Nanoreactors for Aqueous RAFT-Mediated Polymerizations

Nanoreactors provide the ideal setting where selected chemical reactions can take place with high efficiency in controlled environments. Much attention has been focused on carrying out organic reactions in surfactant-based micelles in water.(1) This methodology can provide significant advantages over solution reactions due to compartmentalization of reactants or catalysts, greater heat transfer through the aqueous environment, controlled rates of reaction, better accommodation for potentially explosive or runaway exothermic reactions, and the environmentally friendly nature of the aqueous reaction medium.(2, 3)The concept of nanoreactors is not new to free-radical polymerizations, in which surfactant stabilized micelles are commonly used.(4) However, these micelle-type nanoreactors (termed ab initio emulsion polymerizations) failed to produce well-controlled molecular weight distributions (MWDs) and particle size distributions (PSDs) in reversible addition−fragmentation chain transfer (RAFT) controlled/“living” radical polymerizations (CRP) due to transportation problems of monomer or other compounds (e.g., RAFT agent) from monomer droplet reservoirs to the growing particles.(3) Miniemulsions have solved the transportation problem as each droplet is an isolated bulk nanoreactor, in which all components are located within the droplet.(5) The drawbacks of miniemulsions are (a) the difficulty in isolating pure polymer product due to the high levels of hexadecane (to reduce Oswald ripening) and surfactant and (b) the PSDs are broad with little or no control over the particle size. Other methods of growing amphiphilic diblock copolymers in situ led to poor control of the chain length polydispersity (>1.4, while below 1.2 is considered “good control”), broad PSDs,(6) and very long polymerization times.(7)In our quest to find better alternative water-based methods to carry out RAFT-mediated polymerizations, specially designed nanoreactors have provided the solution to produce narrow and controlled molecular weight and particle size distributions. The nanoreactors used in this work were designed to (a) encapsulate the hydrophobic or water insoluble monomer, (b) selectivity bind the RAFT agent directly in the nanoreactor to avoid transportation problems,(8) (c) provide steric colloidal stabilization of the nanoparticles, (d) produce monodisperse nanoparticles, and (e) be easily removed to obtain the pure synthesized polymer (see Scheme 1). We created the nanoreactor from a diblock copolymer of poly(N-isopropylacrylamide-b-dimethylacrylamide) or P(DMA49-b-NIPAM106), which for the PNIPAM block(9) is hydrophilic below its lower critical solution temperature (LCST 36 °C)(10) and water-soluble and above its LCST forms self-stabilized, through the hydrophilic PDMA block, monodisperse polymer particles in the nanosize range (22.4 nm). The RAFT agent, a low molecular weight PNIPAM18−SC(═S)SC4H9 (MacroCTA), was designed to be highly compatible with the nanoreactor and when self-assembled with the diblock above the LCST of PNIPAM gave a hydrodynamic diameter of 31.5 nm (see Table S1 in Supporting Information). The great advantage of our methodology is that water, P(DMA49-b-NIPAM106), PNIPAM18−SC(═S)SC4H9, styrene monomer, and water-soluble initiator can be added in one pot and heated above the LCST to form the nanoreactor