Melamine–formaldehyde resins: Constitutional characterization by fourier transform 13C-NMR spectroscopy

The random chemical structures of melamine–formaldehyde resins, including methylated melamine–formaldehyde resins and urea–melamine formaldehyde resins, were investigated by ¹³C-NMR spectroscopy (Fourier transform). All the combined formaldehydes, methylol and methyl ether groups, methylene structures, and dimethylene ether structures were assigned. A ¹³C chemical shift of methylene carbon occurred by substitution of other constituents of the methylene group for a proton of the adjacent monosubstituted nitrogen atom, as shown in a ¹³C-NMR spectrum of urea–formaldehyde resins. It was found that the chemical shift of each corresponding carbon of both melamine resins and urea–melamine resins was almost superimposed with that of urea resins.     

Assignments of dissociation constants of primaquine by 13C-NMR spectroscopy

The first and second dissociation constants of primaquine have been determined by titration in acetonitrile-water mixtures and estimated in pure water by using linear regression analysis. The assignments of dissociation constants were unambiguously achieved by studying the ¹³C nmr spectral data obtained on the mono-, di-, and trihydrochloride salts.     

Characterization of phenol-terminated polystyrenes by means of 13C-NMR spectroscopy

In the presence of many alkylphenols, cationic polymerization of styrene by aluminium chloride leads to low-molecular-weight polystyrenes that contain end groups derived from the phenols. The fraction of phenolic end groups in the polymer is estimated by ultraviolet (UV) measurements to be 40–70% dependent on phenol and the reaction conditions. Phenol is incorporated into the polymer over the whole range of molecular weights (up to 7000). At high phenol concentrations, a significant proportion of the product consists of 1:1 and 1:2 phenol-styrene adducts. The position of attack of the growing carbenium ion on the phenol can be determined by ¹³C-NMR spectroscopy. With 2,6-dialkylphenols, such as 2,6-di-tert-butylphenol, the high field aromatic resonance near 119.7 ppm is shifted downfield by about 16 ppm when the phenol is incorporated into the polystyrene as an end group. This is interpreted as an exclusive attack of the growing carbenium ion on the 4-position of the phenol. With 2,4-dialkylphenols, such as 2-tert-butyl-4-methylphenol, a corresponding downfield shift shows that reaction occurs only at the 6-position. The preferred site of attack for phenols such as 2-alkylphenols which lack both ortho- and parasubstituents, is the 4-position. With such phenols attack at the 6-position is not excluded. Low-molecular-weight adducts contain 1-methylbenzyl end groups and ¹³C-spectra are consistent with their presence in the higher-molecular-weight polystyrenes.     

13C-NMR spectroscopy of polyamic acids and polyimides

¹³Carbon chemical shifts are reported for polyimide and polyamic acid prepared from 3-aminophenylacetylene, 1,3-bis(3-aminophenoxy) benzene, and 3,3′,4,4′-benzophenoetetracarboxylic dicanhydride. The shielding parameters are correlated with the structure. A spectral analysis of model compounds, monomers, and other oligomers is also included. This analysis resulted in an analytical method by ¹³C-NMR spectroscopy of determining the amic acid-imide ratio in the partly cured polyamic acid. The development of the method is discussed.     

Stereoregularity and polymerization mechanism of polymethacrylonitriles determined by 13C-NMR spectroscopy

Carbon-13 nuclear magnetic resonance (¹³C-NMR) spectra of polymethacrylonitriles prepared under various conditions were measured. In acetone solution, the α-methyl carbon absorptions were split into triads and partially into pentads, and the methylene carbon absorptions into tetrads. In trifluoroacetic acid solution, the α-methyl carbon absorptions were split into pentads and the cyano carbon absorptions into triads. The triad, tetrad, and pentad tacticities determined from ¹³C-NMR spectra were compared with dyad tacticities determined from proton NMR spectra. The stereoregularity of the polymers which are γ-ray-initiated in liquid phase at temperatures near melting point (?35.8°C) and in the solid state differs from that of the polymers radically initiated at ?20 to 80°C. The stereoregularity and the conversion suggest the existence of an ionic mechanism in the polymerization at low temperatures.     

