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Synlett 2023; 34(20): 2503-2507
DOI: 10.1055/s-0042-1751454
cluster
Special Issue Dedicated to Prof. Hisashi Yamamoto

Smart Hydrogel Reactor of Poly(N-isopropylacrylamide)/Polyethylene Glycol Interpenetrating Polymer Networks for Oxidative Coupling of 2-Naphthol

Minghao Chen
,
Takumi Watanabe
,
Shigeki Habaue∗
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Dedicated to Professor Hisashi Yamamoto on the occasion of his 80th birthday

Abstract

Hydrogels with an interpenetrating polymer network (IPN) structure composed of poly(N-isopropylacrylamide) (poly-NIPAM) gel and a gel containing polyethylene glycol (PEG) chains were synthesized. They showed a typical temperature-responsive volume change in water owing to the constructed poly-NIPAM gel component. Oxidative coupling of 2-naphthol with IPN cryogels and a conventional catalyst, the CuCl2 complex of N,N,N′,N′-tetramethylenediamine, was conducted in water under an O2 atmosphere; the IPN gel prepared from PEG with a larger molecular weight of 11000 afforded a product with a good yield of 73% (91% conv.) during the reaction in basic media. The hydrogel effectively promoted the reaction but hardly produced any product without the catalyst, acting as a reactor vessel in the water. Owing to the low durability of the PEG gel component for hydrolysis, a limitation was also suggested during experiments on the recyclability of the hydrogel.


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Key words

interpenetrating polymer networks - hydrogel reactor - smart gel - catalysis - oxidative coupling

Hydrogels are hydrophilic polymers with three-dimensional network structures that are capable of holding large amounts of aqueous media without dissolving. These soft and versatile materials have good potential for catalyst support, reactors, drug release systems, wound dressings, actuators responding to various stimuli in the biomedical field, and adsorbents/absorbents in environmental applications.[1]

The crosslinked structure of hydrogels effectively restricts agglomeration to act as a template/nanoreactor for the in situ formation and stabilization of metal nanoparticles; thus, various hydrogel–metal nanocomposites containing Co, Ni, Cu, Pd, Ag, etc. have been prepared and used as catalysts for organic reactions in water.[2] [3] The immobilization of metal nanoparticles in a hydrogel facilitates workup and recycling of the catalyst via filtration or centrifugation and often improves the mechanical properties of the gels. Hydrogels with conventional randomly crosslinked network structures generally suffer from poor mechanical properties. Thus, an interpenetrating polymer network (IPN) structure, consisting of two or more polymer networks without covalent bonding, called full-IPN, is attractive because the corresponding IPN gel could have synergistic properties affected by each network and improved gel strength.[4] Therefore, metal-nanoparticle-containing hydrogels with a semi- or full-IPN structure, where a polymer network containing some linear or branched macromolecules is defined as semi-IPN, have also been employed as catalyst systems.[3]

The synthetic methods for full-IPN gels are roughly divided into sequential and simultaneous categories. In sequential construction, one network is swollen in a precursor solution of the other. The precursors are then transformed into their network. In the simultaneous method, the respective precursors of both networks are left to react simultaneously, forming IPNs. Compared to the former, the latter process is more restricted because two reactions producing respective networks must proceed without interfering with each other, but it is a simple and useful preparation.[5] Recently, we reported a novel simultaneous procedure that affords IPN gels, IPNPEG , composed of poly(N-isopropylacrylamide) (poly-NIPAM) and polyethylene glycol (PEG) networks (Scheme [1]).[6] During gelation, the radical polymerization of NIPAM with a crosslinker and the condensation of trialkoxysilyl [–Si(OR)3] groups that end-cap PEG (DS-PEG) to form a siloxane linkage[7] proceed simultaneously.

