Novel unsaturated sucrose ethers and their application as monomers

Article abstract of DOI:10.3390/molecules13040762

Novel unsaturated ethers were synthesised in good yields starting from sucrose, using a two-step mild and efficient procedure based on the Gassman method, which consists in forming a vinyl group by the elimination of ethanol from mixed acetals with trimethylsilyl trifluoromethanesulfonate in the presence of alkyl amines. Mixed acetals are readily obtained from the corresponding alcohols and ethyl vinyl ether, using an acidic catalyst. Conventional etherification involving a primary halide was also examined. The monomers thus obtained were successfully polymerised by a free radical mechanism, yielding unbranched linear and soluble polymers with pending sucrose moieties, and some of their physical properties were determined.

Full text of DOI:10.3390/molecules13040762

Molecules 2008, 13, 762-770
molecules
ISSN 1420-3049
© 2008 by MDPI
www.mdpi.org/molecules
Communication
Novel Unsaturated Sucrose Ethers and Their Application as
Monomers
Carina C. Crucho, Krasimira T. Petrova*, Rui C. Pinto, and Maria T. Barros*
REQUIMTE, Departamento de Química - FCT, Universidade Nova de Lisboa, Quinta da Torre, 2829-
516 Caparica, Portugal
* Authors to whom correspondence should be addressed; E-mails: krasimira.petrova@dq.fct.unl.pt;
mtbarros@dq.fct.unl.pt; Fax: (+)351 212948550
Received: 5 March 2008; in revised form: 26 March 2008 / Accepted: 26 March 2008 / Published:
28 March 2008
Abstract: Novel unsaturated ethers were synthesised in good yields starting from sucrose,
using a two-step mild and efficient procedure based on the Gassman method, which
consists in forming a vinyl group by the elimination of ethanol from mixed acetals with
trimethylsilyl trifluoromethanesulfonate in the presence of alkyl amines. Mixed acetals are
readily obtained from the corresponding alcohols and ethyl vinyl ether, using an acidic
catalyst. Conventional etherification involving a primary halide was also examined. The
monomers thus obtained were successfully polymerised by a free radical mechanism,
yielding unbranched linear and soluble polymers with pending sucrose moieties, and some
of their physical properties were determined.
Keywords: Sustainable chemistry, vinyl sucrose ether monomers, biodegradable
copolymers, natural products copolymers, free radical copolymerisation
Introduction
The demand for biodegradable plastics has increased over the past few decades, not only due to
increasing environmental concerns, but also for their biomedical applications [1]. Sucrose is a low
molecular weight carbohydrate feedstock from which it is possible to elaborate new materials [2], such
as water-soluble and/or amphiphilic, biocompatible polymers [3] and other new compounds owing to

Molecules 2008, 13
763
their low price. Some applications for sugar based polymers are drug delivery systems, dental
medicine, bio implants, contact lenses and tissue engineering [4, 5], and they have the advantage of
being potentially biodegradable [6, 7]. Obtaining polymers and copolymers from carbohydrate derived
monomers is a work with a pronounced ecological aspect. The quest for novel, sustainable, but also
structurally robust materials is gaining momentum as the pressure on our environment is building up
and the progressive changeover of chemical industry to renewable feedstock for their raw materials
emerges as an inevitable necessity. To achieve this goal we had proposed the introduction of sugar
moieties in the macromolecules of conventional polyolefins by copolymerisation with vinyl
carbohydrate derivatives [8, 9]. Similar types of copolymers, obtained by attaching a sugar moiety to
functionalised polyolefins, have been shown to display significantly improved biodegradability [6, 7].
In previous publications [8-14] we have reported the synthesis and applications of different sucrose
esters. Since sucrose has eight chemically active hydroxyl groups, regioselective derivatisation is
important in the selective synthesis of sucrose-containing linear polymers [15, 16]. The route to
selective derivatisation of the 6'-position of the sucrose has been developed in our laboratory [12, 13].
It allowed us to obtain the fully protected sucrose with only the 6’-hydroxyl unprotected. Protection-
deprotection strategies have been discussed by Jarosz et al. [17]. Another difficulty to be taken into
account is the instability of the sucrose glycosidic bond under acidic conditions, which prevented us
from adapting some efficient catalytic methods allowing the direct coupling of natural polyols with
benzyl alcohols over solid acid catalysts, as recently reported [18].
