The effect a Co(II) based catalytic chain transfer agent (CCTA) has on the course of the polymerization and the product properties of an emulsion polymerization is governed by the intrinsic activity and the partitioning behavior of the catalyst. The effect on the conversion time history, the molecular weight distribution and the particle size distribution is evaluated in batch emulsion polymerization of methyl methacrylate for three different CCTAs, which cover a range of intrinsic activities and partitioning behaviors. It was demonstrated that radical desorption from the particle phase to the aqueous phase preceded by chain transfer is the main kinetic event controlling the course of the polymerization and the product properties in terms of the particle size distribution. The experimental results show that the aqueous phase solubility of the CCTA is the key parameter controlling the course of the polymerization and the particle size distribution.

An alternative mass transport mechanism, based on collisions between different entities, is used to explain the performance of an extremely hydrophobic catalytic chain transfer agent (i.e. COPhBF) in emulsion polymerization. Mass transport in emulsion polymerization is generally accepted to proceed via the aqueous phase. COPhBF possesses no detectable water-solubility and would therefore be expected to be inefficient for molecular weight control in emulsion polymerization. However, proper molecular weight control using COPhBF has been demonstrated and results are presented that are consistent with the existence of an alternative mass transport mechanism in emulsion polymerization that circumvents the aqueous phase.

Summary: The use of very hydrophobic monomers in heterogeneous polymerisations is restricted because of insufficient mass transfer as a result of limited aqueous phase diffusion. Experimental evidence for a mass transfer mechanism in miniemulsion polymerisation that is based on the direct interaction between droplets as well as between droplets and particles ('collisions') rather than monomer diffusion is presented. Mass transfer that results in the formation of a copolymer in the miniemulsion polymerisation of a mixture of lauryl methacrylate and 4-tertbutyl styrene, is almost non-existing when physical contact between individual droplets is prevented by a membrane. However, monomer transport by diffusion is observed in reference experiments with more hydrophilic monomers.

It is demonstrated that CCT‐mediated emulsion polymerization can successfully be carried out in a PSPC. Proper molecular weight control is achieved with only minor quantities of a cobalt(II) complex, illustrating the robustness and scalability of CCT. Despite the successful implementation of CCT in the PSPC, differences in the course of the polymerization and the final latex properties (in terms of the PSD/MWD) for batchwise and PSPC operation are observed. Backmixing occurs to some extend in the PSPC, which increases the steady state CCTA concentration in the column. Consequently, a higher chain transfer activity is observed than expected based on the corresponding batch experiments, which affects the course of the polymerization and the final latex properties.

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