Polymeric crystallization, structural requirements

Structurally, plastomers straddle the property range between elastomers and plastics. Plastomers inherently contain some level of crystallinity due to the predominant monomer in a crystalline sequence within the polymer chains. The most common type of this residual crystallinity is ethylene (for ethylene-predominant plastomers or E-plastomers) or isotactic propylene in meso (or m) sequences (for propylene-predominant plastomers or P-plastomers). Uninterrupted sequences of these monomers crystallize into periodic strucmres, which form crystalline lamellae. Plastomers contain in addition at least one monomer, which interrupts this sequencing of crystalline mers. This may be a monomer too large to fit into the crystal lattice. An example is the incorporation of 1-octene into a polyethylene chain. The residual hexyl side chain provides a site for the dislocation of the periodic structure required for crystals to be formed. Another example would be the incorporation of a stereo error in the insertion of propylene. Thus, a propylene insertion with an r dyad leads similarly to a dislocation in the periodic structure required for the formation of an iPP crystal. In uniformly back-mixed polymerization processes, with a single discrete polymerization catalyst, the incorporation of these intermptions is statistical and controlled by the kinetics of the polymerization process. These statistics are known as reactivity ratios. 

A most interesting extension of this type of reaction was performed by Addadi and Lahav (175). Their aim was to obtain chiral polymers by performing die reaction in a crystal of chiral structure. They employed monomers 103. The initial experiments were with a chiral resolved 103 where R1 is (R)- or ( -sec-butyl and R2 is C2H3. This material indeed crystallizes in the required structure, and yields photodimers and polymers with the expected stereochemistry, and with quantitative diastereomeric yield. It was possible to establish that the asymmetric induction was due essentially only to the chirality of the crystal structure and not to direct influences of the sec-butyl. Subsequently they were able, using sophisticated crystal engineering, to obtain chiral crystals from nonchiral 103, and from them dimers and polymers with high, probably quantitative enantiomeric yields. This may be described as an absolute asymmetric polymerization. 

All attempts at crystallization of these species were unsuccessful the only reported Ln crystal structures with these dipicolinate derivatives are of polymeric species obtained under hydrothermal conditions (168), which cannot be directly related to our solution results. Although more thorough analyses will be required for the construction of accurate binding models, we have established that the Pyr nitrogen in dipicolinate and related chelators can play an important part in dictating both lanthanide... 

A munber of surface-confined reactions require a highly ordered prearrangement of the reactive species. The topochemical polymerization of diacetylenes is such a text book example. A critical aspect is the ordering of the diacetylene monomers, both with respect to the distance and the orientation. On the basis of the 3D crystal structures of numerous diacetylene monomers... 

X-ray diffraction techniques are the only way of determining the crystal structure of natural and synthetic polymers, although the x-ray data itself obtained from a crystalline polymeric fiber or film is not sufficient to allow complete refinement of the structure. Conformational analysis and electron diffraction represent complementary methods which will facilitate the determination of the structure. The necessary requirements for the x-ray approach are crystallinity and orientation. X-ray data cannot be Obtained from an amorphous sample which means that a noncrystalline polymeric material must be treated in order to induce or improve crystallinity. Some polymers, such as cellulose andchitin, are crystalline and oriented in the native state.(1 )... 

Many membrane separation applications have already benefitted from membrane modification strategies. Chapter 10 describes how bespoke polymeric membranes have been used to improve the crystallization of biomolecules. Membrane crystallization allows through a careful control of the process parameters the production of crystals with controlled shape, size, size distribution, and polymorphism. Further research is required to provide comprehensive understanding of the complex relationships between membrane process parameters and crystal structure. The control of product polymorphism will continue to be important in the pharmaceutical industry, which, as the range of drugs and their specificity increase, will reqnire improved... 

Fig. 2. Crystal structures of PTS showing the r-stacking of aromatic rings to establish the required translational distance for a topochemically controlled polymerization.

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