**1. Introduction**

Cyclodextrins (CDs) are natural compounds, obtained by the enzymatic modification of starch [1], that consist of at least six glucopyranose units linked by α-1,4 glycosidic bond. They have a truncated cone shape delimiting a relatively hydrophobic cavity and two polar rims bearing primary hydroxyl groups (the narrow rim) and secondary hydroxyl groups (the broader rim) [2]. The main feature of CDs is the formation of noncovalent complexes through host-guest interaction, which is, in fact, a summation of several noncovalent steps and overall represents an entropically driven process [3]. The host-guest interactions are determined by different factors, such as size, shape, charge, or polarity of the molecular actors involved [4]. This type of complexation has been extensively documented since the discovery of CDs, especially for low-molecular-weight compounds, by using a variety of physicochemical methods depending on the properties of the guest molecules [5–7]. These methods refer to nuclear magnetic resonance (NMR), UV-Vis, and fluorescence spectroscopies, as well as calorimetric methods. Although these molecules were reported at the end of the 19th century, the explosion of their applications started in the eighth decade of the 20th century [8]. CDs can be involved in the formation

of noncovalent interactions with polymers, giving rise to special types of assemblies known as rotaxanes or pseudorotaxanes [9, 10].

The other feature of CDs is the presence of numerous primary and secondary hydroxyl groups that allow a facile derivatization in order to obtain new molecules that can be used as building blocks for large assemblies. Owing to the difference in reactivity of the primary and secondary groups, it is possible to control the functionalization, which ensures a selectivity of this process. The easiest way to synthesize monoderivatives is by obtaining monotosylated CD, especially for β-CD. This synthesis was studied in detail. A few syntheses are available in order to obtain pure CDs monotosylated at the primary hydroxyl rim that can be used further as bricks for preparing other derivatives. Monotosylated CD can be easily obtained in aqueous alkaline solution in good yields [11]. This derivative will be easily transformed further into amines [12] or thiols [13] that can be modified through maleimides or iodoacetamides [14]. There are also strategies describing the functionalization of primary and secondary rims that can allow obtaining of large supramolecular assemblies. It can be taken into account that the functionalization of the secondary rim is more difficult than the modification of the primary rim, as the hydroxyls that mark the larger rim require a strong base to become activated. In the review of Liu *et al.*, different ways to functionalize CDs at the secondary rims are described [15]. This method has been used to introduce sensing groups that allow studying supramolecular complexes of CDs by electron paramagnetic resonance (EPR) spectroscopy or fluorescence spectroscopy [14, 16]. The applications of these functionalization reactions will be referred in the cases discussed in this chapter.

In this review, we will focus on the two main features of CDs: to generate large supramolecular assemblies through host-guest interactions and to use functionalization in order to improve the properties of polymeric systems by the incorporation of host CD units. The polymeric systems taken into discussion refer to polyrotaxanes and pseudorotaxanes, the particular cases of sliding gels, the pluronic gels, and the functionalization of various synthetic and natural polymers with CD units. An important part will be focused on noncovalent and covalent hydrogels containing CD units.
