Silylation of poly(vinyl alcohol) with hexamethyldisilazane in liquid ammonia

Silylation of poly(vinyl alcohol) has been performed with hexamethyldisilazane in liquid ammonia in an autoclave. This novel procedure has the advantage that the educt, the reagent, and the product are soluble in ammonia, which works under homogeneous conditions. No salt like by-products are formed and no tedious purification of the product is necessary. The degree of silylation has been adjusted through the ratio of silylating agent to hydroxy groups. Random incorporation of trimethylsilyl groups was proven by the linear dependence of the glass transition temperature from the degree of silylation and the solubility of the partially silylated material. Quantitative investigation of the silylation reaction showed first-order kinetics for the initial and final stages of the reaction indicative of a reaction where k₁ > k₂ > k₃. © 1995 John Wiley & Sons, Inc.     

The solubility of cellulose in liquid ammonia/salt solutions

The dissolution of cellulose in solutions of liquid ammonia and ammonium thiocyanate is discussed. Viscosity measurements on dilute solutions of cellulose in this solvent over a range of shear rates and shear stresses are reported. A four-bulb Ubbelohde suspended level viscometer was used for the measurements. Plots of log [η] versus log M gave Mark-Houwink coefficients of a = 0.95 and K = 6.686 × 10^?5 at 25°C for [η] as dl/g. The Bloomfield equation was used to calculate effective bond lengths (b) from limiting viscosity numbers of cellulose in solutions of ammonia/ammonium thiocyanate and Cuene, respectively. Results indicate that cellulose may have similar configurations in both solvents and also that the ammonia solutions are true cellulose solutions. Miscibility of the cell- ulose/ammonia/ammonium thiocyanate solutions with organic solvents, such as glycerol, is also reported. Further, a few interesting characteristics of the liquid ammonia/ammonium salt solutions, discussed briefly, are the convenient boiling point, the rheological behavior, and the relatively high concentration of cellulose obtainable.     

Silylation of cellulose regiocontrolled by bulky reagents and dispersity in the reaction media

Reaction of cellulose (1) with the bulky thexyldimethylchlorosilane (TDMSCl) leads to regioselectively functionalized silyl celluloses with degree of substitution (DS) up to 2. Uniform 6-O-thexyldimethylsilyl cellulose (2) was obtained in ammonia-saturated aprotic dipolar media, which was unexpected in a heterogeneous reaction. On the contrary, the 6-O-selectivity is low under homogeneous conditions in a N,N-dimethylacetamide (DMA)/LiCl solution, and with an excess of TDMSCl 2,3-di-O-thexyldimethylsilyl cellulose (3) can be synthesized. The polymer structures were characterized by two dimensional NMR spectroscopy after subsequent methylation (polymers 4 and 5 ), desilylation (polymers 6 and 7 ) and acetylation (polymers 8 and 9 ) as well as by HPLC after chain degradation of 4 and 5 to the complementary methyl glucoses.     

Grafting of monodisperse low-molecular-weight polystyrene onto cellulose acetate

Well-defined low-molecular-weight polystyrene was grafted onto cellulose acetate in a homogeneous solution. The grafting was performed by esterifying the free hydroxyls in the cellulose acetate (acetyl DS 2.5) with anionically prepared polystyrene having a carboxylic acid group at one end of the chain. The carboxylic acid end group of the polystyrene was activated by either conversion to the corresponding acid chloride, or by reaction with trifluoroacetic anhydride. Pyridine and the more active 4-dimethylaminopyridine were used as catalysts in the esterifications. The polystyrene contents of the copolymers varied between 10 and 80% and the molecular weights of the polystyrene grafts were 2500, 12,100 and 17,100 (M?_w/M?_n = 1.1).     

Hydrodimerization in liquid ammonia

The hydrodimerization of acrylonitrile (M₁), ethyl acrylate (M₂), and α-methyl acrylonitrile (M₃) in liquid ammonia by Na amalgam was studied. It was found that the concentration of phenol, as protonating agent, affects the rate of electron transfer from the amalgam to the monomer. This was explained by assuming the active species to be the protonated monomer complex MH⁺. Cross hydrodimerization rate constants were measured for the three monomers. The values of the ratios obtained were: k₁₁/k₁₂ = 0.89, k₂₂/k₂₁ = 0.39, k₁₁/k₁₃ = 5.8, k₃₃/k₃₁ = 0.12, k₂₂/k₂₃ = 1.7, k₃₃/k₃₂ = 0.36.     

