Cyclodextrins in polymer chemistry: Enzymatically catalyzed oxidative polymerization of para-functionalized phenol derivatives in aqueous medium by use of horseradish peroxidase

Article abstract of DOI:10.1021/ma021678x

Ethyl 1-[(4-hydroxyphenyl)aminocarbonyl)]-2-vinylcyclopropane carboxylate has been oligomerized using horseradish peroxidase as catalyst. The oligomerization was achieved in the presence of cyclodextrine at room temperature in phosphate buffer (pH 7). The

Full text of DOI:10.1021/ma021678x

7090
Macromolecules 2003, 36, 7090-7093
Cyclodextrins in Polymer Chemistry: Enzymatically Catalyzed
Oxidative Polymerization of Para-Functionalized Phenol Derivatives in
Aqueous Medium by Use of Horseradish Peroxidase
Ya n jie P a n g, Helm u t Ritter ,* a n d Mon ir Ta ba ta ba i
Institute of Organic Chemistry and Macromolecular Chemistry II, Heinrich-Heine-University of
Duesseldorf, Universitaetsstr. 1, 40225 Duesseldorf, Germany
Received November 12, 2002; Revised Manuscript Received May 28, 2003
ABSTRACT: Ethyl 1-[(4-hydroxyphenyl)aminocarbonyl)]-2-vinylcyclopropane carboxylate has been oli-
gomerized using horseradisch peroxidase as catalyst. The oligomerization was achieved in the presence
of cyclodextrine at room temperature in phosphate buffer (pH 7). The oligomer formed was cross-linked
via free radical polymerization.
In tr od u ction
the synthesis and polymerization of VCP derivatives
attached with phenol, which are potential candidates
for dental materials.
The oxido-reductase horseradish peroxidase (HRP)
catalyzes oxidative oligomerization of various electron-
rich aromatic molecules like ortho- and meta-substitut-
ed phenol derivatives. This is a potential alternative
method for preparation of phenol resins without using
toxic formaldehyde. The structure and catalysis mech-
anism of HRP have been intensively studied.1-4
Because of the poor solubility of phenol derivatives,
the HRP-catalyzed oligomerizations are often carried
out in a mixture of aqueous buffer and organic solvent,
such as dioxane or acetone. In the mixtures of acetone
and water (pH 6), Liu et al. have achieved the polym-
erization of azophenol derivatives with functional groups
like CN-, CH3O-, NO2-, SO3-, and COO- at the para-
position.5
In recent studies we have reported that many organic
monomers form soluble host-guest complexes with 2,6-
di-O-methyl-â-cyclodextrin (RAMEB), which can be
polymerized in water.6 In this connection the enzymatic
polymerizations of para- and meta-substituted phenols
in water were achieved, in which RAMEB was used as
host to form soluble complexes with the hydrophobic
phenol monomers.7 A polymerization degree of at least
6-10 with a yield up to about 100% was achieved
through the enzymatic polymerization of these com-
plexes in water (pH ) 7). In contrast, in the absence of
RAMEB, only very low yields of these oligophenols were
obtained.
The enzymatic polymerizations of phenol derivatives
bearing a furamide group, maleimide group, and nitrone
group at the para-position were also studied. These
copolymers were cross-linked via thermal cycloaddition
reactions.8-10
Vinylcyclopropanes (VCP) are interesting monomers
because of low volume shrinkage during the radical
polymerization. 1,1-Disubstituted VCPs undergo radi-
cally 1,5-ring-opening polymerization when the substit-
uents are capable of stabilizing the radical formation.11-13
The lower volume shrinkage exhibited by VCPs is a
consequence of ring-opening polymerization.14 In recent
years many new VCP derivatives have been investi-
gated.15-19 But the VCP derivatives attached with
phenol have not been studied. In this paper we report
Resu lts a n d Discu ssion
The preparation of ethyl 1-{[(4-hydroxyphenyl)amino]-
carbonyl}-2-vinylcyclopropanecarboxylate (5) was car-
ried out by condensation of 1-(ethoxycarbonyl)-2-vinyl-
cyclopropanecarboxylic acid (1) with 4-aminophenol (2)
(Scheme 1). Because the aromatic amino group of
4-aminophenol is relatively inert compared to the
aromatic hydroxyl group, 4-aminophenol was at first
modified with trimethylsilyl chloride, so that the hy-
droxyl group is protected and the amino group is
activated. The formed derivative 4 reacted selectively
with the acid chloride 3 to an intermediate, which at
the next step was hydrolyzed to 5.
The water-soluble complex 6 was prepared simply by
stirring a dispersion of the monomer and RAMEB in
water (1:1) at room temperature. Then water-soluble
HRP was added, and oligomerization was initiated by
H2O2. The clear solution of the monomer complex 6
became immediately turbid after addition of the first
drop of H2O2. The formed oligomer is insoluble in water
but in some organic solvents, such as chloroform,
acetone, ethanol, THF, and DMF. The vinyl group of
the formed oligomer 7 was not affected under these
polymerization conditions and thus would be used to
cross-link the oligomers via free radical polymerization
in the presence of AIBN (Scheme 2).
1
The formation of complex 6 was verified by H NMR
spectra (Figure 1). In comparison to the free monomer
5, the chemical shifts of signals of the cyclopropane ring
and the vinyl group in the host-guest complex 6 were
significantly influenced (δ from 2.50 to 2.42 ppm, e and
f from 1.78 to 2.03 ppm and from 1.64 to 1.95 ppm, c
from 5.67 to 5.76 ppm, a and b from 5.19 to 5.26 ppm
and 5.35 to 5.39 ppm). This indicates that the RAMEB
forms an inclusion complex with 5, in which the vinyl-
cyclopropane group is preferably enclosed in the hydro-
phobic cavity of RAMEB.
The molecular weight of 7 was estimated by MALDI-
TOF MS (Figure 2, with addition of K+). For a better
detection of higher molecular fractions kalium salt was
added. The peaks show exactly the molecular masses
of the oligomers plus kalium, e.g., the octamer peak at
* Corresponding author: e-mail H.Ritter@uni-duesseldorf.de.
10.1021/ma021678x CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/22/2003

