pH-responsive and temperature-responsive polymers are called stimuli-responsive or smart polymers because they can change their physical or chemical properties when exposed to specific environmental conditions. In materials chemistry, these polymers are important because a small change in pH or temperature can produce a large change in polymer structure, solubility, swelling, permeability, or conformation. Such behaviour makes them useful in drug delivery, sensors, tissue engineering, separation processes, coatings, and controlled-release systems. pH-responsive polymers respond to changes in the acidity or basicity of their surrounding environment. Their response mainly occurs because they contain ionizable chemical groups such as carboxylic acid, sulfonic acid, amine, or other weakly acidic or basic groups. When the pH of the surrounding medium changes, these groups can gain or lose protons. This changes the electrical charge of the polymer chains and consequently changes their interaction with water and with other polymer chains. The mechanism of action of an acidic pH-responsive polymer can be understood using a polymer containing carboxylic acid groups, represented as –COOH. At low pH, a high concentration of hydrogen ions is present in the medium. The carboxyl groups remain mainly in their protonated form: –COOH ⇌ –COO⁻ + H⁺ When the pH increases, the polymer loses hydrogen ions and the carboxylic acid groups become negatively charged –COO⁻. The negative charges repel one another along the polymer chain. At the same time, the charged groups have a strong attraction for water molecules. As a result, the polymer chain expands and absorbs more water. In a cross-linked polymer network, this produces swelling. A basic pH-responsive polymer behaves in the opposite manner. Polymers containing amino groups, for example, can become protonated under acidic conditions: –NH₂ + H⁺ ⇌ –NH₃⁺ At low pH, the amino groups become positively charged. The positive charges repel each other and increase the interaction of the polymer with water, causing the polymer network to swell or become more soluble. At higher pH, the amino groups lose their protons and return mainly to the neutral –NH₂ form. The electrostatic repulsion decreases, and the polymer may contract, become less soluble, or precipitate. Therefore, the basic mechanism of pH responsiveness involves three major changes: protonation or deprotonation of functional groups, change in polymer charge, and consequent change in polymer–water and polymer–polymer interactions. These changes can alter the dimensions and properties of the material. The response may be observed as swelling and shrinking in hydrogels, dissolution or precipitation in polymer solutions, changes in membrane permeability, or changes in the release rate of an incorporated substance. The pH at which a significant change occurs depends on the nature of the ionizable groups and their chemical environment. The pKa of the