Trommsdorff effect methacrylates polymerization

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

Radical chain polymerizations are characterized by the presence of an autoacceleration in the polymerization rate as the reaction proceeds [North, 1974], One would normally expect a reaction rate to fall with time (i.e., the extent of conversion), since the monomer and initiator concentrations decrease with time. However, the exact opposite behavior is observed in many polymerizations—the reaction rate increases with conversion. A typical example is shown in Fig. 3-15 for the polymerization of methyl methacrylate in benzene solution [Schulz and Haborth, 1948]. The plot for the 10% methyl methacrylate solution shows the behavior that would generally be expected. The plot for neat (pure) monomer shows a dramatic autoacceleration in the polymerization rate. Such behavior is referred to as the gel effect. (The term gel as used here is different from its usage in Sec. 2-10 it does not refer to the formation of a crosslinked polymer.) The terms Trommsdorff effect and Norrish-Smith effect are also used in recognition of the early workers in the field. Similar behavior has been observed for a variety of monomers, including styrene, vinyl acetate, and methyl methacrylate [Balke and Hamielec, 1973 Cardenas and O Driscoll, 1976, 1977 Small, 1975 Turner, 1977 Yamamoto and Sugimoto, 1979]. It turns out that the gel effect is the normal ... 

Case 3 behavior occurs when the particle size is sufficiently large (about 0.1-1 pm) relative to kt such that two or more radicals can coexist in a polymer particle without instantaneous termination. This effect is more pronounced as the particle size and percent conversion increase. At high conversion the particle size increases and k, decreases, leading to an increase in h. The increase in h occurs at lower conversions for the larger-sized particles. Thus for styrene polymerization it increases from 0.5 to only 0.6 at 90% conversion for 0.7-pm particles. On the other hand, for 1.4-pm particles, n increases to about 1 at 80% conversion and more than 2 at 90% conversion [Chatterjee et al., 1979 Gerrens, 1959]. Much higher values of h have been reported in other emulsion polymerizations [Ballard et al., 1986 Mallya and Plamthottam, 1989]. Methyl methacrylate has a more pronounced Trommsdorff effect than styrene and vinyl acetate, and this results in a more exaggerated tendency toward case 3 behavior for methyl methacrylate. 

In the bulk polymerization of methyl methacrylate (MMA), the Trommsdorff effect sets in after an unperturbed period of about 15-20 % conversion. This enables a separation to be made between unperturbed and autoaccelerated kinetics. Benson and North, O Driscoll, Ito and others have attempted to describe the kinetics of autoacceleration or gelation by introducing a chain-length-dependent value of the termination rate constant... 

Panke, D., Stickler, M. and W. Wunderlich, "Polymerization of Methyl Methacrylate at High Conversions - Experimental Investigation of the Theory of Cardenas and O Driscoll Concerning the Trommsdorff Effect", in press Macromolekulare Chemie (1982). 

We ll come back to that in a minute. First let s look at a typical example of experimental data, such as that obtained for the polymerization of methyl methacrylate. Plots of conversion versus time for solutions of up to 40% in benzene follow the predicted relationship nicely. But, at higher concentrations and in the bulk (i.e., 100% methyl methacrylate), the Trommsdorff or auto-acceleration effect can clearly be seen (Figure 4-19). 

Polymerization of vinyl or methacrylic monomers (especially in conjunction with crosslinking monomers) within the wood often results in an autoacceleration during the latter phase of the polymerization this phenomenon is known as the Trommsdorff or gel effect in homopolymerization reactions (Duran and Meyer, 1972 Trommsdorff et a/., 1948). The gel effect arises from a decrease in the termination rate of the free radical polymerization, caused in turn by the effect of the local viscosity on the diffusion rates of the growing polymer chains. Since the heat of polymerization cannot be removed rapidly enough to maintain isothermal conditions, autoacceleration is characterized by a strong exotherm the intensity of the exotherm depends on the catalyst level, as illustrated in Figures 11.4 and 11.5 (Siau et al., 1968). 

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