Radicals occluded

In the foregoing examples the synthesis of block copolymers was based on the solubility differences between two monomers, of which one is water soluble while the other is emulsified. Another polymerization technique is based on the kinetics of the emulsion polymerization. When a water emulsion of a monomer, such as styrene, is irradiated during a short time, the reaction, continues at a nearly steady rate until practically all the monomer is used up. If a second monomer is then added, it will polymerize, being initiated by the radicals occluded in the polymer particles. Although in this case also the yields of block copolymers are low, nevertheless the physical properties of the final product are markedly different from those of statistical copolymers (4, 5, 151, 176). 

In addition, Bamford, Jenkins and coworkers (19) previously reported on the behavior of occluded radicals in the heterogeneous polymerizations of acrylonitrile, methyl acrylate, methyl methacrylate and vinylidene chloride. From their electron spin resonance studies, they concluded that the degree of occlusion was ... 

Figure 4.3 Effect of a variety of anti-free-radical interventions on reperfuslon-induced ventricular fibrillation In the Isolated perfused rat heart. Regional Ischaemia was induced by occluding a snare around the left anterior descending coronary artery and, after 10 min, hearts were reperfused by releasing the snare. Superoxide dismutase (SOD) (1 x 10° U/l), catalase (CAT) (1 X 10 U/l), mannitol (Mann) (50 mM), l-methlonlne (Methlon) (10 mM), glutathione (Glutath) (10 iiM) or desferrioxamlne (Deafer) (150 iim) were included throughout the experimental time course (n = 15/group). Redrawn with permission from Bernier et af. (1986).

The bulk polymerization of acrylonitrile has been studied by numerous workers (for a literature survey on the problem see ref. j O and JL1J. The kinetic features of this reaction at room temperature are summarized in Table I. It is one of the typical examples of polymerization under heterogeneous conditions in which the anomalies are generally assumed to arise as a result of non-stationary conditions caused by the "occlusion" of growing chains in the precipitated polymer (10). The presence of occluded radicals was indeed demonstrated by 5R measurements (12) and by... 

The bulk polymerization of acrylonitrile in this range of temperatures exhibits kinetic features very similar to those observed with acrylic acid (cf. Table I). The very low over-all activation energies (11.3 and 12.5 Kj.mole-l) found in both systems suggest a high temperature coefficient for the termination step such as would be expected for a diffusion controlled bimolecular reaction involving two polymeric radicals. It follows that for these systems, in which radicals disappear rapidly and where the post-polymerization is strongly reduced, the concepts of nonsteady-state and of occluded polymer chains can hardly explain the observed auto-acceleration. Hence the auto-acceleration of acrylonitrile which persists above 60°C and exhibits the same "autoacceleration index" as at lower temperatures has to be accounted for by another cause. 

Let us compare M-ZSM-5 zeolites with M = H+, Li+, Na, K+, Rb, Cs, AF+, on one hand, and organic electron donors of variable ionization potentials, on the other. Zeolite H-ZSM-5 generates cation-radicals from substrates with an oxidation potential of up to 1.65 V (Ramamurthy et al. 1991). The naphthalene sorption by Al-ZSM-5 zeolites calcified in an atmosphere of oxygen or argon leads to the appearance of two occluded particles—the naphthalene cation-radical and isolated electron. Both particles were fixed by ESR method. Back reaction between the oppositely charged particles proceeds in an extremely slow manner and both the signals persist over several weeks at room temperature (Moissette et al. 2003). 

Other Systemic Effects. In rabbits exposed dermally to isophorone at doses up to 3160 mg/kg, no systemic pathological effects were found by gross necropsy (Hazleton Labs 1964), but histological examinations were not performed. In this study, the site of application was occluded for 24 hours to prevent evaporation of isophorone from the skin. No significant differences between pre-exposure and post-exposure levels of serum electrolytes, blood glucose and sulfhydride radicals, SGOT,... 

