Structure-property relationships in metallosupramolecular poly(p -xylylene)s

Article abstract of DOI:10.1021/ma202312x

The self-assembly polymerization of ditopic monomers via metal-ligand binding is a facile route for the preparation of metallosupramolecular polymers. Here this approach was used for the synthesis of supramolecular poly(p-xylylene)s based on 2,6-bis(1′-me

Full text of DOI:10.1021/ma202312x

Article
pubs.acs.org/Macromolecules
Structure−Property Relationships in Metallosupramolecular Poly(p-
xylylene)s
Mark Burnworth,† Stuart J. Rowan,*,† and Christoph Weder*,†,‡
†Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, Ohio
44106-7202, United States
‡Adolphe Merkle Institute and Fribourg Center for Nanomaterials, University of Fribourg, CH-1700 Fribourg, Switzerland
S
* Supporting Information
ABSTRACT: The self-assembly polymerization of ditopic
monomers via metal−ligand binding is a facile route for the
preparation of metallosupramolecular polymers. Here this
approach was used for the synthesis of supramolecular poly(p-
xylylene)s based on 2,6-bis(1′-methylbenzimidazolyl)pyridine
(Mebip) end-capped telechelic oligomers with a p-xylylene
core and different metal salts. These polymers can be readily
processed from solution and merge the ease of processing of
supramolecular materials with the good thermal stability of the
p-xylylene core. The nature of the metal cation (Fe2+, Zn2+, La3+) and counteranion (ClO4 , OTf−, NTf2 ) was systematically
varied, and a tetrafunctional supramolecular cross-linker was used to probe how these modifications influence the materials’
properties. Interestingly, and in contrast to other metallosupramolecular polymers, where the nature of the metal salt plays a
critical role, only minor property differences were observed for the materials studied. Instead, the properties of the
supramolecular poly(p-xylylene)s investigated appear to be primarily governed by the crystalline nature of the telechelic
oligomer. We note that minor impurities in the latter can exert a significant influence on the metallosupramolecular polymer’s
properties and report a new protocol for the synthesis and purification of Mebip-end-capped p-xylylene telechelic oligomers.
−
−
are employed.7−14 If ditopic telechelic oligomers with low-glass-
INTRODUCTION
■
Metallosupramolecular polymers1 are a class of materials
situated at the interface of metal-containing polymers2 and
supramolecular polymers,3 in which dynamic metal−ligand
interactions serve as a reversible supramolecular polymerization
motif. Metal coordination allows one to polymerize low-
molecular-weight or telechelic cores that carry at least two
ligands, typically at the termini (Scheme 1). Popular ligands of
choice are multidentate ligands, such as terpyridine (terpy),4
bipyridine (bpy),5 and 2,6-bis(1′-methylbenzimidazolyl)-
pyridine (Mebip),6 which can coordinate a range of metal
salts (e.g., Co2+, Fe2+, Zn2+, Eu3+). The variation of nature,
length, and structure of the core employed to connect the
ligands, number, type, and position of the latter, along with the
nature of the metal ion, and counterion should allow one to
tailor the properties of these polymers. While much work has
focused on the synthesis of metallosupramolecular polymers,
the elucidation of their properties in solution, and the study of
“advanced” properties imparted by the metal (e.g., optical,
electrical, magnetic),2 systematic studies of their solid-state
mechanical properties have received comparatively little
attention. Therefore, relatively little is known about struc-
ture−property relationships of such materials. We have
demonstrated that the length of the ditopic core is of great
importance; metallosupramolecular polymers only display
appreciable mechanical properties if telechelic units or
“telechelic oligomers” with molecular weights of a few thousand
transition core (e.g., PEO, poly(THF), poly(ethylene-co-
butylene)) and terminal Mebip ligands are employed, the
metallosupramolecular materials obtained upon addition of 1
equiv of a metal salt can have mechanical properties that lie
between those of highly cross-linked elastomers and ultralow-
density polyolefins, i.e., a storage modulus of the order of 50
MPa, stress at break of the order of 50 MPa, and a strain at
break of around 50%, in some cases after yielding and plastic
deformation.7,12,14 The mechanical properties of these materials
are, at least in part, governed by microphase segregation, where
the metal−ligand complexes form a “hard phase” that physically
cross-links “soft” domains formed by the telechelic cores. The
morphology (and therefore the mechanical properties) depends
on the polarity of the telechelic oligomer as well as the nature
of the metal ion and counterion. With the objective to create
arylene alkylene metallopolymers, which combine ease of
processing with good high-temperature stability,15,16 we also
investigated the metallosupramolecular polymerization of a
crystallizable poly(2,5-dioctyloxy-p-xylylene) telechelic
oligomer end-capped with Mebip ligands and Zn(ClO4)2,
Fe(ClO4)2, and mixtures of these salts and La(ClO4)3. The
resulting polymers are readily solution-processable and exhibit a
Received: October 15, 2011
Revised: December 2, 2011
Published: December 9, 2011
© 2011 American Chemical Society
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reactions. Toluene was distilled from sodium under an inert
atmosphere; diisopropylamine and tripropylamine were distilled
from calcium hydride under an inert atmosphere.
