Oxidative coupling copolymerization for synthesis of poly(phenylene oxide) containing allyl groups in water

Abstract

Poly(2-allyl-6-methyl-1,4-phenylene-co-2,6-dimethyl-1,4-phenylene oxide)s (allyl-PPOs) were synthesized through oxidative coupling copolymerization of 2,6-dimethylphenol and 2-allyl-6-methylphenol in water. The copper(II)/ethylene diamine tetraacetic acid (Cu(II)/EDTA) complex catalyzed copolymerization was conducted in alkaline aqueous solution in an oxygen atmosphere. And low-molecular-weight allyl-PPOs with relatively narrow molecular weight distributions were obtained. FTIR, ¹H and ¹³C NMR spectroscopies were employed to identify the structure of allyl-PPOs. Crosslinking reactions of allyl-PPOs were carried out under UV treatment with the presence of a photo initiator. It is environmentally benign to synthesize allyl-PPO in aqueous solution. And the residual copper ions in the product obtained in water medium were much lower in number than those in the product synthesized in organic solvent. The dielectric constant and dissipation factor of the product produced in this way were very low.

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1. Introduction

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is an important engineering thermoplastic which is widely used for structural parts, electronics, household and automotive items, etc. PPO possesses a low dielectric constant (2.5) and a low dissipation factor (0.0007), which makes it useful in printed circuit boards. PPO was firstly developed through copper–pyridine complex catalyzed oxidative polymerization of 2,6-dimethylphenol in nitrobenzene solution by Hay in 1956 and commercialized by GE in 1960.^1–3 However, the glass transition temperature of PPO is inadequate for solder resistance and PPO is soluble in organic solvents such as aromatics and chlorinated aliphatics. The introduction of allyl group would be a good solution to improve the deficiencies mentioned above. Firstly, allyl modified PPO can maintain its excellent properties since the proportion of allyl group is very low. Besides, the solvent resistance of modified PPO after crosslinking reaction of allyl groups can be greatly improved. A thermosetting poly(aryl ether) with allyl groups on its side chains has been produced by Asahi-KASEI Co. However, the synthetic method included allylation with allyl halide and the lithiation of PPO, which is tedious and dangerous. Furthermore, since the multi-allylation in a PPO unit might easily happen during the process mentioned above, the structure of this kind of allylated PPO could not be characterized clearly.⁴

There have been reports about the synthesis of thermosetting PPO resins in organic solvent through oxidative coupling copolymerization of 2,6-dimethylphenol with 2-allyl-6-methylphenol.⁵ These allyl-PPOs showed excellent solvent resistance, thermal stability and dielectric properties. However, because of the homogeneous system, it is difficult to remove the impurities such as copper ions in the product and the electric properties would be depressed. Thus, the synthesis of the allyl-PPO with a relatively low content of residual Cu catalyst by oxidative coupling copolymerization of these two monomers in a heterogeneous system containing toluene and small amount of water has been reported, but the residual content of Cu ions need to be further reduced.⁶ Moreover, according to the methods mentioned above, allyl-PPO with molecular weight more than 10 000 was obtained, and the amount of organic solvent is quite a lot, which could not meet the demand of green chemistry. In addition, the high-molecular-weight allyl-PPO resin has some disadvantages such as high melt viscosity and poor processability. Thus, the development of a convenient method to prepare allyl-PPO with low molecular weight in an environmentally friendly solvent is essential.

To solve the problems mentioned above, an environmentally benign method was developed to produce low-molecular-weight crosslinkable allyl-PPO in the present paper. Water was used as the medium and copper–amine complex was employed as the catalyst. The impurities, such as catalyst, remained in water during isolation and a purer copolymer could be obtained resulting in good electrical properties. Crosslinking reaction of allyl-PPOs proceeded under UV treatment with photo initiator, thus solvent resistance and thermal stability of cured copolymers were improved.

