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The thermal stability of cellulose/N-methylmorpholine-N-oxide (NMMO) solutions were investigated using UV/VIS spectrometry with a temperature programming cuvette and caloric measurements by means of the Systag calorimeter RADEX (mini-autoclave). Both analytical methods allow to characterize the influences of stabilizers and additives. With the temporal course of the optical density, temperature and pressure thermal runaway reactions with gas evolution and accumulation of chromophoric degradation products were recognized. Kinetic model calculations compared with UV/VIS measurements demonstrate the existence of autocatalytic reactions in cellulose/NMMO solutions. Varying the heating rate autocatalysis can be proved by dynamic caloric measurements as well.

Indigo does not undergo thermal or photochemical trans-cis isomerization in contrast, for example, to thioindigo."] Dry distillation of indigo produces and irradiation in the presence of dissolved oxygen. isatin.13] However, if indigo ( I ) is heated to 450 C in vacuum in the vapor phase it isomerizes to dibenzonaphthyridindione (2) in a suprisingly smooth reaction. 2The UViVis spectra of this isomerization reaction are shown in Figure I . The occurrence of an isosbestic point demonstrates the homogeneity of this reaction (yield about 80% ; in addition to a carbonized residue the only other product detected is aniline). Compound 2 was synthesized for the first time in 1934 in a relatively complicated multistep reaction.r41 It is extraordinarily thermally stable and can be sublimed at about 1000°C without degradation. To obtain a better understanding of the structure and the spectroscopic properties of both isomers, and also the course of the reaction, quantum chemical calculations using the MNDO-PM3 procedurer5' were carried out.For both I and 2 a nonplanar structure (pyramidalization at the N atom) was calculated. According to the calculations, the energy difference between this and a planar structure was 13.4 kJmol-I for 1 and 2.3 kJmol-' for 2. In contrast, in the electronically excited S, state, an almost planar structure resulted for both molecules. The free energy of reaction at 460°C was calculated to be AGR = -62.8 kJmol-'. To explain the mechanism of the isomerization. saddle point calculations were made. A structure for a tranzition state was found and is illustrated in Figure 2. The mechanism of the isomerization can thus be conceived as follows: The bond between TS 2If the orbital symmetry is considered, this process can also be regarded as a dyotrope rearrangement (intramolecular symmetrical transfer of two (CJ bonds).rb1 Such a [D? + 0 2 ] process is forbidden by symmetry. However, by including the n-electron system of 1 in the four-membered transition state. the restriction can be relaxed. The very high value of the activation energy (610 kJmol-') for the calculated transition state TS instead favors a stepwise process. An analogous isomerization was also observed for 5S'-dibromo-. 4,5,4'.5'-tetrabromo-, and 5.7S.7'-tetrabromoindigo. Thioindigo and thionaphthenindigo (2.2') do not isomerize. Heating 1 in a solvent results in practically no isomerization (in DMSO traces of 2 were detected by its very intense fluorescence).The spectroscopic properties (UV/Vis absorption and fluorescence) of I have been frequently investigated and d i s c~s s e d ,~'~ also with respect to the differences in absorption behavior between 1 and 2.[*] A comparison of the absorption and fluores-

A novel polymeric stabilizer consisting of iminodiacetic acid sodium salt (ISDB) and benzyl amine (BSDB) covalently bound to a styrene/divinyl benzene copolymer were studied. Calorimetric, spectroscopic, rheological, and high performance liquid chromatography analysis revealed distinctive improvements of the thermal stability of modified Lyocell solutions compared to the unstabilized solution and to that with conventional NaOH/propyl gallate stabilizer. Segregation processes of the system cellulose/N‐methylmorpholine‐N‐oxide and autocatalytic reactions caused by carboxyl group‐containing additives are suppressed by ISDB/BSDB. Concerning to surface‐active additives, enhanced thermal stability is only received for a weakly reactive charcoal. In the case of nanoscaled carbon black modifier, autocatalytic reactions indicated by isoperibolic measures are prevented by the new polymeric stabilizer system. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1702–1713, 2006

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