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Small-angle neutron scattering (SANS) studies of reversible addition−fragmentation chain transfer (RAFT) microemulsion polymerizations of butyl acrylate and 2-ethylhexyl acrylate with the RAFT agent methyl-2-( O-ethylxanthyl)propionate (MOEP) allow the observed rate retardation to be attributed to slow fragmentation of the macro-RAFT radical. Microemulsion polymerization allows the RAFT mechanism to be investigated in the absence of termination reactions so that the cause of the rate retardation frequently observed in both homogeneous and heterogeneous polymerizations may be isolated. However, the concentration of monomer at the locus of polymerization ( Cmon(part)) must be known as a function of conversion before a mechanistic study of the RAFT reaction can be completed. SANS is uniquely capable of probing the evolving microstructure of both the micelles and polymer particles during the polymerization. Cmon(part)then can be calculated by combining this microstructural information with material balances on the components of the microemulsion. [ABSTRACT FROM AUTHOR]