Covalent Adaptable Networks Using β-Amino Esters as Thermally Reversible Building Blocks

Reversible Building Blocks

Article

Covalent Adaptable Networks Using β‑Amino Esters as Thermally

Christian Taplan, Marc Guerre, and Filip E. Du Prez*

Cite This: J. Am. Chem. Soc. 2021, 143, 9140 −9150

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sı Supporting Information

ABSTRACT: In this study, β-amino esters, prepared by the aza-

Michael addition of an amine to an acrylate moiety, are

investigated as building blocks for the formation of dynamic

covalent networks. While such amino esters are usually considered

as thermally nondynamic adducts, the kinetic model studies

presented here show that dynamic covalent exchange occurs via

both dynamic aza-Michael reaction and catalyst-free transester-

iﬁcation. This knowledge is transferred to create β-amino ester-

based covalent adaptable networks (CANs) with coexisting

dissociative and associative covalent dynamic exchange reactions.

The ease, robustness, and versatility of this chemistry are

demonstrated by using a variety of readily available multifunctional

acrylates and amines. The presented CANs are reprocessed via

either a dynamic aza-Michael reaction or a catalyst-free transesteriﬁcation in the presence of hydroxyl moieties. This results in

reprocessable, densely cross-linked materials with a glass transition temperature (T ) ranging from −60 to 90 °C. Moreover, even for

the low T materials, a high creep resistance was demonstrated at elevated temperatures up to 80 °C. When additional β-hydroxyl

group-containing building blocks are applied during the network design, an enhanced neighboring group participation eﬀect allows

reprocessing of materials up to 10 times at 150 °C within 30 min while maintaining their material properties.

INTRODUCTION

scientists put much research eﬀort in exploring additional

■

2−14

covalent dynamic reactions.

For CANs, applicable

Current times are often referred to as the plastic era in which a

life without polymer materials is unimaginable. However,

plastic pollution accompanying the technological advance-

dynamic covalent reactions are for instance reversible cyclo-

,15,16

17−20

21,22

additions,

transalkylation,

tion

disulﬁde

or silyl ether exchange,

3−25

6,26,27

trans(thio)esteriﬁcation,

of various reactive centers, and others.

Many of those chemistry platforms have also been

transamina-

ments is becoming an ever increasing issue. In particular, one

important polymer material category, referred to as thermoset-

ting materials, which was intentionally designed to be

dimensionally stable and to provide high chemical and

mechanical durability, is nowadays in the focus of research

toward sustainable end-of-life options. Indeed, despite their

superior properties compared to thermoplastics, thermosets are

very challenging to recycle and consequently often simply go

8−33

34−39

40−44

highlighted in recent reviews.

Some of them apply

concepts of internal catalysis or neighboring group partic-

ipation (NGP) to accelerate or enable exchange in the ﬁrst

place. This was for instance applied in the context of the

aforementioned silyl ether exchange by using a γ-positioned

to waste or are being incinerated. Therefore, the scientiﬁc

amino moiety and also, for transesteriﬁcations, by leveraging

community proposed strategies to implement reversible

covalent bonds into the thermoset compositions, which

allowed the creation of three-dimensional covalent networks,

yet with reversible cross-linking points making reprocessing

a β-hydroxyl group. Also, introducing a carboxylic or a

sulfonic acid group in proximity of an ester group was found to

promote a dissociative transesteriﬁcation mechanism when

46,47

terephthalate esters were applied.

Modifying the nucleo-

−6

upon a stimulus possible.

phile in the exchange reaction by making using of β-amino

The introduction of covalent adaptable networks (CANs)

followed a tradition in science and chemistry to consider

molecules and reactions from the past as inspiration to solve

Received: March 31, 2021

Published: June 14, 2021

current problems and push scientiﬁc boundaries. In fact,

already decades ago, it was reported that cross-linked rubbers

using disulﬁde bonds and silyl ethers showed thermally

activated dynamic behavior and thus enabled the creation of

−11

reprocessable covalent networks.

In more recent years,

2021 American Chemical Society

J. Am. Chem. Soc. 2021, 143, 9140−9150

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Article

modiﬁed alcohols also allowed tailoring of the dynamic

behavior.

permanently cross-linked reference materials. Hence, to the

best of our knowledge, β-amino esters were not explored in

CANs despite their vast application potential.

Two coexisting mechanistic pathways were reported when

thiols were used to create thioesters, i.e., an associative and

dissociative one. Other chemical reactions allow for two

RESULTS

■

coexisting mechanistic pathways, making use of diﬀerent

Model Reactions of β-Amino Esters. The ﬁrst research

question was to investigate whether a β-amino ester can

undergo thermally activated dynamic covalent exchange and by

which mechanisms this could occur. For this purpose, N-

methylbutylamine was reacted with 2-ethylhexyl acrylate to

form 2-ethylhexyl 3-(butyl(methyl)amino)propanoate (1)

exchange chemistries separately, adjusting the dissociation

rate with varying substituents, or combining dissociative with

52,53

associative exchange reactions elegantly using thiols.

Yet

combining associative and dissociative dynamic covalent

chemistries in a straightforward fashion is rather rare. Recently,

Dichtel and Elling raised the question of whether the use of a

solely associative exchange mechanism is desirable in view of

material design constraints to meet current processing

Figure 1 and Figure S1 ). This model compound 1 was

challenges.

In this context, here a strategy is presented based on the aza-

Michael addition of amines to acrylates (Scheme 1). First, the

Scheme 1. Display of an Aza-Michael Addition between

Amines and Acrylates with the Formation of a β-Amino

Ester That Provides Possibilities for Associative and

Dissociative Covalent Dynamic Chemistries

Figure 1. (a) Reaction scheme describing the catalyst-free dynamic

exchange with (i) the transesteriﬁcation of the β-amino ester 1 with

benzyl alcohol and (ii) the dynamic (retro) aza-Michael reaction with

benzyl acrylate. (b) Arrhenius plots of the monitored reactions

obtained by following the decay of 1 as a function of time at each of

the investigated temperatures.

created activated esters promote catalyst-free transesteriﬁcation

with alcohol groups as associative exchange mechanism, with

the advantage of maintaining cross-linking points and a gradual

decrease in viscosity. In addition, the formed aza-Michael

adducts allow for a reversible, dissociative breaking point on

the electrophilic side of the dynamic unit. Moreover, possible

side reactions, originating from, e.g., amidation reactions, are

not expected to harm the reprocessability of the network, since

an acrylamide is expected to be more prone to undergo

reversible aza-Michael reaction and the released alcohol is

available for transesteriﬁcation. All of this is achieved in a

straightforward fashion and thus provides an opportunity to

use a myriad of readily available acrylate, amine, and alcohol

functionalities.

obtained with a 95% yield after puriﬁcation. Afterward, kinetic

experiments were conducted in bulk at various temperatures in

the presence of an excess of either benzyl alcohol or benzyl

acrylate to check for transesteriﬁcation or dynamic aza-Michael

reaction, respectively (Figure 1). This excess allowed us to

push the system kinetically to a pseudo-ﬁrst-order reaction so

that linearity during the ﬁrst time points, from which the

reaction constants are obtained, was maintained.

When monitoring thermally activated transesteriﬁcation, the

β-amino ester 1 was combined with 12 equiv of benzyl alcohol

(reaction 1 in Figure 1a). This reaction was monitored via H

The aza-Michael reaction was widely explored and studied in

the context of biomaterials and investigated as for instance

degradable polymer materials, for gene delivery, or as

NMR by following the disappearance of the signal at 3.91 ppm

belonging to 1 and the appearance of the signal at 3.42 ppm

attributed to the newly formed 2-ethylhexyl alcohol. NMR

spectra corresponding to the kinetic study at 100 °C are

provided in the Supporting Information (Figure S2). Notably,

the reappearance of the proton signals between 5.70 and 6.30

ppm, corresponding to the acrylate of the respective amino

55−59

precursors for thermosetting resins.

Yet interestingly,

apart from applying a speciﬁcally designed cross-linker, the

aza-Michael reaction is usually considered to create permanent

bonds in bulk materials and was applied for creating

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amounts of amine protons to acrylates (Figure 2 a).

