47108-G7
Novel Polymeric Amphiphilies for Hydrocarbon Solubilization and Spill Remediation
Project Goal: The overarching aim of this proposal is to determine how changes in polymer architecture relate to, and can enable the optimization of, their ability to act as surfactants at a range of concentrations. In particular, we propose that cyclic polymers with grafted amphiphilic side-chains should provide unique unimolecular micelles with a large capacity to encapsulate hydrocarbons in aqueous environments. This in turn offers potential for an environmentally benign yet effective means for dispersing hydrocarbons, representing a significant improvement over traditional surfactants.
Progress towards this goal can be measure by a series of demonstrated achievements:
1) Optimization of the modular preparation of cyclic substrates: While the preparation of cyclic polymers has been described previously, the ability to tailor their size, polydispersity, and side-chain functionalities has been limited. The click cyclization techinique developed in our labs has been carried out with a range of monomers, including styrene, acetoxystyrene, and methyl acrylate, in addition to biocompatible and biodegradable cyclic esters such as caprolactone. Present studies are investigating hydroxyethyl acrylate and oxazoline monomers to yield polar highly biocompatible cyclic substrates.
2) Synthesis of dendritic analogues: Dendritic cores have also been prepared and optimized to provide an analogous multifunctional, though acyclic, substrate acting as a control for the studies of the unimolecular micelles
3) Attachment of grafts to cyclic substrates: Previous work had demonstrated the ability to divergently grow dendrons from cyclic polymers such as poly(hydroxystyrene). Additional attachment chemistries have been demonstrated to attach polymers or small molecule sidechains, including activated ester couplings, and click couplings.
4) Preliminary exploration of encapsulation behavior: Early studies of encapsulation have been carried out using the dendronized cyclic polymers with pyrene as a non-polar dye. The carriers demonstrated a linear dependence of guest concentration with respect to the carrier, even at very low concentrations, suggesting that they do behave as unimolecule micelles, and therefore can act as effective surfactants independent of dilution.
5) ATRP block copolymerization of side chains: ATRP has been explored as a technique for providing well-defined block copolymers grafted to the appropriate core. Initial polymerizations from dendrimer core macromonomers has been successful, with well-defined control over molecular weight and polydespersity. Alternative, alternative means of grafting sidechains (e.g. click couplings) have also been investigated in a cursory fashion.
Existing challenges that will be the focus of future research:
1) The preparation of biocompatible, biodegradable side chains: While initial proof-of-principle has focused on the preparation of block copolymers that were non-degradable acrylate and styrenic backbones, future research will explore the use of caprolactone and poly(ethylene glycol) as biocompatible/biodegradable alternatives.
2) The preparation of a functional poly( caprolactone) core: While cyclic poly(caprolactone) has been prepared, a method for incorporating functional side chains into cyclic poly(esters) has not yet been explored. Following the work of others with linear analogues, the attachment of a functional sidechain onto the monomer via alpha alkylation (e.g. alkene for thiolene coupling) will be investigated.
3) Assembly of a library of carriers and controls: While initial studies have investigate the stepwise assembly of linear, dendritic and cyclic components, a more rigorous method of determining the critical parameters defining the properties of these compound architectures will involve the use of clickable libraries of azide functionalized arms to alkyne functionalized cores. Preliminary studies suggest this approach will be successful, though efficient purification of excess reactant will be a major challenge to enable high-throughput assembly.
4) Quantification of encapsulation capacity with respect to critical variables. The final aim of this project is to use the tools above to probe the relationship between arm length, block ratio, and core architecture, and the encapsulation efficiency. Studies will be carried out using a variety of hydrocarbon dyes to quantify the encapsulation efficiency for each structure.
Research Impact: The approaches described here provide a further understanding of the fundamental relationship of polymer architecture and its physical properties. In particular, this approach should reveal how polymer architecture can be optimized to yield high capacity surfactants that maintain there function without a critical micelle concentration.
Impact on PI’s Career: The generous funding by the ACS-PRF through the “type G program” has been a vital part of maintaining sufficient funding at an early stage in the PI’s career. This has enabled the flexibility to support graduate students as full-time researchers during the academic year, as well as over the summer semester, which has help yield sufficient results for a number of posters at the last ACS conference, and a number of manuscripts in preparation.
