1 Introduction

Thermosetting acrylic resins are known for their excellent outdoor durability, good adhesion properties to a manifold of substrates, and their moderate cost [1]. They are used for architectural coatings, “special purpose” coatings, and in the original equipment manufacturing market. Cured resins are more resistant to weathering (heat) and solvents than the linear or branched polymers from which they originate. The current thermosetting praxis is based on chemical reactions between entities in the side chains that lead to three-dimensional networks. This crosslinking is often accomplished by combining hydroxyl and carboxyl functional acrylic resins with a co-reacting polyfunctional polymer or hardener, often in the form of polyisocyanates, polyepoxides, or amino resins [1]. They react to give urethanes, etherified amino resins, and a smaller variety of other functionalities. These crosslinks are essential to the final film properties, and the crosslink chemistry is chosen in line with the requirements and application constraints.

Self-crosslinking polyacrylics have the advantage of needing no co-reactant and are therefore easier to handle, often are more economical and hence tend to be the first choice for manufacturers [2]. Resins with such properties generally contain a combination of functional groups that become reactive towards each other upon the action of a trigger (temperature, moisture …) and otherwise should be quite inert. The latter is important for reaching a reasonable “shelf life” that is, a period when the physical and chemical properties of the resin are not changing. The choice of reactive entities leading to crosslinking is limited, effectively to methylolated (meth)acrylamides [3], blocked isocyanates [4], alkoxysilanes [2], keto-dihydrazide systems [5], and metal complexes [6]. The crosslinking can be induced by increasing the temperature or through a reaction with water from the surrounding air. These one-component (1 K) systems have their merits, but also face criticism, for example, for the emission of volatile organic compounds or limits of shelf-lifeA possibly more important weak point may relate to the crosslinking functionality being prone to thermal or photochemical degradation.

It seemed thus of interest to develop a reactive acrylate binder that leads to a network with only ester moieties. This is probably most easily reached through a (trans)esterification of an acid-derived entity and a hydroxyl group in a side chain. However, an esterification between a free acid and a hydroxyl entity has a rather low driving force and shows low rates in an uncatalyzed execution. Similar will hold true for a transesterification involving a hydroxyl functionality [7]. These constraints must be released for an effective 1 K self-crosslinking binder system, which may not be trivial.

Herein, an unusual but simple self-crosslinking acrylic system is presented, yielding an all-ester, potentially UV-resistant coating after thermal curing. In addition, some not-so-common chemistry of covalently bonded perchlorates is described. Alkyl esters of perchloric acid are certainly versatile functional groups, and yet they have barely been applied, neither commercially nor academically. This may originate from their explosive and toxic reputation, in combination with their basically absent commercial availability and also limitations in useful precursors [8, 9]. Indeed, in particular, secondary perchlorates (e.g., sec-pentyl perchlorate) rapidly eliminate perchloric acid (HClO₄) even at room temperature, and tert-butyl perchlorate is a good source of anhydrous perchloric acid [10]. However, primary alkyl perchlorate esters, for example, n-decyl perchlorate, turned out to be thermally quite stable in nonpolar solvents, up to temperatures as high as 140°C. Reasonably stable alkyl perchlorates have, in fact, been used as initiators for cationic polymerization of electron-rich alkenes [11], and ring-opening polymerization of lactones [12] and cyclic ethers [6-9, 13]. Because of the exceptionally good leaving group ability of the ClO₄ ⁻ anion [14], multiple orders of magnitude stronger than mesylates and almost as strong as the esters of triflic acid, perchlorate esters are also potent alkylating agents [15]. They react not only fast with common nucleophiles like amines and alcohols [16], but even with considerably weaker ones like nitriles [17], be it at higher temperatures. Their analogous reactivity towards simple esters can be used for rearranging ester bonds in polyacrylate resins, which is reported upon here.

2 Experimental

3 Results

Covalently bonded perchlorates in (acrylic) polymers appear to be unknown; only polymers with integrated perchlorates in the form of salts are described [20]. A perchlorate-containing resin could be prepared from 4-perchloratobutyl acrylate (pCBA), an acrylic monomer carrying the perchlorate ester functionality. It is obtained from acryloyl chloride, silver perchlorate, and tetrahydrofuran (Scheme 1). Treatment of acryloyl chloride with silver perchlorate in a nonpolar solvent like pentane immediately gives a brownish precipitate (of AgCl) indicative of the formation of acryloyl perchlorate, a highly reactive species readily attacked by any nucleophile. Subsequent addition of THF yields an oxonium ion (2), which precipitates as a white salt and then further converts to pCBA by effectively adding the perchlorate anion to the electrophilic (alpha) carbon atom adjacent to the oxygen. The formation of a minor amount of polyTHF, as was reported under similar conditions [21, 22], cannot fully be avoided in pentane, even when adding THF as a solution in pentane. However, the insoluble, precipitating polyTHF is readily removed by filtration through a small pad of silica gel, and solvent removal under reduced pressure leaves the essential pure liquid monomer in up to 80% yield behind. The more polar carbon tetrachloride may be a good choice for a solvent too (instead of pentane), but was not tried here; the analogous reaction of equimolar amounts of THF and acetyl perchlorate was reported to yield almost quantitatively the perchlorate ester [23].

