The Trommsdorff Effect

This acceleration occurs because the formation of polymer results in a big change in the viscosity of the solution. This does not affect the diffusion of small molecules 

The heat evolved from these exothermic reactions also increases significantly and if steps are not taken to dissipate it, there can be unfortunate consequences (you can blow up the lab, not to mention yourself) This is called the Trommsdorff effect, even though it was apparently originally discovered by two guys named Nonish and Smith. This effect can be explored further by considering another useful quantity we can obtain from kinetics, an equation for the degree of conversion as a function of time. 

Let s go back to our expression for the rate of polymerization, R - d[M]idt. Integrating this will give us an expression for the change in monomer concentration with time. In case you ve just crawled out of bed and haven t had your coffee yet, we will be explicit let the concentration of monomer at time t - 0 be [A/]0, and at time t be [M]. Substituting Equation 4-28 into Rp = - d[M]/dt and integrating we get Equation 4-30. 

The degree of conversion is simply the fraction of monomer that has been used up or converted to polymer (Equation 4-31). 

FIGURE 4-19 Graph of conversion (%) versus time [redrawn from the data of G. V. Schultz and G. Harborth, Macromol. Chem., 1, 106, (1967)]. 

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The rate process for termination is hindered through the Trommsdorff 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. 

The chain termination rate varies inversely with the viscosity of the polymerization medium because of the Trommsdorff Effect (i.e., the reduction of the macroradical mobility with increasing reaction viscosity). This effect significantly influences reaction rate[ ,2, 10]. 

A kinetic study for the polymerization of styrene, initiated with n BuLi, was designed to explore the Trommsdorff effect on rate constants of initiation and propagation and polystyryl anion association. Initiator association, initiation rate and propagation rates are essentially independent of solution viscosity, Polystyryl anion association is dependent on media viscosity. Temperature dependency correlates as an Arrhenius relationship. Observations were restricted to viscosities less than 200 centipoise. Population density distribution analysis indicates that rate constants are also independent of degree of polymerization, which is consistent with Flory s principle of equal reactivity. 

The desorption and termination constants were calculated for a copolymer from the corresponding homopolymer constants as discussed in Nomura and Fujita (12.) The homopolymer desorption coefficients were calculated from the appropriate chain transfer constants and radical diffusivities in the aqueous and polymer phases using an extension of the desorption theory developed by Nomura and Fujita (12.). The homopolymer termination constants were corrected for the Trommsdorff effect by using the Friis and Hamielec (12) correlation. 

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... 

Diffusion of the macroradicals controls can be assumed to be the termination reaction. However, that is not the case the termination rate constant is absolutely independent of the degree of polymerization, as shown in Table I. Therefore, the assumption must be that the diffusion of the segment at the end of the radical chain controls the termination process (as long as the Trommsdorff effect is not rate-determining). 

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). 

The EGDMA addition produces a significant decrease in the conversion at which the Trommsdorff effect makes its appearance. For 30 wt% EGDMA, the pseudo steady-state period has completely disappeared, which means that the concentration of free radicals increases continuously after... 

When vinyl monomers are solutes in solution phase, the morphology and molecular orientation of cellulose can be selectively changed dependent on the composition of the solution. Also, the extent and rate of grafting are influenced by the composition of the solution which is referred to as the Trommsdorff effect ( 3, 27). 

On the other hand, Eq. (136) can be considered as the simple form of the Trommsdorff effect. It is very interesting, because the Trommsdorff effect is not derived from any statistical models of polymerization (as is the statistical polymer). 

Derive the expression for the rate of a free radical polymerization. Using this expression, account for the Trommsdorff effect and the inability of ethylene to polymerize free radically at ordinary temperatures and pressures. 

The lifetime of the growing polymer chain the Trommsdorff effect... 

The major point of Interest in the polymerization curve, Figure 2, is the onset of the Trommsdorff effect, sometimes called autoacceleration. As is well known, the molecular weight increases rapidly after the onset of the Trommsdorff effect. 

This sample was affected by the Trommsdorff effect, and its molecular weight is known with less certainty than the other samples. 

Case 3 n > 0.5. In this scenario, there is not instantaneous termination as in cases 1 and 2 thus, two or more radicals can coexist in a particle. Large particle sizes, high rates of free-radical capture in particles, and the Trommsdorff effect (Chapter 4) favor this scenario. 

Both the Trommsdorff effect and thennal autocatalysis can lead to the autoacceleration in free-radical polymerization. The results indicated that the autoacceleration observed in the smallest test tube was mostly due to the Trommsdorff effect, while both the Trommsdorff effect and thermal autocatalysis strongly affected the onset of autoacceleration in the larger polymerization system. When an exothermic reaction is performed in a larger system, more heat tends to be accumulated in a reactor, since a sur ce-to-volume ratio is decreased as the size of the system increases. There was a critical size in the inner diameter of the test tube at which the behavior of the autoacceleration of the polymerization changes. 

The difTerence in the mechanism of autoacceleration, depending on the size of the test tubes, changed the character of the PMMA produced in the reaction system. As shown in Figure 3, both M and MJM increased in the smallest test tube after the onset of autoacceleration. This is a typical phenomenon for the Trommsdorff effect, since it is led by the retardation of the termination reaction. Increase of M of the polymer, however, was not observed in the larger test tubes. Thermal runaway led to rapid decomposition of the initiator to produce more radicals transiently. This process produced more polymers having a lower degree of polymerization, lowering M,. 

Another type of controlled radical polymerization employs a reversible termination with a nitroxide compound [21]. Rosenfeld et al. [22] reported details of the nitroxide-mediated radical polymerization of styrene and butyl acrylate at 140 °C in a 2.9 m tubular micro-reactor with an inner diameter of 900 gm. Whereas, for the low-heat-producing monomer, styrene, the differences between a batch process and the microtubular reaction were small, in the case of butyl acrylate the difference was high. This situation, which may have been due to the Trommsdorff effect in the batch reaction (Figure 14.11), indicated that the polymerization was no longer under control. By contrast, no such effect was observed in the tubular micro-reactor, and the degree of conversion remained quite low under the applied conditions. 

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