Poly(oxime-ester) Vitrimers with Catalyst-Free Bond Exchange

Article abstract of DOI:10.1021/jacs.9b06668

Vitrimers are network polymers that undergo associative bond exchange reactions in the condensed phase above a threshold temperature, dictated by the exchangeable bonds comprising the vitrimer. For vitrimers, chemistries reliant on poorly nucleophilic bond exchange partners (e.g., hydroxy-functionalized alkanes) or poorly electrophilic exchangeable bonds, catalysts are required to lower the threshold temperature, which is undesirable in that catalyst leaching or deactivation diminishes its influence over time and may compromise reuse. Here we show how to access catalyst-free bond exchange reactions in catalyst-dependent polyester vitrimers by obviating conventional ester bonds in favor of oxime-esters. Poly(oxime-ester) (POE) vitrimers are synthesized using thiol-ene click chemistry, affording high stretchability and malleability. POE vitrimers are readily recycled with little degradation of their initial mechanical properties, suggesting exciting opportunities for sustainable plastics.

Full text of DOI:10.1021/jacs.9b06668

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Poly(Oxime–Ester) Vitrimers with Catalyst-Free Bond Exchange
Changfei He, Shaowei Shi, Dong Wang, Brett A. Helms, and Thomas P. Russell
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06668 • Publication Date (Web): 21 Aug 2019
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Poly(Oxime–Ester) Vitrimers with Catalyst-Free Bond Exchange
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Changfei He,1 Shaowei Shi*1, Dong Wang*1, Brett A. Helms2 and Thomas P. Russell*1, 3, 4
1
Beijing Advanced Innovation Center for Soft Matter Science and Engineering & State Key Laboratory of Organic−Inorganic
Composites，Beijing University of Chemical Technology, Beijing, 100029, China
2
The Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United
States
3
Polymer Science and Engineering Department, University of Massachusetts Amherst, Massachusetts 01003, United States
4
Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United
States
Supporting Information Placeholder
catalyst-free ester-based vitrimer chemistry with intrinsically low
viscosities at high temperatures for processing and high viscosities
at service temperature for creep resistance is desirable, but remains
an outstanding challenge.
ABSTRACT: Vitrimers are network polymers that undergo
associative bond exchange reactions in the condensed phase above
a threshold temperature, dictated by the exchangeable bonds
comprising the vitrimer. For vitrimers, chemistries reliant on
poorly nucleophilic bond exchange partners (e.g., hydroxy-
functionalized alkanes) or poorly electrophilic exchangeable
bonds, catalysts are required to lower the threshold temperature,
which is undesirable in that catalyst leaching or deactivation
diminishes its influence over time and may compromise re-use.
Here we show how to access catalyst-free bond exchange reactions
in catalyst-dependent polyester vitrimers by obviating conventional
ester bonds in favor of oxime–esters. Poly(oxime–ester) (POE)
vitrimers were synthesized using thiol–ene click chemistry,
affording high stretchability and malleability. POE vitrimers were
readily recycled with little degradation of their initial mechanical
properties, suggesting exciting opportunities for sustainable
plastics.
Oxime chemistry provides a promising strategy to prepare
dynamic covalent polymeric materials. Oximes have a lower pKa
than aliphatic alcohols.33 Therefore, we hypothesized that it could
be feasible to carry out transesterification reactions in poly(oxime–
ester) (POE) vitrimers without using an embedded catalyst. Here,
we show that indeed, oxime–ester bonds (Scheme 1a) are
associative dynamic covalent bonds from which to prepare POE
vitrimers that require no embedded catalysts to initiate bond
exchange in the solid-state. POE vitrimers were prepared using a
rapid photo-initiated thio–ene click reaction (Scheme 1b). POE
vitrimers, within the formulation ranges explored here, exhibited
high malleability and excellent reprocessability, in that they are
readily recycled with little degradation of their initial mechanical
properties.
Scheme 1. (a) Transesterification of oxime–ester bonds. (b)
Incorporating oxime–ester bonds into vitrimer networks using
photocuring.
