Butadiene radical attack

Chain propagation occurs by the growing chain free radical attacking either the butadiene or styrene monomer. The active radical chain can react with mercaptan to form a new mercaptyl radical and a terminated chain. The mercaptyl radical then can initiate an additional chain. The molecular weight of the chain P can be controlled by the concentration of mercaptan via this chain transfer mechanism. 

Recently, molecular orbital calculations have been performed on the parent heterocycle.The results of the calculations indicate C-5 and C-7 should be the most reactive sites to electrophilic, nucleophilic, and free-radical attack. The degree of bond fixation evident from bond length calculations suggested that these positions should also act as the termini of a s-butadiene fragment rendering the molecule liable to Diels-Alder-type reactions. 

This is the initiation step of a free radical polymerization process. Free radical polymerizations, like all addition polymerization reactions, produce head to tail polymerization that is, the growing end of the polymer is the most stable possible radical. In the case of 1,3-butadiene the alkoxy radical attacks an end carbon and not an internal carbon. This is because attack of an end carbon forms a resonance stabilized free radical whereas attack of an internal carbon forms a radical with no such stabilization. Hence, the end carbon will be attacked by the alkoxy radical to form the more stcible species. This is shown as ... 

Addition of rubbery materials, however, does improve the impact resistance of polystyrene. This is therefore done extensively. The most common rubbers used for this purpose are butadiene-styrene copolymers. Some butadiene homopolymers are also used, but to a lesser extent. The high-impact polystyrene is presently prepared by dissolving the rubber in a styrene monomer and then polymerizing the styrene. This polymerization is either done in bulk or in suspension. The product contains styrene-butadiene rubber, styrene homopolymer, and a considerable portion of styrene-graft copolymer that forms when polystyrene radicals attack the rubber molecules. The product has very enhanced impact resistance. 

Free-radical polymerizations of 1,3-butadiene usually result in polymers with 78-82% of 1,4-type placement and 18-22% of 1,2-adducts. The ratio of 1,4 to 1,2 adducts is independent of the temperature of polymerization. Moreover, this ratio is obtained in polymerizations that are carried out in bulk and in emulsion. The ratio of trans-1,4 to cis-1,4 tends to decrease, however, as the temperature of the reaction decreases. Polybutadiene polymers formed by free-radical mechanism are branched because the residual unsaturations in the polymeric chains are subjects to free-radical attacks ... 

A common way of representing the situation schematically is indicated in (XXIII) for butadiene. Bond orders are indicated on the bonds and free valences by arrows. It is clear that butadiene has a good deal more residual bonding capacity on its terminal atoms, and this is consistent with the fact that free radical attack on butadiene occurs predominantly on the end atoms. Other examples of correlation between free valence and rate of free radical addition have been reported. A plot of rate data for methyl... 

Chain propagation occurs by the growing-chain free radical attacking either butadiene or styrene monomer. The active radical chain can react with mercaptan to form a new... 

The dependence of relative rates in radical addition reactions on the nucleophilicity of the attacking radical has also been demonstrated by Minisci and coworkers (Table 7)17. The evaluation of relative rate constants was in this case based on the product analysis in reactions, in which substituted alkyl radicals were first generated by oxidative decomposition of diacyl peroxides, then added to a mixture of two alkenes, one of them the diene. The final products were obtained by oxidation of the intermediate allyl radicals to cations which were trapped with methanol. The data for the acrylonitrile-butadiene... 

A second measure of reactivity is the relative rates of attack of a hydrocarbon by the same alkylperoxy radical. The three middle columns of figures in Table VII give such relative rates toward the butadiene peroxy and tetralylperoxy radicals. The 50° and 30°C. results are consistent. The 65°C. results are not as good reasons were considered above, as well as the extensive similar summary by Russell and Williamson (32). 

The monomer addition scheme, shown at the top, requires an initiator which is capable of removing a hydrogen atom from the allylic position of the butadiene, resonance stabilization of the radical from AIBN does not permit this initiator to effect this reaction while benzoyl peroxide is capable of reaction to remove a hydrogen atom and initiate the reaction. On the other hand the polymeric radical addition scheme requires that homopolymerization of the monomer be initiated and this macroradical then attack the polymer and lead to the formation of the graft copolymer. Huang and Sundberg explain that the reactivity of the monomer... 

The rate of H addition to 1-pentene is roughly equal to the addition to 1-butene, of the H addition to cis and trans isomers of 2-pentene as to cis-and frans-butenes. Cycloalkenes add a H atom in a similar way to simple alkenes of comparable structure. H attacks either the terminal or the internal C atom of 1,3-butadiene the first way predominates, probably due to allylic or hyperconjugative stabilization of the generated radical. 

Radical reactions with the n bond of vinyl monomers are not nearly as selective as ionic attack, and free-radical initiators cause the polymerization of nearly all vinyl and vinylidene monomers. (Some of these polymerizations are not elTicient because of side reactions. Propylene is acase in point as described in Section 6.8.5.) Resonance stabilization occurs to some extent with most vinyl monomers but it is important in radical polymerizations only when the monomers contain conjugated C—C double bonds as in styrene, 1,3-butadiene, and similar molecules ... 

We must realize that polar effects are superimposed on effects due to delocalization of the odd electron. Styrene and butadiene, for example, are highly reactive toward any radical since the transition state contains an incipient benzylic or allylic free radical. This high reactivity is modified—enhanced or lowered—by the demands of the particular attacking radical. 

In one of the earliest investigations of regioselectivity in radical addition reactions to polyenes, the addition of hydrogen bromide to 1,3-butadiene was observed to yield mainly the 1,4-addition product in the presence of peroxides. The preference for attack at the Cl position of 1,3-butadiene has subsequently been observed for a large number of radicalsOnly for the addition of the methyl radical has the ratio of addition to the Cl vs C2 actually been measured. A value of Cl C2 = 1.0 0.01 has been found . For all other cases, products arising from attack at C2 have not been reported. This is also true for radical addition to 2,3-dimethyl-l,3-butadiene . Additions to 1,3-pentadiene occur predominantly at the Cl position due to the steric effect exerted by the terminal methyl This is a reflection of the reduced... 

The oxidative process is driven either by oxygen itself or by any source of free radicals. If a polymer backbone is attacked, leading to either a polymeric carbon or oxygen radical, backbone cleavage is possible. For polyethylene, polypropylene and butadiene- or isoprene-containing polymers, this may be accompanied by elimination of formaldehyde or acetaldehyde. For styrene-containing polymers, formaldehyde and benzaldehyde are products from the cleavage [74], Such reactions could take place either in the bulk oil phase or in deposits in which the polymer is physically trapped. 

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