Polyrotaxane preparation

Table 5. Polyrotaxanes prepared from PEG-BA with various molecular weights...

The same types of polyrotaxanes were also prepared by a different method, Method 2 (Figure 6). In this method, a preformed polymer is used and the cyclic is threaded onto the polymer in a melt or in solution. A solution of 28 and polystyrene in THF under reflux afforded a polyrotaxane with an min value of 5.0X1CT4, much lower than those via Method 1 [69]. Threading 28 on to poly (butylene sebacate) afforded poly(ester rotaxane) 33 of Type 4 [70]. Although a laige excess of cyclic was used, 33 only had a min value of 0.0017. This value is 100-fold lower than that for the corresponding polyrotaxane prepared by Method 1 [19]. A possible reason is that the concentration of chain ends is very low and the random coiled-chain conformation of a polymer disfavors threading. 

Polyrotaxanes Prepared by Inclusion Polymerization (Polythiophene Polyrotaxanes)... 

Purification of the polyrotaxanes is one of the important aspects in preparing biomaterials. In the course of the polyrotaxane preparations mentioned above,... 

Such a local arrangement of cydes along die chain is typical both for a statistical polyrotaxane and for a polyrotaxane prepared by styrene polymerization in the presence of swarm complexes of cyclourethanes with ZnClj. For the latter, portions with a higher coiKentratioo of cycles can be attributed to the swarm structure of complexes, while for a statistical polyrotaxane sudi a structure appears to be due to that the initial cyclourethanes are orirated with respect to one another into ordered formations with the aid of intermolecular hydrograi bonds. The lengtii of the format tions is shorter than in swarm cornices. 

Yamaguchi, I., Osakada, K., Yamamoto, T. (2000) A novel crown ether stopping group for side chain polyrotaxane Preparation of side chain polybenzimidazole rotaxane containing aUcyl side chain ended by crown ether—ONa Group. Macromolecules, 33, 2315-2319. 

Table 4 Polyrotaxanes Prepared from PEG-BA with Various Molecular Weights...

Harada et al. started from preparing inclusion complexes by adding an aqueous solution of PEG bisamine (PEG-BA) to a saturated aqueous solution of a-CD at room temperature and then allowing the complexes formed to react with an excess of 2,4-dinitrofluorobenzene. They examined the product by column chromatography on Sephadex G-50, with DMSO as the solvent, and obtained the elution diagram shown in Fig. 46. They identified the first, second, and third fraction, respectively, as the desired product, i.e., a polyrotaxane, dinitrophenyl derivatives of PEG, and uncomplexed a-CD, by measurement of both optical rotation and UV absorbance at 360 nm for the first, UV absorbance at 360 nm for the second, and optical rotation for the third. 

Harada and coworkers proceeded further to obtain a tubular polymer from a PEG-a-CD polyrotaxane by using Scheme 2. The polyrotaxane was prepared... 

Recently, we have also prepared nanosized polymersomes through self-assembly of star-shaped PEG-b-PLLA block copolymers (eight-arm PEG-b-PLLA) using a film hydration technique [233]. The polymersomes can encapsulate FITC-labeled Dex, as model of a water-soluble macromolecular (bug, into the hydrophilic interior space. The eight-arm PEG-b-PLLA polymersomes showed relatively high stability compared to that of polymersomes of linear PEG-b-PLLA copolymers with the equal volume fraction. Furthermore, we have developed a novel type of polymersome of amphiphilic polyrotaxane (PRX) composed of PLLA-b-PEG-b-PLLA triblock copolymer and a-cyclodextrin (a-CD) [234]. These polymersomes possess unique structures the surface is covered by PRX structures with multiple a-CDs threaded onto the PEG chain. Since the a-CDs are not covalently bound to the PEG chain, they can slide and rotate along the PEG chain, which forms the outer shell of the polymersomes [235,236]. Thus, the polymersomes could be a novel functional biomedical nanomaterial having a dynamic surface. 

Supermolecular interlocked macromolecules have been paid much attention as candidates of smart materials. Polyrotaxane (PRX) is a typical example. PEG/ cyclodextrin (CD)-based polyrotaxane was firstly reported by Harada and coworkers by attachment of stoppers to pseudopolyrotaxane (pPRX) consisting of a PEG and CDs [263]. Subsequently, many CD-based PRXs have been designed and prepared as smart materials such as biomaterials, light-harvesting antennae, insulating polymers, stimuli-responsive molecular shuttles etc. [264—268]. 

In the last few years there have been new creative methods of preparation of novel hydrophilic polymers and hydrogels that may represent the future in drug delivery applications. The focus in these studies has been the development of polymeric structures with precise molecular architectures. Stupp et al. (1997) synthesized self-assembled triblock copolymer, nanostructures that may have very promising applications in controlled drug delivery. Novel biodegradable polymers, such as polyrotaxanes, have been developed that have particularly exciting molecular assemblies for drug delivery (Ooya and Yui, 1997). 

