Chain topochemical

Based on the topochemical description of the structure of M02BC = MoB MoC, the typieal B-chain element within the MoB unit becomes eonceivable. 

Later, Tieke reported the UV- and y-irradiation polymerization of butadiene derivatives crystallized in perovskite-type layer structures [21,22]. He reported the solid-state polymerization of butadienes containing aminomethyl groups as pendant substituents that form layered perovskite halide salts to yield erythro-diisotactic 1,4-trans polymers. Interestingly, Tieke and his coworker determined the crystal structure of the polymerized compounds of some derivatives by X-ray diffraction [23,24]. From comparative X-ray studies of monomeric and polymeric crystals, a contraction of the lattice constant parallel to the polymer chain direction by approximately 8% is evident. Both the carboxylic acid and aminomethyl substituent groups are in an isotactic arrangement, resulting in diisotactic polymer chains. He also referred to the y-radiation polymerization of molecular crystals of the sorbic acid derivatives with a long alkyl chain as the N-substituent [25]. More recently, Schlitter and Beck reported the solid-state polymerization of lithium sorbate [26]. However, the details of topochemical polymerization of 1,3-diene monomers were not revealed until very recently. 

Figure 1 summarizes the chemical structures of the topochemically polymerizable 1,3-diene monomers providing stereoregular 1,4-trans polymer (Scheme 6) [ 16]. Most of the polymerizable monomers contain benzyl, naphthylmethyl, and long alkyl-chain substituents in their chemical structures. The (ZyZ)-, (E,Z)-, and ( , )-muconic and sorbic acids as well as the other diene carboxylic acids are used as the ester, amide, and ammonium derivatives. In contrast to this, the carboxylic acids themselves have crystal structures unfavorable for polymerization while they undergo [2-1-2] photodimerization, as has already been described in the preceding sections.

Molecular alignment in the monomer crystals is controlled by several intermolecular interactions, such as strong and weak hydrogen bonds, leading to the formation of various types of stereoregular polymers via a topochemical polymerization process. This approach to the stereocontrol of polymers differs from other conventional ways in the control of the propagating chain end using catalysts or additives in solution polymerization. 

The topochemical polymerization of 1,3-diene monomers based on polymer crystal engineering can be used not only for tacticity but also for the other chain structures such as molecular weight [ 102], ladder [84] or sheet [ 103] structures, and also polymer layer structures using intercalation reactions [ 104-107]. Some mechanical and structural properties have already been revealed with well-defined and highly or partly crystalline polymers [ 108-111 ]. A totally solvent-free system for the synthesis of layered polymer crystals was also reported [112]. 

In the crystal structure of the polymer phase (Fig. 17a), the polymer chains are aligned along the c-axis and the distance (3.71 A) between the centres of adjacent cyclobutane and pyrazine rings corresponds to half the c-axis repeat of the unit cell. For comparison between the monomer and polymer structures, an overlay plot of these structures is shown in Fig. 17b. It is clear that the solid-state reaction is associated with only very small atomic displacements at the site of the [2-1-2] photocyclization reaction (the displacement of the carbon atoms of the C=C double bonds of monomer molecules on forming the cyclobutane ring of the polymer is only ca. 0.8 A for one pair of carbon atoms and ca. 1.6 A for the other pair). Such small displacements are completely in accord with the assignment of this solid-state reaction as a topochemical transformation [124—127] (in which the crystal structure of the reactant monomer phase imposes geometric control on the pathway of the... 

At present, it is common knowledge that not only the photoreactivity, but also the stereochemistry, of the photoproduct is predictable from crystallographic information of starting olefin substrates. This ability of olefinic crystals to dimerize has been widely applied to the topochemical photocycloaddition polymerization of conjugated diolefinic compounds, so called "four-center type photopolymerizations" (7,8). All the photopolymerizable diolefin crystals are related to the center of symmetry mode (centrosymmetric -type crystal) and thus give polymers having cyclobutanes with a 1,3-trans configuration in the main chain on irradiation. 

In an effort to better characterize the topochemical relationship between the hydrophobic pocket in the chymotrypsin binding site and the aromatic side chain in the substrate, 3-azidophenylalanine has been prepared.1161 Conversion of 4-aminophenylalanine (21) into 3-azidophenylalanine (23) was carried out via 4-chloro-3-nitrophenyl intermediate 22 as outlined in Scheme 6. 

The fact that the polyreaction of diacetylenes is topochemically controlled is especially well documented by the polymerization behavior of the sulfolipid (22)23 . (22) forms two condensed phases when spread on an acidic subphase at elevated temperatures (Fig. 10). UV initiated polymerization can only be carried out at low surface pressures in the first condensed phase, where the molecules are less densely packed. Apparently, in the second phase at surface pressures from 20 to 50 mN/m the packing of the diyne groups is either too tight to permit a topochemical polymerization or a vertical shift of the molecules at the gas/water interface causes a transition from head packing to chain packing (Fig. 10), thus preventing the formation of polymer. 

Fig. 10. Surface pressure/area isotherm of (22) at 41 °C, pH 2 23). Topochemical polymerization is impossible in the more densely packed phase (B) where most probably a change from head packing to chain packing of the lipid molecules has taken place...

Due to the topochemical restrictions of diacetylene polymerization, diacetylenic lipids are solely polymerizable in the solid—analogous phase. During the polyreaction an area contraction occurs leading to a denser packing of the alkyl chains. In addition to surface pressure/area isotherms the polymerization behavior of diacetylenic lipids containing mixed films give information about the miscibility of the components forming the monolayer ... 

For explanation of the process of nitration two theories were advanced. Accdg to the 1st of these, the reaction proceeds in topochemical fashion, whereby the nitrating reagent diffuses progressively from the outside to the interior of the fiber, so that the chains on the surface are nitrated first. Accdg to the 2nd theory, the nitrating reagent penetrates uniformly in all parts of the cellulose fiber, and the reaction proceeds quickly thruout the whole micellar system in... 

The phase (or rather reaction ) boundaries of the dimer and chain polymer phases have not yet been determined, and only the reaction coordinates for the two experiments reported are shown in Fig. 18. Also, for C70 the drawing of a reaction map is complicated by the topochemical requirements for polymerization described above. Dimers can be formed in both fee and hep crystals, but ordered chain structures can only form in hep crystals, and different initial structures thus probably also lead to different final structures. Although it has been reported that initially hep C70 reverts to fee after high-pressure treatment (see above), it is not known which of these two structural phases is more stable under pressure and whether a change in the stacking sequence can be induced directly by pressure and/or temperature. 

The topochemical differences in the physical environment of dissolved and crystallized active centers explain easily the difference in reactivity ratios between the two phases. The gain in free energy arising from immediate crystallization of growing chain ends enhances the incorporation of trioxane into the crystalline copolymer. Simultaneous crystallization is considered an important driving force in copolymerization as well as in the homopolymerization of trioxane. On the other hand, dioxolane units do not fit the crystal lattice of polyoxymethylene and reduce the crystallinity of the polymer. This impedes the incorporation of dioxolane units into the crystalline copolymer. 

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