pCBA is moisture sensitive but can be stored as a solution in pentane at room temperature for some days. The identity was established from ¹H, ¹³C NMR, and IR spectra and by elemental analysis. Note that silver perchlorate is explosive and should be handled accordingly in limited amounts and with appropriate precautions. pCBA, in contrast, did not explode in the hammer test [18]. The corresponding methacrylate can be prepared under the same conditions, giving 4-perchlorato butyl methacrylate (3b; pCBMA).

Free radical (co)polymerizations of pCBA in toluene solution are straightforward and yield linear polymers. Bulk polymerizations were not attempted but may be possible at low temperatures. Copolymerization with various acrylates, methacrylates, or vinyl acetate proceeds smoothly at 60°C (60 wt.% in toluene). Polymerizations at temperatures above 100°C lead to significant crosslinking. The copolymerization reactivity ratio of pCBA is, as expected, approximately equal to other short-chain alkyl acrylates, and statistical random copolymers are obtained.

The molecular weights of the copolymers with the perchlorate entity in the side chain are lower than those of homopolyacrylates prepared from alkyl acrylates under the same conditions (Table S1). The free radical polymerization of ethyl acrylate (EA) in the presence of 1 mol% n-decyl perchlorate (relative to EA) resulted, for example, in a polyethylacrylate PEA with M _n of 56 kDa and a broader molecular weight distribution (PDI of 2.7) than homo-PEA (M _n of 66 kDa; PDI of 2.1) obtained similarly in its absence. The perchlorate esters thus show some chain regulating activity. The underlying reactions are not understood and would need more attention. Chain regulation with carbon tetrabromide in the free radical copolymerization with pCBA gives access to polymers of lower molecular weight (M _n < 10,000 g mol⁻¹); mercaptan regulators are too nucleophilic to be used in combination with perchlorate esters.

The obtained polyacrylic resins from pCBA and alkyl acrylates are readily crosslinked with a variety of typical hardeners with hydroxyl and/or amine moieties (results not shown), but these reagents are not necessary for crosslinking the binder. As indicated above, it was unexpectedly found that a simple thermal treatment is sufficient to reach an all-ester (robust) crosslinked coating. Heating thin films of EA/pCBA copolymers over 100°C leads to a thermoset as indicated by swelling studies [19]. The crosslink density (CD) increases for about 4 h while curing at 105°C to reach a constant level for copolymer compositions with less than 8 mol% of pCBA (highest level studied). Curing at 135°C takes a similar course, reaching completion after 1 h (Figure 1a,b).

The experimental CDs are generally in good alignment with CDs that were calculated assuming that all perchlorate entities lead to a crosslink (Figure 1c,d). The crosslinking seems thus quite effective; every mol of incorporated pCBA generates about 1 mol of crosslinks. The glass transition temperatures (T _g) of completely cured resins (Figure S1) were found to increase with increasing pCBA content, as a higher level of crosslinking shortens segments and impedes general mobility. Linear PEA shows a T _g of −23°C; cured P(EA-pCBA0.9–8) has a T _g between −12°C and +20°C (note that alkyl acrylate copolymers containing x mol% of pCBA are designated P(YA-pCBAx), with x the mol% of pCBA in the backbone and Y being either M for methyl, E for ethyl and B for butyl acrylate content).

The mechanism of crosslink formation is envisioned to proceed by the nucleophilic attack of an ester entity at the electrophilic carbon of the perchlorate ester, generating a cationic 1,3-dioxolenium intermediate 4a (Scheme 2). Subsequent liberation of one of the alkyl groups in reaction with the “free” perchlorate entity will regenerate a perchlorate ester, either in the form of the starting material or in the form of a mobile and here also volatile alkyl perchlorate. The equilibrium will shift to the side of the 1,4-butylene ester crosslink 5 when the (volatile) alkyl perchlorate is dynamically removed from the film.

Accordingly, the ¹³C CP MAS NMR spectra of cured films of P(MA-pCBA11) and P(EA-pCBA8) fail to show a signal for covalently bonded perchlorate entities, expected at ~75 ppm. Therefore, new resonances appear at 25, 65, and 181 ppm (Figure 2a,b) [24]. The signals at 25 ppm and 65 ppm are related to the newly established ester crosslink and are assigned to a symmetrical 1,4-butylene unit in between two carboxylic ester moieties. These resonances are also present in the corresponding spectra of independently prepared poly(ethyl acrylate-co-1,4-butanediol diacrylate) resin (Figure 2c).

