Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels

Article abstract of DOI:10.1021/jacs.1c06408

Sterically hindered Lewis acid and base centers are unable to form Lewis adducts, instead forming frustrated Lewis pairs (FLPs), where latent reactivity can be utilized for the activation of small molecules. Applying FLP chemistry into polymeric framework

Full text of DOI:10.1021/jacs.1c06408

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Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels
Utku Yolsal, Thomas A. R. Horton, Meng Wang, and Michael P. Shaver*
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ABSTRACT: Sterically hindered Lewis acid and base centers are unable to form Lewis adducts, instead forming frustrated Lewis
pairs (FLPs), where latent reactivity can be utilized for the activation of small molecules. Applying FLP chemistry into polymeric
frameworks transforms this chemistry into responsive and functional materials. Here, we report a versatile synthesis strategy for the
preparation of macromolecular FLPs and explore its potential with the ring-opening reactions of cyclic ethers. Addition of the cyclic
substrates triggered polymer network formation, where the extent of cross-linking, strength of network, and reactivity are tuned by
the steric and electronic properties of the ethers. The resultant networks behave like covalently cross-linked polymers, demonstrating
the versatility of FLPs to simultaneously tune both small-molecule capture and mechanical properties of materials.
ulky substituents can preclude the close contact required to
Bform thermodynamically favorable bonds. When applied to
Lewis acids (LAs) and bases (LBs), frustrated Lewis pairs
(FLPs) are formed (Figure 1A), promoting activation of small
molecules and subsequent applications in catalysis.1−5 Incorpo-
rating FLPs into polymer frameworks has the profound potential
to extend this concept into responsive materials, as the dynamic
nature of these bonds translates into functionality. The first-ever
such macromolecular FLPs, capable of forming self-healing
polymer networks via small-molecule activation, showed tunable
rheological responses.6−8 Increased Lewis acidity in macro-
molecular FLPs can activate CO2 and facilitate catalytic
formylations,9 while intramolecular and partially macromolec-
ular FLPs promote C−H activation and amine formylation.10,11
Despite these exciting advances, incorporation of FLPs within
polymer networks remains largely unexplored. This limited
scope of reactivity is, in part, driven by the synthetic challenge of
the air-sensitive polymers themselves. New polymeric systems
that unlock a broader range of reactivity could thus significantly
expand this nascent field. The FLP-mediated ring-openings of
cyclic substrates (Figure 1B) are of particular interest; activation
Figure 1. (A) Formation of FLPs. (B) Selected previously reported
reactions between cyclic ethers and FLPs.13,18 (C) Schematic
representation of the preparation of poly(FLP) networks using cyclic
ether substrates.
of these molecules has been reported for small-molecule LA/LB
pairs for tetrahydrofuran, dioxane, and 1,4-thioxane ethers.12−17
In general, the activation of these cyclic substrates proceeds via
heteroatom coordination to the Lewis acidic center, followed by
attack of a Lewis basic species to form a zwitterionic product.
FLP-mediated ring-openings of three- and four-membered
cyclic ethers are rare. In 2018, Slootweg et al. showed that
monosubstituted epoxides undergo regioselective FLP-medi-
ated ring-opening reactions forming zwitterionic species that
were stable at high temperatures.18 The inherent toxicity of
many epoxides and the robustness of the resultant small
molecules sparked interest in the capture of these substrates by
poly(FLP)s (Figure 1C).
of a functional monomer.19,20 While Si−B exchange reactions
are a potentially scalable strategy,21−23 the majority of previous
efforts to prepare air-sensitive LA-containing polymers through
either of these methods has required lengthy and challenging
syntheses that prevented nonexpert adoption. However, hydro-
Received: June 21, 2021
Published: August 13, 2021
To develop this chemistry we recognized the need to rethink
the synthesis strategy for preparing poly(FLP)s. Functionality
can be incorporated into polymer chains using two main
approaches: postpolymerization modification or polymerization
© 2021 The Authors. Published by
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boration reactions are a facile route to organoboranes; although
previously applied to build polymer chains,24,25 their applica-
tions for postpolymerization modifications are rare. We felt that
the residual alkene groups in the backbones of butadiene or
cyclohexadiene (co)polymers could provide the framework for
synthetically accessible, scalable, polymeric Lewis acids.26
This paper thus reports the development of a new, accessible
poly(FLP) system and explores the capture and ring-opening of
1,3-propylene oxide (oxetane, L1), 1,2-propylene oxide (L2),
styrene oxide (L3), and cyclohexene oxide (L4) to form
responsive, functional poly(FLP) networks (Figure 1C). Unlike
the diethyl azodicarboxylate triggered poly(FLP) gels,6,8 which
behaved as supramolecular assemblies, we show that these
poly(FLP) networks behave as covalently cross-linked chemical
networks. A change in these linkers thus directly impacts the
mechanical properties of the resulting polymeric networks,
revealing the versatility of FLPs to simultaneously tune both
reactivity and function.
