A method for preparing a high maleic anhydride content SMA copolymer

Abstract

The application discloses a preparation method of SMA copolymer with high maleic anhydride content and belongs to the technical field of polymer synthesis. The preparation method comprises the following steps: taking 4,4'-azobis(4-cyanopentanoic acid) as raw material, and synthesizing a functional initiator through two-step reactions of acyl chlorination and amidation; and then copolymerizing styrene, maleic anhydride, the functional initiator and tri(pentafluorophenyl)borane to obtain a copolymer with a mass fraction of maleic anhydride of 35%-45%. The application aims at the technical limitation that the maleic anhydride content is generally difficult to exceed 30% due to the charge transfer complex mechanism in the prior art, and proposes that the functional initiator and tri(pentafluorophenyl)borane form a synergistic system through reversible N-B coordination, and the selective activation and local enrichment effect are utilized to realize efficient insertion of maleic anhydride, thereby providing a new path for preparing SMA copolymer with high maleic anhydride content.

Description

Technical Field

[0001] This invention relates to the field of polymer synthesis technology, and in particular to a method for preparing a SMA copolymer with high maleic anhydride content. Background Technology

[0002] Styrene-maleic anhydride copolymers (SMAs) are an important class of functional polymers. Due to their combination of the processability of styrene and the high reactivity of maleic anhydride, they are widely used as polymer compatibilizers, toughening agents, coating resins, and biomedical material matrices. The maleic anhydride (MAh) content in the copolymer directly determines its functionalization potential. SMA copolymers with high MAh content have more reactive sites, stronger hydrophilicity, and better interfacial compatibility, making them particularly valuable in the preparation of ion exchange resins, drug carriers, and high-performance composite materials. Therefore, developing methods for preparing SMA copolymers with high MAh content has always been an important research direction in the field of polymer synthesis.

[0003] Styrene (St) and maleic anhydride are typical electron-donating and electron-accepting monomer pairs. During free radical copolymerization, they readily undergo chain growth via a charge-transfer complex (CTC) mechanism, resulting in a strictly alternating structure in the copolymer composition. Regardless of the monomer feed ratio, the molar fraction of MAh in SMA copolymers obtained through conventional free radical copolymerization is difficult to exceed 50%, corresponding to a mass fraction of only about 20%. This thermodynamically controlled alternating tendency severely limits the preparation of SMA copolymers with high MAh content, becoming a key technical bottleneck restricting the functional applications of this type of material.

[0004] To overcome the limitations of alternating copolymerization, researchers have explored various strategies. Increasing the polymerization temperature to above 100°C and using polar solvents (such as methyl ethyl ketone and dioxane) can partially disrupt charge-transfer complexes, causing the copolymer to shift towards randomization. Semi-continuous feeding processes maintain the concentration of highly reactive MAh monomers throughout the reaction, avoiding low MAh content segments caused by "starvation polymerization" in the later stages. While these process optimizations can increase the MAh mass fraction to 25%–30%, they are approaching their technical limits. In recent years, some studies have attempted to introduce Lewis acids (such as AlCl3 and BF3) into the system to coordinate with MAh and enhance its electron-deficient nature. However, physically mixed Lewis acids are unevenly distributed in the system and struggle to exert a sustained synergistic effect during chain growth, resulting in limited improvement in MAh content (typically only 2–4 percentage points) and a high risk of side reactions. Therefore, there is an urgent need to develop a new method that can efficiently and controllably increase the MAh insertion rate, thereby obtaining SMA random copolymers with higher MAh content while breaking through the limitations of alternating copolymerization. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing SMA copolymers with high maleic anhydride content.

[0006] A method for preparing a high-maleic anhydride content SMA copolymer includes the following steps:

[0007] S1. Preparation of functional initiators:

[0008] S101. In a reaction vessel, add 4,4'-azobis(4-cyanopentanoic acid) and anhydrous dichloromethane at a solid-liquid ratio of 1:8-15 g / mL. Add oxaloyl chloride and anhydrous N,N-dimethylformamide while stirring. The molar ratio of 4,4'-azobis(4-cyanopentanoic acid), oxaloyl chloride, and anhydrous N,N-dimethylformamide is 1:2.5-3.0:0.05-0.1. Stir the reaction at room temperature for 2-3 hours. After the reaction is complete, remove the solvent and excess oxaloyl chloride by vacuum distillation to obtain 4,4'-azobis(4-cyanopentanoic acid).

[0009] First, N,N-dimethylformamide (DMF) acts as a nucleophilic catalyst in an addition reaction with oxalyl chloride: the lone pair electrons on the nitrogen atom of the amide in the DMF molecule attack a carbonyl carbon of oxalyl chloride, forming a tetrahedral intermediate. Subsequently, the chloride ion departs, generating Vilsmeier salt, chloromethylenedimethylamine chloride ([ClCH=N)). + (CH3)2]Cl - The Vilsmeier salt is a strong electrophile, with its methylene carbon carrying a positive charge, and is a key active intermediate in subsequent reactions;

[0010] Next, the carboxyl oxygen in the 4,4'-azobis(4-cyanovaleric acid) (ACVA) molecule acts as a nucleophile, attacking the methylene carbocation of the Vilsmeier salt, resulting in nucleophilic addition and forming an O-acyl Vilsmeier intermediate. At this point, the hydroxyl group in the original carboxylic acid has been replaced by the methylene dimethylamine group of the Vilsmeier salt, forming an unstable ester adduct.

[0011] In the third step, the O-acyl Vilsmeier intermediate undergoes a concerted elimination reaction: the DMF molecule leaves, and at the same time, the chloride ions present in the system (from the dissociation of the Vilsmeier salt or generated by the reaction) attack the carbonyl carbon. After passing through the tetrahedral transition state, the carbon-oxygen double bond is restored, generating the target product 4,4'-azobis(4-cyanopentyl chloride). During this process, the other carbonyl molecule of oxalyl chloride undergoes a decarbonylation reaction, releasing carbon monoxide and carbon dioxide. The DMF is regenerated in the catalytic cycle and can continue to participate in the next round of reaction.

