**2.2 Polymers with pH sensitivity**

pH-Adaptive polymers are a class of stimuli-sensitive polymers that may alter their structural and physical properties in reaction to variations in solution pH, including surface properties, chain conformation, solubility, and arrangement. The phrase "pH-reactive polymers" refers to polymers containing ionizable basic or acidic groups where ionization is affected by the pH of the solution. In the latest days, the topic of

pH-reactive polymers has grown in popularity, with scientific research being published year after year. As either a result, pH-sensitive polymer systems are extremely helpful in a broad array of applications, including gene delivery, drug administration, surfaces, membranes receptors, and chromatography [37–39].

Polymers that respond to pH might be linear, branching, or networked. According to their architectures, polymers may have varied sensitivities to solution circumstances and variable self-assembly tendencies. A pH shift, for instance, might result in the (de)protonation of functional moiety in the polymeric chain. It can produce flocculation, strand collapse-extension, including deposition in homopolymers in certain situations. It also might produce self-assembly in the forms of micelles, unimers, gels, vesicles, swelling, and deswelling, among other things. Surface active behaviors are demonstrated by pH alteration in block (co)polymers, branching (co)polymers, and starry (co)polymers with pH-sensitive block(s). Furthermore, pH changes cause (de)swelling in hydrogel as well as dendrimer-like formations. Surfaces altered with polymers allow for the creation of ionic interfaces with thin/thick layers as a result of pH changes. **Figure 2** depicts the variations in polymers of various topologies caused by pH changes.

pH Adaptive polymers are polyelectrolytes with weakly basic or acidic moieties in their architecture that receive or liberate protons in reaction to variations in the pH of the surroundings. Polymers containing acidic or basic groups, such as carboxyl, pyridine, sulfonic, phosphate, and tertiary amines, are commonly referred to as pH adaptive polymers because of ionization of the molecules with pH variation causes a structural change. Their pH sensibility or ionization allows us to modify its self-assembly behavior, wettability phase segregation, polyelectrolyte character, and other properties, in complement to their biotechnological uses. It is feasible to make a polymer with a pKa ranging from 1 to 14. pH Reactive polymers having basic monomers behave like cationic polymers in acidic conditions, whereas polymers having acidic monomers behave like anionic polymers in basic conditions. Depending on the requirements, a few of these two types or a combination of the two with the appropriate composition is necessary. Natural polymers, as well as manmade polymers, have indeed been thoroughly investigated. Biopolymers are by far the most widely investigated because of their richness in ecology, rapid degradation, bio-compatibility, their potential to be modified. Polypeptides such as poly(histidine), poly(L-glutamic acid), and poly(aspartic acid) can be used to synthesize pH-reactive polymers. Such polymers are biodegradable and biocompatible, just like biopolymers. These biopolymers are quite significant among pH-sensitive polymers [38, 40, 41].

**Figure 2.** *Polymers that respond to pH in a variety of ways.*
