Acrylic Hot-Melt Adhesives Containing Dynamic Covalent Cross-Links

Abstract

Hot-melt adhesives (HMAs) offer several advantages over solvent-based alternatives, such as lower environmental impact and faster processing, yet their mostly thermoplastic nature often limits mechanical strength and solvent resistance. To address this limitation, poly­(n-butyl methacrylate-co-itaconic anhydride) copolymers were designed for facile cross-linking through anhydride ring-opening with alcohols, enabling a dynamic monoester–anhydride exchange within the adhesive. The resulting networks exhibit substantially improved solvent resistance and mechanical performance, resulting in a 10-fold increase in lap-shear strength compared to a thermoplastic reference. Moreover, complex-viscosity measurements reveal a sharp viscosity transition within the typical HMA processing window (120–180 °C), enabling efficient substrate wetting during application while maintaining mechanical integrity at service temperatures. Overall, this work introduces an itaconate-based acrylic platform for dynamically cross-linked HMAs and expands the applicability of reversible anhydride chemistry in adhesive materials.

Hot-melt adhesives (HMAs) have become widely adopted in packaging, assembly, and other high-throughput manufacturing processes. − Owing to their solvent-free application, HMAs enable rapid bonding while avoiding volatile organic compound emissions. , Upon heating, these adhesives soften through melting of crystalline domains or upon exceeding their glass transition temperature, forming low-viscosity melts, typically in the range of 10³ - 10⁶ mPa.s, which effectively wet substrates and subsequently solidify into cohesive joints upon cooling. − Despite these processing advantages and their broad substrate compatibility, most commercial HMAs are, besides additives, fully thermoplastic and based on bulk polymers such as ethylene–vinyl acetate (EVA), polyolefins or polyamides. − Their performance is therefore solely governed by physical interactions rather than covalent cross-links, which generally results in limited mechanical strength and chemical resistance. − In practical applications, this restricts their use for structurally demanding or high-temperature applications, consequently highlighting the need for innovative design strategies that overcome the intrinsic limitations of purely thermoplastic adhesive systems.

To improve the overall properties of HMAs, reversible interactions have been introduced into hot-melt formulations over the last two decades. For instance, supramolecular approaches based on host–guest motifs or strong hydrogen bonding have demonstrated improved cohesion at service temperatures while maintaining melt flow during processing at temperatures between 120 and 180 °C. ,− However, the requirement for tailor-made monomers, together with processing constraints, remains a major hurdle to the practical implementation of such supramolecular materials as HMAs. In contrast to supramolecular interactions, dynamic covalent chemistries offer an attractive alternative approach, combining reprocessability enabled by exchange reactions with the robustness of covalent bonds. − Within this context, thia–Michael exchange, imine and oxime formation, oxime–carbamate exchange, boronic ester or boroxine chemistry, and Diels–Alder chemistry have been incorporated into HMAs, providing enhanced mechanical and chemical resistance as well as thermoreversible cross-linking behavior. ,−

Yet, most of these chemistries rely on non-commodity building blocks, involve multi-step synthesis routes, or exhibit exchange kinetics that impose constraints on processing or service temperatures, thereby continuing to limit their industrial uptake.

Among the dynamic covalent chemistries investigated to date, monoester–anhydride exchange represents a promising yet unexplored approach with high potential for industrial adoption in HMA applications. Delahaye et al. showed that phthalate monoesters undergo catalyst-free exchange through a dissociative mechanism in which intramolecular nucleophilic activation promotes rapid cyclization to a transient anhydride intermediate. This mechanism is particularly attractive for hot-melt processing, as dissociative exchange enables low-viscosity flow at elevated temperatures, whereas covalent network integrity is retained at service temperatures.

Furthermore, a subsequent study highlighted that introducing a tertiary amine at the β-position of the exchanging alcohol creates a synergistic neighboring-group effect on the bond exchange reaction. As a result, dissociative exchange can be accelerated by several orders of magnitude, thereby enabling efficient processing within the range of 120–180 °C, which is usual for HMA applications. Consequently, designing an adhesive based on this dissociative mechanism is postulated to enable adhesive materials with enhanced cohesion, improved thermal stability, and reprocessability while retaining the processing advantages of a HMA.

