Novel applications of poly(ethylene glycol)-bl-poly(propylene sulfide) block copolymers

Block copolymers are comprised of repeating chemical groups (blocks) which commonly contain at least one hydrophilic block coupled to at least one hydrophobic block, forming an amphipathic macromolecule. This molecular arrangement drives the self assembly of these materials in water similar to lipids and low molecular weight surfactants. Unlike these conventional materials, however, block copolymers can typically form a wide variety of morphologies in water such as vesicles, worm like micelles, y-junctions, blackberry micelles, and micelles. This is due to the fact that the large hydrophobic blocks generally display a low mobility inside the core of the aggregate and the initially formed morphologies can be considered as "frozen" structures. Thus, while lipids and other low molecular weight surfactants are largely controlled by thermodynamics, block copolymers are governed by kinetic effects. A large body of work has been produced regarding the physical behavior of block copolymers over the past few decades. Various parameters have been explored including the composition of the dispersing media, the chemical composition of the repeating units of the blocks, the molar fraction of the blocks in solution, the relative and absolute block composition, the polydispersity of the block copolymer, temperature, pressure, and the compatibility of the polymer to the dispersing media. These have been extensively explored both theoretically and empirically for a wide variety of block copolymer systems. Block copolymers of poly(ethylene glycol)-bl-poly(propylene sulfide,) (PEG-PPS,) have recently emerged as a new and interesting block copolymer. This is due both to the low Tg, Tm, and relatively high hydrophobicity of the PPS block. These block copolymers can form polymeric vesicles (polymersomes,) worm like micelles, micelles, or hybrid structures, which are dependent on the relative block lengths of PEG and PPS (∞PEG.) However, the stability of the formed morphologies is directly related to the absolute PPS degree of polymerization, as the hydrophobic effect is the main driving factor towards self assembly. So while morphology is determined by the relative block composition, the stability of the formed aggregates on dilution is determined by the absolute molecular weight of the hydrophobic block. In order to exploit these materials for use as drug delivery vehicles to encapsulate hydrophilic compounds, we have explored the behavior of PEG-PPS by modifying the various parameters described above. We discovered that block copolymers displaying relatively low ∞PEG values are able to form micelles from solvent dispersion using tetrahydrofuran, a good solvent for both blocks, into water. These frustrated micelles, in which the PPS block is tightly packed, can then relax when heated in the presence of a solvent which is capable of swelling the hydrophobic block. This transition, highly dependent on temperature, the nature of the solvent, and the time exposed at elev