Article

Table 1. Overview of Composition and Physical Properties of β-Amino Ester-Based CANs

CAN

BAE-1

BAE-2

acrylate

amine

T_g_D_S_C(°C) T_d_‑₉₅_%(°C) soluble fraction (%) swelling degree (%) E_a₍_f_l_o_w₎(kJ mol⁻¹

)

G′ (MPa)

TMPTA

PTA

Jeﬀamine

Priamine

MBCA

Jeﬀamine

Priamine

MBCA

−57

−13

−59

−2

315

275

280

330

300

295

3.0

5.8

2.4

455

295

163 ± 12

170 ± 14

153 ± 6

242 ± 8

110 ± 4

184 ± 7

0.45 ± 0.05

0.96 ± 0.01

1.38 ± 0.02

0.72 ± 0.01

1.16 ± 0.01

0.89 ± 0.05

BAE-3

BAE-1-OH

BAE-2-OH

BAE-3-OH

DSC glass transition temperature (T

). TGA onset temperatures after 5% weight loss (T

THF. Obtained after swelling in THF for 7 days at rt. No soluble fraction detectable. Obtained from a time sweep measurement at 120 °C.

). Obtained after Soxhlet extraction for 24 h in

gDSC

d‑95%

ester, is detected and pointing to a thermally reversible

behavior of the aza-Michael-adduct (vide infra). Monitoring

diﬀerent temperatures, ranging from 80 to 140 °C, resulted in a

kinetic plot that demonstrates the possibility for the β-amino

ester 1 to undergo thermally activated, catalyst-free trans-

esteriﬁcation (Figure 1b and Figure S4a ).

β-Amino Esters-Based Covalent Adaptable Networks.

This conceptually demonstrated catalyst-free dynamic covalent

exchange chemistry was translated to the design of CANs.

Therefore, several β-amino ester-based networks (BAE) were

synthesized via the aza-Michael addition of small and

macromolecular multifunctional primary amines (4,4′-

methylenebis(cyclohexylamine) (MBCA), Jeﬀamine D2000,

and Priamine 1074) with two types of multifunctional,

technical grade acrylates (trimethylolpropane triacrylate

Furthermore, because 1 provides a tertiary amine and upon

dissociation also a secondary amino moiety, for which a

catalytic eﬀect was earlier described to catalyze the trans-

esteriﬁcation for epoxy networks, the transesteriﬁcation of 1

was tested in the presence of either a tertiary or a secondary

amine as external catalyst. Therefore, 5 mol % of N,N-

diisopropylethylamine (DIPEA) or dibutylamine was added to

the reaction mixture and the reaction was monitored at 120

(

TMPTA) and pentaerythritol triacrylate (PTA)) for which

the radical stabilizer was not removed (Table 1).

PTA provided additional hydroxyl groups, which allowed the

investigation of the BAEs with or without pendent OH-groups

(

referred to as BAE-X-OH and BAE-X, respectively). Network

°C. The obtained reaction rates were not found to be strikingly

synthesis was conducted in bulk, targeting stoichiometric

diﬀerent in the presence of these catalysts, thus not justifying

the use of an external catalyst (Figure S3).

Since the appearance of the acrylate signals was detected at

100 °C in the original model study, in a next step the

possibility for the β-amino ester 1 to undergo a dynamic aza-

Michael reaction was investigated in a temperature window

from 100 to 160 °C (Figure S5 ). In order to target the same

exchange reaction product 2, the reaction was conducted with

benzyl acrylate. Yet, to avoid the inﬂuence of the acrylates’

homopolymerization at such elevated temperatures, the

amount of benzyl acrylate was decreased to 6 equiv. In the

monitored temperature window, the exchange rate to form β-

amino ester 2 appeared to be slower than via the trans-

esteriﬁcation mechanism (Figure 1b and Figure S4b). This

slow exchange rate of the aza-Michael reaction might be

explained by the stability of the adduct, originating from the

addition of an amine to an acrylate. At lower temperatures, the

readily formed adduct is favored; hence dynamic exchange is

unlikely, while at increased temperatures the equilibrium of the

(

retro) aza-Michael reaction is shifted to the dissociated state,

as expected for an addition−elimination type of dynamic

covalent exchange reactions. However, the exchange via the

dynamic aza-Michael reaction follows a higher temperature

dependency, as also expressed by the activation energies of 35

−1

2 kJ mol and 75 ± 6 kJ mol calculated for the monitored

transesteriﬁcation and dynamic aza-Michael reaction, respec-

tively (Figure 1b). Interestingly, while the exchange rates are

very slow at 100 °C, they become moderately high at higher

temperatures with a strong temperature dependence as shown

with an Arrhenius plot. The fact that β-amino esters undergo

dynamic bond exchanges at elevated temperatures, via both an

associative and dissociative dynamic bond exchange, was

intriguing and unique in the context of CANs (vide infra) as

this would allow for potential coexistence of both mechanisms

at material processing temperatures.

Figure 2. (a) Reaction scheme of the formation of β-amino ester

networks. (b) A reprocessed TMPTA/Jeﬀamine D2000 (BAE-1)

sample, leveraging the retro aza-Michael reaction as covalent dynamic

chemistry.

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The mixtures were cured at 70 °C for 72 h for Jeﬀamine

D2000 (BAE-1, BAE-1-OH) and 48 h for Priamine 1074

(

BAE-2, BAE-2-OH) and MBCA (BAE-3, BAE-3-OH)

containing networks (Table 1). The conversion was checked

via ATR-FTIR (Figure S6 ) and network formation veriﬁed via

swelling tests and Soxhlet extraction with THF (Table 1).

Swelling ratios in THF increased when lowering the cross-

linking density and ranged from 455% (BAE-1) to 15% (BAE-

-OH). As expected, when considering hydrogen bonding, the

additional hydroxyl groups in PTA-based CANs consistently

resulted in lower swelling degrees. In addition, even for

Jeﬀamine-based materials showing high swelling ratios,

relatively low soluble fractions were obtained. Moreover, no

soluble fraction was detected for BAE-3 and BAE-3-OH, which

is ascribed to the high cross-linking density of these materials.

Time sweep experiments were performed at 120 °C to limit

the inﬂuence of a dissociation process that is linked to a retro

aza-Michael reaction (vide infra). The elastic shear modulus

(

G′, Table 1), associated with the cross-link density, follows

the same trend as obtained from the swelling degree and T_g

Figure S7 ). The only apparent exception to this trend is the

lower G′-value for BAE-3-OH in comparison to the Priamine

cross-linked network (BAE-2-OH) and to the TMPTA cross-

linked reference materials (BAE-3). This is ascribed to the fact

that the temperature diﬀerence between the T of BAE-3-OH

(

93 °C) and the temperature chosen for the rheological

experiments (120 °C) is much smaller than for the other

materials.

Each of these polymer networks was used to demonstrate

the possibility of creating thermally reprocessable cross-linked

materials on the basis of both catalyst-free dynamic trans-

esteriﬁcation and dynamic aza-Michael reaction. Hence, they

were compression molded into a plate, exempliﬁed for BAE-1

(Figure 2b) and BAE-3 (Figure S8 ) at 180 °C, a temperature

well below the degradation temperature, within 1 h and with

an applied pressure of 2−4 tons. A reference network, obtained

by UV-cured pentaerythritol triacrylate in the absence of β-

amino esters, was not reprocessable, even after 2 h of hot

pressing at 180 °C (Figure S9) and was therefore not suitable

for stress relaxation measurements.

Figure 3. (a) Aza-Michael-based polymer networks originating from

PTA and diﬀerent amines. (b) TGA showing an earlier onset

temperature after 5% wt loss between 230 and 240 °C for butyl-,

octyl-, and 2-ethylhexylamine in comparison to the less volatile

4,4′methylenebis(cyclohexylamine) (295 °C). (c) Representation of

the weight loss versus time of the polymer networks under isothermal

conditions at 200 °C in the course of 1 h.

