Tunable Blocking Agents for Temperature-Controlled Triazolinedione

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Tunable Blocking Agents for Temperature-Controlled Triazolinedione-Based Cross-Linking Reactions Hannes A. Houck,†,‡,§ Kevin De Bruycker,† Christopher Barner-Kowollik,*,‡,§,∥ Johan M. Winne,*,† and Filip E. Du Prez*,† †

Polymer Chemistry Research Group and Laboratory for Organic Synthesis, Department of Organic and Macromolecular Chemistry, Ghent University, Krijgslaan 281 S4-bis, 9000 Ghent, Belgium ‡ Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstraße 18, 76131 Karlsruhe, Germany § Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ∥ School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia S Supporting Information *

ABSTRACT: The remarkable reactivity of triazolinediones (TADs) toward olefin-type substrates marks them as highly attractive click reagents, in particular for polymer modification and cross-linking. Critically, their ultrafast reaction rates result in handling issues and a rather limited shelf life whereas a particular concern for polymer material applications is homogeneous network formation. Herein, we introduce 2phenylindoles as highly promising blocking agents for TADs, giving bench-stable reagents at ambient temperature from which the initial TADs can be released upon heating. A set of 11 indoles with varying substitution patterns was synthesized, leading to a precise control for the temperature of deblocking within a broad range of 35− 100 °C. The established indole−TAD platform was next applied to bivalent TAD reagents to enable on-demand TAD-based cross-linking reactions of a diene-containing polyurethane (Mn = 12.5 kDa). Fine-tuning of the curing temperatures, down to 50 °C, was evidenced via rheological measurements.

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INTRODUCTION The vulcanization of rubber, in which several linear polymer chains are interconnected into one single macromolecule to improve physical and mechanical material properties, has set an important milestone in polymer science and engineering.1 In response to the ever-growing applications, numerous crosslinking chemistries have been developed to generate thermoset materials.2,3 Among these, highly efficient and selective ligation reactions are appealing as they limit inhomogeneities (e.g., dangling chains) introduced into the resulting network and the effect thereof on the final material properties.4,5 To date, isocyanate-based addition reactions are one of the most basic yet versatile cross-linking chemistries to provide tailor-made materials, particularly for foams, coatings, and adhesives.6 However, the high reactivity of isocyanates at room temperature often comes at the cost of a difficult handling and limited shelf life, expressing the need for an “on-demand” and temporal control. As a result, “blocked isocyanates” have been developed in which the highly reactive and moisture sensitive isocyanate functionality can be protected with active hydrogen compounds such as oximes,7,8 caprolactams,8,9 or pyrazoles10 to form a bench-stable adduct at room temperature (see Scheme 1).11−13 Heating of the blocked chemical in the presence of a © XXXX American Chemical Society

Scheme 1. Blocked Isocyanate Strategy Taken as Inspiration for the Development of Blocked Triazolinediones (TADs)

suitable coreactant then results in the in situ deprotection or “deblocking” of the isocyanate, allowing for the cross-linking reaction to proceed. Interestingly, the required deblocking Received: November 29, 2017 Revised: March 12, 2018

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DOI: 10.1021/acs.macromol.7b02526 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 2. Ambient Temperature Reactivity of Triazolinediones (TADs) for the (a) Modification and (b) Direct Cross-Linking of Polymers Containing Isolated or Conjugated Alkenes; (c) When a TAD Cross-Linking Agent Is Blocked with Adamantylidene Adamantane, a Homogeneous Network Formation Becomes Feasible upon Heating via a Delayed CrossLinking Procedure; (d) Reversible Reaction with Indoles Investigated in This Work To Enable Tunable TAD-Blocking Agents for Low-Temperature On-Demand Cross-Linking