Macromolecules, Vol. 36, No. 19, 2003
Cyclodextrins in Polymer Chemistry 7091
F igu r e 1. 1H NMR spectra of (a) complex 6 and (b) monomer 5 (500 MHz, D2O).
Sch em e 1. P r ep a r a tion of th e Mon om er
Sch em e 2. Oligom er iza tion of Vin ylcyclop r op a n e Der iva tives
2773.5 u/e ) 273.3 × 10 + 2 + 39 (K+). The repeat unit
In this work, the structure of the formed oligomer 7
was verified by H NMR (Figure 3) and FT-IR spectros-
copy.
1
corresponds the mass of a monomer unit, e.g., 1953.5 -
1680.4 ) 273.1.
The structures of polyphenols formed via enzymetic
polymerization were already discussed in several pa-
pers. It was confirmed that those oligomer are composed
of either phenylene units6,8,9 or oxyphenylene units.7
Figure 3a shows the broad signals of a typical
polymer. The signals a, a′, b, c, c′, and d indicate that
the cyclopropane ring and vinyl group are undamaged
incorporated into the formed oligomer.

7092 Pang et al.
Macromolecules, Vol. 36, No. 19, 2003
F igu r e 2. MALDI-TOF spectrum of oligomer 7.
F igu r e 3. 1H NMR spectra of (a) oligomer 7 and (b) monomer 5 (200 MHz, CDCl3).
A comparison of the FT-IR spectrum with that of the
monomer demonstrates that a new ether band does not
appear in the range 1080-1280 cm-1 after polymeriza-
tion. Furthermore, the band at 1339 cm-1 suggests the
existence of the OH group in the oligomer. The typical
band at 870 cm-1 indicates a benzene ring with an
isolated H atom, e.g., 1,3,4,5-tetrasubsituted bezene
ring. As described above, the FT-IR spectra prove that
the formed oligomer is composed via directly linked
phenylene units.
Con clu sion
2,6-Di-O-methyl-â-cyclodextrin forms a hydrophilic
host-guest complex with the hydrophobic monomer 5.
At room temperature, horseradish peroxidase acts as
an effective catalyst and hydrogen peroxide as an
effective initiator for oligomerization. Cyclodextrin can
be easily removed by washing with a mixture of acetone
and water. The formed oligomer is soluble in general
organic solvents and can be radically cross-linked. The
obtained polymer is relatively thermally stable.
Free radical polymerization of the formed oligomer 7
using AIBN as initiator at 65 °C leads to formation of a
network. The cyclopropane ring is opened during the
radical polymerization. The signals at 974 cm-1 in the
FT-IR spectrum of the cross-linked polymer 8 are
assigned to the trans vinyl group. The network 8 is a
pale orange hard powder that is insoluble in general
organic solvents like DMSO, DMF, or THF.
Exp er im en ta l Section
Ma ter ia ls. HRP (1044 U/mg) was purchased from Bio-
Chemika. 1,1-Diethoxycarbonyl-2-vinylcyclopropane was pro-
vided by Ivocar-Vivadent (Schaan, EU). RAMEB (degree of
methylation 1.8) was purchased from Wacker-Chemie. The
other reagents were purchased either from Fluka or from
Merck and used as received. The solvents with p.a. quality
were stored over molecular sieves of 3 or 4 Å. The technical

