1993 рік

Rate constants of the reaction of tris(p-methoxyphenyl)vinyi cations with pyridine derivatives were measured in acetonitrile solution at room temperature using the flash photolysis technique. The lowest value (k2 =3.4 x 1041tool -1 s-1) was found for 2,6-di-tert-butylpyridine, which is frequently used as a proton trap in cationic polymerizations. 4-tert-Butylpyridine and pyridine exhibited the highest reactivity towards the vinyl cations: k 2 = 3.2 x 107 and 2.8 x 1071 mol-1 s-1, respectively.

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By selecting solvents with a very weakly concentration-dependent interaction parameter Z with poly(dimethylsiloxane) (PDMS) and using experimental values of the elastic modulus of dry, end-linked PDMS networks, the Flory-Rehner theory and the phantom network assumption we obtained good agreement between the values of X from equilibrium swelling and those obtained from intrinsic viscosity measurements on solutions of linear chains. The solvents used were 2,3-dimethylpentane and 2,2,4-trimethylpentane at 25°C,

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The molten state transesterification reaction of ester groups of ethylene vinyl acetate copolymers and ethylene acrylic ester copolymers leads to a tetrafunctional network. The time and temperature dependence on the network formation is assessed. From the statistical theory developed by Flory on tetrafunctional crosslinking networks, and assuming a log normal distribution of the crosslinkable sites, the theoretical gel network formation is compared with the experimental data assessment of the gel fraction. A good agreement is then obtained between experimental and theoretical values.

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In our previous study, chromium acetylacetonate (Cr(acac)3) was found to have a greater toughening effect on epoxy resin than Co(acac)3. G.p.c. and n.m.r, results indicated that Cr(acac)3 has a tendency to attach to the hydroxy groups of diglycidyl ether of bisphenol A (DGEBA) multimers during mixing. This is due to the fact that the (acac)- ligands of Cr(acac)3 are able to attract protons from hydroxy groups. As the DGEBA epoxy resin incorporating Cr(acac)3 was cured with amine curing agents, each Cr(acac)3 molecule was able to interact with more than one hydroxy group. This reinforced the cured epoxy network unless the amine curing agents exceeded the stoichiometric quantity of epoxy groups.

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The temperature dependence of diffusion of methyl red derivatives through semidilute toluene solutions of poly(vinyl acetate) was studied by forced Rayleigh scattering. The magnitude of the hydrogen bonding effect on probe diffusion was estimated from the activation energy of the diffusion process.

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This paper deals with the behaviour of a superswelling hydrogel based on sodium polyacrylate crosslinked with the allyl ester of carboxymethyl cellulose in solutions of salts of mono- and polyvalent metals. The effect of cooperative sorption of polyvalent ions by the hydrogel was detected. An expression describing the concentration redistribution in the system hydrogel-univalent salt solution was obtained

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The photografting of acrylamide onto cellulose acetate films using light of wavelength > 380 nm is discussed. The semiconducting properties of colloidal Fe20 3 (bandgap 2.2 eV) is used for photosensitizing the reaction. Degree of grafting is dependent upon the concentration of the photosensitizer as well as that of the monomer. Methyl viologen (MV 2?) is used to prevent e--h ? recombination.

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The equilibrium melting temperature, as determined from Hoffman-Weeks plots, has been measured as a function of the molecular weight for several batches of polyhydroxybutyrate. In all the batches there is a significant increase in the equilibrium melting temperature with the molecular weight. However, different polymer batches give non-superimposable Hoffman-Weeks plots and equilibrium melting temperatures. In particular, at the highest molecular weights studied, we can find anomalously high equilibrium melting temperatures. The equilibrium melting temperatures vary from ca. 179°C for polymers with molecular weights as low as 20000 to typically 186°C for polymers with molecular weights ca. 100000; however, for a polymer with a molecular weight of ca. 300 000, we have recorded an equilibrium melting temperature as high as 198°C

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The microstructures of crystalline-amorphous type block copolymers, poly(tetrahydrofuran-b-isoprene), prepared from ultra-dilute solution (0.001 wt%) were studied by transmission electron microscopy, The obtained shape was different from the shape observed in the case of amorphous-amorphous type block copolymers; the shape was deformed by poly(tetrahydrofuran) crystallization

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The effect of the initial molar ratio of the reactive groups, rNc o (= [NCO]0/[OH]o = 1.0 or 0.8), on the gelation of polyurethane networks based on tris(4-isocyanatophenyl)thiophosphate and poly(oxypropylene) diol, was studied. The number-average molecular weight, h4n, in the pre-gel state, the critical conversion at the gel point, P~co, and the critical ratio, r~co, at which gelation occurs, were measured. The decrease in 1/r~co with increasing dilution in xylene suggests that cyclization takes place. The experimental h4, values are always lower than the theoretical ones, with the latter being calculated for the ring-free case. The fraction of bonds that are lost in the cyclization process, s, calculated from the difference between the experimental and theoretical values of/~n, increases with conversion and has a value near the gel point of 0.04--0.06. It is demonstrated both theoretically and experimentally that the values of s calculated from the shift of the gel point conversion are not equivalent to those obtained from Mn, but depend on rNCO.

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