Anhydride

Article pubs.acs.org/Macromolecules

Air Stable and Latent Single-Component Curing of Epoxy/Anhydride Resins Catalyzed by Thermally Liberated N‑Heterocyclic Carbenes Stefan Naumann,† Maria Speiser,† Roman Schowner,† Elisabeth Giebel,‡ and Michael R. Buchmeiser*,†,‡ †

Institute of Polymer Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany Institute of Textile Chemistry and Chemical Fibers, Körschtalstrasse 26, D-73770 Denkendorf, Germany

‡

S Supporting Information *

ABSTRACT: Bisphenol A diglycidyl ether (BADGE) is cured thermally using phthalic acid anhydride (PhA) or hexahydrophthalic anhydride (HHPA) as hardener in the presence of different protected N-heterocyclic carbenes (NHCs), from which the catalytically active NHCs are generated in situ upon heating. It is found that the curing reactions proceed in a well-defined manner, delivering highly cross-linked, high-Tg-thermosets using low catalyst loadings (0.1−1 mol % of NHC precursor). The polymerizations can be conducted under air without loss of activity, employing mild curing temperatures (120−160 °C) and short reaction times. By contrast, at room temperature, polymerizations proceed only very slowly and the mixtures remain processable for weeks, enabling formation of a true singlecomponent composition suitable for applications where large processing windows or storage are required. The curing process was followed in situ by DSC as well as by rheological measurements. On the basis of these observations, the structure of the NHC precursor is correlated with its polymerization activity with regard to latency, temperature profile and polymerization kinetics. The robust and fully homogeneous system consisting of the protected NHC, BADGE, and HHPA was successfully tuned both in terms of activity and pot life by choosing the appropriate protected NHC out of 12 different precatalysts. The most rapid polymerization was effected by N,N′-bis(2,4-dimethoxyphenyl-)tetrahydropyrimidinium-2-carboxylate (6-OMe-CO2), while a dimeric zinc-based NHC-complex (6-Mes-ZnCl2) displayed the longest pot times.

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INTRODUCTION Epoxy-based thermosets owe their success to a very favorable combination of structural diversity, good processing characteristics, and desirable material properties.1 The epoxy resins that are used commercially are monomers or low oligomers, with bisphenol A diglycidyl ether (BADGE, Figure 1) being the

further modulated by curing conditions and ratio of resin to hardener. This allows for the preparation of specifically tailored epoxy compositions for a large range of applications, including coatings, adhesives, electronic applications or composite materials. Though depending on the exact chemical structure, epoxy-based thermosets are known for their high chemical resistance and mechanical strength. However, the buildup of viscosity and premature setting of the material are central difficulties when processing of epoxy resins is considered. This is of special relevance where amines are used as hardeners; here, curing can proceed at room temperature, a tendency that is the most pronounced for aliphatic amines. This can be beneficial in situations where such mild cold-curing conditions are required, but it also mandates a separate storage of both components and severely limits the pot times once the mixture is prepared. Yet a very similar challenge accounts for combinations of epoxy compounds and hardeners that do not react in a noncatalyzed way with each other, as is the case for epoxide- and anhydride functionalities, where even at high temperatures (200 °C) only sluggish polymerization occurs.5 In order to overcome this low reactivity, a suitable

Figure 1. Monomers used in this work.

dominating compound, but other architectures are also frequently encountered.2 An interesting development is the growing research regarding biobased epoxies, driven by the need to circumvent the use of the problematic bisphenol A and by the possibility to create thermosets with novel properties this way.3,4 In principle, the epoxy functionality can react with a large number of hardeners, mainly amines (aliphatic, cycloaliphatic and aromatic), carboxylic anhydrides, carboxylic acids, phenols or thiols. Each of these classes of hardeners imposes different properties on the resulting materials, which can be © XXXX American Chemical Society

Received: May 30, 2014 Revised: June 24, 2014

A

dx.doi.org/10.1021/ma501125k | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. NHC-Based Pre-Catalysts Investigated in This Study and Schematic Deprotection Principle (Bottom Right) Ad = adamantyl, Mes = mesityl

material properties. An elegant method to circumvent this has been published more recently, using “single molecule epoxies” where epoxide groups and amines are present in the same molecule, the latter held in a latent state by thermally labile carbamate links.8 Here, upon heating only CO2 evolves and a very long shelf life is claimed for these compounds. The activation temperatures, however, were quite high as found by in situ DSC, which showed maxima for the exothermic curing process between 200 and 300 °C. In contrast to the former examples, epoxy/anhydride thermosets require a latent catalyst in order to maintain the favorable properties of controlled polymerizations. Typically, strong nucleophiles like tertiary amines, including imidazoles, are used to catalyze the cross-linking reaction between epoxides and anhydrides in a nonlatent manner. Nucleophilic attack of the catalyst on the epoxide and subsequent ring-opening under formation of a zwitterionic species has been shown to initiate an anionic, strictly alternating chain growth.9 Efforts have thus been directed at the development of compounds that are stable under “normal” conditions, but release tertiary amines upon exposure to a suitable trigger, e.g., (UV) light or heat. As an example, thermal dealkylation of N,N′-dialkylimidazolium salts to yield N-alkylimidazoles is a well-known reaction, that has recently received increased attention.10 The simple but versatile structure of these salts (counterion, substituents) and their frequent use as ionic liquids renders them a well accessible source for cheap, thermally latent precatalysts. However, it seems that the curing temperatures have to be rather high over longer times, no doubt due to the stable N−C bond that has to be broken. Photolabile-reduced amidines11 and quarternary ammonium salts12 as well as thermally sensitive aminimides13 have also been employed to generate tertiary amines in situ. All these investigations underline the need to control the liberation

