Polyampholytes

Polyampholytes represent a special class of polyions that contain both positively and negatively charged units on the same macro moleeule. They can be polymeric zwitterions with positive and negative charges off the same backbone, or they can be polybetaines having both charges on a pendent group . By definition, polybetaines have an equal number of anionic and cationic charges. Whereas polyampholytes can be either neutral, having the same number of negative as positive repeat units, or have a net charge of one sign. Polyampholytes are synthesized via free-radical polymerization of anionic and cationic monomers. Due to the electron donor-acceptor nature of some cationic and anionic monomer pairs, they are prone to spontaneous polymerization. Ampholytic monomers can also be copolymerized with other water soluble monomers.

The presence of ionic groups of opposite charges in the same macromoleeule leads to unusual phase-behavior and solution properties that are largely controlled by electrostatic interactions. Unlike polyelectrolytes, polybetaines and neutral polyampholytes are frequently more soluble and show higher viscosities in salt than in deionized water. Polyampholytes solution properties are governed by coulombic attractions between anionic and cationic mer units. As the molar ratio of anionic to cationic species approaches one, coulombic interactions lead to globule-like conformations and, in many cases, insolubility in deionized water. These attractive interactions may be screened by the addition of electrolytes or change in pH, which induces a transition to a random coil conformation, often facilitating solubility. This behavior, known as the antipolyelectrolyte effect, leads to an enhancement in viscosity in the presence of electrolytes . TTiese interactions can be monitored by several external parameters including number and nature of the ionic sites, polymer microstructure, solvent type, pH, and ionic strength. The overall chain conformation that results from the competition between repulsion that tends to stretch the chain (polyelectrolyte effect) and the attraction that tends to collapse it (polyampholyte effect) is highly sensitive to these factors.

Although polyampholytes have not received extensive commercial use, their dual-charge nature facilitates some unique physico-chemical properties that offer diverse application opportunities. Polyampholytes antipolyelectrolyte effect favors their use in high electrolyte solutions, such as biological and seawater applications. For example polyampholytes can be used industrially as thickeners in brine solution, in flocculation and in oil recovery processes. Because of their ability to bind to low molecular weight substances, i.e., metal ions, surfactants, dyes, drugs; polyampholytes can be used as selective chelating agents, as pigment-retention aids, and in paper manufacturing. Their intramolecular associations as a function of pH may be utilized in drug delivery. As we learn more about the unique properties of polyampholytes and use them, it is clear that their applications will continue to expand.

Applications of Specialty Polymers and Areas of Further Research In general, ionic polymers are widely used in various fields of industry, agriculture, medicine, biotechnology, and electronics, as flocculants, coagulants, structurization agents, prolongers of drugs, biocatalysts, and sensors. With the help of gels, membranes, and films of polyelectolytes, it is possible to regulate the water regime in the ground, to purify waste-water, to disinfect ground water from radionuclides, and to create artificial nutrient media and muscles, as well as chemomechanical devices.

Specialty polymers will continue to be an area of great scientific and technological interest, due to their relevance in molecular organization in biological systems and also to various practical applications . These functional polymers provide unique properties to meet many of today's demanding hi-tech requirements by facilitating stimuli-responsive technologies or "smart technologies." "Smart technologies" have the ability to respond to an external stimuli in a desired fashion by altering the chemical, physical, or electrical properties of the system. This response happens in real-time and is often reversible. Examples of desired stimuli-responsive phenomena that are useful for industrial and medical applications, include the ability to respond to changes in temperature, pH, salt concentration, humidity and other environmental changes.

The key to improved specialty polymers is employing a multidisciplinary approach to develop a fundamental understanding of the underlying structure-property relationships. Synthetic specialty polymers provide a unique opportunity to systematically vary the structure of funetionalized macromolecules. The ability to make model compounds of complex macromolecules can facilitate improved understanding of the underlying biological mechanisms. This will enable researchers to design, synthesize, and formulate specialty polymers with desired properties for medical and industrial applications.