Covalency network formers

It is of interest that the order parallels that of increasing degree of covalent character of the oxide as approximated by the coulombic term Zj(R + J a)2, where Rc and Ra are the ionic radii. The assignment of the terms network formers and breakers is obvious from the discussion above. 

Si-C bonds are rather covalent so that they are not broken upon hydrolysis. This is no more valid with transition metals for which M-C bonds are more ionic. Complexing organic ligands such as acetates or acetylacetonates have then to be used as shown previously. However such groups behave as network modifiers rather than network formers therefore decreasing the mechanical properties of the resulting material. They are usually removed upon calcination in order to obtain oxide particles of controlled size. 

The implication then is that the kinetics of low-temperature oxidation depend on the stmeture of the oxide. This can be considered in terms of network formers and modifiers. A network-forming oxide is one in which covalent bonds connect the atoms in a three-dimensional structure. There is short-range order on the atomic scale but no long-range order. For example, oxygen atoms form tetrahedra around... 

If the drug is covalently bound to an insoluble matrix former (e.g., polymer) via hydrolysable bondings, the latter are more or less rapidly cleaved upon water penetration into the device. The rate of this chemical reaction together with the rate of diffusion of water and drug through the polymeric network determine the overall drug-release rate (2). 

Clathrates are host-guest complexes in which a crystalline cage of the host compound holds the guest molecule by weak intermolecular forces. Often, the cavities of the guest molecule are formed by a network of hydrogen bonds between covalently bound compounds. Powell [126] names them clathrates from the Latin word clathratus , which means enclosed. It is interesting to note that the lattice structure of the host in the clathrate is not its normal crystalline form the former becomes thermodynamically more stable than the latter only by the formation of the host-guest complex. 

The formation of intermolecular cross-links, i.e. covalent bonds between different polymer chains, causes an increase in the average molar mass and eventually combines all of the macromolecules into a three-dimensional insoluble network. Cross-linking can be accomplished in various ways. Several methods rely on reactions of electronically excited pendant groups on the polymer chains, others on reactions of various kinds of reactive species in the ground state that are photogenerated in polymeric systems. Typical of the former reaction type are [2-f-2] cycloadditions that occur in the case of linear polymers bear-... 

As already discussed, coal is a solid, three-dimensional, cross-linked network. Converting it to liquids requires the breaking of covalent bonds and the removal of carbon or the addition of hydrogen. The former method of producing... 

---