Synthesis and crystal structures of multifunctional tosylates as basis for star-shaped poly(2-ethyl-2-oxazoline)s

Article abstract of DOI:10.3762/bjoc.6.96

The synthesis of well-defined polymer architectures is of major importance for the development of complex functional materials. In this contribution, we discuss the synthesis of a range of multifunctional star-shaped tosylates as potential initiators for the living cationic ring-opening polymerization (CROP) of 2-oxazolines resulting in star-shaped polymers. The synthesis of the tosylates was performed by esterification of the corresponding alcohols with tosyl chloride. Recrystallization of these tosylate compounds afforded single crystals, and the X-ray crystal structures of di-, tetra-and hexa-tosylates are reported. The use of tetra-and hexatosylates, based on (di)pentaerythritol as initiators for the CROP of 2-ethyl-2-oxazoline, resulted in very slow initiation and illdefined polymers, which is most likely caused by steric hindrance in these initiators. As a consequence, a porphyrin-cored tetratosylate initiator was prepared, which yielded a well-defined star-shaped poly(2-ethyl-2-oxazoline) by CROP as demonstrated by SEC with RI, UV and diode-array detectors, as well as by ¹H NMR spectroscopy.

Full text of DOI:10.3762/bjoc.6.96

Synthesis and crystal structures of multifunctional
tosylates as basis for star-shaped
poly(2-ethyl-2-oxazoline)s
Richard Hoogenboom*1,2, Martin W. M. Fijten1, Guido Kickelbick3
and Ulrich S. Schubert*1,4
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2010, 6, 773–783.
1Laboratory of Macromolecular Chemistry and Nanoscience,
Eindhoven University of Technology, Den Dolech 2, 5612AZ
Eindhoven, The Netherlands, 2Supramolecular Chemistry group,
Department of Organic Chemistry, Ghent University, Krijgslaan 281
S4, 9000 Ghent, Belgium, 3Inorganic Solid State Chemistry, Saarland
University, Am Markt Zeile 3, 66125 Saarbrücken, Germany and
4Laboratory of Organic and Macromolecular Chemistry,
Friedrich-Schiller-University Jena, Humboldtstrasse 10, 07743 Jena,
Germany
doi:10.3762/bjoc.6.96
Received: 09 June 2010
Accepted: 19 August 2010
Published: 09 September 2010
Guest Editor: H. Ritter
© 2010 Hoogenboom et al; licensee Beilstein-Institut.
License and terms: see end of document.
Email:
Richard Hoogenboom* - r.hoogenboom@tue.nl; Ulrich S. Schubert* -
ulrich.schubert@uni-jena.de
* Corresponding author
Keywords:
cationic polymerization; crystal structure; living polymerization;
star-polymer; tosylate
Abstract
The synthesis of well-defined polymer architectures is of major importance for the development of complex functional materials. In
this contribution, we discuss the synthesis of a range of multifunctional star-shaped tosylates as potential initiators for the living
cationic ring-opening polymerization (CROP) of 2-oxazolines resulting in star-shaped polymers. The synthesis of the tosylates was
performed by esterification of the corresponding alcohols with tosyl chloride. Recrystallization of these tosylate compounds
afforded single crystals, and the X-ray crystal structures of di-, tetra- and hexa-tosylates are reported. The use of tetra- and hexa-
tosylates, based on (di)pentaerythritol as initiators for the CROP of 2-ethyl-2-oxazoline, resulted in very slow initiation and ill-
defined polymers, which is most likely caused by steric hindrance in these initiators. As a consequence, a porphyrin-cored tetra-
tosylate initiator was prepared, which yielded a well-defined star-shaped poly(2-ethyl-2-oxazoline) by CROP as demonstrated by
SEC with RI, UV and diode-array detectors, as well as by 1H NMR spectroscopy.
