Synthesis and characterization of well-defined thermo- and light-responsive diblock copolymers by atom transfer radical polymerization and click chemistry

Abstract

We synthesized a series of well-defined, stimuli-responsive diblock copolymers composed of poly(N-isopropylacrylamide) and poly(6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate) by atom transfer radical polymerization and click chemistry. The diblock copolymers were then characterized by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and gel permeation chromatography. When the azobenzene content reached 4.5 mol%, the diblock copolymer became insoluble in water. The aqueous solutions of the polymers exhibited a lower critical solution temperature (LCST) that depended on the amount of incorporated azobenzene. Higher LCSTs were observed after UV irradiation, with a maximum difference of 4.1 °C for the copolymer containing 1.4 mol% azobenzene groups. The photochemical properties of the polymer were also studied by UV-vis spectroscopy.

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1. Introduction

Block copolymers have drawn considerable attention because of their capacity for self-assembling. Self-assembled materials have many potential applications, both as dispersions and as solids, because of their ability to form versatile and functional morphology.¹ Block copolymer systems currently being developed have found numerous applications in biomedical devices,^2–5 biomaterials,^3,6,7 thermoplastic elastomers,⁸ fuel cells,^9,10 and electronics.^11,12

Poly(N-isopropylacrylamide), or PNIPAM, has been the most frequently studied thermosensitive polymer because of its lower critical solution temperature (LCST) of 32 °C, which is close to the temperature of the human body.^13,14 Investigations on the phase transition of PNIPAM have revealed that its macromolecules experience dehydration and collapse from a hydrated, extended coil to a hydrophobic globule; raising the temperature through the cloud point ultimately results in intermolecular aggregation.¹⁵ Functional PNIPAMs have been synthesized and combined with various hydrophobic polymer blocks including polystyrene^16,17 and tetrahydrofuran-protected 2-hydroxyethyl methacrylate,¹⁸ as well as alkyl groups,¹⁹ alkyl chain transfer agents,²⁰ and chromophores.¹⁵

Polymers are no longer responsive to only a single stimulus but show responsive behavior to multiple stimuli. Polymers that are responsive to light and temperature are of special importance. Several reports on thermo-responsive polymers, i.e., polymers containing a light-responsive azobenzene moiety, have been published.^21–29 In all these studies, appropriate azobenzene-containing monomers have been copolymerized with either NIPAM or N,N-dimethylacrylamide yielding light- and thermo-responsive copolymers. In such polymers, the polar cis form of azobenzene exhibits increased solubility in water compared with the relatively nonpolar trans form because a balance between polar and nonpolar moieties is important in LCST control.^21,30–32 Previous studies have also reported the synthesis of a polymer that contains NIPAM and azobenzene moieties by free-radical copolymerization^22,23,27 or polymer analogous reaction.²⁹ However, the diblock copolymers synthesized by these methods generally have higher polydispersity indexes. Click chemistry,³³ (Huisgen's 1,3-dipolar cycloaddition³⁴ between azido and alkynyl groups catalyzed by copper salts) introduced by Sharpless, has recently been proved a very powerful synthetic tool in polymer science. Numerous studies have demonstrated the applicability of this approach in the synthesis of block copolymers with linear-linear^35–42 macromolecular architecture. Atom transfer radical polymerization (ATRP) and click chemistry are employed in the synthesis of diblock copolymers with narrow polydispersity indexes and controlled macromolecular architecture. First, ATRP enables good control over the molecular weight of the polymer and end group, so that precisely defined reactive polymer structures are obtained. Second, click chemistry has been used extensively because of its quantitative yields, high tolerance of functional groups, and insensitivity of the reaction to solvents. Additionally, this method enables the production of copolymers from monomers that would otherwise be impossible to copolymerize.

In this study, we report the preparation of well-defined diblock copolymers containing NIPAM and azobenzene moieties by ATRP and click chemistry. These copolymers are responsive to light and temperature. In contrast to the copolymers produced in previous reports, those in the current work have lower polydispersity indexes (1.13–1.20). The LCST exhibited by the resultant polymers at different amounts of azobenzene is also investigated.

