Orthogonal chain length control in calix[5]arene-based AB-type supramolecular polymers

Article abstract of DOI:10.1016/j.tetlet.2011.09.098

New AB-type supramolecular polymers have been prepared by acid-promoted self-assembly of an ami-nododecyloxy-calix[5]arene monomer precursor. The number-average degree of polymerization has been found to be dependent on the concentration of the salt monom

Full text of DOI:10.1016/j.tetlet.2011.09.098

Tetrahedron Letters 52 (2011) 6460–6464
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Orthogonal chain length control in calix[5]arene-based AB-type
supramolecular polymers
Claudia Gargiulli a, Giuseppe Gattuso a, , Anna Notti a, Sebastiano Pappalardo b, Melchiorre F. Parisi a,
⇑
⇑
a Dipartimento di Chimica Organica e Biologica, Università di Messina, Viale F. Stagno d’Alcontres 31, 98166 Messina, Italy
b Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
New AB-type supramolecular polymers have been prepared by acid-promoted self-assembly of an ami-
nododecyloxy-calix[5]arene monomer precursor. The number-average degree of polymerization has
been found to be dependent on the concentration of the salt monomer and on the nature of the counter-
ion (i.e., chloride, picrate or hexafluorophosphate).
Received 15 August 2011
Accepted 20 September 2011
Available online 22 September 2011
Chain-length regulation experiments have been carried out, employing orthogonal chain stoppers
capable of selectively interacting with a given moiety of the AB-type monomer/polymer. Competitive
calix[5]arene ‘caps’ and n-butylammonium ion ‘plugs’ have been used to control the extent of self-assem-
bly of the polymer, in turn interacting with the ammoniumdodecyloxy or with the cavity end-groups of
the supramolecular calixarene assembly. These experiments, conveniently carried out at a 10 mM con-
centration, can be easily followed by 1H NMR spectroscopy.
Keywords:
Calixarenes
Supramolecular polymers
Chain stoppers
Chain-length control
Host–guest
1H NMR titration
Ó 2011 Elsevier Ltd. All rights reserved.
Supramolecular polymers,1 that is, polymers whose monomeric
units are held together by means of reversible noncovalent interac-
tions, rather than covalent bonds, are the latest frontier in ad-
vanced materials research. Their advantage over classical
covalently-linked counterparts lies in the ease of preparation,
which does not require chemical reactions, but simply relies on
the self-assembly of pre-programmed monomers.2 Supramolecular
polymers have been actively investigated over the past 10 years.
They have been designed around different recognition motifs, by
taking advantage of the most diverse noncovalent interactions,
concentration.6 Because of this, direct control over the degree of
polymerization can in principle be achieved either by indirectly
acting on the association constant (by means of solvent effects7
or, in the case of charged monomers, by selecting appropriate
counterions8), or by varying the monomer concentration.1
However, a third possibility, borrowed from classical polymer
chemistry,5 has recently been proposed, relying on the use
of monofunctional monomers—that is, chain-stoppers—capable of
making the degree of polymerization concentration independent.9
Among the many different building blocks employed for the
construction of self-assembling monomers, calixarenes10 are play-
ing an increasingly significant role. These versatile receptors have
recently been used as a scaffold for the preparation of both hydro-
gen-bonded11 and host–guest type12 polymers. On our part, owing
to the ability of calix[5]arenes locked in a regular cone conforma-
tion13 to act as receptors for linear primary alkylammonium ions,
we have been able to design a number of supramolecular oligo-/
polymers based on the calix[5]arene/alkylammonium recognition
motif.14 AB-type (self-complementary heteroditopic monomer)
aggregates8 have been obtained by the acid-promoted iterative
inclusion of aminododecyloxy-functionalized calix[5]arenes. AA/
BB-type (complementary homoditopic monomer pair) oligomers,15
on the other hand, have been obtained from a series of divergent-
such as hydrogen-bonding,1a,b
interactions.4
p–p
stacking1a,3 or host–guest
When dealing with supramolecular polymers, chain-length regu-
lation is a crucial factor. In fact, as most supramolecular polymeriza-
tions are thermodynamically controlled self-assembly processes,2
they cannot be stopped at will at a given extent of reaction, and
the uncontrolled growth of polymeric chains may ultimately yield
materials which cannot be easily handled.5
Typically, the degree of polymerization of these materials
depends on: (i) the stability constant of monomer–monomer
interactions (i.e., their association constant) and (ii) the monomer
concentration, according to the equation Xn ꢀ (K ꢁ [M])1/2, where
ꢀ
ꢀ
Xn is the number-average degree of polymerization, K is the mono-
mer–monomer association constant, and [M] is the monomer
cavity bis-calix[5]arenes capable of binding
a,x-alkanediyldiam-
monium salts with a (poly)capsular topology.16
In the present Letter, we report on the chain-length control of an
AB-type supramolecular polymer, prepared from a new highly lipo-
philic aminododecyloxy-calix[5]arene monomer precursor (1, see
below). Control over the self-assembly of 1 was equally achieved
⇑
Corresponding authors. Tel.: +39 090 6765241; fax: +39 090 392840 (G.G.); tel.:
+39 090 6765170; fax: +39 090 392840 (M.F.P.).
