Diffusion polymerization rate, effect

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

EB irradiation of polymeric materials leads to superior properties than the 7-ray-induced modification due to the latter having lower achievable dose rate than the former. Because of the lower dose rate, oxygen has an opportunity to diffuse into the polymer and react with the free radicals generated thus causing the greater amount of chain scissions. EB radiation is so rapid that there is insufficient time for any significant amount of oxygen to diffuse into the polymer. Stabilizers (antirads) reduce the dose-rate effect [74]. Their effectiveness depends on the abUity to survive irradiation and then to act as an antioxidant in the absence of radiation. 

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. 

We can tentatively conclude, therefore, that the effect of chain transfer is still making itself felt in the polymerization of vinyl caproate in spite of its low water solubility. Except at the lowest particle concentrations, chain transfer is important. The polymerization in these regions is midway betwen Case I and Case II. When variables are considered separately, there is some dependence of polymerization rate on particle concentration, and also some dependence on initiator concentration. In addition, at constant organic volume, while the rate of polymerization increases as the particle concentration increases (Rp oc 2V- ), the rate per particle decreases as the particles get smaller. This shows that transferred radicals are mainly trapped in the particles, but some diffuse out and can undergo termination with other growing radicals. 

Diffusion-Controlled Free Radical Polymerization - Effect on Polymerization Rate and Molecular Properties of PVC , Abstracts - Third International Symposium on PVC, Case Western Reserve University, Cleveland, August 10-15 (1980). 

If the reaction temperature is below the polymer glass transition temperature and the amount of monomer in the particle decreases far enough, the glass effect may become important. The polymerization rate virtually goes to zero because the particle becomes so internally viscous, essentially glasslike, that the diffusion of monomer to the radicals is limited. The glass transition point varies for different polymers. This effect has also been studied by several authors [34,39,40]. 

The kinetic scheme with constant reaction of the polymer/monomer droplet increases fairly quickly with conversion, and the mobility of the polymer chains rapidly falls below the mobility of the monomer. The reduced diffusion of live polymer chains in the droplet will reduce the rate of termination of polymerization. The associated increase in the number of radicals will cause a rapid increase in the polymerization rate. This phenomenon is well known as the Trommsdorf or gel effect [8,9]. The gel effect causes a growth of the polymer chain length and widening of the molecular weight distribution (Figure 9.5). 

The Increasing rate of polymerization In Interval I was attributed to an Increasing number of polymerizing particles formed by nucleatlon In the mlcroemulslon droplets, similar to the mechanism proposed for mlnlemulslon polymerization by Chamberlain et al. (20). Interval I ends when all mlcroemulslon droplets have disappeared, either by capturing radicals to become polymer particles or losing monomer by diffusion to the polymer particles which have captured radicals. In Interval II, the polymerization rate decreases because of the decrease In monomer concentration In the polymer particles. No gel effect was observed termination occurred Immediately upon entry of the second radical Into the small latex particles which contained a growing radical. 

Most reported values of have been determined in solution polymerization or in bulk systems at low conversions. In bulk at higher conversions, or more generally in viscous media, the rate of termination is lower. The more viscous the reaction medium the more is the termination rate determined by the rate of diffusion of the macroradicals. The result of decreased termination is increased polymerization rate and increased molecular weight. This residt is called the gel effect or Trommsdorff effect. 

Thus, if for any reason the average lifetime of the growing radical increases, then there will be an increase in the polymer chain length. An often-encountered example of this is the Trommsdorff or gel effect that occurs in the polymerization of solutions of high monomer concentration when the viscosity, rj, increases and, after a certain extent of conversion, there is a rapid acceleration in the rate of polymerization. This is interpreted as an indication of the decrease in the rate of termination as this reaction becomes diffusion-controlled. A feature of diffusion-controlled reactions is that the rate coefficient, is not chemically controlled but depends on the rate at which the terminating radicals can collide. This is most simply given by the diffusion-controlled rate coefficient, k, in the Debye equation ... 

The catalytic behavior of enzymes in immobilized form may dramatically differ from that of soluble homogeneous enzymes. In particular, mass transport effects (the transport of a substrate to the catalyst and diffusion of reaction products away from the catalyst matrix) may result in the reduction of the overall activity. Mass transport effects are usually divided into two categories - external and internal. External effects stem from the fact that substrates must be transported from the bulk solution to the surface of an immobilized enzyme. Internal diffusional limitations occur when a substrate penetrates inside the immobilized enzyme particle, such as porous carriers, polymeric microspheres, membranes, etc. The classical treatment of mass transfer in heterogeneous catalysis has been successfully applied to immobilized enzymes I27l There are several simple experimental criteria or tests that allow one to determine whether a reaction is limited by external diffusion. For example, if a reaction is completely limited by external diffusion, the rate of the process should not depend on pH or enzyme concentration. At the same time the rate of reaction will depend on the stirring in the batch reactor or on the flow rate of a substrate in the column reactor. 

Either or both mechanisms may be responsible for phase separation. Table 2 summarizes the anticipated effect of various polymerization conditions on polymer phase separation. Cross-linking monomers coupled with polymerization rates fast relative to lateral polymer diffusion are obvious ways to combat phase separation caused by polymer aggregation. Anchoring the growing polymer chains by employing polymerizable surfactants is also an alternative that should slow lateral polymer diffusion and thereby suppress phase separation. Similar approaches govern some of the features of microemulsion polymerization discussed below. 

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