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.     

The radiation crosslinking and scission of ethylene-propylene copolymers studied by solid-state nuclear magnetic resonance

Methyl groups from chain scission and H-crosslinks have been identified by solid-state nuclear magnetic resonance in amorphous ethylene-propylene copolymers containing 23 and 36 mole % propylene after γ-irradiation to 10 MGy at 30°C. G (scission) and G (crosslink) values determined from the n.m.r. spectra and by extraction are in agreement, which suggests that the crosslinks are not clustered. This may differ from the situation in polyethylene where there is a substantial crystalline content. G(S). G(X) and the ratio G(S)/G(X) increase with increasing propylene content of the copolymers.     

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.     

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.     

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.     

Investigations on esterification reactions of starches in 1‐N‐butyl‐3‐methylimidazolium chloride and resulting substituent distribution

The ionic liquid (IL) 1‐N‐butyl‐3‐methylimidazolium chloride ([C₄mim]⁺Cl^?) was used as solvent for different esterification reactions of the biopolymer starch. Therefore, maize starches with varying content of amylose were used. Different carboxylic acid anhydrides were applied to esterify starch with a degree of substitution (DS) in the range of 0.7–3.0. For example, starch acetates with the mentioned DS are accessible within 30 min at a 105°C‐reaction temperature. The DS distribution of starch acetates synthesized in IL was compared with the common starch acetate synthesis of Mark and Mehltretter. Also, a consideration of starch acetates and cellulose acetates synthesized in [C₄mim]⁺Cl^? is given. The starch esters were characterized by means of Raman spectroscopy for qualitative‐ and nuclear magnetic resonance spectroscopy for quantitative determination of the functionalization pattern. Moreover, the molecular mass distribution was determined after saponification by means of GPC‐MALLS. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Wide-line H n.m.r. study of perdeuterated pyridine imbibed in a premium coal

The motion of perdeuterated pyridine imbibed in Argonne premium high-volatile coal (no. 601) has been studied by wide-line ²H n.m.r. spectroscopy. The coal sample had been saturated with perdeuterated pyridine for several days, then evacuated until the pyridine reached a concentration of 3.5%. The entrapped pyridine molecules undergo two tyes of rotation about the twofold symmetry axis: rapid diffusional rotation with a correlation time of 1 × 10⁻⁷ s; and 180 ° jumps with a residence time of 6 × 10⁻⁶ s. It is estimated that roughly half of the pyridine molecules execute each kind of motion. There is no evidence for a pyridine fraction that executes rapid isotropic rotation. The anisotropic nature of the rotation indicates that the interaction between pyridine and the coal structure is strong enough to prevent isotropic rotation.     

A study of conformation,configuration and phase structure of polyacrylonitrile and their mutual dependence by means of WAXS and 1H BL-n.m.r.

Polyacrylonitrile (PAN) samples have been investigated using wide-angle X-ray scattering (WAXS) and broad-line nuclear magnetic resonance (¹H BL-n.m.r.). The samples differed as far as their tacticity and amorphous and paracrystalline regions amounts are concerned. Structural changes that occur in the analysed PAN samples when they are annealed are discussed, and both structural similarities and differences are presented. An explanation is given to explain the improvement of order within paracrystalline regions in the polymer with an increased amount of isotactic sequences. An attempt is made at determining the dependence between configuration, conformation and phase structure of PAN.     

Studies of the stereospecific polymerization mechanism of propylene by a modified Ziegler-Natta catalyst based on 125 MHz 13C n.m.r. spectra

The mechanism of stereospecific polymerization of propylene (catalysed by a modified Ziegler-Natta system, TiCl₄/MgCl₂/C₆H₅COOC₂H₅/Al(C₂H₅)₃), has been analysed using the values of triad and pentad tacticity determined using 125 MHz ¹³C n.m.r. spectroscopy. The well-known single active-site models, such as the Bernoullian, first- and second-Markovian, and enantiomorphic models, were found to be inadequate to describe the observed tacticity. A two-sites model, in which at one site the stereospecific polymerization proceeds in obedience to the Bernoullian model and at the other proceeds under the control of enantiomorphic-site model, is proposed. It was found that the pentad tacticities of both soluble and insoluble (in boiling heptane) fractions of polypropylene agreed well with the two-sites model.     

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.     

Evaluation of a delayed coking process by H and C n.m.r. spectroscopy: 1. Material balances

The use of ¹H and ¹³C nuclear magnetic resonance (n.m.r.) spectroscopy to evaluate the performance of a delayed coking process is presented in a series of papers, of which this is the first. A meaningful analysis requires an accurate balance, which in turn requires analysis of the various types of atoms in the feed and in all products, including solid coke, as well as the amounts of each. Product yields were determined in a delayed coking pilot plant. Complete n.m.r. (¹H and ¹³C) characterization of feed and all delayed coking products, using a consistent classification, was combined with pilot plant data to obtain the total distribution of carbon and proton atoms in the delayed coking process. Changes in carbon and hydrogen distributions (products versus feedstock) reveal the nature of the thermal cracking reactions involved; the main change is a substantial increase in the number of aromatic and paraffinic carbons at the expense of naphthenic carbons.     