E-mail addresses: ggattuso@unime.it (G. Gattuso), mparisi@unime.it (M.F.
Parisi).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.098

C. Gargiulli et al. / Tetrahedron Letters 52 (2011) 6460–6464
6461
by using either A-type or B-type monofunctional chain stoppers,
that is, a different calix[5]arene, and a n-butylammonium salt.
Monomer amino-precursor 1 was prepared in two steps, start-
ing from the known phthalimidododecyloxy-calix[5]arene deriva-
tive8 2 (Scheme 1). Calixarene 2 was fully alkylated in refluxing
MeCN with 1-bromohexadecane, in the presence of K2CO3, to give
3 in a 62% yield after column chromatography.17 Compound 3 was
then treated with H2NNH2ꢂH2O in refluxing ethanol to give the de-
sired amino-derivative 1, after precipitation from CHCl3/MeOH, in
a 43% yield.18
According to 1H NMR data, the calixarene cavity of derivative 1
is arranged in a regular cone conformation, showing a very high
Scheme 1. Synthesis of amino-calix[5]arene 1.
Figure 1. 1H NMR spectra (500 MHz, CDCl3, 10 mM, 298 K) of: (a) 1, (b) 1ꢂHCl, (c) 1ꢂHPic and (d) 1ꢂHPF6. The asterisk indicates residual solvent peak.

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C. Gargiulli et al. / Tetrahedron Letters 52 (2011) 6460–6464
degree of symmetry, with a single AX system accounting for 10 H
assigned to the ArCH2Ar hydrogens (as opposed to the expected
three AX systems for Cs symmetric calix[5]arenes19), and a single
broad resonance for the aromatic hydrogen atoms.
Exposure of amino-monomer precursor 1 to a variety of acids
(HCl, picric acid—henceforth referred to as HPic—and HPF6)20
caused protonation of the amino group, thus activating the self-
assembly tendency of the newly formed monomer 1ꢂHX. As a
consequence, growth of the expected supramolecular polymeric
species (1ꢂHX)n takes place, by means of an iterative endo-cavity
inclusion process, where the pendant dodecylammonium group
of one monomer penetrates the cavity of the next.
the average degree of polymerization.23 Analysis of the data re-
ported in Table 1 led to two interesting observations: (i) in the con-
centration range examined (2–25 mM in CDCl3) a relatively small
but significant increase in the percentage of complexation (and
hence of the degree of polymerization), in line with the known
behavior of supramolecular polymers;24 (ii) a remarkable depen-
dence of the degree of polymerization on the nature of the counter-
ion, in agreement with the increasing tendency of chloride, picrate,
and hexafluorophosphate anions to form contact ion pairs with
ammonium cations in low polarity solvents—the looser the ion
pair, the higher the Xn value.8,25
ꢀ
Chain-length regulation experiments were carried out by pro-
gressively adding, to a 10 mM solution26 of (1ꢂHPF6)n, of either a
competitive calix[5]arene ‘cap’, able to prevent the chain growth
by binding to the dodecylammonium pendant chains, or an alky-
lammonium ‘plug’, capable of filling the calixarene cavity of the
monomers, thus inhibiting monomer–monomer iterative inclusion.
