Transfer constants to solvents and additives

TABLE 4. COnt d Transfer Constants to Solvents and Additives n/123... 

Many solvents and additives have measurable transfer constants (Table 6.5). The accuracy of much of the transfer constant data in the literature is questionable with values for a given system often spanning an order of magnitude. In some cases the discrepancies may be real and reflect differences in experimental conditions. In other cases they are less dear and may be due to difficulties in molecular weight measurements or other problems. 

Description Styrene monomer, a small amount of solvent and additives are fed to the specially designed reactor (1) where the polymerization is carried out. The polymerization temperature of the reactor is carefully controlled at a constant level to keep the desired conversion rate. The heat of polymerization is easily removed by a specially designed heat-transfer system. 

Generally, the mechanism of the polymerization is described to be a free-radical mechanism and follows theoretical kinetics. In the presence of solvents and additives the chain transfer constants have to be considered. The polymerization methods ean be classified in two broad categories [458] ... 

Transfer Constants to Monomers, Polymers, Catalysts and Initiators, Solvents and Additives, and Sulfur Compounds in Free Radical Polymerization+... 

This equation gives the fundamental correlation of the number average degree of polymerization with the rate of polymerization and the various chain transfer constants. Performing a polymerization experiment with only low conversion of monomer to polymer, the concentration of polymer is often too low to show significant chain transfer. The same holds true for the initiator, which is mainly used in the range of low concentrations. Without addition of solvent and additional chain transfer agent, eqn (1.55) reads after introduction of eqn (1.17)... 

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. 

The micellar surface appears to be less polar than water, based largely on shifts in fluorescence or charge-transfer spectra (Section 1). Although it may not be reasonable to apply bulk solvent parameters such as Z or dielectric constant to submicroscopic species such as micelles, the spectral and kinetic evidence are self-consistent. An additional point is that these reactions have... 

We have seen that crystals can be safely transferred to mixed solvents and that the percentage of organic solvent may often be increased to any desired level provided that its gradual addition is coupled with a gradual reduction in temperature so as to keep the dielectric constant of the medium as near as possible to the value for the normal mother liquor. Such a result deserves explanation and comment about the behavior of the dielectric constant in mixed solvents as a function of temperature. 

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