Angewandte
Chemie
Polymer 6: Monomer 5 (1g, 3.6 mmol) was added to a stirred
cooling step the polymer returns to solution by new formation
of polymer/b-CD complex. In contrast, the b-CD-free poly-
mer is not soluble in water at any temperature in this range.
The interesting polymer-solubility behavior led us to compare
this phenomenon with classical LCST effects. In our case,
because of the reversible complex formation between the
polymer 6 and cyclodextrin, the optical effect is based on
supramolecular interactions. This means that the discovered
pseudo-LCST behavior is a result of noncovalent interactions
between the CD host and polymer guest. Furthermore, in this
system competitive inhibition or control of the LCST is
possible by addition of other suitable guest molecules of low
molecular weight, for example, potassium 1-adamantylcar-
boxylate. CD complexes these molecules preferably, and the
polymer precipitates. This special effect cannot be observed in
“normal” LCST systems.
aqueous solution of b-CD (40 wt%). After the solution had been
flushed with argon for 20 min, a mixture of potassium peroxodisulfate
(24.3 mg) and sodium disulfite (17.1 mg, 2.5 mol%) was added. The
polymerization was carried out at room temperature for 12 h. The
synthesized polymer was purified by several precipitation cycles in a
low concentrated sodium chloride solution first from the crude
solution and then from methanol. NMR and IR data are given in the
Supporting Information.
Received: April 21, 2005
Published online: July 29, 2005
Keywords: cyclodextrins · host–guest systems · LCST · polymers
.
[1] P. R. Washington, O. Steinbock, J. Am. Chem. Soc. 2001, 123,
7933 – 7934.
To prove the postulated dissociation of the polymer/CD
complex at temperatures higher than the clouding point, the
total amount of b-CD in solution was determined by 1H NMR
measurements before polymerization and after filtration of
the precipitated polymer. DMSO was added to the monomer/
b-CD solution as an internal standard. After polymerization,
the solution was heated and the precipitated polymer was
filtered off. Integration of the NMR signals showed the same
amount of b-CD relative to DMSO at each time. The filtered
polymer was nearly free of CD. This is in accordance with the
“pseudo-LCST” effect described above which is caused by
reversible complex formation.
These experiments and results clearly show that the
investigated system of b-CD and polymethacrylamide (5)
reveals new opportunities in the field of thermosensitive
systems based on noncovalent interactions between host and
guest compounds. The noncovalently attached b-CD ring
keeps the insoluble polymer in aqueous solution at lower
temperature. At higher temperatures the CD ring slips off and
the polymer precipitates; this process is reversible. Currently
detailed studies about the temperature-dependent formation
of aggregates and changes in viscosity are underway for a
better understanding of the observed phenomenon and to
investigate the suitability of polymer/b-CD solutions as novel
thermosensitive gels.
[2] J. Zhang, N. A. Peppas, Macromolecules 2000, 33, 102 – 107.
[3] H.-C. Chin, Y.-F. Lin, S.-H. Hung, Macromolecules 2002, 35,
5235 – 5242.
[4] E. Tepper, O. Sadowski, H. Ritter, Angew. Chem. 2003, 115,
3279 – 3281; Angew. Chem. Int. Ed. 2003, 42, 3171 – 3173.
[5] J. H. Han, J. M. Krochta, M. J. Kurth, Y. -L. Hsieh, J. Agric. Food
Chem. 2000, 48, 5278 – 5282.
[6] J. Heo, K. J. Thomas, G. Seong, R. M. Crooks, Anal. Chem. 2003,
75, 22 – 26.
[7] H. Feil, Y. H. Bae, J. Feijen, S. W. Kim, Macromolecules 1993, 26,
2496 – 2500.
[8] H. Ritter, M. Tabatabai, “Cyclodextrin in Polymer Synthesis” in
Encyclopaedia of Polymer Science and Technology, Wiley-
Interscience, New York, 2004, published online.
[9] J. Song, E. Saiz, C. R. Bertozzi, J. Am. Chem. Soc. 2003, 125,
1236 – 1243.
[10] K. Matyjaszewski, T. P. Davis in Handbook of Radical Polymer-
ization, Wiley-Interscience, New York, 2002.
[11] P. Job, C. R. Hebd. Seances Acad. Sci. 1925, 180, 928 – 930.
[12] K. A. Connors, Binding Constants, The Measurement of Molec-
ular Complex Stability, Wiley, New York, 1987.
[13] M. T. Blanda, J. H. Horner, M. Newcomb, J. Org. Chem. 1989,
54, 4626 – 4636.
Experimental Section
For materials and methods, see the Supporting Information. Mono-
mer 3 was synthesized according to the work of Bertozzi and co-
workers.[9]
Monomer 5: 2-Bromoisobutyryl bromide (4; 10.12 mL,
81.9 mmol) was added dropwise to a cold mixture of 3 (8.8 g,
68.2 mmol) and triethylamine (14.1 mL, 100.8 mmol) in 180 mL of
THF over 20 min. The mixture was stirred for 4 h at 08C and 42 h at
room temperature. The precipitated salt was removed by filtration.
The solvent was removed under vacuum, and the resulting yellow oil
was then purified by column chromatography (silica gel, ethyl acetate/
petroleum ether 1:1) (Rf = 0.4). Yield: 85%. For elemental analysis
and NMR and IR data, see the Supporting Information.
Complex 5a: Monomer 5 (1g, 3.6 mmol) was added to a stirred
aqueous solution of b-CD (40 wt%; 7.2 g of b-CD in 10.8 mL of
doubly distilled water). The molar ratio of monomer 5 to b-CD was
1:1.5.
Angew. Chem. Int. Ed. 2005, 44, 5658 –5661
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5661