One-pot synthesis of polyrotaxanes via acyclic diene metathesis polymerization of supramolecular monomers

Article abstract of DOI:10.1021/ja208515r

A one-pot synthesis of polyrotaxanes has been developed. The method employs a supramolecular monomer comprising a polymerizable ammonium salt and crown ether, in combination with dynamic ADMet polymerization. Ultimately, highly efficient complexation, pol

Full text of DOI:10.1021/ja208515r

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One-Pot Synthesis of Polyrotaxanes via Acyclic Diene Metathesis
Polymerization of Supramolecular Monomers
Nebojꢀsa Momꢀciloviꢁc, Paul G. Clark, Andrew J. Boydston,* and Robert H. Grubbs*
Arnold and Mabel Beckman Laboratory of Chemical Synthesis, Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125, United States
S Supporting Information
b
Scheme 1. Synthesis of Dialkenylammonium Salts 2 and
Complexation with DB24C8 To Provide Supramolecular
Monomers 3
ABSTRACT: A one-pot synthesis of polyrotaxanes has
been developed. The method employs a supramolecular
monomer comprising a polymerizable ammonium salt and
crown ether, in combination with dynamic ADMet polym-
erization. Ultimately, highly efficient complexation, polym-
erization, and end-capping were accomplished in a single
operation to yield polyrotaxanes with Mw up to 19.3 kDa
and >80% of the repeat units being complexed.
dvanced supramolecular and mechanically interlocked poly-
A
mers such as polyrotaxanes, polycatenanes, and polypseu-
dorotaxanes offer enticing synthetic targets and the promise of
unique characteristics.1 The ability to construct these compli-
cated architectures in an efficient, scalable, and modular fashion is
essential to realizing their full potential. A particular challenge in
the synthesis of main-chain polyrotaxanes, for example, is
achieving a combination of polymerization, threading or “clip-
ping”, and end-capping to secure the overall interlocked nature of
the ensemble—feats often executed in a stepwise fashion. In-
spired by elegant examples that have successfully accomplished
these objectives,2 we sought a system of high efficiency in which
polyrotaxanes could be generated in one pot from readily
available, modular building blocks via a dynamic polymeriza-
tion/end-capping strategy. We envisioned that incorporating
end-caps during polymerization, while also allowing threading
to occur, would benefit from a rapidly equilibrating polymeriza-
tion such as acyclic diene metathesis (ADMet) polymerization.3
Herein we describe a method to accomplish a one-pot synthesis
of polyrotaxanes via multicomponent ADMet polymerization.
Starting from commercially available tert-butyl carbamate 1,
acyclic dienyl ammonium salts 2 were prepared in three steps as
depicted in Scheme 1. Specifically, alkylation of 1 using NaH and
a bromoolefin in DMF furnished the corresponding dialkyl
carbamate intermediates (not shown). Subsequent deprotection
using TFA in CH2Cl2, followed by anion metathesis with
NH4PF6, provided the desired dialkenyl ammonium salts bearing
6- or 11-carbon chains (2a and 2b, respectively).
Scheme 2. ADMet Polymerization of Supramolecular
Monomers 3 To Form Polyrotaxanes 6 (shown as an idea-
lized, fully threaded polyrotaxane)
ratio of DB24C8/2 to 5:1 (0.01 M in 2, initially) resulted in
quantitative threading. The supramolecular complexes (3a and 3b)
obtained from 1:1 mixtures of 2 and DB24C8 were concentrated
under vacuum and used directly in subsequent ADMet poly-
merizations. It is expected that the concentrated mixtures
comprising 3 contain a higher composition of the supramolecular
complexes than the more dilute solutions used for NMR analysis.
Supramolecular monomers 3 (likely as dynamic mixtures with
the constituent ammonium salt 2 and DB24C8) were found to
readily participate in ADMet polymerization (Scheme 2); key
results are summarized in Table 1. Initially, a solution containing
3a was treated with Ru-alkylidene complex 4 and an end-capping
chain-transfer agent (CTA) 5. Using [3a/4]0 of 40:1 and [3a/5]0
of 2.5:1 (entry 1), the reaction mixture was prepared in dry
CH2Cl2 (initially 0.5 M in 3a), sealed under Ar, and vigorously
Efficient threading of 2 using dibenzo[24]crown-8 ether
(DB24C8) was confirmed via H NMR spectroscopy of a 1:1
1
molar ratio of the two species in CD2Cl2 (0.01 M in each,
initially).4 The CH2 protons alpha to the ammonium moiety in 2
displayed resonances at δ = 3.0 ppm and moved downfield to δ =
3.2 ppm in the presence of DB24C8, indicating threading to
form supramolecular monomers 3 (Scheme 1). Integration of the
two signals correlated to ca. 75% threading, and increasing the
Received: September 8, 2011
Published: October 24, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja208515r J. Am. Chem. Soc. 2011, 133, 19087–19089
|