Reactivity was modeled using zwitterionic small molecules to
probe cross-linking reaction, using CyB(C6F5)2 (1) and PPh3
(2). Complexes of the linker molecules were formed with 1 and
2, forming SM1, SM2, SM3, and SM4 (Scheme 1). The mimics
were characterized using NMR spectroscopy (Table S2, Figures
S8−S23) with tetracoordinate borate centers confirmed by
upfield shifts in 11B NMR spectra and tetracoordinate
phosphonium ions observed in downfield-shifted 31P NMR
peaks. para-Fluorine atoms additionally showed the large upfield
shifts common for a tetracoordinate boron.37 For SM3 and
SM4, two sets of fluorine environments were observed
corresponding to the diastereotopic C6F5 signals. The selectivity
of the ring-opening was probed through 2D NMR studies and
13C−31P coupling constants which revealed that 2 attacks at the
less sterically hindered carbon for SM2 (47 Hz) and at the more
hindered carbon for SM3 (44 Hz). The latter was attributed to
the stabilizing inductive forces of the phenyl ring (see the
Supporting Information).
To meet the steric bulk requirement of FLPs, 1,3-cyclo-
hexadiene (1,3-CHD) was copolymerized with styrene (Figure
1C, feed ratio 1:9), resulting in the formation of copolymer P1
with sterically encumbered cyclohexene units (Supporting
Information section SYN1 and Table S1).27 Treatment with
bis(pentafluorophenyl)borane, a highly Lewis acidic hydro-
borating agent prepared in a single step from commercially
available starting materials,28 gave the desired organoborane
macromolecules (Supporting Information section SYN2). This
work presents one of the rare examples of alkylborane-based
polymeric LAs which have long been considered prone to
degradation via retro-hydroboration.29 However, the presence
of neighboring styrene units flanking 1,3-CHD repeat units can
create π−π interactions with the C6F5 groups that inhibit boryl
migration.28,30−32
Scheme 1. FLP-Mediated Ring-Opening Reactions of L1, L2,
L3, and L4 Using 1/2 Pair
While hydroboration could not be monitored by 11B NMR
spectroscopy due to signal broadening,33,34 complete con-
1
version of (C6F5)2BH was confirmed by H and 19F NMR
Encouraged by these FLP-mediated ring-openings, we
explored the reactivity of L1, L2, L3, and L4 as triggers for
polymeric networks using P2 and P3. Two separate solutions of
these polymers were prepared in toluene with an equivalent
number of B/P units. The frustrated nature of their structure was
confirmed, with no change in solution viscosity observed upon
mixing. The addition of cyclic substrates (2.5 equiv) triggered
immediate gelation (Movies S1 and S2, Figure S30), attributed
to the capture and poly(FLP)-mediated ring-opening of the
substrates (Figure 2A). Although coordination of the cyclic
ether substrate to P2 may be expected to disrupt the π−π
interactions to induce retro-hydroboration, the obtained gels
were stable hinting the rapid and efficient formation of the
borate groups. Unlike our first generation poly(FLP) net-
works,6,8 no chain rearrangement was visually observed. Gel
fractions and swelling capacities of the samples were determined
(Figures S31 and S32). The formation of N1 gave a gel fraction
of ∼1, suggesting all macromolecules react to form a continuous
network structure. The gel fraction then decreased from N1 to
N2, N3, and N4. As expected, the reverse trend, peaking at a
swelling ratio of 16 for N3 and N4 compared to that of the
tighter gel N1 (5). As both solvent and temperature were
controlled during gelation, this swelling ratio can be related to
the cross-link density using the Flory−Rehner equation where
the swelling ratio depends on the molecular weight of the chains
between effective cross-links.38 The cross-link density thus
decreases, and the number of unlinked P/B species increases, in
the order N1, N2, N3 and N4.