temperature (typically exceeding 120 °C) strongly depends on the nature of both the isocyanate, i.e., aliphatic vs aromatic, and the blocking agent and therefore allows for a judicious choice of the blocking system, with regard to the targeted temperature of application.6,14,15 Triazolinediones (TADs) are another type of efficient coupling reagents,16−18 with a very different and somehow complementary reactivity compared to isocyanates. Indeed, TADs are highly enabling synthetic tools in polymer science that exhibit a low-temperature reactivity toward isolated alkenes and conjugated dienes without the need of a catalyst (see Scheme 2a).19−21 As a result, TADs have become well-studied reagents for the modification of a wide range of polydienes and copolymers thereof, following the pioneering works in this field of Butler and Stadler.16,17,22−29 Besides monofunctional triazolinediones (e.g., 1a−b), also bivalent TAD reagents (e.g., 1c−d) have been thoroughly investigated in combination with polyunsaturated (macro)molecules, leading to the swift formation of cross-linked materials at room temperature in a few seconds (refer to Scheme 2b).30,31 However, since the (ultra)fast cross-linking kinetics are directly related to the inherent reactivity of TADs, achieving homogeneous material properties can be troublesome, especially when working under solvent-free conditions.32 Consequently, we were inspired by the well-established blocked isocyanate strategy in order to

control the reactivity of TADs through the development of suitable TAD-blocking agents (cf. Scheme 1). This should not only resolve TAD-handling issues and prolong their shelf life, yet also be highly enabling for temporally controlled curing applications. In fact, a sole example of a blocked TAD cross-linking reagent has already been reported over 30 years ago by Jacobi and Stadler when TADs were first introduced into polymer chemistry. They made use of the thermoreversible [2 + 2]cycloaddition reaction of 1d with adamantylidene adamantane to allow for the controlled cross-linking of polybutadiene under bulk conditions (see Scheme 2c).33−35 Upon heating of a mixture of the polydiene and blocked bivalent TAD cross-linker 1d at 100−120 °C in vacuo, a homogeneous network was formed. Indoles represent an alternative class of promising blocking agents since they exhibit a similar dynamic behavior at elevated temperatures when reacted with triazolinediones36 yet are more readily available than adamantylidene adamantane. Moreover, we recently developed a new class of TAD-reactive indoles by introducing a more bulky phenyl group on the reactive indole 3-position, instead of a methyl, thereby enabling the retroreaction with 1a at lower temperature, i.e., 70 °C rather than 100 °C (see Scheme 2d).27 Since no such structural alterations that would influence the reactivity are possible for the B


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line-3,5-dione (BuTAD, 1a) was used as a benchmark. Thus, indoles 3a−k were reacted with an equimolar amount of 1a to give the corresponding TAD-indole adducts 4a−k (see Figure 1a). All reactions proceeded swiftly at room temperature and gave the addition products in quantitative yield, as expected for a click-type reaction, with reaction times of a few seconds up to 30 min (see Table S1). Subsequently, in line with our previous investigations of reversible TAD-click reactions,21,27 the deblocking behavior of each TAD−indole system was assessed by a 15 min heating of the obtained BuTAD adducts 4a−k in the presence of a conjugated diene, i.e., trans,trans-2,4-hexadien-1-ol (HDEO), which acts as a trapping agent for TADs via an irreversible TAD−diene click reaction (see Figure 1a). Hence, the fraction of the initial indole-blocking agent that is released from the adduct over heating for a period of 15 min can be determined via offline 1H-NMR analysis (details are provided in the experimental procedure section of the Supporting Information and Figure S5). The resulting kinetic thermoreversibility profiles, depicted in Figure 1b, show a broad but finely interspaced array of temperature ranges in which the various BuTAD−indole combinations exhibit their dynamic click reactivity. For the purpose of discussion, we herein define the “TAD deblocking temperature” as the initial temperature at which 5% of indole has been released from the adducts over a 15 min period. This is thus a temperature threshold at which TAD release begins to show a significant reaction rate. Furthermore, we also determine the “half-life temperature” as the temperature at which the indole-blocked TAD system has a 15 min half-life time for its first-order fragmentation reaction (t1/2 = 15 min, intersect with the 50% deblocking line). This is the temperature at which TAD release becomes a relatively fast reaction. Both of these temperatures can be derived from the obtained reversibility plots (see Table 1). With the full kinetic profiling of the TAD-indole release reactivity (cf. Table 1), the influence of the indole substitution pattern with regard to their TAD-blocking capability can be rationalized. In fact, three main classes of blocking indoles can be distinguished. A first class comprises the least dynamic 3-methylindole adducts 4a−e with half-life temperatures between 117 and 131 °C. Electronic substitution of the 2-aryl ring is thus found to have a small but significant influence on this reaction. The electron donating effect of the methoxy group in 4b somewhat lowers the backward reaction barrier (∼6 °C drop compared to 4a, Table 1), while electron withdrawing substituents (cf. 4c−e) significantly shift the thermal reversibility profiles to higher values, up to a half-life temperature of 131 °C (i.e., ∼8 °C rise compared to unsubstituted 4a). The observed reactivity trends correspond to shifts in activation energy barriers of about 10 kJ mol−1 in total (refer to Supporting Information for the calculation of observed activation energies). In a second class of blocked TADs, the introduction of a more bulky 3-phenyl group results in the more dynamic adduct 4f, as observed in a previous study.27 We initially believed the drop in activation barrier for the deblocking reaction (103.8 vs 114.5 kJ mol−1 for 4a) was mainly the result of a steric effect, destabilizing the TAD−indole adducts. However, our earlier theoretical calculations also suggested an additional electronic effect, which has led us to probe the possible influence of substituents in this work. Indeed, a minor but quite notable shift of the reversibility profiles can be observed (cf. 4g-h). However, in terms of the calculated observed activation

