Labile atom

NH4, and in hydroxy compounds such as alcohols and sugars non-labile hydrogen atoms are found in benzene, and in the phos-phinate ion. H2P02. The non-labile atoms in the phosphinate ion... 

The photolysis of labile atoms or groups has also been described. The photobromination of polystyrene occurs essentially in a-position... 

Thus, we may come to a conclusion that dyeing of polymers, containing free OH-groups, by active dyes considerably improves their thermal and photooxidative stability this is connected with substitution of hydrogen labile atom by more volumetric molecule. 

The termination reactions that are more important in polymer production are combination (or coupling) and disproportionation. In termination by combination, two growing polymer chains react with the mutual destruction of growth activity (Equation 2.15), while in disproportionation a labile atom (usually hydrogen) is transferred from one polymer radical to another (Equation 2.16). 

Two free radicals react exchanging a labile atom to give two molecules. 

It was suggested that a basic rule of thumb can be applied to determine which termination reaction predominates in a typical homopolymerization. Thus, polymerizations of 1,1-disubstituted olefins are likely to terminate by disproportionation because of steric effects. Polymerization of other vinyl monomers, however, favor terminations by combination unless they contain particularly labile atoms for transferring. Higher activation energies are usually required for termination reactions by disproportionation. This means that terminations by combination should predominate at lower temperatures. 

The ease with which chain transferring takes place depends upon the bond strength between the labile atoms that are abstracted and the rest of the molecule to which they are attached. For instance, chain transferring in methyl methacrylate polymerization to the solvent occurs in the following order ... 

Examples of molecules that have particularly labile atoms and that contribute readily to chain transferring are mercaptans and halogen compounds, like chloroform, carbon tetrachloride, etc. 

T Chain-transfer agent T can represent the monomer (chain transfer to monomer), the same polymer chain (backbiting), a different polymer chain (branching), or any other molecule present in the system with a labile atom (e.g., surfactant, solvent, etc.). Even water is not inert and can actively... 

Termination, that is the final step in which the propagating chain molecules are transformed to dead molecules by the process of recombination (combination of two active chain ends) or disproportionation (transfer of a labile atom or group from one active chain to another chain). 

In this process, the catalyst produces a cation, usually an ion, which reacts with the n-bond of the electron donating substituted monomer to form the corresponding carbo-cation. Chain propagation is started by the addition of subsequent monomer molecules to this carbo-cation. The chain termination occurs by the transfer of labile atoms or substituents from the catalyst, monomer or other species present in the reaction medium. The whole sequence of the polymerisation reaction is shown in Fig. 1.4. 

And, finely, the third reactivity central in the molecular of PAM is a labile atom of hydrogen of triarylmethane grouping. Availability of this atom allows us to assume about possible inhibition of radical processes because of isolation this hydrogen atom according to the following ... 

Modifiers are species that contain a labile atom (most usually hydrogen or halogen) which can be abstracted by the propagating chain. 

The replacement of chlorine labile atoms by acetoxy groups in PVC has been the subject of several studies. Lewis et al studied this reaction in the presence of crown ethers at 46 °C in the absence of such compounds no substitution is observed, even after three days. When the reaction is carried out in the presence of 18-crown-6,5% of the chlorine atoms are replaced by acetoxy groups, which is far above the usual content of PVC in labile chlorines (ca. 0.1%) it appears that the crown ether is absolutely necessary to produce free active acetate in solution. 

The pair of hexenes 5 and 6 does not interconvert under normal conditions of temperature and pressure. However, the pair of hexenones 7 and 8 is tautomeric, and representations and corresponding search systems must take this into account in order to guarantee finding one when querying for the other. Many systems cannot do this because either they do not consider tautomers or they do not allow carbon atoms in the tautomeric system involving labile atoms and bonds. 

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