Crosslinking investigation of polybutadiene thermal degradation by carbon-13 nuclear magnetic resonance

Thermal degradation of polybutadiene (PBD) in anaerobic atmosphere at 250 °C had been studied by carbon-13 nuclear magnetic resonance spectroscopy (¹³C NMR) before complete crosslinking. In this investigation four types of low molecular weight PBD with different 1,2-vinyl isomer content had been chosen, then pure and mixed samples of PBD were heated in different time periods. ¹³C NMR spectra showed that two kinds of crosslinking mechanisms occur that both of them produce methyl groups. The first mechanism is a reaction between 1,2-vinyl isomers of two PBD chains, and the second one occurs between 1,2-vinyl isomer of one chain via methylene carbon of cis or trans isomer in another chain. Also ¹³C NMR results showed that the presence of 1,2-vinyl isomer in the PBD structure is necessary and without it none of the mentioned reactions will occur. Furthermore isomers sequence is another important parameter which affects crosslinking. Results show that cis or trans isomer which is not adjacent to 1,2-vinyl isomer does not take part in crosslinking reaction. Moreover such cis or trans isomer can take part in second mechanism of crosslinking that 1,2-vinyl isomer was attached from head to cis or trans isomer, thus in this arrangement of isomers second mechanism of crosslinking will become dominant rather than first mechanism of crosslinking.     

Einfluss der Orientierung von Substituenten auf die chemische Verschiebung von 13C. II. 13C-NMR.-spektroskopische Untersuchung von substituierten Spiro [cyclopropan-1′,9-fluorenen]

Effect of the Orientation of Substituents on the Chemical Shift of ¹³ II. ¹³C-NMR. Study of Substituted Spiro [cyclopropane-1′, 9-fluorenes] The ¹³C-NMR. spectra of spiro[cyclopropane-1′, 9-fluorenes] substituted at the cyclopropane ring have been assigned, largely with the aid of selective proton decoupling. The effects of the substituents upon the chemical shifts of the aromatic carbons are orientation dependent. the effects of cis-substituents on γ-carbon atoms are large and negative (inducing high field shifts), those of trans-substituents smaller and dependent on the electron donor/acceptor properties of the substituents. The effects on δ-carbon atoms in close proximity of the substituents are large and positive; they are interpreted in terms of electric field effects.     

13C NMR spectra of 1,3,4-thiadiazole/thiadiazoline isomeric pairs

The ¹³C NMR spectra of four pairs of 1,3,4-thiadiazole/thiadiazoline isomers have been recorded using broad band and off-resonance proton decoupling techniques and in the coupled mode. Differences in the chemical shifts of the heterocyclic ring and the carbonyl carbons allow identification of the isomer present. Their IR and UV spectra are in agreement with literature reports for azole/azoline analogues.     

Line broadening in the 13C NMR spectra of bulk polymers above Tg

The effects of magic-angle sample spinning (MAS), high-power decoupling, and resonance frequency on the ¹³C NMR linewidths of bulk polyisobutylene and bulk trans-polybutadiene are examined. The ¹³C linewidths increase with resonance frequency, are unaffected by high-power decoupling, and are reduced to different extents by MAS. The dominant contribution of the natural linewidth of the polyisobutylene lines is confirmed. The two carbons of trans-polybutadiene have approximately equal linewidths under all conditions, a result that eliminates residual chemical shift anisotropy as a major contributor to the linewidths. The large reduction of the trans-polybutadiene linewidths with MAS, coupled with the above result, suggests that microscopic variation of magnetic susceptibility is the major factor for this semicrystalline polymer. Cross-polarized ¹³C spectra of trans-polybutadiene were obtained with and without MAS. With MAS, resonances due to the crystalline and amorphous components were resolved. The principal components of the chemical shift tensor of the vinylene carbons were obtained from the spectrum without MAS.     

Singlet oxygenation of 1,2-poly(1,4-hexadiene)s

The microstructural changes that occur in cis and trans forms of 1,2-poly(1,4-hexadiene) during methylene blue-photosensitized oxidation were examined by infrared (IR) and ¹³C-NMR spec-troscopy. The singlet oxygenation of these polymers yielded the expected allylic hydroperoxides accompanied by double bond shifts to new vinyl and trans-vinylene double bonds. The photosensitized oxidation exhibited zero-order kinetics; the relative rates for the cis- and trans-1,2-poly(1,4-hexadiene)s were approximately 3.8:1.0.     

