Hydrogen abstraction chain transfer reactions

Intramolecular hydrogen abstraction by primary alkyl radicals from the Si-H moiety has been reported as a key step in several unimolecular chain transfer reactions.59,60 In particular, the 1,5-hydrogen transfer of radicals 14-17 [Eq. (5)], generated from the corresponding iodides, was studied in... 

If hydrogen abstraction from the zinc salt is fast enough to compete with that from the hydrocarbon substrate, then Scheme 2 amounts to a chain-transfer reaction similar to that illustrated in Reaction 9 for phenolic inhibition. Also, an alternative termination via the peroxyalkyl-ester radical (X) is conceivable since this radical might be expected to cyclize... 

The chain transfer reaction played an important role, particularly because of abstraction of the active hydrogen at a-carbon from the allyl group. Moreover, unreacted double bonds were present in the copolymer obtained. The influence of chain transfer reaction could be diminished by applying multimonomers which do not contain allyl groups. This was shown in the example of copolymerization of multimethacrylate prepared by esterification of poly(2-hydroxyethyl methacrylate) with methacryloyl chloride. Copolymerization of the multimethacrylate with vinyl monomers such as styrene or acrylonitrile can be represented by the reaction ... 

Thus, the most direct route to chain-carrying, tertiary butyl carbonium Ions is offered in isobutene-isobutane alkylation (Equation I). When initiating with either a linear butene or propylene, a second step is necessary to form the tertiary butyl carbonium ion, i.e., abstraction of a hydride Ion from an isobutane molecule while forming a molecule of normal alkane. (Equation 2, 2-A, 3, 3-A). Reaction sequences in these equations are often referred to as hydrogen- or hydride transfer reactions and will be discussed subsequently. 

Hydrogen chain transfer reaction, which may occur as intermolecular or intramolecular processes, leads to the formation of oleflnic species and polymeric fragments. Moreover, secondary radicals can also be formed from hydrogen abstraction through an intermolecular transfer reaction between a primary radical and a polymeric fragment. 

It is noteworthy that the observed values are lower than the caleulated ones except for DAT and the decreasing tendency is enhanced with the lower value of Rus,o this may be ascribed to the intramolecular chain transfer reaction accompanying abstraction of an allylic hydrogen from a pendant allyl group by... 

Phenoxyl radicals formed in the inhibition step (eqnation 10) are normally terminated by rapid reaction with peroxyl radicals (eqnation 11). However, phenoxyl radicals, particnlarly nnhindered ones, are also able to participate in chain transfer reactions by hydrogen atom abstraction from hydroperoxides which bnild np (eqnation 21), which is the reverse of eqnation 10, or initiate new reaction chains by hydrogen atom abstraction from substrate (RjH) (eqnation 20). 

Difficulties arise, however, in that a similar rate expression can also be obtained if reactions (9) and (10) are not used. If an overall scheme is used in which these reactions are replaced by chain transfer, reaction (7), by biradical termination, reaction (6), and by termination by hydrogen abstraction, reaction (5), then a similar relationship can be developed. Thus, the assumption that chain transfer is more efficient than reaction (6), leads [9] to the equation... 

The efficiency of a particular amine must depend not only on the rate of the initial hydrogen abstraction, but also on the nature and subsequent reactions of the radical produced. The free radical produced by H transfer may well be stabilised by resonance and may be insufficiently reactive to start a new oxidation chain [40], particularly when the amino group is surrounded by bulky substituents [9]. If the radical does react, then the subsequent rate and nature of the reaction will depend upon the intermediates and on the relative importance of chain termination and chain transfer reactions. Some formal grouping of the factors affecting the efficiency of a given inhibitor and the kinetics of the inhibited reaction is possible. 

On the other hand, chain-transfer reactions between a growing chain and a dead chain may lead to increases in average molecular weight because of the formation of long branches, originating at that link of the de chain from which the hydrogen had been abstracted. 

When we combine this observation with the autoaccelerating tendencies of the system, the chain-transfer reactions to both the monomer and the polymer on one of the several positions which leads to branched-chain formation, and the possible reactivation of dead polymer molecules by hydrogen abstraction with monomeric free radicals [78], the complexity of the kinetics of vinyl acetate polymerization may be appreciated. Similar factors may be involved not only in the polymerization of other vinyl esters, but also in the fiee-radical polymerization of other types of monomers. 

An example of the macromonomer method is the preparation of graft copolymers of PEO by the free radical polymerization of vinyl acetate in the presence of PEO. The growing vinyl acetate radical would abstract a hydrogen atom from the PEO chain, creating a radical at this site. The newly created radical would then polymerize vinyl acetate to form a branch on the chain. The rather randomly occurring chain transfer reaction would form a graft copolymer of PEO and poly(vinyl acetate). 

The branching on chains of low-density polyethylene results from a back-biting reaction in which the radical end group abstracts a hydrogen from the fourth carbon back (the fifth carbon in the chain). Abstraction of this hydrogen is particularly facile because the transition state associated with the process can adopt a conformation like that of a chair cyclohexane. In addition, the less stable 1° radical is converted to a more stable 2° radical. This side reaction is called a chain-transfer reaction, because the activity of the end group is transferred from one chain to another. Continued polymerization of monomer from this new radical center leads to a branch four carbons long ... 

What is missing is a chain-transfer reaction. A growing polymer radical may abstract a hydrogen atom from another polymer chain. 

Chain-transfer reactions can be used to control the molecular weight of a polymer. Certain reagents, such as thiols, have a hydrogen that is easily abstracted. The resulting RS radical is not reactive enough to add to double bonds. Instead, it dimerizes to form a disulfide. 

In addition to termination by combination and disproportionation, chain transfer reaction also terminates a growing chain. Chain transfer occurs by hydrogen abstraction from an initiator, monomer, polymer or solvent molecule [2-4]. 

Random Scission without Depropagation. In radical polymerization, the simple chain-growth mechanism is complicated by side reactions of the propagating free radicals, which may undergo chain-transfer reactions in which the radical activity is transferred from one center to another, typically by hydrogen atom abstraction. 

Transfer Agents. In free radical polymerization, thiols are often employed as chain transfer agents. Chain transfer reactions involving thiols proceed via hydrogen atom abstraction as illnstrated in Figure 4. 

Because organic radicals are highly reactive species, it is not surprising that radical polymerizations are often complicated by unwanted side reactions. A frequently observed side reaction is hydrogen abstraction by the radical endgroup from a growing polymer chain, a solvent molecule, or another monomer. These side reactions are called chain-transfer reactions because the activity of the endgroup is "transferred" from one chain to another. 

It is possible that a radicalized chain abstracts a hydrogen from an adjacent polymer molecule and that the reactive site is then moved from one molecule to another (chain transfer). This leads in most cases to the formation of a molecule with a long-chain branch. The chain transfer reaction may also be intramolecular, which is a well-known mechanism giving branches in high-pressure polyethylene. 

Chain transfer reactions mostly proceed by abstraction of a monovalent atom such as hydrogen or a halogen. The scission of a bond carbon - oligovalent (e.g., H) atom is of interest for the introduction of endgroups into a polymer produced in a free radical reaction. Radically induced vinyl monomer polymerization with the possibility of chain transfer to a polymer of different chemical structure present in the reaction mixture leads to graft copolymers if bond scission occurs outside the main chain, no matter whether a single atom or a grouping is abstracted. Quite a different result is obtained if a radical attack involves a bond in the main chain of the polymer, if this bond scission occurs at a monovalent atom, which must be at the chain end, there is block copolymer formation. If bond scission occurs inside the polymer backbone, either block or random copolymers are produced [63]. 

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