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Ligand-functionalized polymers have attracted great interest due to their ability to form tunable, reversible complexes with metal ions.  The goal of this project is to harness these supramolecular interactions to prepare dynamic polymer materials, with an emphasis on interfacial assemblies.  We have selected bipyridine and terpyridine as ligands, due to the wealth of data available on rate and equilibrium constants for a variety of metal ions.

Over the first year of the project, our work has focused on developing a simpler technique for end-functionalization of anionically-synthesized polymers with bipyridine and terpyridine.  The details of this work have been published in Macromolecules and presented by Ian Henderson, the graduate student who performed the experiments, at the Spring 2010 ACS National Meeting. 

Typical approaches to end-functionalization require the synthesis of a bipyridine- or terpyridine-modified initiator, which can be synthetically demanding.  Instead, we have taken advantage of the well-documented reactivity towards pyridines of nucleophiles such as organo-lithium reagents, specifically by directly adding a solution of living polystyryllithium chains to a solution of bipyridine or terpyridine.  The main limitation of the technique is the possibility of coupling by addition of two chains to the same polypyridine unit, but the fraction of coupled chains can be kept small by using an excess of pyridine end-capping species (which can be recovered and reused).  Reasonably efficient end-functionalization (83 – 92%) of crude products was achieved for polymers with molecular weights of 2 – 13 kg/mol, and we have also developed a simple chromatographic separation that allows these polymers to be purified to near-quantitative functionalization.      

The resulting end-functionalized polymers are distinct from those prepared by existing approaches, in that the polymer chain is bonded to the 6 position of the bipyridine or terpyridine group, whereas previously published routes use the 4 or 4’positions.  This is important since the attached chain may sterically interfere with the ability of the ligand to form metal complexes.  To verify the ability of these polymers to form supramolecular complexes, NMR and UV-Vis characterization was employed.  Titration of a chloroform solution of the terpyridine-funtionalized polystyrene with a methaolic solution of iron(II) chloride revealed  that the terpyridine-functionalized polymer was indeed capable of forming a bis complex.  Surprisingly, ¹H NMR experiments on the bis complexes with iron(II) yielded no proton signal from the pyridine moiety.  While this result remains under investigation, we suspect it reflects some type of exchange between a high-spin sterically hindered complex (in which the proton signals are Knight shifted), and a low-spin diamagnetic state.  To test this assertion, more detailed ESR and magnetic susceptibility experiments are currently being carried out.

The support of this ACS PRF Doctoral New Investigator grant has been instrumental in establishing supramolecular polymer chemistry as a new area of research focus for both the PI and the graduate student.  In addition, the student has learned valuable skills in polymer synthesis and physical characterization, has built equipment for characterization of interfacial activity of polymers, and learned the use of MATLAB software.  This project has already resulted in one published first author paper and a talk at a national ACS meeting, and will certainly result in more over the remainder of the funding period.  The student is well-positioned to finish his PhD in Polymer Science and Engineering in a total of about five years.