NMR studies of polymer solutions. II. Polyethylene

The intramolecular structure of polyethylene in solution was studied by a high-resolution nuclear magnetic resonance technique. Highly purified n-alkanes (99.5%) from C₅H₁₂ to C₃₆H₇₄ were used as its oligomers. The NMR spectra of the polyethylenes (oligomers) are very sensitive to the solvents used. The internal methylene protons of all polyethylenes of various chain length resonance at an identical frequency in carbon tetrachloride. A sharp transition in the NMR spectrum of polyethylene in α-chloronaphthalene at 35°C. was observed at n-C₁₇H₃₆, above which there exist two distinguishable NMR peaks for internal methylene protons, and below which (fewer carbons) only a single peak was seen. The NMR spectra of the internal methylene protons of the polyethylenes (oligomers) taken in benzene are very similar to those taken in pyridine. They are not as easily resolved as those NMR spectra taken in α-chloronaphthalene solutions. The effect of the size of the aromatic solvent molecule on the NMR spectra of the internal methylene protons of the polyethylenes (oligomers) in solutions was demonstrated by using aromatic solvents of various sizes, such as chlorobenzene, α-chloronaphthalene, and 9-chloronathracene. The results indicate that the formation of polymeric structure of the internal methylene groups in the polyethylene chain is very sensitive to the size of the solvent used. The interaction of the solvent with the methylene groups of the polyethylenes varies as a function of chain length; it is stronger for those low member n-alkanes and decreases gradually to an asymptotic value.     

Comparative estimation of the efficiency of structure formation and dispersion interactions in organic solvents of varying nature and polarity

The solubilities of n-tetracosane and dotriacontane in more than 50 solvents (aliphatic and aromatic hydrocarbons, their halo derivatives, alcohols, ethers, ketones, amines, etc.) miscible with liquid aliphatic hydrocarbons were determined at 293±1 K. The increments of the methylene group in hypothetical extraction systems n-octane — solvent and the free energies of methylene group transfer from solvent to n-octane were calculated. The values were found to vary within wide limits. For the overwhelming majority of polar and low-polar solvents, the energies are negative due to self-association of solvents according to the type of spatial structure. Positive energies are due to the closer packing of some low-polar solvent molecules (carbon tetrachloride and cyclohexane) in the structure compared to octane and to the absence of molecular self-association. The closer packing of these molecules leads to higher efficiency of dispersion interactions between the solvents and paraffins compared to octane.     

Extraction of fullerene mixture from fullerene soot with organic solvents

Investigation of extraction of fullerene mixture from the fullerene soot obtained by plasma erosion of graphite rod in helium atmosphere with different solvents such as α-chloronaphthalene, o-dichlorobenzene, o-xylene, toluene, benzene, carbon tetrachloride, and n-hexane at 25°C was carried out. Completeness and effectiveness of extraction as well as relative content of light (C₆₀, C₇₀) and heavy (C₇₆, C₇₈, C₈₄) fullerenes in the extract were evaluated.     

Selective polymeric effects on solvent NMR relaxations

Measurements of the spin-lattice relaxation times (T₁) of solvent protons have been performed on systems containing mixed solvents with and without polymer. It has been found that the motion of solvent is selectively affected by polymers present in the system. Polyisobutylene (10%) in mixed solvents of carbon tetrachloride (or cyclohexane) and dichloromethane at various proportions produces little effect on T₁ values of dichloromethane, but it affects significantly the T₁ values of cyclohexane; whereas poly(methyl methacrylate) (10%) in carbon tetrachloride and dichloromethane (or acetone) selectively associates with dichloromethane (or acetone), resulting in an approximate 50% reduction of the T₁ values for dichloromethane (or acetone). In systems of poly(methyl methacrylate) and three mixed solvents of carbon tetrachloride, dichloromethane, and cyclohexane, the polymer (10%) has a negligible effect on the T₁ values of cyclohexane, but brings about a 50% reduction of the T₁ values of dichloromethane. These phenomena are discussed in terms of local selective interactions between the solvent molecules and the polymeric chain segments.     

