Polystyryl radical

The main reason that the decreases as the polymerization temperature increases is the increase in the initiation and termination reactions, which leads to a decrease in the kinetic chain length (Fig. 17). At low temperature, the main termination mechanism is polystyryl radical coupling, but as the temperature increases, radical disproportionation becomes increasingly important. Termination by coupling results in higher PS than any of the other termination modes. 

Table 5.2 Values of kJkK for Polystyryl Radical Model Systems...

Different initiators have varying transfer constants (Table 3-5). Further, the value of C) for a particular initiator also varies with the reactivity of the propagating radical. Thus there is a fivefold difference in C) for cumyl hydroperoxide toward poly(methyl methacrylate) radical compared to polystyryl radical. The latter is the less reactive radical see Sec. 6-3b. 

The charge transfer radical complex is considered to be the main precursor of the polystyryl radical. 

The graft copolymerization of acrylonitrile onto polystyrene was attempted using benzoyl peroxide, di-/-butylperoxide, and 2,5-dimethyl-2,5-di-(/-butylperoxy)hexane as initiators. In all cases no increase in mass of the polystyrene was observed. Attempts were also made to test whether the polystyryl radical was ever formed by combining the initiator and the polymer or the initiator, polymer and a nitroxide radical trap. In the first case the formation of a radical must lead to cross-linking of the polymer and in the second case the polystyryl radical will be trapped by the nitroxide. ... 

In some but not so rare cases, however, reactivity of macromonomers was found to be apparently reduced by the nature of their polymer chains. For example, p-vinylbenzyl- or methacrylate-ended PEO macromonomers, 26 (m=l) or 27b, were found to copolymerize with styrene (as A) in tetrahydrofuran with increasing difficulty (l/rA is reduced to one half) with increasing chain length of the PEO [41]. Since we are concerned with polymer-polymer reactions, as shown in Fig. 3, the results suggest that any thermodynamically repulsive interaction, which is usually observed between different, incompatible polymer chains, in this case PEO and PSt chains, may retard their approach and hence the reaction between their end groups, polystyryl radical and p-vinylbenzyl or methacrylate group. Such an incompatibility effect was discussed in terms of the degree of interpenetration and the interaction parameters between unlike polymers to support the observed reduction in the macromonomers copolymerization reactivity [31,40]. Similar observations of reduction of the copolymerization reactivity of macromonomers have recently been reported for the PEO macromonomers, 27a (m=ll) with styrene in benzene [42], 27b with acrylamide in water [43], and for poly(L-lactide), 28, with dimethyl acrylamide or N-vinylpyr-rolidone in dioxane [44]. 

It can be seen from Tables 11 and 12 that the reactivity of the a-substituted acrylonitriles and acrylates toward the polystyryl radical (l/r2) increases slightly with the bulkiness of the substituent if the electronic factors are kept constant (COOMe < COOEt or COO/Bu), and also increases regularly with the electro-philicity of the olefin in each series. A noticeable exception to the last rule is methyl a-ethylthioacrylate H2C=C(SEt)COOMe, whose l/r2 value of >100 [70] is higher than the values for the very electrophilic methyl a-cyanoacry-late H2C = C(CN)COOMe (l/r2 = 100) [101] and diethyl methylenemalonate H2C = C(COOEt)2 (l/r2 = 33) [96,98]. As mentioned above its equivalent in the... 

These copolymerization parameters are only slightly influenced by the solvent used (Table 18) [116], suggesting a small solvent effect on the propagation reaction. The reactivity of methyl a-methoxyacrylate towards a polystyryl radical (l/r2) however tends to increase with increasing Ex value or dielectric constant of the solvent. Here again it appears that increased solvent polarity leads to an increased persistency of the captodative radical. 

According to these authors, disproportionation of polystyryl radicals is unimportant. Also, poly(methyl acrylate) radicals almost exclusively combine around 300 K [19]. 

The reason why FR polymerizations are not living is that growing polymer radicals interact with each other, resulting in chain termination. There are several modes whereby polystyryl radicals become terminated, including radical coupling, disproportionation, and chain transfer. Total elimination of these bimolecular processes from an FR polymerization is impossible. However, if one can keep the FR concentration very low, the rate of these termination... 

The abstraction of a hydrogen atom occurs preferentially at the tertiary carbon of the structure, leading to a polystyryl radical. This radical adds to oxygen to form a peroxy radical. By abstraction of another hydrogen atom, the peroxy radical leads to a hydroperoxide. Hydroperoxides have an IR absorption at 3450 cm-1. The decomposition of the hydroperoxide either by photolysis or by thermolysis gives an alkoxy radical that may react in several ways ... 

Photooxidation yields a polystyryl radical as identified formerly in the study of PS photooxidation (Scheme 30.2). 

Two different cases may occur. If this radical is formed in a succession of styrene units (1), it reacts in the same way as in PS. If it is formed on a styrene unit linked to an acrylonitrile unit (2), three reaction pathways may be envisaged. The alkoxy radical resulting from the decomposition of the hydroperoxide formed on this polystyryl radical may react by 3-scission. Scissions (a) and (b) yield chain ketones, acetophenone end-groups and phenyl and alkyl radicals as previously observed in the case of PS photooxidation mechanism. Scission (c) leads to the formation of an aromatic ketone and an alkyl radical. This alkyl radical may be the precursor of acrylonitrile units (identified by IR spectroscopy at 2220 cm-1), or may react directly with oxygen and after several reactions generates acid groups, or finally this radical may isomerize to a more... 

Poly(MMA)-poly(St)-poly(MMA) and poly(BA)-poly(St)-poly(BA) have thus been obtained with good blocking selectivity. This is particularly high in the latter case as the dithiocarbamate radical is substantially unable to initiate the polymerization of acrylates, BA reacting only with the polystyryl radical. 

The relative reactivity of substituted bicyclobutanes toward polystyryl radical was determined by Hall and found to be smaller than that of common vinyl monomers . ... 

Using carbon-14 labeled AIBN as an initiator, a sample of styrene is polymerized to an average degree of polymerization of 1.28x10 . The AIBN has an activity of 8.97x10 counts per minute per mol in a scintillation counter. If 5.0 grams of the polystyrene shows an activity of 315 counts per minute, determine the mode of termination of polystyryl radicals. 

In the bulk polymerization of styrene by ultraviolet radiation, the initial polymerization rate and degree of polymerization are 1.3x10 mol/L-s and 260, respectively, at 30°C. What will be the corresponding values for polymerization at 80°C The activation energies for propagation and termination of polystyryl radicals are 26 and 8.0 W/mol. What assumption, if any, is made in this calculation ... 

In the end of 1960s, Nikolaev et al.29 and Ito et al.30 independently demonstrated an appreciable effect of the reaction medium on the reactivity ratios in the copolymerization of methyl methacrylate and styrene (Table 19). Ito et al. found that the relative reactivity of methyl methacrylate toward the polystyryl radical is correlated with the transition energies ET for the longest wavelength absorption band for pyridinum TV-phenolbetaine in solvents. They suggested that the polarized structure of methyl methacrylate monomer becomes important in the transition state. Bonta et al.32 also demonstrated that there is an appreciable solvent effect on the reactivity ratio in the styrene-methyl methacrylate copolymerization in non-... 

Various chromophore groups, colouring the polymer yellow to brown, are formed during the oxidative degradation of polymers. The production of such groups was identified in the case of polystyrene [2, 255], but their structure is not yet entirely elucidated. Achhammer et al. [2] suggested that the colour of oxidized polystyrene is due to quinomethane formed by the reaction of polystyryl radicals ... 

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