Journal of Polymers
Volume 2015 (2015), Article ID 478729, 6 pages
http://dx.doi.org/10.1155/2015/478729

Research Article

Oxidative Coupling Copolymerization of 2,6-Dimethylphenol and Dihydroxynaphthalene Affording Poly(phenylene oxide) Derivatives

Shigeki Habaue, Ryosuke Ito, Ken Okumura, and Yuki Takamushi

Department of Applied Chemistry, College of Engineering, Chubu University, Kasugai, Aichi 487-8501, Japan

Received 19 May 2015; Revised 3 July 2015; Accepted 7 July 2015

Academic Editor: Anjanapura V. Raghu

Copyright © 2015 Shigeki Habaue et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The oxidative coupling copolymerization between 2,6-dimethylphenol (DMP) and dihydroxynaphthalenes, such as the 2,3- and 2,6-dihydroxynaphthalenes (2,3- and 2,6-DHN), was carried out using a Cu catalyst under an O₂ atmosphere. The polymerization was significantly affected by the structure of the comonomer, and the copolymers were efficiently obtained during the copolymerization of DMP and 2,6-DHN. The obtained copolymers containing hydroxyl groups of the DHN unit were further transformed into polymers containing ester and urethane groups. The obtained copolymers containing various functional groups showed a very different solubility from that of the homopolymer of DMP.

1. Introduction

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is one of the most widely accepted engineering plastics because of its good thermal, mechanical, and electrical characteristics. In addition, the method used for synthesizing PPO, the oxidative coupling polymerization (OCP) of 2,6-dimethylphenol (DMP), is known to be an environmentally benign system due to the fact that the reaction proceeds under mild conditions to produce only water as the by-product. Meanwhile, the OCP mediates the free radical coupling process between highly active phenoxy radicals; consequently, it is very difficult to control the reaction; for instance, the copolymerization leading to PPO derivatives is restricted to some extent [1–3]. Although a typical strategy for the functionalization of PPO should be the use of polymer reactions, such as the bromination of methyl side groups and further transformations of bromosubstituted polymers [3–5], there are few reports about the copolymerization between DMP and other phenolic compounds.

Recently, we reported that the oxidative coupling copolymerization between DMP and 2-hydroxymethyl-6-methylphenol, having a DMP framework with a hydroxyl group, efficiently proceeds to afford a copolymer containing hydroxyl groups [6]. Although this could offer a secure method leading to the functionalized PPO, it is imperative that the comonomer be prepared from o-cresol [7, 8]. Accordingly, the copolymerization between DMP and commercially available phenolic compounds with functional groups is a very attractive topic. On the other hand, the polymers with a rigid backbone often show poor solubility and difficulty in processing, which have been improved by introducing novel segments into the main chain [9–12].

It is known that the OCP of dihydroxynaphthalenes, such as the 2,3- and 2,6-dihydroxynaphthalenes (DHNs), produces polymers, that is, poly(2,3-dihydroxy-1,4-naphthylene) [13–15] and poly(2,6-dihydroxy-1,5-naphthylene) [16–19], respectively. The reaction process is based on the catalytic oxidative coupling of 2-naphthols, which is one of the most facile methods for constructing a 1,1′-bi-2-binaphthol structure [20–22].

In this study, the copolymerization of DMP with the commercial dihydroxynaphthalenes, 2,3-DHN and 2,6-DHN, using a copper catalyst in an O₂ atmosphere was investigated (Scheme 1).

Scheme 1: Oxidative coupling copolymerization between DMP and DHN.

2. Experimental

2.1. General

The ¹H NMR spectra were taken by a JEOL JNM-ECS400 (400 MHz for ¹H) spectrometer. The infrared (IR) spectra were recorded using a Shimadzu FTIR-8700 spectrometer. The size exclusion chromatographic (SEC) analyses were conducted using a Jasco PU-2080 Plus system equipped with a UV (Jasco UV-2075 Plus) detector and Shodex KF-803L and KF-806L columns for CHCl₃ or Tosoh TSK-G7000H and TSK-G3000H columns for tetrahydrofuran (THF) connected in series (temp. = 25°C, flow rate = 0.8 mL/min). Calibration was carried out with polystyrene standards. The UV-vis absorption (UV) spectra were recorded by a V-630 spectrophotometer (Jasco). The thermogravimetric (TG) analyses were performed using a Rigaku Thermo-plus EVO2 apparatus (heating rate: 10°C/min).