Continuity of solvent quality in polymer solutions. Poor-solvent to Θ-solvent continuity in some polystyrene solutions

Critical temperatures for polystyrene/methyl acetate (PS/MA) and polystyrene/ethyl formate (PS/EF) solutions were measured at positive (PS/MA, PS/EF) and negative (PS/MA) pressure. The results confirm that solvent quality is sensitive to pressure; some solvents, designated Θ, at P_nominal ∼ 0, undergo a Θ-to-poor transition at negative pressure, and others, nominally designated “poor,” show a poor-to-Θ transition at positive pressure. Thus, any dichotomous division into sets of “poor” and “Θ” solvents is inaccurate, unless it accounts for the effects of pressure and other variables on solvent quality. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1251–1259, 1997     

Time Programmable Locking/Unlocking of the Calix[4]arene Scaffold by Means of Chemical Fuels

In this work, we report that 2-cyano-2-phenylpropanoic acid and its p-Cl, p-CH₃ and p-OCH₃ derivatives can be used as chemical fuels to control the geometry of the calix[4]arene scaffold in its cone conformation. It is shown that, under the action of the fuel, the cone calix[4]arene platform assumes a “locked” shape with two opposite aromatic rings strongly convergent and the other two strongly divergent (“pinched cone” conformation). Only when the fuel is exhausted, the cone calix[4]arene scaffold returns to its resting, “unlocked” shape. Remarkably, the duration of the “locked” state can be controlled at will by varying the fuel structure or amount. A kinetic study of the process shows that the consume of the fuel is catalyzed by the “unlocked” calixarene that behaves as an autocatalyst for its own production. A mechanism is proposed for the reaction of fuel consumption.     

Conformational behavior of styrene–p-chlorostyrene triblock copolymers in dilute solutions. I. Composition dependence of conformational behavior

Dilute solution properties of (styrene-p-chlorostyrene) triblock copolymers in various solvents were studied over a wide range of molecular weight and composition. Viscosity and osmotic pressure results indicate that the conformational behavior of the B_mA_nB_m and A_mB_nA_m copolymers (A = styrene; B = p-chlorostyrene; m and n denote the number of units) are similar in nonselective solvents such as toluene and 2-butanone, but different in selective solvents such as carbon tetrachloride and cumene. Short-range and long-range interaction parameters of the block copolymers were determined by applying the Stockmayer–Fixman method to viscosity data and also by application of the equation relating the osmotic virial coefficient and the excluded volume. The results show that the unperturbed dimensions of the block copolymers vary linearly with composition, and long-range interaction parameters in nonselective solvents can be expressed by those of the parent homopolymers, the chemical composition, and values of the interaction parameter β_AB between styrene and p-chlorostyrene monomeric units.     

Synthesis and characterization of soluble C60-chemically modified poly(p-bromostyrene)

Addition of bulky C₆₀ moiety, a powerful electron acceptor (EA = 2.6–2.8 eV), to the poly(p-bromostyrene)(PBS) by a novel organometallic reaction considerably changes the chemical and physical properties of this polymer. The product obtained is a “charm-bracelet” non-crosslinked brownish yellow polymer which is easily soluble in many common organic solvents, and has a single glass transition temperature [134.0°C vs. 83.2°C for poly(p-bromostyrene)], this being congruent with its chemical structure. Covalent attachment of C₆₀ to the brominated polystyrene backbone is confirmed by a variety of techniques such as UV-VIS, FT-IR, TGA, DSC, SEM, ESR, GPC, and ¹³C-NMR. The results show that both the stereo-electronic effect and the steric hindrance of C₆₀ have an important influence on the structure and physical properties of polymer. © 1996 John Wiley & Sons, Inc.     

