Occlusion polymerization

The solubility of MN4 macrocyclic compounds can vary from soluble to insoluble in water, although most are insoluble and can be immobilized on electrode surfaces, producing MN4 macrocyclic-based molecular electrodes. The immobilization can be achieved using several different approaches, including vacuum sublimation, vapor deposition, precipitation, adsorption from a solution in an organic solvent or concentrated acid, entrapment or occlusion, polymerization by heat treatment in the... 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

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. 

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 ... 

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-... 

It is generally not recommended to mix medications directly into the EN formula because of concerns that physical incompatibilities between the medications and the formula might lead to tube occlusion. There is some evidence that polymeric formulas are more likely to demonstrate physical incompatibility with medications compared with monomeric formulas, although most of the work in this area has used casein or caseinate-based formulas, and other proteins may act differently.38 The limited data currently available would indicate that acidic syrups and elixirs may be the worst for causing physical incompatibility when admixed with EN formulas. It has been postulated that this incompatibility is due to changes in the protein structure after exposure to acid or alcohol.38... 

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... 

These results conclusively demonstrate that precipitation of polyacrylonitrile as a fine powder and occlusion of growing chains resulting in post-polymerization do not bring about autoacceleration if a highly polar solvent is present in the system. 

From the results presented above it can be concluded that the auto-accelerated conversion curves observed in the polymerization of acrylic acid, methacrylic acid and acrylonitrile are not caused by non-steady conditions arising as a result of the occlusion of growing chains in the precipitated polymer. This occlusion which is responsible for the post-polymerization observed in these systems only contributes to a limited extent to the over-all rates. 

Jenkins, A. D., Occlusion Phenomenon in the Polymerization of Acrylonitrile and Other Monomers, Chap. 6 in Vinyl PolymerizationVol. 1, Part I, G. E. Ham, ed., Marcel Dekker, New York, 1967. 

A characteristic of aldehyde polymerization is the precipitation, often with crystallization, of the polymer during polymerization. Depending on the solvent used, polymerization rate, state of agitation, and other reaction conditions, the polymerization can slow down or even stop because of occlusion of the propagating centers in the precipitated polymer. The physical state and surface area of the precipitated polymer influence polymerization by their effect on the availability of propagating centers and the diffusion of monomer to those centers. 

Bulk polymerization of acrylonitrile initiated by gamma radiation was described by Chapiro at The process is greatly complicated by other reactions. Occlusion of... 

In the associates, polymerization occurs according to the zip-propagation mechanism, and auto-acceleration takes place. In the range of temperatures from 10 to GO C, the system gradually changes from the system dominated by occlusion to the system where template effect determines the kinetic behavior. In this range of temperatures, a significant post-effect occurs. ... 

Lack of adhesion of a dental restoration to tooth structure results in microleakage at tooth-restoration interface. This occurrence can result in discoloration at the margin of the restoration, or in the formation of caries. Occlusal forces on the restoration and differences between the coeffidents of thermal expansion of the cement and tooth material can lead to leakage. In addition, oral fluids and moisture may affect the adhesion. Microleakage of composite resin restorations has been reviewed by Ben-Amar [233]. Microleakage is not as serious a problem with glass-ionomer cements as it is with resin-based restorative materials, due to reduced polymerization shrinkage [234]. 

At this point, the phenomenon of phase inversion occurs and the rubber in monomer phase becomes dispersed as discrete particles in a matrix of the polymer in monomer phase. Usually in a mass polymerization process, the rubber will contain occlusions of polymer/monomer, which serve to swell the volume of the rubber particle. In the course of polymerization, monomer is converted to polymer, the viscosity of the mixture increases and greater power is needed to maintain the temperature and the compositional uniformity throughout the polymerized material (8). 

Overall rates of polymerization are generally rather insensitive to the presence of diluent. Das, Chatterjee and Palit (50) compared rates at 50 mole percent concentration in a variety of liquids. In the range of their experiments, polymerization rate was nearly independent of the choice of diluent. Bamford, Jenkins and White (22) point out that transfer agents reduce the mean degree of occlusion. Since the termination rate is increased under these conditions, the overall polymerization rate is reduced. The extent of polymer swelling will vary from one liquid to another (32, 130), and this obscures the interpretation to be made from limited data. 

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

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