adamantylidene adamantane-blocking agent, indoles can indeed be regarded as a promising class of TAD-blocking agents for on-demand cross-linking applications. Herein, we introduce a library of differentially substituted TAD-reactive indoleswith variation of both the 2- and 3substituents altering their steric and electronic natureand investigated their potential to serve as a blocking agent for aliphatic and aromatic TAD reagents. With the assessment of their characteristic initial deblocking temperatures, clear insights were gained into how often subtle structural alterations can greatly modulate the dynamic properties of the indoleblocked TAD systems. The developed platform of tunable indole-blocking agents was next exploited to gain time-resolved control over the reactivity of bivalent TAD reagents, opening their applicability to on-demand TAD-based cross-linking reactions. The tunability of this delayed cross-linking approach was further demonstrated by simply changing the blocked TAD cross-linker, thereby enabling low-temperature curing of a linear diene-containing polyurethane.

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RESULTS AND DISCUSSION Synthesis and Evaluation of Indole-Blocking Agents. A library of 2,3-disubstituted indoles (3a−k, Scheme 3), Scheme 3. Library of 2,3-Disubstituted Indoles 3a−k, Synthesized via a Fischer Indolization of Their Corresponding Acetophenones 2a−ka

a

The starting ketone is either commercially available or can be synthesized by a Friedel−Crafts acylation (i.e., 2g−j).

containing either electron donating and/or withdrawing moieties, was synthesized via a classical Fischer indolization reaction of the corresponding acetophenones 2a−k in good to excellent yields (i.e., 62−99%).37 The diversity in substituent combinations is easily achieved because of the commercial availability of a wide variety of acetophenones or their straightforward synthesis via a Friedel−Crafts acylation using very simple bulk starting materials (e.g., 2g−j). Briefly, the resulting library of indole-blocking agents comprises on the one hand a series of 3-methyl-2-phenylindole (3a) analogues with varying electron density on the 2-aryl group (3b−e, R1 = OMe, F, Cl, CF3), while on the other hand a series of 2,3-diphenyl(3f) and 2-(4-methoxyphenyl)-3-phenylindoles (3i) with their nitro- (3g and 3j, respectively) and methoxy-substituted (3h and 3k, respectively) counterparts were obtained. Following the synthesis of the novel TAD-blocking indoles, their utility toward TAD-based addition reactions was next investigated. For this, the reaction with 4-n-butyl-1,2,4-triazoC


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Figure 1. (a) TAD addition reaction with indoles 3a−k to give the corresponding blocked adducts 4a−k (R3 = nBu), 5a, 5f, and 5i (R3 = Ph). The deblocking reaction was investigated by heating the resulting adducts ([TAD-indole]0 = 0.04 M, DMSO-d6) for 15 min in the presence of a diene (HDEO) scavenger that traps the in situ released TAD into the irreversible cycloadduct 1-HDEO. (b) Fraction of the released initial indole-blocking agent is determined by 1H-NMR analysis and reflects the amount of deblocking after a 15 min period of heating (refer to Supporting Information and Figure S5 for a detailed experimental procedure). The resulting thermal reversibility profiles of the indole-blocked BuTAD adducts 4a−k allow for a comparison of the deblocking properties of the different indoles investigated. (c) Replacing the aliphatic BuTAD with its aromatic PhTAD counterpart results in a systematic downward shift of the reversibility profiles by 20 °C.