Dilute solution properties of poly(α-methylstyrene–ethylene) “regular” sequence copolymers

“Regular” sequence copolymers having the structure {[? CH₂? C(CH₃)(C₆H₅)? ]_m(CH₂? CH₂)_n}_p with relatively small values of m and n were prepared by means of “living” polymerization techniques. The intrinsic viscosities of fractions of these copolymers were obtained in various solvents including a theta solvent. The molecular weights of these fractions were determined by the Archibald ultracentrifugal method. The results show that the intrinsic viscosity–molecular weight relations of the regular sequence copolymers are affected not only by the average composition of the copolymer, but also by the sequence length in the copolymer molecule. It is suggested that the effective conformation of a chain element in the copolymer is not always the same as that in the homopolymer.     

Structure elucidation with lanthanide induced shifts. 6—solvent effects on bound shifts and association constants

The NMR spectra of acetone, adamantanone and 1-adamantanecarbonitrile have been studied in the presence of Eu(fod)₃ in various solvents. A substantial solvent dependence is found for the association constants between shift reagent and substrate. The magnitude of the association constants is correlated with solvent polarity, and a tenfold decrease in K₁ is observed upon changing from carbon tetrachloride to the more polar dichloromethane. Only a very small solvent dependence is observed for the bound shifts. The relative bound shifts (and therefore the structures of the LS complexes) are found to be solvent independent. The small solvent dependence of the absolute magnitude of the bound shifts for the LS complexes is suggested to result from experimental errors.     

Synthesis and conformation of poly(L-azetidine-2-carboxylic acid-L-proline) and poly([L-proline]3-L-azetidine-2-carboxylic acid)

The synthesis, characterization, and conformational assessment of poL y(L -Aze-L -Pro) and poly[(L -Pro)₃-L -Aze] are reported. The polymers were prepared by using the pentachlorophenol active ester as the polymerizable tetrapeptide derivatives. The copolymer, poly(L -Aze-L -Pro), assumes a Form II helix in polar solvents, and is converted into a form I-like helix at a critical solvent composition of ethanol to trifluoroethanol. The CD spectrum of this Form I-like conformation of poly(L -Aze-L -Pro) is similar to that of poly(trans-5-isopropyl-L -proline), indicating that the rigid four-membered ring at the alternating position can lock in the structure by a mechanism similar to that of a bulky substituent at the trans-5-position of proline. The helix conformation of this copolymer was unfolded in a 0.2M CaCl₂ aqueous solution. In contrast to poly(L -Aze-L -Pro), the copolymer of poly[(L -Pro)₃-L -Aze] contains both cis and trans peptide bond geometry when dissolved in a 90:10 ETOH-H₂O mixture. The conversion of the mixed conformation of poly[(L -Pro)₃-L -Aze)] into a polyproline Form II-like structure occurred in highly polar solvent environments such as water.     

The influence of the solvent on the conformational energy differences due to the anomeric effect

The solvation free energy ΔG^sol of molecules exhibiting the anomeric effect is computed in an approach that considers a continuous distribution for the solvent. A partition of ΔG^sol into separately evaluated contributions confirms that changes in the energy of the systems due to changes in conformation of the solute are ruled by the electrostatic contribution. A comparison with the “exact” values indicates that the approximate expression for the electrostatic contribution to ΔG^sol are not accurate enough to permit a proper modeling of the solvent influence on the anomeric effect. The systems are composed of methanediol, methoxymethanol, dimethoxymethane, and 2-methoxytetrahydropyran in carbon tetrachloride, chloroform, acetone, and water. The calculations have been performed at the SCF level with the STO -3G and 4–31G basis sets.     