2. Experimental

2.1 Materials

N-Isopropylacrylamide (Aldrich, 98%) was recrystallized twice from a hexane/benzene mixture (3/2, v/v). Anisole (SCRC, 99%), as the solvent used in solution polymerization, was purified by distillation from sodium with benzophenone. CuCl (Aldrich, 99%) and CuBr (SCRC, 98.5%) catalysts were washed successively with acetic acid and ether, dried, and then stored under a nitrogen atmosphere. Tris(2-(dimethylamino)ethyl)amine (Me₆TREN) was synthesized from tris(2-amino) ethyl amine (TREN, Aldrich, 99%), according to reported methods.⁴³ Propargyl 2-chloropropionate (PCP) was synthesized from propargyl alcohol (SCRC, 99%), according to previous studies.⁴⁴ All other chemicals were obtained commercially and used as received, unless otherwise stated.

2.2 Characterization instruments

The ¹H nuclear magnetic resonance (NMR) spectra of monomers and polymers in CDCl₃ were obtained on a Varian 300 MHz FT-NMR spectrometer. Molecular weights (M_n) and polydispersity (M_w/M_n) were measured on a gel permeation chromatograph (GPC) using a Waters 510 pump and a Model 410 differential refractometer at 25 °C. THF was used as a mobile phase at a flow rate of 1.0 mL min⁻¹. The UV-vis absorption spectra of the polymers were determined on a Shimadzu-1240 spectrophotometer. Infrared spectra were obtained by measuring samples in KBr disks on a Shimadzu IR-8400S spectrometer.

2.3 Synthesis of 6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate (AzoMA)

The monomer, 6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate (AzoMA), was synthesized following a previous method (Scheme 1).⁴⁵¹H NMR(CDCl₃, ppm): 7.87 (m, 4H), 6.99 (m, 4H), 6.10 (s, 1H), 5.55 (s, 1H), 4.17 (t, 2H), 4.04 (t, 2H), 3.88 (s, 3H), 1.95 (s, 3H), 1.87–1.80 (m, 2H), 1.77–1.70(m, 2H), 1.58–1.45 (m, 4H).

Scheme 1 Synthetic route of the monomer AzoMA.

2.4 Synthesis of 2-azidoethyl 2-chloropropanoate (1)

A solution of 2-azidoethanol (871 mg, 10 mmol) and triethylamine (3.48 mL, 25 mmol) in Et₂O (10 mL) was cooled to 0 °C in an ice bath, after which 2-chloropropionyl chloride (1.90 g, 15 mmol) was added dropwise. After being stirred for 1 h at 0 °C, the solution was stirred further for 20 h at room temperature. The resultant white suspension was filtered, and the filtrate was washed three times with 50 mL of saturated aqueous NaHCO₃ solution. The organic layer was dried over Na₂SO₄, filtered, and then evaporated to obtain a yellow oily crude residue. The obtained oil was purified by flash chromatography (silica column, petroleum ether/Et₂O 20 : 1) to obtain a colorless oil (1.26 g, 71% yield). ¹H NMR (CDCl₃, ppm): 4.48 (q, 1H), 4.37 (t, 2H), 3.56 (t, 2H), 1.75 (d, 3H).

2.5 Synthesis of poly(6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate (PAzoMA) (2)

PAzoMA was synthesized by using a previously reported method (Scheme 2).⁴⁶ A mixture of CuBr (22 mg, 0.15 mmol) and N,N,N′,N′,N′-pentamethyldiethylenetriamine (PMDETA) (33 μL, 0.15 mmol) in anisole (0.8 mL) was placed on one side of an H-shaped glass ampoule and stirred at room temperature. AzoMA (300 mg, 0.75 mmol) and PCP (29 mg, 0.15 mmol) in anisole (1.5 mL) were placed on the other side of the ampoule. Nitrogen was bubbled through both mixtures for 5 min to remove any oxygen. Three freeze-pump-thaw cycles were then performed to degas the solution. Both mixtures were mixed and placed in an oil bath thermostated at 80 °C for 20 h. The polymerization was terminated by exposure to air. The reaction mixture was diluted with THF and passed through a neutral Al₂O₃ column using THF as eluent to remove the copper complex. After precipitation by adding the polymer solution of THF into methanol three times, yellow product 2 was collected by filtration and dried in a vacuum oven overnight (153 mg, 47% yield). M_n = 2100, M_w/M_n = 1.07.

