High-resolution solid-state C nuclear magnetic resonance study of isotactic polypropylene polymorphs

High-resolution ¹³C nuclear magnetic resonance spectra are reported for three solid samples of isotactic polypropylene (i-PP): (i) the -crystalline form (monoclinic), (ii) the β-crystalline form (hexagonal), and (iii) the mesomorphic or smectic form of i-PP. These spectra were obtained using magic angle spinning, high-power proton dipolar-decoupling and cross-polarization techniques, and for the β- and smectic forms of i-PP constitute the first reported high-resolution solid-state spectra. The spectrum of the -crystalline form shows well resolved splittings (1 ppm) of the methyl and methylene carbon resonances, as was reported previously by Bunn et al. These splittings are absent in the spectra observed for the β- and smectic forms of i-PP. Based on a comparison of the chemical shifts and T₁ relaxation times observed for the carbon resonances in these three forms of solid i-PP, we reach the following conclusions: (i) i-PP adopts the same 3₁ helical conformation in each crystalline polymorph, (ii) the packing of i-PP helices in the β-form crystal is closer to the more distant of the two interhelical packings in the -crystalline form, and (iii) the local packings of 3₁ helices are very similar in the β- and smectic forms of i-PP.     

1H and 13C nuclear magnetic resonance studies of the synthesis of linear segmented polyurethane elastomers

Linear segmented polyurethane elastomers have been prepared by reaction of polyethyleneoxide with 4,4′-diphenylmethanediisocyanate and the product was subsequently cured with diamines. Chemical analysis of the prepolymer formed using stoichiometric ratios of polyol and diisocyanate indicated that the product was deficient in ‘free’ isocyanate. A number of side reactions can lead to a reduction in the amount of isocyanate available for the chain extension reaction. Infrared, ¹H- and ¹³C-n.m.r. spectroscopy were used to follow the initial end-capping and subsequent chain extension reactions. In order that an unambiguous assignment of the weak peaks observed in the prepolymer and final elastomer spectra could be made, a number of model compounds associated with the side reactions were synthesized and their spectra characterized. It was concluded that the loss of ‘free’ isocyanate in the formation of the prepolymer can be predominantly attributed to the effects of traces of water in the initial reaction mixture.     

Determination of substituent distribution in cellulose ethers by means of a C nuclear magnetic resonance study on their acetylated derivatives: 3. Hydroxyethylcellulose

The distribution of substituent, i.e. oligo(ethylene oxide) groups, in hydroxyethylcelluloses having various values of molar substitution was determined by means of ¹³C nuclear magnetic resonance after the acetylation of hydroxyl groups both at the unsubstituted position in the anhydroglucose unit and at the substituent end position of the parent hydroxyethylcellulose. The acetyl carbonyl signal of the acetylated hydroxyethylcellulose samples was found to split into a triplet in DMSO-d₆ at 100°C corresponding to the positions of acetyl groups (2, 3 or 6 overlapped with the substituent end) on the anhydroglucose unit. While in CDCl₃ at 40°C, the acetyl carbonyl signal for the substituent end position was observed to be separated from those on the anhydroglucose unit. By combining these results, the distribution of the substituent groups was determined for a series of hydroxyethylcellulose samples of a wide range of molar substitution values.     

C nuclear magnetic resonance spectral analysis of stereosequences in ethylene-propylene copolymer

A number of split peaks dependent on both comonomer sequences and stereosequences were observed in the ¹³C nuclear magnetic resonance (n.m.r.) spectrum of ethylene-propylene (E-P) copolymer. The ¹³C chemical shifts of methylene carbon in stereoisomers of the respective hexad comonomer sequences were predicted by a chemical-shift calculation using the gamma effect on ¹³C chemical shifts and Mark's rotational isomeric state model for E-P copolymer. Assignments of the split peaks that arise from different hexad stereosequences were given by comparison between the observed and calculated chemical shifts. Reference was made to the hexad assignments of comonomer-sequence-dependent peak splittings determined in our previous calculation of ¹³C n.m.r. chemical shifts of stereoregular E-P copolymers. The tacticities were estimated for successive (not separated by ethylene units) propylene units in the hexad sequences.     

Relationship between carbon aromaticities and H/C ratios for bituminous coals

Carbon aromaticities for a selection of 19 bituminous coals and vitrinite concentrates have been obtained by solid-state ¹³C n.m.r. using the inherently quantitative single pulse excitation technique. Within experimental error, the aromaticity values in the range 0.75–0.90 correlate extremely well with the H/C ratios (A² = 0.93); the 11 samples investigated with H/C ratios between 0.74 and 0.77 all have aromaticities in the range 0.75–0.81. Furthermore, the correlation holds for the coals investigated with high inertinite contents.     

