Polymerization methyl methacrylate, diffusion-controlled kinetics

Diffusion-Controlled Kinetics in the Emulsion Polymerization of Styrene and Methyl Methacrylate... 

A valid kinetic model of stage 3 emulsion polymerization must account for diffusion-controlled termination and propagation reactions. Marten and Hamielec (J) have proposed such a model based on a free-volune theory and have confirmed its validity for the bulk polymerization of methyl methacrylate (7). Herein is reported an evaluation of this model for the emulsion... 

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. 

Deviations from ideal kinetics due to size-dependence and diffusion control of termination produce relatively weak effects at low conversions. However, at high conversions these effects are very significant in most radical polymerizations. Thus, instead of the reaction rate falling with time, as would be expected from Eq. (6.24) since the monomer and initiator concentrations decrease with conversion, an exact opposite behavior is observed in many polymerizations where the rate of polymerization increases with time. A typjgal example of this phenomenon is shown in Fig. 6.10 for the polymerization of methyl methacrylate in benzene solution at 50°C (Schulz and Haborth, 1948). An acceleration is observed at relatively high monomer concentrations and the curve for the pure monomer shows a drastic autoacceleration in the polymerization rate. This type of behavior observed under isothermal conditions is referred to as the gel effect. It is also known as the Tromsdorff effect or Norrish-Smith effect in honor of the early researchers in this field. ... 

The onset of the gel effect has been defined in terms of a switch from control of termination by segmental diffusion of polymer radicals to control by translational diffusion. Entanglement of polymer radicals can explain kinetic aspects of the polymerization of methyl methacrylate up to the stage where growth is diffusion controlled. Features of high conversion polymerizations of styrene and benzyl methacrylatehave been explained in terms of diffusion control of termination,... 

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