Scheme 1. Schematic Representation of the
Metallosupramolecular Polymerization of Ditopic Telechelic
Oligomers with Either Transition or Lanthanide Metal Ions
Synthesis of Telechelic Oligomer 1. Telechelic oligomer 2
(3.05 g, 0.469 mmol), p-toluenesulfonyl hydrazide (TSH) (16.01 g,
86.0 mmol), and tripropylamine (TPA) (16.3 mL, 86.0 mmol) were
dissolved in xylenes (350 mL), and the solution was heated under
reflux at 140 °C. After 3 h, additional portions of TSH (16.01 g, 86.0
mmol) and TPA (16.3 mL, 86.0 mmol) were added, and the reaction
mixture was heated under reflux for an additional 2 h. The hot reaction
mixture was passed through a plug of basic alumina, and the plug was
washed with hot toluene. The combined organic fractions were washed
twice with deionized water, and the volume was reduced under
vacuum to produce an orange viscous liquid, to which a small amount
of CHCl3 was added. The orange solution was then precipitated into
well-stirred EtOH (500 mL). After stirring for 2 h, the resulting yellow
precipitate was filtered off, dissolved in a minimal amount of CHCl3
and again precipitated into EtOH (500 mL). The yellow precipitate
was filtered off and subsequently washed with boiling MeOH, EtOH,
CH3CN, and diethyl ether. Drying overnight under vacuum yielded 1
1
as a yellow solid (2.52 g, 0.296 mmol, 63%). H NMR (600 MHz,
CDCl3): δ = 8.35 (s, 4 H, ArH end group), 7.89 (d, 4 H, JH−H = 8.4
Hz, ArH end group), 7.48 (d, 4 H, JH−H = 7.8 Hz, ArH end group),
7.38 (m, 8 H, ArH end group), 6.73 (s, 4 H, ArH end group), 6.68 (s,
2 H, Ar), 4.26 (s, 12 H N−CH3 end group), 3.92 (t, 2 H, JH−H = 6 Hz,
OCH2 end group), 3.89 (t, 4 H, JH−H = 6.6 Hz, OCH2), 3.09 (m, 4H,
Ar−CH2−CH2−Ar end group), 3.04 (m, 4 H, Ar−CH2−CH2−Ar end
group), 2.84 (s, 4 H, Ar−CH2−CH2−Ar), 1.79 (m, 4 H), 1.50 (m, 4
H), 1.38−1.18 (m, 16 H), 0.87 (t, 6 H, JH−H = 6.6 Hz), 0.79 (t, 6 H,
JH−H = 6.6 Hz, CH3 end group). Degree of polymerization (Xn,
determined by NMR) = 28.5; number-average molecular weight (Mn,
determined by NMR) = 11 400 g/mol. 13C NMR (CDCl3, 125 MHz):
δ = 150.7, 149.5, 142.6, 137.2, 129.3, 125.5, 123.5, 122.8, 120.2, 114.2
109.8, 68.9, 36.3, 31.9, 31.3, 29.8, 29.7, 29.5, 29.4, 26.3, 22.7, 12.4.
Synthesis of 2,7-Diiodo-9,9-di(4-iodobenzyl)fluorene (8).19
A solution of 2,7-diiodofluorene (6, 500 mg, 1.120 mmol) in dimethyl
sulfoxide (DMSO, 12 mL) was degassed by sparging with Ar for 30
min, and benzyltrimethylammonium chloride (18 mg, 0.10 mmol) and
aqueous NaOH (50 wt %, 0.6 mL) were added. After 10 min, the
reaction mixture turned red. 4-Iodobenzyl bromide (7, 855 mg, 2.88
mmol) was added, and the reaction mixture was heated to 80 °C for 2
h. The reaction mixture was subsequently cooled to RT, poured into
dichloromethane (DCM, 50 mL), and washed 3 times with H2O.
Recrystallization from boiling DCM yielded 8 as a white powder (800
room-temperature storage modulus of ca. 400 MPa, i.e., a much
higher stiffness than the aforementioned metallosupramolecular
polymers based on a rubbery core. Here, we report an in-depth
study of the structure−property relations in such metal-
losupramolecular poly(p-xylylene)s. The nature of the metal
cation (Fe2+, Zn2+, La3+) and counteranion (ClO4 , OTf−,
−
−
NTf2 ) was systematically varied, and a tetrafunctional
supramolecular cross-linker was used to probe how these
modifications influence the materials’ properties.
1
mg, 0.941 mmol, 79%). H NMR (600 MHz, CDCl3): δ = 7.69 (d,
2H, J = 1.8 Hz, ArH), 7.54 (dd, 2H, J = 8.1, 1.8 Hz, ArH), 7.25 (d, 4H,
J = 8.4 Hz, ArH), 7.10 (d, 2H, J = 8.4 Hz, ArH), 6.33(d, 4H, J = 8.4
Hz, ArH), 3.21 (s, 4H, CH2) ppm. 13C NMR (100 MHz, CDCl3): δ =
149.4, 139.8, 136.8, 136.7, 135.7, 133.7, 132.2, 122.1, 92.3, 92.2, 56.9,
44.8 ppm.
