Poly(para-phenylene ethynylene)s functionalized with Gd(III) chelates as potential MRI contrast agents

A poly(para-phenylene ethynylene) with water-solubilizing groups and Gd(III) chelates conjugated to the polymer backbone was designed and synthesized. Pre- and post-polymerization functionalization approaches were explored and the pre-polymerization approach for the introduction of the Gd(III) chelate was found to be more successful. The UV-vis absorption and fluorescence emission properties of the protected polymers were characterized and were found to be consistent with the results expected for this class of polymers. Removal of the protecting groups followed by chelation of Gd(III) led to a water-dispersible polymer. Relaxivity measurements were performed on this polymer with the aim of evaluating its potential as a new MRI contrast agent, and an r(1) of 1.37 L mmol(-1) s(-1) at 310 K and 20 MHz was determined. These results, along with dynamic light scattering analyses, suggested that the polymers formed micrometre-sized assemblies in aqueous solution. Although the relaxivity was relatively modest, these results provide important insights into the assembly properties of this new class of polymers and into the design criteria for future agents.