Article
Macromolecules, Vol. 43, No. 24, 2010 10247
Scheme 1. Synthesis of the Monomer M4
Scheme 2. Synthesis of Cross-Linkers C6, C7, and C8
hydride. Tetrahydrofuran (THF) was distilled from sodium/
benzophenone. Compound 143 (Scheme 1), compound 544 (Scheme 1),
and third generation Grubbs’ catalyst (G3) (dichloro-di-
(3-bromopyridino)-N,N0-dimethylenoimidazolino-RudCHPh)45
were synthesized according to the literature.
Synthesis of Compound 3. Compound 1 (20 g, 0.12 mol) and
solketal (35.68 g, 0.27 mol) were dissolved in 180 mL of
anhydrous DCM. DMAP (1.47 g, 12 mmol) in 20 mL of
anhydrous DCM was added to the solution dropwise. The
reaction mixture was stirred at room temperature for 12 h to
synthesize compound 2 (Scheme 1). Without purification, the
crude mixture was cooled to 0 °C and EDC (26.84 g, 0.14 mol)
was added. After 1 h, the reaction mixture was warmed to room
temperature and stirred for ∼24 h further. Purification pro-
cesses were followed by washing with 10% sodium bicarbonate
aqueous solution (300 mL), saturated NaHCO3 aqueous solu-
tion (300 mL), and saturated NaCl aqueous solution (300 mL)
successively and then by crystallization from DCM/hexane to
provide the white solid product 3 (Scheme 1) with a yield ca.
1H NMR of C8 (CDCl3) δ (ppm): 6.45 (s, 4H, (-CHdCH-)2),
5.26 (s, 4H, (=CH-CH-)4), 4.35-4.08 (m, 8H, (-COO-
CH2-)4), 3.71-3.65 (t, 396H, (-CH2-O-CH2-)99), 2.85-
2.77 (m, 4H, (-CH-COO-)4), 1.27-1.23 (t, 6H, (-CH3)2).
Homopolymerization of M4. A general procedure: M4 (199.4 mg,
0.6 mmol) was dissolved in 1.0 mL of DMF. After G3 (5.3 mg,
6.0 ꢀ 10-3 mmol) in 0.2 mL of DMF was added, the vial was
sealed and stirred sufficiently at room temperature ca. 12 h. At
timed intervals, samples were withdrawn for measuring the
monomer conversion and molecular weight of poly-M4 by
1H NMR and GPC, respectively.
1
70%. H NMR (CDCl3) δ (ppm): 6.46 (s, 2H, -CHdCH-),
5.28 (s, 2H, =CH-CH-), 4.36-3.70 (m, 10H, -COO-CH2-,
-CH-O-, -CH2-O-), 2.86 (s, 2H, -CH-COO-), 1.43-
1.36 (d, 6H, -CH3).
Synthesis of Monomer M4. Compound 3 (10 g, 24 mmol) was
dissolved in 30 mL of methanol. Activated 50WX2-100 ion-
exchange resin (Aldrich) (10 g) was added to the solution in one
pot, and the reaction was stirred at room temperature for about
24 h. After filtration to remove the used ion-exchange resin, the
filtrate was concentrated and purified by a silica gel column with
ethyl acetate:methanol (5:1, v/v) as eluent. The colorless oil
product M4 (Scheme 1) (1.6 g) was obtained in a yield of 80%.
1H NMR (methanol-d4) δ (ppm): 6.50 (s, 2H, -CHdCH-),
5.22 (s, 2H, =CH-CH-), 4.25-3.99 (m, 4H, -COO-CH2-),
3.87-3.80 (m, 2H, -CH-OH), 3.30-3.32 (m, 4H, -CH2-
OH), 2.94 (s, 2H, -CH-COO-). 13C NMR (75 MHz,
methanol-d4), δ (ppm): 173.7, 137.8, 82.1, 71.1, 67.4, 64.1.
Synthesis of Cross-Linkers C6, C7, and C8. C6, C7, and C8
(Scheme 2) were prepared from Mitsunobu coupling reactions
between compound 5 and diethylene glycol, PEG 600, and PEG
4000, respectively. As an example, compound 5 (5.30 g, 25 mmol),
PEG 4000 (20 g, 5 mmol), and Ph3P (6.56 g, 25 mmol) were
dissolved in anhydrous THF (150 mL). DIAD (5.06 g, 25 mmol)
was added dropwise by a syringe. The reaction mixture was
stirred at room temperature for 24 h. C6 and C7 were purified
by silica gel columns with ethyl acetate:hexane=3:1 (v/v) and
DCM:methanol=7:1 (v/v) as eluents, respectively. C8 was puri-
fied by precipitation into a large amount of ethyl ether three
times. 1H NMR of C6 (CDCl3) δ (ppm): 6.45 (s, 4H,
(-CHdCH-)2), 5.26 (s, 4H, (=CH-CH-)4), 4.35-4.08
(m, 8H, (-COO-CH2-)4), 3.71-3.65 (t, 4H, -CH2-O-
CH2-), 2.85-2.77 (m, 4H, (-CH-COO-)4), 1.27-1.23 (t, 6H,
(-CH3)2). FAB-MS: m/z 495.2 (Mþ), calculated 494.18. 1H
NMR of C7 (CDCl3) δ (ppm): 6.45 (s, 4H, (-CHdCH-)2),
5.26 (s, 4H, (=CH-CH-)4), 4.35-4.08 (m, 8H, (-COO-
CH2-)4), 3.71-3.65 (t, 48H, (-CH2-O-CH2-)12), 2.85-
2.77 (m, 4H, (-CH-COO-)4), 1.27-1.23 (t, 6H, (-CH3)2).
Preparation of Hydrogels. A general procedure: M4 (400.5 mg,
1.21 mmol) and C6 (35.8 mg, 7.24 ꢀ 10-2mmol) were dissolved in 2 mL
of DMF in a 6 mL glass vial. After G3 (5.9 mg, 6.67 ꢀ 10-3 mmol)
in 0.7 mL of DMF was added to the solution and it was stirred
sufficiently at room temperature, the mixture was transferred to a
homemade Teflon mold with dimensions of 48 ꢀ 3 mm (D ꢀ H)
and covered by a glass plate. Twelve hours later, the resultant gels
were washed with large amounts of DMF repeatedly 5 times in
5 days to remove the sol fraction. Then, the same washing
procedure was performed using water to remove the DMF and
replace it with water. The gel fraction was determined as the weight
ratio between the dried gel and the sum of M4, C6, and G3 used in
the synthesis.
Characterization. NMR spectra were recorded on a Bruker
DPX300 spectrometer at room temperature.
Gel permeation chromatography (GPC) was performed by a
Polymer Laboratories PL-GPC50 instrument with two 5 μm
mixed-D columns, a 5 μm guard column, and a Knauer RI
detector. DMF with 0.01 M LiBr was used as eluent at a flow
rate of 1.0 mL/min. Poly(methyl methacrylate) standards were
used for the calibration.
Rheological measurements were performed using a TA
Instruments AR2000 stress-controlled rheometer with a parallel
plate geometry (40 mm diameter, 1-2.5 mm gap distance
depending on the sample) at 25 °C. A solvent trap was used to
minimize water evaporation during testing. A stress sweep at
a constant frequency of 1 Hz was performed first to determine
the linear viscoelastic region for collecting subsequent data.
Frequency sweep tests over a range of 0.01-100 Hz were
performed at a constant stress (0.1-2.0 Pa, depending on the
sample) to measure the elastic modulus (G0) and the viscous
modulus (G00).