Mechanically interlocked mechanophores by living-radical polymerization from rotaxane initiators

Article abstract of DOI:10.1021/ol200801b

Two [2]rotaxane initiators for single-electron-transfer living-radical-polymerization were synthesized and used for the controlled polymerization of methyl acrylate. The mechanically interlocked polymers exhibited distinct responses to mechanical activati

Full text of DOI:10.1021/ol200801b

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2706–2709
Mechanically Interlocked Mechanophores
by Living-Radical Polymerization from
Rotaxane Initiators
Ragnar S. Stoll, Douglas C. Friedman, and J. Fraser Stoddart*
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208-3113, United States
stoddart@northwestern.edu
Received March 26, 2011
ABSTRACT
Two [2]rotaxane initiators for single-electron-transfer living-radical-polymerization were synthesized and used for the controlled polymerization of
methyl acrylate. The mechanically interlocked polymers exhibited distinct responses to mechanical activation by ultrasound. Monitoring the fate
of the rotaxanes’ charge transfer absorption bands provides evidence for preferential mechanical degradation of a midsection rotaxane unit as
compared to a terminal rotaxane entity as a consequence of mechanical forces accumulating in the central region of the polymer chain.
Over the past couple of decades, interest in “smart”
materials1 containing mechanically interlocked com-
ponents2 hasincreased dramatically becauseoftheirability
to function as nanoswitches and nanomachines. Numer-
ous examples of polymers based on rotaxanes and cate-
nanes ; the best known of the mechanically interlocked
molecules (MIMs) ; have been described3 in the litera-
ture. In practice, MIMs can be incorporated either into a
polymer’s main chain or appended to side-chains, attached
to a polymer’s backbone. Based on the constitutions and
sitings of these MIMs, four major architectures can be
distinguished: they are main-chain [n]rotaxanes4 and
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r
10.1021/ol200801b
Published on Web 04/27/2011
2011 American Chemical Society