Vitrimers are a class of thermally re-processable network
polymers, which rely on associative dynamic covalent bond
exchange reactions to flow, relax, and recover. Subsequent to the
pioneering work of Leibler1 and co-workers, a series of vitrimers2
or vitrimer-like3,4 materials have been developed by different types
of
dynamic
covalent
bonding
reactions,
such
as
transesterification,5–10 vinylogous urethane transamidation,11,12
transcarbamoylation,13–15 olefin metathesis,16 imine bond
exchange,17–19 boronic ester exchange,20–22 alkoxysilane exchange
reaction,23–25 Diels−Alder cycloaddition,26,27 reversible radical
chemistry,28,29 oxime−carbamate transcarbamoylation.30 and
diketoenamine exchange.31 Dynamic transesterification reactions,
however, usually rely on large amounts of catalysts, such as
To investigate the bond exchange reaction mechanism (Scheme
S6) of oxime–ester bonds (Figure 1a), we synthesized small
molecule oxime–ester 1 and oxime 2, and mixed them in 1:1
stoichiometric ratio in deuterated chloroform (CDCl3).The 1H
NMR spectrum of the mixture in Figure 1b showed that, at room
temperature, for 1 and 2, the proton signals of the methyl group
near the nitrogen atom were located at 2.38 ppm and 2.26 ppm,
respectively. Upon heating, two new proton signals at 2.34 ppm and
2.30 ppm appeared, giving a clear proof that the exchange reaction
Zn(OAc)2 , Sn(Oct2)5, TBD7, DBU32, which are often toxic or have
1
poor miscibility with vitrimer networks, which limits their
applications. Moreover, the activation barrier for bond exchange
reactions in transesterification reaction systems is usually high,
requiring high temperatures for vitrimer reprocessing, which tends
to degrade the vitrimer on repeated recycling. Developing a
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took place, producing target molecules 3 and 4. This result was
at room temperature showed that all three networks were insoluble
but swollen after 48 h, with a gel fraction of 98.6%, 96.8% and
92.6% for POE-a, POE-b and POE-c respectively (Table S2). By
taking POE-b as an example, differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA) yielded a glass-
transition temperature (Tg) of 7.5 °C and good thermal stability with
a weight loss of 5% at 250 °C (Figure S5 and S6, Table S2).
Dynamic mechanical analysis (DMA) shows that storage modulus
of POE-b is 2.9 GPa in the glass state and then experience an abrupt
drop in the storage modulus in the rubbery state to 4.2 MPa (Figure
S7). As expect, the molecular weight between cross-links (Mc)
increase from 1.7 to 4.5 kg mol−1 as we increase the equivalent of
flexible chain (Table S2). Tensile tests demonstrated that POE-b
had a strain-at-break (εb) of 160 ± 22% and a stress-at-break (σb) of
2.2 ± 0.27 MPa, which are similar to mechanical properties of
elastomers (Table S3, Figure S11).
To study the thermal reversibility of the bulk materials, two
pieces of POE-b were immersed into N-methyl pyrrolidone (NMP)
respectively, while one NMP solvent containing excess
acetophenone oxime. As shown in Figure 2d, after heating the
solvent to 120 °C for 24 h, the POE networks are insoluble in pure
NMP, but can be completely dissolved in NMP with excess
acetophenone oxime (Figure 2c), which suggests such oxime
transesterification in the network is in keeping with the associative
mechanism of vitrimers. On the other hand, as-synthesized linear
POE was treated with 1 equivalent acetophenone oxime at 120 °C
for 2 h. Gel permeation chromatography (GPC) curves shows a
remarkable degradation of linear POE (Figure 2d), further
indicating thermal reversibility of oxime-ester bonds via oxime
transesterification, which is in good agreement with the results of
small molecule models.