[0012]

[0013] The entire reaction can proceed smoothly at room temperature, with a reaction time of 2-3 hours. After the reaction is completed, the solvent and excess oxaloyl chloride are removed by vacuum distillation to obtain 4,4'-azobis(4-cyanovaleryl chloride). The physical state of this product is a pale yellow oily liquid or waxy solid. It is sensitive to moisture and should be used directly in the next step of the reaction.

[0014] S102. Dissolve the obtained 4,4'-azobis(4-cyanovaleryl chloride) in anhydrous tetrahydrofuran at a solid-liquid ratio of 1:5-10 g / mL. Under ice-water bath cooling, slowly add the solution dropwise to an anhydrous tetrahydrofuran solution containing 4-aminopyridine and triethylamine. The molar ratio of 4,4'-azobis(4-cyanovaleryl chloride), 4-aminopyridine and triethylamine is 1:2.5-3.0:2.5-3.0. After the addition is complete, remove the ice bath and stir overnight at room temperature. After the reaction is complete, filter to remove the triethylamine hydrochloride precipitate. Concentrate the filtrate under reduced pressure, add ice water to precipitate the solid, filter, wash, recrystallize to obtain the functional initiator.

[0015] This reaction is a classic N-acylation reaction of acyl chlorides with amines, following a nucleophilic addition-elimination mechanism. The reaction is carried out under ice-water bath cooling to control the exothermic reaction and prevent the azo bond from decomposing due to local overheating. The specific mechanism is as follows:

[0016] First, the amino nitrogen atom in the 4-aminopyridine molecule contains a lone pair of electrons. This nitrogen atom does not participate in the aromatic conjugation system of the pyridine ring and has strong nucleophilicity. In the reaction system, the amino nitrogen acts as a nucleophile, directly attacking the positive charge center of the carbonyl carbon in the 4,4'-azobis(4-cyanovaleryl chloride) molecule, forming a tetrahedral intermediate. At this point, the carbonyl carbon is transformed from sp... 2 Hybridization transforms into sp 3 Hybridization, where the oxygen atom carries a negative charge, while the chlorine atom remains bonded to the carbon atom;

[0017] Subsequently, the tetrahedral intermediate undergoes an electronic rearrangement: the lone pair electrons on the nitrogen atom migrate toward the carbon-oxygen bond, the carbon-oxygen double bond is restored, and at the same time, the chloride ion leaves. The chloride ion combines with triethylamine in the system to form triethylamine hydrochloride precipitate, which can be removed by filtration, and at the same time, it pushes the reaction equilibrium toward the product.

[0018]

[0019] Because the 4,4'-azobis(4-cyanopentyl chloride) molecule contains two acyl chloride groups, the above nucleophilic addition-elimination process occurs twice: after the first acyl chloride group reacts with one molecule of 4-aminopyridine to form an amide bond, the second acyl chloride group continues to react with another molecule of 4-aminopyridine, eventually generating the diamide target product. During the reaction, triethylamine acts as an acid-binding agent, and its amount is equimolar with that of 4-aminopyridine (both are 2.5-3.0 times the molar amount of acyl chloride) to ensure that the hydrogen chloride produced in the reaction is completely neutralized and to avoid the acidic environment from damaging the azo bond.

[0020] After the reaction was completed, the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated under reduced pressure and then ice water was added to precipitate the solid. The solid was washed and recrystallized to obtain a light yellow crystalline powder, which is the functional initiator - bis(4-pyridinamido)azo derivative. The product molecule contains a pyridine group at each end. The nitrogen atom on the pyridine ring did not participate in the reaction and retained its lone pair electrons, which can be used as Lewis base sites for coordination with tris(pentafluorophenyl)borane in subsequent polymerization reactions.

[0021] S2, Preparation of styrene-maleic anhydride copolymer:

[0022] Under free radical polymerization conditions, maleic anhydride, a functional initiator, and tris(pentafluorophenyl)borane are dissolved in a portion of an organic solvent. During the reaction, the mixture is added dropwise at a constant rate at 75-95°C to a reaction system containing styrene and the remaining solvent over a period of 3-6 hours. After the addition is complete, the reaction is continued at this temperature for another 2-6 hours. The molar ratio of styrene monomer, maleic anhydride monomer, functional initiator, and tris(pentafluorophenyl)borane is 1:(1.8-2.5):(0.005-0.02):(0.01-0.08).

[0023] This step involves free radical polymerization using styrene (St) and maleic anhydride (MAh) as comonomers, and a synergistic system of a functional initiator and tris(pentafluorophenyl)borane (B(C6F5)3) to initiate polymerization. A high-maleic anhydride-content styrene-maleic anhydride random copolymer is prepared via a semi-continuous feeding method. The overall reaction formula is:

[0024]

[0025] The above process can be divided into three stages: initiation, coordination activation, and chain growth.

[0026] Initiation stage: The functional initiator (bis(4-pyridinamido)azo derivative) undergoes thermal decomposition under heating conditions of 75-95℃. The azo bond (-N=N-) is homolytically cleaved to generate two carbon-centered free radicals, while releasing one molecule of nitrogen gas. The generated free radicals have appropriate electrophilicity, providing active centers for subsequent chain growth.

[0027]

[0028] Coordination activation stage: The pyridine groups at both ends of the functional initiator molecule act as Lewis base sites, forming a reversible N→B coordination bond with the boron atom of B(C6F5)3. This coordination bond is in dynamic equilibrium at the polymerization temperature: some B(C6F5)3 dissociates from the pyridine and becomes free in the solution; the dissociated B(C6F5)3, acting as a strong Lewis acid, coordinates with the carbonyl oxygen of maleic anhydride to form a B(C6F5)3·MAh adduct. In this adduct, the maleic anhydride double bond... The significantly enhanced electron deficiency greatly improves its free radical addition reaction activity. Because B(C6F5)3 is reversibly coordinated and anchored to the pyridine group of the initiator molecule, and the pyridine group is spatially close to the free radical generation site through a flexible linker arm, when the azo bond decomposes to generate free radicals, the B(C6F5)3 molecule is trapped around the free radical due to the "cage effect", forming a local high concentration region. This makes the density of activated maleic anhydride molecules in the microenvironment of the free radical much higher than the average value of the system.

