Pavel Špaček

Network formation in the free-radical copolymerization of a bismaleimide and styrene

Abstract Network formation during copolymerization of styrene with a model bismaleimide, p -maleimidobenzoic anhydride, or with mixture of p -maleimidobenzoic anhydride and methyl p -maleimidobenzoate was studied. The concentrations of pendent double bonds in copolymers, calculated from the consumption of monomers and copolymer composition, follow the general trend typical for vinyl divinyl copolymerizations. A substantial fraction of pendent maleimide bonds remains in the system at the end of polymerization. The gel point conversions are much higher than for ring-free copolymerizations due to cyclization and steric hindrance of pendent double bonds. The gel point conversions increase with increasing content of the monomaleimide in the copolymerization mixture reaching 100% at 4 mol% of bismaleimide (12 wt% solution). The gel point conversions also increase with increasing dilution reaching 100% for the bismaleimide polymerization in 3 wt% solution. The steric hindrance effect on pendent double bond reactivity seems to be somewhat higher than for conventional vinyl-divinyl copolymerizations.

Influence of the Amount of Bonded Non-Polar Phase and of the Length of Attached Alkyl Chains on Retention Characteristics of Silica-Based Sorbents for Reversed-Phase High Performance Liquid Chromatography

Abstract The retention characteristics of two sets of chemically bonded non-polar silica packings with a high surface concentration of functional groups have been compared in the reversed-phase mode of liquid chromatography: (i) the conventional packings prepared by chemical modification with trimethylsilyl, heptyl, dodecyl and octadecyl groups, and (ii) mixed-phase materials where differences in the amount of organic bonded phase were achieved by bonding octadecyl and trimethylsilyl groups in different proportions. The retention data of two homologous series of solutes show that these two classes of packings are distinctly different; in particular, the capacity factors and selectivities are always higher on the mixed-phase bonded silica packings at the same percentage of carbon.

Cyclization in amine-cured N,N-diglycidylaniline epoxy resins

Abstract The formation of eight-membered cycles (stereoisomers of 1,5-diphenyl-3,7-dihydroxy-1,5-diazacyclooctane) during the reaction of N,N-diglycidylaniline with aniline was followed using high-performance liquid chromatography. The ring compounds were identified by mass spectrometry. The extent of cyclization depends on the ratio of the reaction components, and increases with increasing temperature and upon dilution of the reaction mixture with an inert diluent. The experimental results agree with the theoretical prediction based on the reaction mechanism determined earlier.

Separation of [3H]cholesteryl esters by reversed-phase high-performance liquid chromatography

A method is described for the determination of [3H]cholesteryl esters separated by reversed-phase high-performance liquid chromatography on C18-bonded silica gel. The procedure readily separates esters of saturated fatty acids from those of acids with various degrees of unsaturation. Saturated homologues of straight-chain fatty acids are completely separated from esters of branched-chain higher fatty acids. Picomole amounts of tritiated cholesteryl esters can be detected with a high precision, and the method is suitable for the determination of such compounds synthesized enzymatically in vitro.

Coupling of diffusion flux and chemical reaction in an asymmetric catalytically active membrane

Abstract The behaviour of a catalytically active membrane vectorized by a permanent transversal gradient of catalytic activity has been analysed mathematically. Under the simplifying assumption that the gradient of the catalytic activity profile across the membrane is linear, it is possible to express the steady state concentrations of the reaction components inside the membrane, governed by the diffusion-controlled chemical reaction, by means of tabulated Airy functions. For enzyme-catalysed reactions which follow Michaelis-Menten kinetics, two analytical solutions have been found for high and low substrate concentrations, respectively. If such an asymmetric membrane separates two compartments with the same (and constant) substrate concentrations, it behaves as an active barrier capable of pumping the product against its own concentration gradient. The effects of membrane thickness, slope of the activity profile and diffusion coefficient on the overall rate of product formation and on the pumping efficiency of the membrane are discussed.

Formation of poly(urethane-isocyanurate) networks from poly(oxypropylene)diols and diisocyanate

SummaryReaction conditions leading to the smallest amount of unreacted isocyanate groups for the preparation of poly(urethane-isocyanurate) networks based on 2,4-tolylene diisocyanate and α,ω-dihydroxypoly(oxypropylene) (Mn = 425−) were found. A tertiary amine, POLYCAT* 41, was used as a cyclotrimerization catalyst. The networks were prepared by prepolymer two-stage technique. The content of isocyanate groups in the prepolymers and networks was determined by potentiometric titration. Model reactions in 1,4-dioxane were used for the estimation of possible reaction intermediates during poly(urethane-isocyanurate) synthesis.

Sorption of Flavonoids From Wine and Its Investigation by HPLC in the Reverse Phase

Abstract A screening test of several commercially available sorbents was carried out with the main attention concentrated on flavonoids from wine. of the sorbents studied the most advantageous properties are shown by Amidap, the sorption effects of which were quantitatively investigated by using a number of model compounds. the effect of the added sorbent on qualitative changes of several samples of natural wines was evaluated both gravimetrically and (predominantly) by the HPLC method in the reverse phase. the results of this study show that by applying a suitable amount of sorbent to wine it is possible to affect its flavour and colour properties (with the simultaneous tests by degustators).

Efficiency of Glass CGC Columns in Reversed-Phase HPLC

Abstract The design of metal-jacketed glass columns of CGC type is described, and the dependence of their efficiency on the flow rate is given, both for a single column and for two columns combined in series. The results have been treated in terms of reduced quantities; the optimal values of the reduced plate height h∼2.5 suggest a good quality of the sorbent packing. It is shown that the described combination of columns in series does not reduce the total efficiency of the system.