The reprocessable materials displayed T values from −60 to

0 °C, as measured by diﬀerential scanning calorimetry (DSC)

(Table 1 and Figure S10 ). It should be noted that PTA based

CANs displayed a higher T than their non-hydroxyl group

containing counterpart CANs based on TMPTA, as expected

from the additional hydrogen bond interactions. TGA-analysis

displayed a degradation onset temperature (T_d_‑₉₅_%) in a range

of 275−330 °C (Table 1, Figure S11), well beyond the

processing temperature. Besides, the multifunctional amine

compounds with a boiling point beyond processing temper-

atures are expected to remain part of the network. This is

^e^x^e^m^p^lⁱ^ﬁ^e^d^wⁱ^t^h^B^A^E^-³^-^O^H⁽^Td‑95% = 295 °C), in which the

isothermal mass loss was determined to be less than 1.5%

weight loss after 1 h at 200 °C (Figure 3, black line).

Moreover, when low molecular mass primary amines, i.e.,

butylamine, octylamine, and 2-ethylhexylamine, were used as

reaction partner (with stoichiometric amounts of amine

protons to acrylates) to form dynamic β-amino ester networks,

they did not only show an earlier onset temperature in a TGA

temperature ramp (Figure 3a,b) but also demonstrated a

dramatically higher amount of mass loss under similar

isothermal conditions (Figure 3c). This observation conﬁrms

the retro aza-Michael reaction occurring in such bulk materials,

as demonstrated in the model study described above.

Dynamic Properties of β-Amino Ester Based Materi-

als. Rheological characterization of the created CANs was

performed to investigate the dynamic material properties. First,

when stress relaxation experiments were performed, it was

observed that each of the networks displayed stress relaxation

at 180 °C (Figures S12 −S15). Additionally, a higher exchange

rate (linked to faster stress relaxation) was detected when a

hydroxyl group bearing acrylate (i.e., PTA) was used compared

to TMPTA in elastomeric materials. As even in similar

systems diﬀerences in, for example, cross-link density or

polarity were reported to inﬂuence the dynamic exchange,

including variation of the ﬂow activation energy, the applied

variations among the materials make it diﬃcult to describe a

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faster exchange rate, as exempliﬁed with BAE-1 (Figure 4 a).

Article

trend at this stage. Nonetheless, in the herein presented

system, dynamic aza-Michael and transesteriﬁcation were

concurrently occurring at elevated temperatures, and elasto-

meric materials with pendent hydroxyl groups displayed a

of dynamic, yet creep resistant elastomers, as very recently

reported with addition-type reprocessable elastomers.

Hence, because of the high activation energy derived from

the respective Arrhenius plots (i.e., 163 ± 12 kJ mol and 242

± 8 kJ mol for BAE-1 and BAE-1-OH, respectively) (Figure

b), both elastomers demonstrated strong creep resistance at

−

elevated temperatures (50 and 80 °C). Indeed, upon exposure

to 2 kPa shear stress for 20 min, the materials displayed merely

∼

0.01% (50 °C) and ∼0.02% (80 °C) remaining strain as

permanent deformation after a recovery period of 20 min

(

Figure 4c and Figure S16).

CANs Combining β-Amino and β-Hydroxyl Groups.

To take this approach a step further, the eﬀect of a potential

double neighboring group participation (NGP) on the

exchange rate was investigated. Indeed, in addition to the β-

amino group, which is hypothesized to be responsible for

creating an activation of the ester bond via coordination of the

alcohol proton and the ester-carbonyl group, a β-hydroxyl

group can be simultaneously introduced. Such a β-hydroxyalkyl

ester, was in fact already reported by Leibler and co-workers

in the seminal paper of vitrimers.

Herein, this NGP eﬀect was introduced using a β-

hydroxyalkyl functional bis-acrylate moiety (Figure 5a).

While the β-amino moiety activates the electrophile side of

the ester group, the β-hydroxyalkyl one allows for swift

reactivity on the nucleophile side. For the corresponding

CANs, further referred to as BPAE materials, bisphenol A

glycerolate diacrylate (BPADA) was used in combination with

similar amines as for the β-amino ester networks described

above (Table 2). The synthesized densely cross-linked polymer

networks displayed a T of −40 °C (BPAE-1), 30 °C (BPAE-

) and 70 °C (BPAE-3), leading to elastomeric as well as rigid

polymer samples while being thermally stable beyond 300 °C

(

Table 2, Figures S17 −S21 ).

More speciﬁcally, the network composition BPAE-3 (Figure

a) resulted in a network with a T of 70 °C, a degradation

onset temperature of 310 °C with less than 1% weight loss over

h at 160 °C and less than 2% weight loss over 1 h at 200 °C

Figure S22 ). Moreover, despite having activated ester bonds,

(

BPAE-3 was demonstrated to be hydrolytically resistant when

immersed in demineralized water up to 10 days at room

temperature. This network showed a soluble fraction of less

than 1% while the swelling degree increases with time from

.2%, 11.2% and 18.1%, after 1, 4, and 10 days, respectively.

Yet, when the network BPAE-3 was subjected to boiling water

for 24 h, the network hydrolyzed resulting in a soft and sticky

residue and a turbid dispersion. This indicates the opportunity

to apply optimized hydrolytic conditions in order to recover,

for example, ﬁbers in composite applications.

Figure 4. (a) Stress relaxation of elastomeric dynamic networks based

on Jeﬀamine D2000 BAE-1 (red) and BAE-1-OH (black) at 180 °C.

(

b) Respective Arrhenius plot derived activation energy of ﬂow,

Because of the accelerated dynamic covalent exchange as a

result of an increased number of hydroxyl moieties, decreased

cross-linking density, and a double NGP, reprocessing of the

network BPAE-3 was conducted at shorter times and lower

temperatures compared to the CANs described earlier, i.e.,

within 30 min at 150 °C with a pressure of 4 tons (Figure 5b).

When assessing the rheological behavior, stress relaxation

measurements demonstrate that the thermally activated

network rearrangement allowed for relaxation of the applied

stress in the investigated temperature range from 140 to 180

°C (Figure 5c), with a resulting temperature dependent

viscous ﬂow represented by a ﬂow activation energy of 156 ± 8

obtained from measurements at 180−150 °C. (c) Creep recovery

experiments performed at 80 °C with a remaining permanent

deformation of 0.02%.

This is in line with reported studies, which showed that a

synergetic eﬀect is typically observed when multiple dynamic

chemistries are implemented in a material.

stress relaxation experiments were performed at diﬀerent

temperatures for the obtained materials, a high temperature

dependence was observed, resulting in a ﬂow activation energy

20,50

Besides, when

−

−1

(

E_a₍_f_l_o_w₎) of 110−240 kJ mol

(Table 1). Such high

kJ mol (Figure 5d). This is similarly high as for the above-

temperature dependent dynamic behavior favors the creation

discussed β-amino ester networks and is again ascribed to

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Figure 5. (a) CAN obtained via aza-Michael addition of MBCA to BPADA (BPAE-3), with the potential of two neighboring group eﬀects

originating from a β-amino (green) and a β-hydroxyl moiety (blue). (b) Picture of compression molding of BPAE-3. (c) Normalized stress

relaxation measurements. (d) Corresponding Arrhenius plot as a function of 1000/T. (e) Frequency sweep measurement showing a decreased

elastic plateau modulus with increased temperature. (f) Scheme for the dissociative and associative dynamic covalent bond exchange.

