Synthesis and characterization of coumarin-containing polyphosphazene

Reactive & Functional Polymers 66 (2006) 455–464

REACTIVE & FUNCTIONAL POLYMERS www.elsevier.com/locate/react

Synthesis and characterization of coumarin-containing polyphosphazene Cheng-mei Liu *, Jin-jun Qiu, Rui Bao, Chen Zhao, Xiao-ju Cheng, Yan Xu, Yun Zhou Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, PR China Received 25 April 2005; received in revised form 15 September 2005; accepted 15 September 2005 Available online 23 November 2005

Abstract Novel coumarin-containing poly(organophosphazene) was prepared and its photodimerization characteristics were studied in this paper. Coumarin-containing polymer was synthesized by polymeric substitution reaction of highly reactive poly(dichlorophosphazene) with sodium salt of 7-(2-hydroxyethoxy)-4-methyl-coumarin. Chemical structure of polymer was characterized with FT-IR, NMR, GPC and elemental analysis. The number average molecular weight is 2.16 · 105 and the polydispersity index was 4.45 and Tg of 67.0 C. The polymer was stable up to 280 C. Under UV irradiation, the coumarin rings underwent [2 + 2] cycloadditional reaction to form insoluble curing film. The percentage gel content of irradiated polymer film was increased with increasing irradiation power and time. Upon irradiation with power of 97.2 mW/cm2, 96.7% gel content was reached in 30 mim.  2005 Elsevier B.V. All rights reserved. Keywords: Photosensitive; Polyphosphazene; Ring-opening polymerization; 7-(2-Hydroxyethoxy)-4-methylcoumarin; Characterization

1. Introduction Since the pioneer work on soluble polyphosphazene done by Allcock [1], this kind of inorganic polymer have been attracted more and more attention from 1960s [2]. Its chemical, irradiative and oxidative stability make it valuable in applications such as fuel battery [3,4], polymeric electrolytes [5,6], fire retardant materials [7,8] and other fields. There are two types of polyphosphazenes were widely studied, * Corresponding author. Tel.: +86 27 87544831; fax: +86 27 87543632. E-mail address: [email protected] (C.-m. Liu).

one was reported by Allcock [1] in which all substituents were attached to the phosphorus via phosphorus–oxygen linkage, another was reported by Neilson [9] in which all substituents were attached to phosphorus via direct phosphorus–carbon linkage. In order to reach a better control of polymerization and polymer structure, a new method has been developed to prepare polyphosphazene at room temperature, but it is very difficult to prepare the desired monomer at large scale [10,11]. As a result most researchers adopted AllcockÕs method to prepare the polyphosphazene because it is easy to process. By polymeric substitution reaction, various polyphosphazenes with different chemical

1381-5148/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.09.005

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Nomenclature Tg FTIR DSC TGA NMR GPC UV

glass transition temperature Fourier-transform infrared differential scanning calorimetry thermogravimetric analyzer nuclear magnetic resonance gel permeation chromatography ultraviolet

structures and properties have been prepared. Among them the polyphosphazenes with optical, electronic, and medical properties have been further studied [12–15]. Photosensitive polyphosphazens with cinnamate [16] and chalcon [17] as pendent groups have prepared by AllcockÕs group. These polyphosphazenes could undergo photocross-linking reaction under UV irradiation. Upon UV irradiation, 7-hydroxy-coumarin is another type of photosensitive compound and in which reversible dimerization reaction occurs under UV irradiation either in molecular state or attaching to polymer backbone [18,19]. Matsuda [20] prepared photocurable biodegradable polymer that displayed liquid–solid translation properties. We have previously prepared polyphosphazenes with electroluminescent and conductive properties [21–23]. In this paper, we report the synthesis and photocross-linking characteristics of a novel photosensitive polyphosphazene with 7-hydroxy-4-methylcoumarin as side group. 2. Experimental section 2.1. Materials All chemicals and reagents in this research were purchased from Shanghai Chemical Reagents Company in China and used as received unless noted elsewhere. Hexachlorocyclotriphosphazene was prepared according to previous method [24] and was recrystallized three times from heptane. The crude solid was then sublimed at 50 C under vacuum for three times to afford pure solid (50% yield). Sodium hydride (Fluka, 60% suspension in mineral oil) was used as received. Dioxane for polymer 6substitution reaction was distilled from sodium benzophenone ketyl prior to use. Ethylene carbonate (J&K Chemical Reagent, Ltd.) was purified by vacuum distillation.

