Asymmetric amphiphilic triblock copolymers : synthesis, characterization and self-assembly

Abstract

We developed a synthetic pathway to new amphiphilic ABC triblock copolymers with watersoluble blocks A and C and a hydrophobic middle block B. The synthesis involves a two-step polymerization. The prepolymer AB, constituted of poly(ethylene) oxide –b –poly(dimethyl) siloxane was prepared by anionic ring-opening polymerization of cyclic siloxanes, withandsiloxane units. The polymerization of strained cycles (e.g., D3) leads to polysiloxanes with monodisperse chains; the reaction time for anionic polymerization is lower and the yield of polymerization improved. Finally using the AB diblock copolymers as macroinitiators, a cationic polymerization of 2-methyloxazoline leads to asymmetric ABC triblock copolymers. As a model polymer we used an amphiphilic polyethylene oxide-b-polydimethylsiloxane-b-poly 2-methyloxazoline (PEO-b-PDMS-b-PMOXA) triblock copolymer. In aqueous solutions, this triblock copolymer self-assembles into well defined supramolecular aggregates. For certain compositions, the triblock copolymers form membrane like-superstructures and spherical vesicles in aqueous media. With the help of fluorescently labelled polymers, we were able to prove that the walls of these vesicles are asymmetric, due to the incompatibility between the hydrophilic chains: the blocks A and C are segregated on two different sides of the membrane. In case of nanometer-sized vesicles where membrane-curvature plays an important role we were even able to achieve a control over the membrane orientation, i.e., which of the two hydrophilic block is at the inner and which at the outer surface. This seems to be mainly gouverned by steric considerations: generally the smaller hydrophilic blocks are forced to the inner side where less space is available. Interestingly, the intrinsic asymmetry of the vesicular walls induced a directed insertion of transmembrane proteins. Using Aquaporinas a model system we employed immunoassay, immunofluorescence and immunogold labelling, to quantify the amount and the orientation of these proteins in the walls of the asymmetric ABC-block copolymer vesicles. The results clearly show a direct correlation between the membrane orientation and the prefered direction of the proteins. These studies indicate clearly that amphiphilic ABC triblock copolymers provide a convenient way to come to new materials with a directional functionality. Since they allow even a control over the orientation, they could allow to realize systems with a functionality that is reversed with respect to the biological model.