Trommsdorff gel effect

At the lowest emulsifier concentration there is a long period of constant reaction rate which, however, is terminated at 70 to 80% conversion by a rather high maximum of the reaction rate owing to the strong gel effect (Trommsdorff effect) in the comparably large particles. 

In free-radical polymerization, the autoacceleration (or gel effect, Trommsdorff effect) has been known for a long time 161 168>. 

Here, gt takes into account one interesting feature of the free radical polymerization kinetics the auto acceleration in the rate of reaction (better known as the Trommsdorff or gel effect). The gel effect is taken into account in the model (42) by including the experimental correlation developed in [18] for the termination rate ... 

When polymerizations become viscous, termination slows allowing an increase in the number of growing chains and rate of polymerization. This is known as the gel of Trommsdorff effect. If such reactions are allowed to continue without cooling, explosions are possible. 

Radical chain polymerizations are characterized by the presence of an autoacceleration in the polymerization rate as the reaction proceeds [North, 1974], One would normally expect a reaction rate to fall with time (i.e., the extent of conversion), since the monomer and initiator concentrations decrease with time. However, the exact opposite behavior is observed in many polymerizations—the reaction rate increases with conversion. A typical example is shown in Fig. 3-15 for the polymerization of methyl methacrylate in benzene solution [Schulz and Haborth, 1948]. The plot for the 10% methyl methacrylate solution shows the behavior that would generally be expected. The plot for neat (pure) monomer shows a dramatic autoacceleration in the polymerization rate. Such behavior is referred to as the gel effect. (The term gel as used here is different from its usage in Sec. 2-10 it does not refer to the formation of a crosslinked polymer.) The terms Trommsdorff effect and Norrish-Smith effect are also used in recognition of the early workers in the field. Similar behavior has been observed for a variety of monomers, including styrene, vinyl acetate, and methyl methacrylate [Balke and Hamielec, 1973 Cardenas and O Driscoll, 1976, 1977 Small, 1975 Turner, 1977 Yamamoto and Sugimoto, 1979]. It turns out that the gel effect is the normal ... 

Frequently, even if as little as 20% of the monomer has polymerized, an autoaccelerating polymerization effect will take place. This may manifest itself in an increase in the heat evolved as the process nears completion. Particularly in large-scale, industrial polymerizations, this effect, known as the Trommsdorff effect or gel effect, may be quite dangerous. In fact, serious explosions have... 

This produces a rise in the concentration of active centers and a corresponding increase in the propagation rate. Chains produced at this stage are longer, and this leads to a broadening of the molar mass distribution. The term gel effect is widely used to describe this effect, although no gel is actually formed in the system. The effect is also called the Trommsdorff effect (see Chapter 5). 

These results explain the findings of Blackley and Haynes who also showed that the molecular weight of the polymer formed in the presence of ethyl benzene was lower than that in its absence. Calculation from their experimental data shows that their n varied from 0.005 to 0.039 radicals per particle, well into Case 1. Thus, their explanation on the basis of the Trommsdorff "gel" effect cannot be correct since this requires the mutual termination of two macroradicals in a particle, which obtains only under Case 3 kinetics. Similar experiments on the effect of the diluents on "insitu" (unseeded) and seeded emulsion polymerization indicates that n decreases due to desorption of free radicals from the particles (27). 

The gel or Trommsdorff effect (11) is the striking autoacceleration of the vinyl polymerization reaction as the viscosity of the monomer-polymer solution increases. Chain termination involving the recombination of two free radicals becomes diffusion controlled and this results in a decrease in the rate of termination. The concentration of active free radicals therefore increases proportionally. To sum up the gel effect the rate of Vazo catalyst initiation increases with temperature the rate of propagation or polymerization increases with the viscosity and the rate of termination of the growing polymer chains decreases with the viscosity. This of course also results in an increase in the molecular weight of linear polymers, but this has no practical significance when crosslinking is part of the reaction. 

A chemical instability which can be distinguished based on the gel effect or Trommsdorff effect, or on the ceiling effect. The influence of the gel effect on stability depends on its location. For example, if it occurs entirely within the screws, the increases in viscosity and reaction rate will stabilize the process, but if it occurs close to the die it will mainly affect viscosity in the die, which leads to destabilization. The ceiling effect will generally decrease the reaction speed, and therefore it will be negative for the stability of the process. 

With increasing time, the system will attain and maintain its steady state in the distribution of free radicals in all particles, with the possible exception of growth during a particularly rapid Trommsdorff gel effect. 

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. 

The first part of the log function up to the maximum may be interpreted as follows. At the beginning, the reaction proceeds according to the conception of Harkins. Dispersed PMMA takes up 3 to 4 times its monomer (by weight) contrary to PMMA, dispersed polystyrene dissolves only 1 to 1.5 times its own weight of monomer (Table I). In the case of the emulsion polymerization of MM A, the pure monomer phase therefore disappears at about 30% conversion. From here on, the Trommsdorff (38) or gel effect acts to increase the over-all reaction rate until the maximum is reached. During this period, the bulk of the monomer in monomer-polymer particles is consumed. 

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 ... 

A phenomenon that has a particularly significant effect in the value of and has been studied for many years is the so-called gel effect, which is also known as Trommsdorff or Norrish-Smith effect [14, 15], This consists in an autoacceleration of the reaction rate as the conversion increases, and it is due to an effective decrease in the termination rate as the growing radicals encounter more difficulty in diffusing in the increasingly viscous medium. As the value of decreases by several orders of magnitude in the course of the polymerization as a consequence of this phenomenon, the concentration of growing radicals [P] increases, as well as the polymerization rate (Eq. 4.7,... 

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