Herein, we opted to explore the polymerisation activity of π-ether system attached to sucrose and
we present the synthesis of novel unsaturated sucrose ethers, as well as an example for their successful
free radical copolymerisation with styrene. For obtaining vinyl sucrose ether, we have adopted a new
two-step route via mixed acetals, based on the Gassman method, first reported in 1993 [19], as it
requires readily available reagents, mild conditions, and does not involve the use of heavy metal salts
catalysts, like mercury [20]. It consists in forming a vinyl group by the elimination of ethanol from
mixed acetals with trimethylsilyl trifluoromethanesulfonate (TMS-triflate) in the presence of alkyl
amines. Mixed acetals are readily obtained from the corresponding alcohols and ethyl vinyl ether,
using acidic catalysts.
Results and Discussion
Four unsaturated ether sucrose monomers were synthesised by different methods. The mixed acetal
4 was prepared by treating 3 (obtained as reported [8]) with ethyl vinyl ether in the presence of PPTS
at r.t. for 2 h. No side products were observed, and a mixture of diastereomers was produced, with no
attempt at separation being made. After purification by flash column chromatography, the elimination
reaction was performed by treating 4 with TMS-triflate and triethylamine at r.t. for 2 h, leading to 5
(Scheme 1). The yield of this step was decreased by the competing side reaction of formation of silyl
ether, resulting of a complexation of the trimethylsilyl cation with the sucrose connected oxygen atom,
as it was discussed by Gassman et al. [14] and Hughes et al. [15]. Complexation of the trimethylsilyl
cation with the ethoxy group oxygen produces the desired vinyl sucrose ether.

Molecules 2008, 13
Scheme 1. Synthesis of 1’,2,3,3’,4,4’,6-hepta-O-benzyl-6’-O-vinyl sucrose (5).
764
OBn
OBn
OBn
OBn
O
O
BnO
BnO
CH2=CHOEt, CH2Cl2
PPTS, r.t., 56%
BnO
BnO
OBn
OBn
O
O
OBn
OBn
OEt
O
O
O
OH
4
OBn
3
OBn
OBn
OBn
O
BnO
BnO
TMSOTf, NEt3
OBn
O
OBn
O
CH2Cl2, r.t., 49%
O
5
OBn
With the purpose of comparing the polymerisation ability of the monomer 5, in which the vinyl
group is adjacent to the sucrose moiety, with similar ether compounds in which it is located farther
away, we synthesised vinylbenzyl sucrose ethers 6-8, as previously reported [3]. We tested
conventional etherification involving a primary halide, in our case 4-vinylbenzyl chloride, optimising
the benzylation procedure for protected and unprotected sucrose. In the first case, the intermediate 3
was treated with NaH in DMF in the presence of phase-transfer catalyst at 0°C, with the subsequent
addition of the 4-vinylbenzyl chloride and increasing the temperature to 70°C (Scheme 2).
Scheme 2. Synthesis of 1’,2,3,3’,4,4’,6-hepta-O-benzyl-6’-O-vinylbenzyl sucrose (6).
OBn
OBn
O
BnO
H2C=CHC6H4CH2Cl, NaH
Bu4N+I-, DMF, 700C, 46%
OBn
BnO
O
3
OBn
O
O
6
OBn
In order to obtain hydrophilic and amphiphilic polymers, we needed monomers with unprotected
hydroxyl groups on the sucrose moiety. Those we achieved by reacting sucrose with 4-vinylbenzyl
chloride under similar conditions, as described above. The reaction was monitored by TLC, and
stopped at the first formation of diethers. A mixture of regioisomers of monovinylbenzyl ethers was
isolated, with no attempt at separation being made. Subsequent acetylation was needed for detailed
NMR analysis of the reaction products (Scheme 3).
Scheme 3. Synthesis of monovinylbenzyl sucrose and hepta-O-acetyl-monovinylbenzyl sucrose (8).