Temperature dependent chemical shift change in 4-substituted[2.2.2.2]paracyclophanes

Temperature dependent NMR spectra of 4-substituted tetrakis[2.2.2.2]paracyclophane (4°-PCP) were measured at low temperature. The high field shift at low temperature was remarkable for the protons meta and para to an electron withdrawing and/or bulky substituent. While the chemical shift change was very small for the aromatic protons of 4°-PCP bearing a not so bulky and electron donating substituent. From these observations it is concluded that the rotation or vibration of a benzene bearing an electron withdrawing and/or bulky substituent around C₂-C₃-C₆-C₉ axis was increasingly frozen as the temperature decreased to a more stable “lateral” (or half “lateral”, the intermediary state between “face” and “lateral”⁴) conformation, in an interesting contrast to the favored face conformation of unsubstituted 4°-PCP at low temperature as well as at room temperature.     

Polymerization initiated by an electron donor–acceptor complex. Part IV. Kinetic study of polymerization of methyl methacrylate initiated by the charge-transfer complex consisting of poly-2-vinylpyridine and liquid sulfur dioxide

A kinetic study has been made of the polymerization of methyl methacrylate (MMA) initiated by a charge-transfer complex of poly-2-vinylpyridine (electron donor) and liquid sulfur dioxide (acceptor) in the presence of carbon tetrachloride. It is concluded that the polymerization proceeds through free-radical intermediates, as with the pyridine-liquid sulfur dioxide complex system. The association constants K of acceptor and polymer electron donors which range widely in their molecular weight were determined spectrophotometrically, and it has been found that both K and overall rate of polymerization R_p of MMA decrease with increasing molecular weight of polymer donor; contrary to this, molecular weight of PMMA formed increases with increasing molecular weight of the polymer donor. Other kinetic behaviors was essentially the same as in the pyridine–liquid sulfur dioxide system, i.e., R_p is proportional to the square root of the concentration of the complex and to the 3/2-order of the monomer concentration; R_p is clearly sensitive to the carbon tetrachloride concentration at low concentration of carbon tetrachloride, but for a higher concentration it is practically independent of the carbon tetrachloride concentration. It has been deduced from a kinetic mechanism for the initiation that a primary radical may be produced from the reduction of carbon tetrachloride by an associated complex consisting of liquid sulfur dioxide–polymer donor and the monomer.     

Substituent and solvent effects on the 19F chemical shifts of 3- and 4-XCH2-substituted fluorobenzenes

Substituent effects on the ¹⁹F chemical shifts of a series of 3- and 4-XCH₂-substituted fluorobenzenes have been studied in five different solvents (dimethyl sulphoxide, acetone, chloroform, carbon tetrachloride and benzene). The results show that in both series substituent effects (other than inductive effects) contribute in at least some cases to the overall shifts, but that these showed little solvent dependence. The meta series showed a proportionately greater solvent dependence than the para series, and this is attributed to the greater importance of ‘through solvent’ direct field effects in the former.     

Quantum chemical modelling of the effect of proline residues on peptide conformation

Semiempirical AM1 calculations were performed for quantum chemically optimized minimum-energy conformations of L -alanine oligomers (A)_n at n = 7 and their derivatives containing one, two, or three proline residues at various positions along the peptide chain. The effect of proline residues on the peptide conformation was quantified in terms of the conformational “strain energy” and also analyzed in terms of the spatial compatibility of peptides. The defined “strain energy” corresponds to the transformation of the polyalanine peptide from its minimum conformation to the conformation corresponding to that of the proline-containing peptide. The results of calculations indicate that the “strain effect” of proline residues is additive at all locations along the peptide chain, except at the first and the second positions of its N-terminal part. Also, the regular α-helical polyalanine structure was most significantly altered by the presence of some specific motifs around the proline location in the peptide. This, in turn, has its implications in the prediction of the protein secondary structure, as well as in the design of peptide inhibitors and substrates for enzymes and receptors. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 391–396, 1998     

