Polymorphous modifications

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Calcium Pyrophosphates. As is typical of the pyrophosphate salts of multiple-charged or heavy-metal ions, the calcium pyrophosphates are extremely insoluble ia water. Calcium pyrophosphate exists ia three polymorphic modifications, each of which is metastable at room temperature. These are formed progressively upon thermal dehydration of calcium hydrogen phosphate dihydrate as shown below. Conversion temperatures indicated are those obtained from thermal analyses (22,23). The presence of impurities and actual processing conditions can change these values considerably, as is tme of commercial manufacture. 

Unsubstituted phthalocyanines and also halogenated phthalocyanines show very poor solubility in organic solvents. In the solid state, unsubstituted phthalocyanines exhibit, in most cases, inclined one-dimensional stacking of the planar molecules. Besides other polymorphous modifications PcCu shows an x- and / -arrangement.74-75... 

Stereochemistry. - There has been a far infra-red spectroscopic study of ethyl phosphine,130 and in combination with X-ray fluorescence, further work completed on the conformational analysis of dialkylphenyl phosphines.131 The influence of water on the conformational equilibria of trimethyl phosphate has received attention.132 There has been a low temperature solid state and matrix isolation study of methyl phosphorodichi oridate,133 and a conformational study of polymorphic modifications of diphenyl-phosphinyl acetic acid hydrazide. 34... 

No detailed studies on the crystal structure of methimazole is reported in the literature. Methimazole can be microscopically identified using Kofler s method, the occurrence of polymorphous modification is indicated in methimazole. 

HMX is a white crystalline substance and exists in four polymorphic modifications, the P form being most stable and least sensitive. The a and forms exist at room temperature but all forms transform to 8 polymorph above 160 °C. Octogen is usually obtained in the p form which is less sensitive to impact and has a density of 1.91 gem-3 and m.p. of 291 °C. HMX and RDX are almost alike in chemical reactivity. It is non-hygroscopic, insoluble in water and soluble in organic solvents similar to RDX. They differ only in that ... 

Copper Phthalocyanine Blue. CPC blue exists in several polymorphic modifications, two of which, the red-shade blue alpha and greenshade blue beta form, are of great commercial significance. Beta is the thermodynamically more stable phase and is the product resulting from manufacture by the two basic processes using either phthalonitrilc or phthalic anhydride as starting materials. The alpha form is usually obtained by conversion from the beta form and has to be stabilized to prevent phase reconversion. 

The fact that a simple formula (such as Zn(OH)2) does not always account for all the chemical properties of the substance it represents may be attributed partly to the occurrence of polymorphous modifications. Differences in crystal structure may indeed cause considerable changes in thermodynamic properties (Table V). 

Similar to other binary /(-p-clcment systems, the formation of binary rare earth - antimonides with a simple stoichiometry is a characteristic feature of these systems. The largest number of structure types formed was encountered for the group of RSb2 compounds (4 members). The polymorphic modifications were observed for GdSb2 and TbSb2 as well as the RSb (R = La, Ce) and RsSb (R = Yb, Y and Sc) compounds were noted to undergo the solid state transformations. 

The RMSb compounds crystallize in eight structure types, for some of them the existence of two polymorphic modifications have been reported (table 63). The crystal structure of EuMSb compounds is always different than the structure of other RMSb compounds. 

The Ce-La constitution diagram [19] is presented in Fig. 6. Thermal and micro structural analysis have revealed that 5, y and P modifications of Ce and La form the continuous sequences of solid solutions [20, 21, 22]. The measured lattice parameters [21] have supported the conclusions that fee polymorphous modifications of Ce and La make up the continuous sequence of solid solutions. In addition, a small deviation from Vegard s law is observed. 

Figure 1 (a) Polymorphic modifications of the same building block, [Fe(CsH4COOH)2] ... 

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