Synthesis of Well-Defined Environmentally Responsive Polymer Brushes by Aqueous ATRP

Functionalized anionic polystyrene latex particles with ATRP initiators were synthesized by surfactant-free shell-growth emulsion polymerization of styrene and 2-(2‘-chloropropionato)ethyl acrylate (HEA-Cl). N-Isopropylacrylamide (NIPAM) was polymerized from these particles by surface-initiated aqueous ATRP using PMDETA/CuCl and HMTETA/CuCl catalysts to synthesize poly(N-isopropylacrylamide) (PNIPAM) brushes. The grafted latexes were characterized for molecular weight of the PNIPAM chains, grafting density, and hydrodynamic thickness of the grafted polymer layer. Molecular weights of the grafted PNIPAM chains depended on the monomer concentration, concentration of copper(II) complex, and the presence of external initiator in the reaction medium. Mn of the grafted chains increases with increase in the monomer concentration and decreases with addition of copper(II) complex and external initiator. The HMTETA/CuCl catalyst produces higher molecular weight chains than PMDETA/CuCl. Molecular weights from ∼50 000 to 800 000 with low polydispersities, between 1.25 and 1.4, were achieved. The grafting density of PNIPAM on the surface increases with increasing monomer concentration and decreases with addition of copper(II) catalyst and external initiator. Block copolymerization of N,N-dimethylacrylamide from PNIPAM-grafted latex demonstrated that the chains are terminated with a chlorine atom, and the grafting reactions are taking place by the ATRP mechanism. The hydrodynamic thickness (HT) of the grafted PNIPAM layer scales as DP0.66 (where DP = degree of polymerization) at constant grafting density (chains/nm2). The HT values for PNIPAM brushes are sensitive to temperature and salt concentration. Since the transition from extended coil to collapsed structure occurs over a range of temperature and salt concentration, it follows a second-order transition, as predicted by theory. The thickness of the collapsed brush is sensitive to the type of stimulus used to induce the phase transition.