To translate the internally catalyzed monoester transesterification into an applicable HMA, the choice of the polymer matrix is crucial. While neighboring-group-activated transesterification has already been implemented in alkyd resins and polyisoprene rubbers, its implementation in acrylic polymers, which are widespread in adhesive technologies, remains unexplored. , In other words, extending this dissociative chemistry to acrylic matrices represents a highly promising route toward thermally processable adhesives with potential for industrial adoption. −

In this work, we introduce internally catalyzed anhydride transesterification into an acrylic hot-melt platform by copolymerizing the bulk monomer n-butyl methacrylate (n BMA) with low fractions of itaconic anhydride (IA) (Figure a). The free radical copolymerization is described and their rheological properties for HMA-applications investigated. Furthermore, the influence of IA on the final material properties is studied by varying the ratio of n BMA and IA in the copolymer backbone. The resulting copolymers are cross-linked by reacting the anhydride functionalities with a series of bifunctional alcohols, thereby generating dynamic monoester cross-links capable of thermally activated bond exchange and thus enabling the required flow. The adhesive is designed to target an optimal balance between high-temperature wettability and strong performance at room temperature. Finally, the resulting HMAs are evaluated against a thermoplastic reference cross-linked with a monofunctional alcohol in terms of solvent resistance, thermal stability, and lap-shear strength across different substrates and temperatures.

1.

(a) Overview of the optimized synthesis protocol for copolymerization. (b) Comparison of complex viscosity of copolymers with and without CTA. (c) Effect of CTA on molar mass of the copolymers (see Table for M _n, M _w, and Đ values).

For a straightforward implementation of the monoester–anhydride chemistry in an acrylic polymer matrix, IA was identified as a suitable comonomer that allows for easy access to post-polymerization cross-linking into a dynamic covalent cross-linked polymer (Figure a). Thus, IA was copolymerized with n BMA, a monomer widely used in adhesive formulations, to provide a flexible acrylic backbone. , First, the copolymerization parameters of n BMA and IA were determined. For this, both monomers were dissolved in dioxane at 50 °C, AIBN (1 mol %, dissolved in dioxane) was added as a radical initiator, and the resulting mixture was polymerized at 70 °C for 24 h (Figure a).

Several monomer feed ratios of n BMA and IA were investigated, and the resulting copolymer compositions were determined by ¹H NMR spectroscopy (Figure S1, Supporting Information).

The fraction of IA in the monomer feed was plotted against the corresponding fraction incorporated in the copolymer, after which the Fineman–Ross method was applied to determine the monomer reactivity ratios in the copolymerization. The obtained monomer reactivity ratios (r _IA = 0.089 and r _nBMA = 1.26) indicated preferential incorporation of n BMA into the copolymer (Figures S2 and S3).

Based on those monomer reactivity results, copolymers with different monomer feed ratios were synthesized to obtain acrylic copolymers with varying content of pendant anhydride functionalities. Specifically, three copolymer compositions were targeted with n BMA/IA feed ratios of 0.25/0.75, 0.17/0.83 and 0.08/0.92.

All copolymers were obtained as white solids, and their anhydride content was determined by ¹H NMR spectroscopy (Figure S4), revealing molar fractions of 24, 15, and 8 mol %, respectively (Table ). The copolymers are denoted as IAX in which X represents the molar percentage of anhydride units in the copolymer. Thermogravimetric analysis (TGA) shows that the thermal stability of the copolymers is largely unaffected by the IA content and remains above 260 °C (Table , Figure S5). In contrast, although similar molecular weight values were obtained, the glass transition temperature (T _g) values increased between 45 and 70 °C with increasing IA content (Table , Figure S6).

1. Overview of Copolymer Properties with and without 0.5 wt % of CTA.

	mol % IA	T g (°C)	T d5% (°C)	M n (kg mol–1)	M w (kg mol–1)	Đ
IA24	24	70	263	65	93	1.4
IA15	15	57	269	43	80	1.7
IA8	8	45	269	51	80	1.6
IA24-CTA	24	67	289	25	38	1.5
IA15-CTA	15	53	290	28	43	1.5
IA8-CTA	8	44	293	32	53	1.7

^aDetermined using NMR spectroscopy.

^bDetermined from the second heating in DSC analysis (10 °C·min^–1).

^cDetermined from TGA ramp measurements under N₂ atmosphere.

^dDetermined from SEC measurements in DMAc with PMMA standards.

To further evaluate the rheological behavior of the copolymers, amplitude-sweeps (Figure S7), complex viscosity (Figure S8) and SEC measurements (Figure S9) were performed. As shown in Figure b, the copolymers prepared with 1 mol % AIBN exhibited still high complex viscosity above 150 °C (η* = 5 × 10⁶ – 10⁷ mPa.s), indicating that sufficient flow (<10⁶ mPa·s) would not be achieved within the temperature range typically used for HMAs (vide supra). , Moreover, since previous studies have shown that side reactions associated with the monoester–anhydride exchange may occur above 160 °C, the molecular weight of the copolymers was reduced to promote flow at lower temperatures. For this, 0.5 wt % of 1-dodecanethiol was introduced as a chain transfer agent (CTA). , In line with the previous nomenclature, the resulting copolymers are referred to as IA24-CTA, IA15-CTA, and IA8-CTA (Table ).

