Atomic chain formation

The reverse reaction (ion formation) can occur in two ways internally, by attack of the penultimate polymer oxygen atom, or externally, by attack of a monomer oxygen atom (chain growth). The external process is about 10 times slower than the internal process in bulk THF (1). Since ion formation is a slow process compared to ion chain growth, chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerization process. 

For polymer containing labile hydrogen atoms, chain peroxidation can lead to hydroperoxide formation. The following reactions are expected to take place. 

Fig. 3.1. Formation of IV-atom chain from cyclic chain by breaking bond between n = 0 and N — 1 atoms.

Fig. 3.2. Schematic representation of chemisorption, (a) Before and (b) after formation of /3a-bond between a-atom and n = 0 atom of IV-atom chain.

Br atom which leads to the formation of HBr in steps (ii) and (iii) is also required to continue the cycle. These self-propagating steps are likely to continue through repeated generation of chains of H and Br atoms. Thus, for any initial Br atom, the formation of a large number of HBr would result. 

Confirmation of this hypothesis is obtained by the X-ray examination of soaps and fatty esters by Shearer, Piper, Grindley and Muller (J.O.S. cxxiil. 2043, 1923) (see also Friedrich, Physikal. Zeit. xiv. 317, 1913, De Broglie, G.R. olxxvi 738,1923, Becker and Jahnke, Z. Phys. Ohem. xcix. 242, 1923), who have shown that two types of chain formation occur in carbon compounds in accordance with our preconceived views on the tetrahedral orientation of the valencies of the carbon atom. The spacing of the planes in these two types is indicated in the following diagram ... 

Of the elements which participate in catenation (chain formation) in their elementary states, only carbon retains the property in its compounds to any great extent. The relatively high strength of the single covalent bond between two carbon atoms gives rise to such a large... 

The reaction between Br2 and H2 needs a somewhat elevated temperature because of the weaker reactivity of Br atoms. Chain termination is mostly by Br atom recombination, giving a VI rate dependence. The Repression for quantum yield of HBr formation hbt can be set up from steady state approximation ... 

Similar results are observed in the intramolecular double silylation of alkynes.59cd Both terminal and internal alkynes lead to exocyclic olefin formation. Interestingly, the reaction is not successful for internal alkynes tethered to a disilanyl functionality by a four atom chain, but is accomplished when four atoms link a disilanyl to a terminal alkyne. Internal alkynes with ester or olefin in conjugation with the C-C triple bond undergo chemoselective double silylation, and alkynes with substituents in the tether are also good substrates for the reaction. 

Ethers are susceptible to attack by halogen atoms and radicals, and for this reason they are not good solvents for radical reactions. In fact, ethers are potentially hazardous chemicals, because in the presence of atmospheric oxygen radical-chain formation of peroxides occurs, and peroxides are unstable, explosion-prone compounds. This process is called autoxidation and occurs not only with ethers but with many aldehydes and hydrocarbons. The reaction may be generalized in terms of the following steps involving initiation... 

However, this reaction would lead to the chain formation of water in the H atom initiated ()2-H2 reaction, which is not found (88) except at low oxygen concentrations in systems at low pressures (20). Callear and Robb present evidence that the chain propagating step under these extreme conditions is... 

The reaction occurs rapidly at alkaline pH (7—10), with higher pH values resulting in better yields due to faster Schiff base formation. To ensure complete conversion of available aldehydes to amines, add the ammonia or diamine compound to the reaction in at least a 10-fold molar excess over the expected number of formyl groups present, Diamines that are commonly used for this process include ethylene diamine, diamino-dipropylamine (3,3 -iminobispropylamine), 1,6-diaminohexane, and the Jeffamine derivative EDR-148 containing a hydrophilic, 10-atom chain (Texaco Chemical Co.). 

These workers also showed that polysilanes with nitrogen atoms in the sidechains produced much higher semiconductivity, up to 10-2 Scm-1 (Table l)67. The mechanism is believed to involve oxidation at the nitrogen atoms with formation of Ij, followed by delocalization of the positive charges from the nitrogen atoms to the polysilane chain. 

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