Monomer rich shell model

A number of workers have suggested that emulsion polymerization may not occur homogeneously throughout a polymer particle but either at the particle surface [75] or within an outer monomer-rich shell surrounding an inner polymer-rich core [76]. The latter has been referred to as the shell or core-shell model. The latter model has been proposed to explobserved constant rate behavior up to about 60 percent conversion, which according to Eq. (6.232) requires [M] to be constant, and the considerable experimental evidence which indicates that emulsified monomer droplets (serving as monomer reservoirs) disappear at 25 to 30 percent conversion and the monomer concentration drops thereafter. 

According to the core-shell model, the growing particle is actually heterogeneous rather than homogeneous, and it consists of an expanding polymer-rich (monomer-starved) core surrounded by a monomer-rich (polymer-starved) outer spherical shell. It is the outer shell that serves as the major locus of polymerization and Smith-Ewart (on-off) mechanism prevails while virtually no polymerization occurs in the core because of its monomer-starved condition. Reaction within an outer shell or at the particle surface would be most likely to be operative for those polymerizations in which the polymer is insoluble in its own monomer or under conditions where the polymerization is diffusion-controlled such that a propagating radical cannot diffuse into the center of the particle. 

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