Table 1. Overview of the Indole-Blocked TAD-Systems Investigated with Their Deblocking Temperature (5% Release), HalfLife Temperature, and Experimentally Observed Activation Energies (Ea,obs) for a 15 min Heating Period ([TAD−Indole]0 = 0.04 M, DMSO-d6) 4a 4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 5a 5f 5i a

R1

R2

R3

deblocking temp (°C)

half-life temp (°C)

H OMe F Cl CF3 H H H OMe OMe OMe H H OMe

Me Me Me Me Me Ph 4-NO2Ph 4-MeOPh Ph 4-NO2Ph 4-MeOPh Me Ph Ph

nBu nBu nBu nBu nBu nBu nBu nBu nBu nBu nBu Ph Ph Ph

95 90 98 101 102 71 67 70 62 54 56 78 53 33

123 117 126 127 131 98 94 96 90 83 87 104 77 67

Ea,obsa (kJ mol−1) 114.5 110.0 116.3 118.5 122.0 103.8 102.3 102.3 96.2 90.9 95.1 108.9 98.7 84.2

± ± ± ± ± ± ± ± ± ± ± ± ± ±

4.7 4.2 4.4 4.3 5.1 1.4 1.5 2.0 3.4 0.3 1.3 4.0 0.7 1.9

Refer to the Supporting Information for the calculation of the observed activation energies and standard deviation thereof.

reactivity can be observed. In general, the MeO group on the 2phenyl ring can be said to lower the half-life temperatures by 10 °C (cf. 4i) and the corresponding deblocking reaction barriers by ∼5−10 kJ mol−1. As a result, a deblocking temperature as low as 54 °C is obtained (cf. 4j). Thus, by only making structural alterations to the indole scaffold the temperature range at which deblocking occurs can be tuned from 102 °C down to 54 °C.

barriers, these differences translate into shifts within the margin of error of the measurements. In a third class of indoles, we combined the most activating 2-aryl group from class 1 (4-methoxy-substituted in 4b) with different 3-aryl groups. Here, quite significant further shifts and even apparent synergistic effects were observed. The half-life temperatures are all significantly below that of both 4b and 4f, and when a nitro (4j) or methoxy (4k) group is further introduced on the 3-phenyl ring, more significant changes in D


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Scheme 4. (a) Modification of trans,trans-2,4-Hexadien-1-ol (HDEO) to a Diol Monomer, Which Is Subsequently Reacted with HDI and PPO To Yield a Diene-Functionalized Linear Polyurethane (HDEO-PU, Mn = 12.5 kDa); (b) Addition of an Aliphatic (1c) or Aromatic (1d) Bivalent TAD Leads to a Directly Cross-Linked PU-HDEO

Scheme 5. Schematic Representation of the TAD-Based HDEO-PU Delayed Cross-Linking Experiments: (Left) Treatment of the TAD Cross-Linker (1c or 1d) with 2 equiv of Indole-Blocking Agent (3a, 3f, or 3i) Retains the Cross-Linker’s Reactivity; (Bottom) Blocked TAD Cross-Linkers 6 and 7 Can Now Be Added to the HDEO-PU at Room Temperature without Affecting Gelation; (Right) Only upon Heating, the TAD Cross-Linker Is Released, Thereby Resulting in an On-Demand Cross-Linked Polyurethane System

lines in Figure 1c; also see Table 1). The most reactive blocking indole now corresponds to a deblocking temperature as low as 33 °C and actually has a non-negligible TAD release already at room temperature. This was also observed from the fact that a freshly prepared NMR solution of blocked TAD reagent 5i gradually started to turn pink, indicating free TAD formation. This effect also had an impact on the shelf life of 5i, which had to be stored at 4 °C. In other words, further decreasing of the blocking temperature, which is theoretically possible, would have limited added benefits for the intended applications.