[0029] Although tris(pentafluorophenyl)borane (B(C6F5)3) has significant steric hindrance, this steric structure is not a defect in the synergistic system of this invention; rather, it is a key structural feature for achieving highly selective activation. The three pentafluorophenyl groups in the B(C6F5)3 molecule form a three-dimensional protective umbrella around the boron atom, making it difficult for it to form stable complexes with the larger styrene monomer. Meanwhile, the smaller maleic anhydride can easily approach and coordinate with the boron atom. This steric hindrance selectivity allows B(C6F5)3 to preferentially activate maleic anhydride rather than styrene, precisely improving the reactivity of the target monomer. Simultaneously, the significant steric hindrance results in a moderate coordination bond strength between B(C6F5)3 and the pyridine group, which is suitable for polymerization at temperatures below 7°C. It is in dynamic equilibrium at 5-95℃: it can coordinate stably at room temperature for easy storage and handling, and can also dissociate and release active species during polymerization to form a "breathing" mechanism. In addition, the strong electron-withdrawing effect of the three pentafluorophenyl groups endows the boron atoms with extremely strong Lewis acidity, while the large steric hindrance prevents it from forming an overly stable adduct with maleic anhydride, ensuring that the activated maleic anhydride can participate in chain growth in a timely manner. Therefore, the "large steric hindrance" of B(C6F5)3 is a carefully selected molecular design advantage. It forms reversible coordination with the pyridine group to selectively activate maleic anhydride, and at the same time, it constructs a local enrichment microenvironment through the spatial proximity effect. The synergistic effect of the three factors jointly promotes the efficient preparation of copolymers with high maleic anhydride content.

[0030] Chain growth stage: Chain radicals preferentially add to locally high concentrations of activated maleic anhydride molecules to generate new radicals. Since the addition rate constant of activated maleic anhydride and chain radicals is much greater than that of unactivated maleic anhydride and styrene, the insertion frequency of maleic anhydride in chain growth is significantly increased. At the same time, B(C6F5)3 cycles in coordination-dissociation equilibrium. When coordinating with pyridine, it anchors near the radical. After dissociation, it activates maleic anhydride. After activation, it re-coordinates with pyridine, forming a "breathing" mechanism to ensure that the radicals are always surrounded by activated monomers throughout the chain growth process. The semi-continuous feeding method adds maleic anhydride, functional initiator and B(C6F5)3 to the styrene solution at a constant rate to maintain the concentration of maleic anhydride in the reaction system and avoid "starvation" polymerization in the later stage due to rapid consumption.

[0031] Preferably, the organic solvent is selected from one or more of toluene, tetrahydrofuran, dioxane, and methyl ethyl ketone.

[0032] Preferably, the tetrahydrofuran solution of 4,4'-azobis(4-cyanopentyl chloride) is added dropwise over 30-60 minutes.

[0033] Preferably, the styrene-maleic anhydride copolymer has a number average molecular weight of 20,000-40,000 Da and a polydispersity index of 1.8-2.5.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] 1. This invention synthesizes a functional initiator with pyridine groups at both ends through molecular design. The pyridine nitrogen atom forms a reversible N→B coordination bond with tris(pentafluorophenyl)borane (B(C6F5)3), anchoring the strong Lewis acid B(C6F5)3 near the azo bond (radical generation site) of the initiator. When the azo bond thermally decomposes to generate chain radicals, the B(C6F5)3 molecule is trapped around the chain radical due to the "cage effect," forming a locally high-concentration activation microenvironment. Simultaneously, the dissociation of B(C6F5)3 activates maleic anhydride in the system, ensuring that the chain radical is always surrounded by activated maleic anhydride molecules. This synergistic mechanism achieves highly efficient insertion of maleic anhydride kinetically, increasing the mass fraction of maleic anhydride in the copolymer from the commonly reported 25%–30% range to 35%–45%, significantly enhancing the functionalization potential of the product.

[0036] 2. This invention selects tris(pentafluorophenyl)borane B(C6F5)3 as a synergistic agent. Its three-dimensional protective umbrella structure, formed by three pentafluorophenyl groups surrounding boron atoms, makes it difficult for it to effectively coordinate with the large-volume styrene monomer, while exhibiting selective activation ability for the smaller-volume maleic anhydride. Simultaneously, the moderate coordination strength between B(C6F5)3 and the pyridine group ensures a dynamic equilibrium between the two at the polymerization temperature: it exists in a stable coordinated state at room temperature, guaranteeing stable storage of the initiator system at room temperature; at the polymerization temperature, it undergoes moderate dissociation, releasing the active species of B(C6F5)3 to activate maleic anhydride, forming a "breathing" mechanism that ensures the activated maleic anhydride participates in chain growth in a timely manner. This selective activation strategy based on steric hindrance achieves precise control over the target monomer (maleic anhydride), effectively avoiding side reactions such as monomer homopolymerization and chain transfer caused by the non-selective activation (simultaneous activation of styrene and maleic anhydride) of traditional Lewis acids.

[0037] 3. In this invention, B(C6F5)3 is anchored to the functional initiator through reversible coordination, forming a dynamic "coordination-dissociation" cycle: when coordinating with pyridine groups, B(C6F5)3 is stably anchored near the chain radical; during polymerization, it dissociates and releases, rapidly activating maleic anhydride in the system. After activation, B(C6F5)3 returns to a coordinateable state and re-binds with pyridine groups, forming a continuous and stable "breathing" mechanism. This dynamic cycle ensures that the chain radical is always surrounded by activated maleic anhydride throughout the entire chain growth process, achieving synergistic effects from initiation to chain termination. To further optimize the effect, this invention combines a semi-continuous feeding process to continuously replenish maleic anhydride into the reaction system, maintaining a stable concentration of maleic anhydride in the system and avoiding the "starvation polymerization" problem caused by excessive consumption of maleic anhydride in the later stages of the reaction. It ensures efficient insertion of maleic anhydride from both spatial (local activation microenvironment) and temporal (stable concentration maintenance) dimensions. Attached Figure Description

[0038] Figure 1 The 1H NMR spectrum of the functional initiator prepared in this invention;

[0039] Figure 2 Line graphs showing MAh content and conversion rate for both examples and comparative examples. Detailed Implementation

[0040] The technical solutions of this invention are described below. It should also be noted that, to make the embodiments more detailed, the following embodiments are the best and preferred embodiments; those skilled in the art can also use other alternative methods to implement some well-known technologies.