2. Experimental

2.1 Materials

Analytically pure 2-allyl-6-methylphenol (AMP) and 2,6-dimethylphenol (DMP) were purchased from Aldrich. Acetonitrile, ethylene diamine tetraacetic acid (EDTA), analytically pure sodium chloride (NaCl) and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co. also supplied by Sinopharm Chemical Reagent Co. Analytically pure copper dichloride (CuCl₂·2H₂O) was supplied by Shanghai Zhenxin Chemistry Co. Chemically pure sodium n-dodecyl sulfate (SDS) was purchased from Shantou Chemistry Co. 2-Hydroxy-2-methyl-1-phenyl-1-propanone (UV1173) was obtained from Chengdu Geleixiya Chemical-tech Co.

2.2 Synthesis of poly(2-allyl-6-methyl-1,4-phenylene-co-2,6-dimethyl-1,4-phenylene oxide) in water

The following is a typical procedure for Cu(II)/EDTA complex catalyzed oxidative copolymerization of AMP and DMP in water. The solution (95 mL) containing monomers DMP (0.56 g, 4.5 mmol) and AMP (75 μL, 0.5 mmol), surfactant SDS (0.144 g, 0.5 mmol), and NaOH (0.20 g, 5.0 mmol) was added into a 200 mL flask. Then 5 mL of Cu(II)/EDTA complex solution (CuCl₂·2H₂O: 4.3 mg, 25 μmol; EDTA: 9.3 mg, 25 μmol) was added into the reactor and the mixture was vigorously stirred in the stream of oxygen at 50 °C for 24 h. After the polymerization, the product (0.46 g, M_n = 2800) was easily isolated by filtration after salting out with NaCl. The obtained product was extracted with acetonitrile for 48 h to eliminate byproduct (0.09%) 3,3′,5,5′-tetramethyl-4,4′-diphenoquinone (DPQ), and was determined to be allyl-PPO by FTIR, ¹H and ¹³C NMR.

2.3 UV treatment of copolymer

Crosslinking reaction of copolymer could be carried out by UV treatment after being dissolved in toluene in the presence of photo initiator UV1173 in nitrogen for 30 s.

The cured polymers were extracted with toluene for 48 h to eliminate the copolymer that did not get involved in the crosslinking reaction and the gel contents were determined by weighting the polymers before and after the extraction.

2.4 Characterizations

The content of DPQ was examined by a UV751GW UV-vis spectrophotometer (721, Shanghai Xinyi Instrument Co.). 15 mg of the copolymer without extraction was dissolved in 25 mL toluene, the absorbance (A) of the solution was tested by UV-vis spectrophotometer at 421 nm. The concentration of DPQ (c) in the solution was calculated according to Beer–Lambert's law (eqn (1)).

A = lg(I₀/I) = εcl (1)

here, I₀ represents the incident intensity, I represents the transmitted intensity, and l represents the thickness of the colorimetric utensil. The molar absorption coefficient (ε) of DPQ was calculated to be 6.5 × 10⁴ L mol⁻¹ cm⁻¹ according to Beer–Lambert's law.

The copolymer and copolymer with UV treatment were characterized with a NICOLET 5700 spectrometer (Thermo Electron Corporation, USA) to get their FTIR spectra. A sample of finely crushed KBr was used as the background.

¹H and ¹³C NMR spectrum (JEOL, GSX-270, Japan Electronics Co., Tokyo, Japan) were employed to identify the structure of the copolymer.

The molecular weight and polydispersity of copolymers were determined by gel permeation chromatography (GPC, Waters 1525/2414, Waters Instrument, Milford, Massachusetts) equipped with Waters Styragel HT4/HT3/HR1 columns and a refractive index detector at 30 °C. Toluene was used as the mobile phase and maintained at a flow rate of 1.0 mL min⁻¹. The molecular weight was calibrated with polystyrene standards.

The thermal properties of copolymers were determined by Pyris 1 thermal gravimetric analyzer (Perkin-Elmer Corporation, USA) in N₂ atmosphere. The samples were heated from room temperature to 800 °C at a rate of 10 °C min⁻¹.

