A critical experimental examination of the gel effect in free radical polymerization: Do entanglements cause autoacceleration?

Over the past two decades, a substantial case has been made associating and/or attributing the ''gel'' effect or autoacceleration in free radical polymerization and its onset to entanglement of polymerizing chain radicals, resulting in a marked reduction in the termination rate parameter, k(t). This case has often been made by modeling a limited range of free radical polymerization data using the assumption that in the neighborhood of the gel effect k(t) is controlled by polymer self-diffusion which in turn exhibits entangled polymer dynamics. The present study provides critical experiments involving both bulk methyl methacrylate and styrene polymerizations which contradict the widely held belief that the gel effect onset is related to the formation of chain entanglements. By employing experimental conditions which tend to delay or eliminate the formation of chain entanglements, i.e., high initiator and/or chain transfer agent concentrations and addition of low molecular weight polymer prior to reaction, it is shown that the gel effect readily occurs in the absence of entanglements and that delaying the onset of entanglements does not necessarily delay the onset of the gel effect. Critical examination of the molecular weights produced in these and other experiments often indicates values too low for entanglement formation in solution (polymer plus monomer) and sometimes even in bulk polymer, not only at the onset but also throughout the gel effect. This fact illustrates the importance of understanding the relationship between molecular weight and entanglements in bulk polymer and solution in performing a critical examination of the relationship between the origin of the gel effect and entanglement formation. Furthermore, even under conditions where entanglements are likely to exist, the gel effect onset does not correlate with polymer molecular weight, either of the chains produced or the matrix, in a manner consistent with entanglement arguments. Although the present results indicate that the onset of the gel effect is unrelated to entanglement formation, whether the kinetics during the gel effect may be affected by entanglements if they do form remains an open question; experimental tests to investigate this issue are outlined. Other theories which purportedly explain the origin of the gel effect are also discussed in the context of data from this study and future experimental tests.