Efficient synthetic strategies are described for the preparation of rodlike polyelectrolytes based on the intrinsically rigid poly(p‐phenylene). Uncharged precursors were first prepared via the Suzuki coupling and then characterized by different methods of polymer analysis. Finally, they were transformed into polyelectrolytes using macromolecular substitution reactions. Depending on the substitution pattern, the obtained polyelectrolytes are either soluble or insoluble in water. Using water‐soluble derivatives, the Poisson‐Boltzmann cell model was tested by osmotic measurements and small‐angle X‐ray scattering. It is shown that the cell model provides a good first approximation of the distribution of the counterions around the macroion but still underestimates their correlation. Moreover, the PPP polyelectrolytes show a very pronounced polyelectrolyte effect. Since the rodlike PPPs are very rigid in shape, this observation proves that the polyelectrolyte effect is caused by long‐range intermolecular electrostatic repulsion of the dissolved macroions rather than due to conformational changes.
Pd-catalyzed Suzuki coupling is used to prepare poly(p-phenylene)s (PPPs) bearing oligo(ethylene oxide)s (OEOs) and trialkylamino groups as lateral substituents. The OEO substituents require very specific reaction conditions during monomer synthesis - presumably due to their coordinating oxygen atoms - but do not affect the subsequent Pd-catalyzed polycondensation process. In contrast to this, the lateral amino groups lower the degree of polycondensation of the PPPs considerably when their nitrogen atom is placed in the β-position of the side chains. When there is a longer spacer group between the amino nitrogen and the aromatic ring to be coupled, however, high-molecular-weight PPPs can be obtained. Provided an appropriate substitution pattern and long OEO side chains are used, the resulting PPPs are readily soluble not only in organic solvents but even in aqueous media. For one of these PPPs, the degree of protonation has been determined as a function of pH, using 1H and 13C NMR spectroscopy as well as potentiometry. It is shown that the polymer is completely deprotonated at pH > 10.5 and quantitatively protonated at pH < 4.0.
cross-coupling protocols are applied to synthesize water-soluble poly(p-phenylene) (PPP) derivatives decorated with oligoethylene oxide (OEO) and tertiary amino side groups. It is shown that constitutionally well-defined PPPs can be obtained having degrees of polycondensation, P n , of 30-60. In some cases, however the heteroatoms in the side chains affect the monomer synthesis and prevent proper chain growth through coordination of metal species present in the reaction medium. It is shown that essentially uncharged but still water-soluble PPPs result if (i) the side groups bear their heteroatoms in the right position, and (ii) if the water-soluble segments of the lateral substituents prevent intermolecular hydrophobic interactions of the apolar main chains reliably. In other words, the lateral substituents have to wrap the PPP core structure tightly in a closed outer shell of water-soluble OEO moieties. These PPPs constitute a valuable new pool of model systems for future polyelectrolyte research, being highly complementary to what has been available so far.
The protonation/deprotonation behavior of rodlike poly(p‐phenylene)‐based polyamines is analyzed in aqueous solution using NMR spectroscopy and potentiometry. The observed features cannot be explained by a single acid constant, but shifting electrostatic potential—generated by the protonated ammonium side groups—has to be taken into account additionally. Consequently, the density of ionic groups attached to the polymer backbones and the ionic strength of the solutions contribute to the polybase behavior as well: the shift of basicity as observed with increasing degrees of protonation can be quantified by an apparent ionization constant, pKapp, for each value of pH. Due to intrinsic rigidity of the polymer backbone and true solubility in water effectuated by the highly polar side chains, the electrostatic effects influencing the polybase behavior can be observed free from conformational effects and over the whole range of accessible pH and degree of protonation for the first time.
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