hydrogels. In addition, unreacted azide and/or acetylene groups
can be subsequently functionalized leading to chemical tailoring of
the hydrogels and further demonstrating the formation of new,
diverse crosslinked materials.
Financial support from the NSF MRSEC Program DMR-
0520415 (MRL-UCSB),
a
Program of Excellence in
Nanotechnology Grant (1 U01 HL080729-01) from the NIH,
the GOALI Program (Grant DMI-0217816), Chemistry (CHE-
0514031), ACS PRF (Grant UFS 39964) and IBM is gratefully
acknowledged. L.M. and P.D. are also grateful to ‘‘R e´ gion
Wallonne’’ and the European Community (FEDER, FSE) for
general support in the framework of ‘‘Objectif 1-Hainaut: Materia
Nova’’ and from the Belgian F.N.R.S. and Office of Science Policy
Fig. 1 PEG-based hydrogels formed from 2a and 4 in the presence of (a)
carbon black, (b) 4-phosphonooxy-2,2,6,6-tetramethylpiperidyloxy nitr-
oxide and (c) titanium dioxide nanoparticles.
(PAI-5/3).
occurred at pH = 1, slow degradation (greater than 1 month) was
observed at pH = 7 and complete degradation achieved at pH = 10
after only 3 hours. In contrast, PEG hydrogels could be prepared
from the corresponding diether, 7, and were found to be stable for
greater than 1 month at pH values ranging from 1 to 14.
Notes and references
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2
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3
4
(a) R. Pelton, Adv. Colloid Interface Sci., 2000, 85, 1–33; (b) X. Yin and
H. D. H. St o¨ ver, J. Polym. Sci., Part A: Polym. Chem., 2005, 43,
Another design advantage of Click chemistry is the tolerant and
specific reaction conditions which allows network formation to be
performed at room temperature in the presence of a variety of
additives which would normally retard or terminate polymeriza-
tion under traditional radical or photochemical conditions. To
examine this feature in detail the preparation of PEG hydrogels
based on 2a and 4 was performed in the presence of carbon black
1641–1648.
(a) P. Martens and K. S. Anseth, Polymer, 2000, 41, 7715–7722; (b)
S. Lin-Gibson, R. L. Jones, N. R. Washburn and F. Horkay,
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J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 3932–3944; (d) C. S.
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5 M. P. Lutolf, G. P. Raeber, A. H. Zisch, N. Tirelli and J. A. Hubbell,
Adv. Mater., 2003, 15, 888–892.
(20 wt%), 4-phosphonooxy-2,2,6,6-tetramethylpiperidyloxy nitr-
6
7
8
9
M. Wathier, P. J. Jung, M. A. Carnahan, T. Kim and M. W. Grinstaff,
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The 4-phosphonooxy-2,2,6,6-tetramethylpiperidyloxy nitroxide
was chosen as a water soluble radical trap which would terminate
either a photochemical or thermal free radical process. Similarly
2
the carbon black and TiO particles have reactive surfaces which
can interfere with many polymerization processes while also
adsorbing light, rendering the polymerization mixture opaque and
therefore incapable of undergoing photopolymerization.
Significantly, the gel fraction in each case was essentially the same
as for the non-additive case (0.949) and for the nitroxide example
the mechanical properties were identical to those for 5a. For the
filled samples the modulus improved significantly (25.2 kPa for
1
0 For some recent applications in polymer synthesis, see: (a) D. D. D ´ı az,
S. Punna, P. Holzer, A. K. McPherson, K. B. Sharpless, V. V. Fokin
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4
392–4403; (b) N. V. Tsarevsky, K. V. Bernaerts, B. Dufour, F. E.
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(
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carbon black and 33.4 kPa for TiO ) while the elongation to break
2
decreased less than 5% in both cases. These results demonstrate the
advantages of Click chemistry and the ability to employ a wide
range of reactive additives without any detrimental effects on
network formation.
14942–14949.
In summary, new crosslinked, PEG-based hydrogel materials
have been synthesized by taking advantage of the fidelity of Click
chemistry. The crosslinking in these hydrogels is extremely high,
and results in a more ideal structure leading to improved properties
when compared to traditional photochemically-crosslinked PEG
11 (a) H. Ihre, O. L. Padilla de Jes u´ s and J. M. J. Fr e´ chet, J. Am. Chem.
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12 S. P. Obukhov, M. Rubinstein and R. H. Colby, Macromolecules, 1994,
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2
776 | Chem. Commun., 2006, 2774–2776
This journal is ß The Royal Society of Chemistry 2006