Synthesis of Boronic Acid‐Cross‐Linked Diene‐Based Polymers via Free‐Radical Copolymerization

ABSTRACT

Conjugated diene polymers cross‐linked with boronic acid functionalities were synthesized via free‐radical copolymerization of styrylboronic acid and isoprene or myrcene. The polymerization proceeded with an identical boron incorporation to the comonomer feed in a 1,4‐specific manner. The molecular weight (M _n) of the copolymer reached 46,000–110,000 g/mol, guaranteeing an adequate number of boronic acids in one polymer chain for cross‐linking. To obtain a soluble copolymer, the concentration of styrylboronic acid monomer in THF/H₂O mixture solvent is important to prevent dehydrative boroxine formation during polymerization. The cross‐linking driven by thermal dehydration–condensation of boronic acid moieties was confirmed by the increase in tensile modulus of the molded sample, as well as the rheological measurement and swelling test. The cross‐linked polymer can be hydrolyzed by heating the polymer in wet toluene without losing the amount of incorporated boronic acid functionalities. This method can also be applied to the synthesis of boron‐functionalized other diene‐based polymers containing styrene and acrylonitrile units. Therefore, this synthetic strategy provided a way to introduce reversibly formed cross‐linking points to industrially important rubber materials.

1. Introduction

Polymers containing C═C double bonds in the main chain are an industrially important class of amorphous elastomers, which are used in automobiles, belts, seals, and footwear. These polymers include polybutadiene (PBd), polyisoprene (PIP), the main component of natural rubber, styrene‐butadiene rubber (SBR), and acrylonitrile‐butadiene rubber (NBR). SBR is known to show superior abrasion resistance due to the introduction of aromatic groups. NBR possesses high chemical resistance because of polar substituents. The microstructure of PBd and PIP can be highly regulated to cis‐1,4‐sequence, the origin of high performance as rubber materials, by metal‐catalyzed coordination polymerization [1, 2, 3]. On the other hand, SBR and NBR are generally synthesized by radical polymerization. These diene‐based polymers only exhibit elasticity after forming cross‐links, usually through vulcanization using peracids or sulfur. Such cross‐links consist of monosulfides, polysulfides, or carbon–carbon bonds, which are difficult to selectively cleave without cleaving the polymer main chain [4]. Therefore, this poses a barrier to the recycling process of vulcanized rubber.

Recently, covalent adaptable networks (CAN), have attracted much attention as a cross‐linking method for rubber materials that achieves both sufficient material strength and reprocessing ability [5, 6, 7]. B─O bonds within boronic acids, boroxines, and boronic esters are recognized as one of the most reliable sources of exchangeable bonds and widely applied to CAN [8, 9, 10]. In conjugated diene polymers, cross‐linking using boronic acids and their esters has also attracted particular attention. Most examples introducing boronic acid moieties toward diene‐based polymers apply thiol‐ene reactions and other post‐functionalization reactions [11, 12, 13]. The advantage of this method is the applicability to commercially available pre‐cross‐linked polymers.

Another considerable method is direct copolymerization of monomers containing boronic acid moieties, which enables uniform distribution of cross‐links and structural versatility around the cross‐linking point, based on the monomer design. Here, we investigated radical copolymerization of styrylboronic acid (1) and various conjugated dienes, demonstrating the incorporation of boronic acid into a wide range of industrially important diene‐based polymers such as PIP and SBR/NBR‐like copolymers (Scheme 1). The radical polymerization of boronic acid‐containing vinyl monomers, including 1, can be well‐controlled by adding a small amount of water, preventing the gelation along with the dehydrative condensation of boronic acids [14, 15]. In addition, we have recently developed a process introducing boronic acid into polyolefins through metal‐catalyzed coordination copolymerization, using naphthalenediamide(dan)‐masked comonomers [16, 17, 18]. The boronamide exhibits lower Lewis acidity because the nitrogen lone pair efficiently donates electrons to the vacant p‐orbital of boron, and therefore the boronamide is less poisoning to early transition metal‐based catalysts. Another advantage of boronamide is that it can be easily converted to boronic acid in acidic conditions, whereas pinacol ester, one of the most popular protecting groups, requires oxidation or transesterification to cleave. Boronamides in the polymer can be hydrolyzed by precipitating the polymer into acidic methanol. Related to these studies, we also investigated the radical copolymerization behavior of dan‐masked styrylboronic acid (2) [19] as a method of boron‐incorporated diene‐based polymers, which is free from cross‐linking during the polymerization.