The glass transition temperatures (T_g) of copolymers were measured by differential scanning calorimetry (DSC) measurements performed on a PE DSC-7 (Perkin-Elmer). The samples were heated from room temperature to 250 °C at a rate of 10 °C min⁻¹ in N₂ atmosphere.

The contents of residual copper ions in allyl-PPO were measured by atomic absorption spectroscopy (AAS) (HITACHI 180-50, San Jose, California) after dissolving the copolymers in 40% phenyl sulfonic acid.

To analyze the swelling degree of crosslinked copolymer, the sample was first dried at 60 °C for 12 h in vacuum after crosslinking reaction. The sample was weighed as W₀ after cooling down to 25 °C, then put into the solvent for 48 h, and weighed as W after drying the surface carefully with filter paper. The swelling degree (DS) was calculated as follows

DS (%) = (W/W₀ − 1) × 100%

here, W₀ and W represent weights of sample before and after solvent swelling, respectively. Chloroform and H₂O were used as the swelling solvent, respectively.

Electrical properties were measured by using broad band dielectric spectrometer Concept 40 (Novocontrol Technology GmbH & Co. KG Germany).

3. Results and discussion

3.1 Preparation of allyl-PPO in water

Although the oxidative coupling copolymerization of DMP with AMP to synthesize allyl-PPO was reported before,^5,6 the copolymerization of these two monomers in aqueous solution has not been investigated so far. On the other hand, the oxidative coupling polymerization of DMP in alkaline aqueous solution using copper complexes as catalyst has been studied before,^7–12 and PPO with lower molecular weight and narrow polydispersity was successfully synthesized which is acceptable from the viewpoint of green chemistry.

On the basis of these findings, the synthesis of allyl-PPO was carried out by oxidative coupling copolymerization of DMP and AMP with Cu(II)/EDTA as the catalyst in water (Scheme 1).

Scheme 1 Cu(II)/EDTA complex catalyzed oxidative copolymerization of DMP with AMP in water.

To make the amount of crosslinkable sites in copolymers in a reasonable value, the feeding ratios of DMP and AMP were set as 9/1 and 4/1 (mol/mol), respectively. The copolymerizations were carried out in mild conditions, producing copolymers with low molecular weight. The results are listed in Table 1. The polydispersity of product is narrower than the one synthesized using organic solvent.^5,6 Low-molecular-weight allyl-PPOs were successfully produced and Cu(II)/EDTA complex catalysts showed outstanding C–O/C–C selectivity.

Table 1 Oxidative copolymerization of DMP with AMP in water catalyzed by Cu(II)/EDTA complex^a

Feed rate (DMP/AMP mol)	[Cu(II)/EDTA] (μmol L^−1)	Mn	Mw/Mn	Yield (%)	DPQ (%)
a All the copolymerizations proceeded in water under O2 atmosphere at 50 °C for 24 hours ([Cu(II)] = 1 mmol L^−1, [DMP] + [AMP] = 0.05 mol L^−1, [SDS] = 0.005 mol L^−1, [NaOH] = 0.05 mol L^−1).
Copolymer 1 9/1	50	2400	2.11	64	0.10
	100	2600	2.10	68	0.11
	250	2800	2.20	73	0.09
	500	3200	2.17	81	0.08
	1000	3500	2.19	88	0.10
Copolymer 2 4/1	50	2200	2.33	70	0.09
	100	2400	2.00	73	0.08
	250	2500	2.04	78	0.14
	500	3100	2.07	83	0.10
	1000	3400	2.27	88	0.12

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The amount of Cu(II)/EDTA seriously affected the M_n and yields. It could be found that the molecular weights of copolymers increased with the increasing of catalyst concentration, indicating that higher catalyst concentration leads to the increasing of reaction rates. In addition, the usage of a small amount of catalyst is acceptable from the viewpoint of green chemistry.