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Introduction
Nowadays, well-defined polymer structures are of major impor- The synthesis of star-shaped poly(2-oxazoline)s has been
tance for the development of ever more sophisticated and com- reported using a range of multifunctional electrophilic halide
plex materials, e.g., applications in drug delivery or as adaptive initiators, such as tetrakis(bromomethyl)ethylene, yielding
materials. Star-shaped polymers are especially interesting since 4-armed star-shaped polymers [10], as well as other multi-
their properties are distinctly different from their linear halide initiators based on, e.g., cyclotriphosphazine [11],
analogues with regard to, e.g., number of functional end-groups, silesquioxane [12], porphyrin [13,14] and bipyridine metal
hydrodynamic volume and thermal properties [1,2].
complex [15,16] cores. More recently, Jordan and coworkers
reported the use of multi-triflate initiators for the preparation of
Poly(2-oxazoline)s represent a class of versatile polymer struc- well-defined star-shaped poly(2-methyl-2-oxazoline)s [17].
tures that can be prepared by the (CROP) of 2-substituted-2- However, these multi-halide as well as multi-triflate initiators
oxazoline monomers (see the bottom right corner of Figure 1 are not easily prepared and are not stable upon storage, in par-
for the structure of poly(2-ethyl-2-oxazoline), as an example of ticular in the presence of air. Therefore, we recently reported a
the polymer structure) [3,4]. The versatility of this class of poly- post-modification route for the synthesis of star-shaped poly(2-
mers comes from the living nature of the polymerization, ethyl-2-oxazoline) by coupling of an acetylene-functionalized
allowing control over the length of the polymer with a narrow poly(2-ethyl-2-oxazoline) to a heptakis-azido functionalized
molar mass distribution and allowing the introduction of β-cyclodextrin [18]. However, this method required chromato-
specific end-groups by initiation and termination [4]. Moreover, graphic separation of the star-shaped polymer from the acety-
variation of the 2-substituent of the 2-oxazoline results in a lene-precursor polymer.
variety of the amidic side chains of the poly(2-oxazoline)s [5,6],
which strongly influences the properties of the resulting poly- To overcome the limitations of multi-halide, multi-triflate initi-
mers: ranging from hydrophilic to hydrophobic; and from hard ators and post-modification methods, we investigated the use of
materials, with a high glass transition temperature via crys- multi-tosylate initiators for the CROP of 2-oxazolines as
talline and chiral polymers [7], to soft materials, with a very depicted in Figure 1. The advantages of multi-tosylate initiators
low glass transition temperature [8,9].
are their straightforward syntheses starting from commercially
Figure 1: Schematic representation of the investigated strategy for the synthesis of star-shaped poly(2-ethyl-2-oxazoline)s based on multi-tosylate ini-
tiators.
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available alcohols, their easy purification based on their high The synthesis of the tetra-tosylate (TetraTos; Scheme 1) and
tendency to crystallize [19,20] and their stability under ambient hexa-tosylate (HexaTos; Scheme 1) compounds was based on
conditions. In this contribution, we report the synthesis and tosylation of pentaerythritol and dipentaerythritol, respectively,
crystal structures of various multi-tosylate initiators as well as which are both insoluble in dichloromethane. Therefore, the
their use for the initiation of the CROP of 2-ethyl-2-oxazoline solvent was changed to pyridine, which also acts as a base.
for the formation of star-shaped poly(2-oxazoline)s.
After the reaction, the mixture was poured into acidified water
resulting in the precipitation of the product that could be puri-
fied by recrystallization from a mixture of ethanol and acetone.