catalyst (“accelerator”) has to be added. While absolutely necessary for efficient curing, this catalyst can also induce premature polymerization, thus entailing similar consequences as encountered in amine-hardened epoxy systems. Latent polymerization systems, where reactivity is only induced after application of an external trigger, can obliterate these difficulties and provide considerable benefits during processing.6 Apart from simplifying the production setup by removing the need to control the mixing and homogenization of multiple components, storable single-component (onecomponent) mixtures reduce cost and generally provide a larger processing window where the low viscosity of the resinlike state can be used at great advantage. The application of coatings or the fabrication of composite materials both profit from the higher propensity of low-viscosity resins to wet surfaces or infiltrate fiber-woven reinforcing structures, followed by fast, stimuli-induced polymerization. Exact control over the onset of the polymerization and avoidance of premature reaction is thus a key feature for convenient production of complex polymer materials, and accordingly latent curing techniques for the preparation of epoxy thermosets have been the focus of several investigations. In case of cold-curing amine-hardened thermosets, protecting strategies have to be stochiometric with regard to every single amine functionality present in the mixture. A blocking of amines by conversion into ketimines has been described by Endo and co-workers.7 The imine-containing hardener and the epoxy compounds were stored together and displayed good latency properties. However, efficient curing depended in this case on hydrolysis of the imines to generate free amine functionalities, which proved to be slow. Furthermore, upon hydrolysis ketones are released in stochiometric amounts, which is problematic both environmentally and in view of the B


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Table 1. Curing of BADGE/Anhydride System Using Different Protected NHCs as Pre-Catalystsa no.

precatalyst

hardener

solidification [min]

NHC:BADGE:hardener [molar ratio]

Tgb [°C]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

− 6-Me-CO2 6-iPr-CO2 6-iPr-CO2 6-Cy-CO2 6-Mes-CO2 6-OMe-CO2 5-Cy-CO2 5-tBu-CO2 5-tBu-CO2 5-Ad-CO2 5s-Mes-ZnCl2 − 6-Cy-CO2 6-Cy-CO2 5-Cy-CO2 5-tBu-CO2 5s-Mes-ZnCl2

PhA PhA PhA PhA PhA PhA PhA PhA PhA PhA PhA PhA HHPA HHPA HHPA HHPA HHPA HHPA

− 4 3 6 3 3 3 4 4 6 4 7 − 4 15 7 4 10

0:100:200 1:100:200 1:100:200 1:250:500 1:100:200 1:100:200 1:100:200 1:100:200 1:100:200 1:1000:2000 1:100:200 1:100:200 0:100:200 1:100:200 1:500:1000 1:100:200 1:100:200 1:100:200

− 171 164c 175 163 174 173 164 169 170 173 175 − 135 127d 144 143 132

a Held at 140 °C for 45 min (air). bTg determined via second DSC run using a heating rate of 10 K/min. cCuring time only 5 min. dPostcuring observed in DSC.

were obtained at heating rates of 10 K/min (second DSC run). In situ observation of the curing process was done at scan rates of 5 K/min. To obtain reaction kinetics, an isothermal DSC setup was applied. Hereby, the samples were placed in the DSC and equilibrated for 2 min at 100 °C. The temperature was then ramped up within 1 min to the desired end temperature at which the sample was held for 60 min. Integration to get kinetic data was then accomplished using a simplified linear baseline approach (see Supporting Information for several examples) and t0 = 3 min. For the DSC long-time/latency studies master batches were created (1−2 g of sample in total; stored under air at room temperature) from which small amounts were removed in intervals of 7 days. Samples and master batches for DSC investigations were fully homogenized prior to measurement by stirring and very gentle warming (35−45 °C). Rheology data were collected from a MCR 301 (Anton Paar), using disposable plates (25 mm diameter) with 5 mm plate distance. Measurements were taken at a frequency of 10 Hz beginning at 25 °C, from which the temperature was then ramped to 150 °C at a rate of 30 K/min. 1,3-Dimethyltetrahydropyrimidinium Tetrafluoroborate. A 50 mL round-bottom flask was charged with N1,N3-dimethylpropane1,3-diamine (5.18 g, 50.7 mmol, 1 equiv). Finely granulated ammonium tetrafluoroborate was added (5.32 g, 50.7 mmol, 1 equiv). Subsequently, trimethylorthoformiate (6.73 g, 63.4 mmol, 1.25 equiv) was added to the stirred suspension. The reaction vessel was held at 120 °C for 2 h. A turbid oily liquid formed and the color turned to yellow/orange. After cooling the reaction mixture to room temperature, solid residues were filtered off and all volatile components were removed in vacuo. No further purification was necessary. The product was obtained as a yellow oil (7.24 g, 71.4%). 1 H NMR (400 MHz, DMSO): δ = 1.97 (q, J = 5.9 Hz, 2H, CH2), 3.11 (s, 6H, CH3), 3.26 (t, J = 5.6 Hz, 4H, CH2), 8.17 (s, 1H, N−CH−N). 13 C NMR (101 MHz, DMSO): δ = 18.2 (CH2), 41.1 (CH3), 43.6 (CH2), 153.0 (N−CH−N). IR (cm−1): 2947.87 (w), 1703.24 (s), 1520.80 (w), 1423.69 (w), 1327.31 (m), 1035.27 (m). 1,3-Dimethyltetrahydropyrimidinium-2-Carboxylate (6-MeCO2). Inside a glovebox, 1,3-dimethyl-tetrahydropyrimidin-1-ium tetrafluoroborate (5.17 g, 25.85 mmol, 1 equiv) was dissolved in 50 mL of THF. Under stirring, a solution of freshly sublimed potassium tert-butanolate in THF (3.5 g, 31.19 mmol, 1.2 equiv) was added slowly. A sticky white solid precipitated and the solution turned orange. After 2 h of stirring at room temperature, the suspension was filtered over a pad of Celite to remove precipitated byproducts. The solution was taken out of the glovebox and cooled in an ice bath.

