Synthesis and aqueous solution properties of stimuli-responsive triblock copolymers

Abstract

Two series of poly(di(methylamino)ethyl methacrylate)--poly(di(ethylene glycol)methyl ether methacrylate)--poly(di(methylamino)ethyl methacrylate), pDMAEMA--pDEGMEMA--pDMAEMA, and poly(di(ethylamino)ethyl methacrylate)--poly(di(ethylene glycol)methyl ether methacrylate)--poly(di(ethylamino)ethyl methacrylate), pDEAEMA--pDEGMEMA--pDEAEMA, triblock copolymers have been synthesized using copper-mediated living radical polymerization. Kinetic studies showed that the polymerizations were controlled yielding copolymers with MWt similar to predicted values with narrow polydispersities. UV/Vis experiments in aqueous solution indicate that the incorporation of pDMAEMA hydrophilic segments into the pDEGMEMA polymeric chains causes an increase in the lower critical solution temperature (LCST). Conversely, the incorporation of pDEAEMA hydrophobic segments has the reverse effect. It has been demonstrated that the cloud points of the copolymers increase when blocks pDMAEMA and pDEAEMA are protonated. At acidic pH and above 35-40 °C, the pDEGMEMA inner blocks become insoluble and, consequently, upon contact with water they form micellar aggregates constituted of a pDEGMEMA hydrophobic core with a pDMAEMA or pDEAEMA hydrophilic shell. At pH = 8.3 and 25 °C, the pDMAEMA--pDEGMEMA--pDMAEMA triblock copolymers form ordered aggregates of larger size. In addition under these conditions, in aqueous solution the pDEGMEMA form aggregates of smaller size than those found in the case of copolymers with pDMAEMA. Surface-tension measurements suggest that homopolymers, as well as copolymers, are surface active. The pDEGMEMA polymers present low values of the critical aggregation concentration (cac), due to their amphiphilic properties. The combination of pDEGMEMA thermosensitive segments with pDMAEMA and pDEAEMA blocks sensitive to pH and temperature has given rise to the formation of intelligent materials with responses to both stimuli. Their behavior depends on the MWt and the composition of the copolymers.