Directed one-pot syntheses of crown ether wheel-containing main chain-type polyrotaxanes with controlled rotaxanation ratios

Article abstract of DOI:10.1039/c4cc06943a

The directed synthesis of main chain-type polyrotaxanes possessing crown ether wheels was successfully achieved through two methods, A and B. Method A involved the direct wheel threading of poly(sec-ammonium salt) followed by end-capping with a bulky group, while method B utilized polyaddition of a pseudo[2]rotaxane monomer to facilitate the control of the structure, i.e. the rotaxanation ratio.

Full text of DOI:10.1039/c4cc06943a

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Directed one-pot syntheses of crown ether
wheel-containing main chain-type polyrotaxanes
with controlled rotaxanation ratios†
Cite this: Chem. Commun., 2014,
50, 15341
Received 3rd September 2014,
Accepted 10th October 2014
Kazuko Nakazono, Tomonori Ishino, Tomoyuki Takashima, Daisaku Saeki,
Daisuke Natsui, Nobuhiro Kihara and Toshikazu Takata*
DOI: 10.1039/c4cc06943a
www.rsc.org/chemcomm
The directed synthesis of main chain-type polyrotaxanes possessing synthesis of CE-PRX with controlled structures. We report herein
crown ether wheels was successfully achieved through two methods, the directed one-pot synthesis of CE-PRXs by two reliable synthetic
A and B. Method A involved the direct wheel threading of poly(sec- protocols to yield structure-definite PRXs with controlled number
ammonium salt) followed by end-capping with a bulky group, while of wheel components and effective end-cap groups (Fig. 1), building
method B utilized polyaddition of a pseudo[2]rotaxane monomer to on our recent results on the synthesis of dibenzo-24-crown-8-
facilitate the control of the structure, i.e. the rotaxanation ratio.
ether (DB24C8)-containing PRXs.14 We also describe the simple
modification to nonionic PRX that enables size-exclusion-
The first synthesis of a polypseudorotaxane (PPRX) from cyclodextrin chromatographic (SEC) measurements.
(CD) and a linear polymer by Ogata1 and its end-capping to give a
Fig. 1 shows the two strategies for the synthesis of CE-PRXs.
polyrotaxane (CD-PRX) by Harada2 heralded significant progress Method A consists of the initial wheel threading of poly(sec-
in the science of main chain-type CD-PRX, opening the door to ammonium salt) and subsequent end-cap with a bulky agent. In
various applications3 such as cross-linked gels.4 Efficient syntheses the method B, copolymerization of pseudo[2]rotaxane monomer
of CD-PRX have now led to its actual use.5 However, little is known and appropriate comonomer to CE-PRX by the successive end-cap
about PRXs having other wheel components,6,7 in particular crown to facilitate the control of the structure, i.e. the rotaxanation ratio.
ether (CE) wheels, despite Gibson’s pioneering work on PRXs.8
Method A: direct wheel threading of poly(sec-ammonium salt)
Therefore, the synthesis of PRX (not poly[2]rotaxane9) with a suffi- followed by end-capping. We started with this synthetic protocol
ciently high or controlled CE content and effective end-cap groups is because rotaxane synthesis by this route is usually known to
an important challenge in this area.8a,10 CE–ammonium-salt-type proceed efficiently in one-pot,15 and it seems to be suitable for giving
rotaxane chemistry has enabled the synthesis of end-capped [5]-6d high-molecular-weight PRX. Although poly(sec-ammonium salt)
and [20]rotaxanes;6a,b however, both rotaxanes exhibit low solubility
because of their oligoionic structures. Since CE–ammonium
interaction-driven self-assembly often achieves accurate supra-
molecular systems, the greater ease with which functional
groups can be introduced into CE as compared to CD, as has
been proven in rotaxane chemistry,11 strongly indicates the
potential utility of CE-based main chain-type PRX (CE-PRX),
and hence, its synthesis becomes of paramount importance.
We have long studied the development of an effective synthetic
method for CE-PRX with a controlled structure, and have found
effective synthetic methods.12 A recent Grubbs’ simple approach to
CE-PRX via the polymerization of a CE-based pseudo[2]rotaxane
monomer,13 has prompted us to report our results on the successful
Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan.