PL V V0, R aG Vf TLC

photoluminescence specific volume volume extrapolated to 0 K volume expansion coefficient in the glassy state free volume at Tg thin-layer chromatography

3. Equipment FT-IR spectrum was recorded on Bruker EQUINOX 55 FT-IR system. Photoluminescent properties were recorded on Shimadzu RF-540 fluorescent instrument. UV–Vis spectrum was recorded on Shimadzu UV–VIS 2550 and NMR analysis was carried out on Vanian Mercury Plus-400 instrument with CDCl3 as solvent (31P NMR spectra were referenced to external 85% H3PO4 with positive shifts recorded downfield from the reference. 1H and 13C were referenced to tetramethylsilane standard). Molecular weight and molecular weight distributions were determined by gel permeation chromatography (GPC, Agilent1100 HPLC, Plgel MIXED-C type column, polystyrene standard purchased from Waters, THF as eluent). The elemental analysis was performed on Vario EL elemental analysis instrument. The thermal property of polymer was recorded on PE-7 type thermal analyzer system. For DSC, the heating rate was at 20 C/min and for TGA0 the heating rate was at 10 C/min. High-pressure Hg lamp system (1.0 kW) equipped with an optical filter (Shengyan HB Optical Technology Company, Ltd., China) was used as irradiation resource and has a peak wavelength at 352 nm and half peakwidth 12 nm. In all cases, UV light irradiation was conducted under vacuum. The intensity of the UV light was measured by DM-365HA photometer (Beijing BrightStars Science and Technology Corp, China). 3.1. Synthesis of 7-hydroxy-4-methylcoumarin [25] Into a round-bottomed flask equipped with condenser and stirrer, 22.0 g (0.2 mol) of resorcin, 25.5 ml (0.2 mol) of acetylacetic ester, 60 ml of dioxane and acid catalyst (concentrated H2SO4, 1 ml) were added. The mixture was heated at

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75 C for 4 h under stirring and the reaction monitored by TLC. After disappearance of starting material, the solvent was removed by distillation. The residue was poured into cold water under continuous stirring and the white crystal was collected by filtration. The crude product was washed with cold water and then dried under vacuum to afford 29.2 g(83.0% yield) 7-hydroxy-4-methylcoumarin. The sample for analysis was purified by recrystallization from 70% ethanol twice. Melting point: 187–188 C. 1H NMR (CDCl3, 400 MHz): d 10.50 (1 H, –OH), 7.53(1H, ArH), 6.76(1H, ArH), 6.67(1H, ArH), 6.07(1H, –C@CH), 2.31(3H, –CH3). 3.2. Synthesis of 7-(2-hydroxyethoxy)-4methylcoumarin A mixture of 17.6 g(0.1 mol) of 7-hydroxy-4methylcoumarin, 22.0 g(0.25 mol) of ethylene carbonate, and 16.6 g (0.12 mol) of potassium carbonate in 150 ml of N,N-dimethylacetamide was heated at 95–100 C for overnight. The cooled reaction mixture was poured into water and treated with ether. The organic layer was washed with cold water thoroughly and dried over magnesium sulfate, followed by removal of the solvent. A residual solid was recrystallized from ethanol– water mixture to give colorless prisms (m.p. 90– 91 C) with 69% yield. Anal. Calcd for C12H12O4: C, 65.45; H, 5.49. Found: C, 65.44; H, 5.47%. 1H NMR (400 MHz, CDCl3), d 7.52 (1 H, ArH), 6.90 (1H, ArH), 6.84 (1H, ArH), 6.16 (1H, –C(CH3)CH–), 4.16 (2H, Ar–O–CH2–), 4.03 (2H, –CH2–OH), 2.41 (3H, –C(CH3) CH–), 1.27 (1H, –OH). 3.3. Polymerization of hexachlorocyclotriphosphazene Poly(dichlorophosphazene), [NPCl2]n, was prepared and purified by the thermal ring-opening polymerization of hexachlorocyclotriphosphazene(trimer) at 250 C under vacuum by the AllcockÕs procedure. The percentage of monomer conversion was controlled below 50% to prevent

