Topochemical condensation

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. 

Despite acridine and diphenylacetic acid being achiral molecules, it was found that these two compounds formed a chiral cocrystal on spontaneous crystallization, and further irradiation of this caused photodecarboxylation and then enantio-selective radical coupling to give a chiral condensation product (Scheme 35) [31]. This kind of absolute asymmetric photodecarboxylation is not a topochemical reaction but is accompanied by gradual decomposition of the initial crystal structure as the reaction proceeds. This pathway is different from the four examples of [2 + 2] photocycloadditions described above. 

By analogy with classical systems, the new systems allow the possibility of topochemical reaction (Fig. 5). But their functional groups also permit them to take part in other reactions such as oxidative coupling (chem. or electrochem.) or as dienophiles in addition reactions with dienes, e.g. polyacetylenes and condensed hydrocarbons such as perylene (mono- or disubstituted) ... 

The molecular weight of the condensed silane is also influenced by the same factors. Another factor that is important for the silane coupling effect is the molecular architecture of the siloxane networks. Caged structures and ladder structures are the extreme cases of the different siloxane structures (11,12). The solution pH, topochemical effects of the solid surface, the structure of the organofunctional group, and the concentration of the silane are all believed to influence siloxane structure, although no detailed study has yet been reported. 

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