Radicals polystyrene

The lifetime of polystyrene radicals at 50 C was measuredt by the rotating sector method as a function of the extent of conversion to polymer. The following results were obtained ... 

The Q-e Scheme. The magnitude of and T2 can frequentiy be correlated with stmctural effects, such as polar and resonance factors. For example, in the free-radical polymerization of vinyl acetate with styrene, both styrene and vinyl acetate radicals preferentially add styrene because of the formation of the resonance stabilized polystyrene radical. 

For the 10, 18, and 35% methanol solution, however, the spectra are similar to each other but are distinctly different from that of the dry wool (Figure 9, d), and more closely resemble that of the polystyrene radical previously observed—for example, in the growth of the styrene-popcorn copolymer (2). It is likely, of course, that there will be some... 

It is evident that the values of the transfer constants are dependent on the nature both of the attacking radicals and of the transfer agent itself, and that similar effects should be expected during the synthesis of graft copolymers by chain transfer methods. For example, with respect to toluene the chain transfer constant is a little greater for methyl methacrylate radicals than for styrene radicals on the contrary, with respect to halogenated solvents (CC14) the polystyrene radical is much more effective in the removal of a chlorine atom. Vinyl acetate chains are far more effective than either of the other two polymer radicals. 

The values of relative reactivity of macromonomers (l/r2) in both solution and emulsion were found to decrease slightly with conversion. The r2 values in solution are lower than those for the copolymerization of low-molecular-weight monomers and macromonomers in emulsion. These results support the previous conclusion [95] about the incompatibility of macromonomer with a polymer trunk (polystyrene radical) which suppresses the mutual cross-propagation reactions of comonomers. 

Finally, some termination step occurs, two of which are shown in the scheme. The most common is coupling, in which two radicals combine, leading to one larger macromolecule. Polystyrene radicals typically undergo termination by coupling. Another reaction that is common with some monomers (e.g., methyl methacrylate) is called disproportionation in which on the reaction of two radicals, a hydrogen atom transfers from one species to the other. 

As to polystyrene radicals, they do not react with cumene in the temperature range investigated. This may seem surprising, since, judging by the activation energies of recombination and oxidation processes, polystyrene radicals should not be less active than other radicals. Hense, a direct relationship between the activity of radicals in various reactions and their activation energies does not exist. 

When such comparisons are made it becomes clear that the reactivities of radicals, monomers, or transfer agents depend on the particular reaction being considered. It is not possible to conclude, for example, that polyfvinyl acetate) radical will always react x times more rapidly than polystyrene radical in addition reactions or y times as rapidly in the atom abstraction reactions involved in chain transfer. Similarly the relative order of efficiency of chain transfer agents will not be the same for all radical polymerizations. This is because resonance, sleric, and polar influences all come into play and their effects can depend on the particular species involved in a reaction. 

The thermal volatilization analysis of a mixture of polyvinylchloride and polystyrene is given in Fig. 81. The first peak corresponds to the elimination of HC1 and the second to that of styrene. Dehydrochlorination is retarded in the mixture. The production of styrene is also retarded styrene evolution, in fact, does not occur below 350°C. This contrasts with the behaviour of polyvinylchloride-polymethylmethacrylate mixtures for which methacrylate formation accompanies dehydrochlorination. The observed behaviour implies that, if chlorine radical attack on polystyrene occurs, the polystyrene radicals produced are unable to undergo depolymerization at 300° C. According to McNeill et al. [323], structural changes leading to increased stability in the polystyrene must take place. This could also occur by addition of Cl to the aromatic ring, yielding a cyclohexadienyl-type radical which is unable to induce depolymerization of the styrene chain. 

The symbols used are I for initiator, R for the radical derived from the initiator, S for styrene, and R for growing polystyrene radicals, XH for a source of hydrogen radical, and PS for polystyrene. Thus, polystyrene can be formed in the termination step by chain transfer, disproportionation, and combination. Temperature and chain transfer agents can be used to control molecular weight and molecular weight distribution. Polystyrene resulting from free-radical processes is amorphous. 

Fischer (22) on the other hand showed that the ratio of the rate constants from reaction paths la and lb for an attack by a polystyrene radical on the 1,2- double bond is... 

Addition of rubbery materials, however, does improve the impact resistance of polystyrene. This is therefore done extensively. The most common rubbers used for this purpose are butadiene-styrene copolymers. Some butadiene homopolymers are also used, but to a lesser extent. The high-impact polystyrene is presently prepared by dissolving the rubber in a styrene monomer and then polymerizing the styrene. This polymerization is either done in bulk or in suspension. The product contains styrene-butadiene rubber, styrene homopolymer, and a considerable portion of styrene-graft copolymer that forms when polystyrene radicals attack the rubber molecules. The product has very enhanced impact resistance. 

