A complementary DNA encoding a voltage-gated chloride channel from Torpedo marmorata electric organ was cloned by expressing hybrid-depleted messenger RNA in Xenopus oocytes. The predicted protein has a sequence of 805 amino acids containing several putative membrane-spanning domains. Expression of the protein in Xenopus oocytes shows that it is sufficient for channel function.

The theory of step‐growth polymerizations including the cascade theory is discussed in the light of new results focussing on the role of cyclization reactions. The identification of cyclic oligomers and polymers in reaction products of step‐growth polymerizations has been eased considerably by means of MALDI‐TOF mass spectrometry. Experimental examples concern syntheses of polyesters, polycarbonates, polyamides, polyimides, poly(ether sulfone)s, poly(ether ketone)s and polyurethanes. It was found in all cases that the percentage and molecular weight of the cycles increases when the reaction conditions favor high molecular weights. In the absence of side reactions all reaction products will be cycles when conversion approaches 100%. Cyclization may even take place in the nematic phase but even‐numbered cycles are favored over odd‐numbered ones due to electronic interactions between mesogens aligned in parallel. In contrast to Flory's cascade theory, cyclization also plays a decisive role in polycondensations of abn‐type monomers, and at 100% conversion all hyperbranched polymers have a cyclic core. Furthermore, it is demonstrated that in a2+b3 polycondensations intensive cyclization in the early stages of the process has the consequence that either no gelation occurs or the resulting networks consist of cyclic and bicyclic oligomers as building blocks. Finally, a comparison between cyclization of synthetic polymers and biopolymers is discussed. Schematic representation of a network structure mainly consisting of cyclic oligomers and multicyclic building blocks as derived from “a2” + “b3” polycondensation.magnified imageSchematic representation of a network structure mainly consisting of cyclic oligomers and multicyclic building blocks as derived from “a2” + “b3” polycondensation.

Poly(ether-sulfone)s having an identical backbone were prepared by four different methods. First, silylated bisphenol A (BSBA) was polycondensed with 4,4′-difluorodiphenyl sulfone (DFDPS) and K 2CO3 in N-methylpyrrolidone with variation of the temperature. Second, analogous polycondensation were conducted using CsF as catalyst (and no K2CO3). Third, CsF-catalyzed polycondensations BSBA and DFDPS were conducted in bulk up to 290°C. Fourth, free bisphenol was polycondensed with DFDPS or 4,4′-dichlorodiphenyl sulfone and K2CO3 in DMSO with azeotropic removal of water. MALDI-TOF mass spectroscopy revealed that the first method mainly yielded cyclic poly(ether-sulfone)s which were detected up to masses around 13 000 Da. These and other results suggest that these polycondensations take a kinetically kontrolled course at tempeatures e145°C. This interpretation is also valid for the fourth method where high yields of cycles were obtained with DFDPS. With the less reactive 4,4′-dichlorodiphenyl sulfone lower conversions, lower molecular weights and lower fractions of cycles were found. In contrast to KF (resulting from K 2CO3) CsF cleaves the poly(ether sulfone) backbone at temperatures > 145°C. Smaller amounts of smaller cycles were found in these CsF-catalyzed polycondensations which were in this case the result of thermodynamically controlled "back-biting degradation".

Three different synthetic methods were studied with respect to their usefulness for the preparation of poly(isosorbide carbonate) (PIC). Thermal polycondensations of isosorbide with dimethyl-or diethyl carbonate in bulk proved unsuccessful, regardless of the transesterification catalyst. Polycondensations of isosorbide with diphosgene in pyridine gave polycarbonates, the molecular weights of which depended largely on the excess of diphosgene. In all experiments, OH-terminated linear chains were the main products. Similar results were obtained from pyridine-promoted phosgenations in dioxane. However, polycondensations of equimolar mixtures of isosorbide and isomannide mainly yielded cyclic polymers. Pyridine-promoted polycondensations of isosorbide with isosorbide bischloroformate only gave low molar mass polycarbonates. At low temperatures, even-numbered linear chains were the main products, but higher temperatures gave even-numbered cycles. SEC measurements with triple detection evidenced the formation of high molar mass polycarbonates in the phosgenation experiments and a Mark-Houwink equation was elaborated. The glass transition temperatures varied between 115 and 165 °C depending on the molar mass.

Two series of kinetically controlled polycondensations were conducted yielding polyamides of aliphatic dicarboxylic acids. First, the bis(trimethylsilyl) derivatives of 1,3-diaminobenzene, 4,4‘-diaminodiphenylmethane, 1,12-diamino-4,9-dioxadodecane, and 1,12-diaminododecane were polycondensed with dicarboxylic acid dichlorides in N-methylpyrrodidone below 0 °C. After optimization of the reaction conditions mainly cyclic polyamides were detectable in the MALDI−TOF mass spectra (up to 13 000 Da) of the semiaromatic polyamides in contrast to the Carothers−Flory theory. In the case of silylated aliphatic diamines, side reactions of the acid chlorides prevented a complete conversion of the amino groups, so that the reaction products mainly consisted of cycles and linear chains having two amino end groups. Second, normal interfacial polycondensations were performed either with 1,6-diaminohexane and adipoyl chloride or with 1,12-diaminododecane and decane-1,10-dicarbonyl chloride. When the loss of acid chloride groups by hydrolysis was compensated by an excess of dicarboxylic acid dichlorides cyclic polyamides were again the main reaction products up to masses of 4000−5000 Da. A new version of the “Carothers equation” taking into account the role of cyclization in kinetically controlled step-growth polymerizations is discussed.