Positional distribution of ω3 Fatty acids in marine lipid triacylglycerols by high-resolution13C nuclear magnetic resonance spectroscopy

The positional distribution [α(1,3)-acyl and ß(2)-acyl] of ω3 fatty acids [18:4(n-3), 20:4(n-3), 20:5(n-3), 22:5(n-3) and 22:6(n-3)] in depot fat of Atlantic salmon (Salmo salar), harp seal oil and cod liver oil triacylglycerols has been examined by¹³C nuclear magnetic resonance (NMR) spectroscopy. The positional distribution data can be defined from the spectrum of the carbonyl (C1 carbon) and the methylene (C2 and glyceryl carbon) regions. In depot fat of Atlantic salmon and cod liver oil, docosahexaenoic acid (DHA) was concentrated in the ß-position of the triacylglycerides with 72.6 and 74.4%, respectively. Only 3.2% of DHA and 4.6% of eicosapentaenoic acid (EPA) were esterified to the ß-position of the triacylglycerides in harp seal oil. EPA is nearly randomly distributed in cod liver oil and muscle lipids of Atlantic salmon, with 37.8 and 39.7%, respectively, in the ß-position. In general, the¹³C NMR-derived data were in accordance with corresponding data reported in the literature obtained by conventional techniques.     

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.     

Evaluation of a delayed coking process by H and C n.m.r. spectroscopy: 2. Detailed interpretation of liquid n.m.r. spectra

The ¹H and ¹³C nuclear magnetic resonance (n.m.r.) spectra of a delayed coking feedstock and products, presented in a previous paper, are analysed here in detail by matching results from inspection of both nuclei. The conventional spectra of whole samples are used in combination with elemental analysis data, and substantial agreement is obtained. Differences are explored to yield valuable information. Interpretation of some band assignments is modified to account for new results. This approach results in a detailed quantitative estimation of a few key structures that contain the main functional groups that characterize these petroleum fractions.     

Adsorption and 13C N.m.r. studies of ethene,ethane and carbon monoxide on zn- and cd-exchanged A-zeolites

The adsorption of C₂H₄, C₂H₆ and CO has been studied on Zn²⁺-exchanged A-zeolites. Partial exchange with Zn²⁺ or Cd²⁺ increases the adsorption capacity for C₂H₄ and CO. Complete Zn²⁺ exchange leads to a decrease, indicating a complicated variation of cation positions with exchange level. N.m.r. measurements are consistent with adsorption of C₂H₄ as weak π-complexes on the divalent cations. Weak specific adsorption complexes are also demonstrated for CO.     

13C N.m.r. Spectra of ring-opened polymers of 1-methylbicyclo[2.2.1]hept-2-ene and their hydrogenated products

The ¹³C n.m.r. spectra of the title polymers have been completely assigned for a comprehensive range of cis contents (σ_c = 0–1) and head-tail bias (HT/(HH + TT) = 1 −∞). In the unsaturated polymers (II), cis HH dyads are never found, so that all-cis polymers are also all-Ht, but cis TT dyads are formed in cis-trans polymers, equal to the difference between trans HH and trans TT. The olefinic region consists of seven triplets; two each from cis HT and trans HT, and one each from cis TT, trans TT and trans HH. In the case of trans each triplet consists of a main centre peak due to ttt and ctc sequences and two wing peaks due to ttc and ctt sequences, as defined in the text. The upfield peaks also show cc/ct or tt/tc splittings as well as HH/HT or TT/TH splittings in most cases. In the saturated polymers C⁶ is sensitive to m/r ring dyad sequences. A corrected assignment is given for the hydrogenated polymer of norbornene.     

Molecular motion of polycarbonate in the vicinity of −60°C as revealed by broad-line n.m.r., magic angle spinning n.m.r. and conformational analysis

On the basis of correlation of data obtained by broad-line n.m.r. and magic angle spinning n.m.r. analysis as well as conformational analysis, conclusions have been drawn concerning the nature of molecular motion occurring in a polycarbonate chain in the vicinity of ?60°C. It is postulated that this motion is connected with conformational reorientation of bisphenol in unordered chains with a random conformation.     

Determination of Cis- and Trans-18:1 fatty acid isomers in hydrogenated vegetable oils by high-resolution carbon nuclear magnetic resonance

Carbon nuclear magnetic resonance (¹³C NMR) methods for determining the composition of cis-/trans- and positional isomers in hydrogenated vegetable oils were developed to reduce analytical time. By selecting appropriate olefinic carbon peaks and by measuring individual peak areas subsequent to the identification of isomeric peaks on the NMR spectrum, compositional results of the isomers coincided well with those obtained by conventional gas chromatography (GC). Therefore, it is highly beneficial to choose the ¹³C NMR method when analysis time is limited. Though the proposed ¹³C NMR method is promising, further development is needed. For the time being, combination with the traditional GC method is still encouraged for precise compositional analysis of cis-/trans- and positional isomers.