For the capping of the terminal ammonium group of the mono-
mer, the pentaester, penta-tert-butylpentakis(tert-butoxycarbonyl-
methoxy)calix[5]arene27 (4), was selected, owing to its superior
binding ability—toward alkylammonium ions—with respect to
pentaether calixarene derivatives.14a In addition, alkylammonium
guests penetrate deeper into the cavity of pentaester derivatives
compared with pentaether ones, giving rise to distinct sets of
NMR peaks for the dodecylammonium groups included into 1
and those included into 4.16
The 1H NMR spectra showing the formation of (1ꢂHX)n are re-
ported in Figure 1. In the three cases under study, assembly/disas-
sembly of the ammonium monomer was found to be on a slow
exchange regime on the NMR timescale.14 Supramolecular polymer
formation is consistent with the appearance, in the high-field re-
gion (d = ꢃ2.0–0.8 ppm), of the telltale resonances generated by
the
a-e-methylenes of the newly formed ammoniumdodecyloxy
chain as they experience the shielding effect of the calixarene aro-
matic moieties, along with a broad peak (d = 5.74 ppm) assigned to
the included ANHþ3 moiety involved in hydrogen bonding with the
calixarene phenolic oxygen atoms.8,14,21
1H NMR dilution experiments were carried out on the three pro-
tonated monomers (i.e., 1ꢂHCl, 1ꢂHPic, and 1ꢂHPF6) to gain an in-
sight into the concentration2 and counterion8,22 dependence of
Table 1
a
ꢀ
Percentages of complexation for 1ꢂHCl, 1ꢂHPic, and 1ꢂHPF6 and corresponding calculated number-average degree of polymerization (Xn)
[1ꢂHCl] (mM)
[1ꢂHPic] (mM)
[1ꢂHPF6] (mM)
2
5
10
25
2
5
10
25
2
5
10
25
% complb
Xn
43
1.7
44
1.8
44
1.8
45
1.8
78
4.5
79
4.8
81
5.3
83
5.9
94
16.7
95
20
95
20
96
25
ꢀ
a
ꢀ
For calculation of Xn values see Ref. 23.
b
Determined by 1H NMR (500 MHz) at 298 K in CDCl3. Values derived from the average of three independent measurements. Standard error 610%.
Figure 2. Selected regions of the 1H NMR (500 MHz; CDCl3, 298 K) titration experiment of 1ꢂHPF6 (trace a, 10 mM) with calix[5]arene 4 (b, 1 mM; c, 2 mM; d, 3 mM; e, 5 mM;
f, 10 mM; g, 20 mM; h, 50 mM). n P1, m >1.

C. Gargiulli et al. / Tetrahedron Letters 52 (2011) 6460–6464
6463
Figure 3. Selected regions of the 1H NMR (500 MHz; CDCl3, 298 K) titration experiment of 1ꢂHPF6 (trace a, 10 mM) with n-BuNH3þPFꢃ6 (b, 1 mM; c, 2 mM; d, 3 mM; e, 5 mM; f,
10 mM; g, 20 mM; h, 50 mM). n P1.
Figure 4. Decrease of the number-average degree of polymerization (Xn) of (1ꢂHPF6)n upon addition of: (a) calix[5]arene 4, and (b) n-BuNHþ3 PF6ꢃ
.
