Ferrocene-containing side chain polyrotaxanes obtained by radical copolymerization of styrenes with acrylamide with a [2]rotaxane structure

Article abstract of DOI:10.1246/cl.2009.356

Radical copolymerization of styrenes and a [2]rotaxane monomer composed of ferrocene-containing N,N-dialkylacryl-amide and dibenzo[24]crown-8-ether (DB24C8) yielded side chain polyrotaxanes. Copyright

Full text of DOI:10.1246/cl.2009.356

356
Chemistry Letters Vol.38, No.4 (2009)
Ferrocene-containing Side Chain Polyrotaxanes Obtained by Radical Copolymerization of
Styrenes with Acrylamide with a [2]Rotaxane Structure
Yuji Suzaki, Shintaro Murata, and Kohtaro Osakadaꢀ
Chemical Resources Laboratory R1-3, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503
(Received January 20, 2009; CL-090071; E-mail: kosakada@res.titech.ac.jp)
Radical copolymerization of styrenes and a [2]rotaxane
monomer composed of ferrocene-containing N,N-dialkylacryl-
amide and dibenzo[24]crown-8-ether (DB24C8) yielded side
chain polyrotaxanes.
O
O
-
PF6
O
O
O
+
N
O
Cl
O
O
+
H2
Fe
O
O
R
O
O
R = H, Me
Polyrotaxanes which contain multiple rotaxane units in the
molecule have been reported to exhibit unique thermal and pho-
tochemical properties.1–4 Radical polymerization of acrylate was
employed to introduce the rotaxanes to the side chains of the
polymer; a mixture of acrylic monomers with a bulky stopper
and macrocyclic compounds such as cyclodextrin, crown ether,
and cyclic viologen produces the side chain polyrotaxanes in the
presence of radical initiator (Scheme 1(i)).5–8 These polyrotax-
anes, however, contain branches without rotaxane structure
due to insertion of monomer without pseudorotaxane structure
during the polymer growth. Polymerization of a monomer with
[2]rotaxane structure would produce polymers in which every
monomer unit contains a side chain with interlocked structure
(Scheme 1(ii)). In this paper, we report radical copolymerization
of styrenes and a [2]rotaxane composed of dibenzo[24]crown-8-
ether (DB24C8) and acrylamide derivative to produce the side
chain polyrotaxanes as well as the properties of the rotaxane-
functionalized polystyrenes.
1
ð1Þ
O
O
O
O
iPr2EtN
N
O
O
O
Fe
R
O
O
THF
O
O
O
Ha
2: R = H (48%)
3: R = Me (53%)
Copolymerization of 2 with styrene (4a) in the presence of
AIBN (azobisisobutyronitrile) at 60 ꢂC for 20 h in toluene pro-
duces the polyrotaxane 5a containing the two monomer units
randomly (eq 2). Figure 1b shows the 1H NMR spectrum of
5a. The signals at ꢀ 2.26 (Me), 1.59 (CH2-main chain), and
3.4–4.0 (CH2-DB24C8) indicate copolymerization of 2 and
4a.12 The ratio of the rotaxane unit to styrene unit, x, varies from
0.18–0.41 depending on the ratio of the monomer used, as sum-
marized in Table 1 run 1–4. Molecular weight (Mn) of the poly-
rotaxane 5a ranges from 2000 to 6700 with the molecular weight
distribution (Mw=Mn) of 1.35–1.54.
[2]Rotaxane monomer 2 was synthesized by acylation of the
nitrogen of cationic rotaxane [{(Fe(C5H4)(C5H5))CH2NH2CH2-
C6H4-4-OCH2CH2CHCHCOO-C6H3-3,5-Me2}(DB24C8)]-
(PF6) (1) with acryloyl chloride in THF/i-Pr2EtN (eq 1).9–11
A
O
O
similar reaction of methacryloyl chloride with 1 gave 3. The IR
spectrum of 2 shows peaks at 1728 and 1647 cmꢁ1, which are
assigned to C=O stretching vibration of the ester and amide
groups, respectively. Figure 1a shows the 1H NMR spectrum
of 2. The 1H NMR signals at ꢀ 5.63 and 5.76 are assigned to vi-
nyl hydrogen (Ha in eq 1) of 2 with s-cis and s-trans conforma-
tional structures of the amide group. Rotaxane 2 keeps its inter-
locked structure in toluene-d8 and dmso-d6 ([2] = 5 mM) at
100 ꢂC for 24 h, which was evidenced by 1H NMR spectroscopy.