2.2. Material

The monomers DMP (Kanto), 2,3-DHN (Aldrich), 2,6-DHN (Wako), and 2,7-DHN (Wako), the copper catalyst di--hydroxo-bis[(N,N,N ′,N ′-tetramethylethylenediamine)copper(II)] chloride [CuCl(OH)-TMEDA, TCI] [23, 24], and the ligand bipyridyl (BPy, Wako) were used as received. The dihydroxynaphthalene derivative, methyl 2,3-dihydroxynaphthalene-2-carboxylate (MDHNA), was prepared from the commercially available acid (TCI) by esterification with methanol and sulfuric acid (Scheme 2) [15]. For further transformation of the obtained PPO copolymer, phenylacetyl chloride (Wako) and butyl isocyanate (TCI) [6] were employed.

Scheme 2: DHN monomers used for copolymerization.

2.3. Typical Procedure for Oxidative Coupling Copolymerization of DMP [6]

The copper catalyst was added to a mixture of DMP and a comonomer ([monomers]/[Cu] = 10/1, total amounts of the monomers: 0.8 mmol, CuCl(OH)-TMEDA: 18 mg) in THF (2 mL), and the mixture was stirred at 25°C for 24 h in an O₂ atmosphere. The solvent was evaporated, and dichloromethane (DCM, 2 mL) and excess amounts of phenylacetyl chloride (approximately 6 equivalent of the comonomer used) and pyridine were added. After stirring under an N₂ atmosphere for 24 h, the reaction was quenched with a mixture of MeOH-aqueous 2 mol/L of HCl (9/1, v/v, 50 mL). The polymer was isolated as the MeOH-insoluble fraction by centrifugation, then repeatedly washed with methanol, and finally dried in vacuo.

3. Results and Discussion

The phenolic compounds, DMP, and the 2,3- and 2,6-DHNs, were homopolymerized with two types of conventional copper catalysts, such as CuCl(OH)-TMEDA and CuCl-BPy, in a DCM-MeOH (7/1, v/v) mixed solvent [6] under an oxygen atmosphere. To isolate the products as the MeOH-insoluble part for polymerization of the dihydroxynaphthalenes, the polymer was further acetylated using acetyl chloride and pyridine, as previously reported [9–11]. The results are summarized in Table 1.

Table 1: Homopolymerization of DMP or DHNs.

Run
Monomer
Catalyst
Yield

(%)a
(/)b

1
DMP
CuCl-BPyc,d
70
19.1 (2.5)

2
DMP
CuCl(OH)-TMEDA
77
72.6 (4.9)

3
2,3-DHN
CuCl-BPyc
28e
17.0 (1.3)

4
2,3-DHN
CuCl(OH)-TMEDA
97e
6.4 (1.6)

5
2,6-DHN
CuCl-BPyc,d
88e
—f

6
2,6-DHN
CuCl(OH)-TMEDAd
33e
—f

Conditions: [monomer]/[Cu] = 10/1, solvent = DCM-MeOH (7/1, v/v), time = 48 h, temp. = r.t., and O₂ atmosphere.

aMeOH-insoluble part.

bDetermined by SEC (THF or CHCl₃, polystyrene standard).

c[Monomer]/[Cu] = 5/1.

dPolymerization time = 24 h.

eMeOH-insoluble fraction of acetylated products.

fInsoluble in THF and CHCl₃.

The polymerization of DMP gave a polymer in a good yield (runs 1 and 2), whereas the catalyst system affected the polymer yield during the polymerization of the dihydroxynaphthalenes. The methanol-insoluble polymer was almost quantitatively obtained in 97% yield by the polymerization of 2,3-DHN with the TMEDA catalyst (run 4). The polymerization of 2,6-DHN with CuCl-BPy afforded a polymer, which is insoluble in the common organic solvents, such as THF and CHCl₃, in the high yield of 88% (run 5).

Table 2 lists the results of the copolymerization between DMP and the dihydroxynaphthalenes in THF under various conditions. The unit ratio of the obtained polymer was determined from the ¹H NMR spectra of the peak intensity ratio of the methyl protons around 2 ppm for the DMP unit and methylene protons of phenylacetyl group around 3.5 ppm for DHN unit. The BPy catalyst again resulted in a poor yield during the polymerization of 2,3-DHN (run 1). During the copolymerization with 2,3-DHN of various feed ratios, the copolymers, which mainly consisted of the DHN unit, were produced in low yields (runs 1–4). The polymer yields increased with an increase in the feed ratio of 2,3-DHN. Although the polymer was obtained in 54% yield during the copolymerization with the feed ratio of 30 : 70 (DMP : 2,3-DHN), the unit ratio was estimated to be 2 : 98 (run 4).

Table 2: Copolymerization between DMP and DHNs.