of strong nucleophiles (and strong Brønstedt bases) from well accessible, yet highly latent progenitors. In this regard, we were recently able to show that protected NHCs, that is NHC−CO2 adducts and NHC−metal complexes, can be applied as thermally latent polymerization catalysts in a number of cases, including the preparation of poly(urethane)s,14 poly(methyl methacrylate),15 poly(ε-caprolactone),16 and poly(amide)s.17 Since the protected NHCs displayed excellent latency properties and are known to be much more basic than tertiary amines and at least equally good nucleophiles,18,19 they seemed a suitable choice for one component epoxide thermoset compositions, the more so since their versatility with regard to NHC structure and protecting group should allow for a convenient modulation of key parameters like activation temperature and degree of latency. Accordingly, in the present paper the thermal curing of BADGE/anhydride-resins catalyzed by in situ generated NHCs is described for the first time, and the key parameters of such a system are reported.

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EXPERIMENTAL SECTION

Materials, Synthesis, and Characterization. The preparation of the NHC progenitors used in this study (Scheme 1) was achieved following published procedures.14−16 The synthesis of 6-Me-CO2 is described below, starting from 1,3-dimethyltetrahydropyrimidinium tetrafluoroborate, which was received using the approach described by Kaloustian.20 The final protection step was conducted in a glovebox (Lab Master 130, MBraun, Garching, Germany). The thereby used potassium tert-butanolate was sublimed prior to use, while potassium hexamethyldisilazide (KHMDS) was used as received. Solvents for synthetic steps were predried by a solvent purification system (SPS, MBraun, aluminum oxide/molecular sieve) and stored in the glovebox (CH2Cl2 and THF over activated molecular sieve, pentene, toluene and diethyl ether over Na/K alloy). For curing experiments, commercial grade nonpurified educts were used (stored in glovebox): Bisphenol A diglycidyl ether (BADGE) was received from Aldrich (D.E.R. 332), phthalic anhydride (PhA) was provided by ABCR (99%) and hexahydrophthalic anhydride (HHPA) was purchased from Alfa Aesar (98%). NMR characterization was conducted using a Bruker Avance III 400. For DSC measurements, a DSC 4000 by PerkinElmer (software: Pyris) was used in a temperature range of −50 to +250 °C under nitrogen flow (20 mL/min). Glass transition temperatures (Tg) C


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Under lively stirring, CO2 was bubbled through the solution, whereupon a white precipitate formed immediately. After 20 min, the flask was taken inside the glovebox again. The product was collected on a filter and washed with diethyl ether and THF thoroughly. A white to cream colored powder was obtained (3.64 g, 90%). 1H NMR (400 MHz, MeOD): δ = 2.12 (p, J = 6.0 Hz, 2H, CH2), 3.20 (s, 6H, CH3), 3.43 (t, J = 6.0 Hz, 4H, CH2). 13C NMR (101 MHz, MeOD): δ = 20.0 (CH2), 41.0 (CH3), 46.8 (CH2), 161.7 (N−C−N), 162.1 (O−C−O). IR (ATR, cm−1): 2975.02 (m), 1690.96 (s, CO), 1678.78 (s), 1599.22 (m), 1462.93 (w), 1444.40 (w), 1377.16 (w), 1325.57 (w), 1164.20 (w), 1130.21 (s), 1054.94 (s). HRMS (ESI): m/z calcd for C7H12 N2O2, 179.0791; found, 179.0780. Curing Reactions. Inside a glovebox, the precatalyst (2−20 mg), BADGE (500−1000 mg), and anhydride hardener (400−900 mg) were combined at a molar ratio of epoxy compound to hardener of 1:2 and NHC loadings of 0.1−1.0 mol % (relative to BADGE) in a glass vial with magnetic stirring bar. The sample was then removed from the glovebox and thermally cured by immersion into a preheated oil bath (140 °C). Reactions were conducted under ambient conditions (cap off). After completion of the intended curing time, the sample was cooled to room temperature and the glass vial destroyed to remove the duroplastic polymer for further thermal analysis. In case curing reactions were conducted after prolonged storage time, samples were kept beforehand under air without special precautions.

