Termination kinetics heterogeneous polymerization

Much effort has been devoted during the last 30 years toward understanding the mechanisms operative in the coordination catalysis of ethylene and a-olefin polymerization using Ziegler-Natta systems (metal halide and aluminum alkyl, sometimes with Lewis base modifiers). Aspects of the complex heterogeneous reactions have been elucidated (jL- ) but the intimate mechanistic detail - for example the role of inhibitors and promoters, kinetics and thermodynamics of chain growth, modes of chain transfer and termination - comes primarily from studies of homogeneous catalysts ... 

Ionic polymerizations are not as well understood as radical polymerizations because ionic polymerizations are characterized by a wide variety of modes of initiation and termination. The nature of the reaction media in ionic polymerizations is often not clear since heterogeneous inorganic initiators are often involved. Further, it is extremely difficult in most instances to obtain reproducible kinetic data because ionic polymerizations proceed at very high rates and are extremely sensitive to the presence of small concentrations of impurities and other adventitious substances. 

Since emulsion polymerization is a free-radical addition polymerization, all the kinetic events, namely, initiation, propagation, termination and transfer reactions which have already been described in Chapter 1, are applicable to describe the overall rate of the polymerization and molar mass development of the latex polymer. However, the heterogeneous nature of the polymerization adds some complications due to partitioning of the various ingredients between the phases ... 

The kinetic mechanisms of chain polymerization reactions are relatively complex, with initiation, propagation, termination and transfer being the most significant steps in the overall reactions. However, the kinetic process can be more complex as a result of association or dissociation of the reacting species, viscosity increase or phase separation during polymerization, heterogeneity intrinsic to the process as in emulsion polymerization or some coordination... 

Kinetic analysis of the propagation and termination rate constants in the polymerization of MMA-EGDMA mixtures is more difficult than for MM A. An understanding of the polymerization depends on knowledge of the individual concentrations of C=C bonds that are (1) in monomer molecules and (2) attached to polymer molecules. This information may be obtainable for NMR spectra utilizing differences in the relaxation times of the two types of C=C environments. However, it is likely that the polymerization is heterogeneous with a non spatially random distribution of crosslinks. 

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