Water-soluble polymers

Water-soluble polymers

Polymers that have sufficient number of electronegative atoms and/or func-tional groups that can form hydrogen bonds with water tend to dissolve in water and are called water-soluble polymers. Water-soluble polymers have an ability to increase the viscosity of solvents at low concentrations, to swell or change shape in solution, and to adsorb at surfaces. The rate of dissolution of a water-soluble polymer depends on its molecular weight. Larger the molecules, stronger the forces holding the chains together and lower the rate of dissolution. Greater the degree of crystallinity of the polymer in the solid state, lower the rate of dissolution. This combination of slow dissolution rate and formation of viscous surface layer makes high-molecular-weight hydrophilic polymers suitable for use in controlling the release rate of soluble drugs. For example, high-molecular-weight HPMC is used as a matrix controlled-release drug delivery carrier.

Examples of commonly used water-soluble polymers include the following:

Carboxypolymethylene (carbomer, carbopol)

Carboxypolymethylene, also known as carbomer, carbopol, or carboxyvinyl polymer, is a high molecular weight polymer of acrylic acid, containing a high proportion of carboxyl groups. This polymer is used as a binding agent in tablets and a suspending agent in other pharmaceutical preparations. The car-boxylic groups impart it an acidic character. Thus, its aqueous solutions are acidic. On neutralization with a base, the carboxylic groups become ionized and form stronger hydrogen-bond associations with other polymer chains and the solvent, water. Consequently, carboxypolymethylene solutions become very viscous, with a maximum viscosity at pH between 6 and 11.

Cellulose derivatives

Cellulose itself is insoluble in water. Its partial aqueous solubility is attrib-uted to substitutions, such as methylation and carboxymethylation. Ethyl methylcellulose is soluble in hot and cold water and does not form a gel. Methylcellulose is poorly soluble in water and forms a gel on heating. Sodium carboxymethylcellulose, being an ionized carboxylic acid salt, is soluble in water at all temperatures.

Natural gum (acacia)

Acacia gum, also known as gum arabica, is a complex arabinogalac-tan-type polysaccharide exuded by acacia trees. Acacia solutions are highly viscous in water. It is one of the most widely used emulsifiers and thickeners.

Alginates

Alginates, also called align or alginic acid, is an anionic polysaccharide in the cell walls of brown algae. It forms a viscous gum on binding with water. Alginate solutions are less readily gelled than acacia gum and are used as stabilizers and thickening agents.

Dextran

Dextran is a complex, branched polymer or polysaccharide composed of glucose molecules. Hence, it is also called glucan. Partially hydrolyzed dex-tran reduces blood viscosity and is used as a plasma substitute and a volume expander. It exerts an osmotic pressure comparable with that of plasma. Thus, it is used to restore or maintain blood volume in severe trauma.

Polyvinylpyrrolidone

Polyvinylpyrrolidone (PVP), also known as povidone, is a homopolymer of N-vinyl pyrrolidone (Figure 11.5). It is commonly used as a suspending and dispersing agent. It is also used as binding and granulating agent for tablets and as a vehicle for drugs such as penicillin, cortisone, procaine, and insu-lin to delay their absorption and prolong their action.

Figure 11.5 Chemical structure of poly(ethylene oxide-co-propylene oxide-co-polyethylene oxide) (PEO-PPO-PEO) (commercially known as Pluronic and poloxamer).

Polyethylene glycol

Polyethylene glycol (PEG) is a polyether compound with repeating units of ethylene oxide and a terminal hydroxyl group. The electronegative oxygen confers water solubility on this polymer. Polyethylene glycols have different physical states, depending on their molecular weight, with low-molecular-weight PEGs being liquid at room temperature, while high-molecular-weight PEGs being crystalline solids. Polyethylene glycols are water-soluble and miscible and can dissolve drugs that are not soluble in water. Thus, PEGs are commonly used to increase drug solubility. Polyethylene glycols are also used as plasticizers in coating suspensions to form an elastic film during tablet coating.