Methyl methacrylate diffusion hindered

Deviations resulting from the diffusion control of termination at low conversions of monomer to polymer are the relatively weak effects discussed in the preceding subsection. By contrast, changes in reaction rate resulting from hindered diffusion at high conversions are very important in most radical polymerizations. Figure 6-3 shows rate curves for the polymerization of methyl methacrylate in benzene at 50°C [17]. At monomer concentrations less than about 40 wt % in this case, the rate is approximately as anticipated from the standard kinetic scheme described in this chapter. Rp decreases gradually as the reaction proceeds and the concentrations of monomer and initiator are depicted. 

At high conversion, the growth of viscosity hinders the termination by radical-radical reactions, but it has less effect on the propagation reaction, because diffusion of small monomer molecules is not so much affected by viscosity. Hence, both the rate of polymerization and the molecular weight of the polymer show an increase, there is auto acceleration at high conversion. This effect is known as gel-effect or Tommsdorf-effect, which is usually accompanied by a sudden rise of temperature (Swallow 1973). Such effect was observed in many systems, for instance in the y-ray induced steady-state bulk polymerization of methyl methacrylate (Luthjens et al. 2001). 

---