Results and Discussion
Multi-tosylate preparation
The preparation of tosylates from alcohols is a straightforward Besides the common characterization techniques to prove the
synthetic procedure using p-toluenesulfonic acid chloride (tosyl purity of the compounds, i.e. 1H and 13C NMR spectroscopy
chloride) in the presence of a base. Commonly applied pro- and elemental analysis, the chemical structures of the TetraTos
cedures are performed in dichloromethane using triethylamine and HexaTos were verified by MALDI-TOF MS, revealing
as base or using pyridine both as solvent and base (Scheme 1, only the desired mass peak corresponding to full tosylation
top). The synthesis of diethyleneglycol ditosylate (DiTos-A; (Figure 2). The absence of residual hydroxyl groups is of major
Scheme 1) was not required since this compound is commer- importance for the use of these multi-tosylates as initiators for
cially available. Recrystallization of this compound from the CROP of 2-oxazolines since they lead to side reactions with
ethanol resulted in single crystals suitable for X-ray analysis. the cationic oxazolinium propagating species.
Butane ditosylate (DiTos-B; Scheme 1) was synthesized from
1,4-butanediol and an excess of tosyl chloride using dichloro- Multi-tosylate crystal structures
methane as solvent and triethylamine as base. After 24 h stir- Recrystallization of the prepared multi-tosylates from ethanol or
ring at ambient temperature, ethanolamine was added to this ethanol–acetone mixtures directly gave single crystals suitable
reaction mixture to react with the excess of tosyl chloride for X-ray analysis as we previously also observed for a tosylate
resulting in the water-soluble 1-hydroxy-2-ethyl tosylamide, adduct of 2,2’:6’,2’’-terpyridine [20]. The obtained molecular
which could be removed by washing with 3 N hydrochloric acid structures and the packing diagrams for DiTos-A, DiTos-B,
and brine. Final purification was performed by recrystallization TetraTos and HexaTos are displayed in Figures 3–6,
from ethanol.
respectively. The crystallographic data, selected bond lengths
Scheme 1: General reaction scheme for the preparation of multi-tosylates from multifunctional alcohols (top) and schematic representation of the
structures of the investigated multi-tosylates (bottom). i: triethylamine in dichloromethane; ii: pyridine.
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Figure 2: MALDI-TOF MS spectra of TetraTos a) and HexaTos b) Matrix: dithranol.
Figure 3: Molecular structure a) and packing diagram b) of the structure of diethyleneglyclol ditosylate (DiTos-A).
Figure 4: Molecular structure a) and packing diagram b) of the structure of 1,4-butanediol ditosylate (DiTos-B).
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Beilstein J. Org. Chem. 2010, 6, 773–783.
Figure 5: Molecular structure a) and packing diagram b) of the structure of penthaerythritol tetra-tosylate (TetraTos).
Figure 6: Molecular structure a) and packing diagram b) of the structure of dipenthaerythritol tetra-tosylate (HexaTos).
and angles for the crystal structures can be found in the nitrile, at 140 °C and 10 min for a monomer to initiator ratio of
supporting information. All structures show the expected bond 60 [21,22]; no polymerization was observed at all when using
length and angles. The packing diagrams reveal space filling TetraTos or HexaTos as initiators. By contrast, when the poly-
packing without any π-stacking between the molecules.
merization with TetraTos was performed at a further elevated
temperature of 200 °C, the formation of polymer was observed
by SEC (Figure 7a). However, the resulting polymer had a
Polymerizations
Since the goal of this research was the development of multi- broad molar mass distribution with tailing at the low molar
tosylate initiators for the preparation of well-defined star- mass side. In addition, residual tosylate initiator was still
shaped poly(2-oxazoline)s, TetraTos and HexaTos were utilized observed after heating to 200 °C for 10 min. These results
for the CROP of 2-ethyl-2-oxazoline under microwave irradi- clearly demonstrate that initiation with TetraTos is very slow,
ation. Using the optimal polymerization conditions that were which has also been observed for the polymerization with, e.g.
previously determined for 2-ethyl-2-oxazoline with methyl 1-butyne tosylate, due to the decreased electrophilicity of the
tosylate as initiator, i.e. 4 M monomer concentration in aceto- initiator when compared to methyl tosylate [18]. However, the
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Beilstein J. Org. Chem. 2010, 6, 773–783.
