Molecular reactive precursor

Both effects can be studied quite independently in molecular clusters adding more and more neutral atoms (often Ar) around the reactive precursor can change the thermal bath environment, only weakly changing the energetics. On the other hand, more polar molecules will gradually change the energetics of the reactive... 

As for any other functional polymer, polymeric betaines are accessible by two different synthetic routes (1) the polymerization of zwitterionic monomers or (2) the zwitterionic functionalization of reactive precursor polymers. Both routes have inherent advantages and disadvantages. The polymerization of the zwitterionic monomers leads to polymers with 100% betaine functionality, but their molecular characterization is difficult for several reasons. For instance, the conformation of the polymers in aqueous solution is very sensitive, not only to the ionic strength but also to the type of added salt, and in the case of polycarbobetaines also to the pH. Furthermore, polymeric zwitterions often exhibit strong interactions with other matter, e.g., chromatographic colmnns. Hence, reliable GPC or HPLC measurements are very difficult to perform, if at all. 

Aromatic polyamides are generally made by low-temperature reactions of aromatic diamines and aromatic diacid chlorides in special solvents such as a 1 3 molar mixture of hexamethylphosphoramide A-methylpyrrolidone, as in reaction (4-50). Intensive stirring is required to attain high molecular weights because the polymer precipitates. These macromolecules are very rigid and rodlike. They form oriented liquid crystalline arrays in solution and require little postspinning orientation to produce extremely strong and stiff fibers. The polymer would not be made in the melt because it is infusible. It must be synthesized and handled in solution, and this requires the use of reactive precursors. 

In light of these constraints, an effective chemical modification for anti-stiction treatments involves the application of a molecular film to the micromachine surface. This is most often accomplished through a process known as self-assembled monolayer (SAM) deposition.f Self-assembled monolayers are molecular films that spontaneously form on a (usually pretreated) surface upon exposure to a reactive precursor molecule. SAM precursors generally consist of three main parts a terminal group, a backbone, and a head group. Fig. 3 shows these parts on a model SAM... 

This problem can be solved by addition of the less-reactive precursor to the solution where the second, more-reactive precursor is partially prehydrolyzed, or equahzation of the reactivity by selection of precursors with different molecular structure, or by using a complexing agent to control the hydrolysis rate. For preparation of aluminum silicates with acidic functionality created by entrapment of tetracoordinated aluminum ions in the silica matrix, the prehydrolysis of the less-reactive silicon alkoxide before addition of aluminum alkoxide suppresses the early formation of Al—OH—A1 bridges and subsequent... 

The sol-gel process uses a liquid reactive precursor material that is converted to the final product by chemical and thermal means. This precursor is prepared to form a colloidal suspension or solution (sol) which goes through a gelling stage (gel) followed by drying and consolidation. The process requires only moderate temperatures, in many cases less than half the conventional refractory-ceramic processing temperatures. It also permits close control of the composition and structure on the molecular level.P ... 

Copolymers with a blocky structure have been designed as compatibilizers for immiscible polymer blends. More recently, most research effort has been devoted to reactive compatibilization, with special attention paid to the molecular characteristics of the reactive precursors of the compatibilizer, such as molecular weight, content, and distribution of the reactive groups and kinetics of the interfacial reaction. The interplay between these factors and the various processing factors are important. 

The second approach, sequential additions of different alkoxides to partially hydrolyzed precursors, was invented by Thomas [166] and popularized by Yoldas [167,168]. In this process alkoxides are added in the reverse order of their respective reactivities (least reactive precursors first) and a partial hydrolysis step is performed after each addition. The idea is that the newly added, unhydrolyzed alkoxides will condense with partially hydrolyzed sites on the polymeric species formed by the preceding hydrolysis steps (heterocondensation) rather than with themselves (homocondensation). Unlike the previous method where molecular-level homogeneity is predicted, the homogeneity of the product will depend on the size of the polymeric species to which the last component is added. Two sequential hydrolysis schemes to prepare a multicomponent borosilicate glass composition are shown in Fig. 66 where alkali and alkaline earth metals are introduced as salts or alkoxides. 

Route B corresponds to the use of an initial solution containing molecular reactive sol-gel precursors combined with molecular surfactants or amphiphilic block copolymers. It allows generating mesoporous microspheres via evaporation-induced self-assembly (EISA) [15] or evaporation-induced micelle packing... 

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