O
O
N
O
+
O
O
Fe
O
O
O
Xn
O
O
4a: Xn = -4-H
4b: Xn = -3,5-(CF3)2
4c: Xn = -4-Br
4d: Xn = -4-Me
4e: Xn = -4-t-Bu
4f: Xn = -4-OMe
2
ð2Þ
AIBN
x
1-x
n
Fe
O
Xn
N
O
n
toluene
60 o
O
Random copolymer
O
O
C
+
(i)
O
O
O
O
5a: Xn = -4-H
5b: Xn = -3,5-(CF3)2
O
Polyrotaxane
5c: Xn = -4-Br
5d: Xn = -4-Me
5e: Xn = -4-t-Bu
5f: Xn = -4-OMe
O
O
n
Reaction of 2 in the presence of AIBN results in recovery of
unreacted monomer (run 5), probably due to the steric hindrance
of DB24C8 which prevents homopolymerization of the axle
molecule. Copolymerization of 2 with substituted styrenes
4b–4f yields corresponding polyrotaxanes 5b–5f (run 6–10) in
(ii)
[2]Rotaxane
Scheme 1. Synthetic strategies toward side chain polyrotaxanes.
Polyrotaxane
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.4 (2009)
357
tained polyrotaxane shows higher thermal stability than rotaxane
monomer. This method provides a new means for obtaining side
chain polyrotaxane with high incorporation ratio of macrocyclic
unit to polymer branches.14
Me
(a)
CH2-DB24C8
OCH2-axle
This work was supported by a Grant-in-Aid for Scientific
Research for Young Scientists from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (No.
19750044) and by Global COE Program ‘‘Education and Re-
search Center for Emergence of New Molecular Chemistry.’’
*
C(=O)CHCH
Ha
(b)
References and Notes
1
Reviews on polyrotaxanes: a) H. W. Gibson, M. C. Bheda,
P. T. Engen, Prog. Polym. Sci. 1994, 19, 843. b) F. M.
Raymo, J. F. Stoddart, Chem. Rev. 1999, 99, 1643. c) A.
Harada, Acc. Chem. Res. 2001, 34, 456. d) G. Wenz, B.-H.
8
6
4
2
δ
Figure 1. 1H NMR spectra of (a) 2, and (b) 5a (run 3 of Table 1)
(300 MHz, CDCl3, rt). The peak with asterisk indicates residual
solvent (CHCl3).
Han, A. Muller, Chem. Rev. 2006, 106, 782.
¨
2
3
H. W. Gibson, S. Liu, P. Lecavalier, C. Wu, Y. X. Shen,
J. Am. Chem. Soc. 1995, 117, 852.
a) I. Yamaguchi, K. Osakada, T. Yamamoto, J. Am. Chem.
Soc. 1996, 118, 1811. b) I. Yamaguchi, K. Osakada, T.
Yamamoto, Macromolecules 1997, 30, 4288.
M. Tamura, A. Ueno, Bull. Chem. Soc. Jpn. 2000, 73, 147.
O. Noll, H. Ritter, Macromol. Chem. Phys. 1998, 199, 791.
H. W. Gibson, W. S. Bryant, S.-H. Lee, J. Polym. Sci., Part
A: Polym. Chem. 2001, 39, 1978.
a) T. Takata, H. Kawasaki, S. Asai, N. Kihara, Y. Furusho,
Chem. Lett. 1999, 28, 111. b) T. Takata, H. Kawasaki, N.
Kihara, Y. Furusho, Macromolecules 2001, 34, 5449.
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2004, 36, 927.
Table 1. Copolymerization of styrene derivatives and rotaxane 2a
[styrene] Yield
/%b
Td5
/ꢂCe,f
Run Styrene
Mn (Mw=Mn)c
xd
/[2]
1
2
3
4
5
6
7
8
9
10
4a
4a
4a
4a
4a
4b
4c
4d
4e
4f
4.0
2.3
1.5
1.0
0
1.5
1.5
1.5
1.5
1.5
70
63
93
32
n.r.g
65
50
59
69
2000 (1.51) 0.41
3100 (1.54) 0.24
6700 (1.35) 0.18
2600 (1.38) 0.27
259
277
249
189
—
279
273
283
315
281
4
5
6
—
—
7
3700 (1.56) 0.27
6700 (1.34) 0.25
3600 (1.54) 0.30
5400 (1.58) 0.18
12000 (2.30) 0.18
8
9
75
a) Y. Suzaki, K. Osakada, Chem. Lett. 2006, 35, 374. b) Y.