further confirmed by mass spectroscopy (Figure S2). The integral
area of methyl proton of a’ and b’ increased gradually as
temperature and time increased, which showed that the
consumption of 1 and 2 and the formation of 3 and 4 (Figure 1b and
Figure S1). The consumption of 1 was significantly influenced by
temperature (Figure 1c) and the oxime transesterification rate
constant was calculated to be 0.07 to 0.66 h−1 as temperature
increased from 40 to 70 °C (Figure S1 and details can be found in
SI). Arrhenius analysis of the model reaction gave an oxime
transesterification activation energy (Ea) of 63.5 ± 7.8 kJ mol−1,
which is lower than that of conventional epoxy ester system
containing catalysts (Figure 1d). 1, 5 ,7 Moreover, in order to confirm
that the dynamic exchange reaction mechanism is the oxime
transesterification but not the imine exchange, two oxime–ester of
1 and 8 were also mixed at 1:1 stoichiometric ratio in DMSO-d6
and were heated to 150 °C for 2 h (Figure S3). The NMR results
suggest that there is no formation of 3 and 10, indicating the imine
exchange of oxime ester is negative under 150 °C.
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Figure 1. (a) Proposed oxime–ester exchange reaction. (b) 1H NMR
spectrum of the dynamic exchange reaction of oxime ester at
different temperature for 120 min. (the peaks indicate methyl group
near the nitrogen atom) (c) The consumption of 1 through oxime
transesterification as a function of time at different temperature. (d)
Arrhenius analysis and the activation energy of oxime ester
exchange reaction.
After studying the dynamic nature of the reaction systems based
on small molecules, POE networks using thiol–ene click chemistry
were produced. Oxime–ester monomer 5 was first synthesized by
the reaction between oxime 2 and adipyl chloride. Three cross-
linked POE networks were constructed by photopolymerizing 2,
monomer 5, 3,6-dioxaoctane-1,8-dithiol 6 and pentaerythritol
tetrakis(3-mercaptopropionate) (PETMP) 7, all with different
ratios (Figure 2a, Scheme S1 and Table S1). It should be noted that
the incorporation of oxime 2 provided sufficient oxime hydroxy
groups for transesterification. The complete consumption of the
thio group and incorporation of oxime ester components were
confirmed by FTIR for all of the three cross-linked networks
(Figure S4). The resultant network can then be remolded into ~1
mm thickness of film due to the facile topologic rearrangement via
oxime transesterification (Figure 2b). Swelling experiments in THF
Figure 2. (a) Chemical structure of monomers for building oxime–
ester network. (b) Exchange process of POE network via oxime
transesterification in POEs. (c) The solubility of POE-b in NMP:
swollen (left: 120 °C for 24h) and dissolved (right: 120 °C for 24
h). (d) GPC analysis of thermodynamic depolymerization of linear
POE in N,N-dimethylformamide.
The rheological properties of as-prepared POE vitrimers were
studied by stress-relaxation experiments performed using DMA. In
the linear viscoelastic regime, a 3% strain was applied on the
samples and stress decay was monitored as a function of time. As
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shown in Figure 3a and S8, a full stress-relaxation was observed at
all temperatures despite the highly cross-linked chemical
structures, indicating the highly malleable of POEs. The relaxation
time (τ) is determined when the stress of POEs relax to 1/e of their
original stress. The well-fit line of relaxation time verse
temperature suggests POE networks have Arrhenius flow
characteristics. Activation energies calculated from the slope of
lines are in close agreement with oxime transesterification
activation energy of small molecule models (Figure 3b). The result
indicates that the oxime transesterification dominates the relaxation
behavior in bulk materials, therefore the relaxation time or
viscosity are Arrhenius. It should be noticed that with an increasing
content of free oxime hydroxy group, the relaxation time decreases
for POE-a, from 5102 s to 3897 s, then to 2725 s at 100 °C (Figure
S9), which is another proof that the reversible reaction in the bulk
materials is through oxime transesterification. Creep-recovery
experiments were performed at an applied stress of 0.1 MPa and
the strain was monitored. At low temperature (50 °C), the
reversible exchange reaction is frozen, so the materials behave like
the conventional thermosets, with no deformation and most strain
recovers. While as temperature increases, oxime transesterification
is activated, for example, POE-b exhibits deformation rate of 1%
min−1 at 110 °C, and the residual strain is up to 24% (Figure 3c,
Figure S10). In a word, the flow ability and malleability in bulk
POEs is controlled by associative oxime transesterification.