rate of initiation is further decreased for TetraTos compared to toxy group (mass 1446), next to the utilized matrix dithranol
1-butyne tosylate, which is most likely due to steric hindrance (mass 226), are clearly evident in the spectrum (Figure 8). This
in TetraTos resulting in a decreased accessibility of the initi- latter methoxylated side chain might have been present in the
ating groups. Similar disappointing polymerization results were DiTOs-B. Importantly, no hydroxyl groups remained that might
obtained with HexaTos as initiator as depicted in Figure 7b. In cause side reactions during the polymerizations.
fact, even less of the HexaTos was consumed after 10 min
heating to 200 °C at 4 M monomer concentration as a result of
the further increased steric hindrance. Variation of temperature
or concentration did not improve the polymerization results.
Therefore, it can be concluded that these pentaerythritol based
multi-tosylate initiators are not suitable for the CROP of 2-oxa-
zolines, which is in sharp contrast with the rather similar
pluritriflate initiators reported by Jordan [17]. This difference is
most likely related to both the smaller size and the higher re-
activity of the triflate groups compared to the tosylates.
To circumvent the poor initiation efficiency with the multi-tosy-
late pentaerythritol derivatives, a tetra-tosylated porphyrin
(TetraTos-B) was designed in which the rigid porphyrin keeps
the tosylate groups far apart. Scheme 2 depicts the schematic
path that was followed to synthesize TetraTos-B, and
subsequently, the four-armed star pEtOx. Tetrakis(hydroxy-
phenyl)porphyrin (porphyrin) was used to synthesize the rigid
Figure 8: MALDI-TOF MS spectrum of the tetra-tosylate-porphyrin
(TetraTos-B). Matrix: dithranol.
star-shaped TetraTos-B initiator by reaction with a 20-fold
excess of 1,4-butane ditosylate (DiTos-B), followed by
chromatographic purification. This is not a straightforward syn-
thesis compared to the previously discussed multi-tosylates, Subsequently, this tetrafunctional initiator was applied for the
partly counteracting the advantages of multi-tosylate initiators microwave-assisted cationic ring-opening polymerization of
compared to multi-halides and multi-triflates. Figure 8 depicts EtOx. The polymerization of EtOx with TetraTos-B as initiator
the MALDI-TOF MS spectrum of the porphyrin initiator Tetra- was performed with 2 M monomer concentration in CH3CN at
Tos-B. The formation of TetraTos-B (mass 1582) and a minor 140 °C under microwave irradiation with a [M]/[I] ratio of 200,
fraction, with only three tosylate groups and one methoxybu- corresponding to 50 monomer units per tosylate group. After
Figure 7: SEC traces obtained for the polymerization of 2-ethyl-2-oxazoline initiated with TetraTos a) and HexaTos b). The large signals at 9 to 9.5
min retention time correspond to the multi-tosylate initiators.
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Scheme 2: Schematic representation of the synthesis of a porphyrin initiated four-armed star-pEtOx starting from tetrakis(hydroxyphenyl)porphyrin
(porphyrin).
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20 min polymerization time, the formation of the polymer was (Figure 9b), proving that the porphyrin is incorporated into the
observed and the initiator was completely consumed, which is polymer. The SEC analysis with the UV-detector yielded a Mn
in clear contrast with TetraTos-A, indicating that indeed of 10,700 g/mol and a PDI of 1.18 based on linear
decreasing the steric hindrance significantly improves the initi- poly(ethylene glycol) standards. The Mn is lower than the theo-
ation efficiency of the polymerization. The resulting porphyrin retical molar mass (~21,000 g/mol) due to calibration with
centered star-shaped poly(2-ethyl-2-oxazoline) star-PEtOx was linear standards with a different molecular structure. The hydro-
purified by preparative SEC to remove unreacted monomer dynamic volume, that determines the retention time, will be
since precipitation was unsuccessful due to the small scale of very different for a star-shaped polymer when compared to a
the polymerization. Figure 9a depicts the 1H NMR spectra of linear polymer. Nonetheless, the narrow molar mass distribu-
the tosylate-porphyrin TetraTos-B (bottom) and the star-PEtOx tion indicates that the star-PEtOx was synthesized in a
(top). The porphyrin signals are still present in the star-PEtOx controlled manner. Critical examination of the SEC trace does
spectrum, indicating that indeed a four-armed pEtOx with a por- show a slight shoulder at shorter retention times, indicating the
phyrin core was synthesized. Integration of both the polymer occurrence of minor side reaction leading to star-star coupling.