These results indicate that the growing polystyrene radicals add to the nitro groups on the trunk polymer continually with reaction time, and highly branched graft copolymers can be prepared, when poly(vinyl p-nitrobenzoate) is used as a trunk polymer. 

The graft efficiency increases with increasing amount of the trunk polymer because of the more frequent occurrence of the reaction between growing polystyrene radicals and the nitro groups on the trunk polymer. However, the total conversion, per cent grafting and the molecular weight of nongrafted polystyrene are reduced with the increase in the concentration of poly(vinyl p-nitrobenzoate) due to the retardation of polymerization of styrene by poly(vinyl p-nitrobenzoate) as mentioned above. 

This difference is attributed to the polar effect of the ester bond connecting the polymer backbone and pendant p-nitrophenyl groups. Therefore, several model compounds for trunk polymer with pendant p-nitrophenyl groups were prepared, and the chain transfer constants of polystyrene radicals to these model compounds were determined by use of Kar s equation (5). 

As model compounds, we used isopropyl p-nitrobenzoate for poly(vinyl p-nitrobenzoate), p-nitrophenyl iaobutyrate for poly(p-nitrophenyl acrylate), p-nitrocumene for poly(p-nitrostyrene), and p-nitrophenyl isopropyl ether for poly(p-nitrophenyl vinyl ether). The structure and Hammett s a constants of these model compounds and the chain transfer constants of polystyrene radicals to these compounds are given in Table 5 (3). 

Table 5. Chain Transfer Constants of Polystyrene Radicals to Several Model Compounds for Trunk Polymers...

The chain transfer constant of polystyrene radicals to ethylene-vinyl p-nitrobenzoate copolymer was compared with those to poly(vinyl p-nitrobenzoate) and their model compound, isopropyl p-nitrobenzoate. As expected, the value of chain transfer constant to the copolymer is larger than that to poly(vinyl p-nitrobenzoate) and smaller than that to the model compound isopropyl p-nitrobenzoate obtained according to the method of Mayo et al. (7) (Table 8). 

The reactivities of the propagating polymer-radicals, however, exert greater influence on the rates of propagation than do the reactivities of the monomers. Resonance stabilization of the polymer-radicals is a predominant factor. This fairly common view comes from observations that a methyl radical reacts at a temperature such as 60°C approximately 25 times faster with styrene than it does with vinyl acetate [72]. In homopolymerizations of the two monomers, however, the rates of propagation fall in an opposite order. Also, poly(vinyl acetate)-radicals react 46 times faster with n-butyl mercaptan in hydrogen abstraction reactions than do the polystyrene-radicals [71]. The conclusion is that the polystyrene radicals are much more resonance stabilized than are the poly (vinyl acetate)-radicals. Several structures of the polystyrene-radicals are possible due to the conjugation of the unpaired electrons on the terminal carbmis with the adjacent unsaturated groups. These are resonance hybrids that can be illustrated as follows ... 

Keywords gel permeation chromatography kinetics (polym.) polystyrene radical polymerization... 

Spectrophotometer intensity vs time for the determination of concentration of polystyrene radicals from the FRRPP system (Replotted with permission from Wang et al 1999)... 

As shown in Fig. 4.4.2, it takes a relatively small amount (at least 10%) of ionic component in a molecule for it to exhibit surfactancy. This is achievable with the FRRPP process, because one can always add an acid monomer to react with a hydrophobic polymer radical even in the presence of unreacted hydrophobic monomer. The example is when methacrylic acid was added to polystyrene radicals even at 30% styrene conversion to produce an amphiphilic S-block-(S-stat-MAA) amphiphilic material (see Section 4.2). Methacrylic and acrylic acid monomers are normally reactive to polymer radicals in general, as it can be seen from their reactivity ratios. When amine-PDMS was reacted with the acid groups of the... 

CuCl2 is an even more powerful inhibitor/transfer agent than FeCla- It reacts with growing polystyrene radicals 20 times faster (C = 10,000 vs 500) and with poly(methyl methacrylate) radicals 50 times faster (C = 2000 vs 4) (458). In the presence of a suitable ligand the resulting alkyl hahdes can react reversibly with Cu(I) species and establish an atom transfer equilibrium, which is at the essence ofATRP. 

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