Table 2. Overview of Compositions and Physical Properties of β-Amino Ester-Based CANs Showing a Double NGP Eﬀect

T_g_D_S_C(°C) T_d_‑₉₅_%(°C) soluble fraction (%) swelling degree (%) E_a₍_f_l_o_w₎(kJ mol⁻¹

)

G′ (MPa)

CAN

acrylate

amine

BPAE-1

BPAE-2

BPAE-3

BPADA

Jeﬀamine

Priamine

MBCA

−40

340

310

2.8

5.0

425

180 ± 24

121 ± 7

156 ± 8

0.27 ± 0.003

0.62 ± 0.03

0.62 ± 0.01

T obtained from DSC-analysis, TGA onset-temperatures after 5% weight-loss, obtained after Soxhlet-extraction for 24 h in THF, obtained

after swelling in THF for 7 days at rt. obtained from a time sweep measurement at 120 °C.

originate from a combination of transesteriﬁcation and

dynamic aza-Michael reaction, albeit the transesteriﬁcation

reaction is likely to be dominant in the probed temperature

window. A frequency sweep of the glassy polymer network

crossover was apparent (Figure 5e). This is in accordance with

the observed G (t ) that also decreased with increased

temperature (Figures S23 and S24), which is indicative for a

decrease of cross-linking density in dissociative covalent

(

BPAE-3), in combination with the non-normalized relaxation

dynamic polymer networks, in this particular case related

to the dynamic aza-Michael reaction (Figure 5f).

modulus (G(t)) obtained from stress relaxation measurements,

revealed the coexistence of transesteriﬁcation while also

showing partial de-cross-linking. Indeed, the frequency sweep

showed a decrease of the material’s shear storage modulus (G′)

upon increasing the temperature while a shift of the anticipated

The coexistence of both dynamic aza-Michael and trans-

esteriﬁcation to dissipate applied stress in a material also

resulted in a high ﬂow activation energy for BPAE-1 and

BPAE-2 (Figure S25) and demonstrates the broad applicability

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into diﬀerent matrices. In combination with the results

obtained from the kinetic study (Figure 1), it was concluded

that transesteriﬁcation was likely to occur at temperatures

below 120 °C, whereas the dynamic aza-Michael reaction is

negligible under these conditions. This is supported by the

high creep-resistance at 80 °C for elastomeric network BPAE-1

Figure S26 ). Yet, at elevated temperatures the dynamic aza-

Michael reaction mechanism contributed to the overall

dynamic covalent exchange related network mobility. Accord-

ingly, in a frequency sweep of BPAE-1 upon heating, the elastic

plateau modulus remains almost unaltered up to 120 °C,

whereas a continuous drop in G′ was detected beyond 140 °C

(Figure S27 ).

Reprocessability. Next, network BPAE-3 was reprocessed

up to 10 times with similar conditions (30 min compression at

50 °C with a pressure of 4 tons), further referred to as R up

to R . The network kept its ability for compression molding,

showing each time homogeneous, transparent samples, even

after up to 10 additional reprocessing steps using the

aforementioned processing conditions (Figure 6a, Figure

S28). Subsequently, soluble fraction experiments of a piece

of the material resulted in soluble fractions of 5.0%, 15.2%, and

1.9% for R , R , and R while the swelling degree accounted

0 4 10

to 88%, 97%, and 108%, respectively. The increase in soluble

fraction and swelling ratio is attributed to the partially

dissociative nature of the exchange reactions. Reprocessability

of the samples was conﬁrmed on behalf of BPAE-3 by

examining their stress relaxation properties (Figure S29),

represented by their Arrhenius plots (Figure 6 b) as well as

frequency sweep experiments (Figure 6c, Figures S30 and

S31 ). In addition, the recorded FTIR spectra and thermal

analyses supported the conclusion that the chemical

composition and material properties were maintained after

0 recycling cycles (Figure S32 ).

In the reprocessed sample R , a small decrease in the

storage modulus G′ of the elastic plateau modulus at 180 °C

was observed (Figure 6c), which can again be linked to the

dissociative nature of the retro aza-Michael reaction and partial

reformation of the network upon reprocessing. Indeed, upon

partial dissociation and re-formation of acrylate moieties,

partial homopolymerization/transfer reaction or amidation via

the re-formed secondary amines, particularly at elevated

temperatures, might occur. The homopolymerization of

acrylate groups was experimentally also observed in the

model study (Figure S5 ), and when ﬁve subsequent cycles of

stress relaxation measurements were performed at 150 and 180

Figure 6. (a) Reprocessing of β-amino ester network BPAE-3 up to

0 times. (b) Arrhenius plot derived from the stress relaxation

experiments performed after the initial processing (R (red)) as well

as 4 and 10 times reprocessing (R₄(blue) and R₁₀(green),

respectively). (c) Corresponding frequency sweep at 180 °C.

C (Figure S33 ), a slight increase in relaxation modulus and

shown for highly cross-linked materials with varying T ranging

the respective relaxation time was apparent at 180 °C as a

result of minor side reactions. Nonetheless, reprocessability

tests showed that these reactions barely aﬀected the overall

reprocessing properties of the herein presented networks.

from elastomeric (−60 °C) to glassy materials (90 °C). The

reprocessing ability of the materials was demonstrated for up

to 10 cycles.

This approach allows for polymer material design of tailored

viscosity proﬁles. On the one hand, network connectivity is

reduced with the dissociative part, while on the other hand, a

certain degree of connectivity is maintained with the support of

the associative mechanism. Yet the thermally reversible bonds

are positioned on diﬀerent sides of the electrophilic center of

the ester group, using diﬀerent reaction mechanisms, acting

synergetic and rather orthogonally. In view of the simplicity of

the herein presented approach, the large amount of available

building blocks, and the fact that β-amino esters possess

inherently thermally activated dynamic bonds, these kinds of

CANs are expected to contribute signiﬁcantly to the current

processing and recycling limitations of thermosets. Further-

CONCLUSION

■

In this study, β-amino esters were demonstrated to inherently

behave as dynamic moieties in CANs, with a combination of

dissociative and associative dynamic behavior that is easily

accessible by simply combining acrylic and amino function-

alities. This concept was ﬁrst demonstrated on model

compounds and then in bulk materials solely relying on

dynamic (retro) aza-Michael reaction as well as for trans-

esteriﬁcation in combination with pendent hydroxyl moieties.

Catalyst-free reprocessing of polyester networks was achieved

with β hydroxyl groups at 150 °C within 30 min. This was

146

J. Am. Chem. Soc. 2021, 143, 9140−9150

Journal of the American Chemical Society

Article

more, the robustness and versatility of this system were

demonstrated by combining multifunctional acrylates with

amino groups on primary and secondary carbons of small or

macromolecular entities. It is therefore expected to be rapidly

picked up by interdisciplinary working scientists in many

application areas ranging from (photocurable) coatings and

adhesives to bulk materials. Because poly(β-amino ester)

materials were explored with regard to their degradability, β-

amino esters might also be of importance in the end-of-life

recyclability and degradability of plastic materials. Lastly, this

readily accessible chemistry, bridging from organic chemistry

to polymer materials, can be used for drop-in technologies in

bulk (bio)materials.

applied stress comprised the linear viscoelastic region at the measured

temperatures. For amplitude sweep experiments, the strain was varied

−

from 0.01% to 100% with an oscillating frequency of 5 rad·s . For

time sweep experiments a strain of 0.1% or 1% was applied with a

−

frequency of 5 rad·s for a duration of 300 s at 120 °C and G′ and G″

were followed over time. The reported G′ values were averaged and

taken from the second measurement after an initial 5 min time sweep

to remove a possible thermal history. For stress relaxation

experiments, a strain of 0.1%, or 1% respectively was applied to the

material and the relaxation modulus (G(t)) was followed over time at

a constant temperature. The obtained characteristic relaxation time

(

τ*) was used to calculate an activation energy. For frequency sweep

measurements, a strain of 1% with a normal force of 0.2 N (BPAE-1)

and 1 N (BPAE-3) was applied and a frequency range from 100 to 0.1

−1

rad·s was screened by following the evolution of G′ and G″ at a

constant temperature. Creep recovery experiments were performed by

applying no strain for a duration of 300 s, which was followed by

applying 2 kPa shear stress for 1200 s and a recovery period of 1200 s

at 80 and 50 °C. Creep recovery measurements were preceded by a

measurement at 80 °C to remove a possible thermal history.