EXPERIMENTAL SECTION
■
General Methods. 1H and 13C NMR were acquired in CDCl3
using a Varian 600 MHz spectrometer; chemical shifts are expressed in
ppm relative to an internal tetramethylsilane standard. Ultraviolet−
visible (UV−vis) absorption spectra were obtained on a Perkin-Elmer
Lambda 800 spectrometer. Thermogravimetric analyses (TGA) were
carried out on a TA Instruments TGA Q500 under N2 at a heating rate
of 10 °C/min. Differential scanning calorimetry (DSC) experiments
were carried out on a TA Instruments DSC Q2000 under N2 at a
heating rate of 10 °C/min. Dynamic mechanical thermal analyses
(DMTA) were conducted on a TA Instruments Q800 dynamic
mechanical analyzer at a heating rate of 3 °C/min under N2.
Synthesis of 9. In a nitrogen-filled glovebox, 2,6-bis(1′-methyl-
benzimidazolyl)-4-ethynylpyridine (3, 600 mg, 1.65 mmol), 8 (335
mg, 0.39 mmol), Pd(PPh3)4 (22.7 mg, 0.013 mmol), CuI (3.7 mg,
0.019 mmol), and tetrahydrofuran (THF, 35 mL) were introduced
into a reaction flask, which was subsequently equipped with a reflux
condenser, removed from the glovebox, and connected to a Schlenk
line. (iPr)2NH (15.8 mL) was added, and the reaction mixture was
stirred for 19 h at 45 °C. The reaction mixture was subsequently
poured hot into a saturated aqueous EDTA solution (60 mL), and the
mixture was stirred for 1 h. The organic layer was separated off, and
the aqueous layer was extracted with CHCl3. The combined organic
layers were washed with deionized water and reduced under vacuum
to yield a light-brown solid, which was purified by column
chromatography (CHCl3, neutral alumina) and subsequent recrystal-
lization (twice) from cold DCM. This afforded 9 as a light yellow solid
(310 mg, 0.17 mmol, 44%). 1H NMR (600 MHz, CDCl3): δ = 8.65 (s,
4H, ArH), 8.38 (s, 4H, ArH), 7.89 (d, 4H, J = 7.2 Hz, ArH), 7.83 (s,
2H, ArH), 7.80 (d, 4H, J = 7.8 Hz, ArH), 7.52 (dd, 2H, J = 7.8, 1.2 Hz,
ArH), 7.47−7.41 (m, 10H, ArH), 7.38−7.29 (m, 16H, ArH), 7.17 (d,
4H, J = 7.8 Hz, ArH), 6.73 (d, 4H, J = 8.4 Hz, ArH), 4.28 (s, 12H, N−
Materials. Telechelic oligomer 2 and 2,6-bis(1′-methyl-benzimi-
dazolyl)-4-ethynylpyridine (3) were prepared according to the
previously reported methods.8 Zinc bistriflimide17 and lanthanum
bistriflimide18 were prepared according to literature procedures from
the mixture of either zinc dust or lanthanum oxide with bis-
(trifluoromethane)sulfonimide in water. Unless otherwise stated, all
other reagents, solvents, metal salts (triflate and perchlorate), and
catalysts were purchased from Aldrich Chemical Co., Fisher Scientific,
or Strem Chemicals and were used without further purification.
Spectroscopic grade CHCl3 (passed through a plug of basic alumina)
and spectroscopic grade CH3CN were employed for the optical
absorption as well as for the metallosupramolecular polymerization
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CH3), 4.19 (s, 12H, N−CH3), 3.57 (s, 4H, CH2) ppm. 13C NMR (100
MHz, CDCl3): δ = 150.1, 150.04, 149.98, 149.9, 148.4, 142.9, 142.8,
141.7, 138.0, 137.46, 137.4, 134.2, 134.2, 132.2, 131.5, 130.5, 128.2,
127.2, 127.0, 123.93, 123.86, 123.2, 123.1, 121.1, 120.9, 120.5, 120.1,
110.1, 110.1, 96.7, 96.0, 87.5, 86.4, 57.5, 45.8, 32.8, 32.7 ppm. MALDI
Scheme 2. Synthesis and Structure of the Conjugated
Monomer 2, Its Reduction to the Ditopic Telechelic
Oligomer 1, and Pictures of the Films Obtained upon
Solution-Casting Telechelic Oligomer 1 with Equimolar
a
Amounts of Either Fe(ClO4)2 or Zn(ClO4)2
MS (matrix: HABA): m/z 1791.347 (required for C119H82N20
=
1791.347).
Synthesis of 10. A solution of 9 (75 mg, 0.042 mmol) in o-
dichlorobenzene (o-DCB, 10 mL) was heated under Ar to 130 °C,
before diisopropylamine ((iPr)2NH, 70.4 μL, 0.50 mmol) and p-
toluenesulfonyl hdrazide (TSH, 62.4 mg, 0.33 mmol) were added.
