Balancing Chain Transfer Agent Transport and Reaction in Emulsion Polymerization

ABSTRACT The interrelationship between chain transfer agent (CTA) mass transfer and reaction in emulsion polymerization is examined. Three dimensionless parameters describing these phenomena are proposed. The Damkohler Number ( Da cta ) is defined as the maximum rate of CTA consumption divided by the maximum rate of CTA transfer from the monomer droplets to the polymerizing polymer particles. The probability of mass transfer‐ rather than reaction‐limitation for each CTA‐monomer pair is assessed. S D is the maximum instantaneous rate of consumption of CTA by reaction, divided by the desired instantaneous rate of consumption of CTA, assuming no mass‐transfer limitation on CTA. S D is defined so that it has a value of one when the CTA is consumed at the correct rate in order to control molecular weight and minimize branching and crosslinking over the entire polymerization. It is demonstrated that the desired value of S D in emulsion polymerization is different from that in homogeneous polymerization. S is defined as the maximum rate of CTA mass transfer divided by the desired rate of CTA consumption and is useful if CTA mass transfer is used to “meter” the CTA into the polymerizing polymer particles. Proper selection of CTA, and polymerizer operation are discussed in light of these three dimensionless parameters.