Synthesis of new polyazines by 1,4 photochemical or anionic polymerization of 1,2-diaza-1,3-butadienes

European Polymer Journal 44 (2008) 2545–2550

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Synthesis of new polyazines by 1,4 photochemical or anionic polymerization of 1,2-diaza-1,3-butadienes Orazio A. Attanasi a,*, Maurizio D’Auria b,*, Lucia Emanuele b, Gianfranco Favi a, Paolino Filippone a, Fabio Mantellini a, Rachele Pucciariello b, Rocco Racioppi b a b

Istituto di Chimica Organica, Università degli Studi di Urbino ‘‘Carlo Bo”, Via I Maggetti 24, 61029 Urbino, Italy Dipartimento di Chimica, Università della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy

a r t i c l e

i n f o

Article history: Received 21 April 2008 Received in revised form 13 June 2008 Accepted 13 June 2008 Available online 20 June 2008

Keywords: 1,2-Diaza-1,3-butadiene Photochemical polymerization Anionic polymerization Polyazines Thermal behaviour

a b s t r a c t Photochemical and anionic polymerizations of 1,2-diaza-1,3-butadienes are described. Photochemical polymerization was smoothly performed by irradiation of some 1-aminocarbonyl-1,2-diaza-1,3-butadienes with high pressure mercury arc (k = 300 nm) in the presence of allyltributylstannane. Molecular weights (Mw) in the range 14.6–559  102 g/ mol were obtained. The TGA curve revealed a first weight loss starting at about 200 °C of some 85%, and a second starting at about 300 °C. The DSC showed the glass transition (Tg) at about 34 °C. Anionic polymerization was performed by treatment of some 1-alkoxycarbonyl-1,2-diaza-1,3-butadienes with n-butyllithium. Molecular weights (Mw) in the range 8.44–242  102 g/mol were obtained. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Polybutadiene, a homopolymer of 1,3-butadiene, is the second largest volume synthetic rubber produced, next to styrene–butadiene rubber (SBR). Tires and tire products are responsible for about 70% of worldwide polybutadiene consumption (nearly 2,366,000 metric tons worldwide in 2004) [1–3]. Cured polybutadiene imparts excellent abrasion resistance, and low rolling resistance due to its low glass transition temperature Tg (typically <90 °C) [4]. However, low Tg also leads to poor wet traction properties, so polybutadiene is usually blended with other elastomers like natural rubber or SBR for tread compounds [5]. Polybutadiene also has a major application as an impact modifier for polystyrene and acrylonitrile–butadiene–styrene resin with about 25% of the total volume going into these applications [6]. For a typical polybutadiene, molecular weight (Mn) is usually >100,000 g mol1. This represents a chain that con* Corresponding authors. Tel.: +39 0722 303442; fax: +39 0722 303441. E-mail address: [email protected] (O.A. Attanasi). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.06.014

tains over 2000 butadiene units. Most polybutadienes are made by a solution process, using either a transition metal (Nd, Ni, or Co) complex as catalyst or an alkyl metal, like butyllithium, as initiator [5,6]. On the other hand, polyazines A[N@C(R)AC(R)@NA]xA were subject of some investigations in the past years as a new class of polymeric conductors [7–10]. Also theoretical studies of their electronic properties are reported [11–15]. Polyazines can be obtained from 2,3-butanedione dihydrazone by thermal polymerization, [7] or by condensation of a,b-dihydrazones with a,b-diones under acidic conditions [8]. Unfortunately, there are few studies on such materials because of the experimental difficulties due to instability of the polymers and/or its intermediates. This situation is clearly stated by the absence of data on the thermal behaviour (DSC, TGA) of polyazines. In order to deeply explore the synthetic usefulness of 1,2diaza-1,3-butadienes, we decided to investigate the polymerization reactions of these conjugated heterodienic compounds. The basic skeleton of 1,2-diaza-1,3-butadiene consists of a carbon–carbon double bond conjugated with nitrogen–nitrogen double bond. Thus, the introduction of

