π-Conjugated Polyelectrolytes Derived from 2-Ethynylpyridine: The Effect of Quaternization Agent and Reaction Conditions on the Polymer Structure and SERS Characterization of Nanocomposites with Ag-Nanoparticles

The reaction of 2-ethynylpyridine (2EP) with stoichiometric equivalent of an alkyl halide RX (R is ethyl, nonyl, or hexadecyl and X is Br or I) gives a poly(N-alkyl-2-ethynylpyridinium halide) type ionic polymer that belongs to the family of pi-conjugated polyelectrolytes (CPEs). Reaction conditions significantly influence configuration of the polymer main chains: polymerization in acetonitrile solution gives polymers with high content of cis units while bulk polymerization provides irregular cis/trans polymers. Increased regularity of the high-cis polymers is documented by the NMR and IR as well as SERS (surface enhanced Raman scattering) spectra measured on Ag-nanoparticles/CPE systems. Both polymerization processes give polymers in which 2EP monomeric units are ionized only from ca one half, which exhibit good stability in air and good solubility in polar solvents such as MeOH, DMF, and DMSO and those with N-ethylpyridinium groups even in water.