Thermal reversibility

Secondary amines give only a monosubstituted product. Both of these reactions are thermally reversible. The product with ammonia (3,3, 3 -nitrilottispropionamide [2664-61-1C H gN O ) (5) is frequently found in crystalline acrylamide as a minor impurity and affects the free-radical polymerisation. An extensive study (8) has determined the stmctural requirements of the amines to form thermally reversible products. Unsymmetrical dialkyl hydrasines add through the unsubstituted nitrogen in basic medium and through the substituted nitrogen in acidic medium (9)). 

Diels-Alder reactions with butadiene are generally thermally reversible and can proceed in both gas and Hquid phases. The reactions are exothermic and foUow second-order kinetics first-order with respect to each reactant. 

The reverse reaction, B returning to A, can be driven either by thermal or photochemical energy, or both. When the reversion is photochemicaHy driven, the process is called optical bleaching. Optical bleaching is a general characteristic and is a factor in almost all photochromic systems, even those normally thought of as being thermally reversible. 

Nitro-substituted indolino spiroben2opyrans or indolino spironaphthopyrans are photochromic when dissolved in organic solvents or polymer matrices (27). Absorption of uv radiation results in the colorless spiro compound [1498-88-0], C22H2gN202, being transformed into the colored, ring-opened species. This colored species is often called a photomerocyanine because of its stmctural similarity to the merocyanine dyes (see Cyanine dyes). Removal of the ultraviolet light source results in thermal reversion to the spiro compound. 

By changing the substituents on the ethylenic linkage and exchanging phenyl rings for heteroaromatic rings, photochromic systems that are thermally reversible are transformed into systems that are thermally irreversible but photochemically reversible. The transition between the ben2othiophene-derivative... 

The subject of thermochromism in organic and polymeric compounds has been reviewed in some depth previously (8,16,18), and these expansive overviews should be used by readers with deeper and more particular interest in the subject. Many more examples can be found in the reviews that further illustrate the pattern of association between thermochromism and molecular restmcturing of one kind or another. The specific assignment of stmctures is still Open to debate in many cases, and there are still not many actual commercial uses for these or any of the other thermally reversible materials discussed herein. Temperature indicators have been mentioned, though perhaps as much or more for irreversible materials. 

Reversible HydrocoUoids (Agar). The agar-based impression materials are thermally reversible, aqueous gels (230,231), that become viscous fluids in boiling water and set to an elastic gel when cooled below 35°C. The popularity of agar-based impression materials has diminished with the introduction of elastic impression materials such as alginate-based, polysulfide, silicone, and polyether impression materials, but agar [9002-18-0] materials are still used in substantial quantities. 

JA173) illustrates this possibility (Scheme 34). Thus 3,3,5-trimethyl-3//-pyrazole (371 R = H) on irradiation in pentane solution gives 1,3,3-trimethylcyclopropene (372 R = H) the intermediate diazoalkene (373) has been characterized. The tetramethyl derivative (371 R" = Me) when irradiated at -50 °C in methylene chloride leads to a species believed to be a l,2-dlazablcyclo[2.1.0]pent-2-ene (374). This isomerization is thermally reversible, the 3H- pyrazole being regenerated at room temperature. 

In certain cases, even dimers of certain isocyanates, such as toluene diisocyanate or hexamethylene diisocyanate, can act as blocking agents, thermally reversing to regenerate the isocyanate [16,17]. 

Since amines initiate cyanoacrylate polymerization, the monomer cannot be isolated directly, because a polymer is generated immediately after formation of the monomer. An acid is then added to the polymer, and heat (140-180°C) is applied to the reaction mixture. Because of the relatively low ceiling temperature of the polymer, the pure monomer can be isolated, in greater than 80% yield, by the thermal reversion of the polymer back to the free monomer [4,5]. 