[0041] In the following examples and comparative examples, the organic solvent was a mixture of toluene and tetrahydrofuran in a volume ratio of 9:1. The tetrahydrofuran solution of 4,4'-azobis(4-cyanovaleryl chloride) in S102 was added dropwise over 45 minutes.

[0042] Example 1: Preparation of SMA copolymer with high maleic anhydride content:

[0043] S1. Preparation of functional initiators:

[0044] S101. In a reaction vessel, 4,4'-azobis(4-cyanopentanoic acid) and anhydrous dichloromethane were added at a solid-liquid ratio of 1:11 g / mL. Oxaloyl chloride and anhydrous N,N-dimethylformamide were added with stirring. The molar ratio of 4,4'-azobis(4-cyanopentanoic acid), oxaloyl chloride, and anhydrous N,N-dimethylformamide was 1:2.75:0.075. The reaction was stirred at room temperature for 2.5 h. After the reaction was completed, the solvent and excess oxaloyl chloride were removed by vacuum distillation to obtain 4,4'-azobis(4-cyanopentanoic acid).

[0045] S102. The obtained 4,4'-azobis(4-cyanovaleryl chloride) was dissolved in anhydrous tetrahydrofuran at a solid-liquid ratio of 1:7 g / mL. Under ice-water bath cooling, it was slowly added dropwise to an anhydrous tetrahydrofuran solution containing 4-aminopyridine and triethylamine. The molar ratio of 4,4'-azobis(4-cyanovaleryl chloride), 4-aminopyridine and triethylamine was 1:2.75:2.75. After the addition was complete, the ice bath was removed, and the mixture was stirred overnight at room temperature. After the reaction was completed, the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated under reduced pressure, and ice water was added to precipitate the solid. The solid was filtered, washed, and recrystallized to obtain the functional initiator.

[0046] S2, Preparation of styrene-maleic anhydride copolymer:

[0047] Under free radical polymerization conditions, maleic anhydride, a functional initiator, and tris(pentafluorophenyl)borane were dissolved in a portion of an organic solvent. During the reaction, the mixture was added dropwise at a constant rate at 85°C to a reaction system containing styrene and the remaining solvent over a period of 4 hours. After the addition was complete, the reaction was continued at this temperature for another 4 hours. The molar ratio of styrene monomer, maleic anhydride monomer, functional initiator, and tris(pentafluorophenyl)borane was 1:1.2:0.01:0.04.

[0048] Example 2: Preparation of SMA copolymer with high maleic anhydride content:

[0049] S1. Preparation of functional initiators:

[0050] S101. In a reaction vessel, 4,4'-azobis(4-cyanopentanoic acid) and anhydrous dichloromethane were added at a solid-liquid ratio of 1:11 g / mL. Oxaloyl chloride and anhydrous N,N-dimethylformamide were added with stirring. The molar ratio of 4,4'-azobis(4-cyanopentanoic acid), oxaloyl chloride, and anhydrous N,N-dimethylformamide was 1:2.75:0.075. The reaction was stirred at room temperature for 2.5 h. After the reaction was completed, the solvent and excess oxaloyl chloride were removed by vacuum distillation to obtain 4,4'-azobis(4-cyanopentanoic acid).

[0051] S102. The obtained 4,4'-azobis(4-cyanovaleryl chloride) was dissolved in anhydrous tetrahydrofuran at a solid-liquid ratio of 1:7 g / mL. Under ice-water bath cooling, it was slowly added dropwise to an anhydrous tetrahydrofuran solution containing 4-aminopyridine and triethylamine. The molar ratio of 4,4'-azobis(4-cyanovaleryl chloride), 4-aminopyridine and triethylamine was 1:2.75:2.75. After the addition was complete, the ice bath was removed, and the mixture was stirred overnight at room temperature. After the reaction was completed, the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated under reduced pressure, and ice water was added to precipitate the solid. The solid was filtered, washed, and recrystallized to obtain the functional initiator.

[0052] S2, Preparation of styrene-maleic anhydride copolymer:

[0053] Under free radical polymerization conditions, maleic anhydride, a functional initiator, and tris(pentafluorophenyl)borane were dissolved in a portion of an organic solvent. During the reaction, the mixture was added dropwise at a constant rate at 85°C to a reaction system containing styrene and the remaining solvent over a period of 3 hours. After the addition was complete, the reaction was continued at this temperature for another 4 hours. The molar ratio of styrene monomer, maleic anhydride monomer, functional initiator, and tris(pentafluorophenyl)borane was 1:1.8:0.01:0.04.

[0054] Example 3: Preparation of SMA copolymer with high maleic anhydride content:

[0055] S1. Preparation of functional initiators:

[0056] S101. In a reaction vessel, 4,4'-azobis(4-cyanopentanoic acid) and anhydrous dichloromethane were added at a solid-liquid ratio of 1:11 g / mL. Oxaloyl chloride and anhydrous N,N-dimethylformamide were added with stirring. The molar ratio of 4,4'-azobis(4-cyanopentanoic acid), oxaloyl chloride, and anhydrous N,N-dimethylformamide was 1:2.75:0.075. The reaction was stirred at room temperature for 2.5 h. After the reaction was completed, the solvent and excess oxaloyl chloride were removed by vacuum distillation to obtain 4,4'-azobis(4-cyanopentanoic acid).