EXPERIMENTAL SECTION

■

Material. Acetonitrile (99.9%, HPLC gradient grade), benzyl

alcohol (≥99%), benzyl acrylate (≥99%, contains hydroquinone

monomethyl ether (MEHQ) as inhibitor), bisphenol A glycerolate (1

glycerol/phenol) diacrylate (contains 250−500 ppm MEHQ as

inhibitor), n-butylamine (≥99%), dibutylamine (≥99%), N,N-

diisopropylethylamine (DIPEA, ≥99%) 2-ethylhexyl acrylate (98%,

contains MEHQ as inhibitor), 2-ethylhexylamine (98%), N-

methylbutylamine (≥98%), pentaerythritol triacrylate (PTA, technical

grade, contains 600 ppm of MEHQ as inhibitor), trimethylolpropane

triacrylate (TMPTA, technical grade, contains 300−400 ppm of

MEHQ as inhibitor), n-octylamine (99%) were purchased from

Sigma-Aldrich. Tetrahydrofuran (THF) and n-hexane were purchased

from Acros Organics. 4,4′-Methylenebis(cyclohexylamine) (MBCA,

Reprocessability. To reprocess the network, the polymer was

broken into pieces and placed into a rectangular mold (A, 70 mm ×

0 mm × 2 mm; B, 30 mm × 15 mm × 2 mm) for compression

molding. This assembly was placed in a 150−180 °C preheated

compression press for 1 min under 0.5 metric tons of pressure. Then

the pressure was increased to 3 or 4 tons and kept constant for an

additional 59 min. After 60 min of pressing in total, the sample was

carefully removed from the mold while still heated and in its elastic

state. The temperature and pressing time were adjusted according to

the material based on its T and cross-linking density. Hence, highly

cross-linked materials obtained using tris-functional acrylates were

pressed at 180 °C for 60 min depending on their relaxation behavior,

whereas bis-acrylate based materials were pressed at 150 °C for 30

min. For instance, pentaerythritol triacrylate and trimethylolpropane

triacrylate based networks were processed at 180 °C for 60 min

applying 3 tons (Jeﬀamine) or 60 min with 4 tons (4,4′-

methylenebis(cyclohexylamine)), whereas bisphenol A glycerolate

diacrylate was pressed at 150 °C for 30 min applying 4 tons.

Solubility tests were performed via Soxhlet extraction in reﬂuxing

THF for 24 h with a sample weight of around 100−200 mg. Then, the

solvent was removed, and the sample was dried under vacuum for 2

days at 100 °C. The soluble fraction was calculated using eq 1.

Swelling tests were performed by immersing a sample of 40−60 mg in

98%) was purchased from TCI. Jeﬀamine D2000 was kindly

provided by Huntsman, and Priamine 1074 was kindly provided by

Croda. All reagents were used without further puriﬁcation unless

stated otherwise.

Instrumentation. Nuclear magnetic resonance (NMR) spectra

were recorded on a Bruker Advance Ultrashield 300 MHz

spectrometer. Deuterated chloroform (CDCl ) was used as the

solvent in each sample. Chemical shifts are given in parts per million

(

ppm).

Liquid chromatography−mass spectrometry (LCMS) combined with

electrospray ionization mass spectrometry (ESI-MS) was measured with

an Agilent technologies 1100 series LC/MSD system equipped with a

diode array detector and a single quad MS detector (Agilent G1956B)

with an electrospray source for classic reversed phase LCMS and MS

analysis. Analytic reversed phase high-performance liquid chromatog-

raphy (HPLC) was performed with a Phenomenex Kinetex C18

column (5 μm, 150 mm × 4.6 mm) using a solvent gradient (0−100%

mL of THF at room temperature for 7 days, and the swelling ratio

was calculated using eq 2. Hydrolysis tests were performed by

immersing around 90 mg of BPAE-3 in 3 mL of demineralized water

for 1, 4, and 10 days, drying the samples in a vacuum oven at 120 °C

and determining the respective soluble fraction and swelling ratio

according to eqs 1 and 2. A hydrolysis test at elevated temperatures

was performed by placing a piece of BPAE-3 (110 mg) in a glass vial

acetonitrile in H O in 6 min). The eluting compounds (15 μL

−

injected from 1 mg·mL in acetonitrile) were detected via UV

detection (λ = 214 nm), and ESI-MS results were recorded in positive

mode.

with 5 mL of H O, which was closed with a septum under nitrogen

atmosphere. The material was monitored in water for 24 h at room

temperature and remained visibly intact. Thereafter, the vial was

placed in a heated oil bath (110 °C) for a hot water hydrolysis test.

Attenuated total reflection Fourier transform infrared spectroscopy

ATR-FTIR) spectra were measured using a Perkin-Elmer Spec-

(

trum1000 FTIR infrared spectrometer with a diamond ATR probe.

Thermogravimetric analyses (TGA) were performed with a Mettler

Toledo TGA/SDTA851e instrument under nitrogen atmosphere at a

soluble fraction (%) = 100 × ⁽

m − m )

heating rate of 10 K·min⁻from 25 to 800 °C for the dynamic mode.

Isothermal measurements were conducted under nitrogen atmosphere

at 160 °C for 120 min and 200 °C for 60 min.

^mi

(1)

(m − m )

swelling ratio (%) = 100 ×

Differential scanning calorimetry (DSC) analyses were performed

with a Mettler Toledo instrument 1/700 under nitrogen atmosphere

at a heating rate of 10 K·min⁻either from −100 to 100 °C for elastic

materials or from 0 to 150 °C for glassy materials.

m_i

(2)

with m , m , and m being the initial, swollen, and dry mass,

respectively.

Rheology experiments were performed on an Anton Paar MCR 302.

The experiments were performed in parallel plate geometry using 8

mm sample disks. Unless otherwise speciﬁed, the experiments were

performed using a normal force of 1 N, an oscillating frequency of 1

rad·s , and a strain of 0.1% (for materials containing tris-functional

acrylates) and 1% (for materials containing bis-functional acrylates, or

elastic materials) was applied. For all rheology experiments, the

Synthetic Procedures. 2-Ethylhexyl 3-(Butyl(methyl)amino)-

propanoate (1). N-Methylbutylamine (8.53 g, 0.0979 mol, 1.2 equiv)

was combined with 2-ethylhexyl prop-2-enoate (15.19 g, 0.0824 mol,

1 equiv) and stirred at 70 °C for 72 h. The product was then taken up

in 30 mL of n-hexane (30 mL), washed with water (3 × 20 mL), once

−

with brine (1 × 20 mL), and dried over Mg(SO ), after which n-

hexane was removed and the pale yellow oil was dried in a vacuum

147

J. Am. Chem. Soc. 2021, 143, 9140−9150

Journal of the American Chemical Society

https://pubs.acs.org/doi/10.1021/jacs.1c03316.

Article

oven overnight at 40 °C. Yield = 95% (21.26 g, 0.0783 mol). H

NMR (300 MHz, CDCl , δ ppm): 0.82−0.95 m((−CH −CH ) ),

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

3 3

.12−1.49 m((−CH −CH −CH −CH ) (−CH−CH −CH )

(

−CH −CH −CH (CH )), 1.49−1.64 m(CH −CH(CH )−CH ),

.22 s(N−CH ), 2.30−2.37 m(−CH −CH −N−), 2.43−2.50 and

.65−2.73 m(−N(CH )−CH −CH −(CO)−O−) and 3.99 (dd, J

NMR spectra, LCMS−ESI-MS measurements, IR

spectra, kinetics plots, DSC and TGA thermograms,

stress relaxation data, frequency sweeps, and images of

reprocessed materials (PDF)

5.8, 0.9 Hz; (−(CO)−O−CH −CH(CH )−CH −). C NMR

(

300 MHz, CDCl , δ ppm) 172.91 (CH −(CO)−O−CH ), 66.82

−(CO)−O−CH −CH ), 57.23 (−CH −CH −N−), 52.89

2 2 2 2

−N−CH −CH −), 41.94 (−N−CH ), 38.73 (CH −CH−(CH )−

CH ), 32.62 (CH −CH −(CO)), 30.40 (CH−CH −CH ), 29.44

(

CH −CH CH −N), 28.91 (CH−CH −CH −CH −), 23.77 and

AUTHOR INFORMATION

Corresponding Author

■

2.95 (CH −CH −CH , CH−CH −CH ), 20.60 (CH −CH −

(

CH ) −N), 14.01 (CH−(CH ) −CH , CH −(CH ) −N), 10.97

2 3

+ +

Filip E. Du Prez − Polymer Chemistry Research Group, Center

of Macromolecular Chemistry (CMaC), Department of

Organic and Macromolecular Chemistry, Faculty of Science,

Ghent University, Ghent B-9000, Belgium; orcid.org/

0000-0001-7727-4155 ; Email: ﬁ lip.duprez@ugent.be

(

CH−CH −CH ). ESI-MS m/z: [M + H] for C H NO ;