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any cross-linking. The purified product was stored under vacuum at low temperature (5 C). 3.4. Preparation of coumarin-containing polymer 7-(2-Hydroxyethoxy)-4-methylcoumarin (13.6 g, 0.062 mol) was dissolved in 100 ml freshly distilled dioxane. The solution of 7-(2-hydroxyethoxy)-4methylcoumarin was added dropwise to slurry of NaH (2.06 g, 0.068 mol) in 150 ml freshly distilled dioxane under stirring. The mixture was stirred at room temperature overnight and then was brought to reflux for 12 h to complete the reaction. The mixture was cooled to room temperature and transferred to a dropping funnel via syringe and added dropwise to the solution of poly(dichlorophosphazene) (0.580 g, 0.005 mol) dissolved in 300 ml freshly distilled dioxane at room temperature under agitation. The resulting reaction mixture was brought to reflux for 48–72 h to ensure the complete substitution of all chlorine atoms by coumarin groups (monitored by 31P NMR, till single peak was obtained at 6.8 ppm). The reaction mixture was concentrated on rotary evaporator to remove part of solvent. The residual was added into a large amount of water to precipitate the polymer, which was recovered as a white solid and redissolved in THF again and re-precipitated from water. This procedure was repeated three times and finally was precipitated from methanol. The product was vacuum-dried at 80 C overnight to obtain 1.17 g of polymer. The analysis data are listed in Table 1. 3.5. Photocuring characteristics Photocuring properties of Coumarin-substituted polyphosphazene were studied in film state. About 1.0 g coumarin-containing polymer was dissolved in dichloromethane (100 ml) to get 1% (W/V) polymer solution. Then, the polymer was spin-coated on a weighted silica glass and dried in dark to a constant weight and the weight of polymer was measured in gravity. The polymer film coated on silica was irradiated with UV light for a period of time. The irradiated film was washed with dichloromethane to remove the soluble polymer, the weight of

Table 1 Analysis data of coumarin-containing polymer Mn

Mw

Mw/Mn

Tg (C)

31

C (%) Found (cal)

H (%) Found (cal)

N (%) Found (cal)

2.16 · 105

9.82 · 105

4.55

67.0

6.8

59.73(59.75)

4.41(4.39)

2.87(2.90)

Pd

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insoluble curing polymer was determined. The gel content was expressed as the weight percentage of the insoluble part (gel) (Wg) against that of the coated copolymer (W): Yield ¼

oxy)-4-methylcoumarin [26,27]. Ethylene carbonate was also used as hydroxyethylation reagent in this experiment. At high reaction temperature (refluxing temperature, DMAC as solvent), the reaction mixture turned into dark even under Ar protection, presumably resulting in some by-products. However, the reaction mixture remains clear if the temperature was lowered to 95–100 C. Reaction completed within 12 h. The raw 7-(2-hydroxyethoxy)-4-methylcoumarin was precipitated from water and recrystallized from ethanol–water mixture to give pure product in 63.7% yield. The results of 1H NMR and elemental analysis correlated very well with the calculated values based on chemical structure. 7-(2-Hydroxyethoxy)-4-methylcoumarin reacted with NaH in dioxane at room temperature overnight and was heated at refluxing temperature for 12 h to give a clear solution of the sodium salt of 7-(2-hydroxyethoxy)-4-methylcoumarin. Poly(dichlorophosphazene) was prepared according to the reported procedure [1,2]. The purity of the monomer affected the polymerization time and monomer conversion. So trimer must be carefully purified before polymerization. The purification procedure has been well established by