Structures of Addition Products of Acetylenedicarboxylic Acid Esters with Various Dinucleophiles. An application of C,H-spin-coupling constants

Heterocyclic compounds obtained by addition of acetylenedicarboxylic acid esters to thioureas, cyclic amidines and o-difunctionalized aromatic systems have been studied by ¹³C-NMR. In particular, C, H-spin-coupling constants over two and three bonds were used to differentiate between the various constitutional isomers and to establish the configuration of trisubstituted exocyclic C, C-double bonds. The configurational significance and diagnostic value of vicinal cis and trans C,H-spin coupling is again demonstrated in the present series.     

~(13)C-NMR研究聚丁二烯的序列结构

本文采用~13C-NMR方法研究了不同催化体系制备的聚丁二烯。经分析指出,聚丁二烯分子链的三种序列(顺-1,4-序列、反-1,4-序列和1,2-序列)和十七类脂碳碳核为~13C-NNR谱贡献了十六个谱峰。经数据的解析,得到一组计算聚丁二烯脂碳部分各谱峰化学位移的经验参数。在本文条件下,应用该组参数得到的计算值与实验值符合较好。本文确认了聚丁二烯反-1,4-序列第四峰(T_4)的存在。初步研究了聚丁二烯~13℃-NMR谱图的定量处理问题。     

Nylon 6 structure in solid state as studied by high-resolution 13C-NMR spectroscopy

High resolution ¹³C-NMR spectra of nylon 6 samples crystallized under various conditions and of a drawn sample were measured at room temperature by the cross polarization/magic angle spinning (CP/MAS) and pulse saturation transfer/magic angle spinning techniques. Additionally, ¹³C-NMR spectra of the drawn sample were measured at temperatures from 20 to 100°C by the CP/MAS technique and at 20 and 100°C by the low-power decoupling/magic angle spinning technique. The nylon 6 structure in the solid-state is discussed on the basis of these results. The solid-state ¹³C chemical shift data are used for reference in a study of conformation in solution.     

13C- and 1H-NMR. Assignments for colchicine derivatives

The ¹³C-NMR. spectra of a number of colchicine derivatives are given comprising examples of the normal series ( 4→10 ), iso series ( 11→16 ) and colchicine series ( 17 ), which were either reported in the literature or obtained by partial synthesis or degradation reactions. The ¹³C-NMR. assignments were made by comparisons with known compounds and selective single-frequency offresonance decoupling experiments. Selective proton decoupling experiments have also allowed assignments of the H —C(11) and H —C(12) protons of the iso and colchicine series.     

含硅香原料研究(Ⅵ)——顺(反)-1-取代-3-三甲硅基环己醇的合成、13CNMR谱及其香气评定

合成并分离得到7对1-取代-3-三甲硅基环己醇的顺、反异构体,它们的结构通过¹HNMR、¹³CNMR、MS谱及GC(测其纯度)测定,利用¹³CNMR确定了各对顺、反异构体的构型.评定了各化合物的香气,大部分化合物具有甜香或甜香与木香香气,其中(反)-1-正丁基-3-三甲硅基环己醇具有甜香和木香香气,香气透发,留香持久,质量上乘.     

13C-NMR spectra of fluorinated molecules using 19F-13C polarization transfer

¹³C-NMR spectra of fluorinated molecules are difficult to observe under conventional broad-band proton decoupling. The large coupling constants involved make polarization transfer between fluorine and carbon, using INEPT or DEPT experiments very effective while broad band fluorine decoupling collapses the multiplet pattern.     

Darstellung und Konformationsuntersuchungen von cis- und trans-Isomeren des 1,2,4,5-Tetramethyl-3,6-diphenoxy-3,6-dithioxo-1,2,4,5-tetraaza-3,6-diphosphacyclohexans

Synthesis and Conformational Analysis of Cis- and Trans-Isomers of 1,2,4,5-Tetramethyl-3,6-diphenoxy-3,6-dithioxo-1,2,4,5-tetraaza-3,6-diphosphacyclohexane A mixture of isomers of the title compound could be prepared in much better yields (up to ca. 25%) by the reaction of bis(1,2-dimethylhydrazido) thiophosphoric acid O-phenylester with phenoxythiophosphoryl dichloride in the presence of triethylamine and aluminum chloride as catalyst. Separation of cis- and trans-isomers was achieved by sorting of crystals or in low yields by fractional crystallization. ³¹P- and mainly ¹H-NMR investigations at different temperatures show, that in both isomers the saturated sixmembered ring has a twist-conformation in which sulfur and phenoxy substituents located cis or trans respectively adopt the isoclinal positions. The isomers reveal remarkably different barriers of activation for the interconversion of two enantiomeric twist forms (cis ΔG^# = 63 ± 1 kJ/mol; trans 47 ± 1 kJ/mol).     

