Fourier Transform technique are presented and compared to C. W. experiments. The F. T. technique is especially advantageous for relaxation time measurements. However, since ¹⁴N-nuclei may have widely different relaxation times, it is often not possible to have optimal gain in sensitivity (as compared to C. W. measurements) in a single F. T. experiment for different ¹⁴N-nuclei contained in the same sample. Different experiments have to be performed, optimizing the spectrometer parameters for each ¹⁴N-resonance of appreciably different linewidth. The technique is applied to three different problems. ¹⁴N-relaxation in symmetrical ammonium salts is shown to arise from reorientation of the water dipoles. The use of a double spin-probe, ¹³C-¹⁴N, allows the determination of the electric field gradients in cyclic ammonium salts. The electronic distribution is especially distorted from tetrahedral symmetry in the highly strained N, N-dimethyl-aziridinium cation. Finally, electric and dynamic effects in ion pairing may be studied as shown on the case of tetrabutylammonium iodide in water and in benzene.     

Analyse structurale des méthyl et diméthyl cyclohexanols par chromatographie gaz liquid et résonance magnétique nucléaire du C

Structural analysis of methyl and dimethyl cyclohexanols using gas-lquid chromatography and C¹³ nuclear magnetic resonance

Kováts' retention indices and C¹³ chemical shifts of all twenty dimethyl cyclohexanols have been measured and assigned. It has been found that there are good linear relationships between carbon chemical shifts and the Kováts' indices.     

Étude de l'effet de solvant en résonance magnétique nucléaire—I Caféine et dérivés

The NMR spectrum of caffein shows three N---CH₃ peaks. By study of solvent effect and pH effect on this spectrum, it was possible to make a correct attribution of the absorption peaks. A proof of this attribution was searched in the examination of NMR spectra of theophylline and theobromine. Two new chlorocaffeins were prepared during this work.     

Composés d'addition des halogénures de niobium (V) et de tantale(V): III. Détermination de constantes de stabilité par résonance magnétique nucléaire

In view of a systematic study of the Lewis acidity of niobium (V) and tantalum (V) halides, NMR. methods for the determination of stability constants are discussed. In the case of rapid exchange chemical shifts are used to determine the relative amounts of adduct(s) and free base(s) in equilibrium. In the case of slow exchange the relative concentrations of the species are given by the peak areas.     

Nouvelle méthode de détermination des taux de réativité des copolymères par résonance magnétique nucléaire: Application aux poly(méthacrylate de méthyle–chlorure de vinylidène)

Constant-composition copolymers of methyl methacrylate and vinylidene chloride produced by radical copolymerization are studied by ¹H-NMR at 60 and 250 MHz. The different methods of the literature for the derivation of reactivity ratios from either the copolymer composition or the sequence average lengths, or even the diad distribution, are applied but lead to rather dispersed results. A new graphical method is proposed, based on the use of peculiar values of the triad distribution functions. It allows us to detect a penultimate effect for the vinylidene chloride-rich region. In the same range, a change in tacticity of the diads and triads on the methylmethacrylate sequences, as compared with homopolymers, is observed; it suggests that the anomaly is caused by the competition of the depropagation reaction.     

Etude sur les composés organométalliques. XII [1]. Etude de composés tétraorganiques du titane par spectrométrie de résonance magnétique nucléaire

Tetrabenzyltitanium, tetraphenyltitanium, and tetramethyltitanium were studied by using NMR. spectroscopy. The spectrum of tetrabenzyltitanium depends on the procedure of its preparation. An interpretation of this finding is proposed.     

La résonance magnétique du 77Se dans des composés organiques

⁷⁷Se n.m.r. spectra of twenty-six organoselenium compounds, including some with an endocyclic selenium atom, have been recorded. Our measurements complete the scale of chemical shifts previously established by McFarlane. It seems difficult to define a range of chemical shifts which is characteristic of the valency state of selenium or of its endo- or exocyclic position. Some results are discussed qualitatively. In the case of selenolo[2,3-b]thiophene, evidence has been obtained for a ⁴J[Se, H] coupling across the sulphur atom.     

Polymerisation du methacrylate de methyle par le chlorure de tertio-butylemagnesium en solution dans le tetrahydrofuranne—I Etude de l'equilibre de dissociation du chlorure de tertio-butyle-magnesium

The thermodynamics of the dissociation of tBuMgCl have been carefully studied by PMR taking into account the exact concentrations of the various components. The Schlenk equilibrium should mainly be governed by the formation of different complexes for which degrees of solvation depend on the Grignard concentration. Owing to the estimated values of ΔH and ΔS, at low temperature in THF. MgCl₂ precipitates and only the symmetrical form tBu₂Mg initiates the polymerization of methylmethacrylate.     