Regenerated protein fibers. II. Viscoelastic behavior of silk fibroin solutions

The dynamic viscoelastic behavior of a concentrated solution of silk fibroin dissolved in the “MU” solvent is measured. The dynamic viscosity η′ and dynamic elasticity G′ increase with increasing concentration of silk fibroin at constant frequency; however, the increasing frequency decreases η′ and G′ at a constant concentration of silk fibroin. When the mixing ratio of C₂H₅OH/H₂O in the “MU” solvent is increased at a constant concentration of LiBr·H₂O, η′ and G′ sharply increase at constant frequency. If the LiBr·H₂O concentration is varied in the “MU” solvents whose ratio of C₂H₅OH/H₂O is kept constant at 100 : 0, both η′ and G′ are greater for LiBr·H₂O concentrations of 50% by weight compared to concentrations of 40% by weight. The dependence of η′ on the temperature of the solution can be predicted by Andrade's viscosity equation. Spinnability improves when the SF concentration is increased. © John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1955–1959, 1997     

Charge transfer polymerization of 2-vinyl pyridine initiated by n-butyl amine and carbon tetrachloride

2-Vinyl pyridine (2-VP) can be initiated by a charge-transfer complex formed by the interaction of aliphatic amines such as n-butylamine (nBA) and carbon tetrachloride (CCl₄) in a solvent like NN-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). This article describes the polymerization of 2-VP by n-butylamine (nBA) in the presence of carbon tetrachloride in DMSO at 60°C. The rate of polymerization R_p increases rapidly with carbon tetrachloride (CCl₄) up to a concentration of 3.93 mol/L, but for a higher concentration it is almost independent of the carbon tetrachloride concentration; R_p is proportional to [nBA]^0.5 and [2-VP]^1.5 when [CCl₄]>[nBA]. The average rate constant k is 1.03 × 10^?5 L/mol s. When [CCl₄] < [nBA] the rate constant in terms of [2-VP] was 1.06 × 10^?5 s^?1 at 60°C and the overall rate constant was 1.035 × 10^?5 L/mol s at 60°C.     

High-resolution NMR spectra of the polysulfones of six monolefins

100-MHz NMR spectra are reported for solutions of the 1:1 copolymers of sulfur dioxide with hex-1-ene, cis- and trans-but-2-enes, cyclohexene, cyclopentene, and norbornene, including the deuterated polymers made from hex-1-ene-2-d₁, cis- and trans-but-2-ene-2,3-d₂, and cyclohexene-3,3,6,6-d₄. The resolution of the spectrum of poly-(hex-1-ene sulfone) was very poor in CCl₄ as solvent but good in polar solvents. The main-chain CH₂ protons are nonequivalent, and their chemical shifts show sensitivity to dyad structure; the α-CH₂ protons are also non-equivalent. The spectra of most of the other polymers are discussed in terms of possible preferential modes of addition as well as tacticity effects.     

NH2 stretching vibration absorption and association mechanism of methylamine in n-hexane and carbon tetrachloride

Hydrogen bonding of methylamine in n-hexane is studied by recording the NH₂ stretching vibration absorption as a function of temperature and concentration and comparing it to the absorption in carbon tetrachloride. A continuous or quasi-continuous shift of the symmetric NH₂ stretching vibration on association in hexane and a discontinuous shift by about the same total amount together with an intensity change between a monomeric and an associated band in carbon tetrachloride suggest different association mechanisms in the two solvents. These are related to the enhancement of the proton donor or acceptor ability of the amine resulting from complexation with the solvent in carbon tetrachloride and to a similar influence of the added amine molecules, corresponding to the concept of hydrogen bond cooperativity, in hexane. Differences of 3–5 kJ mol⁻¹ between the mean association energy of methylamine in hexane and of methylamine or its homologues in carbon tetrachloride can be attributed to the energy needed for disruption of the complexes with the chlorine solvent and therefore support the assumption of the complex formation. Further results concern the chain-length dependence of the amine association, the association degrees in the two solvents and the intensity behaviour of the symmetric vibration of the neat compound in i.r. and Raman spectra.