Double melting

To a solution of 180 parts of -benzyl N-benzyloxycarbonyl-L-aspartvI-L-phenylalanine methyl ester in 3,000 parts by volume of 75% acetic acid is added 18 parts of palladium black metal catalyst, and the resulting mixture is shaken with hydrogen at atmospheric pressure and room temperature for about 12 hours. The catalyst is removed by filtration, and the solvent is distilled under reduced pressure to afford a solid residue, which is purified by re-crystallization from aqueous ethanol to yield L-aspartyl-L-phenylalanine methyl ester. It displays a double melting point at about 190°C and 245°-247°C. 

The double melting point has been observed in only one 5 of the previous studies. The initial melting point varies with the state of subdivision and is not a reliable index of purity. Several recrystallizations did not change the melting behavior or the remelt temperature. The checkers did not observe the double melting point with the product initially obtained, but did so with material recrystallized once. 

The melting of a crystalline-amorphous block copolymer of poly(tetrahydro-furan)-poly(isoprene) (PTHF-PI) was investigated using DSC by Ishikawa et al. (1991). They found a double melting peak, which was proposed to result from the semicrystalline structure of the crystalline PTHF layer, with less-ordered crystallites melting before those with well-ordered domains of chain-folded PTHF. Alternative explanations include fractionation of the polydisperse block copolymer or melting of crystals with different fold lengths. 

Mb The double melting point corresponds to two polymorphic forms (reference 97). 

Aspartame (and its (D,D), (D,L), and (L,D) diastereomers) display double melting points. When recrystallized from aqueous ethanol or water, aspartame melts initially at 190°C, and then solidifies and re-melts at 246-247° C [21]. Other sources give only the 246-7°C value [5]. The explanation for this behavior is that intramolecular self-aminolysis and simultaneous demethanolation of aspartame talks place on initial melting, to yield 3-methylenecarboxyl-6-benzyl-2,5-diketopiperazine (DKP) (C12H14N2O4 = 262.26 g/mole). The latter compound has been reported to melt at 259 1.1° C [22]. 

The unusual double melting point behavior of aspartame has been confirmed by differential scanning calorimetry (DSC) [22, 52]. This behavior was interpreted [52] in terms of initial conversion of the methyl ester to I-a-aspartyl-i-phenylalanine, followed by formation of the substituted 2,5-diketopiperazine, although no clear evidence was produced to support this view. However, the alternative explanation that the 2,5-diketopiperazine was produced in a concerted manner from simultaneous self-aminolysis and demethanolation of aspartame seemed more likely to... 

The double-melt endotherms of the silicone-nylon 6,6 IPN do not disappear with thermal cycling. Instead the peaks are further resolved in the DSC when samples are scanned to 50 °C above the melt point, cooled slowly to room temperature, and rescanned (Figures 7 and 8). The stability of the melt endotherm associated with the presence of IPN suggests the presence of an interpenetrated polymer phase of various crystalline structures. 

This complex contains six terminal carbonyl groups and three carbonyl groups bridging the two atoms of iron. When the complex is treated with acetylene at 20-25° and 20-24 atm., acetylene displaces the bridging carbonyls with formation of tropone iron tricarbonyl (2), which is an orange solid of double melting point (63.5-64° and 83-84°). This complex is obtained in only trace amounts with Fe(CO)s and Fe(CO)i. Treatment of the complex (2) with triphenylphosphine affords free tropone (3,69% yield) and a new complex (4), tropone iron dicarbonyltriphenylphosphine. Tropone can also be obtained, if in low yield, by oxidation of (2) with ferric chloride ... 

Phenomena Observed.—In 1888 it was discovered by Reinitzer that the two substances, cholesteryl acetate and cholesteryl benzoate, possess the peculiar property of melting sharply at a definite temperature to milky liquids and that the latter, on being further heated, suddenly become clear, also at a definite temperature. Other substances, more especially j -azoxyanisole and jp-azoxyphenetole, were, later, found to possess the same property of having apparently a double melting-point. On cooling the clear liquids, the reverse series of changes occurred. 

Brevttoxin A, CaH Op, GB-J. Fine prisms from acetoni -trile, mp 197-199 218-220 (double melting point). LClM in guppies 4 ng/ml (Shimizu). 

Re crystallization is a process in which the initial, rather imperfect, lamellae melt and recrystallize to produce thicker and more perfect lamellae that as a consequence melt at a higher temperature. As a result of this process a double melting behavior may be observed. Recrystallization has been observed for neat polymers and blends. 

Several blends prepared by co-precipitation followed by crystallization from the melt exhibit a double melting behavior, due to the occurrence of the secondary crystallization process. The amorphous component causes a retarded crystallization of some of the crystallizable chains, which form lamellae smaller than and located between the primary ones constituting the spherulites (see Figure 3.22). 

Other examples are PPS (which shows a double melting behavior due to the obstructive effect of branching or crosslinking of the molecules on crystallization at high temperature [Mai et al, 1994]), and PEEK. Much controversy exists about... 

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