A polymer is defined as a macromolecule consisting of a number of "building blocks" referred to as monomers. Modern polymer technology has made it possible to build chains of various lengths and compositions by varying the polymerization conditions and the monomer groups incorporated into the structure. When different monomers are used in the polymerization reaction, copolymers are formed. A random copolymer, as the name implies, is one in which there is no order to the structure. Individual monomer units are repeated at random points on the backbone of the polymer or on its branch chains. Graft copolymers have one monomer as the backbone and the other as sidechains off the backbone. Block copolymers have alternating sections of specific monomer chains. The behavior of the polymer results primarily from two factors: its chain length or molecular weight and its functional groups.

The net charge of the functional group determines how the polymer will behave in water. Generally, polymers useful in water treatment have electrolytic properties. That is to say, they are polyelectrolytes, and behave as salt-forming substances when dissolved in water. Polyelectrolytes are usually classified as cationic, anionic or . nonionic. Cationic polyelectrolytes ionize in water to acquire a positive charge and exhibit chemical activity characteristic of cations. Similarly, anionic polyelectrolytes ionize to acquire a negative charge. It is generally held that since nonionic polymers do not ionize in water, they are not really polyelectrolytes. However, nonionics are grouped together in the same class as polyelectrolytes because of the similarity of their uses.

The anionic functional groups are carboxylic acid and sulfonic add; the nonionic groups are amide and alcohol; amine and quaternary ammonium groups comprise the cationic type. Functional groups also enhance the adsorption of the polymer to scale and foulant surfaces. Adsorption is an equilibrium process in which groups are alternately adsorbed and desorbed on the deposit or metal surfaces. Because a polymer has so many functional groups along its chain, the overall equilibrium is shifted toward adsorption; at any one moment, enough functional groups are adsorbed to achieve this status. Although polymeric adsorption is an equilibrium process with respect to the functional groups, it is a non-equilibrium process with respect to the entire molecule.

The length of the polymer chain determines its molecular weight and can be regulated to achieve the properties desired. This is extremely Important with respect to the functions that the polymer is able to perform. Generally, high molecular weight polymers are considered to be those having molecular weights of i,000,000 to 10,000,0000. Low molecular weight polymers, on the other hand, have a molecular weight range of 500 to 20,000. 

There is an optimum molecular weight for each polymer used in a specific application. While it is not possible to have every polymer chain in a given reaction exactly the same length, proper selection of the reaction conditions can optimize the number of polymer chains of desired molecular weight.

Now that we have developed an understanding of polymers and their structure, we can review the major mechanisms by which they and other deposit control agents function.

High molecular weight polymers are adsorbed on several particles and tend to bridge between discrete particles of suspended matter. This bridging results in coagulation or flocculation. With an increase in the overall size of the suspended material, there is a corresponding decrease in the surface area available for attachment, which reduces the extent of deposition possible. It is important to note that the overfeed of a cationic flocculent is normally undesirable because particle surface charges may further increase, causing the particles to flocculate initially and then disperse.

Low molecular weight polymers, those whose chains are not long enough to link particles together via the bridging mechanism, tend to be absorbed on individual particles and on growing crystals of scale. The action of these polymers in preventing scale can be classified as threshold inhibition, whereby substoichiometric quantities of polymeric agents inhibit scale formation. Threshold inhibition can be subdivided into three distinct mechanisms: dispersion, sequestration, and scale crystal distortion.

A polymer can be adsorbed on foulant surfaces - imparting a like charge to them and thereby causing the particles to remain in suspension because of charge repulsion. The molecular weight of the polymer must be kept low to prevent particle bridging and a resultant increase in particle size.

Because most of the fouling material in cooling waters already has a slightly negative surface charge, it is economically sound to add anionic polymers to water. These increase the negative surface charge and keep particles separate. Addition of a cationic polymer is feasible but might involve considerably greater expense. The charge on the suspended particles would neutralize before sufficient positive charge was built up to keep them separated.

Scale crystal distortion is a mechanism whereby the solubility of the growing crystal is increased by altering its growth structure. The inclusion of a relatively large irregularly-shaped polymer in the scale lattice tends to prevent the deposition of a dense uniformly-structured crystalline mass on the metal surface. In theory, these crystals can develop internal stresses which increase as the crystal grows, with the result that the deposit breaks away from the metal surface.

A sequesterant is an agent which prevents an ion from exhibiting its normal properties by complexing with it. Some sequestrants are threshold inhibitors because they function below stoichiometric levels. Other sequestrants which depend on stoichiometric reactions between themselves and deposit components are called chelants. A chelate complex is characterized by two or more coordinate covalent bonds between a cation and electron pair at various points on the chelant structure. A number of divalent and trivalent cations can be controlled in this manner to form soluble complexes with the chelant. Chelants are sometimes referred to as ligands and are classified by their bonding sites (dentates). Five and six membered chelant rings have been found to be the most stable and effective.