SCHEME 1.

Radical copolymerization of styrylboronic acid derivatives studied in this work.

2. Results and Discussion

First, free‐radical copolymerizations of isoprene (IP) and 1 were performed (Table 1). According to the previously reported radical polymerization of isoprene and butadiene [20, 21, 22, 23, 24], a high reaction temperature is needed. Therefore, we applied dicumyl peroxide (DCP) as an initiator, and the polymerization was conducted at 110 °C in an autoclave. Moreover, in these radical polymerizations of conjugated dienes, high monomer concentration or neat condition is required. Therefore, the solvent amount should be minimized to solubilize solid comonomer 1. To prevent the gelation via the dehydration of boronic acids, 10 volume % of water was added to the solvent. We confirmed that THF/H₂O mixed solvent was separated into two phases in the presence of isoprene, but still solubilized comonomer 1, whereas toluene/H₂O did not completely solubilize 1 in the presence of IP. After 132 h, a high molecular weight copolymer was successfully obtained (entry 1). A sole glass transition temperature (T _g), which is slightly higher than that of homo‐PIP, showed the formation of a random copolymer (Figure S10). These copolymerizations proceeded reproducibly without the need for a freeze‐thaw cycle as long as oxygen‐ and moisture‐free grade THF were used.

TABLE 1.

Copolymerization of IP or My and styrylboronic acid derivatives. ^a

Entries	Monomer (mmol)		Solvent (mL)	Yield (%)	Boron incorp. b (mol%)	Diene microstructure b (mol%)			M n c (103)	Đ c	T g d ( °C)
	Diene	Boron				1,4‐	3,4‐	1,2‐
1	IP (80)	1 (0.4)	THF/H2O (7.5/0.75)	31	0.22	90	6	4	110	1.8	−60
2	IP (80)	1 (0.8)	THF/H2O (15/1.5)	22	0.58	90	7	3	58	2.1	−59
3	IP (80)	1 (1.6)	THF/H2O (30/3.0)	18	1.1	89	7	4	46	1.7	−58
4	IP (240)	1 (4.8)	THF/H2O (46/4.6)	31	1.1	89	6	5	57	2.0	−58
5	IP (80)	1 (1.6)	THF/H2O (7.5/0.75)	35	n.d. e	n.d. e	n.d. e	n.d. e	n.d. e	n.d. e	−55
6	IP (80)	1 (1.6)	DMF/H2O (30/3.0)	20	n.d. e	n.d. e	n.d. e	n.d. e	n.d. e	n.d. e	−58
7	My (80)	1 (0.4)	THF/H2O (15/1.5)	22	0.40	87	12	1	75	1.9	−82
8	IP (80)	2 (1.6)	THF (7.5)	23	1.6	90	6	4	38	3.0	−54

^a Reaction conditions: free radical source = DCP (4.0 µmol), temp. = 110 °C, and time = 132 h.

^b Determined by ¹H NMR.

^c Determined by GPC calibrated with polystyrene standards.

^d Determined by DSC.

^e Not determined due to the insolubility of the polymer.

In this copolymerization, the boron incorporation ratio can be increased by increasing the feed ratio of 1 (entries 2 and 3). However, it should be noted that the concentration of 1 is critically important for preventing gelation. The copolymerization can be conducted at a higher concentration of 1 (0.10 M) and on a larger scale, successfully giving 5.0 g of the copolymer (entry 4). However, a much higher concentration of 1 (0.19 M) leads to cross‐linking during polymerization, resulting in the formation of an insoluble solid (entry 5). DMF, one of the most feasible solvents for diene polymerization, only caused a slight improvement in yield and gelation during polymerization (entry 6).

