The epoxy ring in the molecular structure of GMA (with a strain energy of approximately 114 kJ/mol) constitutes the core source of its reactivity. The high electronegativity of the oxygen atom on the ring endows the ring carbon atoms with an electron-deficient nature, which thus requires nucleophilic attack or protonation assistance to achieve ring-opening [8][9]; The conjugated electron-withdrawing effect of the carbonyl groups in the five-membered cyclic anhydride structure of MAH induces a significant decrease in the electron cloud density of the ring carbon atoms, forming strong electrophilic centers that readily undergo nucleophilic addition-elimination reactions with strong nucleophiles [10][11][13].

The nucleophilic ring-opening reaction of GMA requires a relatively high activation energy (60 - 70 kJ/mol), and thus necessitates protonation (e.g., by acidic end groups) or catalysts (e.g., quaternary ammonium salts) to lower the energy barrier [8][9][14][15]; The nucleophilic addition-elimination reaction occurring at MAH has an activation energy of only 30 - 40 kJ/mol and can proceed rapidly in the presence of strong nucleophilic end groups (e.g., -NH₂) [10][14]. From a thermodynamic perspective, the enthalpy change of GMA’s ring-opening reaction stems primarily from ring strain relief, whereas the reaction between MAH and strong nucleophiles exhibits distinct thermodynamic advantages owing to the formation of imide structures with high bond energy (≈350 kJ/mol) [8][10][16].

From a microscopic standpoint, molecular dynamics (MD) simulations provide deep insights into the spatial configuration and kinetic behavior differences between GMA and MAH at the polymer-polymer interface. Experimental evidence suggests that MAH-grafted chains tend to self-assemble into a tightly packed layered arrangement at the interface of polar matrices such as PA6, where the anhydride moieties exhibit a pronounced interfacial enrichment effect. MD simulation results further quantify that the interfacial work of adhesion between MAH and the terminal amino groups of PA is substantially higher than that of GMA, a phenomenon ascribed to the compact molecular architecture of MAH that enables facile penetration into the interchain spaces of PA and subsequent formation of a hydrogen bond-mediated pre-complexation state [10][17]-[21].

In contrast, owing to its longer side chain structure, GMA exhibits a larger free volume at the interface and thus adopts a random coil conformation. MD simulations reveal that, in polyester (PLA/PBAT) systems, GMA can significantly reduce interfacial tension and increase the interfacial width between the two phases via the nucleophilic ring-opening of its epoxy groups [19]. Recent studies have revealed via coarse-grained molecular dynamics (CGMD) simulations that, in PC/ABS blends, the rotational freedom of GMA’s side chains endows it with stronger interfacial healing capacity than MAH under high-temperature shearing, thereby more effectively inhibiting the agglomeration of the rubber phase [22][23].

When GMA is in excess, homopolymerization tends to occur alongside hydroxy-mediated epoxy ring-opening reactions—where hydroxyl groups generated from the initial epoxy ring-opening further induce the ring-opening of residual epoxy groups and trigger cross-linking—thus resulting in a marked decrease in the melt fluidity of the system [8][9]; MAH tends to hydrolyze to form maleic acid under processing humidity, which not only consumes its own reactivity but also may catalyze polymer degradation; additionally, residual monomers can induce material yellowing [10][24]. Factors such as processing temperature, shear rate, system water content, and catalyst type all exert an influence on the reaction selectivity and conversion rate of both substances [15][25]-[27].

For PA series materials (e.g., PA6, PA66, PA6/ABS blends), the reactive end groups are dominated by strong nucleophilic amino groups (-NH₂) and supplemented by carboxyl groups (-COOH), enabling an optimal strong electrophile-strong nucleophile pairing with MAH’s anhydride moieties. This reaction proceeds via two sequential steps: instantaneous nucleophilic addition of -NH₂ to the carbonyl carbon of MAH’s anhydride ring at extrusion temperatures (180˚C - 240˚C, activation energy: 30 - 40 kJ/mol) to yield unstable amic acid, and subsequent dehydration at elevated temperatures to form highly stable imide structures that impart robust interfacial bonding with PA main chains without detectable reverse reactions, as illustrated in Figure 1 [10][11][16]. As documented in relevant literature, the incorporation of MAH-based compatibilizers into PA6/ABS blends not only realizes super-tough room-temperature impact strength but also preserves excellent toughness at a low temperature of −10˚C [27].

Figure 1. Schematic illustration of the reaction between MAH and amide.