Research Focus Method of preparing viscoelastic polymers using crosslinked polyrotaxane inclusion complexes consisting of polyethylene glycol (PEG)-carboxyhc acid and a-cyclodextrin. 

Polyrotaxane derivatives were prepared in five steps summarized below ... 

Preparation of crosslinked polyethylene glycol a-cyclodextrin polyrotaxane... 

Nakajima et al. (1) prepared liquid crystalline polyrotaxane derivatives containing the mesogenic group 4-cyano-4 -hydroxybiphenyl attached to the a-cyclodextrin component of linear polyethylene glycol containing an a-cyclodextrin inclusion complex with an adamantane termini. 

On the other hand, to constrain the cyclic, the blocking group (BG) has to be bulkier than the cavity of the cyclic molecule. Harrison found that a trityl group can only block cyclics up to 28-membered whereas the tris(p-t-butylphenyl)-methyl moiety can effectively constrain 42-membered cyclics [3, 13, 14]. These results have been widely applied as a guide in the preparation of polyrotaxanes e.g., monofunctional [18, 19] and difimctional [19—23] BG based on tri- and tetra-arylmethane derivatives were successfully prepared and used as end groups and in-chain units, respectively, to constrain crown ethers in a variety of polyrotaxanes. 

Similar to those for rotaxanes, different approaches have been employed for poly-rotaxane syntheses these will be summarized in the next section. The most important parameter in polyrotaxanes, the min value, is often employed as a measure of the effectiveness of the preparation method. Because this value mainly depends on the strength of the attractive force between the cyclic and the backbone, this section is again divided into subsections according to the types of driving forces rather than the types of polyrotaxanes. 

In 1979, Maciejewski et al. also explored Method 3 for the preparation of main-chain poly(vinylidene chloride-/ -CD rotaxane) 35 [74, 75]. Radiation polymerization and AIBN-initiated solution polymerization of the complex of vinyli-dene chloride with 21 gave products with min = 0.34 and 0.49, respectively. However, the polyrotaxane via Method 1 had a much lower min (0.087) with much lower CD/monomer feed ratio than for those via the polymerization of the complex (1 1 ratio). Therefore, the reported min values are not comparable, so the difference between the two methods in terms of threading efficiencies cannot be distinguished. Although the authors did not see any threading via Method 2 for the same polyrotaxanes, Ogino and coworkers prepared a true CD-based polyrotaxane of Type 5 using metal complexes as stoppers [76]. It was also found that... 

Harada et al. explored the compatibility of CD with various polymeric backbones including polyethylene oxide) (PEG), polypropylene oxide) (PPG), polyisobutylene (PIB), and polyethylene (PE) [77-87]. The corresponding polyrotaxanes (36 to 47) were prepared by Method 2, simply by mixing a solution of CD and the polymer. The cavity size of CD was found to be the main factor in the threading process. While one a-CD (20) was threaded per two repeat units in PEG (m/n=0.50) and every three repeat units for PE (m/n=0.333), it was too small for PIB and PPG. On the other hand, two PPG units complexed per /(-CD (21). Because the upper limit of the min value is controlled by the depth of the CD cavity, the m/n value remained constant for the same type of backbone, irrespective of the end group. However, the nature and concentration, i.e., polymer... 

Wenz and Keller reported a-CD-based polyrotaxanes 50 and 51 with poly(iminoundecamethylene) and poly(iminotrimethylene iminodecamethylene) as backbones, respectively. These polyrotaxanes were prepared by mixing an acidic solution of the corresponding polymer with CD (Method 2, Figure 6) [88]. Polyrotaxane 50 was also transformed into 52 by attaching BG onto the NH sites. The threading process was monitored by proton NMR spectra and viscosity changes. The purified product had min values from 0.10 to 0.67, depending on the back-... 

Yui and coworkers prepared CD-based polyrotaxane of Type 5 by capping the cyclic on a triblock copolymer of polyethylene glycol) and polypropylene glycol) using a blocking group [92]. When the peptide linkage was between PEO and the BG, the thermal stability of the polyrotaxane was related to disassociation... 

The effective complexation of CD was also used for the preparation of side-chain polyrotaxanes [96-102], All of these side-chain systems are based on Method 4 or Method 5 (Figure 9) and were reported by Ritter and coworkers. For Method 4, the CD was threaded on to a small molecule bearing a functional group Y and with one end blocked, i.e., a hemirotaxane. The reaction of Y with X, a pendant functional group in a preformed polymer, gives thermally stable... 

A side-chain polyrotaxane of Type 10 was also obtained by the reaction of de-protonated poly(benzimidazole) with a long-chain bromide bearing a BG at one end in the presence of / -CD [103], Because the CD was threaded onto the side chain before its reaction with backbone to form a hemirotaxane, the preparation is essentially Method 4. 

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