Unsaturated polyester-styrene resins: Effect of the catalyst concentration on the kinetic rate and activation energy of the radical crosslinking reactions

The T₁ spin lattice relaxation time, measured by pulsed n.m.r., has been used to monitor the effect of the catalyst concentration on the kinetics of the radical crosslinking reaction of unsaturated polyester-styrene resins. The measurements were performed primarily in the intermediate-final part of the reaction under isothermal conditions. The loss of mobility of the protons of the sample has been correlated with the degree of cure and the kinetic constant as well as with the activation energy, and has been evaluated using the semiempirical kinetic expression proposed by Kamal et al. In the concentration range investigated (from 0.05 to 0.3% w/w) for the catalyst cobalt octanoate the decrease of the activation energy for the process was found to be 2.3 kcal mol^?1 of double bonds for each 0.1% w/w of cobalt octanoate.     

Studies on copolymers of acrylonitrile with resins of furfuryl alcohol: 1. Preparation and thermal properties

Copolymers of acrylonitrile with a furfuryl alcohol resin have been prepared by a free radical reaction, and have been characterized by n.m.r. and i.r. spectroscopy. Their t.g.a. has been performed in air and in a nitrogen atmosphere, to permit the recognition of a number of stages in the weight loss process. D.s.c., also in air and in nitrogen, has identified a number of exothermic and one endothermic process in the copolymers. Solid state cross-polarization/magic angle spinning ¹³C n.m.r. spectroscopy and i.r. spectroscopy has been used to obtain some insight into the chemistry of the various chemical changes, which in air are completed by combustion and in nitrogen leave a carbon.     

A spectroscopic investigation of the banding or lithotypes occurring in Victorian brown coal seams

Solid-state ¹³C n.m.r. and infrared spectroscopy were used to investigate the gross differences in chemical structural between the five primary lithotypes or coal bands present in Victorian brown coal seams. Examination of a series of coal lithotypes, along with their insoluble kerogen fractions, indicated significant structural variation between the light and dark samples, particularly in their aromaticities and levels of lignin-derived structures. Comparison of K₂₉₂₀ (cm mg⁻¹) and K₁₇₁₀ (cm mg⁻¹) infrared data revealed that the aliphatic C-H content did not exhibit the level of lithotype dependence shown by carbonyl/carboxyl concentration. Correlation with ¹³C n.m.r. data revealed a direct inverse relation between aromaticity and carbonyl/carboxyl content. Aromaticity and carbonyl/carboxyl content are suggested to be the main gross chemical structural properties affecting lithotype classification. Investigation of a depth profile of the light lithotype revealed that gross structural variation between the samples did not result from rank effects. Rather, the depth profile reflects minor depositional variations which did not affect their lithotype classification. The gross chemical characterization of the lithotypes supports paleobotanical interpretation regarding the different depositional environments and inputs responsible for the observed coal banding.     

Characterization of vitrinite concentrates. 2. Magic-angle 13C nuclear magnetic resonance studies

The fraction aromaticity determined by ¹³C n.m.r. with cross-polarization and magic-angle spinning of 19 vitrinite concentrates obtained from the Lower Kittaning seam shows a range of values from ≈ 0.65 for the samples of lowest rank (83 wt% C (dmmf) to about 0.83 for those of highest rank (91 wt%C (dmmf)). It was determined that the wt% aromatic carbon correlates to the wt% fixed carbon and is in good agreement with the results reported by other authors. The combination of the ¹³C n.m.r. results with FTIR measurements allows a number of coal parameters to be estimated. The atomic ratios of aliphatic hydrogen to carbon were demonstrated to vary from 1.8–2.0 to between 2.4–2.6 and previous assumptions that a single value can be used in calculating structural parameters for coal of any rank are not strictly valid. The calculation of the aromatic $\text{H}\text{C}$ ratio indicates that in mean structural units there is approximately one aromatic hydrogen atom for every six carbons in vitrinites of carbon content 83 wt%C (dmmf) and that this ratio changes progressively with rank to a value of about one aromatic hydrogen for every four carbons for vitrinites of carbon content 91 wt%C (dmmf).     

Nuclear magnetic resonance studies of selectively deuterated aromatic liquid crystalline copolyesters

Deuterium n.m.r. has been used to follow the molecular dynamics of three polyester materials over a temperature range of −150°C to 150°C. Two of the materials are liquid crystalline copolymers, one of perdeuterated hydroxybenzoic acid and hydroxynaphthoic acid and the other of hydroxybenzoic acid, isophthalic acid and perdeuterated hydroquinone. The third sample is crystalline poly(ethylene terephthalate) (PET) in which the benzene rings are deuterated. At the lowest temperatures examined all three materials give n.m.r. lineshapes characteristic of little molecular motion. On heating the liquid crystalline materials, the onset of motion is observed, first in the form of 180^o flips and then unrestricted rotation about the polymer axes. The PET spectra show a small degree of 180^o ring flipping to be taking place above 100°C, but most of the motion is in the form of near random motion that comes to dominate at 150°C. The results confirm the relatively stiff rod-like nature of the liquid crystalline polymers in comparison to the more flexible PET.     