Wg  100. W

4. Results and discussion 4.1. Synthesis of 7-(2-hydroxyethoxy)-4methylcoumarin and coumarin-containing polymer The synthetic strategy for the preparation of 7hydroxy-4-methylcoumarin and polymer was shown in Scheme 1. First the 7-hydroxy-4-methylcoumarin was prepared as reported method with a little modification and 86.7% yield was obtained [25]. Dioxane was chosen as solvent for this reaction and a catalytic amount of concentrated H2SO4 was used. The purity of 7-hydroxy-4-methylcoumarin analyzed by 1H NMR and elemental analysis. These results are well consistent with theoretic values. So far two methods have been reported to convert 7hydroxy-4-methylcoumarin to 7-(2-hydroxyeth-

ONa

OH

HO

O

HO

OH

O

H2SO4

O

O

O

K2CO3

NaH/Dioxane

+ O

CH3

CH 3COCH 2COOCH 2CH 3

O

CH3

CH 3

O

O

O

O

RONa CH3

Cl

O

Cl P

Cl

N

Cl P

Cl

2500C / Vacuum

N

Cl P

P N

Cl

RONa / Reflux N

n

Cl

O P O

O N

n

O O

CH3

poly(dichlorophosphazene)

Coumarin-containing Polymer

Scheme 1. Synthetic route of coumarin derivatives and coumarin-containing polyphosphazene.

O O

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Fig. 1. UV–Vis spectrum of coumarin-containing polymer.

55

No.7 No.6 No.5 No.4 No.3 No.2 No.1

50 45 40 35

C(w/V), 0.002%, 0.006%, 0.006%, 0.02%, 0.02%, 0.06%, 0.06%,

Ex 342nm 438nm 350nm 439nm 358nm 442nm 365nm

30

Intensity

Allcock group [1,2]. The monomer conversion was controlled below 50% to avoid any cross-linking product and the total reaction time was about 24 h. After polymerization was finished, the polymerization tube was first cooled to room temperature and then chilled in liquid nitrogen to stop the polymerization completely. Then, the polymerization tube was opened in a drybox and the unpolymerized monomer was recovered by vacuum sublimation at 50 C.The yield of coumarin-containing Polymer was calculated from the weight change before and after polymerization. Poly(dichlorophosphazene) is a highly reactive polymeric intermedium and needs to be stored under vacuum or under inert atmosphere. It could undergo nucleophilic substitution reaction with various reagents to form different kinds of poly(oragnophosphazene) with desired chemical, physical properties. The coumarin-containing polyphosphazene was obtained by polymeric substitution reaction of highly reactive poly(dichlorophosphazene) with sodium salt of 7-(2-hydroxyethoxy)-4-methylcoumarin. In order to substitute all chloride atoms by coumarin groups, the reaction mixture was refluxed in dioxane over 48 h and phase transfer catalyst (PTC) was added to facilitate the substitute reaction. The reaction was monitored with 31P NMR during whole reaction period. The reaction was completed when single signal of 31P NMR was obtained. The purified polymer was white, fibrous and easily soluble in common organic solvent, such as dichloromethane, chloroform, tetrahydrofuran, toluene, etc. The results of molecular weight and elemental analysis are listed in Table 1. The numeric average molecular weight is 2.16 · 105 and the polydispersity index was 4.55. The glass transition temperature from DSC measurement was 67.0 C. The 31 P NMR spectrum has only one signal at 6.8 ppm.

459

25 20 15 10 5 0 340 360 380 400 420 440 460 480 500 520 540 560

Wavelength nm

Fig. 2. PL spectra of coumarin-containing polymer.