The reaction of GMA with PA suffers from inherent drawbacks: the activation energy required for epoxy ring-opening (60 - 70 kJ/mol) is substantially higher than that of the MAH-amine reaction, while the hydroxyl groups produced can cause local cross-linking, damage melt processability, and prevent the full completion of interfacial reactions under typical short extrusion residence times (<3 min). In addition, investigations into broader polyamide systems including PA12 and PA610 have further validated that MAH outperforms GMA by a significant margin in both grafting efficiency and compatibilization efficacy [8]-[10][14].

The reactive end groups of the polyester series (PBT, PLA, R-PET, PETG, PBS, etc.) are dominated by carboxyl groups (-COOH, acidic) and supplemented by hydroxyl groups (-OH, weak nucleophiles), thus forming a highly efficient reaction system with the epoxy ring of GMA [9][14][28]. This highly efficient reaction is governed by a protonation-assisted ring-opening mechanism: H⁺ donated by carboxyl groups protonates the epoxy ring oxygen atom, which not only reduces the ring strain to below 60 kJ/mol but also amplifies the electrophilicity of the ring carbon atoms, thus facilitating a nucleophilic attack by the resultant -COO⁻. The reaction produces high bond energy ester bonds (≈330 kJ/mol) and new hydroxyl groups, where the latter fails to induce secondary cross-linking as a result of diminished electronic effects [8][13][29].

For biodegradable polyesters such as PLA, GMA can also scavenge the carboxyl groups (-COOH) generated by processing-induced degradation—with the post-processing carboxyl group concentration increasing by 3-5 times—and, as illustrated in Figure 2, achieve chain segment reconnection and chain extension, thereby enhancing the melt strength significantly [9][28][29]. Experimental data demonstrate that when PC/R-PET blends are modified with GMA-based compatibilizers (e.g., E-MA-GMA), both the tensile strength and heat deflection temperature (HDT) exhibit significant enhancements [30]. Since the anhydride group of MAH and carboxyl groups (-COOH) are both acidic (anhydride pKa ≈ 1.5; -COOH pKa ≈ 4 - 5), no effective reaction can occur between them. Furthermore, the conversion rate of its reaction with trace hydroxyl groups (-OH) is less than 10%, representing a reaction efficiency far lower than that of GMA [31]-[33].

Figure 2. Schematic illustration of the reaction between GMA and polyester.

The reactive end groups of PC are phenolic hydroxyl groups (-ArOH, pKa≈10, weakly nucleophilic), and the side reactions of MAH are the key to its inactivation: maleic acid generated from MAH hydrolysis can catalyze the cleavage of PC ester bonds, resulting in a reduction in molecular weight, while its reaction rate constant with phenolic hydroxyl groups (k ≈ 8 × 10⁻²∙s⁻¹) is far lower than that of the hydrolysis reaction (k ≈ 1.2 × 10⁻³∙s⁻¹) [10][16][30].

The reaction between GMA and PC can proceed efficiently under high-temperature shearing conditions. The introduction of quaternary ammonium salt catalysts (e.g., tetrabutylammonium bromide) enables the ionization of phenolic hydroxyl groups into -ArO⁻, enhancing the nucleophilicity by several orders of magnitude; the resultant -ArO⁻ then attacks the substituted carbon of the epoxy ring to form stable ether bonds, without any side reactions occurring. Studies have demonstrated that the addition of trace amounts of quaternary ammonium salt catalysts can increase the molecular weight retention rate of PC in PC/R-PET blends from 70% to 90% [15][30][34].

The differences in reaction efficiency between MAH and GMA across various polymers are summarized in Table 1.

Table 1.Underlying common mechanisms for differences in reaction efficiency between MAH and GMA in polymers.

General-purpose polyolefins (PP/PE) feature low cost and excellent processability yet suffer from insufficient rigidity and poor heat resistance. This drawback can be addressed via melt blending with polar engineering plastics (PA, PET and PBT) combined with modification by MAH/GMA-grafted polyolefin compatibilizers, thus achieving performance complementarity between the two types of polymers. This approach serves as a core strategy for the engineering upgrading of general-purpose plastics and the cost reduction of engineering plastics, and the resulting polyolefin-based alloys have been widely applied in the automotive, home appliance, packaging, and electronic and electrical industries.