Sorption of ammonia and nitrogen on cellulose acetate

Adsorption isotherms of ammonia and nitrogen on cellulose acetate in the temperature range 303–333 K are measured by reversed-phase chromatography. It is established that ammonia specifically interacts with the polymer matrix of the sorbent. The experimental data are described in terms of the Laatikainen-Lindstrom quasi-chemical model of vapor sorption. The enthalpies of sorption are determined, and their dependences on the ammonia and nitrogen concentrations in the sorbent are established.     

Cross-linking of liquid low-molecular-weight siloxane rubbers with orthotitanic acid esters

The kinetics of cross-linking of SKTN liquid low-molecular-weight siloxane rubbers with orthotitanic acid esters (tetraethoxytitanium, tetrapropoxytitanium, tetrabutoxytitanium) was studied. The effective rate constants of the cross-linking at 130, 140, 150, and 160°C and the activation energies of the reaction were calculated. The effect of a catalyst, tin phthalocyanine, on the cross-linking of SKTN liquid rubbers with orthotitanic acid esters was examined.     

Reaction of dibutyl phenylborate with hexamethyldisilazane and hexamethylcyclotrisilazane

A study was carried out on the reaction of dibutyl phenylborate with hexamethyldisilazane and hexamethylcyclotrisilazane in the presence of a catalyst, which leads to B-triphenylborazine and various butoxysilazanes. The formation of a mixed B,N,O heterocyclic compound is also observed in the reaction with hexamethyldisilazane.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1683–1685, July, 1991.     

Reaction of 3,6-disubstituted 4-nitropyridazine 1-oxides with methanolic ammonia and liquid ammonia

Reaction of 3-chloro-6-methyW-nitropyridazine 1-oxide ( 5 ) with methanolic ammonia at 0° led to a replacement of the chlorine atom by a methoxy group as well as by an amino group. Reaction of the 3-methoxy-6-methyl-4-nitropyridazine 1-oxide ( 6 ) with the same reagent led to amino-demethoxylation; this replacement reaction was very slow. Attempts to perform these reactions with liquid ammonia failed. Pmr spectroscopy of solutions of compound 6 in methanolic ammonia revealed that no σ-adduct was present. However in liquid ammonia a 1:1 σ-adduct at C-5 i.e. 12b was formed. 3,6-Dimethoxy-4-nitropyridazine 1-oxide (7) gave with methanolic ammonia an amino-demethoxylation at C-6. No σ-adduct could be detected by pmr spectroscopy. However, in liquid ammonia convincing pmr data were obtained showing the presence of a 1:1 σ-adduct at C-5.     

Photoinjected currents in liquid ammonia

The time evolution of currents following a 10 ns laser pulse on a Ag photocathode has been determined for pure liquid NH₃. The potential dependence of the integrated photoyield follows the “two-slope” pattern characteristic of polar liquids. The decay of the current depends on the potential as well. The increased yield above the second threshold (the “second slope”) is found to be the result of a rapid increase in the duration of the current pulse. Decay constants of several milliseconds are observed. These results are compared with a model of the density of states in polar fluids.     

Reaction of perfluoro ketones with hexamethyldisilazane

Conclusions Perfluoro ketones react with hexamethyldisilazane under mild conditions to give 2-trimethylsilyloxy-2-trtmethylsilylaminoperfluoropropanes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2352–2353, October, 1972.     

Organic reactivity in liquid ammonia

Liquid ammonia is a useful solvent for many organic reactions including aliphatic and aromatic nucleophilic substitution and metal-ion catalysed reactions. The acidity of acids is modified in liquid ammonia giving rise to differences from conventional solvents. The ionisation constants of phenols and carbon acids are the product of those for ion-pair formation and dissociation to the free ions. There is a linear relationship between the pK(a) of phenols and carbon acids in liquid ammonia and those in water of slope 1.68 and 0.7, respectively. Aminium ions exist in their unprotonated free base form in liquid ammonia. The rates of solvolysis and aminolysis by neutral amines of substituted benzyl chlorides in liquid ammonia show little or no dependence upon ring substituents, in stark contrast with the hydrolysis rates of substituted benzyl halides in water which vary 10(7) fold. However, the rates of the reaction of phenoxide ions and amine anions with 4-substituted benzyl chlorides gives a Hammett ρ = 1.1 and 0.93, respectively. The second order rate constants for the substitution of benzyl chlorides by neutral and anionic amines show a single Br?nsted β(nuc) = 0.21 whereas those for substituted phenoxide ions generate a Br?nsted β(nuc) = 0.40. The rates of aromatic nucleophilic substitution reactions in liquid ammonia are much faster than those in protic solvents indicating that liquid ammonia behaves like a typical dipolar aprotic solvent in its solvent effects on organic reactions. Nitrofluorobenzenes (NFB) readily undergo solvolysis in liquid ammonia but oxygen nucleophiles, such as alkoxide and phenoxide ions, displace the fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. The Br?nsted β(nuc) for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative that the decomposition of the Meisenheimer σ-intermediate is rate limiting. The aminolysis of 4-NFB occurs without general base catalysis by the amine and the second order rate constants generate a Br?nsted β(nuc) of 0.36, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate.     