E-mail: ttakata@polymer.titech.ac.jp
Fig. 1 Two independent strategies for preparing CE-based main chain-
type PRXs via one-pot reaction.
† Electronic supplementary information (ESI) available: Experimental procedures,
spectra, SEC profiles, and DSC profiles. See DOI: 10.1039/c4cc06943a
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Table 1 Synthesis of PRX 3 by method A
and the corresponding CE-PRX seemed insoluble in ordinary
organic solvents because of their polyionic structures, we expected
to observe CE-PRX formation because the oligorotaxane is more
soluble than its axle polymer precursor oligo(sec-ammonium salt).6a,b
We chose poly(hexamethyleneammonium PF6) (PHMAP) as the axle
polymer because the hexamethylene spacer distance between the
ammonium groups is appropriate for a high-yield synthesis of
Entry
Solvent
Yield of PRX 3a (%)
RRb (%)
1
2
3
4
CHCl3
ClCH2CH2Cl
CH3CN
33
58
68
76
90
98
92
98
CH3NO2
a
b
Calculated on the basis of 1. Rotaxanation ratio determined by
[3]rotaxane.6e PHMAP was obtained by the reduction of 6,6-nylon 1H NMR (range of error Æ2%).
with BH3–THF complex16 followed by treatment with NH4PF6.
PHMAP was relatively insoluble in organic solvents and the corres-
ponding PPRX was obtained in only a minimal yield with DB24C8 in
a CHCl3–CH3CN mixed solvent system. To address this problem,
benzyl ammonium salt moieties were introduced at both PHMAP
termini to enhance complexation capability.15,17 4-Hydroxymethyl-
benzyl-terminated PHMAP 1 was prepared by condensation of
amine-terminated 6,6-nylon and methyl 4-formylbenzoate followed
by reduction and subsequent treatment with NH4PF6. The molecular
enter into the center of the axle polymer chain. For PRX 3,
neutralization by N-acetylation,19 which is required for estimating
Mn, was incomplete, presumably because CE is densely packed
because of its short CE–CE distance.6c To address this issue, we
turned to method B.
Method B: step polymerization of a reactive pseudo[2]rotaxane
monomer followed by end-capping. We designed this synthetic
protocol because of the reported efficiency of end-capping
pseudo[2]rotaxane to give [2]rotaxane15 in addition to the high
solubility of the monomer.6e,20
1
weight of 1 was calculated by H NMR to be Mn = 3200. One-pot
synthesis of PRX 3 via intermediate PPRX 2 was carried out as
follows (Scheme 1): a heterogeneous mixture of 1 and DB24C8 was
allowed to stand and then 3,5-dimethylphenylisocyanate and a
catalytic amount of dibutyltindilaurate (DBTDL) in solvent at room
temperature was added to it15 (Table 1). The structure of PRX 3 was
confirmed by spectroscopic analyses.
The reactions proceeded under heterogeneous conditions
even with polar solvents such as CH3CN and CH3NO2 (Table 1).
For PRX 3, the yield increased to 76% with increasing solvent
polarity and hence the solubility of PPRX 2 during the end-
capping process. For 1, the rotaxanation ratio (RR) of the NH2
group was sufficiently high (492%) without solvent depen-
dency. These results suggest the following important reaction
features: (i) the formation of PPRX 2 is accelerated by introdu-
cing a more strongly interactive unit (benzyl group) with CE at
the axle termini;18 (ii) complexation between 1 and CE proceeds
under heterogeneous conditions; and (iii) the CE wheel can
We chose a dibenzylammonium-type axle 417 because of the
enhanced stability of the corresponding pseudorotaxane monomer.
One-pot synthesis of PRX 6 was carried out as follows (Scheme 2): a
mixture of pseudo[2]rotaxane (derived in situ from 4 and DB24C8)
and diphenylmethane diisocyanate was treated with DBTDL
in CH2Cl2. The resulting PPRX 5 was treated successively with
3,5-dimethylphenylisocyanate and 3,5-dimethylphenol to give
PRX 6 selectively in high yield. The RR value and Mn were deter-
mined by 1H NMR (Table 2).