were received this way. Reduction of the precatalyst loading from 1 mol % (relative to BADGE) to 0.25 or even 0.1% still allowed for a fast curing of the epoxy resin, though the solidification times lengthened somewhat (compare entries 3/4 and 9/10). Tg values remained high, indicating a high degree of cross-linking that is absolutely comparable to the thermal properties of epoxy thermosets with identical composition that were synthesized using conventional catalysts.24 A drastic reduction of the curing time to only 5 min as found in entry 3 did entail a gentle postcuring exotherm that was observed in DSC analysis (see Supporting Information), but overall the polymerization and cross-linking has to be considered as very rapid at mild 140 °C. Indeed, samples that were cured for 45 min never displayed any clear-cut postcuring effects.25 As is obvious from Table 1, the system BADGE/PhA is very tolerant toward a broad range of different NHC structures; there seems to be little difference in both solidification times and Tg of the resulting materials whether a bulky NHC (for example from 5-Ad-CO2) or a sterically much less congested carbene (i.e., from 6-Me-CO2) is liberated for curing purposes. Likewise, differences between the strongly basic six-membered NHCs and their five-membered counterparts, which are known to have considerably lower pKa-values,18 are only weakly pronounced compared to other incidences of NHC-mediated polymerizations.15,17 Finally, those first results underlined that it was in principle possible to use both NHC-carboxylates as well as NHC-metal complexes (5s-Mes-ZnCl2, entry 7) for a robust curing of epoxy resins. Curing of the BADGE/HHPA System. Having established this, for more detailed investigations HHPA (Figure 1) was chosen as hardener. While BADGE/PhA compositions are sticky, somewhat heterogeneous masses at room temperature, the more soluble and much lower melting HHPA (mp 32−34 °C) enables the formation of fully homogeneous, free-flowing mixtures at low temperatures. These favorable properties allow for in situ observation of the curing process using DSC and rheological measurements. Furthermore, the anhydride functionality in the alkyl-substituted HHPA is less reactive than the one in PhA, which can be useful to pronounce the differences in reactivity of the NHC catalysts. Gratifyingly, the oily matrix of BADGE/HHPA mixtures in a molar ratio of 1:2 proved to be a good enough solvent for the protected NHCs. Clear and homogeneous compositions were obtained after short periods of gentle warming (35 °C) and stirring. The precatalysts 6-MeCO2 and 6-Mes-CO2 required temperatures of 45 °C to go fully into solution. As sole exception, mixtures containing 5sMes-MgCl2 displayed only partially dissolved precatalyst under these conditions. Nevertheless, also in the latter case the liquid phase was successfully hardened after thermal activation. The curing reactions proceeded well and delivered thermoset materials in all cases, while expectedly in the absence of protected NHC no hardening was observed (Table 1, entry 13). To a certain degree, longer solidification times occurred compared to using PhA as hardener and the differences between the precatalysts became more pronounced (compare entries 8/16 or 12/18). Tg values were found to be in the dimension of 135−145 °C, which is again fully in the range of identical conventionally prepared epoxy thermosets.26 The limited variation of the observed Tg values probably stems from the different polymerization properties of the individual precatalysts (see below) and might be modulated by residual monomeric or oligomeric impurities, which can act as softening components. While the high glass transition temperatures of

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RESULTS AND DISCUSSION Curing of the BADGE/PhA System. The precatalysts as depicted in Scheme 1 where synthesized following literatureknown procedures in 1−3 steps starting from commercially available chemicals.14−16,21 The preparation of 6-Me-CO2 can be found in the Experimental Section. As mentioned above, compounds of this type, e.g., NHC−CO2 adducts (“NHCcarboxylates”) and NHC−metal complexes have been used as latent NHC precursors (“protected NHCs”), making use of the thermal lability of the NHC−carbon dioxide and NHC−metal bonds. Similarly, in the present study the carbenes liberated this way were intended to act as curing agents for epoxy resins. To simplify nomenclature and identification, the precatalysts are named according to a sequence of ring size−substituentprotecting group. First experiments using BADGE and PhA together with NHC precursors soon demonstrated the high catalytic activity of the thermally generated free NHCs under curing conditions (see Table 1, top part). The samples were prepared by simple mixing of epoxy compound, hardender, and protected NHC in the desired ratio under protective gas atmosphere. The mixtures were then subjected to thermal treatment at 140 °C for 45 min (under air), which resulted in a clear and stirrable homogeneous mass. While control reactions without any NHC progenitor remained in this state for the rest of the curing time (Table 1, entry 1), the samples containing a protected NHC solidified in a sudden fashion to a hard, glassy thermoset after 3 to 7 min of heating, depending on the applied NHC structure and catalyst loading (entries 2−12). Interestingly, polymerizations conducted under nitrogen did not differ significantly from those that were carried out under ambient conditions in both solidification times and glass transition temperatures of the resulting thermoset materials. In order to further investigate this favorable robustnesswhich is evident in some,23 but by far not all, NHC-mediated polymerizations17ball of the following reactions were carried out without special precautions under air, including storage time under ambient conditions if the compositions were not cured immediately as well as the use of commercial grade, nonpurified monomer. All data presented in this study refer to results that D