Suzaki, K. Osakada, Dalton Trans. 2007, 2376. c) Y. Suzaki,
T. Taira, K. Osakada, M. Horie, Dalton Trans. 2008, 4823.
aReaction conditions: styrene + 2 = 0.100 mmol, AIBN =
6 mmol, solvent = toluene (0.1 mL), reaction time = 20 h, at
60 ꢂC. bThe yields of polymers were calculated on the basis of
the total amount of the used monomers. cDetermined by GPC based
on polystyrene standard. dIncorporation ratio of rotaxane unit
10 a) N. Kihara, Y. Tachibana, H. Kawasaki, T. Takata, Chem.
Lett. 2000, 29, 506. b) Y. Tachibana, H. Kawasaki, N.
Kihara, T. Takata, J. Org. Chem. 2006, 71, 5093.
f
estimated by 1H NMR. e5% weight loss temperature. Td5 of
DB24C8 = 295 ꢂC. gNo reaction.
11 To a solution of 1 (3.23 g, 2.9 mmol) in THF (10 mL) were
added i-Pr2EtN (0.88 g, 6.8 mmol) and acryloyl chloride
(0.65 g, 7.2 mmol). The mixture was stirred for 2 h at room
temperature, followed by evaporation of solvent. Crude
product was dissolved in CHCl3, washed with water and
dried over MgSO4. Further purification by SiO2 column
chromatography (hexane/AcOEt = 5/1 then 1/1) gave 2
(1.48 g, 1.4 mmol, 48%) as a yellow solid.
12 A mixture of 2, styrene 4a, and AIBN (6 mmol) was dis-
solved in toluene (0.1 mL). The total amount of 2 and styrene
was set to 0.10 mmol. The mixture was degassed and stirred
at 60 ꢂC for 20 h, followed by addition of CH2Cl2 (0.1 mL).
The reaction mixture was added to hexane to cause separa-
tion of yellow solid which was collected by filtration and
dried in vacuo to give polyrotaxane 5a as a pale yellow solid.
A part of monomer 2 was recovered by evaporation of the
filtrate.
moderate yields (50–75%) with the incorporation ratio of rotax-
ane unit, x, of 0.18–0.30. Molecular weight of polyrotaxane
varies depending on the substituent group on the aromatic ring.
Copolymerization of 2 with 4f bearing electron-donating para-
methoxy group yields 5f with high molecular weight (Mn ¼
12000, Mw=Mn ¼ 2:30). Methacrylamide with the rotaxane
structure, 3, does not undergo polymerization nor copolymeriza-
tion with styrene in the presence of AIBN.
Thermal gravimetric analyses (TGA) of polyrotaxanes 5a–
5f (run 3 and 6–10) show 5% weight loss in the range of
Td5 ¼ 249{315 ꢂC which is higher than that of monomer 2
(244 ꢂC) and 3 (227 ꢂC), indicating that thermal stability of
rotaxane units was improved by incorporation to the thermally
stable polystyrene.13 The cyclic voltammogram (CV) of 5a in
CH2Cl2 solution of n-Bu4NPF6 shows a redox peak pair of
FeIII/FeII of the ferrocenyl group at E1=2 ¼ 0:08 V (vs Fcþ/Fc
(Fc = ferrocene), scan rate = 0.1 V sꢁ1). The redox potential
is slightly higher than that of 2 (E1=2 ¼ 0:06 V).
13 Degradation of polystyrene (Mn ¼ 3600) was reported to
start above 314 ꢂC. See: B. V. Kokta, J. L. Valade, W. N.
Martin, J. Appl. Polym. Sci. 1973, 17, 1.
14 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
In summary, we succeeded in synthesis of the side chain
polyrotaxanes by using [2]rotaxane monomer bearing a radically
polymerizable acrylamide as a comonomer with styrenes. Ob-
Published on the web (Advance View) March 14, 2009; doi:10.1246/cl.2009.356