RAFT synthesis of sterically stabilized methacrylic nanolatexes and vesicles by aqueous dispersion polymerization.

Emulsion polymerization is widely used for waterborne coatings. Reducing the latex particle size is known to promote coalescence and hence enhance film formation. However, the synthesis of smaller latexes usually requires additional surfactant, which can compromise the quality of waterborne coatings (e.g. poor adhesion and reduced film quality due to migration of excess surfactant). In principle, reactive surfactants offer a potentially decisive advantage over conventional surfactants in emulsion polymerization because they bind irreversibly to the latex and hence cannot migrate during film formation; this allows defect-free coatings to be produced with reduced moisture sensitivity. Over the last two decades, controlled/living radical polymerization techniques have become powerful tools in polymer synthesis. There are many examples of latex syntheses based on these approaches. For example, nitroxide-mediated living radical polymerization has been used by Charleux, El-Aasser, Okubo, and Georges to mediate the miniemulsion polymerization of n-butyl acrylate and styrene. ATRP has been optimized by Matyjaszewski and Okubo for the (mini)emulsion polymerization of (meth)acrylic and styrene monomers. Reversible addition– fragmentation chain transfer (RAFT) polymerization has been extensively exploited in the context of both emulsion and miniemulsion polymerization by Hawkett, Charleux, El-Aasser, Cunningham, and Zhu. There are also a number of RAFT syntheses conducted under nonaqueous dispersion polymerization conditions. However, as far as we are aware, there are only three examples of the application of controlled/living radical polymerization techniques for latex syntheses by aqueous dispersion polymerization. In each case, a relatively expensive speciality monomer was utilized for the latex core, namely N-isopropylacrylamide or N,N’-diethylacrylamide. This relative lack of research is perhaps surprising, because aqueous dispersion polymerization is conceptually much simpler than aqueous emulsion polymerization since the initial reaction solution is homogeneous in the former case. Presumably, the paucity of experimental data merely reflects the fact that there are relatively few vinyl monomers that are suitable for latex syntheses by aqueous dispersion polymerization. Recently, we reported the use of conventional (nonliving) free radical chemistry for the aqueous dispersion polymerization of a commodity methacrylic monomer, 2hydroxypropyl methacrylate (HPMA). The resulting PHPMA latexes were stabilized by poly(N-vinylpyrrolidone) and the mean particle diameter could be varied from approximately 100 to 1000 nm diameter, with good control over the particle size distribution being achieved in most cases. Herein we explore the RAFT synthesis of sterically stabilized PHPMA nanolatexes of 20 to 100 nm diameter by surfactant-free aqueous dispersion polymerization using a poly(glycerol monomethacrylate)-based chain transfer agent (CTA) as the reactive steric stabilizer. Thus both the latex cores and the steric stabilizer chains of the resulting nanolatexes are highly hydroxylated for this prototype formulation. Moreover, varying the length of the targeted PHPMA chains allows the final size of the sterically stabilized nanolatex particles to be controlled quite precisely (see Scheme 1, Table 1, and the Supporting Information).