[n]catenanes,5 and poly-([2]rotaxanes)6 and -([2]catenanes).7
Their physical as well as their chemical properties are influ-
enced largely by the constitutions and relative dispositions of
their component MIMs. By contrast, examples of the in-
corporation of a single MIM component into a polymer
chain are few and far between.8 Such polymers are unique,
however, since they offer the possibility to investigate the
effect of the surrounding polymeric environment upon
MIMs.
a single rotaxane in the middle of the polymer chain,
whereas polymers with a single rotaxane end-group are
accessible by employing the monofunctional initiator 24þ
.
Scheme 1. Synthesis of Initiators 1 4PF6 and 2 4PF6
3
3
In this communication, we describe the synthesis
of cyclobis(paraquat-p-phenylene)-derived9 [2]rotaxane
initiators and their use in the single-electron-transfer
living-radical-polymerization10 (SET-LRP) of methyl
acrylate, leading to well-defined polymers with exactly
one MIM per polymer chain. The transfer of mechanical
force to these single rotaxanes during sonication
experiments11 was investigated in order to probe for
the potential use of MIM-based structures as a new class
of mechanophores.11f
Polymer cleavage by acoustic activation is a nonrandom
process and is known12 to occur preferentially in the
midsection of a polymer chain. To investigate the influence
of the rotaxane’s position along the polymer chain, we
synthesized (Scheme 1) two different mechanically inter-
locked initiators 1 4PF6 and 2 4PF6. SET-LRP using the
3
3
bifunctional initiator 14þ is expected to give polymers with
(7) Main-chain [2]catenanes: (a) Hamers, C.; Raymo, F. M.; Stoddart,
J. F. Eur. J. Org. Chem. 1998, 2109–2117. Side-chain [2]catenanes:
(b) Olson, M. A.; Braunschweig, A. B.; Fang, L.; Ikeda, T.; Klajn, R.;
The rotaxane initiators 1 4PF6 and 2 4PF6 were synthe-
Trabolsi, A.; Wesson, P. J.; Benıtez, D.; Mirkin, C. A.; Grzybowski,
´
3
3
B. A.; Stoddart, J. F. Angew. Chem., Int. Ed. 2009, 48, 1792–1797.
(c) Olson, M. A.; Coskun, A.; Fang, L.; Basuray, A. N.; Stoddart, J. F.
Angew. Chem., Int. Ed. 2010, 49, 3151–3156.
sized by a templated “threading-followed-by-stoppering”
approach13 taking advantage of the high functional group
tolerance and remarkable efficiency of the Cu-catalyzed
azideꢀalkyne cycloaddition (CuAAC) “click” chemistry.14
This synthetic strategy involves (Scheme 1) the initial for-
mation of a pseudorotaxane between the electron-rich 1,5-
dioxynaphthalene (DNP) unit in the half-dumbbell 3 and
the electron-poor tetracationic cyclophane present in either
4 4PF6 or 5 4PF6, followed by reaction with the propargyl
(8) For one of the very few examples, see: Leigh, D. A.; Morales, M.
ꢁ
A. F.; Perez, E. M.; Wong, J. K. Y.; Saiz, C. G.; Slawin, A. M. Z.;
ꢁ
Carmichael, A. J.; Haddleton, D. M.; Brouwer, A. M.; Buma, W. J.;
ꢁ
Wurpel, G. W. H.; Leon, S.; Zerbetto, F. Angew. Chem., Int. Ed. 2005,
44, 3062–3067.
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Stoddart, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27,
1547–1550. (b) Anelli, P.-L.; Ashton, P. R.; Ballardini, R.; Balzani, V.;
Delgado, M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.;
Pietraszkiewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.;
Spencer, N.; Stoddart, J. F.; Vicent, C.; Williams, D. J. J. Am. Chem.
3
3
ether 6 to (i) prevent dissociation of the pseudorotaxane by
stoppering with a bulky aryl group to give either 1 4PF6 or
ꢁ
Soc. 1992, 114, 193–218. (c) Asakawa, M.; Dehaen, W.; L’abbe, G.;
3
Menzer, S.; Nouwen, J.; Raymo, F. M.; Stoddart, J. F.; Williams, D. J.
J. Org. Chem. 1996, 61, 9591–9595. (d) Doddi, G.; Ercolani, G.;
Mencarelli, P.; Piermattei, A. J. Org. Chem. 2005, 70, 3761–3764.
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101, 3689–3745. (b) Percec, V.; Popov, A. V.; Ramirez-Castillo, E.;
Monteiro, M.; Barboiu, B.; Weichold, O.; Asandei, A. D.; Mitchell,
C. M. J. Am. Chem. Soc. 2002, 124, 4940–4941. (c) Ouchi, M.;
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X.; Rosen, B. M.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2010,
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2 4PF6 and (ii) introduce the desired number of R-bromo-
isobutyrate initiation sites.
3
Subsequent polymerization of methyl acrylate using
initiators 1 4PF6 and 2 4PF6 proceeded smoothly to yield
(Scheme 2) well-defined polymers 7 4PF6 and 8 4PF6,
3
3
3
3
respectively. Following the same protocol, polymer 9 was
synthesized as a control compound from dumbbell
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K. S.; Price, G. J. Annu. Rev. Mater. Sci. 1999, 29, 295–326. (c) Cravotto,
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Stoddart, J. F. J. Am. Chem. Soc. 2006, 128, 10388–10390. (b) Miljanic,
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O. S.; Dichtel, W. R.; Mortezaei, S.; Stoddart, J. F. Org. Lett. 2006, 8,
4835–4838. (c) Aprahamian, I.; Dichtel, W. R.; Ikeda, T.; Heath, J. R.;
Stoddart, J. F. Org. lett. 2007, 9, 1287ꢀ1290. (d) Braunschweig, A. B.;
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Dichtel, W. R.; Miljanic, O. S.; Olson, M. A.; Spruell, J. M.; Khan, S. I.;
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43, 466ꢀ474.
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T.; Kato, T.; Stoddart, J. F. Bull. Chem. Soc. Jpn. 2007, 80, 1856–1869.
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der Hoff, B. M. E. J. Macromol. Sci., Part A 1973, 7, 1695–1719.
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Org. Lett., Vol. 13, No. 10, 2011
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Scheme 2. Synthesis of Mechanically Interlocked Polymers 7 4PF6 and 8 4PF6 and the Control Polymer 9
3
3
initiator 10.15 The catalytically active Cu(0) species for
SET-LRP was generated in situ from Cu-wire and tris-
(2-(dimethylamino)ethyl)amine10e (Me6TREN). Gel per-
magnitude higher.16 Sonication (∼6 W, 1 h, 0 °C) of
polymers 74þ, 84þ and 9 in THF induced partial degrada-
tion of the polymer backbones. GPC analyses of the
sonicated samples confirmed (Table 1) decreases in mole-
cular weights for all polymers. Profound differences, how-
ever, in the optical properties of polymers 74þ, 84þ, and 9
were observed. The absorption intensity of polymer 74þ at
529 nm decreased (Figure 2a) by a factor of 0.32 (67%
decrease), indicating a substantial reduction in the CT
interactions. By contrast, a smaller decrease (Figure 2b)
by a factor 0.83 (15% decrease) was observed for polymer
84þ containing a terminal rotaxane. The absorption spec-
trum (Figure 2c) of control polymer 9 remained essentially
unchanged. The larger decrease in the CT absorption band
meation chromatography (GPC) of polymers 7 4PF6,
3
8 4PF6 and 9 in THF confirmed (Figure 1) good control
of their molecular weight distributions during SET-LRP.
The narrow molecular weight distribution (Table 1) of the
3
Table 1. Comparison of polydispersity indices and molecular
weights for polymers 7 4PF6, 8 4PF6 and 9 before and after
3
3
sonication (∼6 W, 1 h, 0 °C) in THF
7 4PF6 8 4PF6
9
3
3
before: after: before: after: before: after:
PDIa
Mn (kDa)a
Mw (kDa)a
1.8
85
1.8
47
85
1.8
180
330
1.7
100
170
1.1
90
94
1.1
62
66
150
a GPC in THF using a refractive index detector.
control polymer 9 and a good match between the targeted
and obtained degree of polymerization (925 vs 1050)
constitute evidence for the living character of the polymer-
ization. Incorporation of the rotaxane initiators in poly-
mers 7 4PF6 and 8 4PF6 was confirmed by UV/vis
3
3
spectroscopy in THF. A characteristic charge-transfer
(CT) absorption associated with the interaction between
the DNP unit and the bipyridinium units on the cyclo-
phane was observed centered on 530 nm. Whereas the
fluorescence from the DNP units is efficiently quenched by
the CT interactions present in 7 4PF6 and 8 4PF6, the
Figure 1. GPC traces of polymers (a) 7 4PF6, (b) 8 4PF6 and (c)
3
3
3
3
9 in THF. The tailing observed at higher molecular weights for
7 4PF6, and 8 4PF6 can be attributed to partial aggregation ꢀ
emission intensity of polymer 9 is at least 2 orders of
3
3
leading (Table 1) to broader molecular weight distributions ꢀ of
(15) Dumbbell initiator 10 was synthesized in good yield by reacting
the known DNP diazide derivative with propargyl ether 6. See ref 12b.
(16) See Supporting Information.
the polycationic rotaxane units in THF.
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Org. Lett., Vol. 13, No. 10, 2011