Investigation of a resorcinol–formaldehyde resin by 13C-NMR spectroscopy and intrinsic viscosity measurement

Various molecular weight fractions and oligomers isolated from a novolac type resorcinol–formaldehyde (RF) wood adhesive resin were studied by ¹³C-NMR, gel permeation chromatography (GPC), and intrinsic viscosity measurements. The ¹³C-NMR results indicate that methylene groups occur mostly between C₄–C′₄ and C₂–C′₄ carbons and in minor amounts between C₂–C′₂ carbons of resorcinol rings. By suppressing the nuclear Overhauser effect (nOe) and using a long delay time for ¹³C-NMR measurements, reasonable methylene/aromatic carbon ratios were obtained from the aromatic C₁ carbon patterns that have resulted from different numbers of methylene groups bonded on the aromatic ring. This analysis results indicate that the RF resin has an appreciable amount of branch structures and compare favorably with the gel permeation chromatography and intrinsic viscosity measurement results. © 1993 John Wiley & Sons, Inc.     

CIS-Trans isomerism of a,2-bistrichlorosilylethylene

Summary The existence of cis and trans isomers of 1,2-bis-rrichlorosilylethylene was detected spectrographically. It was noted that the frequencies of the double bonds of the cis and trans isomers of 1,2-bistrichlorosilyethylene, are unusually far apart as compared with the frequencies found for the cis and trans isomers of other compounds.     

FT-NMR investigation of 13C?11B coupling

The ¹³C FT-NMR spectrum of sodium tetraphenylboron obtained with proton decoupling has a line-shape which suggests the operation of a magnetic-dipole relaxation mode. Coupling constants between ¹¹B and the ring carbons are reported.     

含硅香原料研究(VI)－顺(反)-1-取代-3-三甲硅基环己醇的合成、13C NMR谱及其香气评定

合成并分离得到７对１－取代－３－三甲硅基环己醇的顺、反异构体,它们的结构通过＾Ｈ ＮＭＲ、＾１３Ｃ ＮＭＲ、ＭＳ谱及ＧＣ（测其纯度）测定,利用＾１３Ｃ ＮＭＲ确定了各对顺、反异构体的构型。评定了各化合物的香气,大部化合的具有甜香或甜香与木香香气,其中（反）－１－正丁基－３－三甲硅基环己醇具有甜香和木香香气,香气透发,留香持久,质量上乘。     

Thermal rearrangement study of low molecular weight polybutadiene

Various polybutadienes (PBDs) of low molecular weight were heated below complete crosslinking at 250 °C under anaerobic nonpyrolytic conditions, and the structural changes were investigated. The predominant crosslinking reactions arise from the presence of 1,2-vinyl isomer and the most important one is intermolecular reaction accompanied with methyl group formation. The analysis showed that two crosslinking types as well as two types of methyl groups have been produced in which one was the result of 1,2-vinyl isomer of one chain crosslinked via methylene carbon of another chain of cis or trans isomer, and the second methyl group was the product of the reaction between 1,2-vinyl isomers of two PBD chains. Chain scission also occurred in two pathways due to the presence of 1,2-vinyl isomer, scission at two adjacent 1,2-vinyl isomer and scission at adjacent 1,2-vinyl with cis or trans isomer giving rise to methyl carbons.     

Syntheses of shuttlecock- and bowl-equipped phenylazopyridines and photomodulation of their coordination ability to Zn-porphyrin

Shuttlecock- and bowl-equipped 4-(phenylazo)pyridine derivatives, which bear substituents that allow the pyridine moiety to protrude in the trans form but hinder it in the cis form, have been designed and synthesized. These molecules show cis/trans photoisomerization despite the presence of bulky substituents. ¹H NMR titration with Zn-porphyrin showed that the trans isomers coordinate to Zn-porphyrin much stronger than the cis isomers.     

Stereochemistry and 13C NMR investigation of 9-halotetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones

It was shown that the halogen atom occupies the quasi-axial position in the predominant conformer of the 9-halo derivatives of tetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones. When R³ = Me, the conformational equilibrium is determined by the latter substituent which is always quasi-axial. The effects of the methyl group and the halogen atoms on the ¹³C chemical shifts (SCS values) were used for the identification of cis and trans isomers. Interesting non additivity of substituent effects was found in derivatives bearing quasi-axial substituents at C-6 and C-9: and this was caused by the ring flattening.     

Selective INEPT,a new technique for the 13C NMR assignment of methylene carbons with non-equivalent protons,and its application to the analysis of the 13C NMR spectrum of monensin

A new method based on the combination of selective proton decoupling and INEPT is explained in detail. This technique, named selective INEPT (SEL-INEPT), is a useful and sensitive method for the ¹³C NMR assignment of methylene carbons with non-equivalent protons. The ¹³C NMR spectrum of monensin has been analysed by the application of this technique.