backbone signals (l and m) and the porphyrin signals (a and b) In addition, the specific porphyrin absorption spectrum could be
revealed that 188 EtOx units were incorporated into the detected in the entire molar mass distribution using SEC with a
polymer, corresponding to 47 monomers per arm, which is photodiode-array detector, indicating that the porphyrin is
close to the theoretical number of 50.
indeed incorporated in all polymer chains (Figure 10).
SEC characterization of the TetraTos-B resulted in a negative Conclusion
signal in the RI-detector, indicating that the porphyrin has a The synthesis of various multi-tosylates was successfully
lower RI than the eluent, and a positive signal in the performed by esterification of the corresponding alcohols with
UV-detector at 500 nm, where the porphyrin has a strong tosyl chloride. The tosylation of (di)pentaerythritols was only
UV-absorption (Figure 9b). The Mn was calculated to be 1,580 successful using pyridine as solvent due to the limited solu-
g/mol with a polydispersity index (PDI) of 1.06 (against poly- bility of the respective educts in dichloromethane. Recrystal-
styrene standards). This PDI value results from diffusion of the lization of these tosylate compounds yielded single crystals, and
organic compound in the column since it is almost monodis- the X-ray crystal structures of di-, tetra- and hexa-tosylates were
perse (see MALDI in Figure 8). The star-PEtOx could not be centrosymmetric with ideal space filling packing. The use of
characterized with the RI-detector due to the combination of a tetra- and hexa-tosylates, based on (di)pentaerythritol as initi-
positive signal of the polymer and a negative signal of the por- ators for the living cationic ring-opening polymerization
phyrin. However, detection with the UV-detector at (CROP) of 2-ethyl-2-oxazoline, resulted in very slow initiation
500 nm revealed a relatively narrow molar mass distribution and ill-defined polymers, which is most likely due to the steric
Figure 9: a) 1H NMR spectra (in CDCl3) of the porphyrin initiator TetraTos-B (bottom) and star-pEtOx (top). b) SEC traces of TetraTos-B and star-
PEtOx (in CHCl3:NEt3:2-PrOH; UV-detector at 500 nm).
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Size exclusion chromatography (SEC) was measured on a
Shimadzu system with a SCL-10A system controller, a
LC-10AD pump, a RID-6A refractive index detector, a SPD-
10A UV detector and a PLgel 5 μm Mixed-D column with chlo-
roform: triethylamine:2-propanol (94:4:2) as eluent and the
column oven set to 50 °C (polystyrene calibration). SEC with
photodiode-array detector was measured on a Waters system
with a 1515 pump, a 2414 refractive index detector and a
Waters Styragel HT4 column utilizing DMF containing 5 mM
NH4PF6 at a flow rate of 0.5 mL/min as eluent and the column
oven set to 50 °C (PEG calibration).
MALDI-TOF-MS was performed on a Voyager-DE™ PRO
Biospectrometry™ Workstation (Applied Biosystems) time-of-
flight mass spectrometer using the linear mode for operation
(positive ion mode; ionization with a 337 nm pulsed nitrogen
laser). Elemental analyses were performed on a EuroEA3000
Series EuroVector Elemental Analyzer for CHNS-O.
Figure 10: SEC spectrum obtained for star-pEtOx utilizing a photo-
diode-array detector (eluent: DMF containing 5 mM NH4PF6).