16 34

calculated 272.258; found 272.300. IR (ATR platinum diamond), ν

−1

[

cm ]: 2956 (C−CH ), 2929 (CH ), 2860 (C−CH ), 2788 (N−

CH ), 1735 (CO), 1461 (CH ), 1379 (CH ), 1176 and 1035 (C−

O−C ).

ester

Kinetic Model Study. For kinetic experiments monitoring the

transesteriﬁcation 1 equiv of model compound (1) (400 mg) was

mixed with 12.5 equiv of benzyl alcohol (2 g) in a sealable glass vial,

bubbled with nitrogen at room temperature and kept under nitrogen

atmosphere. The vial was placed in a heated oil bath with the mixture

being stirred. The kinetic experiments were performed at 80, 100,

10, 120, and 140 °C for up to 3 days during which samples were

collected from this mixture, dissolved in CDCl , and the reaction was

followed via H NMR by assessing the disappearance of the signal at

.91 ppm belonging to 1 which was referenced against the appearance

of the signal at 3.42 ppm related to 2-ethylhexyl alcohol. Kinetic

experiments using additional tertiary amine (DIPEA) or secondary

amine (dibutyl amine) as “external” catalyst 5 mol % of the respective

compound compared to 1 were added, and the exchange reaction was

monitored at 120 °C for 2 days. For kinetic experiments monitoring

the dynamic aza-Michael reaction 1 equiv of model compound (1)

Authors

Christian Taplan − Polymer Chemistry Research Group,

Center of Macromolecular Chemistry (CMaC), Department

of Organic and Macromolecular Chemistry, Faculty of

Science, Ghent University, Ghent B-9000, Belgium;

orcid.org/0000-0002-8078-0953

Marc Guerre − Polymer Chemistry Research Group, Center of

Macromolecular Chemistry (CMaC), Department of Organic

and Macromolecular Chemistry, Faculty of Science, Ghent

University, Ghent B-9000, Belgium; Laboratoire des IMRCP,

Université de Toulouse, CNRS UMR5623, Université Paul

Sabatier, 31062 Toulouse, France; orcid.org/0000-0003-

1410-9923

(

400 mg) was mixed with 6 equiv of benzyl acrylate (1.43 g) in a

Complete contact information is available at:

https://pubs.acs.org/10.1021/jacs.1c03316

sealable glass vial, bubbled with nitrogen at room temperature, and

kept under nitrogen atmosphere. The vial was placed in a heated oil

bath with the mixture being stirred. The kinetic experiments were

performed at 100, 130, 140, 150, and 160 °C for up to 2 days during

which samples were collected from this mixture, dissolved in CDCl ,

and the reaction was followed via H NMR by assessing the

Author Contributions

The manuscript was written through contributions of all

authors. All authors have given approval to the ﬁnal version of

the manuscript.

disappearance of the signal at 4.02 ppm belonging to 1 which was

referenced against the appearance of the signal at 4.10 ppm related to

Notes

-ethylhexyl acrylate. Following the assumption of a stationary state,

The authors declare no competing ﬁnancial interest.

consequently mimicking a ﬁrst order reaction, the kinetic reaction

constants (k) were derived from the slope of the linear decay of the

ﬁrst few points of time. The logarithmic reaction constants (ln(k))

ACKNOWLEDGMENTS

We thank Bernhard De Meyer and Jan Goeman for technical

support and Prof. Dr. Johan M. Winne, Dr. Joshua O.

■

_w_e_r_e_t_h_e_n_p_l_o_t_t_e_d_a_g_a_i_n_s_t₁₀₀₀_T−1 in an Arrhenius plot to compare

the relative temperature dependent behavior. An activation energy

was calculated from the obtained slope by eq 3.

Holloway, Dr. Lucas Stricker, Dr. Nezha Badi, and Kamil Unal

for fruitful discussions. C.T. and M.G. acknowledge the

Research Foundation-Flanders (FWO) for Ph.D. (Grant

1S27920N) and postdoctoral (Grant 12Y9918N) fellowships.

F.E.D.P. thanks BOF-UGent (GOA funding).

ln(k) = − ^E

(3)

Network Synthesis. For the herein investigated materials the

multifunctional acrylate and the multifunctional primary amino

compounds were weighed with a ratio of acrylate to primary amine

moieties of 1:0.5 and mixed in a polypropylene cup using a DAC

50.1 FVZ speed mixer (typical conditions of mixing: 2 min with a

speed of 2500 rpm) targeting 3−10 g of the resulting material. When

,4′-methylenebis(cyclohexylamine) was used, the sample was

ABBREVIATIONS

BAE-#, β-amino ester network; BPAE-#, bisphenol A β-amino

ester network; CAN, covalent adaptable network; CDCl ,

■

deuterated chloroform; DIPEA, N,N-diisopropylethylamine;

DSC, diﬀerential scanning calorimetry; E_a₍_f_l_o_w₎, activation

energy of ﬂow; ESI-MS, electrospray ionization mass

spectrometry; FTIR, Fourier transform infrared spectroscopy;

LCMS, liquid chromatography−mass spectrometry; NGP,

neighboring group participation; NMR, nuclear magnetic

resonance; PTA, pentaerythritol triacrylate; TGA, thermogra-

allowed to warm up to 70 °C in an oven in order to melt the

amine. Similarly, when bisphenol A glycerolate diacrylate was used,

the sample was warmed up in an oven at 70 °C to lower the viscosity

of the acrylate for facilitated mixing. Then, the plastic cup was placed

in an oven at 70−80 °C for 48−72 h to complete the network

formation. Hereafter, the polypropylene vial was broken, the sample

was removed, broken into pieces, and pressed separately in steel

molds, following the aforementioned reprocessing procedures.

vimetric analysis; T , glass transition temperature; THF,

tetrahydrofuran; TMPTA, trimethylolpropane triacrylate

148

J. Am. Chem. Soc. 2021, 143, 9140−9150

Journal of the American Chemical Society

Article

22) Tretbar, C. A.; Neal, J. A.; Guan, Z. Direct Silyl Ether

REFERENCES

■

Metathesis for Vitrimers with Exceptional Thermal Stability. J. Am.

Chem. Soc. 2019, 141 (42), 16595−16599.

(

1) Horejs, C. Solutions to plastic pollution. Nat. Rev. Mat 2020, 5

(

9), 641−641.

(23) Obadia, M. M.; Mudraboyina, B. P.; Serghei, A.; Montarnal, D.;

2) Billiet, S.; Trenor, S. R. 100th Anniversary of Macromolecular

Drockenmuller, E. Reprocessing and Recycling of Highly Cross-

Linked Ion-Conducting Networks through Transalkylation Exchanges

of C-N Bonds. J. Am. Chem. Soc. 2015, 137 (18), 6078−6083.

Science Viewpoint: Needs for Plastics Packaging Circularity. ACS

Macro Lett. 2020, 9 (9), 1376−1390.

3) Patrick, J. F.; Robb, M. J.; Sottos, N. R.; Moore, J. S.; White, S.

R. Polymers with autonomous life-cycle control. Nature 2016, 540

7633), 363−370.

4) Chen, X.; Dam, M. A.; Ono, K.; Mal, A.; Shen, H.; Nutt, S. R.;

Sheran, K.; Wudl, F. A thermally re-mendable cross-linked polymeric

material. Science 2002, 295 (5560), 1698−1702.

5) Scott, T. F.; Schneider, A. D.; Cook, W. D.; Bowman, C. N.

Photoinduced Plasticity in Cross-Linked Polymers. Science 2005, 308

5728), 1615−1617.

6) Montarnal, D.; Capelot, M.; Tournilhac, F.; Leibler, L. Silica-like

malleable materials from permanent organic networks. Science 2011,

34 (6058), 965−968.