[0057] S102. The obtained 4,4'-azobis(4-cyanovaleryl chloride) was dissolved in anhydrous tetrahydrofuran at a solid-liquid ratio of 1:7 g / mL. Under ice-water bath cooling, it was slowly added dropwise to an anhydrous tetrahydrofuran solution containing 4-aminopyridine and triethylamine. The molar ratio of 4,4'-azobis(4-cyanovaleryl chloride), 4-aminopyridine and triethylamine was 1:2.75:2.75. After the addition was complete, the ice bath was removed, and the mixture was stirred overnight at room temperature. After the reaction was completed, the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated under reduced pressure, and ice water was added to precipitate the solid. The solid was filtered, washed, and recrystallized to obtain the functional initiator.

[0058] S2, Preparation of styrene-maleic anhydride copolymer:

[0059] Under free radical polymerization conditions, maleic anhydride, a functional initiator, and tris(pentafluorophenyl)borane were dissolved in a portion of an organic solvent. During the reaction, the mixture was added dropwise at a constant rate at 75°C to a reaction system containing styrene and the remaining solvent over a period of 4 hours. After the addition was complete, the reaction was continued at this temperature for another 4 hours. The molar ratio of styrene monomer, maleic anhydride monomer, functional initiator, and tris(pentafluorophenyl)borane was 1:1.8:0.01:0.04.

[0060] Example 4: Preparation of SMA copolymer with high maleic anhydride content:

[0061] S1. Preparation of functional initiators:

[0062] S101. In a reaction vessel, 4,4'-azobis(4-cyanopentanoic acid) and anhydrous dichloromethane were added at a solid-liquid ratio of 1:11 g / mL. Oxaloyl chloride and anhydrous N,N-dimethylformamide were added with stirring. The molar ratio of 4,4'-azobis(4-cyanopentanoic acid), oxaloyl chloride, and anhydrous N,N-dimethylformamide was 1:2.75:0.075. The reaction was stirred at room temperature for 2.5 h. After the reaction was completed, the solvent and excess oxaloyl chloride were removed by vacuum distillation to obtain 4,4'-azobis(4-cyanopentanoic acid).

[0063] S102. The obtained 4,4'-azobis(4-cyanovaleryl chloride) was dissolved in anhydrous tetrahydrofuran at a solid-liquid ratio of 1:7 g / mL. Under ice-water bath cooling, it was slowly added dropwise to an anhydrous tetrahydrofuran solution containing 4-aminopyridine and triethylamine. The molar ratio of 4,4'-azobis(4-cyanovaleryl chloride), 4-aminopyridine and triethylamine was 1:2.75:2.75. After the addition was complete, the ice bath was removed, and the mixture was stirred overnight at room temperature. After the reaction was completed, the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated under reduced pressure, and ice water was added to precipitate the solid. The solid was filtered, washed, and recrystallized to obtain the functional initiator.

[0064] S2, Preparation of styrene-maleic anhydride copolymer:

[0065] Under free radical polymerization conditions, maleic anhydride, a functional initiator, and tris(pentafluorophenyl)borane were dissolved in a portion of an organic solvent. During the reaction, the mixture was added dropwise at a constant rate at 85°C to a reaction system containing styrene and the remaining solvent over a period of 4 hours. After the addition was complete, the reaction was continued at this temperature for another 4 hours. The molar ratio of styrene monomer, maleic anhydride monomer, functional initiator, and tris(pentafluorophenyl)borane was 1:1.8:0.01:0.01.

[0066] Example 5: Preparation of SMA copolymer with high maleic anhydride content:

[0067] S1. Preparation of functional initiators:

[0068] S101. In a reaction vessel, 4,4'-azobis(4-cyanopentanoic acid) and anhydrous dichloromethane were added at a solid-liquid ratio of 1:11 g / mL. Oxaloyl chloride and anhydrous N,N-dimethylformamide were added with stirring. The molar ratio of 4,4'-azobis(4-cyanopentanoic acid), oxaloyl chloride, and anhydrous N,N-dimethylformamide was 1:2.75:0.075. The reaction was stirred at room temperature for 2.5 h. After the reaction was completed, the solvent and excess oxaloyl chloride were removed by vacuum distillation to obtain 4,4'-azobis(4-cyanopentanoic acid).

[0069] S102. The obtained 4,4'-azobis(4-cyanovaleryl chloride) was dissolved in anhydrous tetrahydrofuran at a solid-liquid ratio of 1:7 g / mL. Under ice-water bath cooling, it was slowly added dropwise to an anhydrous tetrahydrofuran solution containing 4-aminopyridine and triethylamine. The molar ratio of 4,4'-azobis(4-cyanovaleryl chloride), 4-aminopyridine and triethylamine was 1:2.75:2.75. After the addition was complete, the ice bath was removed, and the mixture was stirred overnight at room temperature. After the reaction was completed, the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated under reduced pressure, and ice water was added to precipitate the solid. The solid was filtered, washed, and recrystallized to obtain the functional initiator.

[0070] S2, Preparation of styrene-maleic anhydride copolymer:

[0071] Under free radical polymerization conditions, maleic anhydride, a functional initiator, and tris(pentafluorophenyl)borane were dissolved in a portion of an organic solvent. During the reaction, the mixture was added dropwise at a constant rate at 85°C to a reaction system containing styrene and the remaining solvent over a period of 4 hours. After the addition was complete, the reaction was continued at this temperature for another 4 hours. The molar ratio of styrene monomer, maleic anhydride monomer, functional initiator, and tris(pentafluorophenyl)borane was 1:1.8:0.01:0.04.

[0072] Comparative Example 1:

[0073] Compared with Example 5, the copolymerization reaction in Comparative Example 1 did not involve the addition of B(C6F5)3, but only used a functional initiator, with other conditions remaining unchanged.

[0074] Comparative Example 2:

[0075] Compared with Example 5, in Comparative Example 2, a conventional initiator (AIBN, azobisisobutyronitrile) was used instead of the functional initiator, while other conditions remained unchanged.

[0076] Comparative Example 3:

[0077] Compared with Example 5, Comparative Example 3 used AIBN instead of the functional initiator and did not add B(C6F5)3, while other conditions remained unchanged.