For the preparation of polyolefin-based compatibilizers, GMA and MAH are grafted onto the molecular backbone of non-polar polyolefins (PP/PE) with an inert backbone structure via the free-radical initiated melt grafting method, thereby constructing reactive sites for subsequent compatibilization. Among these two monomers, MAH features a compact molecular structure and high electrophilicity, which endow it with high grafting efficiency onto PP/PE backbones—achieving a typical grafting ratio of 1% - 2%—when initiated by peroxides (e.g., dicumyl peroxide). This grafting process follows a “hydrogen abstraction-grafting” mechanism: peroxides decompose at processing temperatures (160˚C - 190˚C) to generate free radicals, which abstract hydrogen atoms from the polyolefin backbone to form macroradicals, and MAH then undergoes addition polymerization with these macroradicals to form grafted chains. MAH-grafted polyolefins (e.g., PP-g-MAH) are widely used in PP/PA blends due to their high grafting efficiency and low cost, as the anhydride groups can react with the -NH₂ end groups of PA to form imide linkages, significantly improving interfacial adhesion.

In contrast, GMA grafting onto polyolefins requires stricter control of processing conditions. The epoxy group of GMA is less reactive toward polyolefin macroradicals compared to MAH, and excessive peroxides may induce epoxy ring-opening side reactions. GMA-grafted polyolefins (e.g., PP-g-GMA) typically have a lower grafting ratio (0.5% - 1.2%) but exhibit higher reactivity toward polyester-based polymers. Their epoxy groups can undergo protonation-assisted ring-opening reactions with the -COOH end groups of PBT/PET, making them more suitable for PP/PBT blend systems where they enhance both interfacial compatibility and thermal stability [11][14][35].

ABS is a ternary copolymer composed of acrylonitrile (AN), butadiene (BD), and styrene (St) segments, with reactive sites including nitrile groups (-CN) in the AN segment and residual hydroxyl groups (-OH) from polymerization initiators. MAH exhibits obvious limitations in ABS-based blends: the anhydride group of MAH can accelerate the thermal oxidative degradation of the BD segment under processing temperatures (220˚C - 260˚C), leading to a decrease in impact strength and yellowing of the material. Additionally, the reaction between MAH and the weak nucleophilic -CN/-OH groups of ABS is inefficient.

GMA, as a functional group, is more suitable in the compatibilizer for ABS/PC blends due to its unique reaction mechanism with ABS. The epoxy group of GMA can react with the terminal groups of PC, GMA can also undergo nucleophilic ring-opening reactions with the -OH groups of ABS (catalyzed by trace acidic impurities) and form dipole-dipole interactions with the -CN groups, thereby improving the interfacial adhesion between ABS and PC, and help for the thermal stability. Moreover, GMA’s longer side chain structure enhances the interfacial toughness and inhibits the agglomeration of the BD rubber phase in ABS under high-temperature shearing, ensuring the retention of impact strength in ABS/PC blends [10][30].

The epoxy groups of GMA undergo ring-opening reactions with the phenolic hydroxyl groups of hindered phenolic antioxidants and the amino groups of amine antioxidants, which consumes the active components of both parties and results in a reduction in compatibilization efficiency; yet the formed grafted antioxidants can enhance the antioxidant durability. In contrast, MAH exhibits a higher reaction priority with amine antioxidants than with polymer end groups: the anhydride group of MAH undergoes nucleophilic addition–elimination reactions with primary amino groups of amine antioxidants to form stable succinimide derivatives. This specific structural formation completely deactivates the antioxidant (as the amino groups responsible for free radical scavenging are covalently bonded), leading to a significant decrease in compatibilization efficiency. In contrast, MAH only forms weak hydrogen bonding interactions with hindered phenols (e.g., 1010), exerting a negligible impact on their antioxidant activity, as illustrated in Figure 3 [9][14]. Thus, in GMA systems, it is advisable to reduce the dosage of hindered phenolic antioxidants (e.g., 1010), increase the proportion of phosphite antioxidants (e.g., 168) or adopt semi-hindered phenols; whereas in MAH systems, phenolic antioxidants should be prioritized and the use of amine antioxidants avoided [10][11].

Figure 3. Reaction of GMA and MAH with antioxidants.

GMA can undergo ring-opening reactions with the amino groups of hindered amine light stabilizers (HALS) and the hydroxyl groups of benzotriazole-based UV stabilizers, anchoring the UV stabilizers onto the polymer chains, reducing their mobility, and thus leading to an increasing trend in the elongation at break retention rate after UV aging. However, excessive usage will competitively consume the active sites of GMA, and some reaction products tend to form quinoid structures, which induce yellowing. On the other hand, the acidity of MAH can accelerate the thermal oxidative degradation of the butadiene phase in ABS; moreover, the acidic environment generated by MAH hydrolysis will deactivate HALS and form salt precipitates, leading to a decrease in material transparency [36]-[39]. Thus, for GMA systems, triazine-based UV stabilizers free of phenolic hydroxyl groups or with high steric hindrance (e.g., UV-1577) are recommended; whereas for MAH systems, the use of HALS should be avoided, and UV absorbers (e.g., UV-327) should be prioritized.

