Structurally dynamic materials based on bis(N-heterocyclic carbene)s and bis(isothiocyanate)s: Toward reversible, conjugated polymers

Article abstract of DOI:10.1021/ma100524g

A study was conducted to demonstrate the formation of structurally dynamic materials based on bis(N-heterocyclic carbene)s (NHC) and bis(isothiocyanate). It was demonstrated that all the challenges involved in the construction and application of self-healing materials were overcome by the chemistry of NHCs substrates that formed reversible adducts with a variety of reagents, including other NHCs, organic electrophiles, and transition metals. The study also explored the feasibility of preparing structurally reversible polymers based on NHCs chemistry. Bis(NHC) was selected as a suitable monomer for the investigations, as its reactive carhene moieties were connected through an extended π-system and featured flexible branched alkyl N-subslituents to enhance solubility in organic sohents.

Full text of DOI:10.1021/ma100524g

Macromolecules 2010, 43, 3591–3593 3591
DOI: 10.1021/ma100524g
Scheme 1. Synthetic Methodology Used To Form a
Structurally Dynamic Polymer Derived from a Bis(NHC) and
Various Bis(isothiocyanate)s
Structurally Dynamic Materials Based on
Bis(N-heterocyclic carbene)s and Bis(isothiocyanate)s:
Toward Reversible, Conjugated Polymers
Brent C. Norris and Christopher W. Bielawski*
Department of Chemistry and Biochemistry,
The University of Texas at Austin, Austin, Texas 78712
Received March 8, 2010
Revised Manuscript Received March 19, 2010
Demand for reconfigurable and self-healing materials in elec-
tronic applications continues to grow at an escalating rate.1 Although
shape memory alloys offer promise for use in some of the afore-
mentioned applications,2 they function at relatively high tempe-
ratures and are challenging to process. As an alternative strategy,
we envisioned polymers with conjugated π-systems capable of
altering their molecular structures, specifically through depoly-
merization to fluidic monomers, in response to changes in exter-
nally applied stimuli. Accessing such reversible, conjugated poly-
mers (RCPs) presents a fundamental challenge in finding sets of
building blocks that are capable of assembling and disassembling
while maintaining unsaturation. For practical reasons, this dyna-
mic process should also exhibit perfect atom economy (i.e., no
byproduct evolution) and function without the need for catalyst(s)
or other exogenous species (i.e., solvents, etc.) Although an
impressive range of dynamic covalent reactions and associated
polymerizations are known,3 their underlying chemistries4 do not
fulfill the aforementioned requirements.
Hereinweshow that ananswer to thesechallenges may lie inthe
chemistry of N-heterocyclic carbenes5 (NHCs), substrates which
can form reversible adducts with a variety of reagents, including
other NHCs, organic electrophiles, and transition metals.6 NHC
coupling reactions with isothiocyanates7 were particularly appeal-
ing because they conserve unsaturation and create formally
conjugated linkages between the two moieties (see eq 1), meeting
two important design criteria for constructing RCPs. Analogous
toNHC dimerization, we reasoned that the NHC-isothiocyanate
coupling reaction may be rendered reversible6,8 through steric
tuning of the N-substituents. Ultimately, we envisioned a new class
of RCPs could be developed by combining aromatic bis-
(isothiocyanate)s with complementary bis(NHC)s.9
and phenylisothiocyanate, with stoichiometric 2,4,6-trideuterio-
phenyl isothiocyanate at 120 °C resulted in the formation of an
equimolar mixture of the starting materials and expected products
after 20 h. The exchange reaction was found to be first order in
adduct and zeroth order in isothiocyanate, and the NHC-
isothiocyanate dissociation process exhibited a positive ΔS‡ (21 (
3 cal mol-1 K-1).11 These results led us to believe that the afore-
mentioned NHC-isothiocyanate exchange reaction proceeded via
adduct dissociation followed by statistical recombination.
Building on these results, we next explored the feasibility
of preparing structurally reversible polymers based on NHC-
isothiocyanate coupling chemistry. Bis(NHC) 3 was selected as a
suitable monomer because its reactive carbene moieties are connec-
ted via an extended π-system and features flexible branched alkyl
N-substituents to enhance solubility in organic solvents.12 To
form high polymer with electronically delocalized structures,
bis(isothiocyanate) fluorene derivative 4,13 which contains two
n-hexyl chains at the 9-position to enhance solubility, was chosen
as the complementary monomer.
Addition of 3 to an equimolar quantity of 4 in DMF ([3]0 þ
[4]0=0.2 M) at ambient temperature resulted in the formation of
a viscous solution within 20 min. Pouring the resulting mixture
into excess diethyl ether caused polymer 5 to precipitate, which
was subsequently collected in 98% isolated yield via filtration.
The polymer was found to be highly soluble in nonpolar as well as
polar organic solvents, which facilitated characterization by
NMR spectroscopy. Compared to its monomers, the diagnostic
signals of 5 were shifted downfield and were in accord with
those expected from the structure shown in Scheme 1. Analysis of
5 using gel permeation chromatography (GPC) (eluent=0.1 M
LiBr in DMF) revealed that this material exhibited a number-
average molecular weight (Mn) of 14.8 kDa and was polydisperse
(PDI = 2.1),14 results typical for a step-growth polymerization
(see Figure 1). Relatively high molecular weight (MW) polymers
(Mn values up to 29.0 kDa) were prepared by performing the
Initial efforts were directed toward investigating the reversible
nature of the covalent bond formed between NHCs and isothio-
cyanates. As shown in eq 2, heating a toluene-d8 solution of adduct
1, which was prepared from 1,3-dineopentylbenzimidazolylidene10
*Corresponding author. E-mail: bielawski@cm.utexas.edu.
r 2010 American Chemical Society
Published on Web 03/29/2010
pubs.acs.org/Macromolecules