High-resolution solid-state 13C-NMR spectroscopy of cellulose nitrates

Solid-state ¹³C-NMR spectroscopy has been used to investigate the structure of cellulose nitrates prepared from cotton linters. The solid-state technique has the advantage that the entire range of substitution can be studied, which is not possible at present by solution methods. The spectra change progressively with increasing degree of substitution (DOS = 3 for cellulose trinitrate), and can be used to quantify the extent of substitution at C6, C2, and C3. Progressive nitration leads to elimination of the original C6 resonances of native cellulose by DOS = 1.39. The first nitration of C6 occurs in the amorphous regions, and this is complete by DOS = 0.50. Substitution at C6 is accompanied by decrystallization, as indicated by the decrease in the resonance assigned to crystalline C4, which also disappears at DOS = 1.39. A new resonance assigned to C1 appears at DOS = 0.28 at 101 ppm; the original C1 resonance for cellulose declines steadily and disappears by DOS = 2.65. This change is assigned to nitration of C2, based on the published solution spectra. Finally, development of intensity at 82 ppm at DOS = 1.83 is assigned to the effect of nitration at C3. There is no indication of any rearrangement of the unsubstituted regions analogous to those that occur on Mercerization of native cellulose.     

The analysis of cured drying oils by swollen state 13C-NMR spectroscopy

Swollen state ¹¹C-NMR spectroscopy is used to obtain information on the structures of cured drying oils used in alkyd paint manufacture. Examination of the individual drying components within these oils suggests that the behaviours of linolenate and linoleate groups are quite different. Low molecular weight materials produced during curing can be examined and their structures compared with those of the bulk insoluble product. The effects of catalyst upon the drying process can also be studied for the more reactive drying oils. Cured oils can be identified but the characteristic features are often reduced to low level components.     

Structural characterization of vulcanized natural rubber by latex state 13C NMR spectroscopy

Structural characterization of vulcanized natural rubber was performed by high‐resolution latex‐state ¹³C NMR spectroscopy. The vulcanized natural rubber latex was prepared by vulcanization of high ammonia natural rubber latex with sulfur and sodium di‐n‐butyldithiocarbamate as vulcanizing agents. High resolution was attained for latex‐state ¹³C NMR spectroscopy even after vulcanization of the rubber latex, as is evident from no background in spectrum and narrow half width of signals, which were independent of vulcanization time. Small signals at 44 and 58 ppm in the carbon region were assigned by measurements of both distortionless enhancement by polarization transfer (DEPT) and attached proton test (APT) to secondary, tertiary, and quaternary carbons of crosslinking points. The assignment was proved by high‐resolution solution‐state NMR spectroscopy of vulcanized liquid cis‐1,4‐polyisoprene as a model, in which DEPT, APT, 2‐dimensional ¹H‐¹³C correlation (HETCOR), and 2‐dimensional heteronuclear multiple bond correlation (HMBC) measurements were applied. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1003–1009, 2007     

In vivo Investigation of Plant-Cell Metabolism by means of natural-abundance 13C-NMR spectroscopy

Based on the natural abundance of ¹³C, in vivo ¹³C-NMR was used for the first time to monitor the metabolism of sucrose and hydroquinone ( 1 ) in cell suspensions of the plant Rauwolfia serpentina (L.) BENTH . ex KURZ . Cells converted sucrose extracellularly into α-D - and β -D -glucose as well as into β -D -fructofuranose and β -D -fructopyranose, respectively. The sugar mixture was completely taken up by the cells after 4 days. Hydroquinone fed at that time resulted in optimum conversion into its β -D -glucoside arbutin ( 2 ) within 10 h. A further metabolite, the primeveroside ( 3 ) of hydroquinone, appeared as a trace compound after 10 h. The formation of this diglycoside can be increased by further addition of sucrose.     

High resolution 13C-NMR spectroscopy of sodium carboxy methyl cellulose

Results of NMR studies on intact sodium carboxy methyl cellulose (SCMC) are presented. Similar studies in the literature are all on partially depolymerized SCMC. The degree of substitution and the relative distribution of substituents at OH-2, OH-3, and OH-6 of anhydro D -glucose residue in intact SCMC were determined by high resolution ¹³C-NMR spectroscopy (125 MHz). It is observed that the degree of substitution at OH-6 is almost equal to that at OH-3. In two SCMC samples, which are widely different in molecular weight and degree of substitution (ds), the relative reactivity order of the hydroxyl groups was found to be OH-2 > OH-6 ? OH-3. The NMR assignments were based on calculated shifts of carbons of anhydroglucose moiety in an oligosaccharide due to substitution.     

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.     