Run
Comonomer
Catalyst
Feed ratio
Yield
Unit ratio

DMP : DHN
(%)a
DMP : DHNb
()c

1
2,3-DHN
CuCl-BPy
50 : 50
11
9 : 91
3.6 (1.2)

2
2,3-DHN
CuCl(OH)-TMEDA
50 : 50
38
5 : 95
4.5 (1.3)

3
2,3-DHN
CuCl(OH)-TMEDA
70 : 30
48
12 : 88
9.5 (1.3)

4
2,3-DHN
CuCl(OH)-TMEDA
30 : 70
54
2 : 98
9.9 (1.3)

5
2,6-DHN
CuCl-BPy
50 : 50
66
17 : 83
4.7 (1.3)

6
2,6-DHN
CuCl(OH)-TMEDAd
50 : 50
29
47 : 53
9.0 (1.3)

7
2,6-DHN
CuCl(OH)-TMEDAe
50 : 50
50
50 : 50
10.3 (1.3)

8
2,6-DHN
CuCl(OH)-TMEDA
50 : 50
91
46 : 54
15.2 (2.4)

9
2,6-DHN
CuCl(OH)-TMEDA
70 : 30
50
68 : 32
10.4 (1.2)

10
2,6-DHN
CuCl(OH)-TMEDA
30 : 70
68
31 : 69
8.8 (1.2)

Conditions: [monomers]/[Cu] = 10/1, solvent = THF, time = 24 h, temp. = r.t., and O₂ atmosphere.

aMeOH-insoluble part of the products after phenylacetylation.

bDetermined by ¹H NMR analysis (CDCl₃).

cDetermined by SEC (THF, polystyrene standard).

dPolymerization time = 3 h.

ePolymerization time = 18 h.

The copolymerization with 2,6-DHN (50 : 50, feed ratio) and the BPy catalyst afforded a polymer mainly consisting of the DHN unit (DMP : DHN = 17 : 83) (run 5); in contrast, the TMEDA catalyst system produced a copolymer in the high yield of 91% with the unit ratio of DMP : DHN = 46 : 54 (run 8). The obtained polymer was fully soluble in THF and CHCl₃. The catalyst system significantly affected the copolymerization with the dihydroxynaphthalenes.

The copolymerizations (50 : 50 feed ratio) with CuCl(OH)-TMEDA were performed for the indicated times of 3 and 18 h (runs 6 and 7). The obtained polymers again showed unit ratios, whose values are almost consistent with that observed for the copolymer prepared by the 24 h polymerization. In addition, the yield and the molecular weight increased with the increasing polymerization time. Accordingly, the copolymerization effectively proceeds through the entire process. Furthermore, the copolymerizations with various feed ratios were conducted, and the observed unit ratios of the obtained copolymers were in good agreement with those in the feed (runs 9 and 10). These results again indicate that the copolymerization successfully takes place during the polymerization using the monomers, DMP and 2,6-DHN, and the catalyst of CuCl(OH)-TMEDA.

Table 3 summarizes the results of the copolymerization between DMP and various 2-naphthol derivatives with the CuCl(OH)-TMEDA catalyst. The monomer, 6-methoxy-2-naphthol (MeDHN) (Scheme 2), which was prepared from 2,6-DHN by the monomethylation with MeI and K₂CO₃, was copolymerized with DMP (runs 1 and 2). The copolymerizations resulted in poor yields, and the methanol-insoluble part obtained by the reaction with the feed ratio of 70 : 30 (DMP : MeDHN) was mainly composed of the DMP unit (96 : 4). This result suggests that both β-hydroxyl groups of 2,6-DHN should contribute to bond formation during the OCP. The copolymerization with 2,7-DHN (50 : 50 feed ratio) was carried out to produce a polymer in a low yield with the DMP unit of 87% (run 3). The ester having a 2,6-DHN structure, MDHNA, afforded a polymer in 96% yield during the copolymerization, and the unit ratio was evaluated to be 58 : 42 when the feed ratio was 70 : 30 (run 4). Although the details of the copolymer structure are not clear at present, the framework of 2,6-DHN plays an important role in constructing the PPO derivatives during the OCP.

Table 3: Copolymerization between DMP and DHNs with CuCl(OH)-TMEDA.

Run
Comonomer
Feed ratio
Yield
Unit ratio

DMP : DHN derivative
(%)a
DMP : DHNb
()c

1
MeDHN
50 : 50
6
—
—

2
MeDHN
70 : 30
11
96 : 4
—

3
2,7-DHN
50 : 50
34
87 : 13
10.0 (2.0)

4
MDHNAd
70 : 30
96
58 : 42
10.8 (1.6)

Conditions: [monomers]/[Cu] = 10/1, solvent = THF, time = 24 h, temp. = r.t., and O₂ atmosphere.

aMeOH-insoluble part of the products after phenylacetylation.

bDetermined by ¹H NMR analysis (CDCl₃).

cDetermined by SEC (THF, polystyrene standard).

dSolvent = DCM-MeOH (7/1, v/v).

