Langmuir 2002, 18, 2453-2454
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Polypseudorotaxanes from Scratch Christopher N. Tait and D. Martin Davies* Division of Chemical Sciences, School of Applied and Molecular Sciences, University of Northumbria at Newcastle, Newcastle upon Tyne, NE1 8ST, U.K. Received December 17, 2001
Introduction In 1990, Harada and Kamachi reported that in aqueous solution an insoluble polypseudorotaxane is formed by the threading of R-cyclodextrin onto poly(ethylene glycol), PEG, with two ethylene glycol units per cyclodextrin.1 Similar polypseudorotaxanes are formed with β-cyclodextrin or γ-cyclodextrin and poly(propylene glycol).2 A review of subsequent developments in the area of cyclodextrin-based catenanes and rotaxanes has been published, and X-ray crystal structures are now available.3,4 Our work on cyclodextrin host-guest interactions5 and kinetics meant that we found of particular interest a recent kinetic study by Baglioni and co-workers, reporting that when aqueous R-cyclodextrin and PEG of molecular weight 3350 (PEG3350) are mixed together there is a certain lag time when the solution remains perfectly clear before the onset of turbidity, measured as the absorbance at 400 nm.6 The lag time depends very strongly on the concentration of the solution. The authors presume that the onset of turbidity marks the end of a complex phenomenon that starts with the threading and sliding of a cyclodextrin onto the linear PEG molecule and ends with the precipitation of large aggregates. It is assumed that during the lag time the cyclodextrin molecules are penetrated by PEG chains and the onset of turbidity marks the aggregation and precipitation of the polypseudorotaxane, and that the whole of the threading process occurs during the lag time, which is defined by the authors as the threading time. A kinetic model is proposed that predicts that the number of cyclodextrin molecules that participate in the formation of the polypseudorotaxane is about 20. This is similar to the total number of cyclodextrin molecules that are capable of threading onto the PEG molecule. Prompted by the above paper and the greater yields reported for complex formation between R-cyclodextrin and poly(ethylene glycol) dimethyl ether, DMPEG, compared to PEG of the same molecular weight,7 we began a study of the kinetics of polypseudorotaxane formation between the cyclodextrin and DMPEG using the lag time approach. This is reported in the present note. Subsequently, a further threading time kinetic study has been (1) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2823. (2) Harada, A.; Kamachi, M. J. Chem. Soc., Chem Commun. 1990, 1322. (3) Nepogodiev, S. A.; Stoddart, J. F.; Chem. Rev. 1998, 98, 1959. (4) Harada, A.; Li, J.; Kamachi, M.; Kitagawa, Y.; Katsube, Y. Carbohydr. Res. 1998, 305, 127. (5) Davies, D. M.; Savage, J. R. J. Chem. Res. (S) 1993, 94. Davies, D. M.; Savage, J. R. J. Chem. Res. (M) 1993, 660. Davies, D. M.; Savage, J. R. J. Chem. Soc., Perkin Trans. 2 1994, 1525. Davies, D. M.; Deary, M. E. J. Phys. Org. Chem. 1996, 9, 433. Davies, D. M.; Deary, M. E. J. Chem. Soc., Perkin Trans. 2 1995, 1287. Davies, D. M.; Deary, M. E. J. Chem. Soc., Perkin Trans. 2 1996, 2415. Davies, D. M.; Deary, M. E.; Wealleans, D. I. J. Chem. Soc., Perkin Trans. 2 1998, 193. Moozyckine, A. U.; Bookham, J. L.; Deary, M. E.; Davies, D. M. J. Chem. Soc., Perkin Trans. 2 2001, 1858. (6) Ceccato, M.; Lo Nostro, P.; Baglioni, P. Langmuir 1997, 13, 2436. (7) Harada, A.; Li, J.; Kamachi, M. Proc. Jpn. Acad. B 1993, 69, 39.
Figure 1. Effect of R-cyclodextrin concentration, (5.0-7.5) × 10-2 M on the inverse lag time for precipitate formation with 2.00 × 10-4 M (lower points) and 5.0 × 10-4 M (upper points) DMPEG2000 at 25 °C.
reported for the formation of polypseudorotaxanes from β-cyclodextrin and poly(propylene glycol)33 bis-2-aminopropyl ether and from γ-cyclodextrin and Pluronic P105, HO(CH2CH2O)34(CH2CH(CH3)O)61(CH2CH2O)34H.8 Experimental Section Poly(ethylene glycol) average molecular weight 2000 (PEG2000) and R-cyclodextrin were obtained from Aldrich, and the dimethyl ether of poly(ethylene glycol) average molecular weight 2000 (DMPEG2000) was from Fluka. Solutions were made in distilled water and mixed to obtain the stated concentrations. Turbidity changes were measured at 25 °C as the absorbance at 400 nm using a Pharmacia Biotec Ultraspec 2000 spectrophotometer equipped with a thermostatic cell holder. The lag time was taken as the time required to obtain a detectable increment in the absorbance as described by Baglioni and co-workers.6 The highest concentration of DMPEG2000 used in the quantitative kinetic study was 5.0 × 10-4 M, which, even assuming the maximum possible cyclodextrin binding to the polymer chain during the binding process would consume only about 1.1 × 10-2 M cyclodextrin, which is a fraction of the lowest total cyclodextrin concentration of 5.0 × 10-2 M, thus ensuring an acceptably constant cyclodextrin concentration during the course of the reaction. Proton NMR spectra were obtained using a JEOL 250 MHz spectrometer at 25 °C in aqueous solution containing 10% deuterium oxide to facilitate signal locking.
