Reversible switching between linear and ring polystyrenes bearing porphyrin end groups

Article abstract of DOI:10.1021/ja108821p

A route to macrocyclic polymers based on a new unimolecular ring-closure process has been investigated. It involves the direct end-to-end coupling of an α,ω-bis[chloroiron(III) meso-tetraphenylporphyrin] telechelic linear polystyrene synthesized by living

Full text of DOI:10.1021/ja108821p

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Reversible Switching between Linear and Ring Polystyrenes Bearing
Porphyrin End Groups
Michel Schappacher and Alain Deffieux*
CNRS and Universit ꢀe Bordeaux, Laboratoire de Chimie des Polym ꢁe res Organiques, ENSCBP, 16 Avenue Pey Berland, 33607 Pessac
Cedex, France
S Supporting Information
b
1
1,23,24
25,26
27
acetal/styrenyl,
carboxy/amino,16,28-3a1cetal/hydroxyl,
ABSTRACT: A route to macrocyclic polymers based on a
new unimolecular ring-closure process has been investi-
gated. It involves the direct end-to-end coupling of an R,
ω-bis[chloroiron(III) meso-tetraphenylporphyrin] teleche-
lic linear polystyrene synthesized by living polymerization
followed by chain-end functionalization. The correspond-
ing macrocyclic polystyrene was obtained readily and selec-
tively by intramolecular condensation of the R,ω-bis-
and azide/alkyne (i.e., click chemistry).
The bimolecular approach is generally more prone to bypro-
duct formation, while the unimolecular route, with sufficiently
high-yielding end-group transformations and couplings, offers
the potential to yield high-purity cyclic polymers by using high
dilution. To extend the scope of unimolecular cyclization reac-
tions, it remains necessary to find new high-yielding end-group
transformations and efficient end-to-end coupling reactions,
resulting the desire to explore alternative approaches.
The present study reports a new selective and reversible
cyclization method based on the coupling of iron(III) porphyrin
moieties into a bis[iron(III) μ-oxoporphyrin] dimer.
of the very few examples of reversible cyclization that we found in
the literature as the most pertinent precedent is the ring closing/
[
chloroiron(III) meso-tetraphenylporphyrin] polymer ends
in the presence of a base to yield a diiron(III)-μ-oxobis-
porphyrin) dimer as ring-closing unit. Addition of dilute
(
3
2-35
HCl was shown to rapidly reconvert the diiron(III)-μ-
oxobis(porphyrin) unit into the initial bis[chloroiron(III)
porphyrin], demonstrating the selectivity and complete
reversibility of the cyclization process. The synthesis and
detailed structural characterization of the R,ω-homodi-
functional precursor and the corresponding macrocyclic
polystyrene along with an analysis of the porphyrin dimer-
ization reaction using NMR spectroscopy and size-exclusion
chromatography coupled with a diode array detector are
presented.
One
36
reopening of thiol-end-capped polystyrene reported by Whittaker.
Our approach consists of the end-to-end ring closure of an
R,ω-homodifunctional polystyrene chain bearing an iron(III)
porphyrin group at each end. Macrocyclization is achieved under
high dilution by coupling of the two iron(III) porphyrin ends
under slightly basic conditions, yielding a polystyrene ring with
a stable bis[iron(III) μ-oxoporphyrin] dimer as closing unit
(
Scheme 1). This unimolecular macrocyclization reaction, which
has been shown to be very fast, is fully reversible by switching the
basicity”/”acidity” of the organic polymer solution. Here the
“
he synthesis of cyclic polymers with controlled molar masses
and low molar-mass dispersity and the study of their physical
strategy of synthesis, the structure of the functional linear and
cyclic polystyrenes, and the characteristics of the cyclization
reaction are presented and discussed.
T
properties have attracted considerable interest in the last 50
1
-5
years.
