Several thioacylating reagents have been tested toward hydrolysis under conditions suitable for protein modifications: 20-35 degrees C and buffered solutions at pH 7.5-8.5. Aliphatic dithioesters are sufficiently stable in aqueous media at room temperature (or below) if protein modification reaction time does not exceed 24 h, whereas at 35 degrees C reaction times must be limited to a few hours. Kinetic data obtained in gelatin thioacylation at room temperature using aliphatic dithioesters and dithio acid are consistent with a second-order reaction rate with respect to amine concentration. The pH dependence of the second-order reaction rate constants indicate that dithioester reacts exclusively with the free amine form of lysine residue, whereas dithiocarboxylate ion reacts with both amine and ammonium ion, probably through a more complex mechanism. Interestingly thioacylation using dithio acids may be obtained in pH near neutrality or in slightly acidic media, thus offering protein modification possibilities at pH 5-9. Thioacylation reaction rates may be expressed as R = -(dAt/dt) = k[H3O+](-b)At2[thioacylating agent] in which At is the amine concentration at time t, constants k and b depending on the reagent nature.

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Thioamidation kinetics have been studied for reaction in solution between mono-and bifunctional amines and dithioesters near room temperature (30-50 °C) using large amine excess. We have used aliphatic reagents as well as several ether mono-and diamines and a diether bis(dithioester), which are able to maintain polythioamides in solution throughout the condensation reaction and thus give rise to significant results in polycondensation kinetics for which sufficient solubility is needed. Pseudo-first-order constants (first order relative to dithioesters) were correlated to amine concentrations and allowed to appreciate the values of secondand third-order reaction rate constants. The influences of solvent nature, structure of reagents, and temperature were examined in numerous examples. Added tertiary amines do not have significant influence on reaction rates whereas the primary amine concentration is an important factor of reaction rates. All the collected data are correlated through kinetic analysis with a reaction mechanism involving first a reversible nucleo-

We have studied the influence of the addition of poly(butylene terephthalate) (PBT) to poly(ethylene‐co‐ethyl acrylate)‐carbon black (EEA‐CB) on the electrical properties of the blends, namely the resistivity and power dissipation of the monophasic and diphasic systems. The morphology of the latter blend is characterized by two interpenetrated immiscible phases which provide a good thermal stability to the composite up to at least 170 °C. Power can be regularly delivered without a negative temperature coefficient (NTC) effect at 100 °C where poly(ethylene‐co‐ethyl acrylate) melts. However, the significant decrease of positive temperature coefficient (PTC) amplitude compared to the monophasic systems reduces the self‐regulating heating ability. The PTC effect amplitude decrease was studied by some of its known origins (thermal expansion, melting of crystalline phase, carbon black localization). Correlation of the composites resistivity with linear expansion, the melting process of EEA, and the volume expansion of PBT, show that none of these parameters can alone explain the electrical properties' evolution with temperature. Our interpretation of the PTC effect amplitude decrease is based on the PBT matrix lower thermal expansion and on the increased carbon black content located at the interface. Copyright © 2003 John Wiley & Sons, Ltd.