Inspection of Table 2 reveals that method B is versatile
and gives the desired CE-based PRXs in high yield. The RR
values were controllable by the DB24C8 feed ratio; for example,
0.5–2.0 equiv. of DB24C8 yielded PRX 6 with RR values in the
range 55–95%. The end-capped structure of PRX 6 was con-
firmed by a decomposition study of PRX 6 in DMSO-d6, which
indicated that no DB24C8 was liberated from PRX 6 at ambient
temperature.21 The structure of PRX 6 was determined using
NMR, IR, and SEC. The 1H NMR spectra clearly indicate the
Scheme 1 Synthesis of PRX 3 and PRX 3Ac (0 r n r 1).
15342 | Chem. Commun., 2014, 50, 15341--15344
Scheme 2 Synthesis of PRX 6 and PRX 6Ac (0 r m r 1).
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Table 2 Controlled synthesis of PRX 6 by method Ba
PRX 6
(NMR) PRX 6Acd (SEC)
DB24C8/NH2 Yieldb RRc Mn/
Mn/
Mw/
Entry (equiv.)
(%)
(%) 104/Da 104 /Da 104/Da PDI
Fig. 3 Photographs of a transparent thin film of PRX 6 (RR = 95%)
prepared by casting from the acetone solution.
1
2
3
2.0
1.0
0.5
93
84
84
95
90
55
2.6
2.5
1.4
2.8
4.2
3.3
6.0
9.3
7.4
3.6
2.2
2.2
a
b
c
4 = 0.5 M. Calculated on the basis of 4. Determined by 1H NMR
In addition, the high molecular weight of PRX 6 was confirmed
by its good film-forming property, which resulted in the for-
mation of flexible transparent films by casting from the acetone
solutions (Fig. 3).22
d
(range of error Æ2%). Estimated by SEC (DMF, PSt) after N-acetylation.
involvement of the rotaxane structure, with DB24C8 localized at the
ammonium moieties (Fig. 2). All signals are reasonably assignable;
the two signals around 4.6 (A) and 4.8 (a) ppm indicate uncom-
plexed and complexed N-benzyl protons, respectively, whose ratio
corresponds to the RR value.
In conclusion, we have successfully accomplished the directed
one-pot synthesis of a challenging target polymer in the PRX family:
CE-based main chain-type PRXs. We showed that CE-PRXs are
unusually soluble in typical organic solvents such as DMF, despite
their polyionic structures. Method B, involving step polymerization
of pseudo[2]rotaxane monomers, enables the controlled synthesis
of PRXs possessing the desired rotaxanation ratios (RR values).
Complete neutralization by the N-acetylation of the ammonium
moieties that promotes the CE translation on the axle23 proceeded
remarkably well. A variety of modifications of CEs established so far
enable versatile functionalization of PRXs. Thus, the present study
heralds continuing rapid progress in the synthesis and application
of CE-PRXs.
To remove the polyionic nature from PRX 6, we subjected PRX
6 to N-acetylation with acetic anhydride and triethylamine.19 The
acetylation reaction proceeded very efficiently to yield PRX 6Ac
(93% yield, Table 2, entry 1). Table 2 shows the SEC results
of PRX 6Acs with different rotaxanation ratios (RR). All PRX
6Acs gave unimodal SEC profiles to support the occurrence of
complete N-acetylation of the ammonium moieties of PRX 6s,
agreeing with other spectral data.22 The H NMR spectrum of
1
PRX 6Acs suggested two sets of signals assignable to the protons
around the urethane moiety, although the axle component is
symmetric. Thus, the conversion of the ammonium moiety caused
the positional change of the crown ether wheel from the ammonium
moiety to the urethane moiety probably due to the hydrogen
bonding interaction as same as the model [2]rotaxane S9.22 This is
the first synthesis of a nonionic DB24C8-based main chain-type
polyrotaxane. The solubility of the PRXs in typical organic solvents
was investigated. PRX 3 was soluble in some polar solvents such as
DMF and CH3CN, whereas PRX 6 and PRX 6Ac showed the higher
solubility to various solvents including acetone, as we expected.22
This work was financially supported by a Grant-in-Aid for Scientific
Research from MEXT, Japan (No. 18064008 and 23245031), and
K. N. thanks the Global COE program (Education and Research
Center for Material Innovation) for the support.
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