Two-dimensional n.m.r. characterization of cycloaliphatic epoxy resins

The cycloaliphatic epoxy resins were studied by various two-dimensional nuclear magnetic resonance (2D n.m.r.) spectroscopies. The ¹³C n.m.r. spectra of the epoxy resins were assigned by using DEPT and 2D INADEQUATE techniques and the ¹H n.m.r. spectra were assigned by using 2D ¹H---¹H and ¹H---¹³C COSY techniques. Complete characterization of the samples synthesized by the oxidation of cyclohexene derivatives not only revealed the existence of the stereochemical isomers resulting from the structural difference of the oxide rings in the samples but also provided the composition ratio of the isomers.     

Monomer sequence determinations of aryl ether sulphone copolymers as determined by 13C n.m.r.

The monomer sequence distributions of three types of aryl ether sulphone copolymers have been determined by ¹³C n.m.r. spectroscopy. Two amorphous aryl ether sulphone copolymers were shown to have transetherified (or ether interchanged) during polymerisation to become random copolymers. The residues in these copolymers were (i)—OøOøSO₂øS—and (ii)—OøSO₂øøSO₂ø—-where ø denotes 1,4-phenylene. Copolymers of 50:50 aryl ether sulphone and aryl ether ketone made from different combinations of monomers were found to have different solubilities in dimethyl sulphoxide, and from ¹³C n.m.r. data this was shown to correlate with the amount of transetherification which had taken place during the polymerisations. The ¹³C n.m.r. chemical shifts for some of the carbon nuclei in these polymers were sensititive to functional groups situated even as far away as three or four aromatic nuclei along the polymer chain.     

Optimum conditions for high resolution C n.m.r. of coal- and oil-derived products

Factors affecting the spin-lattice relaxation times T₁ of ¹³C resonances at 62.9 MHz were investigated for simple mixtures of tetralin, ethylbenzene, pentane and phenol and the prominent resonances of an anthracene oil and a coal-derived hydrogenation oil. Parameters varied were solvent temperature and nature—Cr(acac)₃, Cr(dpm)₃, Cr(hfac)₃, Fe(acac)₃, Fe(dpm)₃—and concentration of a paramagnetic relaxation agent (PARR). Effects of PARRs on relaxation rates are strongly dependent on neighbouring acidic or basic centres, but are much more uniform for Cr(hfac)₃ than for Cr(acac)₃. The presence of basic or acidic compounds does not significantly affect the relaxation of ¹³C of the neutral components. Optimum experimental conditions are characterized by nearly uniform T₁ values of all resonances to be detected. While 2D spectra allow disentanglement of overlapping lines, choosing optimum conditions for the neutral constituents may lead to a loss of signals from the acidic or basic compounds.     

Structural analysis of Aleksinac oil shale kerogen by high-resolution solid-state C n.m.r. spectroscopy

A sample of kerogen from Aleksinac oil shale was examined by high-resolution solid-state ¹³C n.m.r. spectroscopy. The presence and relative proportions of kerogen structural units were estimated using a combination of NQS and T_1C methods with a peak-synthesis technique applied to the ¹³C CP—MAS spectrum. Relaxation parameters from these experiments were used to estimate differences in relative ‘mobility’ of various structural units. The kerogen was found to be highly aliphatic and to contain 79% long-chain aliphatic plus alicyclic structures, as well as 9% aromatic structures. These findings are in good agreement with the characterization of the same kerogen from its oxidation products.     

Nuclear magnetic resonance spectral analysis of polymer-supported reagents and catalysts

High resolution and cross-polarisation/magic angle spinning nuclear magnetic resonance spectroscopy of crosslinked polystyrene and of reagents and catalysts supported on crosslinked polystyrene and silica gel are reviewed. The materials studied include ionexchange resins, resins in solid phase peptide synthesis, organic synthetic reagents, phase transfer catalysts, transition metal catalysts, and liquid chromatography column packings. ¹³C and ³¹P n.m.r. spectra have been used to identify structures and to study macromolecule and micromolecule dynamics. New spectra of crosslinked polystyrenes and supported reagents are reported. Procedure to obtain qualitative and quantitative spectra are recommended.     