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Scheme 1 Preparation of IPNPEG from PEG. ICPTMS: 3-(Trimethoxysilyl)propyl isocyanate; BIS: N,N′-methylenebisacrylamide; AIBN: azobisisobutyronitrile

A hydrogel that can typically change in volume when exposed to external stimuli, such as pH, temperature, and chemicals, is called a smart hydrogel.[8] Poly-NIPAM hydrogel is a well-known thermo-responsive smart material with a volume phase transition temperature (VPTT) at approximately 32 °C; it can switch between the states of swollen below and shrunken above this temperature.[9] The synthesized IPNPEG demonstrated a gentle temperature-responsive volume change in water owing to the constructed poly-NIPAM gel component. In addition, a unique color-change response to chemical stimuli, such as CuCl2 and AgNO3 in water, was observed. Thus, it was a smart hydrogel with dual-responsive functionalities.[6]

Herein, we report preliminary results of a novel strategy for organic reactions in water and the function of the IPNPEG hydrogel as a reactor vessel/flask, which does not show any catalytic activity but effectively promotes an oxidative coupling reaction. The large surface area inside the hydrogel could provide organic conditions even in aqueous media. Consequently, this hydrogel may provide a useful and versatile alternative to conventional hydrogel composite catalysts. Many studies have been conducted on the oxidative coupling of 2-naphthol to afford 1,1′-binaphthalene-2,2′-diol (BINOL).[10] Among them, several procedures have been developed in water, where designed systems, such as catalysts supported on Al2O3, SiO2, or a biopolymer, are generally needed for the success of the oxidative coupling.[11] Thus, we focused on this reaction, employing typical copper complexes with a diamine, such as N,N,N′,N′-tetramethylenediamine (TMEDA), as the catalyst and molecular O2 as the oxidant (Scheme [2]).[12]

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Scheme 2 Oxidative coupling reaction of 2-naphthol in water in the presence of hydrogel

The synthetic procedure for IPNPEG has been previously reported.[6] Herein, for the synthesis of DS-PEG, PEG with a large molecular weight of 11000 (PEG11k) was used to produce gels with flexible network structures by decreasing the crosslinked points. Table [1] lists the yields of the IPNs synthesized by using DS-PEG11k , NIPAM, a crosslinker (BIS), and a radical initiator (AIBN). Furthermore, the preparation of the single-network poly-NIPAM gel (GNIPAM ) is presented in Table [1] (entry 1). Gelation was conducted under conditions with 0.90 g of NIPAM ([NIPAM]/[BIS]/[AIBN] = 100/4/1), while the feed ratio of DS-PEG was varied from 0.18 to 0.70 g. Opaque white hydrogels were effectively obtained after washing in excess water; they were dried overnight under reduced pressure at 80 °C or freeze-dried from water to prepare a cryogel (Figure S1 in the Supporting Information). The yields of IPNPEG11k increased with the increase in DS-PEG11k feed amounts. For comparison, gelation with DS-PEG2k (0.53 g), derived from PEG2k (molecular weight = 2000), produced 1.12 g of a tetrahydrofuran- and water-insoluble product (IPNPEG2k ).[6]

Table 1 Simultaneous Synthesis of IPN Gels with DS-PEG11k a

Entry

DS-PEG11k (g)

Gel abbreviation

Yield (g)b

1c

0

GNIPAM

0.84

2

0.18

IPNPEG11k1

0.96

3

0.35

IPNPEG11k2

1.26

4

0.53

IPNPEG11k3

1.41

5

0.70

IPNPEG11k4

1.65

a Reaction conditions: DS-PEG11k (35 wt%, THF), NIPAM (0.90 g), [NIPAM]/[BIS]/[AIBN] = 100/4/1, 60 °C, 48 h.

b The obtained gel was immersed in THF for two days and distilled water for two days.

c 24 h.

The structures of the prepared IPNPEG gels were determined by using infrared (IR) spectroscopy and scanning electron microscopy (SEM) (Figure S2 and Figure S3 in the Supporting Information). The IR spectra demonstrated the characteristic absorptions of poly-NIPAM, siloxane, ether, and urethane bonds; the absorption strength increased with the increase in DS-PEG feed amounts. The SEM images showed that the IPN cryogels have sponge-like porous structures with significantly different pore sizes between each other and that of GNIPAM .[6] Notably, the pore size was considerably smaller in IPNPEG11k3 and IPNPEG11k4 (Table [1], entries 4 and 5), relative to that of the other gels, such as GNIPAM and IPNPEG11k2 (entries 1 and 3). These observations indicate that the IPN structure was successfully constructed during gelation.