3.2 Characterization of copolymer

Copolymers were identified as the expected ones using FTIR, ¹H and ¹³C NMR, respectively. Fig. 1 is the FTIR spectrum of allyl-PPO, the absorption at 1193 cm⁻¹ is ascribed to the ether bond of Ar–O–Ar, and the peaks at 990 and 914 cm⁻¹ are due to the plane bending of –CH=CH₂.

Fig. 1 FTIR spectrum of allyl-PPO.

The ¹H NMR spectrum (Fig. 2) of copolymer shows that the multiple signals at 4.99 (h) and 5.85 (g) ppm are assigned to the protons of CH₂=CH–, respectively. The signal at 2.08 (b and f) ppm is ascribed to hydrogens of –CH₃ and signal at 6.47 (a, c, and d) ppm is assigned to phenyl hydrogens. The small signals (i) around 7.1–7.2 ppm represented the phenyl hydrogens on the end the copolymer chains. All above suggests that allyl-PPO was successfully prepared. The ratio of two monomer units could be estimated from the integral values of signal at 2.1 ppm (b and f) and signal 4.99 ppm (h) in ¹H NMR spectrum, and showed a good agreement with the feeding ratio of two monomers. The signals and their corresponding carbons are showed in ¹³C NMR (Fig. 3), which also confirms that the allyl-PPO was successfully obtained.

Fig. 2 ¹H NMR of allyl-PPO (DMP/AMP = 4/1) in CDCl₃ at 20 °C.

Fig. 3 ¹³C NMR of allyl-PPO (DMP/AMP = 4/1) in CDCl₃ at 20 °C.

T_g values were determined by DSC measurement. The relationship between T_g and M_n at relatively low-molecular weight could be expressed with the equation proposed by Flory and Fox.^13–15

T_g = T_g,∞ − K/M_n (2)

here, T_g represents the glass transition temperature of the polymer with M_n, T_g,∞ is T_g of a polymer of infinite chain length (PPO: T_g,∞ = 490 K), and K is the constant related to the volume of chain ends (PPO: K = 12.73 × 10⁴ g K mol⁻¹).¹⁶

The T_gs measured by DSC are plotted in Fig. 4 with 1/M_n as the x-coordinate for allyl-PPO (DMP/AMP = 4/1), and the straight line which demonstrates how T_g is dependent on M_n according to Fox and Flory is also provided in Fig. 4. It can be seen clearly that T_g decreases as M_n decreases. The measured T_gs of copolymers with molecular weight ranging from 2000 to 3500 are well consistent with the ones calculated from eqn (2).

Fig. 4 T_g as function of 1/M_n and T_g measured by DSC.

3.3 Crosslinking reaction of allyl-PPOs

Crosslinking reactions of allyl-PPOs proceeded under UV treatment after dissolving in toluene in nitrogen. The crosslinking reaction could easily be carried out in the presence of UV1173 (3 wt%) by UV treatment for 30 s resulting in a gel content at about 96.4%. Although copolymers synthesized in water can dissolve easily in such as aromatics and chlorinated aliphatics, these crosslinked copolymers are insoluble due to the network structure. FTIR spectrum was used to track the decrease of absorption at 914 cm⁻¹ ascribed to –CH=CH₂ group, which indicated the crosslinking reaction of allyl-PPO. The vinyl groups were not completely consumed up due to the steric hindrance (Fig. 5).

Fig. 5 The consumption of –CH=CH₂ followed by FTIR spectrum.

The thermal properties of allyl-PPOs before and after crosslinking reaction were determined by TGA, which showed the T_d10% of the copolymer 1 (M_n = 3400) was about 380 °C, and after the crosslinking reaction the T_d10% of cured copolymer can reach 400 °C, indicating the better thermal stability. Furthermore, T_g of cured copolymers with the same crosslinking density (4/1) were all about 234 °C, showing that the molecular weight of copolymers has little influence on the T_g of cured copolymers (Scheme 2).^12,17

Scheme 2

The crosslinked copolymers were swelled in a good solvent—chloroform and a poor solvent—H₂O for 48 h, respectively, to estimate the swelling capacity of crosslinked allyl-PPO. As shown in Table 2, these crosslinked copolymers possessed good ability in solvent resistance and low water uptake amount. For Sample 2, the polymer network could only absorb 7.62% in chloroform and the water uptake amount of it could be as low as 0.16%.

