Trommsdorf 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. 

Each differential equation contains a flow term identified by Q/V (flow rate/reactor volume) and also a reaction term which can be identified by a rate of reaction or equilibrium constant (k, K, k ). These reaction and equilibrium constants are functions of temperature which, in this study, was fixed. The viscosity dependence of the equilibrium constant (relating reactive species to total polymer) shown in Equations 6 and 7 was observed experimentally and is known as the Trommsdorf effect (6). Table I lists values and units of all parameters in Equations 1-7. 

Trixylenyl phosphate, 22 493, 494 Trombone coolers, 23 779 Trommsdorf Effect, 16 281-282 in styrene polymerization, 23 382 Trona, 5 785t... 

As the direct test, let us employ equations derived above for the reproduction of the Trommsdorf effect. 

Samer also demonstrates the existence of multiple steady states in isothermal miniemulsion polymerization in a CSTR. This is not surprising, since multipH-city is a function of gel or Trommsdorf effect, and not of nucleation mechanism. 

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. 

Trommsdorf effect n. The acceleration of a reaction such as polymerization which continues to increase uncontrollably without external stimulus (also referred to as gel effect). Lenz RW (1967) Organic chemistry of high polymers. Interscience Inc., New York. 

Additional work in this area has been carried out by Hill et al. " They suggested that if a second polymer radical is added to DST, a crosslink will be formed, which will ultimately cause the formation of a gel network due to the Trommsdorf effect. Accordingly, the prepolymer chain length controlled the free-radical termination reaction and resulted in materials with desirable properties. A large variety of products with commercial applications were thus obtained. 

At high conversions, the solvation of the rubber and the gel effect (Trommsdorf effect) cause an increase in the molecular weight of the grafted polystyrene. The viscosity change with styrene conversion in the manufacture of HIPS is shown in Fig. 10 with typical rubber particle morphology [34]. 

The autoacceleration effect (Trommsdorf effect) is less pronounced in solution polymerization than in bulk or suspension polymerization due to lower viscosity of the polymerizing solution. To prevent a thermal runaway reaction, the reactants are often added gradually to the reactor. The polymer molecular weight is controlled through the use of a chain transfer agent and by initiator concentration and type. Monomer concentration, solvent type, and reaction temperature also affect the molecular weight. 

---