A kinetic investigation of seeded emulsion polymerization of styrene using reversible addition-fragmentation chain transfer (RAFT) agents with a low transfer constant

The mechanisms were investigated for the seeded emulsion polymerization of styrene mediated by xanthate-based reversible addition-fragmentation chain transfer (RAFT) agents with low transfer constants (ca. 0.7). gamma-Radiolysis relaxation measurements were used to determine the exit rate coefficient directly; this increased with the amount of RAFT, consistent with the chain-transfer characteristics of RAFT and the standard transfer/diffusion mechanism for exit. Combining the exit data with measurements of the steady-state rate with chemical initiation showed that the entry rate coefficients decreased with the amount of RAFT, while with gamma initiation the entry rate coefficients were unaffected, unless a very high RAFT concentration was used. This behavior is inconsistent with conventional assumptions about RAFT and the aqueous-phase propagation model for entry (which predicts the entry rate coefficient successfully for ordinary emulsion polymerization systems). It is postulated that the RAFT agents used in this work are surface active, which is not unexpected given the canonical forms of these RAFT agents. The dramatic decrease in entry rate coefficient for the chemically initiated system can be taken into account within the framework of the standard aqueous-phase model for entry if it is assumed that the surface activity gives an increased concentration of RAFT near the surface, which leads to increased transfer to radicals which desorb instead of entering, and whose subsequent reentry is "frustrated" by this event recurring, until they eventually undergo aqueous-phase termination.