¹H NMR spectrum of the obtained copolymer showed a small signal at 7.62 ppm and 7.18 ppm (H_g and H_h in Figures 1, S1–S3, and S5), which corresponds to protons located at ortho‐ and meta‐ positions of boronic acid, respectively. Besides, some boronic acids have already formed boroxine while the polymer maintained its solubility in chloroform. This is evidenced by the additional signal in ¹H NMR, corresponding to protons on boroxine observed at 8.12 ppm (H_g ’). The aromatic protons located on the meta‐ position of boroxine would be overlapped with the solvent peak. Protodeboration of 1 is not likely to have occurred, because no signal was observed upfield of 7.1 ppm, indicating the absence of an unsubstituted styrene unit. From the integral ratios of the aromatic signals toward the other signals of olefin protons, the boron incorporation ratio can be determined to be in the range of 0.22–1.1 mol%. The average number of incorporated boron moieties per polymer chain, calculated from the number‐averaged molecular weight (M _n, 46,000–110,000), molecular weight of isoprene (68), and molar incorporation ratio of 1, was 4.2 – 9.1. These results show that the obtained copolymers can be cross‐linked by multiple boronic acids after dehydrative condensation. The ratio of 1,4‐/3,4‐/1,2‐ microstructures of the copolymer can be determined by the integral ratios of three regions, which correspond to vinyl proton in 1,2‐sequence (5.8–5.7 ppm, H_d ), internal olefinic proton of 1,4‐sequence (5.13 ppm, H_a ), and terminal olefinic protons of 3,4‐sequence (4.8–4.5 ppm, H_b and H_b’ ). The ratio was almost the same as the previously reported polyisoprene obtained by radical polymerization [25, 26]. Further ¹³C NMR microstructure assignment of a representative copolymer (entry 3) based on the previous literature [27, 28] showed a trans‐rich structure (Figure S12). In addition, a significant amount of the regioirregular sequences like head‐to‐head and tail‐to‐tail 1,4‐linkage were detected.

FIGURE 1.

¹H NMR spectrum of IP/1 copolymer obtained in Table 1, entry 3 (in CDCl₃, 25 °C, 500 MHz).

The comonomer reactivity ratios were determined to be r _IP = 2.3 and r _{comonomer 1} = 15.7 from Kelen–Tüdös plots (Figure S13), suggesting that this copolymerization tends to give block‐like microstructure. Copolymerization of myrcene (My), one of the representative plant‐derived diene monomers, can be copolymerized in a similar way with isoprene (entry 7). Copolymerization of IP and boronamide monomer 2 also proceeded and gave a high molecular weight copolymer (entry 8). In the ¹H NMR spectrum of the copolymer, characteristic signals assigned to N─H protons and protons at the 2,7‐position of the naphthalene skeleton were observed at 6.02 and 6.39 ppm, respectively (Figure S4). The calculated comonomer incorporation ratio of 2 was higher than the incorporation of 1 at the same comonomer feed ratio (Figure S5). The rigid structure of boronamide 2 may be reflected in the slightly higher T _g of the obtained copolymer.

The sample sheets of the obtained IP/1 copolymers for mechanical testing were prepared by heat pressing to promote cross‐linking via the dehydration condensation of boronic acid during molding. The samples containing 0.20 mol% of boronic acid, prepared at (80 °C, 120 °C, 140 °C, or 160 °C) for 10 min, showed an increased tensile modulus and tensile strength along with the increased temperature, suggesting that cross‐linking had thermally occurred (Table 2, entry 1, Figure 2). All the samples showed a similar glass transition temperature as the copolymer before cross‐linking (Figure S11), probably because cross‐linking density is so low that the molecular weight between cross‐linking point is comparable or larger (6200–26,000) than that between entanglement point (M _e ∼ 6800) [29], and the cross‐linking point would give crucial effect on the micro‐Brownian motion of the polymer chain. Tensile modulus also increased when the boron content increased (entries 2 and 3, Figure 3). Relatively low tensile strength and high Young's modulus of the polymer with the highest boron incorporation may be because some pre‐cross‐linked boronic acid existed before the molding process.

