On the protonation of partially quaternized poly-4-vinylpyridines in aqueous solution

The ionic equilibria for free pyridine rings in water-soluble macromolecules of poly-4-vinylpyridine (PVP), partially quaternized with either benzylchloride (PP_yBz) or bromoacetone (PP_yAc), were studied by potentiometry and spectrophotometry. Peculiarities of protonation in aqueous solution were observed for the pyridine rings of these polymers compared with the analogue 4-ethylpyridine (EtP_y). The values of intrinsic pK₀ in the absence of protonated pyridine residues for PP_yBz and PP_yAc were found to be 2·4–3·3, and are abnormally low compared with pK_a of EtP_y (6·1). The pK₀ depends insignificantly on the degree of quaternization in PVP but more markedly on ionic strength. However, increase of NaCl concentration (from 0 to 0·1 M) does not cause pK₀ to rise to pK_a of the analogue. The titration data of the base group and keto-enol tautomerism of acetomethylene fragments in PP_yAc allow one to assume the effect of positively charged atoms in α-position of the ring on the proton binding to nitrogen atom of the same ring, thereby decreasing the value of pK₀. The intensification of electrostatic interactions between these charges in the polymer pyridine compared with that of the analogue is likely to be due to partial dehydration of the microregion near to nitrogen atom (microenvironmental effect).