Polypseudorotaxanes via ring-opening metathesis polymerizations of [2]catenanes

Article abstract of DOI:10.1021/ja806122r

A bis-phenanthroline [2]catenane copper complex, consisting of one olefinic macrocycle and one nonolefinic macrocycle, underwent an entropy-driven ring-opening olefin metathesis polymerization (ROMP) to provide a polypseudorotaxane. The polymerization featured an average degree of polymerization of ca. 63 wherein the polymer was effectively saturated with threaded macrocycles. Removal of the copper led to near complete release of the macrocycles from the polymer backbone. Copyright

Full text of DOI:10.1021/ja806122r

Published on Web 10/22/2008
Polypseudorotaxanes via Ring-Opening Metathesis Polymerizations of
[2]Catenanes
Songsu Kang, Brandon M. Berkshire, Zheng Xue, Manav Gupta, Christianah Layode,
Preston A. May, and Michael F. Mayer*
Department of Chemistry and Biochemistry, Texas Tech UniVersity, Box 41061, Lubbock, Texas 79409-1061
Received August 4, 2008; E-mail: mf.mayer@ttu.edu
The synthesis of polymers with well-defined mechanical en-
tanglements1 is a state-of-the-art challenge that is essential to master
for greater control over macroscopic materials properties. In our
efforts to render such supramolecular polymeric architectures more
accessible, we became interested in the novel possibility of
preparing polypseudorotaxanes via ring-opening olefin metathesis
polymerizations (ROMPs) of olefinic [2]catenanes, eq 1. The
realization of this concept is an important advance because, notably,
with suitable variation of the starting materials, one may also access
daisy-chain polymers as well as rubbery network polymers that
feature mobile, slip-link, cross-linkssa materials theorist’s dream.2
Macrocycle 1 and product 5 were examined by analytical size
exclusion chromatography (SEC), Figure 1. Macrocycle 1 exhibited a
single, relatively sharp peak with a retention time of 18.0 min. In
contrast product 5 displayed a broad peak (max. at 12.4 min) with a
ROMP reactions are typically performed with low molecular
weight, strained-ring, olefinic monomers, where ring-opening
relieves ring strain, thus driving the process.3 In contrast, macro-
series of small shoulders from 15 to 17 min, indicative of a polydisperse
oligomer of 1. The ratio of the total polymer and oligomer (5) to
cyclic olefins are generally uncommon starting materials for these
reactions because they are essentially free of ring strain and thus
not currently well-suited for living polymerizations and preparation
of low polydispersity polymers, block copolymers, etc. ROMP
starting material 1 was calculated to be 93:7. Relative to a set of
monodisperse polystyrene standards, polymer 5 was estimated to
possess a molecular weight (Mw) of 116 000 amu, an Mw/Mn (PDI) of
1.8, and an average degree of polymerization of ca. 83.
reactions of macrocyclic olefins are not without precedence however
With data to support that macrocycle 1 was reasonably amenable
as a few examples employing macrocyclic monomers have been
to ROMP, we synthesized the Sauvage-type5,7 [2]catenane 9 as the
recently reported.4 In these cases, the olefin metatheses are dynamic
key test monomer for our experiments. Using a 1:1 ratio of 3 to 7
processes termed entropy-driven ring-opening metathesis polymer-
izations (ED-ROMPs);4e they have allowed direct access to
(30-membered ring), intermediary [2]pseudorotaxane copper com-
polymers with unusual main-chain functionality.4a-c Herein, we
communicate the first report of a ROMP of an olefinic [2]catenane
to access polypseudorotaxanes.
plex 8 was formed and isolated in 95% yield. Complex 8 (3.2 mM)
was subsequently treated with 10 mol% 4 to effect RCM; the
resulting catenane 9 (E/Z ratio of 5:1) was isolated in 91% yield,
eq 4. The RCM yield of 9 was significantly higher than the yield
of 1. This result is consistent with previous catenane syntheses via
RCM of bis-phenanthroline copper complexes and is presumably
due to preorganizational effects of the bis-phenanthroline complex.5,8
We initially sought to attempt a ROMP of a model, uncatenated,
olefinic macrocyclesone, however, that could be readily interlocked
with another macrocycle, if desired. An attractive candidate for this
test polymerization was the 1,10-phenanthroline-based olefinic mac-
rocycle 1 since catenanes of 1-like compounds have been prepared
with very high yields.5 Thus 46-membered macrocycle 1 (E/Z ratio
of 5:1) was synthesized from known diol 25 via a double etherification
to yield intermediary R,ω-diolefin 3 which underwent ring-closing
olefin metathesis (RCM) when treated with Grubbs’ second-generation
catalyst6 (4) under dilute conditions (10 mM), eq 2.
In a test of the feasibility of ROMP of such a macrocycle, a
concentrated DCM solution of olefinic macrocycle 1 (0.5 M) was
treated with 1 mol% initiator 4, eq 3. Over the course of stirring
for 4 h at 40 °C, the reaction mixture became significantly more