TABLE 2.

Mechanical properties of IP/1 copolymers.

Entries	Boron incorp. a (mol%)	M n b (x103)	Đ b	Pressing temp. ( °C)	Tensile modulus (MPa)	Tensile strength (MPa)	Elongation at break (%)	T g c ( °C)
1	0.22	110	1.8	80	0.25 ± 0.03	0.58 ± 0.04	1740 ± 140	−60
	0.22	110	1.8	120	0.27 ± 0.05	0.57 ± 0.07	1950 ± 350	−60
	0.22	110	1.8	140	0.40 ± 0.05	1.11 ± 0.48	1320 ± 620	−60
	0.22	110	1.8	160	0.42 ± 0.06	1.58 ± 0.44	1100 ± 300	−60
2	0.58	58	2.1	160	0.59 ± 0.04	1.49 ± 0.32	890 ± 180	−59
3	1.1	46	1.7	160	0.98 ± 0.03	0.99 ± 0.01	240 ± 2	−58

^a Determined by ¹H NMR.

^b Determined by GPC calibrated with polystyrene standards.

^c T _g after heat pressing determined by DSC.

FIGURE 2.

Representative stress–strain curves of IP/1 copolymer obtained in Table 1, entry 1 pressed at different temperatures.

FIGURE 3.

Representative stress–strain curves of IP/1 copolymers pressed at 160 °C with different boron content.

The cross‐linking densities of heat‐pressed IP/1 copolymers were measured by the swelling test in toluene (Table 3). The measured cross‐linking density (ρ_{c_meas}) was calculated from the fraction ratio of the polymer in the swelled sample (Φ _s) using Flory–Rhener equation. The calculated values were increased by the increase in boron content in the polymer. The value was about 20%–30% of the estimated values (ρ_{c_est}), which is calculated from the density of polyisoprene (0.92 g/cm³) and boron incorporation.

TABLE 3.

Swelling test of IP/1 copolymers.

Entries	Boron incorp. a (mol%)	Φ s	ρc_meas (mol/m3)	ρc_est (mol/m3)
1	0.22	0.052	3	10
2	0.58	0.063	5	26
3	1.1	0.10	14	50

^a Determined by ¹H NMR.

To reveal the detailed dehydrative cross‐linking process, the IR spectra of IP/1 copolymer were measured after processing the polymer sample at various temperatures (Figure 4). All the spectra showed two absorbance peaks, which are assigned to boronic acid (911 cm⁻¹) and boroxine (670 cm⁻¹) [30], which indicated these two states are in equilibrium. Especially, a large absorbance of the boroxine peak was observed above 80 °C, suggesting that formation of boroxine started at this temperature. In addition, the linear rheological behavior change along with the heating was traced (Figure 5). The storage modulus (G’) initially decreased slightly from room temperature to 140 °C and turned to increase until 170 °C, indicating that elimination of water took place in this temperature region, and the result explains the result of the tensile test in Figure 2. The frequency sweep of G’ at 30 °C after the heating process was significantly higher than that before heating, supporting the occurrence of stable cross‐linking (Figure S14). These results well explained the mechanical property change of the copolymer prepared at various temperatures.

FIGURE 4.

IR spectra of IP/1 copolymer (entry 3) after heat processing at a certain temperature.

FIGURE 5.

Rheological behavior of IP/1 copolymer (entry 1) during heating process (heating rate: 1.0 °C/min).