The methods of preparation of cyclic polymers can be
Iron(III) chloride meso-tetraphenylporphyrin telechelic poly-
styrene (5) was prepared by the procedure described in
Scheme 2. Linear bis(hydroxymethyl) telechelic polystyrene
precursors (3) were synthesized by living anionic polymerization
of styrene using isopropylidene-2,2-bis(hydroxymethyl)-1-(2-
lithiobutyl)butane as an initiator, yielding (1) and isopropyli-
dene-2,2-bis(hydroxymethyl)-1-(2-chlorobutyl)butane as a
functional chain-terminating agent to form the bisacetal-ended
polystyrene (2). The latter was converted into (3) by acidic
hydrolysis of the acetal ends and then trans-acetalized in presence
of diethylacetal-functionalized tetraphenylporphyrin in an acidic
medium to yield (4). Finally, the iron(III) chloride meso-tetra-
phenylporphyrin telechelic polystyrene (5) was obtained by
classified into three main categories: (a) ring/linear chain equil-
6
-8
9,10
ibrations,
(b) ring-expansion polymerizations,
and (c)
1
1-15
end-to-end cyclization reactions.
The last category is one of the most explored and versatile
routes. It relies on the ring closure of an R,ω-difunctional linear
polymer precursor generally prepared by living/controlled poly-
16
merization techniques. Cyclization was first achieved by the
bimolecular coupling, under high dilution, of an R,ω-homodi-
functional linear polymer, typically an anionically prepared
polystyrene or polydiene in which a difunctional electrophilic
molecule had been introduced in perfectly stoichiometric
1
1-13
proportion.
interfacial condensation
been used. One original approach proposed by Tezuka
Other bimolecular coupling reactions, such as
17-19
20
metalation of (4) using FeCl . The experimental degree of
and radical coupling, have also
2
21,22
polymerization (DPn) of 65 for the polystyrene chain (2)
determined from SEC data was found to agree well with
con-
sists of a dipolar precyclization step before formation of a
covalent linkage. A second ring-closing approach involves the
intramolecular coupling of R,ω-heterodifunctional precursors
bearing two complementary reactive end groups, for instance,
Received: September 30, 2010
Published: January 19, 2011
r 2011 American Chemical Society
1630
dx.doi.org/10.1021/ja108821p
|J. Am. Chem. Soc. 2011, 133, 1630–1633

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Scheme 1. Reversible Switching between Linear and
Macrocyclic Polystyrenes Induced by Iron(III)
Scheme 2. Strategy of Synthesis of Telechelic Polystyrene
with Iron(III) meso-Tetraphenylporphyrin Chloride Ends
(εH O Denotes a Trace of H O)
meso-Tetraphenylporphyrin Coupling (εH O Denotes
2
2
2
a Trace of H O)
2
theoretical values calculated on the basis of complete initiation of
styrene polymerization by the lithioacetal derivative, whereas the
polymer molar-mass dispersity (MWD) was very low (1.05).
The relative number of meso-tetraphenylporphyrin end groups
per polystyrene chain (4) was determined from their relative
1
H NMR peak areas on the basis of the polystyrene DPn of 65.
The chain-end functionality was found to be very close to 2. Experi-
mental reaction conditions, characterization data, and proton
NMR spectra are given in the Supporting Information.
Cyclization of (5) was performed in dichloromethane under
high dilution by slow addition of a small amount of sodium
methanolate/methanol solution as ring-closing catalyst. The
crude size-exclusion chromatography (SEC) chromatograms of
both the linear precursor (5) and the cyclized polystyrene (6) are
presented in Figure 1.
and (2) ring closing via condensation of the two OH ligands to
form a μ-oxo bridge with liberation of one molecule of water,
As expected, the cyclized polymer (apparent number-average
molecular weight Mn,app = 4700 as determined by SEC) eluted at
2TPPFeOH f ðTPPFeÞ O þ H2O
ð2Þ
2
a higher volume than the linear precursor (M = 6800 as
n
which corresponds to an unimolecular cyclization process.
determined by SEC). This corresponds to a ratio of the two
In tetrahydrofuran, the red-brown-colored iron(III) meso-
tetraphenylporphyrin chloride-functionalized linear polystyrene
precursor exhibits characteristic metal-to-porphyrin absorption
bands at 370.1 nm, 419.4 nm (charge-transfer band), and 507.7 nm
SEC signals (M /M ) of ÆGæ = 0.7, which agrees well with
pc
pl
37-39
ÆG æ values previously reported for macrocyclic polystyrenes.