In our previous investigations, we also found an influence of the substituent and the TAD reagent itself,27 which led us to here also investigate the deblocking of the 4-phenyl TAD reagent 1b (PhTAD). Indoles 3a, 3f, and 3i were selected as a representative blocking agent for each of the above established indole classes and reacted with 1b to yield the respective adducts (5a, 5f, and 5i, Figure 1a). The corresponding reversibility curves of the aromatic PhTAD−indole systems show a consistent and significant downward shift by about 20 °C compared to their aliphatic counterparts (dotted vs full E


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Figure 2. (a) Rheological measurements during the delayed cross-linking of HDEO-PU with indole-blocked TADs 6−7a,f,i upon heating from 25 to 150 °C at a rate of 2.25 °C min−1. (b) Low-temperature curing of HDEO/7i at 50 °C and sol−gel transition with a gelation time of 110 min, here defined as the intersect of the storage modulus G′ and the loss modulus G′′.

Table 2. Overview of the Conducted HDEO/(Blocked) TAD Cross-Linking Experiments with the Temperature Range of the Sol−Gel Transition upon Heating from 25 to 150 °C at 2.25 °C min−1 and the Glass Transition Temperature (Tg, DSC) and Onset Temperature of Degradation (Td, TGA; Defined at 5% Weight Loss) of the Cured Networks cross-linking system 1c 1d 6a 7a 6f 7f 6i 7i

R1

H H H H OMe OMe

R2

R3

sol−gel transition temp (°C)

Tg (DSC) (°C)

Td,5% (TGA) (°C)

Me Me Ph Ph Ph Ph

(CH2)6 4,4′-(PhCH2Ph) (CH2)6 4,4′-PhCH2Ph (CH2)6 4,4′-PhCH2Ph (CH2)6 4,4′-PhCH2Ph

rt rt 120−150 110−140 95−115 85−105 90−110 80−100

−57 −57 −52 −53 −36 −37 −34 −39

253 278 225 232 253 242 260 253

less than 1 min. However, as a result of these fast curing times, the cross-linking reaction lacks any form of control and readily results in highly inhomogeneous networks. Thus, three representative indole-blocking agents (i.e., 3a, 3f, and 3i; cf. Figure 1) were used to generate six different blocked aliphatic and aromatic cross-linking agents, i.e., 6a,f,i and 7a,f,i, respectively (Scheme 5, left). Subsequent addition of these blocked TADs to the HDEO-PU solution resulted in benchstable formulations (Scheme 5, bottom). Only upon heating of the resulting mixture are the TAD click reagents released from their indole adducts, and the temporally controlled TAD−diene cross-linking reaction is initiated (see Scheme 5, right). For the analysis of the progress of these on-demand polymer network formations, delayed cross-linking experiments with the blocked TAD cross-linkers 6 and 7 were carried out in a rheometer under oscillatory shear mode, monitoring the viscoelastic response upon continuous heating from 25 to 150 °C at 2.25 °C min−1 (refer to Figure 2a and Figure S1). The torque was measured throughout the heating process from which the storage and loss moduli (G′ and G′′, respectively) were determined. Strain sweeps confirmed that all measurements were performed in the linear viscoelastic regime (Figure S2). All blocked TAD/HDEO-PU mixtures (0.1 M blocked TAD/0.4 g mL−1 HDEO-PU solutions in DMF) were

In summary, the rational design of a library of substituted 2phenylindoles allows for a meticulous regulation of the backward TAD−indole reaction. This enables the use of indoles as highly tunable blocking agents for TADs, making it possible to carry out temporally controlled TAD-based reactions in a broad temperature interval, ranging from approximately 100 °C down to just 35 °C. On-Demand Cross-Linking with Various Blocked TAD Reagents. Following the establishment of a versatile toolbox of tunable TAD-blocking reagents, we aimed to validate their use in polymer chemistry by exploiting the temporally controlled release of TAD-based cross-linkers. For this, HDEO, the highly TAD-reactive diene building block used in the above model study, was therefore incorporated into a linear polyurethane, after it was first converted into a diol monomer via an esterification. The HDEO−diol was used as a comonomer (15 wt %) with poly(propylene oxide) (PPO, Mn = 2.0 kDa) and hexamethylene diisocyanate (HDI) to give a linear polyurethane (HDEO-PU, Mn = 12.5 kDa; see Scheme 4a). The resulting HDEO-PU thus contains diene side chains that can be easily clicked with a TAD reagent. Addition of either an aliphatic or aromatic TAD cross-linking agent (i.e., 1c or 1d; refer to Scheme 4b) to the dienecontaining polyurethane at ambient temperature resulted in ultrafast gelation ( −57 °C), with a more pronounced effect for the 3-arylindole systems (cf. 6f,i and 7f,i; see Table 2 and Figure S3). This effect was found to be reproducible (see Table S2 and Figure S3). Moreover, when a Soxhlet extraction was carried out on the isothermally cured HDEO-PU/7i network, the indole-blocking agent was successfully removed from the network (see Figure S4 for 1HNMR spectrum of the soluble fraction) and the Tg decreased to