¹H NMR spectroscopy shows that the amount of itaconic anhydride incorporated in the polymer backbone remains unchanged after introduction of the CTA (Figures S10–S12), while the intrinsic polymer properties, such as T _g and thermal stability, remain comparable to those of the initial copolymers (Table , Figures S13 and S14). As intended, SEC analysis demonstrated a significant reduction in weight-average molecular weight (Figure c, Table ). Considering that the thermoplastics used in commercial HMAs span a broad range of weight-average molecular weights (M _w), with low M _w (<15 kg/mol) facilitating processing and higher M _w (>50 kg/mol) yielding stronger adhesives, the molecular weight values obtained using 0.5 wt % of CTA (38–53 kg/mol) fall within the latter category. , Correspondingly, rheological measurements revealed an earlier viscosity drop for all CTA-containing copolymers (Figure b, Figures S15 and S16).

Based on these results, the CTA-containing protocol using 1 mol % AIBN and 0.5 wt % of CTA was further selected as it enables the viscosity reduction to occur within the targeted HMA processing window. Having established a copolymer series with three varying ratios of IA, the next step was to investigate whether the characteristic viscoelastic flow required for hot-melt processing could be retained in a dynamically cross-linked architecture. Hence, the three copolymers were cross-linked with a bifunctional diol to generate dynamic covalent networks (Figure a). For this, in line with previous work, the copolymers were reacted with 1,6-hexanediol (HDO) in toluene at 80 °C using an alcohol-to-anhydride functionality ratio of 1:1, followed by drying to obtain the cross-linked materials (IAX-Y in which Y indicates the cross-linker employed). ,

2.

(a) Scheme of the dynamic cross-linking chemistry. (b) Temperature-dependent FTIR spectra demonstrating the presence of dynamic bonds in IA8-HDO. (c) Lap-shear strength values measured on an aluminum substrate for three different feed ratios.

The formation of a well-cross-linked network was first established by FTIR spectroscopy (Figure b), which showed near-complete conversion of the anhydride groups (1775 and 1860 cm^–1) into ester (1725 cm^–1) and acid functionalities (1750 cm^–1) (Figure S17). Additionally, soluble fraction values below 5 wt % were obtained in acetone, isopropanol (rt and 80 °C), and water (Figures S18 and S19), confirming the targeted solvent resistance (vide supra). Cross-linking had a negligible effect on the T _g of the materials, which remained comparable to that of the precursor copolymers (Table , Table , Figure S20). Likewise, TGA measurements showed that the overall thermal stability of the networks remained similar to that of the copolymers (Table , Table , Figure S21). However, isothermal measurements at 150 °C - the targeted application temperature - revealed that the thermal stability decreases with increasing anhydride content in the polymer backbone (Table , Figure S22).

2. Overview of the Thermal and Mechanical Properties of the Materials.

	T g (°C)	T d5% (°C)	% weight loss	lap-shear strength values (MPa)
IA24-HDO	61	240	2.9	<1
IA15-HDO	59	263	1.1	1.9 ± 0.3
IA8-HDO	43	291	0.6	4.0 ± 0.2
IA8-CHDM	41	253	1.1	2.7 ± 0.3
IA8-MDEA	41	229	3.5	4.0 ± 0.3
IA8-REF	40	284	0.7	0.4 ± 0.2

^aDetermined from the second DSC heating run (10 °C/min).

^bDetermined from TGA ramp measurements under N₂ atmosphere.

^cDetermined from TGA isothermal measurements at 150 °C for 2 h.

^dDetermined from lap-shear strength measurements based on ASTM D1002 conditions.

Subsequently, the rheological behavior of the networks was evaluated using temperature-sweep experiments, from which the complex viscosity was extracted (Figures S23–S25). Such measurements are particularly relevant for HMAs, which are expected to exhibit a viscosity decrease at elevated temperatures to ensure proper substrate wetting. The lowest cross-linked material (IA8-HDO) exhibits a pronounced viscosity drop (<10⁶ mPa·s) within the desired processing window (vide supra). In contrast, IA15-HDO and IA24-HDO show only minor changes in viscosity, with the complex viscosity remaining largely constant up to 170 °C, indicating that the network structure remains largely intact within the measured temperature range.