Additional portions of TSH and (iPr)2NH were added after 4 h (31.2
mg, 0.16 mmol TSH; 35.2 μL, 0.25 mmol (iPr)2NH), 5.5 h (31.2 mg,
0.16 mmol TSH; 35.2 μL, 0.25 mmol (iPr)2NH), and 7.5 h (62.4 mg,
0.33 mmol TSH; 70.4 μL, 0.5 mmol (iPr)2NH), and the reaction
mixture was stirred at 130 °C for a total of 26.5 h. The hot reaction
mixture was then passed through a plug of basic alumina, and the plug
was washed with hot toluene (150 mL). The organic layers were
combined, the solvent was evaporated, and the product was dried
under vacuum to yield 10 as a yellow solid (60 mg, 0.033 mmol, 78%).
1H NMR (600 MHz, CDCl3): δ = 8.37 (s, 4H, ArH), 8.10 (s, 4H,
ArH), 7.82 (d, 8H, J = 7.8 Hz, ArH), 7.42 (d, 4H, J = 7.8 Hz, ArH),
7.37−7.26 (m, 22H, ArH), 7.10 (s, 2H, ArH), 7.07 (d, 2H, J = 7.8 Hz,
ArH), 6.74 (d, 4H, J = 7.8 Hz, ArH), 6.54 (d, 4H, J = 8.4 Hz, ArH),
4.19 (s, 12H, N−CH3), 4.14 (s, 12H, N−CH3), 3.17 (s, 4H, CH2),
3.13 (s, 8H, CH2), 2.86 (m, 4H, CH2), 2.74 (m, 4H, CH2) ppm. 13C
NMR (100 MHz, CDCl3): δ = 153.5, 153.4, 150.59, 150.57, 149.9,
149.6, 149.0, 142.8, 139.2, 138.6, 138.5, 137.4, 137.3, 135.4, 130.7,
127.4, 127.3, 125.7, 125.5, 125.2, 123.7, 123.7, 123.0, 120.38, 120.37,
119.8, 110.1, 110.0, 56.4, 45.1, 38.3, 37.5, 37.3, 36.2, 32.7 ppm.
MALDI MS (matrix: HABA): m/z 1809.274 (required for C119H98N20
= 1809.274).
Typical Preparation of Metallosupramolecular Polymer
Films. A solution of Fe(ClO4)2 in CH3CN (180.4 μL of a 64.8
mM solution) was added to a stirred solution of telechelic oligomer 1
(99.4 mg, 11.7 μmol) in CHCl3 (2 mL, 5.84 mM). The viscosity of the
mixture increased noticeably, and the color changed to purple. The
solution was transferred to an aluminum solution caster (diameter 3
cm) with a Teflon base. The solvent was evaporated overnight under
ambient conditions in a well-ventilated hood, and the resulting films
were subsequently dried at room temperature to produce a solid
purple film. Yellow films of metallosupramolecular polymers made
with different Zn2+ salts were prepared by the same method. Yellow
films of metallosupramolecular polymers made with La(NTf2)3 were
prepared by the same method, but the telechelic oligomer:metal salt
ratio was adjusted to account for 3:1 Mebip:La binding.
General Procedure for UV−vis Titration with Metal Salts. A
solution of 1 (50 μM) in a mixture of CHCl3/CH3CN (9/1 v/v, 2
mL) was titrated with 25 μL aliquots of a solution of the metal salt (ca.
300 μM) and 1 (50 μM) in the same solvent mixture. The addition
was done incrementally; after each addition of an aliquot of the metal
salt solution the samples were characterized by UV−vis spectroscopy.
The Mn of 1 was calculated from the ion titration experiments with
Fe(ClO4)2: Mn ≈ 8500 g/mol, Xn ≈ 21.
RESULTS AND DISCUSSION
a
■
The properties of the resulting films depend on the procedure used
Synthesis and Characterization of Telechelic
Oligomer 1. The synthetic protocol utilized to prepare the
2,6-bis(1′-methylbenzimidazoyl)pyridine end-capped 2,5-dia-
lkoxy-p-xylylene telechelic oligomer (1) follows our previously
published route9 and involves the Hahn diimide reduction of
the parent conjugated 2,5-dialkoxy-p-phenyleneethynylene 2,
which was prepared in greater than 90% yield via the
Sonogashira coupling reaction of acetylenes 3, 5, and aryl
diiodide 4 (Scheme 2). Our original purification of 1 involved
precipitating the product into MeOH and washing with boiling
MeOH, EtOH, CH3CN, and room-temperature MeOH.9
When this procedure was repeated in the context of the
to purify 1.
1
present work, close examination of the H NMR spectrum
showed a peak around 2.32 ppm (Supporting Information,
Figure S2), which can also be found in the published spectra of
1.9 The signal, along with matching resonances of aromatic
protons, is indicative of a small amount (ca. 2% w/w) of
residual p-toluenesulfinic acid, stemming from the reduction
process. In order to probe if this impurity, albeit present only in
a small concentration, might impact the properties of the
targeted metallosupramolecular polymers, the purification
procedure was modified to completely remove this byproduct.
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The new purification method involves repeated precipitation in
EtOH and washing with boiling MeOH, EtOH, CH3CN, and
diethyl ether to isolate 1 in 63% yield which was free of the
characteristic p-toluenesulfinic acid peak at 2.32 ppm
(Supporting Information, Figures S1 and S2).
