Types of Cyclodextrin Polymers

Cyclodextrin (CD) polymers, referred to as CDPs, are polymers or networks containing CD units in the molecular skeleton. Besides maintaining the macrocyclic structure and functional hydroxyl groups of CDs, CDPs also have the properties of polymers or cross-linked networks, showing a high degree of integration and synergy. They exhibit potential for many applications, such as drug carriers. Based on their structure, CDPs can be divided into four main types: CD-based polyrotaxanes (CD-PRs), grafted CD polymers (G-CDPs), cross-linked CDPs (C-CDPs), and CD-based star polymers (star-CDPs). Here we mainly focus on the first two types of CDPs and their structures are shown in Fig. 1 ^{[1], [2]}.

Fig. 1 The first two types of CDPs: (a) CD-PRs; (b) G-CDPs ^[2].

CD-PRs

CD-PR consists of three parts: CD host molecules, guest polymer chains and end-capping groups. In CD-PRs, CD molecules are threaded into guest polymers, and then the polymer ends are capped by end-capping groups or stoppers to prevent CD molecules from detaching from the polymer chain. The common guest polymers for CD-PRs are polyethylene oxide (PEO) and poly(p-phenylene oxide) (PPO), while the common end-capping groups are end amino groups, end carboxyl groups, and end hydroxyl groups.

• Types of CD-PRs

In order to enable CD molecules to slide or rotate along the polymer chains and increase the solubility of CD-PRs, CD-PR derivatives have been designed and synthesized, which can be mainly divided into six types: (1) CDs on the PRs modified with small groups; (2) CDs on the PRs modified with polymers; (3) PRs with polymers as end-capping groups; (4) chemically crosslinked CDs on PRs; (5) star-polymer-based PRs; (6) CDs on PRs are physically cross-linked through hydrogen bonds. Their structures are shown in Fig. 2 ^[1].

Fig. 2. Six types of CD-PR derivatives ^[1].

• Synthesis of CD-PRs

"Threading" and atom transfer radical polymerization (ATRP) approaches are two common synthetic strategies for CD-PRs. The former approach starts with CD-forming polypseudorotaxane (PPR) with polymers through self-assembly, an essential building block for CD-PRs, and then end-caps the product, as shown in Fig. 3. In the latter approach, the PPR initiators are prepared first, and then vinyl monomers are polymerized in situ at both ends via the ATRP method, resulting in final end-capping polymer PRs. The latter approach has the ability to modify the PR without changing the CD ^[2].

Fig. 3 "Threading" approach for CD based PRs ^[2].

G-CDPs

G-CDP refers to a polymer system in which CD molecules are grafted onto a polymer chain or onto a substrate through chemical bonds. CD molecules in G-CDP are flexible and can freely adjust their position in the solvent due to their grafted structure. It is worth noting that the CD cavity in G-CDP is vacant which can include guest molecules. G-CDPs have high solubility, high biocompatibility, and can self-assemble into micelles or supramolecular hydrogels, making them a good candidate to form smart hydrogels with multi-responsive properties ^[1].

• Types of G-CDPs

G-CDP can be divided into three categories based on differences in grafting structures: (1) CDP grafted from zero-dimensional (0D) nanoparticles; (2) one-dimensional (1D) polymer chains and nanowires; and (3) two-dimensional (2D) nanosheets. The second type of G-CDP can be summarized as G-CDPs with linear chains, which are the most widely synthesized and commonly used G-CDPs. Fig. 4 demonstrates the structures of these three types of G-CDP ^[2].

• Synthesis of G-CDPs

G-CDP is obtained by grafting CD onto polymeric materials. As CD molecules have evolved into CD derivatives substituted by thiols, aminos, azides, and carboxyls, the synthesis of G-CDP has become easier. Of the three types of G-CDP, the most frequently synthesized are those with linear chains. They can be synthesized either by polymerizing monomers with CD moieties or by covalently grafting CD into linear polymers as side chains. Polymers such as alginate, poly(ethyleneimine) (PEI), and poly(N-(2-hydroxyethyl)acrylamide) (PHEAm) are often used as linear components in G-CDP synthesis.

Fig. 4. Three types of G-CDPs: (a) 0D core G-CDPs; (b) 1D linear chain G-CDPs; and (c) 2D plate G-CDPs ^[1].

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