7) Zou, W.; Dong, J.; Luo, Y.; Zhao, Q.; Xie, T. Dynamic Covalent

Polymer Networks: from Old Chemistry to Modern Day Innovations.

Adv. Mater. 2017, 29 (14), 1606100.

8) Tobolsky, A. V.; Stern, M. D. Stress-Time-Temperature

Relations in Polysulfide Rubbers. Rubber Chem. Technol. 1946, 19

4), 1178−1192.

9) Zheng, P.; McCarthy, T. J. A surprise from 1954: siloxane

equilibration is a simple, robust, and obvious polymer self-healing

mechanism. J. Am. Chem. Soc. 2012, 134 (4), 2024 −2027.

10) Osthoff, R. C.; Bueche, A. M.; Grubb, W. T. Chemical Stress-

Relaxation of Polydimethylsiloxane Elastomers1. J. Am. Chem. Soc.

954, 76 (18), 4659−4663.

11) Tobolsky, A. V.; Prettyman, I. B.; Dillon, J. H. Stress Relaxation

of Natural and Synthetic Rubber Stocks. Rubber Chem. Technol. 1944,

7 (3), 551−575.

12) Bowman, C. N.; Kloxin, C. J. Covalent adaptable networks:

reversible bond structures incorporated in polymer networks. Angew.

Chem., Int. Ed. 2012, 51 (18), 4272−4274.

13) McBride, M. K.; Worrell, B. T.; Brown, T.; Cox, L. M.; Sowan,

Drockenmuller, E. Tuning the Viscosity Profile of Ionic Vitrimers

(24) Obadia, M. M.; Jourdain, A.; Cassagnau, P.; Montarnal, D.;

Incorporating 1,2,3-Triazolium Cross-Links. Adv. Funct. Mater. 2017,

(

7 (45), 1703258.

25) Hendriks, B.; Waelkens, J.; Winne, J. M.; Du Prez, F. E.

Poly(thioether) Vitrimers via Transalkylation of Trialkylsulfonium

(26) Capelot, M.; Montarnal, D.; Tournilhac, F.; Leibler, L. Metal-

Salts. ACS Macro Lett. 2017, 6 (9), 930−934.

(

catalyzed transesterification for healing and assembling of thermosets.

J. Am. Chem. Soc. 2012, 134 (18), 7664−7667.

(27) Wang, C.; Goldman, T. M.; Worrell, B. T.; McBride, M. K.;

Alim, M. D.; Bowman, C. N. Recyclable and repolymerizable thiol −X

photopolymers. Mater. Horiz. 2018, 5 (6), 1042−1046.

(28) Guerre, M.; Taplan, C.; Nicolay, R.; Winne, J. M.; Du Prez, F.

E. Fluorinated Vitrimer Elastomers with a Dual Temperature

30) Taplan, C.; Guerre, M.; Winne, J. M.; Du Prez, F. E. Fast

Response. J. Am. Chem. Soc. 2018, 140 (41), 13272−13284.

(29) Denissen, W.; Rivero, G.; Nicolay, R.; Leibler, L.; Winne, J. M.;

(

Du Prez, F. E. Vinylogous Urethane Vitrimers. Adv. Funct. Mater.

015, 25 (16), 2451−2457.

processing of highly crosslinked, low-viscosity vitrimers. Mater. Horiz.

020, 7 (1), 104−110.

31) Taynton, P.; Ni, H.; Zhu, C.; Yu, K.; Loob, S.; Jin, Y.; Qi, H. J.;

Zhang, W. Repairable Woven Carbon Fiber Composites with Full

Recyclability Enabled by Malleable Polyimine Networks. Adv. Mater.

016, 28 (15), 2904−2909.

32) Whiteley, J. M.; Taynton, P.; Zhang, W.; Lee, S. H. Ultra-thin

Solid-State Li-Ion Electrolyte Membrane Facilitated by a Self-Healing

Polymer Matrix. Adv. Mater. 2015, 27 (43), 6922−6927.

(33) Taplan, C.; Guerre, M.; Bowman, C. N.; Du Prez, F. E. Surface

(

Modification of (Non)-Fluorinated Vitrimers through Dynamic

Transamination. Macromol. Rapid Commun. 2021 , 42 (7), 2000644.

N.; Wang, C.; Podgorski, M.; Martinez, A. M.; Bowman, C. N.

Enabling Applications of Covalent Adaptable Networks. Annu. Rev.

Chem. Biomol. Eng. 2019, 10 (1), 175−198.

(34) Jin, K.; Li, L.; Torkelson, J. M. Recyclable Crosslinked Polymer

Networks via One-Step Controlled Radical Polymerization. Adv.

Mater. 2016, 28 (31), 6746−6750.

(

14) Fortman, D. J.; Brutman, J. P.; De Hoe, G. X.; Snyder, R. L.;

Dichtel, W. R.; Hillmyer, M. A. Approaches to Sustainable and

Continually Recyclable Cross-Linked Polymers. ACS Sustainable

Chem. Eng. 2018, 6 (9), 11145−11159.

(35) Fortman, D. J.; Brutman, J. P.; Cramer, C. J.; Hillmyer, M. A.;

Dichtel, W. R. Mechanically activated, catalyst-free polyhydroxyur-

ethane vitrimers. J. Am. Chem. Soc. 2015, 137 (44), 14019−14022.

(

15) Terryn, S.; Brancart, J.; Lefeber, D.; Van Assche, G.;

(36) Snyder, R. L.; Fortman, D. J.; De Hoe, G. X.; Hillmyer, M. A.;

Vanderborght, B. Self-healing soft pneumatic robots. Sci. Robot

017, 2 (9), eaan4268.

16) Chapelle, C.; Quienne, B.; Bonneaud, C.; David, G.; Caillol, S.

Diels-Alder-Chitosan based dissociative covalent adaptable networks.

Carbohydr. Polym. 2021, 253, 117222.

Dichtel, W. R. Reprocessable Acid-Degradable Polycarbonate

(37) Lu, Y. X.; Tournilhac, F.; Leibler, L.; Guan, Z. Making insoluble

Vitrimers. Macromolecules 2018, 51 (2), 389−397.

polymer networks malleable via olefin metathesis. J. Am. Chem. Soc.

012, 134 (20), 8424−8427.

38) Rottger, M.; Domenech, T.; van der Weegen, R.; Breuillac, A.;

(

17) Rekondo, A.; Martin, R.; Ruiz de Luzuriaga, A.; Cabanero, G.;

Grande, H. J.; Odriozola, I. Catalyst-free room-temperature self-

healing elastomers based on aromatic disulfide metathesis. Mater.

Horiz. 2014, 1 (2), 237−240.

Nicolay, R.; Leibler, L. High-performance vitrimers from commodity

thermoplastics through dioxaborolane metathesis. Science 2017, 356

(6333), 62−65.

(

18) Ruiz de Luzuriaga, A.; Martin, R.; Markaide, N.; Rekondo, A.;

Caban

ero, G.; Rodríguez, J.; Odriozola, I. Epoxy resin with

(39) Zhang, B.; Digby, Z. A.; Flum, J. A.; Chakma, P.; Saul, J. M.;

Sparks, J. L.; Konkolewicz, D. Dynamic Thiol −Michael Chemistry for

Thermoresponsive Rehealable and Malleable Networks. Macro-

molecules 2016, 49 (18), 6871−6878.

(40) Scheutz, G. M.; Lessard, J. J.; Sims, M. B.; Sumerlin, B. S.

Adaptable Crosslinks in Polymeric Materials: Resolving the

Intersection of Thermoplastics and Thermosets. J. Am. Chem. Soc.

2019, 141 (41), 16181−16196.

(41) Van Zee, N. J.; Nicolay, R. Vitrimers: Permanently crosslinked

polymers with dynamic network topology. Prog. Polym. Sci. 2020, 104,

101233.

exchangeable disulfide crosslinks to obtain reprocessable, repairable

and recyclable fiber-reinforced thermoset composites. Mater. Horiz.

016, 3 (3), 241−247.

19) Lei, Z. Q.; Xiang, H. P.; Yuan, Y. J.; Rong, M. Z.; Zhang, M. Q.