Mesophase formation and intrinsic viscosity for some low-molecular-weight cellulose acetate samples

The intrinsic viscosity [η] of cellulose acetate (CA) has been determined in two solvents: acetic acid (AA) and N,N-dimethylacetamide (DMAC). The samples of this cellulose derivative have a rather weak mean molecular weight MW, with a mean degree of polymerization DP ranging from 213 to 47 and the same origin. We use the theory of Yamakawa for a worm-like chain to obtain the hydrodynamic diameter D and the persistence length q for these samples of CA, and the Mark-Houwink equation to estimate the parameters K and α. These results were compared with the same parameters obtained with previous experimental published results for CA with a high MW. The measurements were also carried out at different temperatures; the intrinsic-viscosity temperature dependence is discussed. The D and q values calculated for the low-MW range allow estimation of the critical concentrations C* for the solutions, which are compared with experimental results.     

Reactions of organylhalosilanes with 1,1,3,3-tetra- and hexamethyldisilazane

Organylchlorosilanes, and also SiCl₄, decompose 1,1,3,3-tetra- and hexamethyl-disilazanes with formation of hitherto unknown organylchlorosilazanes of general formulas R_4–nSiCl_n–1NHSiH(CH₃)₂ and R_4–nSiCl_n–1NHSi(CH₃)₃ (n=2–4) in yields of 54–98%. The IR and mass spectra of the prepared compounds were studied.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1159–1162, May, 1991.     

Reactivity of ytterbium in liquid ammonia

A solution of metallic ytterbium in liquid ammonia reacts readily with various carbonyl complexes of metals to form the corresponding lanthanide carbonylmetallates. The reaction of an excess of Yb in liquid NH₃ with [CpFe(CO)₂]₂ gave (THF)₄Yb[Fe(CO)₂Cp]₂ in 42% yield. It was suggested that the resulting complex contains two equivalent Yb−Fe bonds. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1883–1885, October, 1997.     

Rearrangement during reaction of sodiumo-carboranes with epichlorohydrin in liquid ammonia

Conclusions The 1,2-epoxy-3-(alkyl(aryl)-o-carboranyl)propanes when reacted with sodiumo-carboranes or sodium amide in liquid ammonia are isomerized to the trans--(alkyl(aryl)-o-carboranyl)allyl alcohols.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2338–2339, October, 1973.     

Modification of silica with mixtures of hexamethylcyclotrisilazane and hexamethyldisilazane

Summary For chromatographic purposes the use of mixtures instead of individual silylating agents can produce better deactivated surface than those shown by silicas modified by hexamethylcyclotrisilazane or hexamethyldisilazane separately.     

The structural organization of liquid low-molecular-weight epoxy oligomers

The possibility of the existence and the behavior of molecular aggregates in low-molecular-weight epoxy resins were studied. The globular morphology of all liquid resins was determined using transmission electron microscopy. The lamellar morphology of crystallized ED24 was established by additionally using atomic-force microscopy. The thermal behavior of oligomeric epoxides was studied over the temperature range ?100–100°C. The T _g and T _ll transitions were recorded for liquid resins by differential scanning calorimetry. For solid ED24, the T _m peak of resin melting situated between T _g and T _ll was observed. The existence of the T _ll transition was substantiated by positron annihilation measurements. The morphological picture was thermally unstable and became completely degenerate at T _ll. The morphology and thermophysical parameters of the resins changed substantially after their vibrational treatment. The morphology of oligomers was shown to be “hereditarily” (genetically) related to that of the corresponding network polymers.