R1=vinylbenzyl, R2=R3=H
R2=vinylbenzyl, R1=R3=H
R3=vinylbenzyl, R1=R2=H
OR3
OH
OR2
H2C=CHC6H4CH2Cl, NaH
OH
HO
HO
O
O
HO
HO
Bu4N+I-, DMF, 700C
OH
OH
O
O
OH
OH
O
O
OH
OR1
7
OH
OH
OR3
OR2
Ac2O, py, r.t.
overall 43%
O
AcO
AcO
OAc
O
R1=vinylbenzyl, R2=R3=Ac
R2=vinylbenzyl, R1=R3=Ac
R3=vinylbenzyl, R1=R2=Ac
OAc
O
OR1
8
OAc

Molecules 2008, 13
765
The regioselectivity question is a difficult one because of the complex chemistry of sucrose. In
general, it is accepted that bulky substituents like TBDPS, are introduced at the primary positions in
the order 6-OH ≈ 6’-OH > 1’-OH [1]. Molecular modeling of sucrose reveals the persistence of an
intramolecular hydrogen bond of the 2-O···HO-1’ type in aprotic solvents [21], hence the 2-OH should
be the most readily deprotonated under basic conditions, affording the 2-substituted derivative as the
major product. In addition, the regioselectivity also depends on the nature of the electrophilic reagent,
on the catalyst used for promoting the reaction and, not the least, on the solvent used.
1
In our case, when we compared the H-NMR and COSY spectra of the product and sucrose octa-
acetate, the signals for the 2-H had exactly the same chemical shifts, while there were significant
differences for the signals of the six methylene protons at the three primary positions. Although this
assignment is not precise as we are dealing with a mixture of possible regioisomers, we concluded that
most probably the substitution has taken place at the primary positions, the easiest accessible ones,
although not the most reactive. The reason for that could be the presence of the phase transfer catalyst
Bu4N+I-, or a partial separation of the primary ethers, achieved during the purification in 43% global
yield.
With thus obtained monomers example copolymerisation experiments were performed in organic
medium, using toluene as a solvent, at 70°C in the presence of AIBN as a radical initiator (Table 1).
The molar mass of the copolymers was measured by dynamic light-scattering, as Mark-Houwink-
Sakurada constants were estimated by calibration with commercial polystyrene standards. Copolymer
compositions were calculated from 1H NMR spectra by comparing the peak areas of aromatic styrene
protons with the 14 sucrose unit protons.
Table 1. Experimental results for copolymerisation in toluene solutions of 5, 6, and 8
with styrene and the radical initiator AIBN, 70°C, 48 h.
[M]0/
[M]/
[S][b]
Yield [%][c] MW[d] [g/mol] ϕ [e]
[nm]
654
α [f]
D[g] [cm2/s]
[a]
[S]0
5/st
6/st
8/st
0.095
0.090
0.103
0.012
0.040
0.091
31
36
51
31900
6700
0.288
0.208
0.251
5.57.10-9
1.26.10-8
3.25.10-9
289
89500
1121
[a] Initial mol ratio of the monomers. [b] Mole ratio of the comonomers in the copolymer,
1
determined by H-NMR. [c] Overall conversion of the monomers. [d] Calculated from the MHS
equation from the diffusion coefficient D, measured by light-scattering. MHS coefficients were
estimated by calibration with monodisperse polystyrene standards, a=-0.52, K=1.245.10-6. [e]
Effective diameter, determined by light-scattering. [f] Polydispersity (value, provided by the light-
scattering method, it is close to zero for monodisperse samples, small (0.020 to 0.080) for narrow
distributions, and larger for broader distributions). [g] Diffusion coefficient.

Molecules 2008, 13
766
As we can see, copolymers with good sucrose derivative incorporation rates and high molecular
mass were obtained. With monomer 5, in which the double bond is very close to the bulky sucrose
moiety and as expected, is impeding the polymerisation, we have lower incorporation of the sugar
monomer, but nevertheless high molecular mass for this type of polymers. Under the conditions
chosen, we achieved good overall conversions, compared to the reported in the literature [3], lower
incorporation of sugar, which is probably result of the longer polymerisation times, and higher average
molecular mass. The optimisation of the polymerisation procedure with a view to possible
applications, as well as the biodegradability of copolymers, containing sucrose moieties side groups, is
under investigation.
Conclusions
In summary, novel unsaturated ethers of sucrose were synthesised using a two-step mild and
efficient procedure based on the Gassman method, as well as conventional etherification involving a
primary halide. As an example application, the products were successfully polymerised by a free
radical mechanism, yielding unbranched linear polymer materials with pending sucrose moieties, and
some of their physical properties were determined.
Experimental
General
Reagents and solvents were purified before use [22]. NMR spectra were recorded at 400 MHz on a
Bruker AMX-400 instrument using CDCl3 as solvent. Chemical shift values (δ) are given in ppm
downfield from TMS. The IR spectra were recorded on a Perkin-Elmer FTIR-1600 spectrophotometer.