X-ray crystal structures were measured by mounting selected
crystals on a Bruker-AXS APEX diffractometer with a CCD
area detector. Graphite-monochromated Mo-Kα radiation
hindrance of the multiple tosylate groups in these initiators. (71.073 pm) was used for the measurements. The nominal
Therefore, a porphyrin-cored tetra-tosylate initiator with signifi- crystal-to-detector distance was 5.00 cm. A hemisphere of data
cantly reduced steric hindrance was successfully prepared by was collected by a combination of three sets of exposures at 292
reaction of 1,4-butane-ditosylate with 5,10,15,20-tetrakis(4- K. Each set had a different Φ angle for the crystal, and each
hydroxyphenyl)porphyrin. Utilization of this star-shaped exposure took 20 s and in steps of 0.3° in ω. The data were
initiator yielded a well-defined star-shaped poly(2-ethyl-2-oxa- corrected for polarization and Lorentz effects, and an empirical
zoline) by CROP.
absorption correction (SADABS) was applied [23]. The cell
dimensions were refined with all unique reflections. The struc-
tures were solved by direct methods (SHELXS97). Refinement
was carried out with the full-matrix least-squares method based
Experimental
Materials
Solvents were purchased from Biosolve Ltd. Acetonitrile (size on F2 (SHELXL97) [24] with anisotropic thermal parameters
3 Å) was dried over molecular sieves. CH2Cl2 was distilled for all non-hydrogen atoms. Hydrogen atoms were inserted in
over potassium. All other solvents were used without further calculated positions and refined riding with the corresponding
purification. EtOx (Aldrich) was distilled over barium oxide atom.
(BaO) and stored under argon. Methyl tosylate (Aldrich) was
distilled over P2O5 and stored under argon. Diethyleneglycol Synthesis of 1,4-butanediol ditosylate (DiTos-B)
ditosylate (DiTos-A; Aldrich) was recrystallized from ethanol To a solution of 1,4-butanediol (4.5 g, 50 mmol) and triethyl-
and 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (porphyrin; amine (6.07 g, 60 mmol) in dry CH2Cl2 (100 mL), a solution of
Aldrich) was used without further purification.
tosyl chloride (23.8 g, 125 mmol) in CH2Cl2 (100 mL) was
added dropwise over 75 min. The resulting solution was stirred
for 24 h under argon and subsequently, ethanolamine (6 mL)
Instrumentation
Polymerizations were carried out in an Emrys Liberator was added to react with excess tosyl chloride. The resulting
(Biotage, formerly PersonalChemistry) with capped reaction mixture was poured into water (200 mL). The aqueous layer
vials. All microwave polymerizations were performed with was extracted with CH2Cl2, and the combined organic layers
temperature control (IR sensor).
were washed successively with 3 N HCl (2 × 100 mL) and brine
(150 mL). After drying with MgSO4 and filtration, the solvent
NMR spectra were recorded on a Varian AM-400 spectrometer was evaporated under reduced pressure. Recrystallization of the
or on a Varian Gemini 300 spectrometer. Chemical shifts are product from ethanol yielded the desired 1,4-butanediol ditosy-
given in ppm relative to TMS or residual solvent signals.
late as white platelets in 58% yield (11,5 g, 28,9 mmol).
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Beilstein J. Org. Chem. 2010, 6, 773–783.
1H NMR (CDCl3): δ 7.74 (d, 8.3 Hz, 4H, o-CH), 7.33 (d, (100 mL), saturated sodium hydrogen carbonate solution
8.3 Hz, 4H, m-CH), 3.97 (t, 5.5 Hz, 4H, OCH2), 2.43 (s, 6H, (100 mL) and brine (100 mL). After drying with MgSO4 and
CH3), 1.68 (t, 5.5 Hz, 4H, OCH2CH2). 13C NMR (CDCl3): δ filtration, the solvent was removed under reduced pressure. The
144.8 (CCH3), 132.7 (CS), 129.8 (m-C), 127.7 (o-C), 69.2 resulting solid was purified by column chromatography (SiO2
(OCH2), 24.9 (OCH2CH2), 21.5 (CCH3).
with CH2Cl2) and preparative size exclusion chromatography
(biobeads SX-1 in CH2Cl2) resulting in the title compound 3
Synthesis of pentaerythritol tetra-tosylate (TetraTos) (38 mg, 0.024 mmol, 10% yield).