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influence of deprotection (the generation of the free NHC must be expected to be strongly temperature dependent)30 as well as the inherent reactivity of the free NHC toward BADGE and HHPA, it is not surprising that the different precatalysts yield individually different thermograms. To give a better overview about the performance of all latent NHCs, Figure 3 shows Tmax for every precatalyst structure in this study (see Supporting Information for full thermograms). Interestingly, the application of NHC−CO2 adducts seems to entail a lower Tmax compared to the use of NHC-metal complexes. This is also valid in case structurally identical NHCs are liberated: 6-MesCO2 and 6-Mes-ZnCl2 differ considerably in this property with values of Tmax= 150 and 172 °C, respectively. This indicates that CO2 is lost more easily from this NHC precursor compared to ZnCl2, which forms a dimeric precatalyst (see Scheme 1).14 However, carbon dioxide can leave the curing setup irreversibly, while in contrast the metal component cannot. ZnCl2 will remain in the polymerization, where it can in principle stay in a temperature dependent equilibrium with the free NHC and thus “block” a certain percentage of the catalyst. Furthermore, it must be considered that Lewis acids like ZnCl2 are noninnocent protecting groups, which are generally able to activate the monomer functionalities in the curing system and thus potentially facilitate the polymerization process.31 Overall, these results fit earlier investigations regarding the polymerization of ε-caprolactone, where 6-Mes-ZnCl2 was found to have a high thermal stability.16 The influence of the protecting groups also becomes apparent when the three homologues 5sMes-MCl2 (M = Mg, Zn, Sn) are compared. In all cases, Tmax is significantly different. Although the Mg(II)-NHC complex can surely be expected to be the most labile,16 its Tmax is highest, indicating clearly that dissociation ability is not the single decisive attribute for the reaction profile of these three precatalysts (see also Supporting Information). The low Tmax as observed by the action of 6-Cy-CO2 can be related to geometric reasons: as a six-membered NHC, its NCN angle is larger than those existent in its five-membered analogues, which forces the N-substituents closer to the CO2 moiety, decreasing the stability of the adduct this way.30b Indeed, removing most of the steric pressure by introducing small methyl substituents instead of the larger cyclohexyl groups raises the value to 162 °C (6-Me-CO2, see Scheme 1). Consistently, sterically congested six-membered NHC-carboxylates lead to curing

the cured materials indicate a high degree of cross-linking and high monomer conversion, more detailed information were won from in situ DSC experiments, using the oily compositions directly to observe the thermal changes in the sample upon polymerization.27,28 Indeed, temperature scans (5 K/min from −30 °C up to +250 °C) revealed a strong exothermic peak in all runs containing protected NHCs; consecutive cooling and reheating cycles of these postcure samples then displayed only a glass transition temperature (fitting to those in Table 1) and no further exothermic peaks (see Supporting Information for exemplary thermograms), which obviously means that in the first run complete cross-linking and monomer consumption took place. Analysis of the peaks delivered curing exotherms of 300−360 J/g, with the majority of all samples in a much narrower range of 330−345 J/g. These results nicely match literature-known values and again strongly indicate a quantitative NHC-mediated curing of the epoxy resin.29 Apart from these similarities, the use of the different precatalysts heavily influenced the shape and position of the reaction exotherms. In Figure 2, the first DSC heating scan of

Figure 2. Characteristic curing exotherms for three different precatalysts as observed via dynamic DSC scans.22

6-Cy-CO2, 5-tBu-CO2, and 5s-Mes-ZnCl2 with BADGE/ HHPA is compared. The temperature of maximum negative heat flow, Tmax, is shifted considerably depending on the NHC, with 6-Cy-CO2 delivering the lowest Tmax at 146 °C and 5sMes-ZnCl2 effecting a relatively high Tmax of 169 °C. As the curing rates will be effectively determined by the combined

Figure 3. Precatalyst structures and their resulting Tmax values in the curing system. NHC/BADGE/HHPA = 1/100/200. E


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Figure 4. Isothermal DSC scans at different temperatures including peak integration.