[0078] Comparative Example 4:

[0079] Compared to Example 5, Comparative Example 4 used 4,4'-azobis(4-cyanopentanoic acid) without pyridine modification as the initiator, while other conditions remained unchanged.

[0080] Comparative Example 5:

[0081] Compared with Example 5, in Comparative Example 5, B(C6F5)3 was replaced with conventional Lewis acid AlCl3, while other conditions remained unchanged.

[0082] Comparative Example 6:

[0083] Compared with Example 5, in Comparative Example 6, all MAh, initiator, and B(C6F5)3 were added to St and organic solvent at once without semi-continuous addition, and other conditions remained unchanged.

[0084] Performance testing:

[0085] The maleic anhydride content was determined by chemical titration. 0.25 g of the polymer sample was accurately weighed and dissolved in 30 mL of acetone. 20 mL of 0.2 mol / L sodium hydroxide standard solution was added, and the mixture was saponified at room temperature for 4 hours. Then, using phenolphthalein as an indicator, the mixture was back-titrated with 0.1 mol / L hydrochloric acid standard solution. The mass fraction of maleic anhydride was calculated based on the saponification value. This method can accurately determine the actual maleic anhydride content in the copolymer and is used to verify whether the present invention reaches the target range of 35%–45%.

[0086] Conversion rate determination method: The total monomer conversion rate was determined by gravimetric method. After the reaction was completed, the polymer solution was poured into excess anhydrous methanol to precipitate, filtered, and dried under vacuum at 50°C to constant weight. The mass of the obtained copolymer was then weighed. The conversion rate was calculated as follows: Conversion rate (%) = (Copolymer mass / Total monomer feed mass) × 100%. This method can quickly assess the polymerization reaction efficiency.

[0087] Molecular weight and its distribution were determined using gel permeation chromatography. Tetrahydrofuran was used as the mobile phase at a flow rate of 1.0 mL / min. Narrow-distribution polystyrene was used as the standard. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) of the copolymer were determined at 35 °C. This method was used to confirm whether the molecular weight of the copolymer was controlled within the range of 20,000–40,000 Da, and whether the molecular weight distribution was uniform.

[0088] Differential scanning calorimetry (DSC) was used for thermal property analysis. 5-10 mg of sample was weighed and placed in an aluminum crucible, heated from room temperature to 200 °C at a heating rate of 20 °C / min under a nitrogen atmosphere, and the glass transition temperature (Tg) of the copolymer was determined. This method was used to characterize the effect of high maleic anhydride content on the thermal properties of the copolymer.

[0089] The copolymer sequence structure was analyzed using carbon nuclear magnetic resonance spectroscopy (NMR). 13 C NMR. 50-80 mg of polymer sample was dissolved in 0.5 mL of deuterated dimethyl sulfoxide (DMSO-d6), and spectra were acquired at frequencies above 100 MHz. The carbonyl carbon region (170-175 ppm) and the methylene / methoxycarbon region (35-50 ppm) were analyzed in detail. The relative contents of maleic anhydride-maleic anhydride, maleic anhydride-styrene, and styrene-styrene ternary units were calculated by peak fitting to verify the random copolymerization characteristics of the copolymer. This method was used to confirm that the product obtained in this invention is a random copolymer rather than an alternating copolymer.

[0090] The test results of the above embodiments and comparative examples are shown in Table 1 below. Mn represents the number-average molecular weight, reflecting the average length of the polymer molecular chain; PDI represents the polydispersity index, reflecting the width of the molecular weight distribution. The closer the PDI is to 1, the more uniform the molecular weight; Tg represents the glass transition temperature, characterizing the thermal properties of the copolymer; MA-MA represents maleic anhydride-maleic anhydride, MA-St represents maleic anhydride-styrene, and St-St represents styrene-styrene. The appearance of MA-MA units is the key microscopic evidence for determining the breakthrough of the alternating copolymerization limitation.

[0091] Table 1 Performance test data of the examples and comparative examples

[0092] Group MAh content Conversion rate Mn PDI Tg MA-MA content MA-St content unit wt% % ×10³ - ℃ % % Example 1 33.8 78 29.5 2.2 138 20 76 Example 2 36.5 81 27.8 2.3 145 26 72 Example 3 42.1 72 33.2 2 158 42 56 Example 4 29.6 76 31.5 2.4 132 12 80 Example 5 38.2 83 28.6 2.1 149 32 66 Comparative Example 1 26.4 75 29.8 2.3 126 6 82 Comparative Example 2 24.8 73 31.2 2.5 123 4 80 Comparative Example 3 21.5 71 32.5 2.4 121 0 78 Comparative Example 4 25.2 70 27.6 2.6 124 5 80 Comparative Example 5 28.3 68 24.5 2.8 128 10 80 Comparative Example 6 23.8 65 35.8 2.9 120 2 80

[0093] Based on the performance test data of the embodiments and comparative examples in Table 1 above, it can be found that:

[0094] Figure 1 For functional initiators 1The 1H NMR spectrum clearly shows the corresponding characteristic absorption peaks, confirming the successful preparation and structural integrity of the initiator. In the spectrum, hydrogen atoms on the pyridine ring exhibit characteristic multiplets in the 7.2-8.5 ppm range, corresponding to hydrogen protons at different positions on the pyridine ring, proving that 4-aminopyridine has been successfully grafted into the initiator molecule. In the 4,4'-azobis(4-cyanopentanoic acid) skeleton, hydrogen atoms of the methylene group (-CH2-) exhibit multiplets in the 1.5-2.5 ppm range, while hydrogen atoms of the methylene group adjacent to the cyano group exhibit a singlet in the 2.8-3.2 ppm range due to electron-withdrawing effects. Hydrogen atoms surrounding the azo group (-N=N-) exhibit characteristic peaks in the 3.5-4.0 ppm range. All characteristic peaks are symmetrical, without obvious impurities, and the peak area ratio is consistent with the theoretical ratio of the functional initiator's molecular structure, indicating high product purity and the absence of unreacted raw material impurities, making it suitable for subsequent styrene-maleic anhydride copolymerization.

