Propagating effect, chain

This evidence indicates that the principal chain-breakers in these solutions are free ions and that the 1 1 complexes which are formed are much less ionised and are much less effective chain-breakers than the compounds (probably 2 1 and 1 2 complexes) which are prevalent on either side of the neutralisation point. This matter is discussed further in Example 5 below. The authors concluded from their results that the propagating species is also a free ion rather than an ion-pair. However, whilst this may be true, it does not follow from this evidence, since the cation in an ion-pair may well be able to react with a free anion. 

Radiation-Induced Cross-Linking in the Presence of CTFE/Butadiene Mixture. On the basis of the results mentioned in the previous section, it is concluded that in the irradiation of polyethylenes in the presence of CTFE the polyethylenes are mainly cross-linked through the addition reaction of the unsaturated groups contained in the main and the side chains of the polymers to the propagating graft chain radical of CTFE. Therefore, the radiation-induced cross-linking of polyethylene is expected to be accelerated by the presence of the mixture of CTFE and a diene monomer effectively than the presence of pure CTFE. 

This produces a rise in the concentration of active centers and a corresponding increase in the propagation rate. Chains produced at this stage are longer, and this leads to a broadening of the molar mass distribution. The term gel effect is widely used to describe this effect, although no gel is actually formed in the system. The effect is also called the Trommsdorff effect (see Chapter 5). 

In recent years, considerable work has been devoted to polymerization reactions of vinyl monomers at higher conversions which permit useful quantitative interpretation of the results. A useful review of studies of the gel effect, chain transfer reactions, and new theoretical postulates and studies at elevated conversions has been presented by Gladyshev and Rafikov (24). This accumulated work has demonstrated the effects of conditions at elevated conversions not only on the termination rate constant, but on initiator efficiency, propagation rate constants, and therefore, the concentration of macroradicals. A rigorous quantitative theory, however, has not yet been developed. 

In addition to these difficulties, water is not the best diagnostic additive since it can terminate both carbonium ion and carbanion propagations effectively, although apparently it is specific for ionic as opposed to free radical chain reactions. 

As in the case of thiiranes, also in the cationic polymerization of 4-mem-bered cyclic sulfides, thietanes, chain transfer to polymer effectively competes with propagation. Intramolecular chain transfer, leading to formation of branched structures is well documented in these systems, because branched ions has been observed directly by H NMR [160] ... 

Eqs. (1) to (3) indicate that conversion studies under conditions where thermal polymerization prevails can only yield e and the product nyo, whereas the photon-induced reaction provides information on the product nq. To disentangle chain initiation and chain propagation effects an independent determination of the kinetic chain length is required. 

A similar method for cyclic ketone synthesis via an intramolecular carbonyl ad-dition/elimination strategy has been reported by Kim and Jon, who used acylsul-fides and acylselenides as radical acceptors (Scheme 4-21) [41]. As has previously been observed in the cases of sulfonyl oxime ethers which liberate sulfonyl radicals, thiyl and selenenyl radicals react with ditin to propagate the chain. Generally acylselenides are more efficient substrates than acylsulfides due to the better leaving ability of the phenylseleno group. As shown in the third example in Scheme 4-21, a tandem cyclization leading to a tricyclic ketone has also been effected. 

One of the most distinguishable features of radical polymerisation is its tolerance to water, relative to the ionic counterparts, however the effective polymerization should be performed conventionally under stringent conditions without protonic or basic impurities to insure effective chain propagation and therefore desired polymer growth without unnecessary inhibition and premature termination. Because of their unique features, suspension, dispersion... 

None of the above reactions terminates the kinetic chain. All are treated as chain transfer reactions since there is reinitiation of new propagating chains. The relative extents of the various termination reactions depend on the monomer, identity and concentrations of the initiator components, temperature, and other reaction conditions. There are considerable differences in the efficiencies of chain transfer to different Group I-III metal components for example, diethylzinc is much more effective in chain transfer compared to triethylaluminum. Molecular hydrogen is a highly effective chain-transfer agent and is commonly used for molecular weight control in the industrial production of polypropylene. 

Conductivity studies of quaternary ammonium salts and polymerization systems, experiments with additions of neutral salts, and investigations on the counterion effect make it possible to state that free ions and ion pairs serve as active centers in the polymerization of nitrogen-containing cycles and also to separate the contributions of the former and the latter to the effective chain propagation rate constant (see Fig. 8). 

That antiknocks are effective in retarding liquid phase oxidations that are propagated by chain reactions is shown by the fact that the rate of oxidation of 5 cc. of benzaldehyde was reduced from 1.5 cc. of oxygen absorbed per minute to 0.005 cc. of oxygen absorbed per minute by the addition of one drop of lead tetraethyl to 5 cc. of aldehyde.158 The effect of oxidation inhibitors, as phenol or aniline on gaseous oxidation of fuels is similar to their effect on liquid phase oxidation of benzaldehyde.17... 

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