1 with stoichiometric amounts of either Fe(ClO4)2, Zn(ClO4)2,
Zn(OTf)2, or Zn(NTf2)2 (designated as 1·[Fe(ClO4)2]1.0,
1·[Zn(ClO4)2]1.0, 1·[Zn(OTf)2]1.0, and 1·[Zn(NTf2)2]1.0, re-
spectively). In addition, a film of 1 with 0.66 mol equiv of
La(NTf2)3 (1·[La(NTf2)3]0.66) was prepared, assuming the
binding of three Mebip ligands per metal ion (Scheme 1).9 Our
selection of metal salts is rationalized with the objective to
cover a range of binding strengths and dynamics (from the
dynamic, labile La3+ over Zn2+ to the kinetically slower Fe2+)
and to explore bistriflimide and triflate anions as thermally
more stable alternatives to the perchlorate anion. Already a
qualitative comparison of the 1·[Fe(ClO4)2]1.0 and 1·[Zn-
(ClO4)2]1.0, films made by old and new protocols (Scheme 2),
revealed that the new processes results in metallosupramo-
lecular polymer films with much better mechanical properties
than the previous materials: the original 1·[Fe(ClO4)2]1.0 was
very brittle and 1·[Zn(ClO4)2]1.0 did not form self-supporting
films, very much in contrast to the corresponding materials
prepared here. The comparison shows that the protocols
reported herein afford metallosupramolecular polymers with
significantly improved mechanical properties, presumably on
account of a better metal:ligand stoichiometry, which is
concomitant with higher self-assembled molecular weight
aggregates than previously attainable. The properties of the
new materials were hence studied in greater detail.
The molecular weight of the metallosupramolecular poly-
mers under investigation can be maximized if telechelic
oligomer 1 and Fe2+ or Zn2+ salts are combined so that the
molar ratio of Mebip ligand to metal ion is exactly 2:1 (Scheme
1). In our original study on metallosupramolecular poly(p-
9
1
xylylene)s we employed H NMR spectroscopy-based end-
group analysis to establish the molecular weight of 1. On the
basis of our experiences with other ditopic telechelic oligomers,
we here conducted UV−vis titration experiments to determine
the concentration of the ligands11 by slowly titrating a solution
of 1 (50 μM) with a solution of Fe(ClO4)2, while maintaining a
constant concentration of 1 (Figure 1). As the metal ion was
Characterization of 1 and Its Metallosupramolecular
Polymers. The thermal stability of 1 and the metal-
losupramolecular polymers 1·[Fe(ClO4)2]1.0, 1·[Zn(ClO4)2]1.0,
1·[Zn(OTf)2]1.0, 1·[Zn(NTf2)2]1.0, and 1·[La(NTf2)3]0.66 were
investigated by thermogravimetric analysis (TGA) (Figure 2).
It was found that the neat telechelic oligomer 1 was stable up to
a temperature of ca. 350 °C, above which it degraded with a
90% weight loss at 500 °C. 1·[Fe(ClO4)2]1.0 exhibited a 2.5%
loss at ca. 190 °C, while 1·[Zn(ClO4)2]1.0 displayed a 2.7%
weight loss at 285 °C. Consistent with previous studies, these
events are attributed to the degradation of the perchlorate
counterion.9 Both perchlorate-based metallosupramolecular
polymers display a major weight loss in the range of 350−
500 °C, similar to the telechelic oligomer. Expecting a higher
thermal stability from this counterion, we also explored series of
Figure 1. UV−vis absorption spectra acquired upon titration of 1 (50
μM) with Fe(ClO4)2. Shown are spectra at selected Fe2+:1 ratios
ranging from 0:1 to 1.5:1. The inset shows the normalized absorption
at 350.5 nm as a function of Fe2+:1 ratio.
added to the telechelic oligomer, the absorption spectrum
changed from that of the free Mebip ligand with an absorption
maximum, λmax, of 300.5 nm to that of the Mebip2:Fe2+
complex, which shows additional maxima at 350.5, 366, and
580 nm. The inset of the figure shows that the changes of the
absorbance spectrum level off when all Mebip ligands have
been complexed with Fe2+. Using the assumption that the
telechelic oligomer 1 is fully end-capped with Mebip, the Fe2+
concentration at which the optical changes leveled off was used
to back-calculate the number-average molecular weight (Mn)
and number-average degree of polymerization (Xn) of 1. The
values thus determined (Mn = 8500 g/mol, Xn = 21) are
−
salts with bistriflimide (NTf2 ) as the anion. Metallosupramo-
lecular polymers 1·[Zn(OTf)2]1.0, 1·[Zn(NTf2)2]1.0, and 1·[La-
(NTf2)3]0.66 all showed similar trends with an onset of weight
loss occurring ca. 350 °C and a single weight loss peak
occurring from 350 to 500 °C. Thus, these metallosupramo-
lecular polymers display a higher thermal stability than the
corresponding materials comprising perchlorate as the counter-
ion.
