Synthesis and Self-Assembly of Poly(ferrocenyldimethylsilane-b-dimethylaminoethyl methacrylate): Toward Water-Soluble Cylinders with an Organometallic Core

Abstract

Samples of the asymmetric block copolymer poly(ferrocenyldimethylsilane-b-dimethylaminoethyl methacrylate), PFS-b-PDMAEMA (block ratio ≈ 1:5), with a hydrophobic organometallic block and a hydrophilic organic block have been synthesized via a two-step anionic polymerization methodology. To study the self-assembly of these block copolymers in aqueous media, micelles were prepared by two different methods: selective precipitation involving the addition of water to PFS-b-PDMAEMA in dioxane and directly mixing water with PFS-b-PDMAEMA. In the selective precipitation experiments, the rate of water addition was found to be a crucial factor in determining the morphology of the resulting micellar structures. Rapid addition of water resulted in a monomodal distribution of solid spheres with an overall diameter of 20 nm, while slow addition created vesicle-like structures with a broad size distribution. Direct mixing of water with PFS-b-PDMAEMA led to even more complex micellization behavior. After PFS-b-PDMAEMA was initially dissolved in water, polydisperse spherical micelles with Rh = ∼65 nm were formed. However, over time with constant stirring, a range of morphologies including spheres, rods, cylinders, and hollow structures were identified by electron microscopy, whereas, under quiescent conditions, the diameter of the spherical micelles gradually decreased to ca. 25 nm. Quaternization of the PDMAEMA blocks in PFS-b-PDMAEMA rendered a dramatic simplification to the micellization process. Only spheres were observed after the quaternized block copolymers were directly mixed with water. In contrast, alcohol solvents such as EtOH and iPrOH are able to induce PFS-b-PDMAEMA to self-assemble into cylindrical micelles by simply dissolving the block copolymers in the solvent. Aqueous colloidal solutions of cylindrical micelles could then be prepared via the dialysis of PFS-b-PDMAEMA in ethanol against water. The studies indicated that the solubility characteristics of the PDMAEMA corona chains in the micellization media exert a major influence on the micellization process and the resulting micellar morphologies.