systems with the lowest Tmax (6-Mes-CO2, 6-OMe-CO2), followed by five-membered imidazolium-based compounds bearing large N-substituents (5-Ad-CO2, 5-tBu-CO2). DSC studies also enabled further insights concerning monomer consumption and polymerization rates. To this end, an isothermal DSC setup was used (see Experimental Section).28 In Figure 4, the monomodal reaction exotherms of 5-tBu-CO2 and 5s-Mes-ZnCl2 at different temperatures are compared. Befitting their differently shifted Tmax values (compare Figure 3), 5-tBu-CO2 effects a significantly faster polymerization than 5s-Mes-ZnCl2 in a temperature range of 140−160 °C. It should be noted that both compounds at both 140 and 160 °C deliver nicely matching integrals of the exotherms (Figure 4), underlining that in all four cases complete monomer conversion takes place. The isothermal peaks show “tailing”, which becomes more pronounced with falling temperature. This is most probably due to the rapidly growing viscosity during polymerization and much more prominent in case the curing temperature is below (120 °C) or roughly in the range (140 °C) of the typical Tg of HHPAhardened BADGE-based epoxy resins, which is in agreement with the diffusion-controlled kinetics that are usually observed in the later stages of epoxide curing reactions.28,32 At temperatures higher than 160 °C, the reactions were difficult to quantify, because polymerization already started before the target temperature was reached. In view of the strictly alternating nature of the polymerization9 and the quantitative monomer consumption as described above, it is possible by way of stepwise integration to convert the exothermic peaks into monomer consumption plots, which give a better measure of the speed of polymerization (Figure 5 and Figure 6). While notably all NHCs entail complete conversion, the rate of polymerization is modulated by the structure of the liberated NHC and its protecting group. Under identical conditions, at 160 °C bulky and electron-rich 6-OMe-CO2 is revealed as the most active precatalyst, reaching conversions of more than 50% after 1 min and 94% after only 4 min of curing time. A close second with regard to catalyzing power is found in the structurally related 6-Mes-CO2. Contrasting this, 6-Me-CO2 displays an already somewhat attenuated reactivity. The relatively slowest polymerization is achieved by using dimeric 6-Mes-ZnCl2, where a conversion of 36% is found after 4 min and more than 90% completion is reached after 12 min. Precatalysts 5-tBu-CO2, 6-Cy-CO2, 5-Ad-CO2, and 5s-MesSnCl2 all performed very similarly, so in Figure 6 only the

Figure 5. Monomer conversion plots derived from isothermal DSC analysis for three different precatalysts at 160 °C (blue) and 140 °C (red). NHC/BADGE/HHPA = 1/100/200.

Figure 6. Monomer conversion at T = 160 °C for curing reactions using different protected NHCs. NHC/BADGE/HHPA = 1/100/200.

monomer consumption of 5-tBu-CO 2 is plotted (see Supporting Information for a convoluted graph). Note again that 5s-Mes-SnCl2 behaves differently from its ZnCl2- and MgCl2-masked homologues, entailing much sharper exotherms at 160 °C and a correspondingly faster polymerization, which again hints at a noninnocent involvement of the tin(II) species. Latency of the Precatalysts in the BADGE/HHPA System. As described above, the curing reactions mediated by the thermally latent, protected NHCs proceeded nicely and rapidly at elevated temperatures; contrasting this, at room temperature, no immediate reaction occurred. To investigate the degree of latency and its dependence on the chemical structure of the precatalyst, long-time studies (28 days) at room F


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temperature under air were undertaken. Samples (1−2 g) were prepared, fully homogenized and stored under ambient conditions. From those, in intervals of 7 days, small amounts were removed for DSC analysis. Isothermal measurements at 160 °C delivered exotherms that were compared to results directly obtained after the sample had been mixed. Interestingly, in no case deactivation of the protected NHCs was observed, but contrary the polymerizations proved to be excellently reproducible with no or very minor changes evident in the exotherm profiles over a time of 4 weeks (see Figure 7

Figure 8. Time-dependent relative loss of curing exotherm for different precatalysts.

min) first resulted in a drop of viscosity to 0.6 Pa·s, clearly indicating that no significant molecular weight had been built up, followed by a rapid increase as the polymerization started (Figure 9).

Figure 7. Observation of time-dependent curing properties of 5-tBuCO2.

for an example, exhaustive data can be found in the Supporting Information). With increasing pot times, a slow loss of generated heat per weight unit of the resin upon curing was observed. The magnitude of this effect was found to be strongly controlled by the precatalyst’s structure: compounds which imposed a low Tmax on the system and provided fast crosslinking at lower temperatures consistently suffered from a faster decrease of the reaction exotherm under storage conditions at room temperature and vice versa. A loss of curing exotherm directly translates into growing viscosity (the relative difference between curing exotherm at any given time and the original ΔH value is equal to the percentage of ester bonds formed prematurely). Indeed, using 1% precatalyst loading, compounds like 6-Cy-CO2 or 5-tBu-CO2 entailed a loss of the “freeflowing” ability of the curing mixture after about 2 weeks. By contrast, NHC progenitors like (dimeric) 6-Mes-ZnCl2, which liberates two NHCs per molecule and was thus used on a 0.5% precatalyst level, allowed for the formation of compositions with much longer pot times, that were still free-flowing after 28 days. 6-Me-CO2 expectedly proved to possess a considerable higher degree of latency than its bulkier counterpart 6-Cy-CO2, mirroring the reduced stability of the latter CO2-adduct. In general, the propensity to polymerize slowly at room temperature moved within a corridor having precatalysts 6Mes-CO2 as lower and 6-Mes-ZnCl2 as upper limits (Figure 8; for a table containing numeric data of all precatalysts see Supporting Information). Apart from DSC experiments, in situ rheology enabled a complementing view on the NHC-mediated curing process. Using a larger scale sample (110 mL overall) with 0.5% precatalyst loading (6-Cy-CO2), measurements at 25 °C yielded a viscosity of 0.5 Pa·s 2 days after the composition had been prepared. Three weeks later, the sample had become slow-flowing and displayed an increased value of 160 Pa·s under the same conditions. Nevertheless, heating this sample to 150 °C (30 K/

Figure 9. Viscosity development of a three week old sample (0.5% loading of 6-Cy-CO2) at room temperature and after thermal activation at 150 °C.