Figure 2. UV/vis spectra of (a) 7 4PF6, (b) 8 4PF6 and (c) 9 before (dotted lines) and after (solid lines) sonication (THF, ∼6 W, 1 h,
3
3
0 °C). The spectra were recorded in THF and are corrected for the concentration of the polymer (insets: zoom in region from 400ꢀ700 nm).
intensity for polymer 74þ compared with that of polymer 84þ
can be ascribed to the higher efficiency of mechanical activa-
tion in the midsection of the polymer chain, resulting in
increased mechanical degradation and hence breaking of the
centrally located rotaxane unit.12 The rotaxane end-group of
84þ is less prone to degradation since mechanical forces are
less efficiently transferred to the mechanically interlocked unit.
Interestingly, the substantial decrease in CT absorption
by 74þ is not reflected in a corresponding increase of emission
intensity arising from a potentially liberated DNP unit. It is
not unlikely that alternative deactivation pathways for the
emission lead to this observation since GPC provides clear
evidence (Table 1) for a decrease in the molecular weight
during sonication and absorption spectra (Figure 2) support
a substantial interruption of the CT interactions originally
present in 74þ. Furthermore, the emission intensity of
control polymer 9 decreased by only 19% after sonication,
presumably because of the partial destruction of the central
DNP unit under these conditions.15 Assuming a similar
extent of sonication-induced degradation of liberated DNP
moieties during sonication of polymer 74þ ; and to a
smaller extent in the case of polymer 84þ ; a much higher
emission intensity would be expected in the absence of
alternative deactivation pathways. We conclude, therefore,
that the mechanically interlocked midsection of polymer 74þ
is efficiently destroyed during application of mechanical
stress, leading to a loss of CT interactions with a concomitant
destruction of the DNP unit. Consequently, a less efficient
to cause an even smaller increase in fluorescence intensity.
Nonetheless, destruction of polymer chains is also observed for
84þ and 9.
In summary, we designed, synthesized and character-
ized poly(methyl acrylate) polymers grown from mech-
anically interlocked initiators under SET-LRP condi-
tions with good control over molecular weights and
polydispersities. The potential of mechanically inter-
locked moieties as mechanophores has been investigated
and efficient transfer of mechanical forces to a rotaxane
incorporated into the midsection of a polymer chain has
been demonstrated.
Acknowledgment. This research was supported by the
Non-Equilibrium Energy Research Center (NERC), an
Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award Number DE-SC0000989 and
also by the Air Force Office of Scientific Research (AFOSR)
under their Multidisciplinary University Research Initiative
(MURI) award number FA9550-07-1-0534. R.S.S. is in-
debted to the Alexander von Humboldt-Foundation for a
postdoctoral fellowship. We thank Matthew Duch and the
research group of Professor Mark Hersam at Northwestern
University for access to sonication equipment.
Supporting Information Available. Experimental details,
spectroscopic characterization data for all new compounds,
and GPC analysis results. This material is available free of
charge via the Internet at http://pubs.acs.org.
degradation of the rotaxane end-group in 8 4PF6 is expected
3
Org. Lett., Vol. 13, No. 10, 2011
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