Radical compartmentalization effect

Compartmentalization of radicals into polymer particles may yield a unique MWD for the linear chains, as discussed in Sect. 3.1, except when the dominant chain termination mode is the chain transfer reaction. Branched polymer molecules are assemblies of linear polymer chains (called primary chains), and compartmentalization effects on the primary chain length distribution must be properly accounted for. 

The compartmentalization of radicals may produce another important effect when large-sized branched polymer molecules are formed by chain transfer to polymer plus combination termination. As clarified in Sect. 4.1, when the n value is small, the frequency of bimolecular termination reactions between large polymer radicals drops significantly compared to models that do not account for compartmentalization of radicals. From this fact, it is easy to see that the size of branched polymer molecule is smaller than that calculated without considering compartmentalization effects [281]. 

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. 

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. 

Asymmetrically substituted dibenzyl ketones (ACOB) have been employed frequently as precursors of geminal triplet radical pairs in studies of cage effects in constrained or microphase-compartmentalized media. ... 

For the transfer- and exit-dominated system shown in Fig. 8, it is found that P = 2.0. This result shows that compartmentalization does not affect the MWD of polymer in transfer-dominat systems, vis-a-vis their bulk/so-lution counterpart. Briefly, the reason for this is that the cha m-stopping mechanism does not involve free radicals other than those on the growing chain, and so compartmentalization has no effect. 

This problem was first treated in detail by Haward (1949). He considered the case of a bulk polymerization that has been compartmentalized by subdividing the reaction system into a large number of separate droplets, each of volume v. Radicals are generated exclusively within the droplets and always in pairs. An example would be the polymerizatiim of styrene in emulsified droplets dispersed in water initiated the thermal decomposition of an oil-soluble initiator which partitions almost exclusively within the monomer droplets. In the model considered by Haward, radicals are unable to exit from the droplets into the external phase. The only radical-loss process is in fact bimolecular mutual termination. It therefore follows that all the droplets must always contain an even number (including zero) of propagating radicals, and that the state of radical occupancy will change in increments of 2. The conclusion reached by Haward is that in this case the effect of compartmentalization is to reduce the overall rate of polymerization per unit volume of disperse phase. The f ysical reason for this is that, as the volume of the droplets is reduced, so are the opportunities for a radical to escape from the others—and hence to avoid mutual... 

In this system the number of radicals in a particle is relatively so high that the polymerization resembles bulk polymerization. The average number of radicals per particle, h, is, almost, always greater than 0.5. Compartmentalization has no effect on the kinetics of a pseudo-bulk system, and termination, which is rate determining, is always diffusion controlled. 

A pseudo-bulk system is one in which the compartmentalized nature of the locus of polymerization has no effect on any kinetic property (rate, molar mass or particle size distributions). A system in which n is appreciably greater than 0.5 will always be pseudo-bulk there are so many radicals in a particle that the polymerization will be indistinguishable from the equivalent bulk one. However, a system with a low value of n can also be pseudo-bulk, if (for example) radical desorption results in the desorbed radical suffering no other fate except to re-enter another particle [1,3]. It is then apparent that the polymerization process will not see the walls between particles. Because pseudo-bulk kinetics can occur in systems where n 0.5, a pseudo-bulk system is different from the Smith-Ewart Case 3. 

In this section, the concept of compartmentalization is illustrated by assuming the presence of one segregated entity, which for simplicity is referred to as a polymer particle. Macroscale effects are also neglected for simplicity. The number of polymer particles Np is taken to be constant, as well as the partiele diameter dp and particle volume Vp. A distinction is made between the calculation of the polymerization rate and the CLD characteristics as a function of polymerization time, on the one hand, and a free radical polymerization (HIP) and CRP (NMP) reaction scheme on the other hand. 

In summary of this part, the average number of radicals per compartment is the centerpiece of heterophase polymerization kinetics. Its value depends mainly on the rate with which active, propagating centers appear inside the particles either by decomposition of monomer-soluble initiators or by entry from the continuous phase. Furthermore, particle size and overall concentration of compartments influence h in such a way that it increases with both increasing D and decreasing N. If the viscosity inside the particles is so high that termination by radical recombination is hindered, h increases as an expression of the gel effect in compartmentalized polymerization systems. 

The yields of the products change upon solubilization of the reactants because of the change of the effective polarity and microviscosity of the micellar phase as compared with the bulk phase. There are reactions for which the solubilization itself changes the yields of the photoproducts due to the compartmentalization of the reactants and intermediates of the reaction. For example, the photolysis of the asymmetric ketones in homogeneous solutions results in different radicals, which form a number of products as a result of a bulk recombination [22] ... 

The term compartmentalization in dispersed polymerization systems refers to the effects of the isolation of radical spedes in partides ( compartments ). There are two distinct and opposite effects of compartmentalization. ... 

The kinetics of catalytic reduction of 4-BB in CTAB solutions involves micellar catalysis of the electron transfer (ET) between 9-PA anion radical and 4-BB (Equation 2) in a thick layer of surfactant at the electrode. The effective rate constant for this ET in 0.1 M CTAB increased more than three orders of magnitude compared (Table 1) to the same reaction in surfactant-free DMF. The rate-determining step in CTAB was not Equation 2 as in DMF. In CTAB decomposition of the 4-BB anion radical (Equation 3) became kinetically important. The major cause of the kinetic alterations was compartmentalization of the reactants in high concentrations in surfactant aggregates at the surface of the electrode. The same catalytic reaction was not as successful in non-ionic igepal micelles, which did not provide good stabilization for 9-PA anion radicals. 

Delaitre, G. and Charleux, B. 2008. Kinetics of in situ formation of polyfacrylic acid)-b-polystyrene amphiphilic block copolymers via nitroxide-mediated controlled free-radical emulsion polymerization. Discussion on the effect of compartmentalization on the polymerization rate. Macromolecules 41 2361-7. 

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