To have a better chemical understanding of the observed decrease in viscosity for IA8-HDO, FTIR spectroscopy was performed at elevated temperature (Figure b). The ester peak at 1725 cm^–1, originating from the polymer backbone, was used to normalize the anhydride bands at 1775 and 1860 cm^–1. At room temperature, nearly complete ring-opening of the anhydrides is observed, indicating efficient network formation. Upon heating above 100 °C, the increasing intensity of the anhydride bands relative to the normalized ester signal confirms dynamic activation of the monoester–anhydride exchange at elevated temperatures.

Lap-shear strength measurements were carried out at room temperature for the additive-free materials. IA8-HDO exhibits a lap-shear strength of 4.0 ± 0.2 MPa, whereas IA15-HDO shows a lower strength of 1.9 ± 0.2 MPa. Both display cohesive failure (Figure S26), while IA24-HDO fails adhesively at values below 1 MPa (Table ). The reduced adhesion observed for the higher cross-linked networks is attributed to their limited ability to flow during application, which results in poor substrate wetting. In addition, higher IA content increases the rigidity and brittleness of the network, further contributing to the lower lap-shear strength. Nevertheless, these first lap-shear strength results highlight the potential of the dynamically cross-linked monoester–anhydride platform for HMA applications, particularly when compared to EVA, the most widely used thermoplastic matrix for HMAs, for which lap-shear strength values below 1 MPa are typically reported in the absence of additives under comparable testing conditions. ,

To further investigate the applicability of the IA-containing copolymers for HMA applications, the IA8 copolymer was cross-linked using alternative bifunctional alcohols (Figure a). To this end, N-methyldiethanolamine (MDEA) was selected because its tertiary amine functionality can catalyze bond exchange reactions, while 1,4-cyclohexanedimethanol (CHDM) was investigated due to its cyclic structure, which is expected to enhance the rigidity and strength of the resulting material. To provide a reference system and assess the effect of cross-linking on the material properties, also a non-cross-linked material (REF) was prepared by ring-opening the anhydride groups of IA8 with 1-hexanol with an alcohol-to-anhydride functionality ratio of 1:1.

3.

(a) Overview of the different cross-linkers, as well as the reference. (b) Picture of IA8-HDO showing transparency. (c) Swelling ratio and soluble fraction of each material measured in acetone and isopropanol at rt for 24 h. (d) Complex-viscosity experiments.

IA8-HDO and IA8-CHDM were obtained as transparent materials, whereas the material prepared with MDEA appeared faint yellow (Figure b, Figure S27). FTIR spectroscopy confirmed near-complete conversion of the anhydride groups, indicating successful cross-linking for all HMAs (Figure S28). In terms of solvent resistance, IA8-HDO and IA8-CHDM exhibit significantly greater stability in acetone and isopropanol than IA8-MDEA. Nevertheless, all cross-linked materials display substantially improved solvent resistance relative to the reference material, which is fully soluble in both acetone and isopropanol at 80 °C (Figure c, Figure S29).

Table shows that the T _g is not significantly affected by the nature of the cross-linker, remaining around 40 °C and comparable to the corresponding copolymer. Complementary T _g values derived from rheological temperature-sweep measurements are provided in the Supporting Information (T _g values between 55 and 70 °C; Figure S30). On the other hand, IA8-HDO and IA8-CHDM exhibit a thermal stability similar to IA8-REF, whereas IA8-MDEA shows significantly reduced thermal stability (Table , Figures S32 and S33). This behavior is consistent with previous observations reported by Delahaye et al., where the use of MDEA as β-aminoalcohol cross-linker accelerates dynamic exchange processes, which in turn promotes faster thermal degradation.

Rheological measurements further highlight the significant influence of the cross-linker on the dynamic response of the network. In line with the results observed for IA8-HDO (vide supra), IA8-MDEA exhibits a pronounced viscosity drop upon heating (Figure d), which can be attributed to the catalytic activity of the tertiary amine that accelerates the monoester–anhydride exchange. In contrast, the material cross-linked with CHDM does not display such a viscosity transition, likely due to the increased rigidity introduced by the cyclic cross-linker. In general, the dynamically cross-linked hot-melts exhibit a G′-dominated response above T _g, accompanied by a sharper rheological transition from solid-like to liquid-like behavior compared to the thermoplastic reference IA8-REF, for which the complex viscosity decreases more gradually with increasing temperature (Figure d).

