Norrish 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. 

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. 

The Tromsdorf effect, also called the Norrish effect or gel effect, is associated with exothermic reactions during bulk polymerization. Autoacceleration of the polymerization rate can occur with medium to high polymerization conversions. This phenomenon inhibits termination. Strength of the Tromsdorf effect is calculated as the gel effect index [12]... 

There is ample experimental evidence to support the relationship Rp) n t ix [M]()[I]o for most systems. However, when [M]o is high there often is observed a sharp increase in Rp as the conversion of monomer increases. This phenomenon is known as autoacceleration or the Tromm-sdorff-Norrish effect or the gel effect and is demonstrated in Fig. 2.5 by some experimental conversion curves for polymerization of methyl methacrylate (MMA) at different [MMA]q. Since Rp is proportional to the... 

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. 

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