Aspects of the molecular structure of a bituminous vitrinite. Nuclear magnetic resonance study of the effects of imbibed pyridine

The effects of imbibed pyridine on the molecular structure of a high-volatile bituminous vitrinite have been studied at room temperature by proton nuclear magnetic resonance pulse techniques. Both the free induction decay and nominal solid echo signals, recorded before and after addition of fully-deuterated and non-deuterated pyridine, could be decomposed into two or three components corresponding to protons with different degrees of mobility. The results confirm that pyridine produces substantial mobilization of the bituminous coal structure, thereby indicating the role of polar interactions in determining the molecular conformation of the coal. Analysis of the data indicates that two mobile proton populations can be distinguished in the presence of deuterated pyridine. Evidence is provided for strong coupling between protons and free-radical unpaired electrons in the vitrinite and for a reduction in this interaction due to the mobilizing and swelling effects of imbibed pyridine.     

Hydrocarbon structural group analysis of Athabasca asphaltene and its g.p.c. fractions by C n.m.r.

Asphaltene from the Athabasca oil sand bitumen separated according to molecular weight by gel permeation chromatography was examined by high resolution solution-state ¹³C and ¹H and solid state ¹³C n.m.r. Integration of the inverse gated decoupled solution state and CP/MAS ¹³C n.m.r. spectra gave the aromaticity of each fraction as well as the relative number of carbon atoms responsible for well resolved signals in the aliphatic portion of the spectra. A two-pulse spin-echo ¹³C n.m.r. sequence in the solution state, and dipolar dephasing in the solid state, permitted the assignment of each aliphatic carbon signal to a methyl, methylene, or methine carbon; no quaternary aliphatic carbons were observed. These signals show that the aromatic core of the asphaltene is surrounded by alkyl chains with a mean length of 7.7 for the lowest molecular weight fraction (MW 1200), increasing to 12 for the highest MW fraction (MW 16900). Aromaticity of the fractions obtained from solution spectra ranged from 34 to 48%, the highest aromaticity being associated with the lowest molecular weight asphaltenes. In the solid state, the aromaticity ranged from 31 to 42 %. The degree of branching ranged from 0.5 per chain for the low MW asphaltene to 1.0 for the high MW fraction. The highest naphthenic carbon content was found in the high MW fractions; the 1200 MW fraction appeared to be nearly devoid of such carbon, this fraction having a much higher aromaticity than the second-lowest MW fraction.     

A chemical demonstration of the randomness of epoxidation of natural rubber

During the development of chemical methods for determining the epoxide content of epoxidised natural rubber (ENR) two methods were found to give results which were identical but which showed a progressive divergence from values obtained by absolute instrumental methods as the level of epoxidation increased. Statistical analysis of the sequence distribution of the epoxide groups, based on a random epoxidation, showed a good correlation between the value obtained by chemical analysis and the number of epoxide blocks rather than the number of epoxide groups. Two further instrumental methods for measuring the total epoxide content, using infra red spectroscopy and differential scanning calorimetry, were developed. Analytical procedures and calibration data are supplied. A combination of one chemical and one instrumental method can thus provide the absolute level of epoxidation and indicate whether the reaction has proceeded randomly or not, a point of crucial importance in the large scale manufacture of this material.     

A 13C-n.m.r. study of radiation-induced racemization in isotactic polypropylene

Unexpectedly large changes in the stereochemistry of isotactic polypropylene have been observed by ¹³C-n.m.r. following γ-irradiation at 25°C up to 250 kGy. The observed G-value of 64 for the loss of isotactic triads is much higher than previously reported G-values for scission, crosslinking or hydrogen formation in irradiated polypropylene. A mechanism of initial chain scission, racemization and subsequent recombination is indicated. The pattern of pentad sequences is inconsistent with the occurrence of isolated random racemization events; this suggests that the energy released on recombination promotes a second racemization within a range of 1–3 monomer units.     

Compositional and configurational sequence determination of methacrylonitrile–vinylidene chloride copolymers by nuclear magnetic resonance spectroscopy

Methacrylonitrile–vinylidene chloride (M/V) copolymers of different composition were prepared by bulk polymerization using benzoyl peroxide as an initiator under nitrogen atmosphere in a sealed tube. The copolymer composition was determined from quantitative ¹³C[¹H] NMR spectra. The reactivity ratios for M/V copolymers obtained from a linear Kelen–Tudos method and nonlinear error‐in‐variables method are r_M = 2.47 ± 0.14, r_V = 0.40 ± 0.02, and r_M = 2.43, r_V = 0.39, respectively. The complete spectral assignment in term of compositional and conformational sequences of these copolymers were done with the help of distortionless enhancement by polarization transfer, two‐dimensional heteronuclear single‐quantum coherence spectroscopy. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1865–1874, 2005