Scheme 2 Synthesis of PAzoMA by ATRP.

2.6 Synthesis of N₃-PNIPAM (3)

Four precursors of end-functionalized PNIPAM containing the azide group (Table 1) were synthesized (Scheme 3). The procedure for N₃-PNIPAM production is described as follows. A mixture of CuCl (10.0 mg, 0.1 mmol) and Me₆TREN (24 μL, 0.1 mmol) in 1 : 1 (v/v) DMF/H₂O (1.0 mL) was placed on one side of an H-shaped glass ampoule and stirred at room temperature. NIPAM (1.13 g, 10 mmol) and 1 (17.8 mg, 0.1 mmol) in 1 : 1 (v/v) DMF/H₂O (1.5 mL) were placed on the other side of the ampoule. Nitrogen was bubbled through both mixtures for 5 min to remove any oxygen. Three freeze-pump-thaw cycles were then performed to degas the solution. Both mixtures were mixed and left to stand at 25 °C for 30 min. The polymerization was terminated by exposure to air. The reaction mixture was diluted with THF and passed through a neutral Al₂O₃ column using THF as eluent to remove the copper complex. After precipitation by adding the polymer solution of THF into hexane three times, white polymer 3a was collected by filtration and dried in a vacuum oven overnight (0.98 g, 87% yield). M_n = 10600, M_w/M_n = 1.13.

Table 1 Characteristics of PAzoMA and PNIPAM

	Sample	[M]0/[I]0	M n,GPC	PDIGPC^a
a Polydispersity index.
2	PAzoMA5	5	2100	1.07
3a	PNIPAM105	100	10600	1.13
3b	PNIPAM226	200	23400	1.09
3c	PNIPAM357	350	36000	1.12
3d	PNIPAM510	500	52000	1.11

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Scheme 3 Synthesis of PNIPAM by ATRP.

2.7 Synthesis of PAzoMA-b-PNIPAM by click chemistry

A Schlenk flask was charged with alkyne-functionalized 2 (51 mg, 0.024 mmol), 3a (127 mg, 0.012 mol), and CuBr (6.9 mg, 0.048 mmol). The flask was closed with a rubber septum, evacuated, then backfilled with nitrogen several times. PMDETA (10 μL, 0.048 mmol) and deoxygenated DMF (3 mL) were added using a nitrogen-purged syringe; the flask was degassed further by three freeze-pump-thaw cycles. The reaction mixture was stirred under a nitrogen atmosphere at 40 °C for 24 h. The mixture was diluted with THF and then passed through a neutral Al₂O₃ column using THF as eluent to remove copper salts. The solvent was evaporated and the resultant mixture was dialyzed in DMF using a cellophane tube (Spectra/Por 6 Membrane; MWCO:10000). Finally, the solvent was evaporated and a yellow product was obtained (130 mg, 85% yield). M_n = 13000, M_w/M_n = 1.22. The synthesis of PAzoMA-b-PNIPAM copolymer is shown in Scheme 4.

Scheme 4 Synthesis of PAzoMA-b-PNIPAM by click chemistry.