exp
The cyclization appeared to be almost quantitative. The forma-
tion of high-molar-mass polycondensates was not detected, and
only a small SEC peak attributed to polystyrene cyclic dimer (see
below) was observed. As confirmed by UV-vis spectroscopy
(
Q band). After addition of sodium methanolate, the polymer
solution turns green and exhibits new absorbance bands at 320.2 nm,
09.7 nm (Soret band), and 572.1 and 613.6 nm (Q bands),
4
(Figure 2), the cyclization mechanism (see Scheme 1) involves
35,40,41
characteristic of the (FeTPP) O dimer
(see Figure 2).
the intramolecular coupling of the two iron(III) meso-tetraphe-
2
Because of the different UV-vis absorbance spectra of the
monomeric and dimeric iron(III) meso-tetraphenylporphyrin
species, the cyclization reaction could be analyzed for polymer
fractions eluting at different volumes using a UV-vis diode-array
detector coupled to the SEC instrument. The rapid and almost
complete disappearance of the initial iron(III) bands for all of the
cyclized polymer fractions indicated that conversion into the
diiron(III)-μ-oxobis(porphyrin) is very fast and close to quanti-
tative (see Figure 3).
nylporphyrin polystyrene ends to yield a diiron(III)-μ-oxobis-
(
porphyrin) dimer as ring-closing unit. This reaction is triggered
under basic conditions by methanolate anions or more likely by
hydroxide ions present in the methanolate/methanol solution.
33
According to the literature, the coupling proceeds in two steps: (1)
-
substitution of the two Cl ligands by attack of OH from the basic
medium,
-
-
TPPFeCl þ OH f TPPFeOH þ Cl
ð1Þ
1
631
dx.doi.org/10.1021/ja108821p |J. Am. Chem. Soc. 2011, 133, 1630–1633

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Figure 1. SEC chromatograms of linear (blue trace) and cyclized (black
trace) polystyrene obtained using a using a refractive index detector.
Characterization data: (linear) M = 6800, MWD = 1.05; (cyclic) Mn,app =
n
Figure 3. Soret bands of cyclized polystyrene fractions taken at different
SEC elution volumes (see Figure 1) using a UV-vis diode-array
detector.
4
700, MWD = 1.08. Arrows 1-6 refer to cyclized polymer fractions
analyzed using a UV-vis diode-array detector. The indicated data refer
to the observed λ position and the calculated percentage of linear
max
contaminant (see Figure 3).
Figure 2. Characteristic wavelengths (in nm) of main metal-to-por-
phyrin absorption bands of (a) R,ω-terminated iron(III) meso-tetra-
phenylporphyrin chloride polystyrene (blue spectrum) and (b) the
Figure 4. SEC chromatogram of cyclized polystyrene showing the
relative amount and location of the linear polystyrene contaminant.
corresponding cyclic polystyrene with
a
diiron(III)-μ-oxobis-
(porphyrin) dimer as a ring-closing unit (black spectrum).
35,40
regenerating the chloroiron(III) tetraphenylporphyrin molecule:
Although not observable using refractive-index detection
Figure 1), the presence of traces of uncyclized linear precursor
could be deduced from the shift of the Soret band to higher
(
ðTPPFeÞ O þ 2HCl f 2TPPFeCl þ H2O
2
wavelength, characteristic of Fe(III)ClTPP or Fe(III)OHTPP
Under similar acidic conditions, the polystyrene macrocycles readily
“re-open”, giving back the linear telechelic polystyrene with identical
M and MWD. For instance, addition of few drops of HCl/CH Cl to
42
(
which have Soret bands at 419.4 and 415.0 nm, respectively).
The fractions eluting at the peak maximum of the cyclic polymer
arrows 1 and 2 in Figure 1) and the one centered on polystyrene
n
2
2
(
the green cyclic polystyrene solution yielded a rapid color change of
the solution to red-brown. The selective reformation of linear
polystyrene (5) with iron(III) meso-tetraphenylporphyrin chloride
end groups was confirmed by the shift of the SEC peak to higher
elution volume and the location of the Soret band at 419.4 nm.
