Cyclodextrin Complexes of Polymers Bearing Adamantyl Groups: Host–Guest Interactions and the Effect of Spacers on Water Solubility (original) (raw)
2007, Angewandte Chemie International Edition
Polymer-inclusion complexes (PICs) with cyclodextrins (CDs) show a broad structural variety [1-4] and are of interest for many different applications, for example, drug delivery systems [5] and stimuli-responsive hydrogels. [6, 7] We have demonstrated that the lower critical solution temperature (LCST) of N-isopropylacrylamide(NIPAAM)based copolymers bearing adamantyl groups can be influenced through complexation of the adamantyl moieties by cyclodextrins (CDs). [8, 9] Recently, we reported on the synthesis of a PIC consisting of randomly methylated b-CD and a polymethacrylamide which showed a reversible phase transition in aqueous solution as a result of a dissociation/ complexation process. [10] Unfortunately, this system was not optimal in terms of chemical stability and the guest moiety used. We report herein on the pseudo-LCST behavior of PICs consisting of poly(adamantylacrylamide)s and Me-b-CD. In contrast to the previously reported system, the newly synthesized polyacrylamides are expected to be much more stable against hydrolysis. Additionally, the incorporated adamantyl moieties are well suited for inclusion into b-CD. With this optimized system we will demonstrate the influence of spacer groups and concentration on the phase-transition process. It is known that Me-b-CD-complexed hydrophobic monomers can be polymerized in water by means of a free-radical mechanism by use of water-soluble azo or redox initiators. [11] In most cases, the Me-b-CDs slip off the growing macroradicals which leads to precipitation of the polymeric material. In contrast, polymerization of Me-b-CD-complexed 1-adamantylacrylamide (1 a) and 6-acryloylaminohexanoic acid 1-adamantylamide (3 a) resulting the formation of the water-soluble polymer/Me-b-CD-complexes 2 a and 4 a, respectively (Scheme 1). The polymerization was carried out in water at 25 8C using 1 mol % of the redox initiator system K 2 S 2 O 8 /Na 2 S 2 O 5. The molecular weights of the purified Me-b-CD-free polymers 2 and 4 were determined by MALDI-TOF mass spectrometry (Figure 1). Interestingly, the obtained polymer/ Me-b-CD complexes 2 a and 4 a show thermosensitive solubility properties in water that strongly depend on the distance between the Me-b-CD-complexed adamantyl groups and the polymer backbone.
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