Also, a set of minor reactions was identified to take place during the curing. The intramolecular variant of the transesterification process, the nucleophilic attack of the butyl perchlorate carrying ester, will regenerate a coordinated THF-oxonium ion 6, and subsequently, THF and a polymer-bound analog of 1, acyl perchlorate 7, are expected to be formed [21, 25]. The reaction of 7 with water results in the formation of a carboxylic acid substituent and perchloric acid. The resonance at 181 ppm likely originates from free carboxylic acids formed by partial hydrolysis in the film (from the air during curing). Further evidence for the presence of carboxylic acids was found in IR spectra of cured polymers with emerging bands at ~3280 cm⁻¹ (broad) and ~ 1715 cm⁻¹ (Figure 3). Spectra of P(MA-pCBA11) cured in the absence of water do not show these features (Figure S2). It may be expected that the formed carboxylic acids are even more favorable for crosslink formation, as they are more nucleophilic than the corresponding esters. In addition, the presence of acid functionalities in the final polymer can be advantageous as they increase the adhesion of the resin to metals and glass substrates. Some moisture can thus be tolerated in the perchlorate ester-based crosslinking chemistry.

The curing process is conveniently monitored by ATR FT-IR spectroscopy of the films (Figure 3). The solitary band at around 585 and 707 cm⁻¹ of the perchlorate anion in the spectra of the copolymers is a good probe to detect changes related to the perchlorate ester. They show the same changes as the characteristic absorptions of covalently bonded perchlorates [8] at around 1260, 1230, and a broad signal from 1100 to 1000 cm⁻¹, but lack the partly strong overlap with other absorptions. The signal maxima show an approximate linear dependence on the pCBA concentration in the virgin polymers up to 10 mol% (using the CO vibration at ~1700 cm⁻¹ as an internal standard).

Heating of P(BA-pCBA10) thin films at 105°C leads to a loss of perchlorate ester entities in a period of about 8 h. The loss has approximately a first-order dependence on the perchlorate concentration. Similar results are found for curing at 115°C resp. 125°C but at higher rates (Figure S3). Other aliphatic acrylic copolyesters with pCBA show this behavior too, with increasing rates in the order of BA, EA, and MA Figure (S4). The perchlorate entities disappear from the films in the form of volatile perchlorate ester alkyl derivatives and not by decomposition. This is in accordance with the thermal stability of primary alkyl perchlorates; some have been distilled at temperatures above 100°C [26]. As mentioned above, it was found that n-decyl perchlorate is stable in 1,1,2,2-tetrachloroethane at 140°C for several hours.

Thermographic analyses (TGA FT-IR) of copolymers of pCBA with MA, EA, or BA reveal a two-stage loss of mass, starting at around 140°C and extending to 220°C (Figure 4) and at 300°C to 500°C when heated at 10 K min⁻¹ under nitrogen (Figure S5). The decomposition at the high-temperature range is the general thermal decomposition of polyacrylates [27]. The loss of mass starting at about 140°C is consistent with the loss of alkyl perchlorates from the samples. The onset temperatures (T _ON) have the same order as the rate of perchlorate loss from the films: 145°C for P(MA-pCBA11), 154°C for P(EA-pCBA10), and 171°C for P(BA-pCBA11) (Figure 4).

The loss of mass is higher than calculated, presuming solely alkyl perchlorate loss from the film (Table 1; Figure S5). It correlates, as expected, with the pCBA content (Figure S6) and with the mass of the side chain in the corresponding comonomer. The superseding loss of mass has a proportionality to the (molecular) mass of the comonomer and also to the onset temperature. However, overall, the feature is not well understood: it indicates that some polymer backbone decomposition takes place, but little evidence is found.

The IR spectra of the off-gasses are coincident with that of a mixture of the respective alkyl perchlorates, THF, and CO₂ (Figure 5). In addition, products like alcohols and some ethers were detected. The formation of the mixture of alkyl perchlorates and THF was corroborated by analyzing the volatiles from P(EA-pCBA10) and P(BA-pCBA11) while heating to 200°C under a dynamic vacuum and collecting the volatiles in a liquid nitrogen trap. ¹H NMR spectra of the liquids indicate the presence of ethyl perchlorate or butyl perchlorate, respectively, and some THF (Figure S7). The extent of THF formation (i.e., isolation from the trap), however, is not enough to account for the missing mass. CO₂ may (in part) originate from partial decomposition of gaseous alkyl perchlorates in the heated transfer line (200°C) connecting the TG unit and the IR instrument. Separate thermogravimetric analysis-mass spectrometry (TGA-MS) on benzene with a mixture of benzene and ethyl perchlorate gave similar results. This observed reactivity of ethyl perchlorate (and other low molecular weight alkyl perchlorates) under such conditions is consistent with prior reporting [26, 29]. The decomposition of ethyl perchlorate C₂H₅O₄Cl yields CO₂, dihydrogen, and HCl in the appropriate stoichiometry. Additional calorimetric data obtained from differential scanning calorimetry (DSC) are in alignment with the thermogravimetric data and reveal an exothermic peak in the relevant temperature range of 140°C–230°C (Figure S8).