viscous. The reaction was then quenched with ethyl vinyl ether,
and a solid residue (5) was obtained upon removal of the solvent.
The 1H and 13C NMR spectra of 5 appeared similar to the spectra
of macrocycle 1 except each signal was somewhat broadened and
the E/Z ratio of the olefinic signals was found to increase to 12:1.
To probe and ensure that the bis-phenanthroline copper complex
in 9 was sufficiently robust to endure ROMP without dissociation
of the two phenanthroline moieties, we treated a dilute DCM
9
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10.1021/ja806122r CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. SEC chromatograms of (A) 1 and (B) 5.
Figure 2. SEC chromatograms of (A) 11 and (B) 12.
macrocycles (7) from the covalent polymer chain (∼5). Such release
demonstrates that the preponderance of ROMP product 10 cannot
be cyclic.9
solution of [2]catenane 9 (5 mM) with 5 mol% 4 for 6 h at 40 °C.
After quenching the reaction with ethyl vinyl ether, we recovered
96% of [2]catenane 9. Fortunately, even under dilute ring-opening,
ring-closing metathesis (RORCM) conditions, the copper(I) ion was
able to maintain association of the two phenanthroline ligands of
9, thus boding well for the synthesis of a polypseudorotaxane with
a high, predictable, and controlled density of threaded macrocycles.
For the title reaction, we treated a concentrated DCM solution
of olefinic [2]catenane 9 (>0.135 M) with 1 mol% initiator 4, eq
5. Over the course of stirring for 4 h at 40 °C, the reaction mixture
again became significantly more viscous. The reaction was then
quenched with ethyl vinyl ether, and a solid product (10) was
isolated upon removal of the solvent.
The demetallated [2]catenane (11) exhibited a single SEC peak,
albeit with a tail, with a retention time of 17.3 min, Figure 2a. In
contrast the demetallated polypseudorotaxane mixture (12) displayed
a broad polymer peak (max. at 13.1 min) in addition to two other
peaks corresponding to [2]catenane 11 (17.4 min) and macrocycle 7
(18.5 min),9 Figure 2b. The bare covalent polymer backbone of 10
(∼5) was estimated to have a molecular weight (Mw) of 93 000 amu,
a PDI of 1.9, and an average degree of polymerization of ca. 63. The
ratio of polymer/catenane/macrocycle 7 was calculated to be 47.6:6.6:
45.8. Although the measured ratio of polymer to released 7 was near
the 1:1 expectation, the experimental bias may be indicative of a minor
amount of macrocyclic 7 being mechanically trapped in the polymer,
possibly as a result of backbiting during the polymerization process;
i.e., a small amount of threaded cyclic polymer may be present. In
summary, we have shown that an ED-ROMP reaction of an olefinic
Sauvage-type [2]catenane leads to a reasonably high molecular weight
main-chain polypseudorotaxane. This approach takes advantage of
using a prethreaded monomer to access polymers that are effectively
saturated with threaded macrocycles.
The 1H and 13C NMR spectra of 10 analogously appeared similar
to the spectra of [2]catenane 9 except each signal was slightly
broadened and the E/Z ratio of the olefinic signals was again found
to increase.9 Importantly, all upfield shifted phenanthroline signals,
characteristic of bis-phenanthroline copper complexes, maintained
their upfield shift.9 This similarity in the NMR spectra of 9 and 10
is strong evidence that the polymer chain produced during the
ROMP retained, at least, the vast majority of the threaded
macrocycles 7. For example, in a control experiment, when
macrocycle 7, alone, was treated with 0.5 equiv of Cu(I), the
solution did not acquire the dark red color that is characteristic of
Acknowledgment. This research was supported by the donors
of the American Chemical Society Petroleum Research Fund
(42943-G1), by the Texas Advanced Research Program (003644-
0013-2006), and in part by an HHMI grant through the Under-
graduate Science Education Program to TTU.
Supporting Information Available: Experimental procedures and
characterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
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1
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the phenanthroline groups would have ceased and resulted in more
complex spectra. From such components, in this case of slow
exchange,9 the only arrangement in which to achieve clean spectra,
where all phenanthroline ligands are involved in bis-phenanthroline
copper complexes, is for the macrocycles (7) to remain threaded
on the polymer backbone of 10.
Direct examination of catenane 9 and polypseudorotaxane 10
by analytical SEC was complicated due to the ionic nature of these
compounds and subtle solubility issues. Instead, the demetallated
compounds were studied by SEC. Both 9 and 10 were readily
demetallated when treated with aq. KCN in MeCN/DCM,9 resulting
in near complete loss of color in both cases, while for 10
demetalation additionally resulted in concomitant release of the
JA806122R
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