Results Figure 1 shows the effect of cyclodextrin concentration on the lag time at two different concentrations of DMPEG2000. The data in Figure 1 conform to eq 1, which shows an approximately sixth order dependence of the lag time on cyclodextrin, CD, concentration and third order on DMPEG concentration. The values are ( one standard deviation.
log10 τ-1 ) (15.1 ( 0.5) + (6.3 ( 0.3) log10[CD] + (2.9 ( 0.1) log10[DMPEG] (1) Experiments with PEG2000 for direct comparison with the DMPEG2000 data led to much longer lag times. For (8) Lo Nostro, P.; Lopes, J. R.; Cardelli, C. Langmuir 2001, 17, 4610.
10.1021/la015737w CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002
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example, with [PEG2000] ) 5.0 × 10-4 M and [CD] ) 6.0 × 10-2 M, the lag time is about 10000 s as compared with 200 s under identical conditions with DMPEG2000. These lag times with PEG2000 were much longer than anticipated from those of Baglioni and co workers with PEG3350.6 To resolve the difference, we carried out the following experiments using freshly mixed solutions of PEG2000 and R-cyclodextrin of the same concentrations as above. (1) A volume of 2.0 mL of the solution was seeded with 0.050 mL of a well mixed suspension of precipitated solution previously obtained under the same conditions. This resulted in almost immediate precipitation. (2) A small volume of the solution was put in a Pyrex beaker that was then scratched with a soda glass rod. This induces precipitation much more rapidly. Precipitation was visually apparent after about 600 s of continuous scratching, whereas it took about 10,000 s in a similar control beaker that was not scratched. To test whether the scratching and seeding was causing the precipitation of polypseudorotaxane from a supersaturated solution we attempted to detect polypseudorotaxane in solutions containing 5.0 × 10-4 M DMPEG2000 and 4.5 × 10-2 M R-cyclodextrin prior to precipitation, which occurred at about 60 min. No significant changes in the cyclodextrin or the polymer proton NMR shifts in comparison to single component spectra were detected. Discussion The effect of DMPEG and cyclodextrin concentrations on the inverse lag time for precipitate formation shown in Figure 1 and quantified in eq 1 is clearly indicative of a cooperative process. Interpretation of these results along the lines of the threading time kinetic model of Baglioni and co-workers6 would relate the order of the reaction with regard to the reactants to the composition of the activated complex for polypseudorotaxane formation. The reaction is approximately sixth order in cyclodextrin concentration and third order in DMPEG concentration, and while it is conceivable that a preequilibrium process involving six or so cyclodextrin molecules in solution leads
Notes
to the activated complex for polypseudorotaxane formation, it is difficult to envisage an activation process involving a preequilibrium of three DMPEG chains in solution. The lag time for PEG2000 is much greater than that for DMPEG2000 under the same conditions, suggesting that hydrogen bonding of the PEG terminal OH groups with the primary or secondary OH groups of cyclodextrin somehow inhibits the precipitation process. It seems likely that after the initial threading of a cyclodextrin onto the PEG the sliding process, allowing further cyclodextrin molecules to thread onto the chain, is inhibited by the relative stability endowed to the initial complex by this hydrogen bonding. This is of course not possible with DMPEG where the sliding process is not inhibited by hydrogen bonding within the initial cyclodextrin-polymer complex. The scratching and seeding experiments indicate that nucleation is an important step in the precipitation of the polypseudorotaxane and that the whole of the threading process is not completed at the end of the lag time in undisturbed solutions. Hence the assumptions made by other authors6,8 in the derivation of the threading time kinetic model cannot hold. Our inability to detect a significant interaction between cyclodextrin and the polymer by NMR suggests that it is unlikely that a supersaturated solution of the polypseudorotaxane is formed. It is probable that, as suggested by Weickenmeier and Wenz, in a paper that dealt with cyclodextrin threading onto a polyester formed from octanedicarboxylic acid and poly(ethylene glycol),9 that the inclusion of poly(ethylene glycol) by R-cyclodextrin is mainly driven by the precipitation of the polypseudorotaxane. Acknowledgment. We thank Warwick International Group Ltd. for providing a Research Studentship to Christopher N. Tait. LA015737W (9) Weickenmeier, M.; Wenz, G. Macromol. Rapid Commun. 1997, 18, 1109.