As ethyl perchlorate is the only perchlorate ester identified when P(EA-pCBA) is cured, it is concluded that the ethyl entity must originate from ethyl acrylate units. The curing mechanism thus involves a transesterification substituting the butyl perchlorate for an ethyl perchlorate. THF will originate from the butyl pCBA units (Scheme 2). The formation of THF in a side reaction would go along with the formation of a net equivalent of a perchlorate anhydride 7 resp. of a consecutive product (Scheme 1). Swelling measurements suggest, however, that 7 ultimately forms a crosslink. The reaction of 7 with an ester entity could lead to a bridging anhydride and an alkyl perchlorate. The putative formation of an anhydride, however, could be ruled out. The ¹³C NMR spectrum of a prepared poly(ethyl acrylate-co-acrylic anhydride) was not coincident with that of the cured product. Alternatively, a formed perchloric acid (e.g., after reaction of 7 with water) may either also evaporate from the film or, more likely, react in an equilibrium reaction with ester entities under the formation of an alkyl perchlorate and a further carboxylic acid [30]. The generated free carboxylic acid would then readily be alkylated by perchlorate esters, as demonstrated in model reactions (also leading to free perchloric acid). A crosslink is again generated in such a case when an intermediate 1,4-bis-perchlorato butane would be involved. The intermediate formation of 1,4-bisperchlorato butane can be envisioned along the same route that leads to crosslink motive 5 (Scheme 2). If such a crosslinking proceeds between two pCBA units, the outcome is 5 and 1,4-bis-perchlorato butane. The latter would also be able to regenerate a pCBA unit under the elimination of an alkyl perchlorate. Such a process also distributes the crosslinks dynamically over the film.

The importance of reactions that are coupled to the formation (elimination) of THF, which we expect to be the initiating cause of the superseding loss of mass (faster at higher temperatures) however, remains unclear at this moment. It may at least be concluded that the initial alkylation to intermediates 4a and 6 represents the key steps in the crosslink formation as THF and alkyl perchlorate are liberated simultaneously from the heated polymer despite having considerably different boiling points (Figures S9–S11). The onset of curing is higher for longer alkyl chains, indicating that the reaction leading to 4a is slower with sterically larger entities on the ester moieties, making the formation of 6 more competitive and thus inducing the partial breakdown of the binder (Figure S4).

4 Conclusion

Alkyl esters of perchloric acid were incorporated in poly(meth)acrylates by copolymerization with 4-perchloratobutyl (meth)acrylate (pCBA/pCBMA). Polymers were found to self-crosslink upon heating between 105°C and 135°C, giving a thermoset material. NMR studies revealed that perchlorate esters react with carboxylic esters in a transesterification to form a new type of crosslink in 1 K curable binders, a butylene unit connecting two carboxylic esters. TGA-FT-IR measurements confirmed that volatile alkyl perchlorates by-products formed during the transesterification quickly evaporate from the polymer surface, effectively shifting the equilibrium to the product side. Some THF was also detected leaving the polymer film, which suggests that additional reactions take place, leading to carboxylic acids and possibly to crosslinks (of unknown structure). The butylene crosslink motive 5 constitutes a nonpolar crosslink that does not provide a target for chemical attack as its esters do not differ from the esters of the acrylic backbone, and unless esters are cleaved, the butylene crosslink remains intact. Such a nonpolar crosslink, even though its formation comes with some downsides related to the perchlorate, constitutes a unique crosslink that stands out in the field of self-crosslinking acrylic resins. It opens up opportunities for application in the high-end coatings industry where robust polyacrylates are required. Of course, the potential hazards that come along with volatile alkyl perchlorates may be an obstacle for wide application. A desirable catalytic variant would have the option of minimizing such risk; this is under development.

Author Contributions

G. A. Luinstra: conceptualization (lead), formal analysis (supporting), resources (lead), supervision (lead), writing – review and editing (lead). P. Wienefeld: conceptualization (supporting), data curation (lead), formal analysis (lead), investigation (lead), methodology (lead), validation (lead), visualization (lead), writing – original draft (lead), writing – review and editing (supporting).

Conflicts of Interest

The authors declare no conflicts of interest.