3592 Macromolecules, Vol. 43, No. 8, 2010
Communication
Figure 2. UV-vis spectra of polymer 5 before (red) and after (black)
the addition of 1.0 equiv of 4 (per repeat unit) as well as model com-
pounds 6 (green) and 7 (blue) in DMF.
Figure 1. Gel permeation chromatograms (eluent: 0.1 M LiBr in DMF;
rate: 1.0 mL/min): (a) polymer 5; (b) polymer 5 with 1.0 equiv of 4
(per repeat unit); (c) solution obtained from (b) after the addition of 1.0
equiv of 3 (per repeat unit); (d) solution obtained from (c) after the
addition of 1.0 equiv of 4 (per repeat unit); (e) solution obtained from
(d) after the addition of 1.0 equiv of 3 (per repeat unit).
aforementioned polymerization reactions at higher concentrations
([total monomer]0 up to 0.8 M).
Figure 3. Synthesis of the model compound 7 used to evaluate the elec-
tronic structure of polymer 5. R = CH2C(CH3)2C2H5. R0 = n-C6H13
.
We next probed the structurally dynamic characteristics of 5.15
Heating a DMF solution of this polymer (Mn=14.8 kDa; PDI=
2.1) to 120 °C in the presence of 1.0 equiv of 4 (relative to the
polymer’s repeat unit) resulted in a significant decrease in the
polymer’s MW. Analysis of the crude reaction mixture by GPC
resolved a series of low-MW peaks ranging from 1.5 to 6.1 kDa,
of which the predominant peak was attributed to 6 (isolated from
the reaction mixture and compared to an authentic sample; see
below). Subsequent addition of 3 to the aforementioned reaction
mixture resulted in the re-formation of polymer 5 with a Mn =
18.0 kDa (PDI = 2.9). Owing to the high fidelity of the NHC-
isothiocyanate coupling reaction, these depolymerization/polym-
erization cycles were repeated twice on the same samples. Ana-
logous results were obtained when 5 was depolymerized with 3
and then treated with 4 to re-establish the formation of polymer.
In parallel with molecular weight measurements, the afore-
mentioned depolymerization/polymerization reactions were
monitored via UV-vis spectroscopy (see Figure 2). Polymer 5
exhibited a λmax =410 nm (in DMF). However, upon depolym-
erization via treatment of 5 with excess 4, as described above, this
value shifted hypsochromically to 392 nm. Qualitatively, the
longer λmax of the polymer suggested that the effective conjugation
length of these materials exceeded that of their lower MW
analogues.
To gain additional insight into the electronic structure of this
polymeric material, model compounds 6 and 7, which approxi-
mate two possible repeating units of polymer 5, were synthesized.
The former was prepared by adding an excess (4.0 equiv) of 4 to a
THF solution (14 mM) of 3, followed by purification via column
chromatography. As shown in Figure 3, compound 7 was synthe-
sized by treating 1,3-di(2,2-dimethylbutyl)benzimidazolium tetra-
fluoroborate with sodium tert-butoxide (to generate the res-
pective NHC in situ) in THF followed by the addition of 0.3 equiv
of 4. Analysis of 6 via UV-vis spectroscopy revealed a λmax at
382 nm (DMF), a result which indicated that its effective chromo-
phore is shorter than that of 5. In contrast, the UV-vis spectrum
of 7 (λmax = 409 nm in DMF) was found to be similar to that
exhibited by the polymer. Hence, although polymer 5 is formally
conjugated, it appears to be comprised of repeating units that
are related to 7 and that these repeating units exhibit minimal
π-overlap with each other.16 The limited long-range electronic
communication does not prohibit the polymer’s ability to con-
duct electrical charge, however. For example, while thin (5 μm)
films of 5 were found to be nonconductive (σ < 10-10 S/cm),
exposure to iodine vapor increased the material’s electrical
conductivity to σ = 1.7 mS/cm, as determined using a four-point
probe technique.17 We are currently investigating other dopants
which could provide additional insight into the charge transfer
mechanisms exhibited by these doped materials.18
In conclusion, we have developed a new class of aromatic poly-
mers using NHC-isothiocyanate coupling chemistry. Simple
combination of bis(NHC)s with complementary bis(isothio-
cyanates)s produced polymers that were found to be structurally
dynamic. These results expand the utilities of free bis(NHC)s as
polymer building blocks and create a new design strategy for
accessing polyelectrolytes with charges as integral components of
their main chains.19 The polymerization reaction reported herein
is one of the few examples where chemical unsaturation is
maintained as monomer is converted to polymer and remains
poised for adaptation into a system that affords a structurally
reversible, conjugated polymer. Efforts toward these goals will
focus on increasing the effective conjugation length in the afore-
mentioned materials, principally via desymmetrization of the
NHC N-substituents as well as through varying the electronic
properties of the bis(NHC) and bis(isothiocyanate) monomers.
Acknowledgment. We are grateful to the ONR (N00014-08-1-
0729), the NSF (CHE-0645563), the Welch Foundation (F-1621),
and the Beckman Foundation for their generous financial support.
C.W.B. is a Cottrell Scholar of Research Corporation.
Supporting Information Available: Detailed experimental
procedures and characterization of all new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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Macromolecules, Vol. 43, No. 8, 2010 3593
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