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CONCLUSIONS In summary, we prepared a range of 11 indoles from simple starting materials containing varying sterically demanding substituents as well as electron withdrawing and/or donating groups. The blocking capacity of these indoles toward aliphatic and aromatic triazolinediones was assessed by monitoring their deblocking kinetics at elevated temperatures. Thus, deblocking temperatures, half-life temperatures, and observed activation energies could be determined which spanned an unusually wide range. This led to the identification of three structurally distinct classes of indole-blocking agents, which allowed for the careful modulation of the temperatures needed to promote the TAD− indole deblocking reaction from above 100 °C down to 35 °C. We have thus shown that indoles present a versatile class of TAD-blocking agents, providing temporal control over TAD reactions and paving the way for bench-stable TAD derivatives for use in polymer modifications. The applicability of indole-blocking agents was herein demonstrated for bivalent TAD-based cross-linking reagents, which allowed for the on-demand curing of a diene-containing polyurethane at distinct temperatures. Temporally controlled TAD-based gelation was further evidenced at temperatures as low as 50 °C, and the influence of the released indole-blocking agents on the thermal material properties was investigated, showing an unusual increase in Tg. In addition to delayed network formation, the tunability of the backward TAD−indole reaction opens its exploitation in the design of dynamic materials in which the reactivity of the TAD-click reagents can be triggered at a user-defined temperature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02526. Additional figures, experimental procedures, instrumentation, synthetic procedures, and analysis of the compounds used (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: fi[email protected] (F.E.D.P.). *E-mail: [email protected] (J.M.W.). *E-mail: [email protected] (C.B.-K.). ORCID

Christopher Barner-Kowollik: 0000-0002-6745-0570 Filip E. Du Prez: 0000-0001-7727-4155 Notes