The reprocessability of the cross‐linked polymer sample was investigated. First, we attempted remolding of the finely cut polymer at 160 °C. However, only a part of the fractured surfaces of the sample fused after 1 h, resulting in the incomplete recovery of the sample specimen. Stress relaxation of the specimen at 190 °C reached only a 20% reduction of the initial stress after 15 min, suggesting that dynamic bond exchange between boroxines is mostly suppressed and the cross‐links behave as a permanent network (Figure S15). This result well explained the incomplete thermal reprocessing result of the copolymer. On the other hand, heating the sample in wet toluene (containing ca. 200 ppm of water) at 90 °C completely dissolves the polymer within 24 h, and the following reprecipitation in methanol quantitatively recovers the pre‐cross‐linked polymer. The ¹H NMR spectra of the recovered copolymer showed no loss of incorporated boronic acids (Figure S9). These results indicate that the wet solvent swelling process is essential for the cleavage of the boroxine cross‐link. The GPC traces of the as‐synthesized polymer and recovered polymer mostly matched with each other, indicating that the cross‐links were formed only with boronic acids, and C═C bonds in the main chain did not participate in the cross‐link formation (Figure 6). The sample specimen prepared by re‐cross‐linking of recovered copolymer by heat pressing showed lower tensile modulus and higher elongation ratio, showing lower cross‐linking density than the original one, but still showed elastic property (Figure 7 and Table S1). This result successfully demonstrated that the boroxine cross‐links can be cleaved by hydrolysis and formed again by heating.

FIGURE 6.

GPC traces of IP/1 copolymer (entry 4) immediately after synthesis and after cross‐link‐decross‐link cycle.

FIGURE 7.

Mechanical properties of as‐synthesized and recycled IP/1 copolymer (entry 4).

Terpolymerization of isoprene, 1, and styrene or acrylonitrile was investigated to demonstrate the applicability of boron‐cross‐linking to SBR or NBR‐like rubber materials (Table 4). These polymerizations proceeded in a similar manner with the copolymerization of isoprene and 1, yielding high‐molecular‐weight polymers (entries 9–11). Similar to the case of the IP/1 copolymer, these polymers exhibited a narrow molecular weight distribution and a single glass transition temperature, indicating that copolymerization proceeded randomly. The T _g was higher than that of IP/1 copolymer due to the incorporation of St/AN units. In IP/St/1 terpolymerization, the incorporation ratio at the same comonomer feed ratio calculated from the ¹H NMR spectra (Figures S6–S8) was comparable to IP/1 copolymerization, whereas IP/AN/1 terpolymerization incorporated more 1, probably because electron deficient AN chain end is more reactive toward electron‐rich 1.

TABLE 4.

Copolymerization of IP, 1, and other vinyl monomers. ^a

	Monomer (mmol)					Diene microstructure b (mol%)
Entries	1	Comonomer	Yield (%)	Comon. incorp. b (mol%)	Boron incorp. b (mol%)	1,4‐	3,4‐	1,2‐	M n c (× 103)	Đ c	T g d ( °C)
3	1.6	None	18	— e	1.1	89	7	4	46	1.7	−58
9	1.6	St (16)	23	10.7	0.91	90	7	3	50	2.5	−42
10	0.8	St (16)	46	12.6	0.38	91	6	3	74	2.0	−42
11	1.6	AN (16)	25	11.9	1.6	92	5	3	39	2.2	−37

^a Reaction conditions: [IP] = 80 mmol, free radical source = DCP (4.0 µmol), temp. = 110 °C, solvent = THF/H₂O (30 + 3.0 mL), time = 132 h.

^b Determined by ¹H NMR.

^c Determined by GPC calibrated with polystyrene standards.

^d Determined by DSC.

^e Not determined.

The pressed sample of the terpolymer showed the elastic properties (Figure 8 and Table 5). In IP/St/1 terpolymer, more boron content increased the tensile modulus, similarly with the IP/1 copolymer. IP/AN/1 terpolymer showed relatively low tensile modulus and tensile strength with high elongation, typically observed in the material with low cross‐linking density, as shown in Figure 2. Here, the presence of polar nitrile groups can promote the incorporation of water in the polymer matrix, and the insufficient dehydrative cross‐linking based on the equilibrium would lower the mechanical properties.