spectra (Figures S2 and S3), with the disappearance of CHD
olefin peaks and broadening of the aryl fluorine resonances. A
concomitant increase in polymer Mn (P2), observed by gel
permeation chromatography, correlated to the addition of boron
moieties (Table S1). The Lewis acidity matched that of a small-
molecule mimic, as confirmed by a 31P NMR study using the
Gutmann−Beckett method (acceptor number = 68 vs 69 for
CyB(C6F5)2, 1; see Supporting Information sections SYN4 and
SYN5).35,36 Line width (ω1/2) of the 31P resonances also
increased from 50 (1) to 230 Hz (P2), further confirming the
polymer-supported nature of the LAs. While this paper focused
solely on this highly Lewis acidic variant, the ease of synthesis
and large variety of commercially available hydroborating agents
suggest this is a flexible synthetic route to tunable polymeric
LAs.
To prepare the LB polymer, 4-styryldiphenylphosphine was
copolymerized with styrene using anionic polymerization. In
earlier poly(FLP) publications, blocky polymers have been
reported using reversible-addition−fragmentation-chain-trans-
fer (RAFT) copolymerization,6,9 meaning that functional
monomers were not distributed along a polymer chain. Anionic
copolymerization eliminates the presence of any donating RAFT
agents on the polymer chains. Unwanted interactions with the
Lewis acidic borane units can therefore be minimized while
maintaining a well-controlled polymerization (Table S1) and
potentially afford a more random copolymerization and thus
increase potential sites for capture and cross-linking.
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(Figures 2B−E and S35−S38). N1 revealed a very dense
microstructure, with pore diameters from 100 to 400 nm and an
irregular continuous polymer mesh. These relatively small pore
sizes support the smaller solvent uptake capacity of this polymer
network. N2 showed a distinctive internal pattern compared to
that of N1, with closed-cell pores and thin polymer walls
resulting in pores roughly 5 times larger in diameter. N3 and N4
showed a much wider range of pore diameters, from several
hundred nanometers to micrometers. The random distribution
and irregular structures confirm that steric hindrance and
basicity influence cross-linking at a structural level.
Figure 2. Illustrative representation of cross-linking chemical structures
(A, left) and an example gel picture of N1 (A, right) and the SEM
images to show the microstructure of the freeze-dried gels of N1 (B),
N2 (C), N3 (D), and N4 (E).
Figure 3. Rheological characterization of the prepared poly(FLP)
networks. Storage and loss moduli values were tested against various
frequencies (A), amplitudes (B) and temperatures (C), and the creep-
recovery behaviors (D) were recorded under constant applied stress.
Computational studies of small molecules showed the
importance of both proximity and geometry of the LA and LB
in ring-opening epoxides.18 The trend in cross-link density can
be similarly explained: As LA coordination constitutes the initial
step in ring opening,14 basicity and inductive effects play a
dominant role at the beginning of gelation where chains are
unencumbered. Increased basicity (L1 < L2 < L3 < L4) results
in stronger cyclic ether−LA coordination and induces rapid
cross-linking, limiting polymer chain rearrangement and
generating a nonhomogeneous microstructure. A lower energy
barrier to ring opening has a similar impact as this also
accelerates cross-linking over chain rearrangement. As the
phenyl group on L3 can resonance stabilize the positive charge
generated by LA coordination and similarly L4 through
hyperconjugation,39 they promote the fastest cross-linking and
lowest density. As gelation progresses and chains become more
restricted, substrate steric influences predominate. Bulkier side
chains (L2 vs L1) hinder further cross-linking and thus preclude
higher cross-linking densities. While in our first-generation
systems, this rapid cross-linking was then followed by rearrange-
ment to thermodynamically optimized gels,4,5 cyclic ether ring
opening forms kinetically trapped gels (vide infra).
The mechanical properties of these poly(FLP) gels were
characterized using rheology. With all the polymeric networks,
the storage (G′) moduli dominated over the loss (G′′) moduli
over all the tested frequencies under small-amplitude oscillatory
shear (Figure 3A). This suggests that most of the deformation
energies were stored elastically within the networks, suggesting
that a covalently cross-linked polymer network is formed. This
was attributed to the release of ring strain preventing the
thermodynamically unfavorable reverse reaction, leading to a
behavior that is dramatically different from all previous
poly(FLP) gels that act as reversible supramolecular assemblies
with a strong frequency dependency.8 This enabled the
preparation of poly(FLP)s with tunable mechanical properties.