GMA can undergo reactions with the P-H bonds of phosphorus-based flame retardants (e.g., DOPO), the hydroxyl groups of hydroxyl-containing flame retardants (e.g., APP), and the amino groups of nitrogen-based flame retardants (e.g., melamine), thereby enhancing the flame-retardant synergistic effect. This elevates the Limiting Oxygen Index (LOI) of PC/ABS blends and enables them to achieve the UL-94 V-0 rating. However, excessive amounts of flame retardants will impair the compatibilization efficiency and even catalyze the cross-linking of GMA. As for MAH, its esterification reaction with the hydroxyl groups of phosphorus-based flame retardants facilitates the dispersion of the latter; yet its inherent acidity will catalyze the hydrolysis of organophosphorus flame retardants, leading to a reduction in flame-retardant performance [10][40]-[45]. Reactive phosphorus-based flame retardants are recommended for GMA systems; whereas acid scavengers (e.g., hydrotalcite) can be incorporated into MAH systems to inhibit hydrolysis.

GMA undergoes ring-opening reactions with hydroxyl-containing lubricants (e.g., stearyl alcohol), leading to diminished lubricating efficacy; thus, fatty acid ester-based lubricants (e.g., butyl stearate) are recommended as alternatives. Whereas MAH exhibits no significant reactions with the ester groups in plasticizers, its inherent acidity may accelerate plasticizer migration, and it is therefore necessary to select acid-resistant plasticizers (e.g., epoxidized soybean oil) [10][14].

In conventional blending systems such as PC/ABS alloys, the reaction priority sequence follows the order below under high-temperature (240˚C - 260˚C) and high-shear conditions: the reaction of MAH with trace moisture/polar additives (the highest priority) > the ring-opening addition of GMA to the terminal carboxyl groups of PC (the second priority) > the in-situ crosslinking between GMA and MAH (the third priority) > the direct reaction between MAH and PC (the lowest priority), as illustrated in Figure 4 [14][30]. This regularity is jointly governed by activation energy and functional group concentration: the high sensitivity of MAH toward polar substances leads to its preferential consumption; the protonation-catalyzed effect of the terminal carboxyl groups of PC on GMA expedites the reaction; and the electronegativity mismatch between MAH and PC renders their direct reaction difficult to proceed [9][10].

Figure 4. Reaction sequence and mechanism of anhydride and GMA in the system.

Notably, variations in shear rate can significantly alter this reaction priority sequence by regulating mass transfer and diffusion kinetics. At low shear rates (<200 s^-1), diffusion limitations become dominant: trace moisture in the system tends to aggregate at the interface rather than uniformly disperse, reducing the collision frequency between MAH and moisture molecules. Meanwhile, the prolonged diffusion time allows MAH to migrate toward the polymer end groups (e.g., -COOH of PC), increasing the probability of MAH-polymer reactions. Under such conditions, the reaction priority may shift to: ring-opening addition of GMA to PC terminal carboxyl groups > reaction of MAH with PC terminal carboxyl groups > reaction of MAH with moisture/polar additives > in-situ crosslinking between GMA and MAH. This shift is particularly pronounced in systems with low moisture content (<0.1 wt%), where diffusion limitations override the thermodynamic advantage of MAH-moisture reactions. Conversely, at moderate shear rates (300-500 s^-1), the balance between diffusion and reaction kinetics maintains the original priority sequence, while at ultra-high shear rates (>1000 s⁻¹), excessive shear-induced chain scission may generate additional polar groups, further influence the priority of these reactions [10][46].

Appropriate blending of GMA and MAH (typically at a mass ratio of 3:1 to 5:1) enables the formation of a reactive interfacial layer, which enhances impact strength while significantly improving the heat distortion temperature (HDT). However, when their mass ratio approaches 1:1, severe gelation tends to occur, resulting in the loss of melt processability and a drastic reduction in elongation at break. In addition, GMA can scavenge the acidic species generated by MAH hydrolysis to inhibit polymer degradation, whereas MAH is capable of consuming the hydroxyl groups formed by excessive GMA to mitigate the risk of over-crosslinking, thus exhibiting a complementary synergistic effect [47][48].

