Thermally Triggered Assembly of Cationic Graft Copolymers Containing 2-(2-Methoxyethoxy)ethyl Methacrylate Side Chains

Abstract

Thermoresponsive copolymers continue to attract a great deal of interest in the literature. In particular, those based on ethylene oxide-containing methacrylates have excellent potential for biomaterial applications. Recently, some of us reported a study of thermoresponsive cationic graft copolymers containing poly(N-isopropylacrylamide), PNIPAm, (Liu et al., Langmuir, 24, 7099). Here, we report an improved version of this new family of copolymers. In the present study, we replaced the PNIPAm side chains with poly(2-(2-methyoxyethoxy)ethylmethacrylate), PMeO(2)MA. These new, nonacrylamide containing, cationic graft copolymers were prepared using atom transfer radical polymerization (ATRP) and a macroinitiator. They contained poly(trimethylamonium)-aminoethyl methacrylate and PMeO(2)MA, i.e., PTMA(+)(x)-g-(PMeO(2)MA(n))(y). They were investigated using variable-temperature turbidity, photon correlation spectroscopy (PCS), electrophoretic mobility, and (1)H NMR measurements. For one system, four critical temperatures were measured and used to propose a mechanism for the thermally triggered changes that occur in solution. All of the copolymers existed as unimolecular micelles at 20 °C. They underwent reversible aggregation with heating. The extent of aggregation was controlled by the length of the side chains. TEM showed evidence of micellar aggregates. The thermally responsive behaviors of our new copolymers are compared to those for the cationic PNIPAm graft copolymers reported by Liu et al. Our new cationic copolymers retained their positive charge at all temperatures studied, have high zeta potentials at 37 °C, and are good candidates for conferring thermoresponsiveness to negatively charged biomaterial surfaces.