ꢀ
1H NMR spectra relating to this chain-length regulation experi-
ment (Fig. 2) showed that in the high-field region, upon increasing
the concentration of the chain stopper 4 (from 0.1 to 5 equiv), a
new set of signals for the included methylene hydrogen atoms ap-
peared and progressively grew, matched by the progressive disap-
pearance of the signals belonging to (1ꢂHPF6)n. This observation is
compatible with the formation of (1ꢂH+)nꢄ4 assemblies, in which
the terminal dodecylammonium groups reside within the cavity
of the chain stopper 4. In addition, in the region where the included
2 (Fig. 4a, see below).29 Similar behavior has previously been ob-
served for hydrogen-bonded supramolecular polymers.9c–e
n-Butylammonium hexafluorophosphate was selected as a po-
tential ‘plug’ for the ‘cavity-plugging’ chain-length control experi-
ment. 1H NMR spectra detailing the outcome of this titration are
shown in Figure 3. In this case, discrimination between monomer–
monomer cavity-included and n-BuNH3 ꢄ(1ꢂH+)n was made
+
possible by the appearance, for the latter species, of the distinctive
resonance of the methyl group of the included n-BuNHþ3 ion
+
NH3 hydrogen atoms resonate, the peak for the pentaether-in-
(d = ꢃ0.34 ppm), given that the peaks of the
a-, b-, and c-CH2s are
cluded ammonium group (d = 5.54 ppm) progressively decreased
in intensity, whereas two overlapping peaks (d = 6.10–6.14 ppm)
grew.28 Integration of the relevant peaks indicates that the average
degree of polymerization rapidly decreases during the early stages
of the titration, and then slowly stabilizes toward the final value of
either partiallyor fully overlapping. As expected, addition of increas-
ing amounts of n-BuNHþ3 PF6ꢃ led to the progressive filling of the cav-
ities, as demonstrated by the replacement of the (1ꢂH+)n diagnostic
peaks with those of n-BuNH3þꢄ(1ꢂH+)n. In this case, however, the in-
+
cluded NH3 group gave rise to a single new resonance. Again,

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C. Gargiulli et al. / Tetrahedron Letters 52 (2011) 6460–6464
13. Stewart, D. R.; Krawiec, M.; Kashyap, R. P.; Watson, W. H.; Gutsche, C. D. J. Am.
Chem. Soc. 1995, 117, 586–601.
integration of the relevant peaks showed a trend consistent with a
decrease in the average degree of polymerization, similar to that ob-
served in the ‘capping’ experiment (Fig. 4b). Similarly, beyond the
14. (a) Arnaud-Neu, F.; Fuangswasdi, S.; Notti, A.; Pappalardo, S.; Parisi, M. F.
Angew. Chem., Int. Ed. 1998, 37, 112–114; (b) Cafeo, G.; Gattuso, G.; Kohnke, F.
H.; Notti, A.; Occhipinti, S.; Pappalardo, S.; Parisi, M. F. Angew. Chem., Int. Ed.
2002, 41, 2122–2126; (c) Gattuso, G.; Notti, A.; Pappalardo, S.; Parisi, M. F.;
Pilati, T.; Resnati, G.; Terraneo, G.; Pilati, T. CrystEngComm 2009, 11, 1204–
1206.
15. (a) Garozzo, D.; Gattuso, G.; Kohnke, F. H.; Notti, A.; Pappalardo, S.; Parisi, M. F.;
Pisagatti, I.; White, A. J. P.; Williams, D. J. Org. Lett. 2003, 5, 4025–4028; (b)
Gattuso, G.; Notti, A.; Pappalardo, A.; Parisi, M. F.; Pisagatti, I.; Pappalardo, S.;
Garozzo, D.; Messina, A.; Cohen, Y.; Slovak, S. J. Org. Chem. 2008, 73, 7280–
7289.
point of equivalence the cavities of the monomers are almost fully
+
‘plugged’ with n-BuNH3 chain stoppers (i.e., Xn ¼ 2).30
ꢀ
In conclusion, new calix[5]arene-based supramolecular poly-
mers—that is, (1ꢂHCl)n, (1ꢂHPic)n, and (1ꢂHPF6)n—have been synthe-
sized, and their concentration- and counterion-dependence have
been investigated. In addition, we have demonstrated that the
chain-length control of these AB-type supramolecular polymers
can be efficiently achieved by independently acting on both termi-
nals of the monomeric unit. Addition of either a competitive recep-
tor—a polymerization-inert calix[5]arene—or a competitive guest—
a polymerization-inert linear alkylammonium ion—resulted in a
progressive decrease in the number-average degree of polymeriza-
tion, opening the way for greater control over the macroscopic prop-
erties of these novel materials. Further studies aimed at the
elucidation of the behavior of the chain stoppers at higher concen-
tration ranges are currently in progress.