Mechanistic Considerations. Typical accelerators for the curing of epoxy resins (like tertiary amines, including imidazoles) act as initiators, most probably by attack on the epoxide functionality (Scheme 2, top).9,24,33 A limited regeneration of the free tertiary amine may also play a role, thus enabling it to start more than one growing chain.34 In principle, plausible mechanisms for the in situ generated NHCs to mediate the curing process could likewise include ringopening of the epoxy moieties or the anhydride functional group with subsequent alternating copolymerization. Additionally, a reaction of the NHCs, which possess a considerable Brønstedt basicity,18 with impurities like traces of water or phenolic hydroxyl groups is feasible, creating initiating hydroxide or phenolate ions, respectively. However, the findings discussed above indicate that impurities do not significantly influence the polymerization process: for one, the curing exotherms do not alter position or shape after week-long contact with atmospheric humidity (during which time the water content should build up). Second, if deprotonation of water or phenolic residues (the latter originating from Bisphenol A-based impurities) were the dominating process, clearly the precatalyst liberating the most basic NHCs would display the highest activity and fastest curing. Such a clear-cut tendency, which would favor Nalkylated six-membered NHCs over N-arylated ones, is not to G


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hydrolysis of the NHC−CO2 precatalysts to azolium hydrogen carbonates has to be considered.38 This class of compounds, while also room temperature active, is able to release free NHCs upon heating.39 Furthermore, the anion (hydrogen carbonate) must be expected to be potentially able to generate hydroxide ions from contact with water, thus inherently being a source for slow curing at room temperature. A minor contribution of this mechanism to the premature curing at room temperature cannot be ruled out at present, however, significant formation of hydrogen carbonates can be excluded on the basis of the constant properties that show in the curing exotherms (shape, position) even after prolonged exposure to atmospheric humidity; thermal generation of free NHCs from the azolium salts instead from the CO2-adducts would without doubt be connected to a significant change in the DSC profiles. This is not found.

Scheme 2. Generally Accepted Mode of Initiation for Conventional, Imidazole-Based Accelerators (Top) and Proposed Mechanism for NHC-Mediated Curing of Epoxide/Anhydride Resins

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CONCLUSION

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ASSOCIATED CONTENT

In this work, the first NHC-mediated thermal curing of epoxy resins has been described. It was shown that NHCs are especially well suited to cure anhydride/BADGE mixtures fully and fast, applying mild curing conditions (140−160 °C). The system proved gratifyingly tolerant toward changes in the catalyst structure, so that notably all compounds that were part of this study were able to effect quantitative monomer consumption, based on DSC results. The thermosets prepared this way were found to have high glass transition temperatures, absolutely comparable to conventionally derived materials. Furthermore, the application of protected, thermally latent NHCs as precatalysts strongly retarded the undesired premature setting at room temperature and enabled the preparation of stable, ready-to-cure compositions with pot times of several days or weeks. The characteristics of the curing systems could be readily tuned by choice of suitable NHCs and protecting groups, depending on whether longest pot times (best achieved with dimeric 6-Mes-ZnCl2) or the most rapid curing at lower temperatures (using 6-OMe-CO2) was targeted. The homogeneous curing setup consisting of protected NHC, BADGE and HHPA exhibits a remarkable robustness, which allowed for the use of low precatalyst loadings (0.5−1.0%) and commercial grade monomers, in addition to convenient handling without special precautions under atmospheric conditions. Overall, protected NHCs seem to emerge as a powerful alternative to the commonly used tertiary amine catalysts, and currently further investigations in our laboratories are progressing to map out the potential of this new class of latent curing agents in the fabrication of fiber-based composite materials.

be found. Interestingly, introduction of phenolic functionalities via substitution of PhA for the frequently used hardener phloroglucinol (1,3,5-trihydroxybenzene), applying standard curing conditions (6-iPr-CO2, 1 mol % catalyst loading, 140 °C), requires a much longer solidification time (15 vs 3 min, compare entry 3, Table 1). Furthermore, the homopolymerization of epoxides like propylene oxide by NHCs, both free35 and generated in situ from NHC−carboxylates,36 has been shown to proceed sluggishly and to be very sensitive toward changes in the NHC structure. This contrasts sharply with the findings of the BADGE/HHPA system described here, where essentially all NHCs were able to bring about a full curing in short reaction times only modulated by the precatalyst structure. In sum, above points suggest that the NHC-mediated curing of BADGE/anhydride resins is governed by initial attack of the carbene on the anhydride moiety (Scheme 2; see also the Supporting Information for further illustration). However, this has to be understood as a tentative interpretation of above findings and is complicated by the superimposed effects of deprotection, individual reactivity of the NHCs and potential involvement of noninnocent protecting groups. Finally, it should also be noted that imidazoli(ni)um- and tetrahydropyrimidinium-based NHCs, e.g., five- and six-membered compounds, are prone to behave very differently in a number of polymerizations and may follow differing mechanistic pathways, as was recently demonstrated.15−17 The slowly proceeding polymerization at room temperature is most certainly caused by a very small degree of deprotection or dissociation of the precatalysts under these conditions. This can be easily understood for the NHC complexes, where, however miniscule, a dissociation constant must exist. Crossover and labeling experiments with 13CO2 have demonstrated that also NHC−carboxylates may coexist with their respective free NHCs under certain conditions.30c,37 Notably, recent findings underlined that the CO2 adducts themselves, being carboxylates after all, can possess the ability to polymerize βbutyrolactone directly by attack of the −COO− moiety on the cyclic ester.23b However, so far this behavior has not been observed with epoxy or anhydride groups and the direct influence of the steric pressure on the CO2-moiety on the latency properties (compare 6-Me-CO2 and 6-Cy-CO2, Figure 8) indicates that adduct stability is the dominant factor. Finally,

