T. E. Kristensen, F. K. Hansen, T. Hansen
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more Et2O (200 mL), each time allowing the piece of polymer to
sit for some time with occasional stirring. The white polymer was
separated once more and dried under vacuum in a desiccator over
concentrated H2SO4 at room temperature for 72 h and then in air
at the same temperature for 2 d to give poly(O-acryloyl-trans-4-
hydroxy--proline hydrochloride) (10, 10.36 g, 93%) as a crystalline
solid. 1H NMR (D2O, 200 MHz, reported relative to residual 2-
propanol at δ = 4.02 ppm): δ = 5.45–5.65 (m, 1 H, 4-H), 4.59 (t, 3J
= 8.4 Hz, 1 H, 2-H), 3.75–3.95 (m, 1 H, 5-H), 3.55–3.75 (m, 1 H,
5-H), 2.35–2.80 (m, 3 H, 2ϫ3-H and H on acrylic backbone),
1.55–2.30 (m, 2 H, 2ϫ H on acrylic backbone) ppm. IR (KBr): ν
˜
= 3429, 2968, 1732, 1632 cm–1. Molecular weight distribution (Mn
= 29600, Mw = 686000) was determined by GPC analysis in aque-
ous phosphate buffer by using the sodium salt of poly(acrylic acid)
as calibration standard. A portion of poly(O-acryloyl-trans-4-hy-
droxy--proline hydrochloride) (10, 5.11 g) was dissolved in water
(40 mL) at room temperature to give a clear viscous solution, and
Et3N was added dropwise whilst stirring with a spatula. A white
piece of polymer started to separate from the solution and the ad-
dition was continued until the pH of the aqueous phase stabilized
at ca. 6 (close to the isoelectric point for hydroxyproline). The poly-
mer was separated and sequentially submerged into another por-
tion of water (100 mL), MeOH (100 mL), MeOH (50 mL) and fi-
nally Et2O (50 mL), each time allowing the polymer to sit for some
time with occasional stirring. The polymer was separated once
more and dried under vacuum in a desiccator over concentrated
H2SO4 at room temperature for 26 h and then in air at the same
temperature for 24 h to give poly(O-acryloyl-trans-4-hydroxy--
proline) (11, 3.67 g, 86%) as a transparent and brittle, crystalline
solid (soluble in formamide and water/glycerol mixtures). IR (KBr):
[4]
ν = 3436, 2978, 2943, 1738, 1634 cm–1. Elemental analysis: C 46.55,
[5]
˜
H 5.52, N 6.83.
Acknowledgments
This work was generously supported by Birkeland Innovasjon (the
Technology Transfer Office of the University of Oslo). We thank
Osamu Sekiguchi of the Department of Chemistry, University of
Oslo, for obtaining the mass spectra and to Steinar Pedersen of
Ineos for useful discussions. We would especially like to acknow-
ledge Evonik Degussa GmbH, Exclusive Synthesis & Catalysts, for
kindly donating a generous sample of trans-4-hydroxy--proline.
[6]
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Although widely and incorrectly perceived as expensive, the
current market price of trans-4-hydroxy--proline as of 2008 is
60 j/kg, compared to 20 j/kg for -proline (numbers kindly
provided by the marketing and sales department of Evonik De-
gussa GmbH).
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