The temperature dependencies of the swelling behaviors are depicted in Figure [1]. The typical swelling degree (Q) was used as an evaluation index for the swelling property.[13] The GNIPAM hydrogel swells at 3 °C with a Q value of >12, which reaches <0.5 at 45 °C, along with a drastic shrinking in accordance with the VPTT. The IPNPEG hydrogels demonstrated a similar behavior to that of the GNIPAM hydrogel with temperature, depending on the poly-NIPAM gel component in the IPNPEG structure, indicating that they have a functional response to temperature with a volume change in water. In the lower temperature region, the Q values of the IPN hydrogels were smaller than that observed for GNIPAM owing to suppression of swelling with the double-network structure of the IPN hydrogels, whereas, at higher temperatures, they showed a tendency to have slightly higher Q values than GNIPAM . This is because PEG is water soluble but does not have a VPTT. The swelling property of IPNPEG11k3 hydrogel was more improved than that of IPNPEG2k , although these gels were prepared with the same feed ratio of DS-PEG. Because IPNPEG11k3 has a lower crosslinking density of the DS-PEG gel component, it is more flexible than IPNPEG2k . The Q values observed for the IPNPEG11k gels gradually decreased with the increase in the DS-PEG11k content of the feed.

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Figure 1 Temperature dependencies of swelling degree (Q) for (a) GNIPAM (Table [1], entry 1), (b) IPNPEG11k3 (entry 4), (c) IPNPEG2k ,[6] (d) IPNPEG11k1 (entry 2), (e) IPNPEG11k2 (entry 3), and (f) IPNPEG11k4 (entry 5).

The oxidative coupling of 2-naphthol with a conventional catalyst, CuCl(OH)–TMEDA (10 mol%),[12] in the organic solvent THF under an O2 atmosphere at 25 °C for 12 h was performed to produce BINOL in 80% yield, whereas the reaction in water under the same conditions resulted in a yield of 8%.[11b] [14] Table [2] summarizes the reaction results with various cryogels in an aqueous medium.[15] The GNIPAM hydrogel did not show any effect for the reaction with CuCl2 in water (entry 1) because almost the same result was observed as for that without any additive (3% yield). When IPNPEG11k3 was used, the yield increased to 15% (entry 2), suggesting that the IPN structure has some effect on the reaction in water. During the reactions with TMEDA as a ligand, IPNPEG11k3 produced BINOL with an improved yield of 27%; in contrast, the IPNPEG2k hydrogel reduced the yield to 8% (entries 3 and 4) owing to the better hydrophilicity and flexibility of IPNPEG11k3 relative to those of the IPNPEG2k hydrogel.

Table 2 Oxidative Coupling of 2-Naphthol in Aqueous Media with Hydrogelsa

Entry

Cu catalyst

Gel, quantity (mg)b

Solvent

Yield [conv.] (%)c

1

CuCl2

GNIPAM , 30

water

 3

2

CuCl2

IPNPEG11k3, 30

water

15 [58]

3

CuCl2–TMEDA

IPNPEG2k , 30

water

 8 [63]

4

CuCl2–TMEDA

IPNPEG11k3, 30

water

27 [66]

5

CuCl2–TMEDA

IPNPEG11k3, 99

water

42 [72]

6

none

IPNPEG11k3, 92d

water

 2

7

CuCl2–TMEDA

IPNPEG11k3, 90d

water

40 [70]

8

CuCl2–TMEDAe

IPNPEG11k3, 102

NaOH aq.f

73 [91]

9g

CuCl2–TMEDAe

IPNPEG11k3, 100

NaOH aq.f

60 [93]

a Reaction conditions: 2-Naphthol (182 mg), Cu catalyst (10 mol%), [CuCl2]/[TMEDA] = 1/1, 25 °C, 48 h, O2 atmosphere.

b Cryogels were used.

c Yield of isolated product.

d Recovered gel (entry 5) was used.

e 20 mol%.

f 0.1 mol/L.

g 0 °C, 72 h.