[0095] Figure 2 The graph shows line graphs of MAh content and conversion rate for the examples and comparative examples. The horizontal axis represents each group (Examples 1-5, Comparative Examples 1-6), and the vertical axis represents MAh content (wt%) and conversion rate (%). The double lines correspond to the changing trends of the two indicators. The graph clearly shows that the MAh content (29.6%-42.1%) and conversion rate (72%-83%) of the example groups are generally higher than those of the comparative examples (MAh content 21.5%-28.3%, conversion rate 65%-75%). Among them, the MAh content of Examples 2, 3, and 5 reached the target of 35%-45%, with Example 3 having the highest MAh content (42.1wt%) and Example 5 having the highest conversion rate (83%). This intuitively demonstrates the advantages of the functional initiator synergistic system with B(C6F5)3 and the semi-continuous feeding process.

[0096] Example 5 employed an optimized feed ratio and a semi-continuous feeding process, with a slight excess of B(C6F5)3 relative to the initiator and a suitable polymerization temperature. Under these conditions, the reversible coordination equilibrium of pyridine-boron in the system was fully established. B(C6F5)3 could both coordinately anchor itself near the initiator and moderately dissociate to activate maleic anhydride, forming an effective local enrichment effect. During chain growth, a high concentration of activated maleic anhydride was maintained around the free radicals, resulting in a significantly higher insertion frequency of maleic anhydride than styrene, ultimately leading to a high level of maleic anhydride content in the copolymer. Simultaneously, the semi-continuous feeding ensured a stable monomer concentration and controlled molecular weight distribution within a narrow range, making the polymerization process stable and controllable.

[0097] In Example 1, reducing the maleic anhydride feed ratio significantly decreased the maleic anhydride content in the copolymer, and consequently reduced the proportion of MA-MA continuous units, while increasing the proportion of MA-St alternating units. This indicates that a lower maleic anhydride feed ratio makes the system closer to the characteristics of alternating copolymerization. Although the synergistic effect of the functional initiator and borane remains effective, insufficient monomer supply limits the formation of the MA-MA sequence. The molecular weight and distribution showed little change, indicating that the initiation efficiency was not significantly affected.

[0098] In Example 2, the maleic anhydride content decreased after the dropping rate was increased. The faster dropping led to greater fluctuations in the instantaneous concentration of maleic anhydride in the reaction system, making it difficult to maintain a high local concentration of activated monomers around the free radicals. This increased the competitive addition opportunity of styrene during some chain growth processes, resulting in a decrease in the net insertion amount of maleic anhydride in the copolymer.

[0099] In Example 3, the maleic anhydride content reached its highest value among all examples after the polymerization temperature was lowered, and the proportion of MA-MA continuous units was also correspondingly the highest. The lower temperature slowed down the chain growth rate, causing the coordination equilibrium of B(C6F5)3 with pyridine to shift towards the anchored state. At the same time, the decomposition rate of the azo initiator slowed down, and free radical generation was more stable. These factors together resulted in a higher local concentration and longer duration of activated maleic anhydride around the free radicals, thus leading to more complete insertion of maleic anhydride. However, the trade-off was a longer overall reaction time and a decrease in monomer conversion.

[0100] In Example 4, when the amount of B(C6F5)3 was reduced to the same molar amount as the initiator, the maleic anhydride content decreased significantly, and the proportion of MA-MA units also decreased substantially. This indicates that insufficient borane content resulted in some pyridine coordination sites not being effectively occupied, weakening the reversible coordination anchoring effect. The reduced concentration of free borane led to a decrease in the activation efficiency of maleic anhydride, making it difficult to fully realize the local enrichment effect.

[0101] During the copolymerization reaction, in Comparative Example 1, without the addition of B(C6F5)3, the maleic anhydride content decreased to a low level, the MA-MA units almost disappeared, and the copolymer composition approached an alternating structure. This directly proves the irreplaceable role of borane in the synergistic system; without the activating effect of borane, the functional initiator alone cannot overcome the limitations of alternating copolymerization.

[0102] In Comparative Example 2, even with the presence of borane, the maleic anhydride content remained low after replacing the functional initiator with conventional AIBN, and the proportion of MA-MA units was significantly reduced. This indicates that without pyridine groups as anchoring sites, borane cannot form local enrichment around free radicals, and the activation effect of freely distributed borane on maleic anhydride is limited, making it difficult to achieve continuous and efficient maleic anhydride insertion.

[0103] In Comparative Example 3, after removing both the functional initiator and borane, the copolymer composition exhibited typical alternating characteristics, with the complete disappearance of MA-MA units and the maleic anhydride content reduced to a minimum. This result verifies the synergistic nature of the two indispensable elements of the present invention: the functional initiator and borane.

[0104] In Comparative Example 4, when unmodified 4,4'-azobis(4-cyanopentanoic acid) was used as the initiator, although borane was still present, the maleic anhydride content was close to that of Comparative Example 2, and the MA-MA unit ratio was also very low. This further confirms the crucial role of the pyridine coordinating group. Without the pyridine nitrogen atom forming a reversible coordination with borane, borane cannot be anchored near the initiator molecule, and the local enrichment effect cannot be established.

[0105] In Comparative Example 5, replacing B(C6F5)3 with the traditional Lewis acid AlCl3 resulted in a limited increase in maleic anhydride content and an extremely low proportion of MA-MA units. Although AlCl3 can coordinate with maleic anhydride, its excessively strong Lewis acidity and lack of steric selectivity make its coordination with pyridine too stable and difficult to dissociate. Furthermore, it easily triggers side reactions such as chain transfer, leading to a wider molecular weight distribution and decreased conversion rate. In contrast, the sterically hindered structure and moderate coordination strength of B(C6F5)3 make it superior in the "anchoring-activation-regeneration" cycle.