3. Results and discussion

3.1 Synthesis and characterization

To prepare a well-defined diblock polymer, synthesizing the two precursors at a narrow molecular weight distribution is necessary. These distributions are normally realized by the ATRP method. 2-Azidoethyl 2-chloropropanoate was used as the initiator for ATRP of NIPAM in DMF/H₂O solution at room temperature, yielding the azide group-ended PNIPAM. The M_n values of the four products were determined by GPC using polystyrene as the standard. The results are presented in Table 1. The data in the table show that all the samples had narrow molecular weight distributions in the range 1.09–1.13. Alkyne-functionalized PAzoMA was successfully prepared in anisole using the PCP/CuBr/PMDETA system at 80 °C for 20 h. Because of the high hydrophobicity of the azobenzene moiety, only diblock copolymers with a long hydrophilic NIPAM block and a short azo block (<10 monomer units) were soluble in pure water.

The click reaction between alkyne-functionalized PAzoMA and PNIPAM was carried out in DMF at 40 °C using CuBr/PMDETA complex as the catalytic system (Scheme 4). Excess alkyne-functionalized PAzoMA was used to facilitate the completion of the click reactions. Excess PAzoMA was removed by dialysis, and a yellow solid product was obtained.

The obtained block copolymers were characterized by ¹H NMR, GPC, and Fourier transform infrared (FT-IR) techniques, all of which can confirm the occurrence of the click reaction between the alkyne and azide groups. Fig. 1 shows the ¹H NMR spectra of PNIPAM with the azido end-group, PAzoMA with alkyne, and the resultant product. Fig. 1(A) shows that the methylene protons next to the alkyne group exhibited a signal at ∼4.6 ppm. This characteristic signal was not observed in the click product but a new signal at ∼5.3 ppm appeared in Fig. 1(C). This signal is attributed to methylene protons next to 1,2,3-triazole. Fig. 1(B) shows the ¹H NMR spectrum of PNIPAM. The signal of the methylene protons adjacent to the azido group at ∼3.5 ppm completely disappeared, as shown in Fig. 1(C).

Fig. 1 ¹H NMR spectra in CDCl₃ of the polymers PAzoMA, PNIPAM, and PAzoMA-b-PNIPAM.

The IR spectra reveal that the characteristic absorption band of the azide group at ∼2100 cm⁻¹ of PNIPAM was missing in the product (Fig. 2). A representative GPC trace showed a lower elution time for the block copolymer compared with the two precursors, indicating the increase in the molecular weight of the former (Fig. 3). However, the molecular weight distribution did not change appreciably. All the results indicate that the product was the targeted PNIPAM-b-PAzoMA with a triazole ring.

Fig. 2 FT-IR spectra obtained for (A) PAzoMA, (B) PNIPAM-N₃, and (C) PAzoMA-b-PNIPAM.

Fig. 3 GPC traces of precursors (2 and 3a) and the resultant product P1.

3.2 Thermo- and light-responsive characterization

Aqueous PNIPAM solutions show a LCST at 32 °C. This phenomenon has been attributed to the association of polymer molecules; this association creates large aggregates through intermolecular hydrogen bonds and non-polar bonds. When PNIPAM contains different amounts of azobenzene moieties, LCST changes. The isomerization of azobenzene always entails a change in the dipole moment of the molecular structure. Azobenzene groups have a dipole moment of 0 Debye in trans-configuration, whereas azobenzene molecules in cis-configuration have a dipole moment of 3 Debye;⁴⁷ this indicates that the LCST shifted during UV irradiation in the copolymers containing azobenzene groups. Because of the increased dipole moment of cis-azobenzene, the respective copolymers showed higher LCSTs for cis-azobenzene-containing polymers than copolymers containing trans-azobenzene.