Although G′ is not completely independent of frequency, as
would be expected from a perfectly covalently cross-linked
polymer network, its dependency is plateau-like suggesting an
imperfectly cross-linked network with dangling polymer
chains.40 The power law dependency of G′ to ω was determined
(Figure S41) and increased from N1 to N4, with higher values
correlating with more imperfections in the network structures, in
agreement with the SEM results. Moduli values can be related to
the stiffness of the polymeric networks and ranged from 105 to
102 Pa with N1 being the stiffest, followed by N2, N3, and N4,
respectively. The affine network model, where the modulus of a
polymer network increases with its cross-link density,40 suggests
The microstructures of the poly(FLP) networks were
characterized using scanning electron microscopy (SEM). The
cross-sectional morphology of all samples showed noticeably
clear continuous 3D polymer mesh and porous structure
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that cross-linking density decreases from N1 to N4, in
agreement with the swelling data.
M13 9BL, United Kingdom; orcid.org/0000-0002-7152-
6750; Email: michael.shaver@manchester.ac.uk
Amplitude sweep tests (Figures 3B and S43−S44) further
support these mechanically tunable networks, as N1 has the
highest yield stress (3200 Pa) and the lowest yield strain (28%)
(Table S5), followed by N2 (810 Pa, 52%), N3 (410 Pa, 73%),
and N4 (62 Pa, 98%). Only marginal changes were observed in
the moduli values when temperature was varied from 0 to 40 °C
at a constant frequency and under a small strain (Figure 3C),
demonstrating the stability of the polymeric gels over the tested
range. Creep-recovery behaviors of N1, N2, and N3 confirmed
the strong viscoelastic behaviors of these poly(FLP) networks
(Figures 3D and S46). Creep strain values of 3, 17, and 36%
were observed for N1, N2, and N3, respectively. An increased
number of junctions precludes macromolecular movement,
preventing strain from increasing.
Authors
Utku Yolsal − Department of Materials, School of Natural
Sciences, University of Manchester, Manchester M1 3BB,
United Kingdom; orcid.org/0000-0003-3183-9100
Thomas A. R. Horton − Department of Materials, School of
Natural Sciences, University of Manchester, Manchester M1
3BB, United Kingdom; orcid.org/0000-0001-6719-7440
Meng Wang − Department of Materials, School of Natural
Sciences, University of Manchester, Manchester M1 3BB,
United Kingdom; orcid.org/0000-0002-5926-4970
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c06408
While covalently linked, the gels remain responsive. Triggered
degradation of N1 and N2, our most robust polymer networks, is
achieved through addition of either a stronger Lewis acid (BCl3)
or Lewis base (pyridine). LA-induced degradation is particularly
rapid, with gels degrading within minutes back to polymeric
species (Figures S49−S51, Movie S3) and pyridine degradation
occurring over days.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
While this first disclosure of the system precluded full
development of an application of these materials, interesting
avenues for future investigations are found in the observation of
macroscopic changes in the gels. Upon the addition of excess
epoxides, unique color patterns were observed, suggesting future
opportunities as chemical sensors (Figure S59A,B), while for
oxetane-based networks, lowering the cross-linking density
accesses L1-linked polymer networks that serve as triggerable
adhesives (Figure S59C).
In summary, we report the first cyclic-ether-triggered P,B-
based polymer networks. This new chemistry exploited a novel,
easily prepared, polymeric Lewis acid using the versatility of
hydroboration chemistry. The ring-opening reactions of cyclic
ethers gave poly(FLP) gels which behaved as covalently cross-
linked networks, contrasting with previous systems, and show
that both reactivity and function can be tuned. Our current
efforts are exploring the reactivity of these captured epoxides and
an increased diversity of small-molecule triggers.
ACKNOWLEDGMENTS
■
We kindly thank the Leverhulme Trust (81420) and the
University of Manchester for financial support. This work was
supported by the Henry Royce Institute for Advanced Materials,
funded through EPSRC grants EP/R00661X/1, EP/S019367/
1, EP/P025021/1, and EP/P025498/1 and the Sustainable
Materials Innovation Hub, funded through the European
Regional Development Fund OC15R19P. The authors also
thank Dr. Elisabeth Francis for the SEM images.
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* Supporting Information
The Supporting Information is available free of charge at
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AUTHOR INFORMATION
■
Corresponding Author
Michael P. Shaver − Department of Materials, School of
Natural Sciences, University of Manchester, Manchester M1
3BB, United Kingdom; Sustainable Materials Innovation Hub,
Henry Royce Institute, University of Manchester, Manchester
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