Transfer with Solvents

Reciprocal degrees of polymerization of polystyrenes prepared by thermal polymerization at 100°C in hydrocarbon solvents are plotted against [ 8]/[itf] in Fig. 16. Conversions were sufficiently low to permit the assumption of constancy in this ratio, which is taken equal to its initial value. The linearity of plots such as these, including those for numerous other monomer-solvent pairs which have been investigated, affords the best confirmation for the widespread occurrence of chain transfer and for the bimolecular mechanisms assumed. It is 

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. 

In all of the examples included in Table XIII, with the probable exceptions of benzene and chlorobenzene, f chain transfer evidently takes 

The results of chain transfer studies with different polymer radicals are compared in Table XIV. Chain transfer constants with hydrocarbon solvents are consistently a little greater for methyl methacrylate radicals than for styrene radicals. The methyl methacrylate chain radical is far less effective in the removal of chlorine from chlorinated solvents, however. Vinyl acetate chains are much more susceptible to chain transfer than are either of the other two polymer radicals. As will appear later, the propagation constants kp for styrene, methyl methacrylate, and vinyl acetate are in the approximate ratio 1 2 20. It follows from the transfer constants with toluene, that the rate constants ktr,s for the removal of benzylic hydrogen by the respective chain radicals are in the ratio 1 3.5 6000. Chain transfer studies offer a convenient means for comparing radical reactivities, provided the absolute propagation constants also are known. 

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When vinyl and aHyl monomers undergo chain transfer with solvents, so-caHed telomers may form, generaHy of rather low molecular weight (1). 

Scraped-surface exchangers are particularly suitable for heat transfer with crystalhzation, heat transfer with severe folding of surfaces, heat transfer with solvent extraction, and heat transfer of high-viscosity fluids. They are extensively used in paraffin-wax plants and in petrochemical plants for ciystallization. 

The gegen ion, K+, has been dismissed from the scheme in consideration of the comparatively high dielectric constant of the liquid ammonia medium.) The associated kinetic equations are equivalent to Eqs. (44) and (45). It will be observed that the termination step (51) is essentially a chain transfer with solvent. A similar process has... 

Yushchenko DA, Shvadchak W, Bilokin MD, Klymchenko AS, Duportail G, Mely Y, Pivovarenko VG (2006) Modulation of dual fluorescence in a 3-hydroxyquinolone dye by perturbation of its intramolecular proton transfer with solvent polarity and basicity. Photochem Photobiol Sci 5 1038-1044... 

When chain transfer with solvent occurs, what effect does this have on the average DP ... 

In an inert atmosphere, alkyl radicals are converted to alkanes by hydrogen transfer with solvent. Radicals can also undergo electron transfer oxidation by the metal oxidant and afford products (alkene, ester, etc.) ascribable to car-bonium ion intermediates,237 249, 288, 333 namely,... 

Poly(styrene)s containing acylperoxide groups are thus obtained by selective photolysis of the azo moieties at 350 or 371 nm. These prepolymers are successively used as macronitiators for the free radical polymerization of vinyl chloride at 70 °C. Styrene/vinyl chloride block copolymers are thus produced [55] by the above two-step route, although relevant amounts (50-60%) of poly(styrene) and poly(vinyl chloride), due to both low peroxide content ( 0.6 groups per macromolecule of polystyrene) and chain transfer with solvent and monomer, are also pre t. 

Termination by chain transfer with solvent or epoxy resin... 

Examination of the reactions of 4-picoline 1-oxide with labelled acetic anhydride and labelled butyric anhydride, both of which cause substitution at the methyl group and at C(3>, suggested three processes to be at work first, homolytic cleavage of the N—O bond in the anhydro base followed by radical recombination within a solvent cage second, similar cleavage followed by recombination after some radical transfer with solvent acid third, heterolytic cleavage of the N-O bond followed by nucleophilic attack of n-butyrate on the anhydro base. The relative importance of each of these three processes varies with the solvent used24i ... 

For oxygen and nitrogen acids and bases, proton transfer with solvent participation is very common. Typical results for symmetrical proton transfer (from an acid to its conjugate base) in water and methanol are listed in Tables 1 and 2. Further results, including reactions in t-butyl alcohol-water mixtures, anhydrous t-butyl alcohol, and anhydrous acetic acid, will be described later. 

Proton transfer with solvent participation always competes with direct bimolecular proton transfer, at least in principle, and comparison of the two rates is of theoretical interest. Unfortunately, data are available only in rare instances. The most complete series is that for the methylamines in water second-order rate constants for reaction with (equation 3) and without (equation 4) water participation are compared in Table 3 (p. Ill)... 

Thus, when bimolecular proton transfer is relatively slow, we surmise that the displacement of water molecules from the solvation shells requires an activation energy and is the rate-determining step for reaction. The fact that proton transfer with solvent participation remains fast when k2 becomes small, suggests that the proton-transfer step is inherently fast and the fact that 2 is small also for the reaction of (CH3)3N with NH4 suggests that steric hindrance is not the primary factor [32]. 

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