Average molecular weights were determined using a Brookhaven model BI-90 dynamic light-
scattering apparatus, with operational angle 90°, calculated from the diffusion coefficients of polymer
solutions suspended in ethanol, at 20°C. All solvents used for dilution were filtered through a 0.2
micron membrane filter. 2,500 experimental cycles were used for each run, and the dust cut off, above
21, had no effect on the measurement, indicating a reasonably dust-free sample. The calibration was
performed with monodisperse polystyrene standards purchased from Aldrich.
Synthesis of the mixed acetal 1’,2,3,3’,4,4’,6-hepta-O-benzyl-6’-O-(1-ethoxy)ethyl sucrose (4)
To a solution of 3 [8] (1.016 g; 1.045 mmol) in dry CH2Cl2 (10 mL) were added PPTS (10 mg) and
ethylvinyl ether (1.5 eq, 0.5 mL). The reaction mixture was stirred at r.t. for 2 h. NaHCO3 (1 g) was
added, and stirred for 10 min more, filtered, concentrated and purified by flash column
chromatography on silica gel 60 (0.04-0.06 mm), eluent: hex/EtOAc 3:1. Compound 4 was obtained as
a light yellow oil (0.610 g, 56%); Rƒ 0.40 (hex/EtOAc 3:1); IR: νmax (CH2Cl2) 3088, 3040, 3030, 2976,
1
2897, 2867, 2000-1600, 1496, 1454, 1363, 1266, 1208, 1082, 1028, 736, 698 cm-1; H-NMR: δH
(CDCl3) 9.79 (1H, qd Ј 2.13 Hz, CH), 7.33-7.20 (32H, m, Ar-H), 7.12-7.10 (2H, m, Ar-H), 5.54 (1H, d
J 2.5 Hz, H-1), 4.92-4.28 (16H, m, CH2-Ph, H-5’ and H-3), 4.09-4.06 (1H, m, H-6), 4.03-3.97 (2H, m,

Molecules 2008, 13
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H-4’ and H-3’), 3.87-3.80 (1H, m, H-6’), 3.73-3.63 (4H, m, H-2, H-1’ and OCH2CH3), 3.61-3.51 (4H,
m, H-5, H-1’, H-6’ and H-4), 3.48 (1H, d J 3.03 Hz, H-6), 2.20 (1H, d J 2.13 Hz, CHCH3), 1.27-1.19
13
(4H, m, CH2CH3) ppm; confirmed by COSY and HMQC; C-NMR: δC (CDCl3) 128.3 (Ph), 128.0
(Ph), 127.9 (Ph), 127.8 (Ph), 127.7 (Ph), 127.5 (Ph), 103.8 (C-2’), 91.1 (C-1), 83.6 (C-5’), 81.8 (C-3’),
81.2 (C-4’), 79.5 (C-3), 79.4 (C-5), 77.3 (C-2), 75.6 (CH2-Ph), 74.9 (CH2-Ph), 73.5 (CH2-Ph), 73.4
(CH2-Ph), 73.3 (CH2-Ph), 72.9 (CH2-Ph), 72.5 (CH2-Ph), 71.3 (C-4), 71.2 (C-6’), 67.9 (C-1’), 61.2 (C-
6), 60.6 (OCH2CH3), 20.0 (O2CHCH3), 15.3 (OCH2CH3) ppm; confirmed by DEPT.