Pentaerythritol (1.36 g; 10 mmol) and pyridine (20 mL) were
weighed into a round-bottom flask and cooled to 0 °C. Subse- 1H NMR (CDCl3): δ 12.3 (s, 2H, NH), 8.89 (s, 8H, CHpor), 8.20
quently, solid tosyl chloride (9.5 g; 50 mmol) was added (d, 8.5 Hz, 8H, OCCHCH), 7.90 (d, 8.2 Hz, 8H, o-CHtos), 7.42
portionwise ensuring that the temperature remained below 5 °C. (d, 8.2 Hz, 8H, m-CHtos), 7.21 (d, 8.5 Hz, 8H, OCCH), 4.26 (t,
The resulting solution was stirred overnight, during which time 5.6 Hz, 8H, COCH2), 4.15 (t, 7.2 Hz, 8H, SOCH2), 2.48 (s,
it was allowed to warm slowly to ambient temperature. The 12H, CH3), 2.03 (m, 16H, OCH2CH2CH2). 13C NMR (CDCl3):
formed white-pinkish slurry was poured into 125 mL of a 6M δ 158.3, 144.5, 135.3, 134.4, 132.9, 129.6, 127.7, 119.4, 112.3,
HCl solution yielding a white precipitate that was collected by 70.0, 66.8, 25.6, 25.2, 21.3. C88H86N4O16S4: calcd. C 66.73, H
filtration. This solid was washed with water (2 × 100 mL). 5.47, N 3.54, S 8.10; found C 66.22, H 5.38, N 3.67, S 7.74.
Further purification was performed by recrystallization from a GPC (CHCl3:NEt3:2-PrOH = 94:4:2; UV detector at 500 nm):
mixture of ethanol (100 mL) and acetone (100 mL) yielding Mn = 1,580 g/mol; PDI = 1.06. MALDI-TOF-MS: m/z [M+]
4.7 g (62%) of the desired product as white crystals. Partial 1582, [M+-tosyl] 1446.
evaporation of the acetone (~75 mL) from the filtrate yielded
another 1.6 g (22%) of crystals, resulting in a total isolated yield General polymerization procedure
of 84%.
The polymerizations of 2-ethyl-2-oxazoline with TetraTos and
HexaTos as initiators were performed under microwave irradi-
1H NMR (CDCl3): δ 7.68 (d, 8.2 Hz, 8H, o-CHtos), 7.36 (d, ation. Before use, the microwave vials were heated to 105 °C,
8.2 Hz, 8H, m-CHtos), 3.82 (s, 8H, SOCH2), 2.47 (s, 12H, CH3). allowed to cool to ambient temperature and filled with argon
13C NMR (CDCl3): δ 145.3, 131.0, 129.8, 127.6, 65.2, 42.9, prior to use. Subsequently, the initiator, monomer and aceto-
21.4. C33H36O12S4: calcd. C 52.65, H 4.82, S 17.03; found C nitrile were weighed in so that a 1.0 mL polymerization mix-
52.87, H 4.89, S 17.21.
ture was obtained in which the ratio of monomer per tosylate
group is 25 and the desired monomer concentration is 1, 2 or
4 M. This polymerization mixture was heated by microwaves to
the desired temperature for a fixed time (5 min at 140 °C; 2 min
Synthesis of dipentaerythritol hexa-tosylate
(HexaTos)
This compound was prepared in a similar manner as TetraTos at 160 °C; 30 seconds or 10 min at 200 °C). After heating, the
using the following amounts: dipentaerythritol (2.5 g; polymerization mixtures were investigated by size exclusion
10 mmol), pyridine (20 mL) and tosyl chloride (14.3 g; chromatography.
