Trommsdorff effect, free radical polymerization

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 ... 

If ki and k.i are much larger than kj, the reaction Is controlled by kj. If however, ki and k.i are larger than or comparable to kz, the reaction rate becomes controlled by the translational diffusion determining the probability of collisions which Is typical for specific diffusion control. The latter case Is operative for fast reactions like fluorescence quenching or free-radical chain reactions. The acceleration of free-radical polymerization due to the diffusion-controlled termination by recombination of macroradicals (Trommsdorff effect) can serve as an example. 

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. 

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

A lot of papers have been published on the effect of solvent on free radical polymerization rate. Studies on this effect have greatly been stimulated by (1) Norrish-Trommsdorff effect, (2) Q, e-scheme in copolymerization, (3) Retardation of the polymerization rate of vinyl acetate, (4) radical complex. 

Free-radical polymerizations of certain monomers exhibit autoacceleration at high conversion via an additional mechanism, the isothermal gel effect or Trommsdorff effect (23-26). These reactions occur by the creation of a radical that attacks an unsaturated monomer, converting it to a radical, which can add to another monomer, propagating the chain. The chain growth terminates when two radical chains ends encounter each other, forming a stable... 

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. 

Polymerization of vinyl or methacrylic monomers (especially in conjunction with crosslinking monomers) within the wood often results in an autoacceleration during the latter phase of the polymerization this phenomenon is known as the Trommsdorff or gel effect in homopolymerization reactions (Duran and Meyer, 1972 Trommsdorff et a/., 1948). The gel effect arises from a decrease in the termination rate of the free radical polymerization, caused in turn by the effect of the local viscosity on the diffusion rates of the growing polymer chains. Since the heat of polymerization cannot be removed rapidly enough to maintain isothermal conditions, autoacceleration is characterized by a strong exotherm the intensity of the exotherm depends on the catalyst level, as illustrated in Figures 11.4 and 11.5 (Siau et al., 1968). 

Due to their unique features, microsystems truly represent new process tools for the synthesis of polymer through free radical polymerization. Phenomena such as thermal runaway, Trommsdorff effect and segregation, which are commonly encountered in conventional polymer reactors, can be reduced or alleviated when microreactors and micromixers are employed. Moreover, successful implementation of microsystems, in an already-existing production line as well as the numbering approach have proved that despite their small internal volume microsystems can be considered for large scale polymer production. 

Seth, V. Gupta, S.K. (1995) Free radical polymerizations associated with the Trommsdorff effect under semibatch reactor conditions an improvement model. Journal of Polymer Engineering, 15(3-4), 283-323. 

There is also the case of reaction-controlled diffusion (briefly discussed in Section 1.2.4), closely associated with the Trommsdorff effect [55, 56], which leads to the loss of control even under isothermal conditions because the slow diffusion of radicals drastically decreases the rate of termination. This subsequently increases the concentration of radicals, as well as the rate of propagation relative to termination. Under these circumstances, polydispersity can increase significantly, easily reaching PDIs in excess of 10. In fact, the solutions found for polydispersity in a steady-state system in Section 1.2.7 generally underestimate the PDI values expected by a polymerization engineer due to various effects at high conversion and other deviations from steady-state conditions. It has also been recently shown that nanocon-fmement of a free radical polymerization can actually lower the polydispersity [57-59]. 

Ray, A. B., D. N. Saraf, and S. K. Gupta, Free Radical Polymerizations Associated with the Trommsdorff Effect under Semihatch Reactor Conditions I. Modeling, Polym. Eng. Sci., 35, 1290-1299, 1995. 

Autoacceleration, Glass and Zutty (S) and Burnett and Melville 9) reported an increase in the rate and average degree of polymerization with increasing solution viscosity, heterogeneous conditions and chain coiling for free radical, vinyl polymerizations. Autoacceleration is also called Trommsdorff. (10) effect. 

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. 

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