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Table 1. Data for ADMet Polymerization of 2 To Form
Polyrotaxanes 6a
Scheme 3. One-Pot ADMet Polymerization To Form
Polyrotaxanes
entry monomer yield (%)b threading (%)c Mw (kDa)d PDId
1
3a
3b
2b
2b
85
94
80
50
72
82
82
80
13.2
11.0
19.3
14.9
1.58
1.42
1.18
1.37
2
3e
4f
a Reactions were conducted under Ar atmosphere at 50 °C using [2/4]0
or [3/4]0 = 40:1 and [2/5]0 or [3/5]0 = 2.5:1. b Isolated yield after
c
1
precipitation, based on 2 or 3 accordingly. Determined by H NMR
spectroscopy of the isolated product. d Determined by GPC
using MALLS. e Using [2b/DB24C8]0 = 1:1. f Using [2b/DB24C8]0 =
5:1.
stirred in an oil bath at 50 °C. The viscosity of the solution
quickly increased, and after ca. 30 min the mixture was removed
from heat and placed under vacuum to remove all volatiles,
including ethylene byproduct formed during the polymerization.
Fresh CH2Cl2 was then added under Ar, and the sealed reaction
mixture was again heated at 50 °C. The process of adding CH2Cl2
and subsequently removing the solvent under vacuum was re-
peated after 1, 2, and 12 h intervals. After a total reaction time of ca.
24 h, the mixture was concentrated under vacuum to give a thick
tan foam. Analysis of the polyrotaxane via 1H NMR spectroscopy
indicated that ca. 72% of the repeat units remained threaded, and
gel permeation chromatography (GPC) analysis using multiangle
laser light scattering (MALLS) revealed a Mw of 13.2 kDa.
To improve the efficiency of the ADMet polymerization, we
next focused on monomer 3b, bearing longer undecenyl groups
in comparison with 3a. The reduced viscosity of the resulting
reaction mixture indeed appeared to facilitate the polymeriza-
tion, reaching full monomer conversion in ca. 2 h (cf. 90%
conversion at 6 h when 3a was employed). After ca. 12 h and
three cycles of solvent removal/addition, the polyrotaxane was
concentrated to yield a thick, viscous oil (entry 2). Analysis as
before revealed ca. 82% threading and Mw = 11.0 kDa.
Encouraged by the ability to efficiently polymerize the con-
gested supramolecular monomers via ADMet, and the rapid
threading observed from ammonium salts 2 and DB24C8, we
next attempted the polyrotaxane synthesis without discrete
preassembly to form 3 (Scheme 3). Accordingly, 2b, DB24C8,
and CTA 5 were combined as a heterogeneous mixture, and a
solution of catalyst 4 in CH2Cl2 was added. The polymerization
and solvent cycling were conducted as described above. Analysis
of the resulting polyrotaxane revealed similar threading (82%)
and an increased Mw of 19.3 kDa (entry 3) in comparison with
using the preassembled 3b monomer (entry 2). Notably, the
amount of threading did not benefit from the addition of 5 equiv
of DB24C8 relative to monomer 2b (entry 4), suggesting that
the maximum amount of threading for this particular monomer
structure had been reached.
Figure 1. DOSY spectra of polyrotaxane 6a (blue) and free DB24C8
(red) at concentrations of 1 mg/mL (top) and 10 mg/mL (bottom).
1
DP (and corresponding Mn values) via H NMR analysis by
comparing the integration of the CH2 protons alpha to the
ammonium moiety with that of the dimethoxyarene end-cap.
Using the Mw values from MALLS analysis in combination with
1
the Mn values obtained via H NMR analysis gives PDIs for
entries 1À4 (Table 1) of 1.91, 1.58, 3.55, and 1.65, respectively,
that are more consistent with other ADMet polymerizations.
The end-capped, mechanically locked nature of the polyrotax-
anes was confirmed via 2-D DOSY NMR experiments.5,6 As can
be seen in Figure 1, the diffusion rates for both the polyammo-
nium backbone of 6a and the DB24C8 moieties are correlated
to one another, with a diffusion rate value of ca. 2.5 Â 10À9 m2/s.
Moreover, this diffusion rate is distinct from that of free
DB24C8, which displays a faster diffusion rate of ca. 3.5 Â
10À9 m2/s. This supports the successful incorporation of the
end-caps but may also have been ascribed to strong ammo-
niumÀDB24C8 interactions. To further support the end-capped
nature of 6a, we compared the DOSY spectrum upon 10-fold
dilution, which again revealed consistent diffusion rates of the
polymer backbone and the interlocked DB24C8 moieties.
In conclusion, we have developed a simple strategy for a one-
pot, multicomponent synthesis of polyrotaxanes using ADMet
polymerization that advances the synthetic capabilities toward
advanced macromolecular structures. The efficiency and ease
with which these mechanically interlocked macromolecules can
be assembled should facilitate rapid modulation to achieve
versatile polyrotaxane architectures, and can be readily adapted
to incorporate a variety of “metathesis-friendly” substrates.
The polydispersity index (PDI) values resulting from the step-
growth polymerizations were lower than expected when deter-
mined using MALLS analysis (Table 1). It is evident that the
polymerizations do not reach full equilibrium, considering the
feed ratio of monomer (2 or 3) to end-cap (5) of 2.5:1 would
result in an average degree of polymerization (DP) of 5, and
those obtained were greater than 5. The PDI values, however,
likely reflect poor resolution of the polyelectrolyte structures
during elution on the GPC columns that would manifest in
artificially low PDIs. For comparison, we determined the average
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’ ASSOCIATED CONTENT
S
Supporting Information. Detailed experimental proce-
b
dures and characterization of all new compounds. This materials
is available free of charge via the Internet at http://pubs.acs.org
’ AUTHOR INFORMATION
Corresponding Author
boydston@chem.washington.edu; rhg@caltech.edu
’ ACKNOWLEDGMENT
Financial support of this research by the National Science
Foundation (CHE-0809418), Army Research Office (W911NF-
09-1-0512), and the Department of Energy (DE-FG02-
05ER46218). A.J.B. thanks the National Institutes of Health
for a postdoctoral fellowship. We are grateful to Dr. David
VanderVelde for his assistance with NMR spectroscopy.
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