Synthesis Of Cyclodextrin Polyrotaxanes And Sliding Supramolecular Polymer Brushes Via Click Chemistry

Cyclodextrin (CD) is regareded as one of the most important supramolecular hosts, which is widely used in the fields of food additive, drug delivery, etc. due to its significant merits, such as broad source, low toxicity, good complexation ability, facile functionalization via the reaction of hydrogen groups, and so on.As one kind of supramolecular polymers, polyrotaxanes (PRs) in whichα-cyclodextrins (α-CDs) are threaded onto poly(ethylene glycol) (PEG) chains, have attracted great interest over the past decades, and till now, a large number of these PRs with different stoppers have been prepared. However, there are still some problems or challenges in this field: (1) lack of efficient methods for the synthesis of PRs with high coverage ratio in large scale; (2) the difficulty in the functionalization of PRs due to the hydrogen bond network among the PR molecules; (3) rare application studies of the as-prepared and functionalized PRs. This thesis aims to work out the solutions for these problems by employing the fast Cu (I)-catalyzed"click"reaction of azides and alkynes. Accordingly, a one-pot strategy for facile and scalable preparation of PRs in water via click end-capping is presented and demonstrated, opening the door for larege-scale and cost-effective production of PRs. The"Click"coupling is also used as a powerful method for the functionalization of PRs with various amino acids, and a one-step click parallel"grafting onto"strategy is proposed to prepare the amphiphilic sliding supramolecular polymer brushes (SSPBs) with novel self-organization behaviors. A preliminary research on the application of PRs as templates for synthesis of metallic nanoparticles is explored, showing the promising application of PRs in nanotechnology. The details are described as follows.1. Synthesis of Fluorescent PRs via Click ChemistryMost of the reported PRs were prepared via the two-pot strategy, which involved the formation of pseudo-PRs in water and the end-capping reaction in organic solvent (DMF is the most often used solvent). Herein, the fluorescent PR was prepared via the two-pot click strategy, demonstrating the possible use of azide-alkyne click chemistry as an end-capping reaction.A fluorescent polyrotaxane PR consisting of PEG (Mn=4600) andα-CDs was facilely prepared by azide-alkyne click chemistry with rhodamine B (RhB) as the stoppers. Scalable synthesis approach to diazido-PEG (N3-PEG-N3) with double ester bonds at each end was set up by esterification of commercialized dihydroxy-PEG (HO-PEG-OH) and 4-(2-azidoethoxy)-4-oxobutanoic acid that was cost-effectively accessed by the facile reaction between 2-azidoethanol and succinic anhydride. Stirring of N3-PEG-N3 andα-CDs in water for 4 days afforded diazido pseudo-PR, which could react with alkyne-functionalized RhB (RhB-alk) in the presence of CuBr/1, 1, 4, 7, 7-pentamethyldiethylenetriamine (PMDETA) to give rise to the PR. Atomic force microscope (AFM) was used to observe the morphology of the fluorescent PRs under ambient environment and a nanowire-like morphology less than 1 nm in height (0.41 nm±0.10nm) was visualized. Interestingly, it was found that the fluorescence emission intensities of RhB-capped PEG (RhB-PEG-RhB) and the PR were much higher than that of neat RhB or RhB-alk with the same concentration of RhB fluorophore in both dimethylsulfone and alkaline aqueous solutions. Fluorescence lifetimes were detected as 3.59, 3.31, 2.99, and 2.99 ns for neat RhB, RhB-alk, RhB-PEG-RhB, and the PR, respectively. The PRs with unique fluorescence properties might have further applications in the fields of biomedicine and bionanotechnology.2."Full CD"PR: One-Pot Facile Synthesis via Click Chemistry and Use as Templates for Platinum NanowiresThe successful synthesis of fluorescent PR demonstrated that the click reaction could work as an end-capping reaction for the preparation of PRs. However, the click end-capping reaction did not bear obvious advantages over the reported one with the two-pot strategy. Since this reaction was efficient and tolerant of water, herein, we developed an alternative one-pot strategy for the preparation of PR in water.? ?Various PRs, in whichα-CDs were threaded onto PEG chain capped withβ-CDs, were prepared facilely and efficiently by click chemistry via one-pot strategy in the water solution of pseudo-PR directly at room temperature. The yield was high, up to 320 mg/100 mg PEG axis with a short reaction time (only 2 h). The terminalβ-CD cavity could be recognized by phenolphthalein and utilized to form a supramolecular block copolymer with alternate rod and coil segments via the formation of a host-guest inclusion complex with diadamantyl-terminated PEG. The prepared PR could further work as a novel template for the in situ deposition of platinum (Pt) nanoparticles (NPs) to fabricate metallic nanowires. TEM and SEM observations showed that the resulting polycrystalline nanowires with length of 50-200 nm and diameter of ca. 12 nm were composed of close-packed uniform Pt NPs with diameter of ca. 2.5 nm. The catalytic activity of the Pt nanowires was demonstrated by the reduction of 4-nitrophenol. The fascinatingβ-CD-capped PR is promising in a wide variety of fields such as supramolecular chemistry and bionanotechnology due to its facile and salable availability and biocompatibility.?3. Facile Functionalization of PR with Amino Acides via Click ChemistryThe efficient accessibility of PRs in large scale promised their further functionalization.? The click reaction was also proved to be an efficient modification method for PRs by the successful synthesis of various biocompatible PR conjugates composed ofβ-CD-capped azido-functionalized PR backbone and amino acids protected by tert-butyloxycarbonyl (Boc) groups, that is, Boc-Gly-OH, Boc-Lys(Boc)-OH, and Boc-Phe-OH. The alkynyl groups were introduced into the aforementioned amino acids via DCC/DMAP condensation with propargyl alcohol. The click coupling reaction was demonstrated to be extremely efficient and high conversion of azido groups, approximate 100%, was achieved in a short reaction time (within several minutes) in the cases of Boc-Gly-OH and Boc-Lys(Boc)-OH. As to the bulky Boc-Phe-OH, only part of the azido groups of the backbone was converted though the coupling reaction time was prolonged (3 h). The corresponding amino acids-PR conjugates were soluble in a wide variety of organic solvents, such as N,N-dimethylformamide, acetone, ethanol and so forth. Other alkyne-functionalized compounds involving 4-(2-azidoethoxy)-4-oxobutanoic acid and propargyl alcohol were also attached to the PR backbone successfully. This click strategy would be a promising general method for the functionalization of PRs.4. One-Step Parallel Click Synthesis of Sliding Supramolecular Polymer BrushOn the basis of the successful coupling of small molecules of amino acids, theβ-CD-capped ?PRs were utilized as the backbones to prepare"sliding supramolecular polymer brushes"(SSPBs) with controllable ratios of hydrophilic poly(ethylene glycol) (PEG) to hydrophobic palmitoyl (C16) side miktochains via one-step parallel"grafting onto"strategy. Click reaction was employed as the efficient coupling reaction as well and high grafting density, approximate 100%, was achieved in a short reaction time (within 3 h). Dye-labeling method and fluorescence measurements showed that the SSPB with close proportion of PEG and C16 was amphiphilic and tended to aggregate into a Janus film at the interface of hexane and water. The SSPBs could also assemble into microporous films on mica surfaces via spin-coating method. The construction of the films was proved to be affected by the factors of relative humidity, rotational speed of spin-coating, and composition of SSPBs.