1
significantly lower than those determined by H NMR end-
Differential scanning calorimetry (DSC) was used to probe
thermal transitions of the neat telechelic oligomer 1, and the
metallosupramolecular polymers 1·[Zn(ClO4)2]1.0, 1·[Zn-
(OTf)2]1.0, 1·[Zn(NTf2)2]1.0, and 1·[La(NTf2)3]0.66. The neat
telechelic oligomer 1 (Figure 3a) displays a glass transition
temperature (Tg) of ca. 50 °C and a reversible endothermic
transition with a maximum at 164 °C. Likewise, all metal-
losupramolecular films tested displayed similar thermal
transitions with Tg occurring at ca. 50 °C and a reversible
group analysis (Mn = 11 400 g/mol, Xn = 28.5). The Mn thus
determined is consistent with the targeted Mn (8300 g/mol)
based on the stoichiometry of the starting materials (3, 4, and
5). The titration data show that using the Mn determined by
NMR end-group analysis would lead to an underestimation of
the amount of metal salt required for stoichiometric binding to
the ligands that end-cap the telechelic oligomer. Therefore, we
used the Mn determined by the UV titration (8500 g/mol) as
the molecular weight of 1.
endotherm at 164
4 °C (Figure 3b and Supporting
In order to explore how the above-described changes (new
purification method and use of UV titration to achieve
stoichiometric ratios) influence the properties of metal-
losupramolecular polymer based on 1, a series of thin films
were prepared by solution-casting CH3CN/CHCl3 mixtures of
Information; no DSC data were acquired for 1·[Fe(ClO4)2]1.0
on account of the lower degradation temperature associated
with this material). The reversible endotherm displayed by both
neat 1 and the resulting metallosupramolecular polymers is
ascribed to the melting of the p-xylylene core of the telechelic
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Figure 3. Differential scanning calorimetry (DSC) traces of the neat
telechelic oligomer 1 (a) and 1·[Zn(NTf2)2]1.0 (b). Traces from first
heating (solid), first cooling (dashed), and second heating scans (dot)
are shown. The experiments were conducted at a heating rate of 10
°C/min under N2.
Figure 2. (a, b) Thermogravimetric analysis (TGA) traces of the neat
telechelic oligomer 1 (black), 1·[Fe(ClO4)2]1.0 (red), 1·[Zn(ClO4)2]1.0
(green), 1·[Zn(OTf)2]1.0 (blue), 1·[Zn(NTf2)2]1.0 (magenta), and
1·[La(NTf2)3]0.66 (olive). The experiments were conducted at a
heating rate of 10 °C/min under N2.
oligomer.16 The crystalline nature of the materials was
b
confirmed by polarized optical microscopy experiments
(Supporting Information). Thus, neither the nature of the
counterion nor the metal ion exerted a dominant influence on
the thermal transitions of the metallosupramolecular polymers
studied, which, instead, appear to be largely governed by the
crystalline morphology imposed by the crystalline telechelic
oligomer.
The mechanical properties of 1·[Fe(ClO4)2]1.0, 1·[Zn-
(ClO4)2]1.0, 1·[Zn(OTf)2]1.0, and 1·[Zn(NTf2)2]1.0 were
investigated by dynamic mechanical thermal analysis
(DMTA). The thin films were heated at a rate of 3 °C/min
and tested at a frequency of 1 Hz under N2. All materials
showed very similar mechanical properties with room temper-
ature storage moduli of 440, 390, 420, and 470 MPa,
respectively (Figure 4). As in the case of similar systems
based on Mebip-terminated telechelic oligomer with a p-
phenylene ethynylene core,8,11 a steady decrease of the storage
modulus is observed with increasing temperature, consistent
with the formation of linear polymers (as opposed to physically
cross-linked structures). The 1·[La(NTf2)3]0.66 film proved to
be more brittle than the samples based on either Fe2+ or Zn2+
and could not be characterized by DMTA temperature sweeps.
The brittle nature of the 1·[La(NTf2)3]0.66 could be the result
of the weaker, more dynamic nature of the Mebip:La3+ bond,
which has resulted in weaker materials as has been seen in
previous studies,12−14 but it may also be related to highly cross-
linked nature of this material since the La3+ ion acts as a cross-
linker by binding to three Mebip ligands.
Figure 4. Dynamic mechanical thermal analysis (DMTA) temperature
sweeps of 1·[Fe(ClO4)2]1.0 (red), 1·[Zn(ClO4)2]1.0 (green), 1·[Zn-
(OTf)2]1.0 (blue), and 1·[Zn(NTf2)2]1.0 (magenta): storage modulus
(solid line), loss modulus (dashed line), and tan δ (dotted line). The
experiments were conducted at a heating rate of 3 °C/min and a
frequency of 1 Hz under N2.
