A method is developed to enable emulsion polymerization to be performed under RAFT
control to give living character without the problems that often affect such systems: formation of an oily
layer, loss of colloidal stability, or loss of molecular weight control. Trithiocarbonate RAFT agents are
used to form short stabilizing blocks from a water-soluble monomer, from which diblocks can be created
by the subsequent polymerization of a hydrophobic monomer. These diblocks are designed to self-assemble
to form micelles. Polymerization is initially performed under conditions that avoid the presence of monomer
droplets during the particle formation stage and until the hydrophobic ends of the diblocks have become
sufficiently long to prevent them from desorbing from the newly formed particles. Polymerization is then
continued at any desired feed rate and composition of monomer. The polymer forming in the reaction
remains under RAFT control throughout the polymerization; molecular weight polydispersities are
generally low. The number of RAFT-ended chains within a particle is much larger than the aggregation
number at which the original micelles would have self-assembled, implying that in the early stages of
the polymerization, there is aggregation of the micelles and/or migration of the diblocks. The latexes
resulting from this approach are stabilized by anchored blocks of the hydrophilic monomer, e.g., acrylic
acid, with no labile surfactant present. Sequential polymerization of two hydrophobic monomers gives
completely novel core−shell particles where most chains extend from the core of the particles through
the shell layer to the surface.
Amphiphilic, RAFT-capped, (acrylic acid) x (styrene) y diblock copolymers (x = 10, y ) 10, 5, 0) were synthesized and used as stabilizers in emulsion polymerization. Above the critical micelle concentration (cmc) of the diblocks and under appropriate reaction conditions micelles of the more hydrophobic diblocks were sufficiently nonlabile to be nucleated and act as seed particles for latex particle formation. The key parameters which allow control over the system are diblock hydrophobicity and initiator concentration. A homogeneous nucleation mechanism is most likely to operate below the cmc of the diblocks.
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