Norrish-Smith effect

Studies of the copolymerization of VDC with methyl acrylate (MA) over a composition range of 0—16 wt % showed that near the intermediate composition (8 wt %), the polymerization rates nearly followed normal solution polymerization kinetics (49). However, at the two extremes (0 and 16 wt % MA), copolymerization showed significant auto acceleration. The observations are important because they show the significant complexities in these copolymerizations. The auto acceleration for the homopolymerization, ie, 0 wt % MA, is probably the result of a surface polymerization phenomenon. On the other hand, the auto acceleration for the 16 wt % MA copolymerization could be the result of Trommsdorff and Norrish-Smith effects. 

Some typical examples of this autoacceleration are (Figure 5) Norrish and Smith ( 2) polymerized methyl methacrylate in bulk and in the presence of various precipitants and measured the polymerization rates dilatometrically. They determined that autoacceleration of the precipitation polymerizations was larger than that observed for the Trommsdorf effect in bulk polymerization. 

A superficially related dependence of on the medium has been observed by Norrish and Smith working with methyl methacrylate, and by Burnett and Melville with vinyl acetate. Rates in poor solvents are high, and determination of by the rotating sector method reveals what appears to be a decrease in kt in the poor solvents. This apparent decrease in kt accounts for the increased rate of polymerization. Actually, precipitation of the polymer seems to be responsible for the effect. The growing radicals become imbedded in precipitated droplets, presumably of very small size. The termination reaction is suppressed owing to isolation of the chain radical in one droplet from that in another. This gel effect is fairly common in systems yield-... 

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 ... 

Norrish and Smith f have studied two reactions, namely, the interactions of trimethylamine with m- and with p-nitrobenzyl chloride in non-polar solvents, with a view to correlating the absolute rates of change with the values for the energy of activation as calculated from the temperature coefficients. They find here also that there is a marked deactivating effect of the solvent, which, as they point out, is not surprising, since in solution the mean free path is of the same order as the molecular diameter, and nearly every collision between potentially reactant solute molecules must therefore of necessity partake of the nature of a ternary collision at least, in which the third body is a solvent molecule . 

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... 

Deviations from ideal kinetics due to size-dependence and diffusion control of termination produce relatively weak effects at low conversions. However, at high conversions these effects are very significant in most radical polymerizations. Thus, instead of the reaction rate falling with time, as would be expected from Eq. (6.24) since the monomer and initiator concentrations decrease with conversion, an exact opposite behavior is observed in many polymerizations where the rate of polymerization increases with time. A typjgal example of this phenomenon is shown in Fig. 6.10 for the polymerization of methyl methacrylate in benzene solution at 50°C (Schulz and Haborth, 1948). An acceleration is observed at relatively high monomer concentrations and the curve for the pure monomer shows a drastic autoacceleration in the polymerization rate. This type of behavior observed under isothermal conditions is referred to as the gel effect. It is also known as the Tromsdorff effect or Norrish-Smith effect in honor of the early researchers in this field. ... 

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,... 

When the concentrations of monomers are high in solution or bulk polymerizations, typical autoaccelerations of the rates can be observed. This is known as the gel effect or as the Trammsdorff effect, or also, as the Norrish—Smith effect [66]. The effect has been explained as being caused by a decrease in the rate of termination due to increased viscosity of the medium. Termination is a reaction that requires two large polymer-radicals to come together and this can be impeded by viscosity. At the same time, in propagation the small molecules of the monomer can still diffuse for some time to the radical sites and feed the chain growth. 

The pronounced decrease of (kt) in the TD regime is associated with the occurrence of the so-called gel-effect. " Also known as the Trommsdorff, Norrish-Smith or Norrish-Trommsdorff effect, this effect can cause problems within both an industrial and scientific context ranging from a product mixture to reactor explosion, due to its exothermic nature. " " Increasing polymer content induces overlap of polymer chains and decreases the mesh-size in between the polymer chains beyond a critical limit. As a consequence, TD may become the rate-determining step in Scheme 1.21 for the majority of macroradicals, thus (kt) decreases by orders of magnitude in some cases. It is important not to confuse the gel effect with the auto-acceleration that is observed when a polymerization is carried out under non-isothermal conditions, so that the reaction temperature increases with increasing monomer conversion, due to the exothermic nature of the polymerization reaction. The gel effect is observed under isothermal reaction conditions. The cause of the gel effect has been discussed extensively and various theories have emerged which can explain all or part of the experimental data (excellent reviews on the topic can be found in ref. 150 and 151). 

---