The LCSTs of the copolymer solutions were determined by turbidimetry; the optical transmittance of a light beam (600 nm) through the sample cell in the photospectrometer was monitored as a function of temperature. The concentration of all the copolymer solutions was 0.5 mg mL⁻¹ and the heating rate was 1 °C min⁻¹. The cloud points were measured before and after irradiation with UV light at 365 nm. LCST is defined as the temperature at which 90% transmission was observed. The results are shown in Table 2. At 4.5 mol% azobenzene moiety, the polymer was insoluble in water. The LCSTs of the aqueous solutions of the copolymers were dependent on the content of azobenzene moieties. In all cases, higher LCST values were observed after irradiation with UV light. From the copolymer P2–P4, the highest LCST shift was observed (4.1 °C) for 1.4 mol% azobenzene (P3) compared with the LCST shift of 0.6 °C (2.2 mol% azobenzene) for P2 and 1.9 °C (0.97 mol% azobenzene) for P4. At an azobenzene content higher than 1.4 mol%, the temperature difference decreased. This phenomenon was previously described and reported by Irie and Kungwatchakun.²⁷ By incorporating azobenzene chromophores into the polymer, the hydrophobic interaction was induced, and the solubility of the polymer decreased. In one case, the azobenzene concentration range displaying a photoinduced LCST change was quite narrow; in another, the LCSTs of all the copolymers decreased with increasing azobenzene.

Table 2 Characteristics of PAzoMA-b-PNIPAM diblock copolymers

	Sample	M n,GPC	PDIGPC	Amount of azobenzene (mol%)	LCST before irradiation (°C)	LCST after irradiation (°C)	ΔLCST (°C)
P1	PAzoMA5-b- PNIPAM105	13000	1.22	4.5
P2	PAzoMA5-b- PNIPAM226	25800	1.15	2.2	19.4	20.0	0.6
P3	PAzoMA5-b- PNIPAM357	37800	1.17	1.4	23.6	27.7	4.1
P4	PAzoMA5-b- PNIPAM510	53500	1.18	0.97	27.5	29.4	1.9

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The UV-vis spectra of PAzoMA₅-b-PNIPAM₂₂₆ (P2) were recorded for aqueous solutions and films before and after UV exposure (365 nm, 8W) (Fig. 4). The polymer exhibited one strong absorption band and one weak band; the bands are related to the π–π* and n–π* transition bands of the trans- and cis-azobenzene, respectively. The spectra changed after different irradiation times. The change in the UV-vis absorption spectra of the P2 in aqueous solution is shown in Fig. 4(A). The intensity of the π–π* transition band at about 338 nm decreased, whereas that of the n–π* transition band at about 450 nm gradually increased with prolonged irradiation time. The photostationary state was realized after irradiation for 12 min. In the dark, the trans-azobenzene content of the polymer was recovered completely.

Fig. 4 (A) Spectral changes in P2 in aqueous solution during UV irradiation (365 nm, 8W). Irradiation time: (a) 0 min, (b) 2 min, (c) 6 min, (d) 8 min, and (e) 12 min. (B) Spectral changes in P2 on quartz glass under UV irradiation. Irradiation time: (a) 0 min, (b) 2 min, (c) 6 min, and (d) 9 min.

The P2 film was cast from the THF solution (1 wt%) onto a quartz surface, and dried at 50 °C in a vacuum oven to remove the solvent. When the P2 film was irradiated, the spectra on the film were similar to those in the aqueous solution; the intensity of the trans-isomer absorbance decreased, whereas that of the cis-isomer gradually increased. However, the absorbance at about 338 nm corresponding to the π–π* transition was blue-shifted on the solid film compared with the spectra in aqueous solution. In polar solvent, the π–π* absorption band of the aromatic compounds exhibited a red shift.

4. Conclusions

Well-defined diblock copolymers of NIPAM and azobenzene methacrylate monomer were successfully synthesized via ATRP and click chemistry. The copolymers exhibited thermo- and light-responsive properties as well as narrow polydispersity indexes. The copolymers P2–P4 exhibited low LCSTs in aqueous solution; the LCSTs strongly depended on the amount of incorporated azobenzene. Moreover, the reversible isomerization of the azobenzene groups in the copolymers influenced the LCST. After irradiation, the LCST values increased. In the temperature region between the LCST of the non-irradiated and the irradiated solutions, a change in the light-controlled reversible solubility was observed. The polymer P2 exhibited a typical trans-cis photoisomerization reaction under UV irradiation. In the film form, the trans-cis isomerization of the azobenzene chromophore resembled the light-responsiveness in solution. Various multifunctional polymers may be designed through the proposed synthetic strategy.

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