FIGURE 8.

Representative stress–strain curves of terpolymers obtained in Table 4. Samples were prepared by pressing at 160 °C, 30 MPa.

TABLE 5.

Mechanical properties of terpolymers obtained in Table 4.

Entries	Boron incorp. a (mol%)	Comon. incorp. a (103)	M n b (103)	Đ b	Tensile modulus (MPa)	Tensile strength (MPa)	Elongation at break (%)
3	1.1	0	52	2.0	0.98 ± 0.03	0.99 ± 0.01	240 ± 2
9	0.91	10.7	50	2.5	0.94 ± 0.05	1.77 ± 0.14	500 ± 60
10	0.38	12.6	74	2.0	0.52 ± 0.07	1.87 ± 0.40	1290 ± 340
11	1.6	11.9	39	2.2	0.57 ± 0.11	0.61 ± 0.04	2000 ± 270

^a Determined by ¹H NMR.

^b Samples were prepared by pressing at 160 °C, 30 MPa.

3. Conclusion

Radical copolymerization using styrylboronic acid 1 as a comonomer successfully incorporated boronic acid functionality into various conjugated diene‐based copolymers oriented toward industrially important rubber materials, such as PIP and analogues of SBR and NBR. Molecular weight of the obtained copolymer was up to 10⁵ orders with a narrow molecular weight distribution. The cross‐linking of the copolymers via dehydration of boronic acid took place above 140 °C, as evidenced by the rheological study, to give elastic materials. The tensile modulus of the copolymer increased when the processing temperature or boron incorporation ratio was increased, showing the increase in cross‐linking density. By the swelling test, the final cross‐linking density was comparable to the calculated value from the boron content. The cross‐linked polymer cannot be decross‐linked or reprocessed simply by heating, but heating in wet toluene completely solubilizes the polymer via the hydration of boroxine cross‐links. The reprocessed polymer still showed an elastic property, although the tensile modulus got lower than the original one. These results demonstrated the on‐demand reprocessability of boroxine‐cross‐linked polydienes.

4. Experimental Section

4.1. Materials and Methods

All operations were performed in a standard Schlenk technique under a nitrogen atmosphere. Solvents were purified using an organic solvent purifier (glass contour) or dried over molecular sieves 3A before use. Naphthalene‐1,8‐diaminoborane was synthesized according to the literature [31]. Isoprene and styrene were distilled over calcium hydride and stored in the refrigerator under nitrogen. Other materials were used as purchased.

NMR spectra were measured on a Varian system 400 or a system 500 NMR spectrometer. The obtained spectra were referenced to the signal of a residual trace of the partially protonated solvents [¹H: δ = 7.26 ppm (CHCl₃), solvents [¹³C: δ = 77.1 ppm (CDCl₃)]. The molecular weights of polymers were determined by gel permeation chromatography (GPC) on a Tosoh HLC‐8320 chromatograph (T = 40 °C; eluent: THF) calibrated with polystyrene standards (concentration of the injected polymers: 2–3 mg/mL; injection volume: 0.2 mL). To prevent the interaction between column and boronic acid, pinacol coexisted in the sample solution [32, 33]. Differential scanning calorimetry (DSC) measurement was performed using a SHIMADZU DSC‐60 system from −100 °C to 100 °C. The heating rate was 10 °C/min.

Sample sheets for mechanical and rheological property measurements with 1 mm thickness were prepared by pressing the polymer sample at 160 °C, 30 MPa for 20 min. Mechanical properties of the polymer samples were measured using an A&D RTC‐1210A tensile tester at 20 °C and a constant crosshead speed of 50 mm min⁻¹. Viscoelastic analysis was performed on an Anton Paar MCR302 rheometer using an 8 mm φ grid surface stainless‐steel parallel plate with a 1 mm distance between the grid surface plates. Temperature dependence was measured with a heating of 1.0 °C/min, with an applied strain (γ) of 0.1% and a frequency of 6.28 rad/s for linear viscoelastic measurements. A frequency sweep was performed in the range of 6.28 × 10⁻²–1.99 × 10² rad/s (36 plots for each data set) with an applied strain of 0.1%. The stress relaxation test was performed on a Metravib DMA+1000 dynamic mechanical analyzer with the initial strain of 1%.