In feet this method is similar to the synthesis of block copolymers using trapped radicals produced by polymerization of a monomer in a non-solvent. The existence of occluded radicals with a relatively long life-time was already postulated several years ago by Melville, in the photopolymerization of gazeous methyl methacrylate (149). The polymer deposited on the walls of the vessel was able to initiate the polymerization of chloroprene and of methylisopropenylketon (44,150). 

More recently, electron paramagnetic resonance has been used in place of DPPH to estimate radical concentrations (10, 80). Considering the uncertainties attendant on the use of DPPH the agreement is rea-sonablygood. Under conditions where Bamford and J enkins using DPPH found 5 x 1016 radicals per ml, Ingram, Symons and Townsend reported 3.5 0.05 x 1017 with e.p.r. (80). The latter authors point out that polyacrylonitrile is far more efficient in occluding radicals than are any of... 

The interior phase. Precipitated polymer chains are expected to be tightly coiled. A coiled radical would have reduced reactivity since there is a high probability that the radical end will be occluded within the coil. Trapped radicals are presumed to be deeply buried in the interior phase. 

Very few radicals exist in tissues at rest . Two important exceptions are melanin radicals and tyrosine radicals. The former exist in low concentrations in samples of melanin, which is a high polymer made up of units which include potential semiquinone units. It is reasonable to expect some orf/io-semiquinone formation from such structures. The radicals can be thought of as occluded, being sterically protected by surrounding polymer. So far as we know, they have no special chemical significance. 

Thrombolysis aims to reduce the volume of infarcted brain by recanalizing the occluded vessel and restoring blood flow. Restoration of blood flow may not necessarily always be beneficial. First, studies in animals suggest that reperfusion of acutely ischemic brain may actually be harmful, through the release of free radicals and toxic products into the circulation. Second, thrombolysis will probably not be of benefit if infarction is completed or if the ischemic penumbra is small. Finally, thrombolysis may cause hemorrhagic transformation of the infarct or extracranial bleeding. 

Following preparation of the rubber feed solution, the HIPS process will use one or two stirred reactors to form the rubber phase. In a single CSTR system, the reactor operates above phase inversion. Here, the steady-state effluent is a continuous polystyrene matrix, containing unreacted styrene monomer and distinct rubber particles with occluded polystyrene and styrene monomer. In a two-CSTR system, the first vessel operates below phase inversion to provide superior grafting. In this case, the continuous rubber phase and its reactive sites are readily available for styrene radical grafting reactions to occur. The second vessel operates above phase inversion, as in a single CSTR system, and controls the size of the rubber particles. 

The morphology of the rubber-modified polystyrenes system involves some complex aspects, such as particle size, size distribution, occlusions of polystyrene inside the rubber phase, interfacial bonding between the rubbery particles and the brittle matrix, etc. Many authors have observed that some of the most important factors in controlling the mechanical properties of HIPS and ABS are rubber particle size [49], volume fraction of the rubbery phase (rubber + occluded polystyrene) [50,51] and the degree of graft [52]. Grafting occurs during the polymerization of styrene when some of the free radicals react with the rubber... 

The polymer concentration at which autocatalytic effects are significant varies from system to system. When a polymer is insoluble in its own monomer, radicals can be occluded in the precipitated polymer-rich phase where they are prevented from terminating by their low mobilities. Bulk polymerization of vinyl chloride provides an example of this. 

Net reaction between hydrophilic oxidants and reductants separated by bilayer membranes has been convincingly demonstrated in numerous asymmetrically organized vesicle and planar bilayer systems [2a-c]. Particularly compelling examples are those in which electrogenic electron transport is accompanied by net translocation of lipophilic cations or anions, as is required to maintain electroneutrality. For example, MV + occluded within the inner aqueous phase of DHP vesicles can be completely reduced to the MV+ radical cation by 8204 ion in the bulk phase, but only if an equal or greater amount of viologen is initially present in the external environment [40]. Neither MV-+ nor 8204 is membrane permeable. The reaction is biphasic, with rapid bimolecular reduction of external being followed by... 

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