75 mmol). Recrystallization of the crude product from ethanol
yielded the desired product as white crystals (~8 g; 68%).
Microwave synthesis of 5,10,15,20-tetrakis(pEtOx)-
21H,23H-porphyrin (star-PEtOx)
1H NMR (CDCl3): δ 7.66 (d, 8.2 Hz, 12H, o-CHtos), 7.35 (d, A mixture of porphyrin tosylate 3 (7.92 mg, 0.005 mmol) and
8.2 Hz, 12H, m-CHtos), 3.77 (s, 12H, SOCH2), 3.14 (s, 4H, EtOx (100 mg, 1 mmol) in CH3CN (0.4 mL) was heated to
OCH2) 2.44 (s, 18H, CH3). 13C NMR (CDCl3): δ 145.4, 131.7, 140 °C for 20 min under microwave irradiation. After heating,
130.0, 127.8, 67.8, 66.5, 43.6, 21.5. C52H58O19S6: calcd. the solvent and residual monomer were evaporated under
C 52.96, H 4.96, S 16.31; found C 53.30, H 5.10, S 15.97. vacuum and the resulting residue was purified by preparative
size exclusion chromatography (biobeads SX-1 with CH2Cl2),
Synthesis of 5,10,15,20-tetrakis(4-hydroxybutyloxy
tosylate)-21H,23H-porphyrin (TetraTos-B)
resulting in 60 mg of polymer 4 (56% yield).
A mixture of 5,10,15,20-tetrakis (4-hydroxyphenyl)porphyrin 1 1H NMR (CDCl3): δ 12.23 (s, 2H, NH), 8.85 (br, 8H, CHpor),
(170 mg, 0.25 mmol), 1,4-butanediol ditosylate 2 (2 g, 5 mmol) 8.10 (br, 8H, 8H, OCCHCH), 7.23 (br, 8H, OCCH), 4.35–4.17
and potassium carbonate (190 mg, 1,37 mmol) in dry CH3CN (br, 16H, COCH2 + SOCH2), 3.75–3.20 (br, 752H, NCH2),
was refluxed for 75 h. After this period, the solvent was evapo- 2.57–2.05 (br, 395H, COCH2 + OCH2CH2CH2), 1.11–1.09 (br,
rated under reduced pressure and the residue was redissolved in 574H, CH3). GPC (CHCl3:NEt3:2-PrOH = 94:4:2; UV detector
CHCl3. This solution was washed successively with water at 500 nm): Mn = 10,700 g/mol; PDI = 1.18.
782

Beilstein J. Org. Chem. 2010, 6, 773–783.
20.Hoogenboom, R.; Nakahashi, A.; Kickelbick, G.; Chujo, Y.;
Supporting Information
Schubert, U. S. Acta Crystallogr., Sect. E 2007, 63, o2311–o2313.
doi:10.1107/S1600536807017229
Supporting Information File 1
21.Wiesbrock, F.; Hoogenboom, R.; Abeln, C. H.; Schubert, U. S.
Macromol. Rapid Commun. 2004, 25, 1895–1899.
doi:10.1002/marc.200400369
Details of the reported crystal structures.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-6-96-S1.pdf]
22.Wiesbrock, F.; Hoogenboom, R.; Leenen, M. A. M.; Meier, M. A. R.;
Schubert, U. S. Macromolecules 2005, 38, 5025–5034.
doi:10.1021/ma0474170
23.Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33–38.
doi:10.1107/S0108767394005726
Acknowledgements
RH (Veni-grant) and USS (Vici award) are grateful to the
Netherlands Scientific Organisation (NWO) for financial
support.
24.SHELX97 Programs for Crystal Structure Analysis, Release 97-2;
Universität Göttingen: Göttingen, Germany, 1998.
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