Synthesis and Characterization of Cross-Linker 10. In
order to test the effect of cross-linking without using
“trifunctional” lanthanide salts (which, as discussed above,
may not afford materials with good mechanical properties on
account of weaker binding), a tetrafunctional cross-linker (10)
carrying four Mebip units was prepared (Schemes 3). It was
synthesized via the Sonogashira coupling of the ethynyl-
derivatized Mebip (3) to a tetraiodofluorene derivative (8)
followed by diimide reduction of the resulting ethynylene
precursor 9. Initial attempts to carry out the alkyne reduction
using literature procedures,20 namely toluene sulfonyl hydrazide
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Macromolecules
Article
Hz under N2 (Figure 5). Interestingly, it was found that at a
Scheme 3. Synthesis of Cross-Linker 10
content of 10 mol % the cross-linker 10 had little effect on the
Figure 5. Dynamic mechanical thermal analysis (DMTA) temperature
sweeps of 1·[Zn(OTf)2]1.0 (blue), and [10.9100.1]·[Zn(OTf)2]1.0 (dark
blue): storage modulus (solid line), loss modulus (dashed line), and
tan δ (dotted line). The experiments were conducted at a heating rate
of 3 °C/min and a frequency of 1 Hz under N2.
materials’ mechanical properties. Thus, the result confirms the
above conclusion that, unlike other metallosupramolecular
polymer systems based on low-Tg telechelic oligomers, the
mechanical properties of this film appear to be based primarily
on the crystalline polymeric core, with little effect from metal
ion, counterion or cross-linking.
CONCLUSIONS
■
Here we have shown that mechanically robust films (storage
moduli of ca. 400 MPa) of metallosupramolecular polymers can
be created, in which the mechanical properties are primarily
governed by the crystalline nature of the telechelic oligomer. As
such, the solid-state properties of these materials are much less
sensitive to the metal ion salt used in their assembly than
comparable metallosupramolecular polymers based on tele-
chelic oligomers with low-Tg cores. Even the introduction of a
moderate amount of a tetrafunctional cross-linker did not affect
their properties in an appreciable manner. Similarly, the
replacement of the perchlorate counterion with either triflate
or bistriflimide counterions does not significantly alter the
mechanical properties, but the latter materials offer higher
thermal stability. We have also shown that the solid-state
properties materials are sensitive to certain impurities, which
presumably compete with the metal ligand binding and/or an
offset of the ideal metal-to-ligand binding stoichiometry, as
would be expected for a step-growth-like assembly process.
Thus, this highlights important design criteria that need to be
taken into account and/or utilized while developing structure/
property relationships in metallosupramolecular polymers,
especially toward the goal of obtaining systems with enhanced
mechanical properties.
(TSH) and tripropylamine (TPA) in toluene, yielded only
mixtures of partially reduced products (Supporting Informa-
tion). The systematic variation of solvent and amine used
allowed us to develop reaction conditions (use of o-
dichlorobenzene as solvent and diisopropylamine as a less
basic amine than TPA; see Experimental Section and
Supporting Information) to reduce precursor 9 to cross-linker
10 in good yield. The improvement is presumably the result of
a slower decomposition of TSH under the new reaction
conditions,21 which dramatically increases the effectiveness of
the reduction.
Dynamic Mechanical Thermal Analysis of Cross-
Linked Films. To test the effect that 10 has on the mechanical
properties of the metallosupramolecular polymers under
investigation, a film composed of 90 mol % of 1 and 10 mol
% of 10 was prepared with a stoichiometric amount of
Zn(OTf)2 as the metal component ([10.9100.1]·[Zn(OTf)2]1.0).
The mechanical properties of the thin film were tested by
DMTA with a heating rate of 3 °C/min and a frequency of 1
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of 1; DSC traces of 1·[Zn(ClO4)2]1.0,
1·[Zn(OTf)2]1.0, and 1·[La(NTf2)3]0.66; polarized optical
microscopy images of 1·[Zn(OTf)2]1.0. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Article
(14) Burnworth, M.; Tang, L.; M.; Kumpfer, J. R.; Duncan, A. J.;
Beyer, F. L.; Fiore, G. L.; Rowan, S. J.; Weder, C. Nature 2011, 472,
334.
AUTHOR INFORMATION
■
Corresponding Author
(15) (a) Beach, W. F. Xylylene Polymers. In Encyclopedia of Polymer
Science and Technology, 3rd ed.; Kroschwitz, J., Ed.; John Wiley &
Sons: New York, 2004; Vol. 12, p 587 ff. (b) Greiner, A.; Mang, S.;
*E-mail: stuart.rowan@case.edu (S.J.R.); christoph.weder@
unifr.ch (C.W.).
Schafer, O.; Simon, P. Acta Polym. 1997, 48, 1.
(16) Steiger, D.; Tervoort, T.; Weder, C.; Smith, P. Macromol. Rapid
̈
ACKNOWLEDGMENTS
■
Commun. 2000, 21, 405.
This material is based upon work supported by the U.S. Army
Research Office (W911NF-09-1-0288 and W911NF-06-1-
0414) and the Swiss National Science Foundation (200021-
135405).
(17) Earle, M. J.; Mcauley, B. J.; Ramani, A.; Seddon, K. R.;
Thomson, J. M. PCT Pub. No. WO02/072260, Sept 19, 2002.
(18) Bhatt, A. I.; May, I.; Volkovich, V. A.; Collison, D.; Helliwell,
M.; Polovov, I. B.; Lewin, R. G. Inorg. Chem. 2005, 44, 4934.
