Polymers with Ionic Functions

Polymer molecules with just one or a few ionic groups, in most cases terminal and anionic, are called macroions. They are encountered primarily in living polymers, polymer molecules present in a polymerizing reaction system that will grow as long as monomers (e.g., esters or nitriles of methacrylic acid) continue to be supplied. The ionic charge of the macroion always transfers to the last monomer added, keeping the macroion ready for the next such addition. 

Polymers with a sizable number of ionic groups and a relatively nonpolar backbone are known as ionomers. The term was first used for copolymers of ethylene with carboxylated monomers (such as methacrylic acid) present as salts, and cross-linked thermoreversibly by divalent metal ions. Such polymers are useful as transparent packaging and coating materials. Their fluorinated forms have been made into very interesting ion-exchange membranes (considered further below). 

Synthetic examples include the poly(meth)acrylates used as flocculating agents for water purification. Biological examples are the proteins, nucleic acids, and pectins. Chemically modified biopolymers of this class are carboxymethyl cellulose and the lignin sulfonates. Polyelectrolytes with cationic and anionic substituents in the same macromolecule are called polyampholytes. 

Solutions of polyelectrolytes contain polyions and the free (individual) counterions. The dissociation of a polyacid or its salt yields polyanions, and that of a polybase or its salt yields polycations, in addition to the simple counterions. The polyampholytes are amphoteric their dissociation yields polyions that have anionic and cationic functions in the same ion and often are called zwitterions (as in the case of amino acids having HjN and COO groups in the same molecule). Such an amphoter will behave as a base toward a stronger acid and as an acid toward a stronger base its solution properties (particularly its effective charge) will be pH dependent, and an isoelectric point (pH value) exists where anionic and cationic dissociation is balanced so that the polyion s charges add up to zero net charge (and solubility is minimal). 

Inorganic polyelectrolytes exist, too polyphosphates, -0-P(=0)(-0H)-, are the most common (soluble) ones. 

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The ion-exchangers used in LC consist either of an organic polymer with ionic functional groups, or silica coated with an organic polymer with ionic functional groups. The types of functional groups used are the same as described in Chapter 18. Since IEC can be carried out with an aqueous mobile phase near physiological conditions, it is an important technique in the purification of sensitive biomolecules such as proteins. 

Most ion exchangers are solid synthetic polymers with ionic functional groups. The... 

In theory, it is possible to form block, graft, or crosslinked copolymers by ionic associations. However, in practice, telechelic polymers with ionic functionality at the chain ends are uncommon. Therefore, the majority of reported examples involve crosslinked copolymer formation between two immiscible polymers bearing pendent ionic groups. 

Polyelectrolytes are polymers with ionic side groups. The coil expansion of polyelectrolytes is not only determined by the usual parameters molar mass, solvent quality and temperature, but also by the degree of dissociation of the ionic groups. An example for a polyelectrolyte is the poly(acrylic acid) (PAAc) or its sodium salt (PAAcNa). The reduced viscosity of this polyelectrolyte is plotted as a function of the concentration c in Fig. 5.14. At high concentrations, the reduced viscosity decreases with a decreasing concentration as expected for a polymer solution. However, at lower concentrations the reduced viscosity levels into a plateau and increases again with a decreasing concentration. 

Polymers are limited to several ionic polymers due to the design of the coprecipitation process as well as the available safety data of polymers. Excipient manufacturers are striving to design additional polymers with different functional groups exhibiting different solubility and interaction potential. This certainly will broaden MBP applications. As discussed earlier, the authors observed the drug-polymer interaction as one of the key elements in the success of MBP. Increased pool of polymers may expand the usefulness of MBP and move the technology beyond the current limitations. 

Meanwhile, the R-R coupling (see Sect. 2.2) has evidently found general acceptance as the main reaction path for the electropolymerization of conducting polymers The ionic character of the coupling species explains why polar additives such as anions or solvents with high permittivity accelerate the rate of polymerization and function as catalysts. Thus, electropolymerization of pyrrole is catalyzed in CHjCN by bromide ions or in aqueous solution by 4,5-dihydro-1,3-benzenedisulfonic acid The electrocatalytic influence of water has been known since the work... 

Non-ionic polymers have also been blended with ionic block copolymers. Poly(vinyl phosphanate)-l7-polystyrene and PS-l -SPS have been blended with PPO. In both cases, improvements were seen in MeOH permeability over that of fhe unmodified block copolymers and conductivity values dropped as a function of increasing PPO confenf. PVDF has been blended wifh SEES in order fo improve its mechanical and chemical stability, but aggregation was found fo be a problem due fo incompafibility between components. However, it was found that a small amount (2 wt%) of a methyl methacrylate-butyl acrylate-methyl methacrylate block copolymer as com-patibilizer not only led to greater homogeneity but also improved mechanical resistance, water management, and conductivity. ... 

The formation of complexes is not restricted to mixtures of polyectrolytes and surfactants of opposite charge. Neutral polymers and ionic surfactants can also form bulk and/or surface complexes. Philip et al. [74] have studied the colloidal forces in presence of neutral polymer/ionic surfactant mixtures in the case where both species can adsorb at the interface of oil droplets dispersed in an aqueous phase. The molecules used in their studies are a neutral PVA-Vac copolymer (vinyl alcohol [88%] and vinyl acetate [12%]), with average molecular weight M = 155000 g/mol, and ionic surfactants such as SDS. The force measurements were performed using MCT. The force profiles were always roughly linear in semilogarithmic scale and were fitted by a simple exponential function ... 

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