D. Doskočilová

Conformational structure of poly(ethylene terephthalate). Infra-red, Raman and n.m.r. spectra

Abstract Infra-red and Raman spectra of unoriented samples of poly(ethylene terephthalate) in the crystalline and amorphous states and in solution were measured. The dissolved polymer was also studied by n.m.r. spectra. Digital separation of the vibrational spectra of amorphous and crystalline components made possible the detection of some new bands, the interpretation of which is presented. By comparison with the vibrational spectra of model compounds combined with analysis of n.m.r. spectra, the conformational forms present in the amorphous and liquid states of poly(ethylene terephthalate) were characterized and their populations were determined. Bands in vibrational spectra which are characteristic of various conformational forms were defined.

Study of thermal degradation of polybutadiene in inert atmosphere: 1. Evidence of temperature and time of heating in i.r. and n.m.r. spectra

Abstract The degradation of polybutadiene by heating in inert atmosphere at various temperatures for different time intervals was characterized by i.r. and 1H n.m.r. methods. It was established that by i.r. analysis it is possible to assess both the temperature and the duration of the heating to which the sample had been subjected.

Motional restrictions and chain conformation in various swollen crosslinked polystyrene gels from 1H n.m.r. line-shape analysis

Abstract 1 H high resolution (HR), broad-line (BL) and magic-angle rotation n.m.r. spectra of polystyrene crosslinked with 2–10 mol% divinylbenzene or ethylene dimethacrylate, swollen to equilibrium in CCI 4 , CDCI 3 and benzene, were measured. The spectra were analysed as a folding of a narrow line-shape function approximated by the MAR n.m.r. spectrum, and an orientation-dependent dipolar broadening function. A molecular interpretation of the parameters obtained is presented. From MAR n.m.r. spectra, the rate of conformational transition depends on solvent, temperature and degree of crosslinking, but it remains very rapid even at 10% crosslinking. The second moment of the dipolar broadening function depends on the degree of crosslinking and is almost independent of temperature and solvent. The linear dependence of the dipolar broadening parameter on degree of crosslinking indicates that, in gels swollen to equilibrium, the statistical-average conformation of inter-crosslink chains is independent of their length, and is equal to that of linear polystyrene chains in dilute solution.

Study of thermal degradation of polybutadiene in inert atmosphere: 2. Characterization of thermal crosslinking in polybutadiene by high resolution solid state 13C and 1H magic angle spinning n.m.r. spectroscopy

Abstract The chemical structure of polybutadiene thermally crosslinked at temperatures up to 250°C was characterized by high resolution solid state 13C n.m.r. spectra in combination with high resolution 1H and 13C magic angle spinning n.m.r. spectra of the swollen gel. Crosslinking is shown to involve the reaction of a vinyl group with the methylene group in 1,4-cis 1,2 sequences.

Dynamic structure of swollen crosslinked poly(ethylene oxide) gels from n.m.r. line-shape analysis and magic angle rotation n.m.r.

Abstract Chemically crosslinked poly(ethylene oxide) gels swollen to equilibrium in CDCl 3 (22.3 w/w% of polymer) and CCl 4 (77.5 w/w% of polymer) were characterized by combination of the results of 1 H n.m.r. spectra measured with magic angle rotation ( MAR -n.m.r.) and numerical analysis of static 1 H n.m.r. line shapes. From MAR -n.m.r., activation energies of rapid segmental motions were found to be 1.7 ± 0.3 kcal mol −1 in CDCl 3 and 9.4 ± 0.6 kcal mol −1 in CCl 4 . Static line-shape analysis yields a parameter which characterizes the deviation of segmental motions from isotropy in space. This parameter was found to be independent of temperature and equal in both systems.

Conformational structure of N-ethylacetamide and N-isopropylacetamide. NMR shift reagent study

Abstract The structure of the stable conformers of N-ethylacetamide and N-isopropylacetamide was determined by NMR spectroscopy with lanthanide shift reagents. N-ethylacetamide was found to exist in the form of two mirror-image isomers, with the ethyl group twisted out of the plane of the amide bond by the angle ψ XXX ± 105°. Similarly N-isopropylacetamide exists in the form of two mirror-image isomers, with the isopropyl methyl groups in positions with ψ1 XXX ± 90° and ψ2 XXX ∓ 150°.

Conformational study of methylethers of the type CH3OCH2R: an NMR and vibrational spectroscopic study

Abstract Analysis of 13C NMR spectra of methylethers of the type CH3OCH2R, with R = CH3 (I), CH2CH3 (II), CH(CH3)2 (III) and C(CH3)3 (IV) has shown that the trans form strongly predominates on the OCH, bond, the gauche forms appearing only in those cases where close contact between methyl groups can be avoided. Analysis of both 1H and 13C NMR spectra indicates that all staggered forms are populated on the CH2R bond. In vibrational spectra, the strong Raman bands near 800 cm−1 were found to be very sensitive to conformational structure. For I, III and IV, the bands of various conformers in this range are separated, permitting ΔH to be determined.

NMR study of cis3̄trans isomerism in N-methylated lactams with 11- and 13-membered rings

Abstract From 1H-NMR spectra of 1-methyl-azacyclo-undeca-2-one and 1-methyl-azacyclo-trideca-2-one, the bands corresponding to the cis and trans forms have been assigned and analyzed; based on this analysis, conformational structures about the C-C bond next to nitrogen are proposed. By analysis of the relative areas and shapes of the N-methyl bands measured for the two lactams in 1,1,2,2-tetrachloroethane-d2, over a broad temperature range, the equilibrium and thermodynamic parameters characterizing the cis—trans isomerism of the amide bond in these lactams have been determined. Peaks corresponding to the cis and trans forms in the 13C-NMR spectra of these lactams have also been assigned.

Structure of amorphous polyethylene from n.m.r. line shape analysis and MAR-n.m.r.

Abstract Based on results of proton nuclear magnetic resonance measurements with magic angle rotation (MAR n.m.r.), the line of amorphous polyethylene in conventionally measured proton n.m.r. spectra is described as a convolution of a basic narrow line-shape function S(v) with an orientation-dependent dipolar broadening function G(v). With this approach it is possible to describle to describe the broadline n.m.r. spectrum of polyethylene as a superposition of the crystalline component and of a single amorphous phase. The fit of experimental and computed spectra, and the parameters obtained by this type of line-shape analysis are discussed for a series of polyethylene samples of different crystallinity.

Conformational structure of N-methylpropionamide and N-methylisobutyroamede. NMR shift reagent and IR study

Abstract The structures of the stable conformers of N-methyjpropionamide and N-methyliso-butyroamide in CCl4, solution were determined by a combination of IR spectroscopy and NMR spectroscopy with lanthanide shift reagents. N-methyl-propionamide was found to exist in the form of two rotational isomers, 1 and 2, with the ethyl group twisted out of the plane of the amide bond by the angles Ψ = 140 and 20°, respectively. For these two conformers, the enthalpy difference is ΔH = 2.13 ± 0.08 kcal mole−1 and the entropy difference ΔS = 7.81 ± 0.55 cal mole−1 grad−1. N-methylisobutyroamide exists in a single form, with the two C-methyl group positions very close to those found in the two isomers of N-methylpropionamide.