Propagation free radical reactions

In the presence of the proper initiation factors, these reactions can be induced within the hydrophobic matrix of the membrane ( ). Once these reactions have started, there can be considerable cell damage (49). The orderly array of fatty acids present in membranes permits maximum interaction of the individual molecules and thus, readily propagates free radical reactions. 

Transfer constants for polystyrene chain radicals at 60° and 100°C, obtained from the slopes of these plots and others like them, are given in the second and third columns of Table XIII. Almost any solvent is susceptible to attack by the propagating free radical. Even cyclohexane and benzene enter into chain transfer, although to a comparatively small extent only. The specific reaction rate at 100°C for transfer with either of these solvents is less than two ten-thousandths of the rate for the addition of the chain radical to styrene monomer. A fifteenfold dilution with benzene was required to halve the molecular weight, i.e., to double l/xn from its value (l/ rjo for pure styrene (see Fig. 16). Other hydrocarbons are more effective in lowering the degree of polymerization through chain transfer. 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

The use of free-radical reactions in organic synthesis started with the reduction of functional groups. The purpose of this chapter is to give an overview of the relevance of silanes as efficient and effective sources for facile hydrogen atom transfer by radical chain processes. A number of reviews [1-7] have described some specific areas in detail. Reaction (4.1) represents the reduction of a functional group by silicon hydride which, in order to be a radical chain process, has to be associated with initiation, propagation and termination steps of the radical species. Scheme 4.1 illustrates the insertion of Reaction (4.1) in a radical chain process. 

It was shown from the study of styrene and MMA in a KNOa-DMF system using tracer techniques for the composition analysis that the free-radical contribution is apparently confined to the first few percent of polymerization, whereas at later stages the reaction is anionic (34). This is consistent with the inhibitor studies. On the same monomer pair with a variety of solvents with ammonium salts, only with tetrahydrofuran did a possible free-radical reaction accompany the anionic propagation, with the others, DMF and dimethylacetamide, as well as without solvent, an anionic reaction accompanying the free radical one was assumed (35). 

In the atmospheric free radical reactions involving hydrocarbon species, molecular products of interest are formed via either radical chain propagation or termination steps. 

As a consequence of the fact that free-radical reactions are chain processes, they are very well suited for the preparation of polymers rather than single products. That is, products are obtained whose size is determined by the number of propagation cycles that occur before a termination event stops the growing chain. 

The value of kp now reported for propagation by free cation is approximately five orders of magnitude greater than that for the corresponding free radical reaction in THF (20). Similar differences have already been noted for free cation (42) and free anion (37) propagation of styrene. 

Methyl methacrylate (Fig. 1-4) and methacrylonitrile (6-5) are allylic-type monomers that do yield high molecular weight polymers in free radical reactions. This is probably because the propagating radicals are conjugated with and stabilized to some extent by the ester and nitrile substituents. The macroradicals are... 

For simplicity, the simple copolymer equation will be developed for the case of free radical reactions. The four possible propagation reactions with monomers M and M2 are... 

Most emulsion polymerizations are free-radical reactions. The main difference from alternative free-radical polymerizations, such as those in bulk, solution, and suspension systems, is that the propagating macroradicals in emulsion reactions are isolated from each other. Encounters between macroradicals are hindered as a consequence, and termination reactions are less frequent than in comparable systems in which the reaction mixture is not subdivided. Emulsion polymerizations thus often yield high-molccular-wcight products at fast rates when suspension or bulk reactions of the same monomers are inefficient. 

Initiation is the slow reaction in the initiation-propagalion-termination sequence in free radical reactions whereas selected initiation reactions in anionic systems can be very rapid compared to the subsequent propagation reaction. This facilitates the preparation of anionic polymers with narrow molecular weight distributions, if the polymerization is conducted carefully. 

Anionic polymerizations are generally much faster than free-radical reactions although the A p values are of the same order of magnitude for addition reactions of radicals and solvated anionic ion pairs (free macroanions react much faster). The concentration of radicals in free-radical polymerizations is usually about 10 -10 M while that of propagating ion pairs is 10 -10 M. As a result, anionic polymerizations are lO -lO times as fast as free-radical reactions at the same temperature. 

Copolymerizations analogous to free-radical reactions occur between mixtures of monomers which have more or less the same e values [Table 9-1). The copolymerizations of styrene and dienes have been particularly studied in this connection. The simple copolymer equation (Eq. 7-13) applies to most of these systems, but the reactivity ratios will vary with the choice of solvent and positive counterion because these factors influence the nature of the propagating ion pair. 

Vitamin E is the major hydrophobic chain-breaking antioxidant that prevents the propagation of free radical reactions in the lipid components of membranes, vacuoles and plasma lipoproteins. 

Pathways of the overall reaction sequence are constructed using instances of free-radical-reaction. Information associated with the individual instances initiation-reaction, transfer-reaction, propagation-reaction, branching-reaction, and termination-reaction facilitates this task. With this information, free-radical-reaction is able to construct individual free radical pathways. 

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