In polymer modification, the selection of primary compatibilizers is generally determined by the type of polymer terminal groups: MAH is preferred for PA systems, while GMA is the optimal choice for polyester/PC systems. A small amount of the other type of compatibilizer is added as a supplement (accounting for 20% - 30% of the primary compatibilizer dosage). In terms of processing conditions, the processing temperature should be controlled (230˚C - 250˚C for GMA systems and 220˚C - 240˚C for MAH systems), along with a dwell time of 2 - 3 min. For MAH systems, the relative humidity of the processing environment must be strictly controlled below 5%. A small amount of catalyst (e.g., 0.1% - 0.3% quaternary ammonium salt catalyst) can be incorporated into PC systems, whereas 0.5% - 1% acid scavenger is recommended for MAH systems. Furthermore, the selection of additives such as antioxidants and UV absorbers is also a key factor for high-performance modification (as shown in Table 2) [10][11][14][15][30].

Table 2.Optimization strategies of compatibilizers and additives for polymer modification systems.

When PA/ABS blends are applied to automotive interior components, the incorporation of MAH-based compatibilizers can increase the impact strength by more than 40%, thereby satisfying the requirements for low-temperature toughness [27][49]; When PC/PBT (PET) alloys are applied to automotive exterior components, GMA-based compatibilizers can suppress the transesterification reaction, increase the heat distortion temperature (HDT) to above 180˚C, and significantly improve the weather resistance [2][50]-[52].

When PC/ABS alloys are used for electronic device housings (e.g., laptop casings, router shells), the synergistic application of GMA and reactive phosphorus-based flame retardants (e.g., DOPO) can achieve a flame retardant rating of UL-94 V-0 while maintaining favorable impact strength and tensile strength. The addition of GMA also improves the dielectric strength, suitable for the insulation requirements for electronic enclosures. When PLA/PBS biodegradable materials are applied to packaging (e.g., chip carriers), GMA-based compatibilizers can enhance the melt strength and increase the tensile modulus, enabling the materials to meet the dimensional stability requirements of blow molding and injection molding processes [4][9][30].

When PA/EVOH blend barrier materials are applied in food packaging, MAH-based compatibilizers can improve interfacial adhesion, resulting in a reduction of the oxygen transmission rate (OTR) by more than 30%. When PLA blend materials are used in food-contact packaging, GMA-based compatibilizers can enhance the stability of mechanical properties without the release of hazardous substances [11][34][48].

In polymer systems, real-time monitoring methods for the microscopic reaction mechanisms of GMA/MAH are inadequate, and in-situ characterization techniques to clarify dynamic reaction processes are lacking. Moreover, methodologies for evaluating reaction selectivity and conducting quantitative analysis of GMA/MAH in complex additive systems are scarce. Specific examples of such complex additive systems include PC/ABS alloy systems containing a combination of hindered phenolic antioxidants (1010), triazine-based UV stabilizers (UV-1577), reactive phosphorus flame retardants (DOPO), and fatty acid ester lubricants (butyl stearate); PLA/PBS biodegradable blend systems with semi-hindered phenol antioxidants, benzotriazole UV absorbers (UV-327), and hydrotalcite acid scavengers; and others. The effects of these complex systems are highly complex, and there is a lack of methodologies for qualitative analysis. Additionally, technologies for enhancing reaction efficiency under green processing conditions (i.e., low temperature, low shear rate, and solvent-free conditions) remain to be broken through [10][14][30][34].

Developing multifunctional compatibilizers suitable for polymer systems (e.g., GMA-MAH composite grafted compatibilizers) that integrate functions such as compatibilization, antioxidation, and flame retardancy; utilizing molecular simulation techniques (e.g., DFT calculations) to predict reaction efficiency and interfacial interactions—specifically, DFT can quantify the activation energy discrepancies between GMA (60 - 70 kJ/mol) and MAH (30 - 40 kJ/mol) in reactions with different polymer end groups, verify the effect of protonation/catalysts on reducing GMA’s energy barrier, and clarify the thermodynamic advantage of MAH-imide formation (≈350 kJ/mol) to resolve uncertainties in Section 2.2—for guiding the rapid optimization of formulations; developing in-situ characterization technologies to reveal microscopic reaction mechanisms and the evolution law of interfacial structures; and expanding application research in bio-based polymer systems to adapt to green development requirements will become the research hotspots.