Reversible commutation between linear and macrocyclic
polystyrene was checked by switching the solution from acidic
to basic over several cycles. The color change from red-brown to
green and from green to red-brown observed upon alternative
addition of a few drops of methanolate and of 1 M HCl to the
solution as well as the corresponding shifts in the Soret band
strongly support the reversibility of the process.
dimers (arrow 6) show a Soret band position in agreement with
the almost unique presence of diiron(III)-μ-oxobis(porphyrin)
species and the quantitative formation of cyclic polystyrene and
cyclic polystyrene dimer. In contrast, in the peak tail correspond-
ing to polymer fractions 3-5, the amount of linear contaminant
is significant.
From the estimated percentage of linear polystyrene in the
different polymer fractions and their relative weights, the total amount
of linear contaminant, mostly in the SEC peak tail, was estimated to be
∼
9% (see Figure 4). Incomplete cyclization likely results from the
presence of some nonfunctionalized chains.
As described in the literature, in presence of a Brønsted acid
HCl, for instance), the μ-oxo-Fe(III)TPP dimer readily breaks,
In conclusion, we have described a new approach for the
synthesis of macrocyclic polymers based on the rapid intramolecular
(
1
632
dx.doi.org/10.1021/ja108821p |J. Am. Chem. Soc. 2011, 133, 1630–1633

Journal of the American Chemical Society
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conversion of the iron(III) meso-tetraphenylporphyrin chloride
chain ends of a linear polymer precursor into a diiron(III)-μ-
oxobis(porphyrin) dimer as a ring-closing unit. This process,
which is readily catalyzed by the addition of a base, involves a
unimolecular chain-end coupling reaction that shows very little
dependence on the dilution conditions and is very efficient for
the end-to-end cyclization of polymers. The reversibility of the
cyclization is another important and original feature of this
process.
(24) Schappacher, M.; Deffieux, A. Macromolecules 2001, 34, 5827–
5832.
25) Kubo, M.; Hayashi, T.; Kobayashi, H.; Tsuboi, K.; Itoh, T.
Macromolecules 1997, 30, 2805–2807.
26) Kubo, M.; Takeuchi, H.; Ohara, T.; Itoh, T.; Nagahata, R.
J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2027–2033.
27) Schappacher, M.; Deffieux, A. Macromol. Chem. Phys. 2002,
03, 2463–2469.
28) Laurent, B. A.; Grayson, S. M. J. Am. Chem. Soc. 2006, 128,
238–4239.
29) Hoskins, J. N.; Grayson, S. M. Macromolecules 2009, 42, 6406–
6413.
(
(
(
2
(
4
(
’
ASSOCIATED CONTENT
(30) Eugene, D. M.; Grayson, S. M. Macromolecules 2008, 41, 5082–
S
Supporting Information. Experimental conditions for
5084.
b
(31) Lonsdale, D. E.; Bell, C. A.; Monteiro, M. J. Macromolecules
the synthesis of meso-tetraphenylporphyrin derivatives, prepara-
2
010, 43, 3331–3339.
(32) Liu, Q.; Gong, Y. Z.; Gong, C. J.; Li, Q. H.; Guo, C. C.
tion of end-functionalized linear polystyrene and of polystyrene
1
macrocycles, and their characterization by H NMR and UV-vis
J. Porphyrins Phthalocyanines 2009, 13, 854–858.
33) Fielding, L.; Eaton, G. R.; Eaton, S. S. Inorg. Chem. 1985, 24,
309–2312.
34) Fleischer, E. B.; Srivastava, T. S. J. Am. Chem. Soc. 1969, 91,
403–2405.
35) Ghosh, S. K.; Patra, R.; Rath, S. P. Inorg. Chem. 2010, 49, 3449–
460.
36) Whittaker, M. R.; Goh, Y. K.; Gemici, H.; Legge, T. M.; Perrier,
spectroscopy. This material is available free of charge via the
Internet at http://pubs.acs.org.
(
2
2
3
(
’
AUTHOR INFORMATION
(
Corresponding Author
deffieux@ipb.fr
(
S.; Monteiro, M. J. Macromolecules 2006, 39, 9028–9034.