Determination of microstructure and glass-transition temperature of acrylonitrile-methyl acrylate copolymers by 13C-NMR spectroscopy

Acrylonitrile-methyl acrylate (A/M) copolymers of different monomer compositions were prepared by bulk polymerization using free radical initiator (benzoyl peroxide). Copolymer compositions were determined by elemental analyses and comonomer reactivity ratios were determined by the nonlinear least squares errors-in-variables methods (EVM). Terminal and penultimate reactivity ratios have been calculated using the observed monomer triad sequence distribution determined from ¹³C{¹H}-NMR spectra. The triad sequence distribution was used to calculate diad concentrations, conditional probability parameters, number-average sequence lengths, and run number in the copolymers. The observed triad sequence concentrations determined from ¹³C{¹H}-NMR spectrum agreed well with those calculated from reactivity ratios. Glass transition temperatures (T_g) of various copolymers determined from DSC gave good agreement with those obtained from NMR. © 1992 John Wiley & Sons, Inc.     

Study of the microstructure of tetrahydrofuran-3,3 dimethyloxetane copolymers by 13C-NMR spectroscopy.

The microstructure of tetrahydrofuran (A)-3,3 dimethyloxetane (B) copolymers was studied by ¹³C-{¹H}-NMR spectroscopy. Only the methyl carbons corresponding to the 3,3 dimethyloxetane unit appear as a singlet, whereas the other carbons present a more complicated spectral pattern than it would be expected if ? effects were negligible. The assignment of the resonance signals allowed the determination of the values of the probabilities of the different triads, which were in good agreement with those obtained from the reactivity ratios.     

Stereoregularity of fractionally crystallized poly(propylene oxide) samples by 13C-NMR spectroscopy

Several stereoregular poly(propylene oxide) (PPO) samples were synthesized employing either trimethyl aluminum hydrolyzates or Pruitt–Baggett type catalysts. Dilute isooctane solutions of these samples were fractionally crystallized by stepwise cooling from 333 K down to 273 K. The fractions which dissolve at 273 K were noncrystallizable and were further fractionated by GFC. The isolated fractions were characterized by ¹³C-NMR, DSC, and viscometry techniques. Both structural (head-to-head and tail-to-tail linkages) and steric (syndiotactic dyads) irregularities were found in otherwise isotactic chains and their abundance apparently increased as the melting point and the chain length of the fractions decreased.     

The 13C-NMR solid state spectroscopy of various classes of coals

The ¹³C-NMR spectra of various classes of coal obtained in the solid state show two resonances, one of which is assigned to aromatic carbon and the other to aliphatic carbon. The resonances are very broad with the high field resonance centered at about 7 ppm below tetramethylsilane and a low field resonance centered at about 140 ppm below tetramethysilane. Based on our previous solid state ¹³C-NMR studies of graphite and diamond, the high field resonance is typical of a sp³ carbon whereas the low fields resonance is assigned to a sp² carbon whereas the low fields resonance is assigned to a sp² carbon. It is found that the antracitic coals have more aromatic (sp²) carbons than the bituminous, subbituminous and lignite coals. The analytical implications of this technique are briefly discussed.     

Characterization of propylene-ethylene block copolymers using solid state 13C-NMR spectroscopy

Commercial block copolymers of propylene with ethylene (PEBC) are multiphase systems comprising block and random copolymers as well as small amounts of homopolymers. At present, no satisfactory method exists for characterizing the “blocky” structure of these copolymers. This article aims to fulfill this need. Accordingly, the block and random copolymers of propylene with ethylene have been investigated using ¹³C CP/MAS NMR with high-power dipolar decoupling. Comparisons have been made between the spectra of block and random copolymers and it is shown possible to distinguish between them by means of an additional signal, appearing at 32.5δ, in block copolymers (attributable to block junctions). © 1992 John Wiley & Sons, Inc.     

Stereoregularity of poly(2-vinylpyridine) determined by 1H-NMR and 13C-NMR spectroscopy

In order to determine the stereoregularity of poly(2-vinylpyridine), 2-vinylpyridine-β,β-d₂ was synthesized. The ¹H-NMR spectra of the deuterated polymer in D₂SO₄ and o-dichlorobenzene solutions showed three peaks, which were assigned to triad tacticities. Since the absorptions of heterotactic and syndiotactic triads of methine protons overlap those of methylene protons in nondeuterated polymers, only isotactic triad intensities can be obtained from the ¹H-NMR spectra of nondeuterated poly(2-vinylpyridine). The ¹³C-NMR spectra of poly(2-vinylpyridine) were obtained in methanol and sulfuric acid solutions. In methanol solution the absorption was split into three groups, which cannot be explained by triads, and in sulfuric acid solution several peaks were observed. These splittings may be due to pentad tacticity. The results show that poly(2-vinylpyridine) obtained by radical polymerization is an atactic polymer.