4.2. Characterization of coumarin-containing polymer Figs. 1 and 2 present the UV–Vis and fluorescent spectrum of coumarin-containing polymer in chloroform solution. From Fig. 1, it was found that the absorptive property of polymer depended on coumarin side group. There are two peaks at 315 and 210 nm, which are very similar to those of pure 7-hydroxy-4-methylcoumarin, which suggested that the P@N backbones do not have any absorption

in this region. Based on this property, it is possible for us to design different poly(organophosphazene)s with optical characteristics only depending on the side organic groups. When excited at desired 365 nm, the emission spectrum of polymer solution changed with variation of concentration (Fig. 2). At higher concentration, the fluorescence was very weak due to the concentration effect. At lower concentration, the emission strength was increased and some new peaks appeared.

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FT-IR spectrum of coumarin-containing polymer was shown in Fig. 3. The characteristic peak at 1678 cm1 corresponds to carbonyl group, 1602 cm1 corresponds to the double band adjacent to carbonyl group, 1017 cm1, 1348 cm1 due to P@N double bond, 713 cm1, 750 cm1 attribute

4000

3500

3000

2500 2000 1500 Wavenumber cm-1

1000

500

Fig. 3. FT-IR spectrum of coumarin-containing polymer.

to P–N single bond, 988 and 1050 cm1 to P–O–C bond, 1150 cm1 to C–O–C bond. All those data suggested that coumarin groups was attached to P@N main chain by P–O–C bond. NMR results were showed in Figs. 4 and 5. For 1H NMR, d 2.405 ppm corresponds to hydrogen atom of methyl group and d 4.00–4.16 ppm was related to hydrogen atom of –OCH2CH2O–, d 6.15 ppm corresponds to the hydrogen atom of double bond (C@CH) adjacent to carbonyl group, d 6.83–6.90 ppm and d 7.50–7.53 ppm corresponds to the hydrogen atom of benzyl ring. In 13C NMR spectrum, there are 12 peaks at d 18.7, 29.5, 61.0, 69.7, 101.6, 112.1, 112.5, 113.9, 125.7, 152.7, 155.3, 161.5 ppm, Those data match very well with the chemical structure of the polymer. Thermal properties of synthesized polymer were studied with DSC and TGA. The Tg of coumarincontaining polymer was 67.0 C from DSC measurement (Table 1). The polymer showed very high thermal stability both in nitrogen and air atmosphere and the results were shown in Fig. 6. In

Fig. 4. 1H NMR spectrum of coumarin-containing polymer.

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Fig. 5.

13

C NMR spectrum of coumarin-containing polymer.

110

Weight Percentage %

100 90

Nitrogen Air

80 70 60 50 40 30 0

100

200

300

400

500

461

600

700

800

Temperature oC

Fig. 6. TGA curves of coumarin-containing polymer.

air, the polymer is stable up to 180 C and decomposed quickly above 200 C. At 680 C the decomposition nearly completed and some black solid left. The percentage of residue was 38% at 750 C. From the chemical formula of polymer, the total amount of phosphor and nitrogen is 9.3%. Such high content of phosphor and nitrogen atoms make the coumarin-containing polyphosphazene is a typical fire-retardant polymer [2]. The details of fireretardant properties will be reported elsewhere. In air the polymer combust incompletely and left a black char. In nitrogen, the onset temperature of polymer was 300 C and showed continuous degradation characteristics in the temperature region of 300–750 C. Because the ether linkage was unstable at high temperature, no cross-linking reaction occurred in inert atmosphere.

