ω-Unsaturated methacrylate macromonomers as reactive polymeric stabilizers in mini-emulsion polymerization

Polymer latexes of poly(benzyl methacrylate) P(BzMA) were synthesized by mini-emulsion polymerization, using hexadecane as the hydrophobe and omega-unsaturated methacrylate-based macromonomers as a reactive stabilizer. The amphiphilic macromonomers were synthesized by catalytic chain transfer emulsion polymerization (CCTP) and subsequent chain extension via sulfur-free reversible addition-fragmentation chain transfer (SF-RAFT). Their critical micelle concentration (CMC) was determined by dynamic light scattering (DLS), and micelle size was measured using DLS and small angle X-ray scattering (SAXS). The surface activity of the stabilizers was measured by pendant drop tensiometry and compared to modelled behaviour. For the mini-emulsion polymerizations, macromonomer stabilizers were added at a range of concentrations, with respect to the dispersed phase. Using less than 5 wt% stabilizer, SEM micrographs showed many of the particles were bowl-shaped. This morphology was studied in depth and we propose that monomer transport occurs between particles during polymerization towards the smaller particles as a direct result of compartmentalization. At concentrations of 5 wt% and higher, bimodal droplet and particle distributions were observed by DLS and SEM. We propose shear-dependent depletion flocculation as the explanation. Lastly, the effectiveness of the reactive stabilizers was tested in terms of latex stability and molecular weight control. Resistance to coagulation during freeze-thaw cycles and prolonged dialysis were tested. Examination of P(BzMA) reaction kinetics and molecular weight indicated that the incorporation of macromonomer is gradual and less than quantitative at the end of the polymerization process, in agreement with the mechanistic understanding.