The authors declare no competing financial interest. G


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(15) Wang, J.; Xu, Y. Z.; Fu, Y. F.; Liu, X. D. Latent curing systems stabilized by reaction equilibrium in homogeneous mixtures of benzoxazine and amine. Sci. Rep. 2016, 6, 38584. (16) Butler, G. B. Triazolinedione Modified Polydienes. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19 (4), 512−528. (17) Butler, G. B. Modification of diene polymers and polymer synthesis by reaction of triazolinediones with olefinic bonds. Polym. Sci. U. S. S. R. 1981, 23 (11), 2587−2622. (18) De Bruycker, K.; Billiet, S.; Houck, H. A.; Chattopadhyay, S.; Winne, J. M.; Du Prez, F. E. Triazolinediones as Highly Enabling Synthetic Tools. Chem. Rev. 2016, 116 (6), 3919−3974. (19) Cookson, R. C.; Gilani, S. S. H.; Stevens, I. D. R. 4-Phenyl-1,2,4Triazolin-3,5-Dione - a Powerful Dienophile. Tetrahedron Lett. 1962, 3 (14), 615−618. (20) Pirkle, W. H.; Stickler, J. C. Reaction of 1,2,4-Triazoline-3,5Diones with Mono-Olefins. Chem. Commun. 1967, 15, 760−761. (21) Billiet, S.; De Bruycker, K.; Driessen, F.; Goossens, H.; Van Speybroeck, V.; Winne, J. M.; Du Prez, F. E. Triazolinediones enable ultrafast and reversible click chemistry for the design of dynamic polymer systems. Nat. Chem. 2014, 6 (9), 815−821. (22) Butler, G. B.; Williams, A. G. Low-Temperature Modification of Dienic Polymers Via the Ene Reaction with 4-Substituted-1,2,4Triazoline-3,5-Diones. J. Polym. Sci., Polym. Chem. Ed. 1979, 17 (4), 1117−1128. (23) Stadler, R.; de Lucca Freitas, L. Thermoplastic elastomers by hydrogen bonding 1. Rheological properties of modified polybutadiene. Colloid Polym. Sci. 1986, 264 (9), 773−778. (24) Türünç, O.; Billiet, S.; De Bruycker, K.; Ouardad, S.; Winne, J.; Du Prez, F. E. From plant oils to plant foils: Straightforward functionalization and crosslinking of natural plant oils with triazolinediones. Eur. Polym. J. 2015, 65, 286−297. (25) van der Heijden, S.; De Bruycker, K.; Simal, R.; Du Prez, F.; De Clerck, K. Use of Triazolinedione Click Chemistry for Tuning the Mechanical Properties of Electrospun SBS-Fibers. Macromolecules 2015, 48 (18), 6474−6481. (26) Vlaminck, L.; De Bruycker, K.; Turunc, O.; Du Prez, F. E. ADMET and TAD chemistry: a sustainable alliance. Polym. Chem. 2016, 7 (36), 5655−5663. (27) Houck, H. A.; De Bruycker, K.; Billiet, S.; Dhanis, B.; Goossens, H.; Catak, S.; Van Speybroeck, V.; Winne, J. M.; Du Prez, F. E. Design of a thermally controlled sequence of triazolinedione-based click and transclick reactions. Chem. Sci. 2017, 8 (4), 3098−3108. (28) De Bruycker, K.; Delahaye, M.; Cools, P.; Winne, J.; Prez, F. E. D. Covalent Fluorination Strategies for the Surface Modification of Polydienes. Macromol. Rapid Commun. 2017, 38 (11), 1700122. (29) Becker, G.; Vlaminck, L.; Velencoso, M. M.; Du Prez, F. E.; Wurm, F. R. Triazolinedione-″clicked” poly(phosphoester)s: systematic adjustment of thermal properties. Polym. Chem. 2017, 8 (28), 4074−4078. (30) Chattopadhyay, S.; Du Prez, F. Simple design of chemically crosslinked plant oil nanoparticles by triazolinedione-ene chemistry. Eur. Polym. J. 2016, 81, 77−85. (31) Vonhören, B.; Roling, O.; De Bruycker, K.; Calvo, R.; Du Prez, F. E.; Ravoo, B. J. Ultrafast Layer-by-Layer Assembly of Thin Organic Films Based on Triazolinedione Click Chemistry. ACS Macro Lett. 2015, 4 (3), 331−334. (32) Saville, B. Bis-(Para-3,5-Dioxo-1,2,4-Triazolin-4-Ylphenyl)Methane - Highly Reactive Bifunctional Enophile. J. Chem. Soc. D 1971, 12, 635−636. (33) Jacobi, M. M.; Stadler, R. Synthesis of Elastomer Networks of Defined Structure - Crosslinking Via Masked Bis(1,2,4-Triazoline-3,5Dione)S. Makromol. Chem., Rapid Commun. 1988, 9 (10), 709−715. (34) Seymour, C. A.; Greene, F. D. Mechanism of triazolinedioneolefin reactions. Ene and cycloaddition. J. Am. Chem. Soc. 1980, 102 (20), 6384−6385. (35) Cheng, C. C.; Seymour, C. A.; Petti, M. A.; Greene, F. D.; Blount, J. F. Reaction of electrophiles with unsaturated systems: triazolinedione-olefin reactions. J. Org. Chem. 1984, 49 (16), 2910− 2916.

ACKNOWLEDGMENTS H.A.H. and K.D.B. thank the Research Foundation-Flanders (FWO) for the funding of their PhD Fellowship. Bernhard De Meyer and Timothee Courtin (Ghent University) are gratefully acknowledged for extensive DSC, TGA, and NMR measurements. C.B.-K. acknowledges key funding from the Queensland University of Technology (QUT), the Australian Research Council (ARC) in the form of a Laureate Fellowship, and the Karlsruhe Institute of Technology (KIT) in the context of the BIFTM or STN programs of the Helmholtz Association.

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ABBREVIATIONS DSC, differential scanning calorimetry; HDEO, trans,trans-2,4hexadien-1-ol; HDI, hexamethylene diisocyanate; 1H NMR, proton nuclear magnetic resonance; TAD, triazolinedione; BuTAD, 4-n-butyl-1,2,4-triazolinedione; PhTAD, 4-phenyl1,2,4-triazolinedione; PPO, poly(propylene oxide); PU, polyurethane.

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