S Supporting Information *

Several DSC analyses of cured thermosets, in situ thermograms both dynamic and isothermal, additional graphs on latency of different precatalysts, monomer conversion plots, and monomer activation by Lewis acids. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(M.R.B.) E-Mail: [email protected]. H


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Notes

(23) For examples of relatively robust NHC-based polymerizations, see: (a) Marrot, S.; Bonnette, F.; Kato, T.; Saint-Jalmes, L.; Fleury, E.; Baceiredo, A. J. Organomet. Chem. 2008, 693, 1729−1732. (b) Brulé, E.; Guérineau, V.; Vermaut, P.; Prima, F.; Balogh, J.; Maron, L.; Slawin, A. M. Z.; Nolan, S. P.; Thomas, C. M. Polym. Chem. 2013, 4, 2414−2423. (24) Steinmann, B. J. Appl. Polym. Sci. 1990, 39, 2005−2026. (25) However, since the curing temperature (140 °C) is below the typical glass transition temperature of the BADGE/PhA system, minor exothermic transitions were observed in DSC in several cases at about 200 °C, most probably originating from relaxation processes. This is to be expected if the polymer chains are “frozen” during polymerization by the rapidly increasing Tg. If postcuring effects occur in this range, they are much more pronounced because of the distinctly exothermic formation of new ester bonds (examples for both can be found in the Supporting Information). (26) Kretzschmar, K.; Hoffmann, K. W. Thermochim. Acta 1985, 94, 105−112. (27) Yousefi, A.; Lafleur, P. G. Polym. Compos. 1997, 18, 157−168. (28) Barton, J. M. In Epoxy Resins and Composites I; Springer Berlin Heidelberg: Berlin and Heidelberg, Germany, 1985; Vol. 72; pp 111− 154. (29) Peyser, P.; Bascom, W. D. J. Appl. Polym. Sci. 1977, 21, 2359− 2373. (30) On the release of NHCs from NHC−CO2 adducts, see: (a) Naumann, S.; Buchmeiser, M. Catal. Sci. Technol. 2014, 4, 2466− 2479. (b) Van Ausdall, B. R.; Glass, J. L.; Wiggins, K. M.; Aarif, A. M.; Louie, J. J. Org. Chem. 2009, 74, 7935−7942. (c) Denning, D. M.; Falvey, D. E. J. Org. Chem. 2014, 79, 4293−4299. (31) On this dual/ cooperative catalysis of NHCs and Lewis acids, see discussion in ref 30a and: (a) Cohen, D. T.; Scheidt, K. A. Chem. Sci. 2011, 3, 53−57. (b) Piedra-Arroni, E.; Amgoune, A.; Bourissou, D. Dalton Trans. 2013, 42, 9024−9029. (32) Xu, J.; Holst, M.; Wenzel, M.; Alig, I. J. Polym. Sci., B: Polym. Phys. 2008, 46, 2155−2165. (33) Matějka, L.; Lövy, J.; Pokorný, S.; Bouchal, K.; Dušek, K. J. Polym. Sci., Polym. Chem. Ed. 1983, 21, 2873−2885. (34) (a) Mauri, A. N.; Galego, N.; Riccardi, C. C.; Williams, R. J. J. Macromolecules 1997, 30, 1616−1620. (b) Riccardi, C. C.; Dupuy, J.; Williams, R. J. J. J. Polym. Sci., B: Polym. Phys. 1999, 37, 2799−2805. (35) Raynaud, J.; Ottou, W. N.; Gnanou, Y.; Taton, D. Chem. Commun. 2010, 46, 3203. (36) Lindner, R.; Lejkowski, M. L.; Lavy, S.; Deglmann, P.; Wiss, K. T.; Zarbakhsh, S.; Meyer, L.; Limbach, M. ChemCatChem. 2014, 6, 618−625. (37) Duong, H. A.; Tekavec, T. N.; Arif, A. M.; Louie, J. Chem. Commun. 2004, 112−113. (38) Fèvre, M.; Pinaud, J.; Leteneur, A.; Gnanou, Y.; Vignolle, J.; Taton, D.; Miqueu, K.; Sotiropoulos, J.-M. J. Am. Chem. Soc. 2012, 134, 6776−6784. (39) Fèvre, M.; Coupillaud, P.; Miqueu, K.; Sotiropoulos, J.-M.; Vignolle, J.; Taton, D. J. Org. Chem. 2012, 77, 10135−10144.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.Sc. Christina Lienert, M.Sc. Laura Widmann, and M.Sc. Johannes Klein (all University of Stuttgart) are gratefully acknowledged for support in DSC measurements and synthesis. This work was supported by the University of Stuttgart, Germany.

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