1’,2,3,3’,4,4’,6-hepta-O-benzyl-6’-O-vinyl sucrose (5)
To a solution of 4 (0.797 g; 0.763 mmol) in dry CH2Cl2 (7 mL) were added Et3N (0.12 mL, 0.839
mmol) and TMSOTf (0.15 mL, 0.839 mmol) at 0°C. After 10 min it was allowed to warm to r.t. and
stirred for 2 h. The reaction mixture was neutralised with 1N NaOH (4 mL), and extracted with diethyl
ether (3 x 20 mL). After drying and concentrating, it was purified by flash column chromatography on
silica gel 60 (0.04-0.06 mm), eluent: hex/EtOAc 3:1. Compound 5 was obtained as a light yellow oil
(0.250 g, 31%); Rƒ 0.5 (Hex/AcOEt 3:1); IR:νmax (CH2Cl2) 3088, 3064, 3030. 2909, 2869, 1737, 2000-
1600, 1497, 1454, 1361, 1243, 1209, 1096, 910, 737, 698 cm-1; 1H-NMR: δH (CDCl3) 7.31-7.26 (35H,
m, Ar-H), 7.16-7.14 (1H, m, Ar-H), 6.49 (1H, dd Ј 10.73 e 5.1 Hz, OCH=), 5.67 (1H, d J 2.6 Hz, H-1),
4.93 (1H, d J 8.16 Hz, CH2-Ph), 4.83 (1H, d J 8.19 Hz, CH2-Ph), 4.77 (1H, d J 8.16 Hz, CH2-Ph),
4.68-4.38 (12H, m, CH2-Ph and H-5’), 4.20 (1H, dd J 10.8 and 1.41 Hz, =CH2trans), 4.18-4.15 (1H, m,
H-3’), 4.10-4.05 (1H, m, H-4), 4.01 (1H, dd J 5.1 and 1.44 Hz, =CH2cis), 3.97-3.92 (1H, m, H-4’),
3.86-3.81 (1H, m, H-6), 3.75 (1H, m, H-6), 3.64 (1H, t J 7.17 Hz, H-2), 3.56-3.49 (3H, m, H-5, H-1’
and H-6’), 3.42 (1H, dd J 7.1 and 1.2 Hz, H-6’) ppm; confirmed by COSY and HMQC; 13C-NMR:δC
(CDCl3) 151.6 (OCH=), 128.3 (Ph), 128.3 (Ph), 127.9 (Ph), 127.8 (Ph), 127.7 (Ph), 127.6 (Ph), 127,5
(Ph), 104.6 (C-2’), 90.2 (C-1), 86.9 (=CH2), 83.9 (C-5’), 81.8 (C-3’), 81.2 (C-4’), 79.5 (C-3), 79.4 (C-
5), 77.3 (C-2), 75.6 (CH2-Ph), 74.8 (CH2-Ph), 73.4 (CH2-Ph), 72.9 (CH2-Ph), 72.6 (CH2-Ph), 72.3
(CH2-Ph), 71.0 (C-6’), 70.6 (C-4), 69.1 (C-1’), 69.5 (C-6) ppm; confirmed by DEPT.
1’,2,3,3’,4,4’,6-hepta-O-benzyl-6’-O-vinylbenzyl sucrose (6)
To a solution of 3 (0.5 g, 0.514 mmol) in DMF (12.5 mL) were added a catalytic amount of
Bu4N+I- and NaH (0.07 g, 1.0 eq) at 0°C. After 20 min, 4-vinylbenzyl chloride (0.09 mL, 1.0 eq) was
added, the ice-bath removed, and the reaction mixture was heated at 70°C for 4 h. Concentrated and
purified by flash column chromatography on silica gel 60 (0.04-0.06 mm), eluent hex/EtOAc 3:1.
Compound 6 was obtained as a light yellow oil (0.260 g, 46%); Rƒ 0.7 (hex/AcOEt 3:1); 1H-NMR: δH
(CDCl3) 7.28-7.12 (39H, m, Ar-H), 6.65 (1H, dd Ј 13.2 e 8.2 Hz, PhCH=), 5.71- 5.66 (2H, m, H-1,
=CH2trans), 5.20 (1H, dd, J 10.8 e 2.11 Hz =CH2cis), 4.86-4.35 (16 H, m, 7 (CH2-Ph), H-5’, CH2-Ph),
4.22-3.84 (4H, m, H-6’a, H-6’b, H-5, H-4’, H-3), 3.71-3.25 (7H, m, H-2, H-6a, H-6b, H-1’a, H-1’b, H-4,
13
H-3’) ppm; confirmed by COSY and HMQC; C-NMR: δC (CDCl3) 138.9 (Ar-C), 138.6 (Ar-C),
138.2 (Ar-C), 138.0 (Ar-C), 137.9 (PhCH=), 136.8 (Ar-C), 136.5 (Ar-C), 128.3 (Ar-CH), 128.3 (Ar-
CH), 128.0 (Ar-CH), 127.9 (Ar-CH), 127.7 (Ar-CH), 127.6 (Ar-CH), 127.5 (Ar-CH), 126,2 (Ar),
113.7 (CH2=), 104.6 (C-2’), 89.9 (C-1), 83.9 (C-5’), 82.4 (CH2-Ph), 81.9 (C-3), 81.9 (C-4’), 79.8 (C-

Molecules 2008, 13
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3’), 79.6 (C-5), 77.6 (C-2), 75.5 (CH2-Ph), 74.8 (CH2-Ph), 73.4 (CH2-Ph), 73.0 (CH2-Ph), 72.5 (CH2-
Ph), 72.2 (CH2-Ph), 71.4 (C-6’), 71.2 (C-6), 70.6 (C-4), 68.4 (C-1’) ppm; confirmed by DEPT.