Macromolecules, Vol. 36, No. 19, 2003
Cyclodextrins in Polymer Chemistry 7093
solvents for flash column chromatography were distilled before
usage.
was repeatedly vacuumized under refrigeration in fluid nitro-
gen for three times, so that it is thoroughly degassed. The
ampule was then sealed under vacuum and heated at 65 °C
for 2 days. The formed nonsticky mixture was powdered and
washed with methanol. 204 mg of yellow hard powder was
collected.
Mea su r em en ts. The 200 MHz 1H NMR spectra were
measured on a Bruker Avance DRX 200, IR spectra were
recorded with a Nicolet 5SXB FTIR spectrometer, and MALDI-
TOF spectra were measured with a Micromass TofSpec-MS
spectrometer. DSC measurements were carried out with a
Mettler DSC 30, and the melting points were measured with
a Bu¨chi 510.
Ack n ow led gm en t. We thank Mr. Bahm for the
measurement of 1H NMR spectra, Ms. Schmelzer (Uni-
versity Mainz) for GPC measurements, and Ms. Bach
(University Mainz) for the measurement of MALDI-
TOF spectra.
Syn th esis of 3. 1-(Ethoxycarbonyl)-2-vinylcyclopropanecar-
boxylic acid (1) was prepared from 1,1-diethoxycarbonyl-2-
vinylcyclopropane in 70% yield in accordance with the proce-
dure reported earlier.20
7.62 g (60 mmol) of oxalyl chloride was slowly dropped to a
solution of 7.95 g (43 mmol) of 1 in 20 mL of dichloromethane,
which was placed in a three-necked flask and cooled in an ice-
water bath. The mixture was then heated at 40 °C for 4 h.
The solvent and unreacted oxalyl chloride were removed under
reduced pressure (ca. 30 mbar). The residual acid chloride (3)
was used directly for the following preparation of 5.
13.04 g (120 mmol) of trimethylsilyl chloride was added
dropwise at 5 °C to a solution of 4.70 g (43 mmol) of
4-aminophenol and 12.14 g (120 mmol) of N,N,N-triethylamine
in 200 mL. The mixture was heated and refluxed for 2 h, and
then the solution of crude 3 in some dichloromethane was
added. Subsequently, the solution was stirred at room tem-
perature (RT) for 20 h, the formed precipitate was removed,
and the solution was hydrolyzed by stirring with saturated
NaHCO3 solution. The collected organic phase was hydrolyzed
again with water (pH 5-6). The product was collected as usual
and purified by flash column chromatography using acetone/
CHCl3 (1:8 v/v) to give 7.57 g of colorless crystalline solid 5.
Yield (in comparison with 1) 64%, colorless crystalline solid,
mp 85-86 °C.
Refer en ces a n d Notes
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1H NMR (200 MHz, CDCl3): δ 10.48 (br s, NH), 7.42 (ddd,
2H), 6.80 (ddd, 2H), 5.74 (m, 1H), 5.41 (m, 1H), 5.25 (m, 1H),
4.29 (q, 2H), 2.68 (m, 1H), 2.22 (m, 1H), 2.01 (m, 1H), 1.36 (t,
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134.14, 131.97, 123.52, 121.14, 116.79, 62.86, 39.04, 35.69,
22.77, 15.31.
MS: m/z (relative intensity): 229 (100), 275 (77.2), 202 (13),
167 (12.8), 135 (33.7); 121 (75.7, 109 (92.9), 95 (35.8), 65 (35.4),
41 (19.2).
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Rf: 0.46 (chloroform:acetone 1:8 v/v)
P r ep a r a tion of 7 in th e P r esen ce of RAMEB. 550 mg
(2 mmol) of 5, 2.7 g (2 mmol) of RAMEB, and 10 mL of
phosphate buffer (pH 7) were stirred at RT until a homoge-
neous solution was formed. After addition of 1.5 mg of HRP,
0.21 mL of 30% hydrogen peroxide solution was added slowly
at intervals (0.1 mL/5 min). The mixture was stirred at RT
for 15 h. The formed precipitate was collected by centrifuga-
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washing with acetone/water.
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P r ep a r a tion of 8. 243 mg of 7, 8.2 mg (0.05 mmol) of AIBN,
and some THF were introduced in an ampule. The solution
MA021678X