Radical occlusion

Polymerization Rate and Radical Occlusion. In modeling the rate of precipitation polymerization, the reaction can occur at three different loci in the diluent, at the surface of the particles, or in the interior of the particles. 

The predominant mode of polymerization is in the interior of the particles and this leads to a reduction of macroradical mobility, usually referred to as radical occlusion, and a marked autoacceleration of the polymerization rate. 

Similarly, Garcia-Rubio and Hamielec (17) conducted bulk polymerizations of acrylonitrile at various temperatures and initiator levels in glass ampoules. Their plots of the rate of polymerization as a function of conversion are typical of the extensive radical occlusion in this very glassy polymer. 

Since the extent of radical occlusion varies from one precipitation polymerization to the next, it is nearly impossible to develop a generalized polymerization rate equation. As a result, rate expressions are most often determined from experi-... 

The photospeed increases linearly with incident light intensity (Figure 4). The linear relationship between the photospeed and the incident light intensity is explained by considering that in this viscous monomer the main termination mechanism is radical occlusion instead of bimolecular reaction between macroradicals. We will comment more extensively on the intensity... 

It has been known for some time that UV photopolymerization of multifunctional monomers does not obey the classical rate expression, Rp proportional to I0 5, but follows an approximately first-order relationship [196,197]. These results have been explained by postulating that, in these viscous monomers, radical occlusion competes with bimolecular termination. 

Although the polymerization of AN in bulk and also in the presence of 5-15% alcohol was heterogeneous, that of MMA in the presence of the alcohol remained homogeneous. Radical occlusion undoubtedly took place in the AN polymerizations, but this complication has not been considered in detail. 

The NMMAm monomer is,for the most part, converted into living propagating radicals apparently a unimolecular termination by polymer radical occlusion occurs. Presumably, only a small portion of the living polymer radicals function as active centers for the polymerization, while the others are dormant in the microspheres. Otherwise, MW and hence ] of the poly(NMMAm) would increase with the conversion. 

On the other hand, even a complete unimolecular termination by occlusion of polymer radicals may be expected to result in an apparent initiator order lower than unity, because the rate of occlusion depends on that of polymer formation, Rp. An increase of the initiator concentration causes an increase of R and consequently of the occlusion rate. That is, the rate of unimolecular termination due to polymer radical occlusion increase with the initiator concentration. In fact, the molecular weight of poly(NMM Am) decreased with increasing initiator concentration, as shown in Fig, 7, First order in the initiator concentration should be observed only if the lifetime of the active centres were independent of the initiator concentration. The Rp dependence on the monomer concentration was not linear but increased with increasing NMMAm concentration. Since poly(NMMAm) is considered to have a higher affinity to the monomer than to benzene (nonpolar solvent), swelling of the polymer microspheres is accelerated by increased NMMAm concentration. This... 

Pistoia has used electrochemically generated nitrate radicals to effect the bulk polymerization of acrylonitrile the system shows a remarkable postpolymerization effect which is affected by such factors as the anode material, current, temperature, stirring, electrolysis time, and HNO, concentration. Radical occlusion phenoma and the formation of oligomers limit the monomer to >oly-mer conversion. Pistoia has also reported the polymerization of acrylonitrile by the oxidation of sulphuric acid at the anode and has extended the work to the anodic polymerization of methyl methacrylate in methanol-sulphuric acid... 

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 ... 

Decreased cerebral blood flow, resulting from acute arterial occlusion, reduces oxygen and glucose delivery to brain tissue with subsequent lactic acid production, blood-brain barrier breakdown, inflammation, sodium and calcium pump dysfunction, glutamate release, intracellular calcium influx, free-radical generation, and finally membrane and nucleic acid breakdown and cell death. The degree of cerebral blood flow reduction following arterial occlusion is not uniform. Tissue at the... 

Ohnishi (Sakamoto etal., 1991) has described an oligomeric derivative of prostaglandin Bi (PGB2) and ascorbic acid. In a rat bilateral carotid occlusion-reperfiision injury complicated by haemorrhagic hypotension, this compound reduced a-phenyl-r-butyl nitrone (PBN) spin-trapped radicals and thiobarbituric acid-reactive products (TBARs) (a measure of lipid peroxidation) in isolated... 

Idebenone, an inhibitor of lipid peroxidation, was shown to prolong survival time and delay the onset of ischaemic seizures in a bilateral carotid occlusion model in rats. It is marketed in Japan as a therapy to improve cerebral metabolism and performance after a stroke (Suno and Nagaoka, 1984). Cerebral protective effects after an ischaemic insult in dogs and rabbits have been seen with the hydroxyl radical scavenger, mannitol (Meyer et al., 1987). 

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... 

Cardiovascular heart diseases (CHD) are considered as the clinical expression of advanced atherosclerosis. One of the initial steps in atherogenesis is the oxidative modification of LDL and the uptake of the modified lipoprotein particles by macrophages, which in turn become lipid laden cholesterol-rich cells, so-called foam cells [159]. An accumulation of foam cells in the arterial wall is the first visible sign of atherosclerosis and is termed fatty streak, the precursor to the development of the occlusive plaque [160]. It is well known that oxidation of LDL can be initiated in vitro by incubating isolated LDL particles with cells (macrophages, lymphocytes, smooth muscle cells, or endothelial cells), metal ions (copper or iron), enzymes, oxygen radicals, or UV-light. However less is known about the mechanisms by which... 

It is now generally admitted that the occlusion of radicals must be considered as an ultimate consequence of the lack of mobility of the large... 

The concentration of trapped radicals and the degree of occlusion (how deeply they are buried) have been studied extensively by Bamford and Jenkins (9). They determined the approximate radical concentration by swelling or dissolving polymer in solutions of diphenylpicryl hydrazyl (DPPH), a violet substance that becomes colorless on reaction with radicals. Nitrobenzene was generally used as the medium to dissolve the hydrazyl and to swell the polymer /3-propiolactone, a solvent for the polymer, was used also. 

Studies in polymerization. IX. The occlusion of free radicals by poly-... 

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