Hepta-O-acetyl-monovinylbenzyl sucrose (8).
To a solution of sucrose (1.000 g, 2.9 mmol) in DMF (25 mL) were added a catalytic amount of
Bu4N+I- and NaH (0.14 g, 1.1 eq) at 0°C. After 20 min, 4-vinylbenzyl chloride (0.45 ml, 1.1 eq) was
added, the ice-bath removed, and the reaction mixture was heated at 70°C for 4 h. After removing the
solvent, the residue was dissolved in pyridine, acetic anhydride (3 mL) was added, and the mixture
stirred overnight. Concentrated and purified by flash column chromatography on silica gel 60 (0.04-
0.06 mm), eluent hex/EtOAc 1:1. Compound 8 was obtained as a light yellow oil (0.934 g, 43%); Rƒ
0.7 (hex/AcOEt 1:2); IR:νmax (CH2Cl2) 3059, 2962, 2121, 1746, 1513, 1432, 1370, 1230, 1168, 1104,
1046, 910, 829, 737, 703, 601 cm-1; 1H-NMR: δH (CDCl3) 7.56-7.43 (2H, m, Ar-H), 7.27-7.21 (2H, m,
Ar-H), 6.71 (1H, dd Ј 13.2 e 8.2 Hz, PhCH=), 5.75 (1H, dd J 13.2 e 4.02 Hz, =CH2trans), 5.72-5.65
(1H, m, H-1), 5.56-5.5 (1H, m, H-4’), 5.42 (1H, t Ј 6.56 Hz, H-3), 5.40-5.36 (1H, m, H-3), 5.30 (1H, s,
PhCH2), 5.27 (1H, dd J 9.14 e 3.48 Hz, =CH2cis), 5.05 (1H, m, H-4), 4.88-4.81 (1H, dd, H-2), 4.66-
4.45 (2H, m, H-6’a, H-6’b ), 4.38-3.97 (7H, m, H-1’a, H-1’b, H-6a, H-6b, H-5, H-5’), 3.61-3.5 (1H, m),
2.16-1.90 (21H, m, CH3(C=O)) ppm; confirmed by COSY and HMQC; 13C-NMR: δC (CDCl3) 170.7-
169.8 (CH3(C=O)), 137.5 (Ar), 137.3 (Ar), 137.0 (Ar), 136.7 (Ar), 136.4 (PhCH=), 128.8 (Ar), 128.5
(Ar), 128.3 (Ar), 128.0 (Ar), 126.4 (Ar), 126.3 (Ar), 114.0 (CH2=), 113.8 (CH2=), 104.5 (C-2’), 103.6
(C-2’), 90.3 (C-1), 89.6 (CH2-Ph), 78.8 (C-5’), 78.5 (C-5’), 75.8 (C-3), 75.4 (C-4’), 74.6 (C-3’), 73.4
(C-6’), 72.5 (C-6), 71.7 (C-2), 70.2 (C-1’), 68.5-68.3 (C-4 and C-5), 63.7-63.3 (C-6 and C-6’), 61.9
(C-1’), 61.7 (C-1’), 20.7 (CH3(C=O)) ppm; confirmed by DEPT.
General procedure for free radical copolymerisations of monomers 5, 6, and 8 with styrene
Copolymerisations were carried out in anhyd. toluene solutions (0.1 M) in the presence of AIBN as
radical initiator (1 % by weight with respect to the monomer mixture). Dissolved oxygen was removed
from the solutions by sparging with argon. They were then heated at 70°C for 48 h, and the solutions
were then cooled to r.t. and the product precipitated in cold EtOH. The white solid was filtered and
washed several times with cold EtOH. The polymers were purified by repeated dissolution in toluene
and reprecipitation in cold EtOH and dried under vacuum.
Acknowledgements
This work has been supported by Fundação para a Ciência e a Tecnologia (SFRH/BPD/9474/2002
and PTDC/QUI/72120/2006).
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Sample availability: Samples of all compounds synthesised herein are available from the authors upon
request.
© 2008 by MDPI (http://www.mdpi.org). Reproduction is permitted for noncommercial purposes.