Crystal concomitant

Despite this tremendous versatility, there is still a fundamental limitation for these random copolymers melting point and modulus (stiffness) are inextricably coupled to the density (or percentage short-chain branching from LAO comonomer) as shown in Fig. 1. The same method employed to lower modulus (incorporation of comonomer) results in a thinning of the polyethylene crystals, concomitant with a lowering of the melting point, according to a relationship established by Flory [8],... 

By making use of Volmer s equations some attempts have been made by Becker and Doring (1935), Stranski and Totomanov (1933), and Davey (1993) to explain the Rule in kinetic terms. In doing this, it becomes apparent that the situation is by no means as clear cut as Ostwald might have us believe. Figure 2.11 shows the three possible simultaneous solutions of the nucleation equations which indicate that by careful control of the occurrence domain there may be conditions in which the nucleation rates of two polymorphic forms are equal, and hence their appearance probabilities are nearly equal. Under such conditions we might expect the polymorphs to crystallize concomitantly (see Section 3.3). In other cases, there is a clearer distinction between kinetic and thermodynamic crystallization conditions, and that distinction may be utilized to selectively obtain or prevent the crystallization of a particular polymorph. 

An example of a tt-bonded complex is the remarkable cyanine oxonol system, 3-XVI 3-XVII, for which at least fourteen different polymorphs or solvates have been identified (Etter et al. 1984). Two of these, a gold and a red form (each containing a molecule of CHCI3 solvent per 1 1 complex, and hence true polymorphs) crystallize concomitantly and have been structurally characterized (Etter et al. 1984). Three of these polymorphs are shown in Fig. 3.2. Despite the fact that both of these dye molecules are known to be individually self aggregating (Cash 1981) the two... 

Also worthy of note here is the fact that both a yellow and an orange modification of a 2,2-di(p-nitrophenyl) hydrazine derivative of 6-XVI crystallize concomitantly from dichloromethane, with very similar molecular conformations (Hong et al. 1991). The colour difference vide infra) has been attributed to a difference in molecular packing. 

Figure 3-11. DSC of forms 1-3 of sulfonamide 7 polymorphs. Form 1 crystallizes concomitantly with a minor amount of thermodynamic form 3. Form 2 undergoes phase transition to form 3 at ca. 140°C. Form 3 exhibits a sharp melting endotherm at 159°C as the only phase transition...

CO3) XH2O and Ce202(C03) may also be present. Thermal decomposition causes loss of crystallization water followed by the formation of ill-defined hydroxy- and oxy-species and eventually, at C, Ce02. Discrete intermediates are not seen because of the concomitant oxidation of Ce(III) to Ce(IV). 

Morphology. A crystal is highly organized, and constituent units, which can be atoms, molecules, or ions, are positioned in a three-dimensional periodic pattern called a space lattice. A characteristic crystal shape results from the regular internal stmcture of the soHd with crystal surfaces forming parallel to planes formed by the constituent units. The surfaces (faces) of a crystal may exhibit varying degrees of development, with a concomitant variation in macroscopic appearance. 

Eeactive crystalli tion addresses those operations ia which a reaction occurs to produce a crystallizing solute. The concentration of the solute formed generally is greater than that corresponding to solubiHty. In a subset of systems, the solubiHty is nearly zero and, concomitantly, the supersaturation produced by reaction is large. These are often referred to as precipitation operations, and crystal size distributions from them contain a large fraction of fine crystals. 

It is known the case of i-PP, for which the copolymerization with small amounts of ethylene tends to stabilize the y form [84] for instance, by melt crystallization of a copolymer with 6% by mol of ethylene more than 80% of the crystalline phase is in the y form [85], It is also known that the obtainment of the y form by melt crystallization, is also favored for samples of low molecular mass [86, 87] and for stereoblock fractions [88]. This seems to suggest that, whenever the preferential crystallization of the y-form is observed, there is the concomitant occurrence of a reduction in the polymer of the length of the chain stretches with polypropylene head to tail constitution and isotactic configuration. 

A crystal structure of the C02 derivative of (8), K[Co(salen)( 71-C02)], haso been reported in which the Co—C bond is 1.99 A, the C—O bonds are both equivalent at 1.22 A and the O-C-O angle is 132°.125 Carboxylation of benzylic and allylic chlorides with C02 in THF-HMPA was achieved with (8) electrogenerated by controlled-potential electrolysis,126 in addition to reductive coupling of methyl pyruvate, diethyl ketomalonate and / -tolylcarbodiimide via C—C bond formation. Methyl pyruvate is transformed into diastereomeric tartrates concomitant with oxidation to the divalent Co(salen) and a free-radical mechanism is proposed involving the homolytic cleavage of the Co—C bond. However, reaction with diphenylketene (DPK) suggests an alternative pathway for the reductive coupling of C02-like compounds. 

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