Transformations of the naphthol groups of poly(DMP/2,6-DHN) were carried out (Scheme 3). The unprotected poly(DMP/2,6-DHN) was prepared by the copolymerization with CuCl(OH)-TMEDA with the feed ratio of DMP : 2,6-DHN = 90 : 10, and after washing with methanol, the polymer was collected in 50% yield. The copolymer was treated with an excess amount of phenylacetyl chloride and pyridine [poly(DMP/2,6-DHN)-COBn] or an excess amount of butyl isocyanate in the presence of the catalyst, dibutyltin dilaurate, at room temperature for 24 h [poly(DMP/2,6-DHN)-CONH-Bu] [6]. The former was isolated by washing with MeOH, and the unit ratio was determined to be 86 : 14 (, ). The latter was collected as the hexane-insoluble part, because it was soluble in MeOH. The ¹H NMR spectrum of the obtained polymer is shown in Figure 1. The peaks are assigned as shown in the figure, and the unit ratio was estimated to be 87 : 13 (, ), which is identical to the value for the former.

Scheme 3: Transformations of the obtained copolymer.

Figure 1: ¹H NMR spectrum of poly(DMP/2,6-DHN)-CONHn-Bu (CDCl₃, 50°C).

Figure 2 depicts the IR spectra of poly(DMP) (PPO) and poly(DMP/2,6-DHN)-COBn (Table 2, run 9). The absorption for the introduced carbonyl group clearly appeared at 1751 cm⁻¹. The TG analyses were also conducted for these polymers. The thermal stability of the copolymer (the latter) significantly decreased in comparison with that of the homopolymer (the former), and the 5% weight-loss temperature was estimated to be 382 for the former and 237°C for the latter.

Figure 2: IR spectra of (a) poly(DMP) and (b) poly(DMP/2,6-DHN)-COBn (KBr).

The UV absorption spectra of poly(DMP) and poly(DMP/2,6-DHN)s in CHCl₃ are shown in Figure 3. The onset positions of the copolymers are at a much longer wavelength than that of the homopolymer of DMP, indicating that the copolymers have a higher extended π-conjugation than PPO. In addition, the 2,6-DHN unit ratio in the copolymer significantly affected the π-conjugation.

Figure 3: UV spectra of the obtained polymers: (a) poly(DMP) ( M), (b) poly(DMP/2,6-DHN) (unit ratio, DMP : 2,6-DHN = 84 : 16) ( M), and (c) poly(DMP/2,6-DHN) (unit ratio, DMP : 2,6-DHN = 60 : 40) ( M) (CHCl₃).

The solubility of the obtained polymers was examined and the results are summarized in Table 4. The DMP homopolymer, PPO, is soluble in chloroform, whereas the copolymer with 2,6-DHN and the functionalized ones, poly(DMP/2,6-DHN)-COBn and poly(DMP/2,6-DHN)-CONHn-Bu, are soluble in THF as well as chloroform. In addition, poly(DMP/2,6-DHN)-CONHn-Bu has a solubility in methanol, as already mentioned. The copolymer bearing naphthol groups, poly(DMP/2,6-DHN), showed a solubility in aqueous alkaline ethanol. The copolymer and its derivatives demonstrated quite different solubilities from that of poly(DMP) [9, 12].

Table 4: Solubility of the obtained copolymers.

Solvent
Poly(DMP/2,6-DHN)
Poly(DMP)b

(-OH)a
(-OCOBn)
(-OCONHn-Bu)

THF
S
S
S
I

CHCl₃
S
S
S
S

MeOH
I
I
S
I

HFIP
S
—
—
I

EtOH
+ NaOH aq.c
S
—
—
I

I: insoluble; S: soluble.

aDMP unit : DHN unit = 51 : 49. = 7.26 × 10⁴, / = 4.9 (measured by SEC).

cEtOH/0.1 M NaOH aq. = 10/1.

4. Conclusion

The oxidative coupling copolymerization of DMP and dihydroxynaphthalenes was carried out, and the preparation of PPO with functional groups was successfully achieved. The copolymerization was affected by the catalyst and the comonomer structure and produced the copolymer, especially, by using 2,6-dihydroxynaphthalene in an effective manner. The PPO derivatives obtained by the copolymerization and further transformations showed significantly different characteristics in solubility from that of the original PPO.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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