Trommsdorf-Norrish effect

In experimental work, however, it is frequently noticed that after a linear decrease in with u the rate of polymerization again increases and goes through a maximum, only to fall again to zero (at u = 1). This effect is seen at 60°C with methyl methacrylate for yields as low as 20% with styrene, by contrast, it does not occur until 65%. The effect is also observed when reactions are carried out isothermally. Therefore, it cannot primarily be caused by liberation of heat. The effect is accentuated when the medium is more viscous (addition of otherwise inert polymer, low initiator concentrations, poor solvent). Therefore, it must originate from some kind of diffusion control, and is called the gel effect or Trommsdorf-Norrish effect. 

Retardation is sometimes observed in RAFT polymerizations when high concentrations of RAFT agent are used and/or with inappropriate choice of RAFT agent. Some decrease in polymerization rate is clearly attributable to a mitigation of the gel (or Norrish-Trommsdorf) effect.384" 94 However, it is also clear that other effects are important. 

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. 

Changes in the population of propagating species and the increase in the polymer concentration mean that the rate coefficient for radical-radical termination will decrease with conversion. The moderate conversion regime is characterized by the autoacceleration phenomenon known as the gel or Norrish-Trommsdorf effect. Various empirical relationships defining or the rate of diffusion of long chains in terms of either the viscosity or the free volume have been proposed which enable the onset of the gel effect (Figure 5.3) to be predicted for a number of polymer systems. 

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