Radicals propagating

One contributing factor, which seems to have been largely ignored, is that the ring closed radical (in many cases a primary alkyl radical) is likely to be much more reactive towards double bonds than the allyl radical propagating species. This species will also have a different propensity for degradative chain transfer (a particular problem with allylamines and related monomers - see 6.2.6.4) and other processes which complicate polymerizations of the monoencs. 

Ring-opening provides a thiyl radical propagating species. Although the polymers have a double bond on the backbone there is little or no crosslinking (Scheme 4.34, Scheme 4.35). There is, however, evidence of reversible addition... 

The depth of light energy penetration for a photo cross-linking (photo-initiated free-radical propagation reaction) process can be calculated as... 

Monomer addition under radical propagation conditions leads to mainly an atactic configuration. As a consequence, radical polymerisations of asymmetric vinyl polymers usually lead to amorphous materials. However, if the substituent is small enough to enter into the crystal cell, atactic vinyl polymers can crystallise (an example is poly(vinyl fluoride)). 

When free radicals appear in a system, two basically different types of reactions are possible reactions with conservation of free valence and reactions in which radicals (or atoms) interact with each other without conservation of free valence. For example, the peroxyl radical propagates the chain by the reaction... 

In the initial period the oxidation of hydrocarbon RH proceeds as a chain reaction with one limiting step of chain propagation, namely reaction R02 + RH. The rate of the reaction is determined only by the activity and the concentration of peroxyl radicals. As soon as the oxidation products (hydroperoxide, alcohol, ketone, etc.) accumulate, the peroxyl radicals react with these products. As a result, the peroxyl radicals formed from RH (R02 ) are replaced by other free radicals. Thus, the oxidation of hydrocarbon in the presence of produced and oxidized intermediates is performed in co-oxidation with complex composition of free radicals propagating the chain [4], A few examples are given below. 

Another factor complicating the situation in composition of peroxyl radicals propagating chain oxidation of alcohol is the production of carbonyl compounds due to alcohol oxidation. As a result of alcohol oxidation, ketones are formed from the secondary alcohol oxidation and aldehydes from the primary alcohols [8,9], Hydroperoxide radicals are added to carbonyl compounds with the formation of alkylhydroxyperoxyl radical. This addition is reversible. 

The peroxide bond in the product is weak and readily cleaves to form additional radicals. Because more radicals are formed, any further reaction proceeds by a chain reaction, termed radical propagation, until all the petrol has been consumed. 

In the propagation steps shown above, the radical propagates a further radical by causing fission of a single bond in the substrate. Many important radical reactions actually involve compounds with double bonds as substrates, and the n bond is cleaved during the radical addition reaction. 

Finally, when we are running out of cyclohexane, the process terminates by the interaction of two radical species, e.g. two chlorine atoms, two cyclohexyl radicals, or one of each species. The combination of two chlorine atoms is probably the least likely of the termination steps, since the Cl-Cl bond would be the weakest of those possible, and it was light-induced fission of this bond that started off the radical reaction. Of course, once we have formed cyclohexyl chloride, there is no reason why this should not itself get drawn into the radical propagation steps, resulting in various dichlorocyclohexane products, or indeed polychlorinated compounds. Chlorination of an alkane will give many different products, even when the amount of chlorine used is limited to molar ratios, and in the laboratory it is not going to be a particularly useful process. 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

Anionic propagating centers are transformed into radical centers in the presence of a second monomer that undergoes radical propagation, by reaction with trimethyl lead chloride... 

The effect of temperature on r is not large, since activation energies for radical propagation are relatively small and, more significantly, fall in a narrow range such that En Eu is less than 10 kJ mol-1 for most pairs of monomers. However, temperature does have an effect, since E 2 — E is not zero. An increase in temperature results in a less selective copolymerization as the two monomer reactivity ratios of a comonomer pair each tend toward unity with decreasing preference of either radical for either monomer. Temperature has the greatest... 

More generally, low-temperature combustion relies heavily on the tendency of radical propagation to yield chain-branching reactions, a phenomenon first explored... 

From these simple examples we see that chain-radical-propagated combustion processes frequently have very comphcated kinetics. 

The present calculations confirm Thomas conclusion that termination via Reaction 13 must be relatively slow. A significant fraction of the CH300 radicals propagate rather than terminate chains under many conditions. 

The results are surprising since they apparently infer that monomer radical propagation starts at one end of the template and continues to the other end of the template when termination occurs. According to the authors this needs not necessarily be the case. The explanation is illustrated in Figure 4.4. 

Figure 4.7. A three-step radical propagation sequence (top to bottom) that has two serious possible side reactions.

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