Lap-shear testing was selected to probe the cohesive strength of the adhesive layer and thereby isolate the effect of dynamic cross-linking for evaluation. As expected from the rheological observations, IA8-HDO and IA8-MDEA exhibit similar lap-shear strength results, which are higher than those of IA8-CHDM (Table , Figure S35). This behavior can be attributed to the improved wetting enabled by activation of the dynamic chemistry during heating, illustrated by a cohesive failure. All cross-linked materials outperform the thermoplastic reference, which shows a lap-shear strength of only 0.4 ± 0.2 MPa under the same conditions. These results demonstrate that the dynamic cross-linking enhances the strength of the resulting HMA up to 10 times compared to the reference. This combination of mechanical robustness and flowability, while being an additive-free adhesive, clearly outperforms conventional thermoplastic alternatives, consistent with the retained elastic response above T _g and the sharper transition to the liquid state (Figures S30 and S34). Notably, the retained G′-dominated rheological behavior also contributes to the lap-shear performance at elevated temperatures. To demonstrate this, lap-shear strength measurements were conducted at room temperature, near the glass transition temperature (T _g ≈ 40 °C), and above T _g (60 °C), thereby capturing the relevant temperature range for HMAs. The adhesive maintains significant mechanical performance, with values exceeding 2.25 MPa at temperatures above room temperature (Figure S36).

Finally, adhesion was evaluated on different substrates, including polycarbonate (PC), wood, and aluminum (Figure ). The adhesive performance was retained on wood, whereas barely no adhesion was observed on PC, a substrate known for its inherently challenging adhesion behavior. Nevertheless, the additive-free formulation still demonstrates promising performance.

4.

Lap-shear strength measurements of IA8-HDO on different substrates.

In summary, this work demonstrates that the inherent limitations of thermoplastic HMAs, namely limited solvent resistance and mechanical strength, can be overcome through dynamic cross-linking. Copolymers based on n BMA and IA were successfully synthesized, and their copolymerization parameters were first determined using the Fineman–Ross method. Based on these results, three copolymers with varying anhydride content (IA24, IA15, and IA8) were prepared. To ensure sufficient melt flow within the typical HMA processing window (120–180 °C), 0.5 wt % of dodecanethiol was introduced to lower the average molecular weight. Subsequently, the copolymers were cross-linked using different bifunctional alcohols, yielding networks with excellent solvent resistance and good thermal properties. Rheological investigations revealed that the dynamic monoester–anhydride exchange enables a sharp viscosity transition upon heating, allowing efficient substrate wetting during application while maintaining mechanical integrity at lower temperatures.

Among the investigated hot-melts, IA8-HDO emerged as the most suitable material. Its performance was benchmarked against a thermoplastic reference and further evaluated through lap-shear strength testing at elevated temperatures and on different substrates. The dynamically cross-linked adhesive exhibits significantly improved solvent resistance and a 10-fold increase in mechanical strength compared to the thermoplastic reference, addressing key limitations of conventional HMAs. Overall, introducing the monoester–anhydride transesterification chemistry into dynamically cross-linked poly­(n-butyl methacrylate-co-itaconic anhydride) copolymers represents a promising alternative to traditional thermoplastic HMAs, offering much improved durability while retaining the desired processing advantages of HMAs.

Glossary

Abbreviations

nBMA

n-butyl methacrylate

CHDM

1,4-cyclohexanedimethanol

CTA

chain transfer agent

DSC

differential scanning calorimetry

EVA

ethylene vinyl acetate

FTIR

Fourier-transform infrared spectroscopy

HDO

1,6-hexanediol

HMA

hot-melt adhesive

IA

itaconic anhydride

MDEA

N-methyldiethanolamine NMR, nuclear magnetic resonance

SEC

size-exclusion chromatography

TGA

thermogravimetric analyses

T _g

glass transition temperature

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmacrolett.6c00189.

• Details about instrumentation, synthesis and experimental procedures, NMR, SEC, FTIR, Rheology, as well as DSC and TGA thermograms (PDF)

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These authors contributed equally (J.D. and J.B.). J.D.: Conceptualization, Data curation, Formal analysis, Investigation, Validation, Writing – original draft. J.B.: Data curation, Formal analysis, Investigation, Validation, Writing – original draft. T.M.: Conceptualization, Writing – review and editing. J.Beniers: Investigation, Writing – review and editing. H.H.: Writing – review and editing, Supervision, Resources, Funding acquisition. F.D.P.: Writing – review and editing, Supervision, Resources, Funding acquisition.

The authors declare no competing financial interest.