The zero-one and pseudo-bulk dichotomy

In free-radical polymerisations, rates are controlled by the processes of initiation, propagation, transfer and combination. Although these same processes operate in an emulsion polymerisation, the kinetics in an emulsion polymerisation particle are in general different 

It is nowwell established (e.g. Maxwell etal, 1991 van Berkel etal, 2003) that entry arises when a z-meric radical irreversibly attaches itself to a particle. There is also strong evidence (e.g. Morrison et al, 1994) that exit occurs by transfer within a particle to a monomer, producing a radical which is slightly soluble (e.g. it will be recalled that the water solubility of styrene monomer is 4 mM, and it is reasonable to suppose that a styrene radical will have a similar solubility). This new monomeric radical can diffuse away from the parent particle (Nomura etal., 1970). 

A system obeying pseudo-bulk behaviour is one wherein the kinetics are such that the rate equations are the same as those for polymerisation in bulk. In these systems, n can take any value in a pseudo-bulk system. Common cases are (a) when the value of n is so high that each particle effectively behaves as a microreactor, and (b) when the value of n is low, exit is very rapid and the exited radical rapidly re-enters another particle and may grow to a significant degree of polymerisation before any termination event. (This case is not the same as Smith and Ewart s Case 3 kinetics, because these were applicable only to systems with n significantly above. ) 

As will be seen, this simple dichotomy permits data to be interpreted in a way that avoids having so many fitting parameters that no meaningful mechanistic information can be obtained. 

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