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Figure 4. (a) Temperature dependence of thermal expansion of
POE networks with a heating rate of 2.5 K min−1. (b) Recycling
POE-b from pieces to reshaped transparent film (scale bar: 1cm).
(c) Stress-strain curves of POE-b and recycled POE-b.
Dilatometry experiments provide information of volume
changes as a material undergo thermal transition.1, 7 It is well-
known that the thermal expansion coefficient of a polymer network
without reversible chemical bonds remains constant above Tg.
However, for a polymer network with reversible covalent bonds,
segmental motion is triggered by the breaking and reforming of
bonds. Once topological rearrangement is activated upon heating,
an obvious increase in the expansion coefficient of networks is
observed and the network becomes malleable. As shown in Figure
4a, a notable increase of expansion coefficient was observed at
temperature of 134, 122 and 109 °C for POE-a, POE-b and POE-c
respectively. The different temperatures where the POE networks
become malleable is attribute to differences in the cross-link
density and it provides guidance for reprocessing temperatures.
To test the reprocessability of POE vitrimers, the networks were
cut into pieces and then molded into bulk materials. (Figure 4b).
The reprocessed samples were characterized by DMA, DSC, FTIR
and tensile testing. The mechanical properties including stress,
Young’s moduli and strain-at-break of the reprocessed samples are
very close to those of the initial samples (Table S3 Figure S11).
DMA confirmed the reprocessed sample have the same moduli at
rubbery plateau (Figure S14), which indicates the density of the
cross-links are constant during the remolding process. No Tg
variation was observed in recycled samples (Figure S13) and no
chemical change was measured by ATR-FTIR (Figure S12). In
addition, welding experiments were performed by cutting a
rectangle bar (1mm (T) × 6mm (W) × 30mm (L)) into two pieces
and then placing them in contact at 120 °C for 10 min. Optical
microscopy images show that the cracks almost disappear and the
welded sample can be loaded up to 0.2 kg (Figure S15).
Figure 3. (a) Normalized stress-relaxation curves of POE-b at
different temperature. (b) Arrhenius analysis and the activation
energy of POE networks. (c) Tensile creep and recovery of POE-b
with an applied stress of 0.1 MPa at different temperature. (d)
Angell fragility plot showing viscosity as a function of inverse
temperature normalized to Tv of polystyrene34, silica35 and POEs.
The topology freezing transition temperature, Tv, corresponding
to the transition from the solid to the liquid state, is determined by
the point when the viscosities becomes higher than 1012 Pa s1. For
three POE networks, Tv is in the range of 12−43 °C (Table S2),
which is a little higher than their Tg, indicating POE networks are
highly malleable in major rubbery plateau. The well-fit Angell
fragility plot shows the viscosity of POEs follows an Arrhenius law
over a very broad range temperature, which is similar to inorganic
compounds (e.g. silica35), rather than organic polymers (e.g.
polystyrene34) (Figure 3d).
In summary, we have designed and synthesized a new catalyst-
free POE vitrimers by using photo-initiated “thio–ene” reactions at
room temperature. The UV-polymerization of vitrimers shows a
great advantage for vitrimer manufacturing, which sets up a whole
range of processing techniques access to a networked architecture
facile and efficient. Similar to thermosets, POE networks are
insoluble in normal organic solvents with good structural stability.
However, at high temperature, owing to the oxime
transesterification, POE networks exhibit excellent malleability
and reprocessability. This new dynamic network was found to be
easily reprocessed and recycled numerous times without
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the ACS
Publications website.
Experimental details and characterization data
AUTHOR INFORMATION
Corresponding Author
*E-mail: shisw@mail.buct.edu.cn
*E-mail: dwang@mail.buct.edu.cn
*E-mail: russell@mail.pse.umass.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by National Natural Science Foundation
of China (51673016) and Grant No. 2194083 of Beijing Natural
Science Foundation. B.A.H. acknowledges support from the
Molecular Foundry, which is supported by the Office of Science,
Office of Basic Energy Sciences, of the U.S. Department of Energy
under Contract No. DE-AC02-05CH11231.
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