Étude des Paramètres de Résonance Magnétique Nucléaire et de la Stéréochimie de Thiono-2 R-2 Dithiaphosphorinanes-1,3,2

The ¹H, ¹³C and ³¹P n.m.r. spectra of eleven 2-R-2-thiono-1,3,2-dithiaphosphorinanes with various R groups attached to the phosphorus atom have been recorded and analysed. In the compounds where R=Cl, OMe or N, the molecule adopts a chair conformation with the R group axially oriented. When the molecule adopts a chair conformation with equatorially oriented R. In the case of the derivatives with R=phenyl, Me, N(Me)₂ or NHt-Bu, a chair–chair conformational equilibrium was observed. The hindered rotation about the P? N bond was also examined in the derivative with      

Etude par Résonance Magnétique Nucléaire de dérivés anthracéniques: II—Anisotropie magnétique du phényle et effets steriques

The peri effect induced by the phenyl group has been studied in the anthracene series by means of ¹H and ¹³C n.m.r. The chemical shifts of overcrowded protons can be explained by a combination of magnetic anisotropy and steric effects. Steric contributions amount to c. 25% of the phenyl induced shift at the peri position. Amongst published ring-current theories, only the model of Johnson and Bovey is capable of describing correctly the shielding region of the phenyl group. The unexpected shieldings and deshieldings, observed by ¹³C n.m.r. in the case of very hindered derivatives, is probably due to distortions of the anthracene skeleton.     

Etude par Résonance Magnétique Nucléaire du carbone 13 de complexes du cobalt (III) et de la diméthylglyoxime

From a carbon magnetic resonance study of several alkylcobaloximes RCo(DMG)₂B (DMG = dimethylglyoximate monoanion), it was possible to estimate the α, β and γ effects of the Co(DMG)₂B group on the chemical shifts of the carbon atoms of various alkyl groups R. The chemical shifts of the carbon atoms belonging to the equatorial ligands and to the axial base B are not significantly affected by structural modification of the R groups. Values of δ in benzylcobaloximes XC₆H₄CH₂Co(DMG)₂B agree with a donor effect of the ? CH₂Co(DMG)₂B radical. Values of ¹J(¹³C? H) coupling constants, measured in ¹³C enriched methylcobaloximes, do not vary appreciably when B is changed (J(¹³C? H) = 137 ± 1 Hz) and are close to the value obtained for methylcobalamine.     

Reaction de redistribution entre le tripropylbore et ses produits d'oxydation: Etude par résonance du Noyau 11B

A study by B¹¹ NMR of the system tripropylborane–boron trioxide shows that scrambling reactions occur between the alkylborane and its oxidation products; this involves a redistribution of the mono-(? C₃H₇) and difunctional (? O? ) substituents attached to boron, leading to molecules of varying size and structure.     

Résonance magnétique nucléaire de 13C et 17O d'éthers aliphatiques. Effets γ entre les atomes d'oxygène et de carbone

The ¹³C- and ¹⁷O-chemical shifts of 31 aliphatic ethers are measured and discussed. The ¹⁷O-chemical shifts of the ethers ROR′ correlate with chemical shifts for the methylene groups of the corresponding alkanes RCH₂R′. The constant of proportionality can be related to the orbital expansion term 〈r^?3〉_2p. The δ_c for carbon atoms can also be correlated with δ_c for the corresponding alkanes. The origin of the correlation is discussed taking into account the conformational modifications resulting from introduction of an oxygen atom in an alkyl chain.     

Polymerisation du methacrylate de methyle par le chlorure de tertio-butylemagnesium en solution dans le tetrahydrofuranne—II Comportement de la polymerisation en fonction de la temperature et de la composition du magnesien

The polymerization of methylmethacrylate initiated by tert butyl-magnesium chloride in tetrahydrofuran has been studied in terms of yield and balance-sheet. It has been shown that only the symmetrical form of the Grignard reagent is efficient in initiating the polymerization. Initiation, propagation and termination mechanisms have been proposed.     

Polymerisation du methacrylate de methyle par le chlorure de tertio-butylemagnesium en solution dans le tetrahydrofuranne—III: Parametres thermodynamiques des propagations iso- et syndiotactique en fonction de la composition du magnesien

The thermodynamics of the polymerization of methylmethacrylate have been carefully studied for various MgCl₂/tBu₂Mg ratios. Assumptions have been made on the stereospecificity of the anionic propagation of MMA. The complex behaviour of the propagation mechanisms may result from possible (donor-acceptor) equilibria between monomer and ether on the one hand, and different magnesium compounds on the other.     

Spectres RMN haute resolution des homopolymeres et copolymeres de l'acrylate de glycidyle avec le chlorure de vinyle

The HR-NMR spectra of the glycidyl acrylate homopolymer and (glycidyl acrylate-vinyl chloride) copolymers are described. The assignment of the different resonance groups allows quantitative determination of the compositions of the copolymers.