Radical compartmentalization

Considerations of radical compartmentalization and higher polymer concentration effects are not sufficient to describe the processes that build branched polymer molecules in emulsion polymerization, and the effects of limited space must be properly taken into account [266-269]. 

In bacteria and plants, the individual enzymes of the fatty acid synthase system are separate, and the acyl radicals are found in combination with a protein called the acyl carrier protein (ACP). However, in yeast, mammals, and birds, the synthase system is a multienzyme polypeptide complex that incorporates ACP, which takes over the role of CoA. It contains the vitamin pantothenic acid in the form of 4 -phosphopan-tetheine (Figure 45-18). The use of one multienzyme functional unit has the advantages of achieving the effect of compartmentalization of the process within the cell without the erection of permeability barriers, and synthesis of all enzymes in the complex is coordinated since it is encoded by a single gene. 

Compartmentalization (or segregation) of a propagating radical in a polymer particle isolates it from other propagating radicals and allows growth to higher molecular weight than might be achieved in solution polymerization, provided that the rate of entry of another radical from the aqueous phase is sufficiently slow. 

The role of oxygen in metabolism involves a paradox. Combustion of food to release and store its energy content requires a stepwise four-electron reduction of oxygen to produce harmless water, carbon dioxide, and ammonia (Figure 10.1). The first electron produces superoxide anion radical, the second produces peroxide anion, the third produces hydroxyl radical, and the fourth produces water. When this process is compartmentalized... 

Ito s group [83] reported the micellar polymerization mechanism was operative during the radical polymerization of PEO macromonomers in cyclohexane and water under similar reaction conditions. The reaction medium has an important effect on the polymerization behavior of macromonomers. Cyclohexane was chosen as a nonpolar type of solvent. The polymerization was found to be independent of the lengths of p-alkyl group (R) and the PEO chain in benzene. On the other hand, the rate of polymerization in cyclohexane increased with increasing number of EO units. This may be attributed to the formation of aggregates (micelles) and/or compartmentalization of reaction loci,i.e., polymerization in distinct aggregates (polymer particles). The C12-(EO)14-MA macromonomer polymerized faster in bulk than in benzene but far slower than in water. 

These chains cannot undergo bimolecular termination and so grow unhindered until a second free radical enters the latex particle. This environment is manifestly different from that in other growing latex particles and from that in the bulk system. This argument also explains why compartmentalization has no effect on the MWD if termination is by chain transfer because the chains containing one free radical can still undergo the transfer process, just as they do in the bulk system. 

Theory of Compartmentalized Free-Radical Polymerization Reactions... 

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