The effect of chain transfer agent on the entry of free radicals in emulsion polymerization

The model of Maxwell et al. (Macromolecules 24, 1629 (1991)) for initiator efficiencies in emulsion polymerization has been extended to take into account the effect of added chain transfer agent (CTA). The model supposes that the rate-determining step for radical entry into latex particles is aqueous-phase propagation of the primary free radicals to a critical chain length z required for entry (forming species such as .M(z)SO4-, where M is a monomer entity and peroxodisulfate S2O8(2-) is the initiator). The effect of CTA on the entry rate is assumed to occur by facilitating the production of aqueous-phase free-radical species (CTA.) by transfer between species such as .M(n)SO4- (where n < z) and CTA in the aqueous phase. The CTA. will be formed at a reasonable rate provided the CTA is not too water-insoluble (e.g. C12H25SH); it should also be capable of entering the latex particles rapidly because of its relative insolubility. If the monomer-derived .M(n)SO4- tend to suffer aqueous-phase termination rather than entry, the overall rate of entry (and hence initiator efficiency) will be increased. This can explain the accelerating effect of intermediate molecular weight CTA's on emulsion polymerization of monomers such as butadiene, where z is large and hence initiator efficiency is very low in the absence of CTA, because most .M(n)SO4- undergo termination rather than entry into the latex particles