4.2. Preparation of Monomer 2

This synthetic procedure is a direct installation method of B(dan) moieties on alkyl/aryl halides [34]. Under a nitrogen atmosphere, magnesium (1.0 g, 42 mmol) and THF (34 mL) were placed in a two‐necked round‐bottomed flask. To this was added 4‐bromostyrene (4.9 mL, 38 mmol) dropwise over 30 min at room temperature and then stirred for 1 h. The THF (4.0 mL) solution of naphthalene‐1,8‐diaminoborane (6.3 g, 38 mmol) was added to the prepared Grignard reagent and stirred for 3 h at room temperature. The reaction was quenched by the addition of water, and the aqueous phase was extracted with dichloromethane (20 mL × 3). The combined organic layer was dried over anhydrous sodium sulfate, and the solvent was removed in vacuo. Recrystallization of the crude product from boiling hexane gave 2‐(4‐vinylphenyl)‐2,3‐dihydro‐1H‐naphtho[1,8‐d,e][1,3,2]diazaborinine (2) as a colorless solid (6.0 g, 60%). The chemical shifts of the ¹H NMR spectrum were identical to those of previously reported [19].

4.3. Copolymerization of Isoprene and Monomer 1

The representative procedure for Table 1, entry 3 is described here. Isoprene (8.0 mL, 80 mmol), 4‐vinylphenylboronic acid (0.24 g, 1.6 mmol), dicumyl peroxide (containing ca. 60% CaCO₃, 53 mg, 4.0 µmol) were weighed in a 100 mL autoclave equipped with a magnetic stirring bar and dissolved in a mixture of THF (30 mL), and H₂O (3 mL). The mixture was heated to 110 °C and stirred for 132 h. The polymeric product was collected by precipitation into excess methanol containing 5 wt% of 2,6‐di‐tert‐butyl‐4‐phenol (BHT) and dried under vacuum overnight to give 1.0 g of white solid (18%). A similar procedure was applied for terpolymerization with styrene or acrylonitrile.

4.4. Calculation of Cross‐Link Density

In a 30 mL test tube, 100 mg of finely cut copolymer was charged, and 10 mL of toluene was added. After leaving it for 24 h at room temperature, the swollen polymer samples were collected and weighed. The cross‐link density was calculated by the difference of weights of the dried and swollen samples, using the Flory–Rhener equation for the three functional cross‐linked case shown in the following [35]:

$$\nu =-\frac{1}{V_{\mathrm{tol}}}\left[\frac{\ln \left(1-V_R\right)+V_R+\mu {V_R}^2}{V_R^{1/3}-2V_R/3}\right]$$

where V _R is reciprocal of the weight ratio of gel fraction (Φ _s), V _tol is molecular volume of toluene (1.06 × 10⁻⁴ m³ mol⁻¹) and μ is solvent‐polymer interaction term (0.476), which is calculated from experimental or the empirical Hildebrand solubility parameter (18.2 MPa^1/2 for toluene and 16.4 MPa^1/2 for polyisoprene [36, 37]).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

The file contains NMR spectra and DSC thermograms of the obtained copolymers and additional rheological data of the cross‐linked copolymer.

Supporting File: asia70572‐sup‐0001‐SuppMat.docx.

Kawamoto M., Kihara S., Dolui S., et al. “Synthesis of Boronic Acid‐Cross‐Linked Diene‐Based Polymers via Free‐Radical Copolymerization.” Chemistry – An Asian Journal 21, no. 2 (2026): e70572. 10.1002/asia.70572

Data Availability Statement

The data support the findings of this study are available in the supplementary material of this article.