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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.

2) Analysis of the degradation behavior of cyclic polyesters: Because the polyesters are the favored substrate for biocompatible applications (e.g. environmental remediation) studies were carried out and published detailing the degradation behavior of the cyclic poly(caprolactone) relative to identical linear analogues.

3) Synthesis of dendronized cyclic polymers: Using a poly(hydrostyrene) substrate dendritic side chains have been grafted to enable tuning of both the solubility and the flexibility of the cyclic core

4) Attachment of grafts to cyclic substrates: Using a cyclic polystyrene backbone with alkyne side chains, a diversity of azide modified side chains were attached to the cyclic core. The coupling yields were nearly quantitative and enables access to a diversity of architectures.

5) 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.

6) 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 polyfunctional dendritic macroinitiators has been successful, with well-defined control over molecular weight and polydispersity.

7) Click technique for building modular biocompatible star amphiphiles: In order to provide stars that were both biocompatible and biodegradable, polycaprolcatone stars were polymerized from dendritic cores, and then PEG terminal chains couple to the star chain ends using click chemistry, yielding biocompatible carriers that efficiently encapsulate non-polar hosts.

8) The preparation of a functional polycaprolactone core: Initial results, in collaboration with the Emrick research group (UMass) demonstrate the ability to incorporate pendant alkynes onto polycaprolactone cores. Attempts to cyclize are ongoing,

Existing challenges that will be the focus of future research:

1) Assembly of a library of carriers and controls: Preliminary studies have verified the ability of each of the diverse architecture described above to encapsulate non-polar molecules in aqueous media. A quantitative study is underway to determine which structures exhibit the lowest critical micelle concentrations, and which exhibit the highest weight percent loading of guest per carrier molecule.

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 for two successive ACS conferences. In addition, one manuscript has been recently published and two manuscripts in preparation which were funded in part by ACS PRF support.