[0106] In Comparative Example 6, the maleic anhydride content was significantly reduced and the molecular weight distribution broadened considerably after the monomer was fed in a single batch instead of a semi-continuous feeding method. Single-batch feeding resulted in excessively high maleic anhydride concentration and vigorous reaction in the initial stage. Later, after the maleic anhydride was depleted, chain growth relied mainly on styrene self-polymerization. Therefore, the copolymer not only had a low total maleic anhydride content but also contained longer styrene segments, leading to a broader molecular weight distribution. Semi-continuous feeding, by continuously replenishing maleic anhydride, effectively avoided this problem.

[0107] A comparative analysis of the data from the above examples and comparative examples shows that the present invention successfully achieved efficient insertion of maleic anhydride into styrene-maleic anhydride copolymers through a reversible N→B coordination synergistic system of a functional initiator and tris(pentafluorophenyl)borane, combined with a semi-continuous feeding process. Specifically, Example 5 (standard example) achieved balanced performance in terms of maleic anhydride content, conversion rate, molecular weight, and distribution under suitable temperature, optimized feed ratio, and excess borane conditions. Although Example 3 achieved the highest maleic anhydride content due to low-temperature polymerization, the reaction efficiency was somewhat reduced. In Examples 1, 2, and 4, the maleic anhydride content and the proportion of MA-MA continuous units all decreased to varying degrees due to changes in feed ratio, dropping rate, or borane dosage. Comparative Examples 1-6 further verified the synergistic necessity of the various technical features of this invention: the absence of borane, the absence of pyridine anchoring sites, the use of the traditional initiator AIBN, the use of unmodified ACVA, the use of AlCl3 to replace B(C6F5)3, or the use of a one-time feeding method all led to a significant decrease in maleic anhydride content, enhanced alternating structural features, or an increase in side reactions. In summary, the synergistic system of the functional initiator and B(C6F5)3 is the core technical means by which this invention significantly improves the maleic anhydride content.

[0108] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a high-maleic anhydride content SMA copolymer, characterized in that, The method includes: under free radical polymerization conditions, subjecting a reaction mixture containing styrene monomer, maleic anhydride monomer, a functional initiator, and tris(pentafluorophenyl)borane to a copolymerization reaction, wherein the copolymerization reaction is carried out in a semi-continuous feeding manner: maleic anhydride, the functional initiator, and tris(pentafluorophenyl)borane are dissolved in a portion of an organic solvent and added dropwise at a constant rate to a reaction system containing styrene and the remaining solvent during the reaction, to obtain a styrene-maleic anhydride copolymer with a maleic anhydride mass fraction of 35%–45%; wherein the structure of the functional initiator is: .

2. The method for preparing a high maleic anhydride content SMA copolymer according to claim 1, characterized in that, The molar ratio of the styrene monomer, maleic anhydride monomer, functional initiator and tris(pentafluorophenyl)borane is 1:(1.8-2.5):(0.005-0.02):(0.01-0.08).

3. The method for preparing a high maleic anhydride content SMA copolymer according to claim 1, characterized in that, The copolymerization reaction is carried out using a semi-continuous feeding method: maleic anhydride, functional initiator and tris(pentafluorophenyl)borane are dissolved in a portion of an organic solvent and added dropwise at a constant rate to the reaction system containing styrene and the remaining solvent, with a dropping time of 3-6 hours; the reaction is carried out in the organic solvent, which is selected from one or more of toluene, tetrahydrofuran, dioxane, and methyl ethyl ketone.

4. The method for preparing a high maleic anhydride content SMA copolymer according to claim 1, characterized in that, The reaction temperature for the copolymerization reaction is 75-95℃.

5. The method for preparing a high maleic anhydride content SMA copolymer according to claim 1, characterized in that, The functional initiator is synthesized through the following steps: Preparation of S1, 4,4'-azobis(4-cyanopentyl chloride): In a reaction vessel, 4,4'-azobis(4-cyanopentanoic acid) and anhydrous dichloromethane were added, and oxalyl chloride and anhydrous N,N-dimethylformamide were added with stirring. The reaction was stirred at room temperature for 2-3 hours. After the reaction was completed, the solvent and excess oxalyl chloride were removed by vacuum distillation to obtain 4,4'-azobis(4-cyanopentanoic acid). S2. Preparation of functional initiators: The obtained 4,4'-azobis(4-cyanopentyl chloride) was dissolved in anhydrous tetrahydrofuran and slowly added dropwise to an anhydrous tetrahydrofuran solution containing 4-aminopyridine and triethylamine under ice-water bath cooling. After the addition was complete, the ice bath was removed, and the mixture was stirred overnight at room temperature. After the reaction was completed, the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated under reduced pressure, and ice water was added to precipitate the solid. The solid was filtered, washed, and recrystallized to obtain the functional initiator.

6. The method for preparing a high maleic anhydride content SMA copolymer according to claim 5, characterized in that, In step S1, the molar ratio of 4,4'-azobis(4-cyanopentanoic acid) to oxaloyl chloride is 1:2.5-3.0, the amount of anhydrous N,N-dimethylformamide is 0.05-0.1 times the molar amount of 4,4'-azobis(4-cyanopentanoic acid), and the solid-liquid ratio of anhydrous dichloromethane to 4,4'-azobis(4-cyanopentanoic acid) is 1:8-15 g / mL.

7. The method for preparing a high maleic anhydride content SMA copolymer according to claim 5, characterized in that, In step S2, the molar ratio of 4,4'-azobis(4-cyanovaleryl chloride), 4-aminopyridine, and triethylamine is 1:2.5-3.0:2.5-3.0, and the solid-liquid ratio of anhydrous tetrahydrofuran to 4,4'-azobis(4-cyanovaleryl chloride) is 1:5-10 g / mL; the dropwise addition is performed by adding the tetrahydrofuran solution of 4,4'-azobis(4-cyanovaleryl chloride) dropwise over 30-60 minutes.

8. A method for preparing a high maleic anhydride content SMA copolymer according to any one of claims 1-4, characterized in that, The obtained styrene-maleic anhydride copolymer has a number average molecular weight of 20,000-40,000 Da and a polydispersity index of 1.8-2.5.

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