16. Garozzo, D.; Gattuso, G.; Kohnke, F. H.; Malvagna, P.; Notti, A.; Occhipinti, S.;
Pappalardo, S.; Parisi, M. F.; Pisagatti, I. Tetrahedron Lett. 2002, 43, 7663–7667.
17. 31-[12-N-(Phthalimidododecyl)oxy]-32,33,34,35-tetra-hexadecyloxy)-
5,11,17,23,29-penta-tert-butylcalix[5]arene 3: Calix[5]arene
2
(200 mg,
0.18 mmol), 1-bromohexadecane (652 mg, 2.14 mmol) and K2CO3 (295 mg,
2.14 mmol) were suspended in anhydrous CH3CN (25 mL) and refluxed for
16 h, under vigorous stirring. Inorganic salts were filtered off and washed with
CHCl3. The combined filtrates were evaporated to dryness under reduced
pressure, and the residue was purified by column chromatography (SiO2,
petroleum ether-Et2O 8:1) to give 3 (225 mg, 62%) as a white powder. Mp 71–
73 °C; 1H NMR (300 MHz, CDCl3) d 0.90 (t, J = 7.0 Hz, CH2CH3, 12 H), 1.04, 1.05
and 1.08 (3 ꢁ s, ratio 1:2:2, C(CH3), 45 H), 1.20–1.55 (m, CH2, 122 H), 1.69 (bq,
J = 7.0 Hz, CH2,
2 H), 1.91 (bq, J = 7.0 Hz, CH2, 8 H), 3.26 and 4.56 (AX,
J = 14.0 Hz, ArCH2Ar, 10 H), 3.60–3.70 (m, NCH2 and OCH2, 12 H), 6.92, 6.94 and
6.97 (3 ꢁ s, ratio 1:2:2, Ar, 10 H), 7.69–7.72 and 7.84–7.87 (2 ꢁ m, Pht, 4 H)
ppm; 13C NMR (75 MHz, CDCl3), d 14.1, 22.7, 26.4, 27, 0, 28.7, 29.3, 29.4, 29.7,
29.8, 29.9, 30.0, 30.1, 30.2, 30.3, 30.6, 31.4, 31.9, 33.9, 38.0, 74.0, 123.1, 125.3,
125.4, 132.2, 133.8, 133.9, 144.4, 152.8, 168.4 ppm. ESI MS m/z (rel. int. %)
2045.5 ([M+Na]+, 100), and 2061.6 ([M+K]+, 78). Anal. Calcd for C139H225NO7: C,
82.55; H, 11.21; N, 0.69. Found: C, 82.31; H, 11.33; N, 0.68.
Acknowledgment
We are indebted to Dr. G. Grasso (CNR IBB, Catania, Italy) for the
acquisition of ESI MS spectra. MIUR (PRIN-2009A5Y3N9 project) is
gratefully acknowledged for financial support.
