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Norrish-Tromsdorff effect

With growing viscosity, the diffusion rate of propagating macromolecules decreases, the probability of their effective collision is diminished, and thus also the rate of bimolecular termination. As the rate of initiation is not appreciably affected by increasing viscosity, in a certain phase some radical polymerizations pass from a stationary to a non-stationary state in which the number of radicals increases. The polymerization is appreciably accelerated. This situation is called the gel effect or the Norrish-Tromsdorff effect [55]. As the change in the rate of termination contributes considerably to the gel effect, it will be discussed in detail in Chap. 6, Sect. 1.3. [Pg.250]

Norrish and Smith [29] and later Tromsdorff et al. [30] described a polymerization of methyl methacrylate, the rate of which increased from a certain conversion. The number of monomers of similar behaviour was extended by methyl acrylate [31 ], butyl acrylate [32] and other acrylates [33] and methacrylates [34], and vinyl acetate. The effect was explained by the reduction of the termination rate caused by hindered macroradical mobil-ity in viscous medium it was called the gel effect, or the Norrish-Tromsdorff effect. The gel effect is clearly manifested in radical polymerizations of weakly transferring monomers in bulk. It is significant also in the presence of a good solvent. The gel effect is suppressed by the presence of poor solvents++ and by... [Pg.396]

Diffusion control of termination is presently beyond any doubt. Various authors differ, however, in their opinion concerning the type of diffusion and the quantitative contribution of diffusion in the collection of processes manifested as the gel effect. Attempts to explain the Norrish-Tromsdorff effect on a molecular level are still of empirical character [8, 12, 36-38],... [Pg.397]

All the described procedures can reproduce surprisingly well the autoac-celerating course of the rate and the degrees of polymerization for the products of radical polymerizations in bulk or in concentrated solutions. From the point of view of the technological control of these systems it is a success. On the other hand, this very agreement hinders our approach to a real understanding of the basis of the Norrish-Tromsdorff effect because it is difficult to abandon incorrect ideas that have, nevertheless, been successful. [Pg.398]

Polymerizations as part of liquid-phase organic reactions are also influenced by mass and heat transfer and residence time distribuhon [37, 48]. This was first shown with largely heat-releasing radical polymerizations such as for butyl acrylate (evident already at dilute concentration) [49]. Here, a clear influence of microreactor operation on the polydispersity index was determined. Issues of mass transfer and residence time distribution in particular come into play when the soluhon becomes much more viscous during the reachon. Polymerizahons change viscosities by orders of magnitude when carried out at high concentration or even in the bulk. The heat released is then even more of an issue, since tremendous hot spots may arise locally and lead to thermal runaway, known in polymer science as the Norrish-Tromsdorff effect. [Pg.121]

In a completely different way run the polymerization at its final stage, when the degree of conversion of substrates is large. In this case a selfacceleration takes place and a departure from first order law is observed. This phenomenon is called the gel or Norrish-Tromsdorff effect. It is mainly due to the increased viscosity of the polymerization medium and the diminished probability of meeting macroradieals and their recombination. In that case one observes also a decrease of thermal conductivity of the system. It results in the temperature increase, thus in the acceleration of reaction. The gel effect leads also to the increase of molecular mass of polymer, because the inhibition of the termination process does not affect the growth rate of the chain. [Pg.269]

In view of Eq. (6.26) for ideal polymerization kinetics one would normally expect the reaction rate to fall with time, since the monomer and initiator concentrations decrease with conversion. However, the exact opposite behavior is observed in many polymerizations where the rate of polymerization increases with time. A typical example of this phenomenon is shown in Fig. 6.10 for the polymerization of methyl methacrylate in benzene solution at 50°C [49], At monomer concentrations less than about 40 wt% in this case, the rate (slope of conversion vs. time) is approximately as anticipated from the ideal kinetic scheme described in this chapter, that is, the rate decreases gradually as the reaction proceeds and the concentrations of monomer and initiator are depleted. An acceleration is observed, however, at higher monomer concentrations and the curve for the pure monomer shows a dramatic autoacceleration in the polymerization rate. Such behavior is referred to as the gel effect. (The term gel used here is different than the usage in Chapter 5 as it refers only to the sharp increase in viscosity and not to the formation of a cross-linked polymer.) The autoaccelerative gel effect is also known as the Tromsdorff effect or Norrish-Smith effect after pioneering workers in this field. It should be noted that the gel effect is observed under isothermal conditions. It should thus not be confused with the acceleration that would be observed if a polymerization reaction were carried out under nonisotherraal conditions such that the reaction temperature increased with conversion due to exothermicity of the reaction. [Pg.518]


See other pages where Norrish-Tromsdorff effect is mentioned: [Pg.416]    [Pg.397]    [Pg.2617]    [Pg.2520]    [Pg.194]    [Pg.416]    [Pg.397]    [Pg.2617]    [Pg.2520]    [Pg.194]    [Pg.43]   
See also in sourсe #XX -- [ Pg.194 ]




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