When the amount of hydrogel used was increased, the reaction afforded the product in 42% yield (entry 5). To examine the role of the hydrogel, it was recovered after the reaction and reused without addition of CuCl2 and TMEDA (entry 6). After being washed, the recovered gel was slightly colored (Figure S1), suggesting the presence of a trace amount of residual copper in the recovered hydrogel. However, reuse of the recovered IPN gel demonstrated that it holds hardly any copper catalyst because the reaction resulted in a poor yield. Reuse of the recovered hydrogel with fresh copper catalyst gave the product with a successful yield (entry 7). The experiments also showed the recyclability of the gel (Figure S4), although it has a limitation, which will be discussed later. These results indicate that the IPNPEG11k hydrogel acts as a reactor vessel that promotes the reaction in water without itself having any catalyst activity.

The coupling reaction with catalyst (20 mol%) was performed under basic conditions to improve the solubility of 2-naphthol in water (entries 8 and 9). In the presence of IPNPEG11k3 gel, BINOL was obtained in 73% yield during the reaction, whereas the reaction in the absence of the gel resulted in a poor yield of 5%. The recyclability of the hydrogel was examined under the same conditions as the reaction shown in entry 8 of Table [2], and the recovered gel was used along with additional CuCl2 and TMEDA (Figure [2]). The gel showed good yields (69% and 66%) for at least two cycles. However, the results were accompanied by negligible reductions in the gel recovery, which were 96 mg/102 mg after the first cycle and 90 mg/96 mg after the second. The gel, especially the DS-PEG gel component, can suffer basic hydrolysis. The IR spectrum of the gel after three reaction cycles showed a reduction in the absorption intensities of the urethane and ether bonds in comparison with those of the as-synthesized IPNPEG11k3 (Figure S5). Therefore, some decomposition appears to occur during the reaction in basic media, most likely owing to hydrolysis of the urethane and crosslinked siloxane bonds.

Zoom Image
Figure 2 Results of oxidative coupling with recycled IPN gel (Table [2], entry 8) in NaOH aq. (25 °C, 48 h, O2 atmosphere)

In summary, we have introduced a novel hydrogel reactor with an IPN structure composed of poly-NIPAM and PEG networks for the oxidative coupling of 2-naphthol under an O2 atmosphere. The hydrogel prepared from PEG with a molecular weight of 11000 showed better functionality as a reactor and smart thermo-responsiveness with a volume change than those of the gel derived from PEG2k. The developed hydrogel reactor showed no catalyst activity but effectively promoted the coupling reaction in collaboration with a conventional copper catalyst, although a limitation in the recyclability of the hydrogel in basic media was revealed. Further investigations into the smart IPN hydrogel reactor and its functions are underway.


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Conflict of Interest

The authors declare no conflict of interest.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/s-0042-1751454. Figures S1–S5 are included.
  • Supporting Information

Corresponding Author

Shigeki Habaue
Department of Applied Chemistry, College of Engineering, Chubu University
1200 Matsumoto-cho, Kasugai, Aichi 487-8501
Japan   

Publication History

Received: 20 March 2023

Accepted after revision: 11 April 2023

Article published online:
30 May 2023

© 2023. Thieme. All rights reserved

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Zoom Image
Scheme 1 Preparation of IPNPEG from PEG. ICPTMS: 3-(Trimethoxysilyl)propyl isocyanate; BIS: N,N′-methylenebisacrylamide; AIBN: azobisisobutyronitrile
Zoom Image
Scheme 2 Oxidative coupling reaction of 2-naphthol in water in the presence of hydrogel
Zoom Image
Figure 1 Temperature dependencies of swelling degree (Q) for (a) GNIPAM (Table [1], entry 1), (b) IPNPEG11k3 (entry 4), (c) IPNPEG2k ,[6] (d) IPNPEG11k1 (entry 2), (e) IPNPEG11k2 (entry 3), and (f) IPNPEG11k4 (entry 5).
Zoom Image
Figure 2 Results of oxidative coupling with recycled IPN gel (Table [2], entry 8) in NaOH aq. (25 °C, 48 h, O2 atmosphere)