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the soluble polymer became partially insoluble in common organic solvent and finally gel formed. This reaction was characterized with gel formation during the irradiation. Fig. 7 showed the correlation of irradiation time vs. gel amount at different irradiation power. Under fixed irradiation power, the gel amount increased with extended irradiation time. After 60 min irradiation, the gel content reached to a constant value. The highest gel percentage was nearly 92% at irradiation power 97.2 mW/ cm2. The DSC result of the irradiated polymer film was shown in Fig. 8. There was no glass transition region in the curve due to the cross-linking reaction. According to free-volume theory of glass transition developed by Flory and Cohen [30,31], Tg was an iso-free-volume state. Simha [32] postulated that the free volume at Tg should be defined as:

4.3. Photocross-linking reaction of coumarincontaining polyphosphazene Coumarin is well known for its optical properties and has numerous applications in polymer design and industry applications. This topic was reviewed thoroughly by Long [28]. Coumarin and derivatives underwent reversible photodimerization reaction under UV irradiation. This [2 + 2] cyclodimerization reaction was first reported by Ciamician and Silber [29], and related photochemical and photophysical properties have been intensively investigated. It is well known that direct irradiation (UV > 300 nm) of coumarin leads exclusively to photochemical dimerization resulting in cyclobutane-type dimers which revert to the starting compound upon irradiation with light of shorter wavelengths (UV < 300 nm) [28]. Recently, applications of this reaction were explored where coumarin and its derivates have been used as photoreactive materials in photorecording and photoresists, and as photolabile protecting groups in biological applications [28]. When coumarin group was attached to polyphosphazene backbone, this reaction can take place smoothly under irradiation of 354 nm light (Scheme 2). The cross-linking will occur at both intra and inter polymer chain. After cycloadditional reaction,

V  ðV 0;R þ aG T g Þ ¼ V f . In the expression above, V is the specific volume, V0, R is the volume extrapolated to 0 K, aG is the volume expansion coefficient in the glassy state and Vf is free volume at Tg. Cross-linking affected strongly the local polymer segmental relaxation though the restrictions imposed by network junction. The restriction reduced the configurational degree of freedom or the free volume, and thereby increased the Tg.

CH3 O

O

O O

O O

O

O CH3

UV irradiation

O O

O CH3 O

O H3C

O

O

O

Scheme 2. Photocuring reaction of polymer under UV irradiation.

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463

100

Gel Percentage (%)

80

60

40

B: 97.2mW /cm2 C: 80.1mW /cm2 D: 70.3mW /cm2 E: 59.4mW /cm2

20

0 0

10

20

30

40

50

60

Irradiation Time (Min)

Fig. 7. Gel changes with irradiation time at different irradiation intensity.

90

Fig. 9. Absorption spectra of coumarin-containing polymer before and after irradiation.

Heat Flow Endo Up (mV)

80 70

These results were very similar to those reported by Allcock [17]. The absorption maximum of those two polymers at 320 nm closely matches the emission spectrum of medium pressure mercury lamp. This improved spectral match should provide greater photosensitivity than in the cinnamate system [16], and without the use of photosensitizer.

60 50 40 30 20

5. Conclusion

10 0

100

200

300

400

500

600

o

Temperature ( C)

Fig. 8. DSC curve of irradiated polymer.

Highly cross-linking density would completely confine the movement of polymer chain and resulted in disappearing of glass transition region. During the irradiation, the absorption property of polymer also changed remarkably due to the photodimerization reaction. This optical change was shown in Fig. 9. With increasing the irradiation time, the absorption at 320 nm decreased rapidly due to the disruption of aromaticity of the coumarin (Scheme 2). After 40 min irradiation, the absorption peak disappeared and indicated the completion of cyclodimerization.

A novel photosensitive polyphosphazene with coumarin as side group was synthesized through polymeric substitute reaction. The polymer underwent [2 + 2] photodimerization reaction under UV irradiation (352 nm light). The gel content increased rapidly with increasing irradiation intensity and time due to the photocross-linking reaction. This polymer may have potential application in photoresists and photorecording. Acknowledgements This work was supported by the National Science Foundation of China (20374022) and the authors express their grateful appreciations to Prof. JianHua Mo for his valuable experimental support on UV irradiation.

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