18. 31-[(12-Aminododecyl)oxy]-32,33,34,35-tetra-hexadecyloxy-5,11,
penta-tert-butylcalix[5]arene 1: A stirred mixture of calix[5]arene 3 (202 mg,
0.10 mmol) and hydrazine monohydrate (49 L, 1.0 mmol) in EtOH (20 mL)
17,23,29-
References and notes
l
was refluxed for 3 h. The solvent was evaporated under reduced pressure. The
residue was dissolved in CHCl3, washed with aqueous NaOH (5% w/w), dried
(Na2SO4), and the solution was evaporated to dryness. Precipitation of the
residue from CHCl3/MeOH afforded 1 (81 mg, 43%). Mp 78–80 °C; 1H NMR
(300 MHz, CDCl3) d 0.88 (t, J = 6.7 Hz, CH2CH3, 12 H), 1.02, 1.03 and 1.05 (3 ꢁ s,
ratio 1:2:2, C(CH3), 45 H), 1.20–1.60 (m, CH2, 122 H), 1.80–2.00 (m, CH2, 10 H),
2.67 (t, J = 6.8 Hz, CH2NH2, 2 H), 3.24 and 4.54 (AX, J = 14.1 Hz, ArCH2Ar, 10 H),
3.60–3.70 (m, OCH2, 10 H), 6.91, 6.92 and 6.93 (3 ꢁ s, ratio 1:2:2, Ar, 10 H)
ppm; 13C NMR (75 MHz, CDCl3), d 14.1, 22.7, 26.4, 27.0, 29.3, 29.4, 29.7, 29.8,
29.9, 30.0, 30.1, 30.2, 30.6, 31.4, 32.0, 33.9, 42.3, 74.1, 125.3, 125.4, 133.9,
144.4, 152.8 ppm. ESI MS m/z (rel. int. %) 1893.3 ([M+H]+, 100), and 1915.7
([M+Na]+, 3). Anal. Calcd for C131H223NO5: C, 83.15; H, 11.88; N, 0.74 Found: C,
82.87; H, 11.99; N, 0.69.
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19. The isochronous nature of these signals likely reflects the similar size of the
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20. A solution of the amino precursor 1 (30 mg in 5 mL of HCl-free CHCl3 was
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picric acid in H2O for 1ꢂHPic and 1 M HPF6/H2O for 1ꢂHPF6). The organic layer
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ꢀ
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NMR monitoring of the decrease in the average degree of polymerization.
27. Barrett, G.; McKervey, M. A.; Malone, J. F.; Walker, A.; Arnaud-Neu, F.; Guerra,
L.; Schwing-Weill, M.-J. J. Chem. Soc., Perkin Trans. 2 1993, 1475–1479.
28. Owing to the fact that the higher-field peak firstly grows in intensity and then
disappears, in favor of the lower-field one, these two peaks have been
tentatively assigned to the pentaester-included ammonium moieties of the
1ꢂH+ꢄ4 complex and the differently-sized (1ꢂH+)mꢄ4 (m >1) assemblies,
respectively.
10. Gutsche, C. D. Calixarenes
– An Introduction, 2nd ed. In Monographs in
Chemistry, Vol. 6, Stoddart, J. F. Ed.; The Royal Society of Chemistry: Cambridge,
2008.
11. (a) Castellano, R. K.; Nuckolls, C.; Eichhorn, S. H.; Wood, M. R.; Lovinger, A. J.;
Rebek, J., Jr. Angew. Chem., Int. Ed. 1999, 38, 2603–2606; (b) Xu, H.; Rudkevich,
D. M. Chem. Eur. J. 2004, 10, 5432–5442; (c) Podoprygorina, G.; Janke, M.;
Janshoff, A.; Böhmer, V. Supramol. Chem. 2008, 20, 59–69.
12. (a) Haino, T.; Matsumoto, Y.; Fukazawa, Y. J. Am. Chem. Soc. 2005, 127, 8936–
8937; (b) Ishihara, S.; Takeoka, S. Tetrahedron Lett. 2006, 47, 181–184; (c)
Haino, T.; Hirai, E.; Fujiwara, Y.; Kashihara, K. Angew. Chem., Int. Ed. 2010, 49,
7899–7903; (d) Guo, D.-S.; Chen, S.; Qian, H.; Zhang, H.-Q.; Liu, Y. Chem.
Commun. 2010, 2620–2622.
ꢀ
29. As far as the Xn determination is concerned, chain stoppers are considered
ꢀ
equivalent to monomers, so that a Xn ¼ 2 represents the complete capture of
the monomers by the relevant chain stoppers, with the exclusive formation of
1:1 1ꢂH+ꢄ4 complexes in this case.
30. It should be mentioned that, due to the poor solubility of n-BuNHþ3 PF6ꢃ, the last
two spectra of the titration (i.e., 20 mM, 1:2; 50 mM, 1:5) were recorded in the
ꢀ
presence of some undissolved salt. However, this had no influence on the Xn
calculations.