Room-Temperature Self-Healable and Remoldable Cross-linked

Polymer Based on the Dynamic Exchange of Disulfide Bonds.

Chem. Mater. 2014, 26 (6), 2038−2046.

20) Fortman, D. J.; Snyder, R. L.; Sheppard, D. T.; Dichtel, W. R.

Rapidly Reprocessable Cross-Linked Polyhydroxyurethanes Based on

Disulfide Exchange. ACS Macro Lett. 2018, 7 (10), 1226 −1231.

21) Nishimura, Y.; Chung, J.; Muradyan, H.; Guan, Z. Silyl Ether as

a Robust and Thermally Stable Dynamic Covalent Motif for Malleable

Polymer Design. J. Am. Chem. Soc. 2017, 139 (42), 14881−14884.

M.; Martinez, A.; Dobson, A.; Bongiardina, N. J.; Bowman, C. N.

(42) Podgorski, M.; Fairbanks, B. D.; Kirkpatrick, B. E.; McBride,

Toward Stimuli-Responsive Dynamic Thermosets through Continu-

149

J. Am. Chem. Soc. 2021, 143, 9140−9150

Journal of the American Chemical Society

ous Development and Improvements in Covalent Adaptable Net-

Article

works (CANs). Adv. Mater. 2020, 32 (20), 1906876.

Aza-Michael Addition Chemistry. Macromolecules 2016, 49 (20),

7814−7824.

43) Guerre, M.; Taplan, C.; Winne, J. M.; Du Prez, F. E. Vitrimers:

directing chemical reactivity to control material properties. Chem. Sci.

020, 11 (19), 4855−4870.

44) Winne, J. M.; Leibler, L.; Du Prez, F. E. Dynamic covalent

(61) Stukenbroeker, T.; Wang, W.; Winne, J. M.; Du Prez, F. E.;

Nicolay, R.; Leibler, L. Polydimethylsiloxane quenchable vitrimers.

(62) Altuna, F. I.; Hoppe, C. E.; Williams, R. J. J. Epoxy vitrimers

Polym. Chem. 2017, 8 (43), 6590−6593.

with a covalently bonded tertiary amine as catalyst of the

transesterification reaction. Eur. Polym. J. 2019, 113, 297−304.

chemistry in polymer networks: a mechanistic perspective. Polym.

Chem. 2019, 10 (45), 6091−6108.

(63) Spiesschaert, Y.; Taplan, C.; Stricker, L.; Guerre, M.; Winne, J.

(45) Van Lijsebetten, F.; Holloway, J. O.; Winne, J. M.; Du Prez, F.

M.; Du Prez, F. E. Influence of the polymer matrix on the viscoelastic

E. Internal catalysis for dynamic covalent chemistry applications and

behaviour of vitrimers. Polym. Chem. 2020, 11 (33), 5377−5385.

polymer science. Chem. Soc. Rev. 2020, 49 (23), 8425 −8438.

(64) Li, L.; Chen, X.; Jin, K.; Rusayyis, M. B.; Torkelson, J. M.

(46) Delahaye, M.; Winne, J. M.; Du Prez, F. E. Internal Catalysis in

Arresting Elevated-Temperature Creep and Achieving Full Cross-Link

Density Recovery in Reprocessable Polymer Networks and Network

Composites via Nitroxide-Mediated Dynamic Chemistry. Macro-

molecules 2021, 54 (3), 1452−1464.

Covalent Adaptable Networks: Phthalate Monoester Transesterifica-

tion As a Versatile Dynamic Cross-Linking Chemistry. J. Am. Chem.

Soc. 2019, 141 (38), 15277−15287.

(47) Zhang, H.; Majumdar, S.; van Benthem, R. A. T. M.; Sijbesma,

(

65) Jourdain, A.; Asbai, R.; Anaya, O.; Chehimi, M. M.;

R. P.; Heuts, J. P. A. Intramolecularly Catalyzed Dynamic Polyester

Networks Using Neighboring Carboxylic and Sulfonic Acid Groups.

ACS Macro Lett. 2020, 9 (2), 272−277.

Drockenmuller, E.; Montarnal, D. Rheological Properties of Covalent

Adaptable Networks with 1,2,3-Triazolium Cross-Links: The Missing

Link between Vitrimers and Dissociative Networks. Macromolecules

(48) Delahaye, M.; Tanini, F.; Holloway, J. O.; Winne, J. M.; Du

2020, 53 (6), 1884−1900.

Prez, F. E. Double neighbouring group participation for ultrafast

exchange in phthalate monoester networks. Polym. Chem. 2020, 11

(

32), 5207−5215.

(

49) Podgorski, M.; Mavila, S.; Huang, S.; Spurgin, N.; Sinha, J.;

Bowman, C. N. Thiol-Anhydride Dynamic Reversible Networks.

Angew. Chem., Int. Ed. 2020, 59 (24), 9345−9349.

(50) Chen, M.; Zhou, L.; Wu, Y.; Zhao, X.; Zhang, Y. Rapid Stress

Relaxation and Moderate Temperature of Malleability Enabled by the

Synergy of Disulfide Metathesis and Carboxylate Transesterification

in Epoxy Vitrimers. ACS Macro Lett. 2019, 8 (3), 255−260.

(51) Anaya, O.; Jourdain, A.; Antoniuk, I.; Ben Romdhane, H.;

Montarnal, D.; Drockenmuller, E. Tuning the Viscosity Profiles of

High-Tg Poly(1,2,3-triazolium) Covalent Adaptable Networks by the

Chemical Structure of the N-Substituents. Macromolecules 2021, 54

(

7), 3281−3292.

52) Podgórski, M.; Spurgin, N.; Mavila, S.; Bowman, C. N. Mixed

mechanisms of bond exchange in covalent adaptable networks:

monitoring the contribution of reversible exchange and reversible

addition in thiol −succinic anhydride dynamic networks. Polym. Chem.

(

020, 11 (33), 5365−5376.

53) Van Herck, N.; Maes, D.; Unal, K.; Guerre, M.; Winne, J. M.;

Du Prez, F. E. Covalent Adaptable Networks with Tunable Exchange

Rates Based on Reversible Thiol-yne Cross-Linking. Angew. Chem.,

Int. Ed. 2020, 59 (9), 3609−3617.

(54) Elling, B. R.; Dichtel, W. R. Reprocessable Cross-Linked

Polymer Networks: Are Associative Exchange Mechanisms Desirable?

ACS Cent. Sci. 2020, 6 (9), 1488−1496.

(55) Anderson, D. G.; Tweedie, C. A.; Hossain, N.; Navarro, S. M.;

Brey, D. M.; Van Vliet, K. J.; Langer, R.; Burdick, J. A. A

Combinatorial Library of Photocrosslinkable and Degradable

Materials. Adv. Mater. 2006, 18 (19), 2614 −2618.

(56) Lynn, D. M.; Langer, R. Degradable Poly(β -amino esters):

Synthesis, Characterization, and Self-Assembly with Plasmid DNA. J.

Am. Chem. Soc. 2000, 122 (44), 10761−10768.

(57) Genest, A.; Portinha, D.; Fleury, E.; Ganachaud, F. The aza-

Michael reaction as an alternative strategy to generate advanced

silicon-based (macro)molecules and materials. Prog. Polym. Sci. 2017,

(

2, 61−110.

58) González, G.; Fernández-Francos, X.; Serra, A.; Sangermano,

M.; Ramis, X. Environmentally-friendly processing of thermosets by

two-stage sequential aza-Michael addition and free-radical polymer-

ization of amine −acrylate mixtures. Polym. Chem. 2015, 6 (39),

59) Zhang, H.; Bre, L. P.; Zhao, T.; Zheng, Y.; Newland, B.; Wang,

987−6997.

W. Mussel-inspired hyperbranched poly(amino ester) polymer as

strong wet tissue adhesive. Biomaterials 2014, 35 (2), 711−719.

(60) Baruah, R.; Kumar, A.; Ujjwal, R. R.; Kedia, S.; Ranjan, A.;

Ojha, U. Recyclable Thermosets Based on Dynamic Amidation and

150