Table 2 Swelling degrees of crosslinked copolymers in different solvents^a

DS (%)	Chloroform	H2O
a All the DS are measured at 25 °C.b Sample 1 is the crosslinking product of copolymer with DMP/AMP = 9/1, Mn = 3500.c Sample 2 is the crosslinking product of copolymer with DMP/AMP = 4/1, Mn = 3400.
Sample 1^b	11.34	0.21
Sample 2^c	7.62	0.16

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3.4 Cu ion content of allyl-PPO

The oxidative coupling copolymerization of DMP with AMP catalyzed by Cu(II)/EDTA complex in water medium is a successful method for the synthesis of allyl-PPO. Since the Cu(II)/EDTA catalyst can easily dissolve in aqueous solution, the catalyst could remain in the solution when the product is separated by precipitation,^7,9 thus the residual copper ions in the in the product could be greatly reduced. The residual contents of Cu ions in product obtained with different catalyst concentration are summarized in Table 3. The contents of residual copper in the copolymer synthesized in water is much lower than the ones synthesized in organic solution,⁷ such as SA 120, whose residual Cu content is 120 ppm. The results indicated that water medium is a better choice to produce low-molecular-weight allyl-PPO with high purity. Since the high-residual Cu content would negatively influence the dielectric properties of PPO when used in the field of high speed and frequency printed circuit board, it is suitable to synthesize allyl-PPO with outstanding dielectric properties in water.

Table 3 Residual Cu(II) content of allyl-PPOs synthesized in water^a

[Cu(II)/EDTA] (μmol L^−1)	Residual Cu content (ppm)
a Allyl-PPOs were dissolved in 40% phenyl sulfonic acid and residual Cu(II) contents were determined by AAS.
1000	1.16
500	0.93
250	0.77
100	0.63
50	0.56

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The obtained allyl-PPO showed good dielectric properties due to the reduction of copper ions. The Cu ion contents in copolymers were around 1 ppm, indicating that the contents of Cu ions in the copolymers should cause little influence when allyl-PPO obtained in this method is applied as an insulating material.^18–20 The ε and dissipation factor of copolymers at 10 GHz were measured as about 2.3 and 0.0016, respectively. As shown in Table 4, the Cu ion contents in copolymers obtained from water are low and close to each other, thus, the dielectric constant and dissipation factors are similar, respectively. And the copolymers synthesized in water possess smaller dielectric constants and dissipation factors compared to that in organic solution since their residual Cu contents are much lower. Therefore, the oxidative copolymerization using Cu(II)/EDTA as the catalyst in aqueous solution is a convenient way to prepare low-dielectric-loss crosslinkable PPO.

Table 4 Relationship between residual Cu content and dielectric property

Residual Cu content (ppm)	ε	Dissipation factor
120	2.5	0.001900
1.16	2.307	0.001612
0.93	2.298	0.001607
0.77	2.296	0.001598
0.63	2.295	0.001598
0.56	2.295	0.001598

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4. Conclusions

Low-molecular-weight crosslinkable allyl-PPOs have been synthesized by oxidative coupling copolymerization of DMP and AMP in water with Cu(II)/EDTA complex as the catalyst, producing copolymers with relatively narrow molecular weight distributions (M_w/M_n ≈ 2). The residual copper content of allyl-PPO synthesized in water was much lower than the ones synthesized in organic solvent, which result in excellent electrical property. Crosslinking reactions of allyl-PPOs were conducted by UV treatment and initiated by UV1173. Crosslinked copolymers possessed outstanding solvent resistance and thermal stability. Therefore, these materials with good processability and excellent thermal stability after crosslinking reaction would be of great use when applied in the field of high frequency and speed printed circuit board.

Acknowledgements

This study was supported by Natural Science Foundation of China (20974100).

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