Norrish-Trommsdorff effect

For the major duration of a chain polymerisation the reaction is first-order in monomer concentration. However, at high conversions of monomer to polymer using either undiluted monomer or concentrated solutions there is a significant deviation from first-order kinetics. Under such circumstances the rate of reaction (and also molar mass of pol) mer) increases considerably. This so-called autoacceleration is sometimes referred to as the Trommsdorff-Norrish effect, after two of the pioneers in the study of polymerisation kinetics, who first noticed its occurrence. 

The kinetic results displayed in Figure 1 exhibit two other important features first an autoacceleration (the Trommsdorff-Norrish or gel effect) is apparent if sufficient monomer is initially present second, the bulk polymerization apparently proceeds to a limiting fractional conversion that is less that 1.00 (in this case ca. 0.85). It is generally agreed (1) that the Trommsdorff-Norrish effect is a consequence of a reduction is k with increasing Wp, but the physical origin(s) of this decrease has not yet been established definitively. 

In many polymerizations, a marked increase in rate is observed toward the end of the reaction instead of the expected gradual decrease caused by the depletion of the monomer and initiator. This auto-acceleration is a direct result of the increased viscosity of the medium, and the effect is most dramatic when polymerizations are carried out in the bulk phase or in concentrated solutions. The phenomenon, sometimes known as the Trommsdorff-Norrish or gel effect, is caused by the loss of the steady state in the polymerization kinetics. 

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). 

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. 

Chain transfer to polymer is reported as a major complication and is thought to be unavoidable in the polymerization of alkyl acrylates.200 202 The mechanism is believed to involve abstraction of a tertiary backbone hydrogen (Scheme 6.32). It has been proposed that this process and the consequent formation of branches may contribute to the early onset of the gel or Norrish-Trommsdorff effect in the polymerization of these monomers. At high temperatures the radicals formed may undergo fragmentation. 

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 ... 

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. 

It is well known that rates of polymerizations can increase markedly w ith the degree of conversion or with the polymer concentration. Some workers have attributed this solely or partly to a template effect. It has been proposed that adventitious template polymerization occurs during polymerizations of AA, MAA and AN, and that the gel or Norrish-Trommsdorff effect observed during polymerizations of these monomers is linked to this phenomenon. However, it is difficult to separate possible template effects from the more generic effects of increasing solution viscosity and chain entanglement at high polymer concentrations on rates of termination and initiator efficiency (Section 5.2.1.4). 

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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