CD-based polymers

Fig. 20 Synthesis of P-CD-based polymer. Reproduced with permission from [141]. 2001 American Chemical Society...

While native CDs are appropriate for the molecule recognition of a wide range of snbstrates, specific applications reqnire the use of more elaborated CD-structures. Concurrently to mono- and polysnbstituted CDs, CD-based polymers appear as a powerful tool. For example, covalent polymer networks containing CDs are of great interest because their cross-linked macromolecnlar strnctures reveal a cooperative action between the CD cavities or between the cavities... 

FIGURE 2.4 Cross-linked (a) and linear (b) CD-based polymers. 

Different parameters can be changed for the modification of the properties of the CD-based polymers type of the CDs, the nature of the cross-linker or the polymer and the ratio between hydrophobic (cross-linker or polymer) and hydrophilic part (CDs) used for the synthesis. The tuning of these parameters gives polymers with different characteristics (chemical and mechanical properties, surface area, etc.) [3]. 

Urethane cross-linker Urethane CD-based polymers are obtained by reaction of the CD hydroxyl groups with diisocyanates via polycondensation reaction. The most widely used diisocyanate cross-Unkers are tolylene-2,4-diisocyanate (TDl) and hexamethylene diisocyanate (HDI) [4]. It is also possible to synthesize polymers with specific properties with exotic diisocyanates such as isophorone diisocyanate (IPDI) (Scheme 2.1) [5]. 

Ether cross-linker Polyether CD-based polymers are mostly prepared from epoxides opening in basic conditions [7]. The most widely used epoxides are epichlorohydrin (EPI) and its nontoxic equivalent, the ethylene glycol diglycidyl ether (EGDE) (Scheme 2.3). 

Two different approaches can be used to synthesize linear CD-based polymers. The first one consists in initially synthesizing CD-monomers that can be engaged in a polymerization process to obtain CD-based polymers. Classic CD functionalizations are generally made with acrylates or vinyl derivatives (see next section) [8]. The second method consists in reacting native or modified CDs with an already synthesized polymer to functionalize it (see next section). 

In this part, we will focus on the synthesis of CD-based polymers and their use in mass transfer promoters. Their beneficial impact on the catalytic performance is especially described through relevant examples of the literature. 

Pioneered works related to CD-based polymers for catalysis were reported by Nozakura s group. The model reaction was the hydrolysis of various p-nitrophe-nol esters [18]. The CD-based polymers synthesis was realized in a controlled two steps procedure. First, CD-acryloyl monomers were synthesized and purified by gel ehromatography through Sephadex G15. The monomers were then polymerized in an H20/Me0H mixture (v/v 1/1) using 2,2 -azobis (isobutyroni-trile) as an initiator to give polyacryloyl-CD polymers (Scheme 2.4). 

These CD-based polymers were engaged in hydrolysis reaction of various p-nitrophenol esters derivatives. The reaction was monitored by UV at 400 mn. Among the CD-based polymers, only poly- 3-CD-A showed a high hydrolysis activity. The pseudo first-order rate constant k measured for the ester hydrolysis catalyzed by poly- 3-CD-A is compared to the rate constant measured using the native 3-CD. The rate enhancements resulting from the use of poly- 3-CD-A are presented in Table 2.1. 

SCHEME 2.4 Two-step synthesis of polyacryloyl CD-based polymers. (Adapted from Ref. [18a] with permission of Wiley.)... 

SCHEME 2.11 Synthesis of the RAME-p-CD-based polymers. (Adc tedfrom Ref [24] with permission of American Chemical Society.)... 

The catalytic results showed that high-substituted CD-based polymers were more effective than low-substituted polymers. For 1-decene that could interact with only one CD cavity, polymers 5 and 6 (high CD-substituted polymers) were as efficient as RAME-(3-CD alone. In that case, each CD covalently attached to the polymer chain acted more or less as a CD alone. For the longer 1-hexadecane substrate, the low CD-substituted polymer 2 gave approximately the same yield of RAME-(3-CD ( 40%). In contrast, polymers 5 and 6 gave much higher yields (-80%). This results from the synergic cooperation of two CDs in close vicinity on the polymer backbone. 

Another application is related to the ability of cross-linked CD-based polymers to produce metal-loaded solids. Different approaches led to high concentrations of metal salts in polymeric frameworks, among them a reticulation in the presence of metal ions solution [60] and the reflux of a polymer suspension in a metal salt solution [61] or a physical mixture in a planetary ball mill [62]. 

Thus, the use of metal-loaded cross-linked CD-based polymers is part of a green chemistry approach for organic synthesis, as much for the versatility of the potential catalytic applications as for their use under microwave assistance and their recyclability. Furthermore, the high absorption of organic compounds in nanosponges, associated with their chiral environments, place this material for promising applications in sustainable chemistry. 

In conclusion, three different types of CD-based polymers (cross-linked polymers, polymers bearing pendant CDs, imprinted CD-based polymers or nanosponges) have been shown to be efficient materials in biphasic or aqueous... 

Thus, concurrently with other CD-based materials for catalysis (such as CD-based metallic nanoparticles [65], 3-CD-silica hybrid [66], and more recently CD-based metal-organic frameworks (MOF) [67]), CD-based polymers also appear to be very effective materials to improve catalytic performance in aqueous media. 

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