The thermal reversal of the photochemical a-cleavage, i.e., the direct recombination of the resulting radical pair or diradical, can be recognized as such only when at least one of the a-atoms is chiral and is epimerized in the process. In fact, the frequently rather low quantum yields observed in the phototransformations of nonconjugated steroidal ketones may be largely due to the reversal of a-cleavage. 

A few examples are known of the thermally reversible, photoisomerization of 4//-azepines, e.g. 22, to l-azabicycIo[3.2.0]hepta-2,6-dienes, e.g. 23.83 The yields are high (65 95%) and the alternative isomers, the 6-azabicycIo[3.2.0]hepta-3,6-dienes, resulting from C2-C6 ring closure, do not appear to be formed. 

Pochan DJ, Schneider JP, Kretsinger J et al (2003) Thermally reversible hydrogels via intramolecular folding and consequent self- assembly of a de novo designed peptide. J Am Chem Soc 125 11802-11803... 

D. Miscellaneous.—A further study of the reactions of diphenyl-phosphine oxide with tetracyclone has confirmed that the reaction yields the oxide (30) under mild, basic conditions, and that the reaction is thermally reversible. The displacement of halogen from phosphorus by amino-compounds has been used in the synthesis of a number of aminofluoro-phosphine oxides (31), and of A-methyl-AA-bis(dichlorophosphinyl)-amine (32). ... 

Many compounds can have a gelling action on poly (vinyl alcohol) solutions and in some cases the gels are thermally reversible. Gelling by means of polyhydroxy aromatic compounds has been of interest in photographic emulsions ( 7). 

It has been established 25) that a firm, thermally reversible gel of poly (vinyl alcohol) can be formed by mixing it with polyhydric phenols, 1-naphthols, and dihydric naphthols. Monohydric phenols have no action. Compounds such as catechol, phloroglucinol, and 1-naphthol are typical gelling agents. 

Thermally reversible gels can be prepared from poly (vinyl alcohol) and alkali metal salts of o-hydroxybenzal derivatives having benzenoid groups at both ends 24). Colored gels can be obtained, depending on the type of ketone used (Figure 4). 

The property of thermal, reversible gelation is obtained by the addition of water-soluble proteins and protein degradation products to an aqueous solution of poly (vinyl alcohol) 2). Protein products such as albumin, gelatin, glue, a-amino acids, and their condensation products—diketopiperazines—may be used. A typical formulation for the preparation of a thermally reversible gel is ... 

The mixture is heated to boiling for a few minutes and is spread out to dry at 20" to 60° C. The dried material is insoluble in cold water, but it swells to form a tough gel which is thermally reversible and transparent. Another... 

Hydroboration is thermally reversible. B-H moieties are eliminated from alkyl-boranes at 160° C and above, but the equilibrium still favors of the addition products. 

Compounds such as o-nitrotoluene which undergo photochemical reactions that are rapidly thermally reversed are called photochromic or phototropic. Nitro compounds comprise a very important group of compounds which exhibit this behavior. Further examples are as follows(112,113) ... 

AS Hoffman, A Afrassiabi, LC Dong. Thermally reversible hydrogels II. Delivery and selective removal of substances from aqueous solutions. J Controlled Release 4 213-222, 1986. 

A Afrassiabi, AS Hoffman, LA Cadwell. Effect of temperature on the release rate of biomolecules from thermally reversible hydrogels. J Membrane Sci 33 191-200,... 

TG Park, AS Hoffman. Immobilization of Arthrobacter simplex in thermally reversible hydrogels Effect of gel hydrophobicity on steroid conversion. Biotech Prog 7 383-390, 1991. 

Lane 5 Reverse orientation of pLD-OC-40Pris Lane 6 Purified extract of E. coli strain XL-1 Blue containing no plasmid. Two rounds of thermally reversible phase transition purification were performed. 

Ketenimine 3b undergoes thermal reversion to triplet lb over time in solution at ambient temperature.38 In addition, triplet lb and 3b can be photo-chemically interconverted in a cryogenic matrix.43 These results are summarized in Scheme 12. 

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