Synthesis of variable bandgap conjugated polyelectrolytes via metal catalyzed cross-coupling reactions

Metal catalyzed coupling reactions such as the Stille, Suzuki, and Sonagashira (Heck) have become useful tools for the organic chemist over the past two decades for the formation of carbon-carbon bonds. Tolerance of functional groups, reasonable reaction temperatures, and high yields have allowed these techniques to be applied to the synthesis of conjugated polymers. These syntheses offer access to a wide variety of conjugated backbone structures that have previously been difficult to reach using traditional polymerization techniques.; Poly(p-phenylene) [PPP], poly(p-phenylene- co-thiophene) [PPT], and poly(p-phenylene- co-ethynylene) [PPE] electrolytes have been prepared by using one of the aforementioned coupling techniques. A methodology was applied whereby charge neutral polymers were first synthesized and then converted to the corresponding cationic polyelectrolyte. This “pre-polymer” technique allows for studies comparing neutral polymer properties (i.e., absorption, luminescence, solubility) to those of the polyelectrolyte. Significant changes in the polymers' visible absorption and emission wavelengths occur between the differing backbone structures. The polyelectrolytes' optical transitions are shifted to higher energies (blue-shifted) versus the absorption and emission of the neutral version within the same polymer backbone type.; The effects of halogenation of the monomer, solvent type, and palladium catalyst on the molecular weight were determined for each set of neutral polymers by monitoring chain extension by gel permeation chromatography. In the case of the PPP derivatives, it was found that the Suzuki polymerization proceeds the fastest to maximum molecular weight in a DMF/aqueous media using PdCl ₂(dppf) catalyst with di-iodinated monomer. Polymerizations using di-brominated monomers reached similar molecular weight values but only after longer reaction times. Polymer chain growth in this system was limited by the precipitation of polymer from the reaction solution and not the reactivity of the halogenated monomer. PPT polymers synthesized using the Stille reaction proceeded to highest molecular weight values in anhydrous DMF using PdCl₂(PPh₃ )₂ catalyst and di-iodinated monomer. Triethylamine/THF solvent systems using PdCl₂(PPh₃)₂ catalyst with a small amount of CuI co-catalyst and di-iodinated monomer were the best conditions for the PPE Sonagashira polymerizations. Di-brominated monomers were ineffective in reaching polymeric materials when used in either Stille or Sonagashira polymerizations. The conversion procedure to the polyelectrolyte was determined to be sufficiently mild not to induce breakages of the backbone, thus allowing the molecular weight characteristics for the neutral species to be roughly applied to the polyelectrolyte.