Cyclodextrin Complexes of Polymers Bearing Adamantyl Groups: Host–Guest Interactions and the Effect of Spacers on Water Solubility

Abstract

Polymer-inclusion complexes (PICs) with cyclodextrins (CDs) show a broad structural variety [1][2][3][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. Figure 2 shows the transmittance of an aqueous solution of 2 a as a function of temperature. The measurement was performed at a concentration of 100 g L À1 using a heating/ cooling rate of 1 K min À1 . It can be seen that in the heating run the transmittance of the solution drops from 100 to 0 % within a temperature range of about 1-2 8C around the cloud point of 44.6 8C. This effect is caused by the dissociation of the polymer/Me-b-CD complex 2 a and the precipitation of the uncomplexed, more hydrophobic polymer; the Me-b-CD molecules remain in the aqueous solution. During the cooling run the transmittance did not return back to the starting level. However, after the solution had been stirred for several days at 5 8C, it became transparent again, indicating...

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