’
REFERENCES
(37) Hadziioannou, G.; Cotts, P. M.; Ten Brinke, G.; Han, C. C.;
Lutz, P.; Strazielle, C.; Rempp, P.; Kovacs, A. J. Macromolecules 1987, 20,
(
1) Deffieux, A.; Borsali, R. Controlled Synthesis and Properties of
Cyclic Polymers; Wiley-VCH: Weinheim, Germany, 2007.
2) Kricheldorf, H. R. J. Polym. Sci., Part A: Polym. Chem. 2010, 48,
51–284.
3) Roovers, J. In Cyclic Polymers, 2nd ed.; Semlyen, J. A., Ed.;
Kluwer Academic Publishers: Dordrecht, The Netherlands, 2000; pp
493–497.
(38) Roovers, J. J. Polym. Sci., Polym. Phys. Ed. 1985, 23, 1117–1126.
(
(39) Schappacher, M.; Billaud, C.; Paulo, C.; Deffieux, A. Macromol.
2
Chem. Phys. 1999, 200, 2377–2386.
40) Ghosh, S. K.; Patra, R.; Rath, S. P. Inorg. Chim. Acta 2010, 363,
791–2799.
41) Manso, C. M. C. P.; Neri, C. R.; Vidoto, E. A.; Sacco, H. C.;
(
(
2
3
47-360.
(
(
4) Clarson, S. J.; Dodgson, K.; Semlyen, J. A. Polymer 1985, 26,
Ciuffi, K. J.; Iwamoto, L. S.; Iamamoto, Y.; Nascimento, O. R.; Serra,
O. A. J. Inorg. Biochem. 1999, 73, 85–92.
930–934.
(
5) Huang, W.; Frisch, H. L.; Hua, Y.; Semlyen, J. A. J. Polym. Sci.,
(42) Quinn, R.; Nappa, M.; Valentine, J. S. J. Am. Chem. Soc. 1982,
Part A: Polym. Chem. 1990, 28, 1807–1812.
104, 2588–2595.
(
(
(
(
6) Dodgson, K.; Semlyen, J. A. Polymer 1977, 18, 1265–1268.
7) Semlyen, J. A. Adv. Polym. Sci. 1976, 41–75.
8) Beevers, M. S.; Semlyen, J. A. Polymer 1972, 13, 385–390.
9) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. Science 2002, 297,
2
6
1
041–2044.
10) Kricheldorf, H. R.; Lee, S. R. Macromolecules 1995, 28, 6718–
725.
(
(
11) Deffieux, A.; Beinat, S.; Schappacher, M. Macromol. Symp.
997, 118, 247–253.
(
(
12) Geiser, D.; H €o cker, H. Macromolecules 1980, 13, 653–656.
13) Hild, G.; Kohler, A.; Rempp, P. Eur. Polym. J. 1980, 16, 525–
5
6
2
27.
(
14) Chen, R.; Ling, J.; Hogen-Esch, T. E. Macromolecules 2009, 42,
015–6022.
(
15) Hogen-Esch, T. E. J. Polym. Sci., Part A: Polym. Chem. 2006, 44,
139–2155.
16) Laurent, B. A.; Grayson, S. M. Chem. Soc. Rev. 2009, 38, 2202–
213.
17) Cho, D.; Masuoka, K.; Koguchi, K.; Asari, T.; Kawaguchi, D.;
Takano, A.; Matsushita, Y. Polym. J. 2005, 37, 506–511.
(
2
(
(18) Ishizu, K.; Ichimura, A. Polymer 1998, 39, 6555–6558.
(19) Ishizu, K.; Kanno, H. Polymer 1996, 37, 1487–1492.
(20) Takano, A.; Kushida, Y.; Aoki, K.; Masuoka, K.; Hayashida, K.;
Cho, D.; Kawaguchi, D.; Matsushita, Y. Macromolecules 2007, 40, 679–
681.
(
(
(
21) Tezuka, Y. Macromol. Symp. 2003, 192, 217–226.
22) Tezuka, Y. Chem. Rec. 2005, 5, 17–26.
23) Deffieux, A.; Schappacher, M.; Rique-Lurbet, L. Polymer 1994,
35, 4562–4568.
1
633
dx.doi.org/10.1021/ja108821p |J. Am. Chem. Soc. 2011, 133, 1630–1633