(19) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mullen,
̈
REFERENCES
K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001,
123, 946.
■
(1) For recent reviews see: (a) Lohmeijer, B. G. G.; Schubert, U. S. J.
Polym. Sci., Part A: Polym. Chem. 2003, 41, 1413. (b) Dobrawa, R.;
Wurthner, F. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4981.
(20) Beck, B. J.; Kokil, A.; Ray, D.; Rowan, S. J.; Weder, C.
Macromolecules 2002, 35, 590.
(21) Nakagawa, T.; Makotot, O. J. J. Polym. Sci., Part A: Polym. Chem.
̈
(c) Friese, V. A.; Kurth, D. G. Coord. Chem. Rev. 2008, 252, 199.
(d) McKenzie, B. M.; Rowan, S. J. The Encyclopedia of Supramolecular
Chemistry; Taylor and Francis: New York, 2007. (e) McKenzie, B. M.;
Rowan, S. J. In Molecular Recognition and Polymers: Control of Polymer
Structure and Self-Assembly; Rotello, V. M., Thayumanavan, S.,
Hoboken, N. J., Eds.; Wiley: New York, 2008; Chapter 7, p 157.
(2) (a) Whittell, G. R.; Hager, M. D.; Schubert, U. S.; Manners, I.
Nature Mater. 2011, 10, 176. (b) Wang, X. S.; McHale, R. Macromol.
Rapid Commun. 2010, 31, 303. (c) Holliday, B. J.; Swager, T. M. Chem.
Commun. 2005, 23.
1968, 6, 1795.
(3) For some reviews see: (a) Fox, J. D.; Rowan, S. J. Macromolecules
2009, 42, 6823. (b) Wojtecki, R. J.; Meador, M. A.; Rowan, S. J. Nature
Mater. 2011, 10, 14. (c) Wilson, A. J. Soft Matter 2007, 3, 409.
(d) Serpe, M. J.; Craig, S. L. Langmuir 2007, 23, 1626. (c) Brunsveld,
L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. Rev. 2001,
101, 4071.
(4) (a) Kelch, S.; Rehahn, M. Macromolecules 1999, 32, 5818.
(b) Chiper, M.; Hoogenboom, R.; Schubert, U. S. Macromol. Rapid
Commun. 2009, 30, 565. (c) Meier, M. A. R.; Hofmeier, H.; Abeln, C.
H.; Tziatzios, C.; Rasa, M.; Schubert, D.; Schubert, U. S. e-Polym.
2006, 16, 1. (d) El-ghayoury, A.; Hofmeier, H.; de Ruiter, B.; Schubert,
U. S. Macromolecules 2003, 36, 3955. (e) Schwarz, G.; Bodenthin, Y.;
Geue, T.; Koetz, J.; Kurth, D. G. Macromolecules 2010, 43, 494.
(5) (a) Kelch, S.; Rehahn, M. Macromolecules 1998, 31, 4102.
(b) Archar, S.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 1994,
1895. (c) Achar, S.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1996,
15, 43. (d) Kaes, C.; Hosseini, M. W.; Rickard, C. E. F.; Skelton, B. W.;
White, A. H. Angew. Chem. 1998, 110, 970; Angew Chem., Int. Ed.
1998, 37, 920.
(6) (a) Beck, J. B.; Rowan, S. J. J. Am. Chem. Soc. 2003, 125, 13922.
(b) Zhao, Q. Y.; Beck, J. B.; Rowan, S. J.; Jamieson, A. M.
Macromolecules 2004, 37, 3529. (c) Rowan, S. J.; Beck, J. B. Faraday
Discuss. 2005, 128, 43. (d) Weng, W.; Beck, J. B.; Jamieson, A. M.;
Rowan, S. J. J. Am. Chem. Soc. 2006, 128, 1163. (e) Weng, W.; Li, Z.;
Jamieson, A. M.; Rowan, S. J. Soft Matter 2009, 5, 4647. (f) Kumpfer, J.
R.; Wie, J. J.; Swanson, J. P.; Beyer, F. L.; Mackay, M. E.; Rowan, S. J.
Macromolecules 2011, DOI: 10.1021/ma201659d.
(7) Beck, J. B.; Ineman, J. M.; Rowan, S. J. Macromolecules 2005, 38,
5060.
(8) Knapton, D.; Rowan, S. J.; Weder, C. Macromolecules 2006, 39,
651.
(9) Knapton, D.; Rowan, S. J.; Weder, C. Macromolecules 2006, 39,
4069.
(10) Burnworth, M.; Knapton, D.; Rowan, S. J.; Weder, C. J. Inorg.
Organomet. Polym. Mater. 2007, 17, 91.
(11) Burnworth, M.; Mendez, J. D.; Schroeter, M.; Rowan, S. J.;
Weder, C. Macromolecules 2008, 41, 2157.
(12) Kumpfer, J. R.; Jin, J. Z.; Rowan, S. J. J. Mater. Chem. 2010, 20,
145.
(13) Kumpfer, J. R.; Rowan, S. J. J. Am. Chem. Soc. 2011, 133,
12866−12874.
132
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