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trometer and are reported in reciprocal centimeters (cm1). UV spectra were recorded at room temperature in a double-ray UV–Vis–NIR 05E Cary Varian spectrophotometer, in 1.0 cm optical path quartz cuvettes. Thermal analyses of the sample have been carried out through a Differential Scanning Calorimeter DSC7 Perkin-Elmer. The temperature scale was calibrated from the melting point of high purity chemicals (lauric and stearic acids). Samples (ca. 10 mg) were weighed and scanned at 20 °C/min from 80 °C to 20 °C, in a nitrogen atmosphere. Three consecutive scans were performed at 20 °C/min for each sample: heating/cooling/heating. The actual value for the Tg was estimated as the temperature of intersection of the initial tangent with the tangent drawn through the point of inflection of the trace and the temperature of intersection of the tangent drawn through the point of the inflection with the final tangent. Data have been analyzed through the software Pyris 1, running under Windows NT 4.0. As usually, the melting temperature has been taken as the maxima of the corresponding endothermic peaks and the glass transition temperature has these corresponding to the point where half of the increase of the heat capacity occurred. The error is ±0.5 °C. The apparatus has been calibrated using the melting temperatures of indium and zinc and the heat of fusion of indium. Before each experiment the baseline in the range of interest has been optimized, then it has been subtracted from the corresponding calorimetric curves. The thermal-gravimetric analyses have been performed by a Thermal-gravimetric analyzer TGA7 Perkin-Elmer, operating either in a nitrogen atmosphere or in air from 0 to 800 °C at a scanning rate of 20 °C/min. The samples were analyzed by GPC, using a Hewlett-Packard HPLC1000 with H–P Plgel 5 micro column. Spectrophotometric grade THF was used as mobile phase. The analyses were performed at room temperature. The chromatograms were obtained using UV detector at 280, 254, 230, and 260 nm. Using a calibration through polystyrene samples, the conversion from elution time to molecular weight has been performed.

this monomeric structure could offer interesting opportunities for the characteristic and/or properties of the polymeric materials. Although the heterodiene system of 1,2-diaza1,3-butadienes has been shown to give 1,4-conjugated additions in a similar way of the homodiene systems, [16–18] however, to the best of our knowledge no data are reported in the literature on the thermal, photochemical or ionic polymerization of these compounds. In this paper we report the first preparation of polyazines from substituted 1-aminocarbonyl- or 1-alkoxycarbonyl1,2-diaza-1,3-butadienes [19] by using both photochemical [20–22] or anionic polymerization [23–27]. 2. Experimental 2.1. Materials and measurements All reactions were carried out under an inert atmosphere. Photochemical reactions were performed in an immersion apparatus with a 125 W high-pressure mercury arc (Helios-Italquartz) surrounded with a Pyrex water jacket, unless otherwise indicated. All the commercially available reagents and solvents were used without further purification. 1,2-Diaza-1,3-butadienes 1a–h were synthesized as a mixture of E/Z isomers as previously reported [19]. Column chromatography was performed on silica gel (60200 lm) and analytical thin-layer chromatography was carried out using 250 lm commercial silica gel plates (Kieselgel 60). 1H and 13C NMR spectra were recorded on a 400 (Varian Mercury) or 500 MHz (Varian Inova) magnetic resonance spectrometer in CDCl3. For 1H NMR, tetramethylsilane (TMS) served as internal standard (d = 0) and data are reported as follows: chemical shift, multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad signal) integration and coupling constants in Hz. For 13C NMR, CDCl3 was used as internal standard (d = 77.0) and spectra were obtained with complete decoupling. Infrared spectra were recorded on a FT infrared spec-

O 1

R

N

N

hν

NHR2 + Bu3Sn

benzene

O

N COR1

CONHR2 N n

2a−c

1a−d

Scheme 1. Photochemical polymerization of 1-aminocarbonyl-1,2-diaza-1,3-butadienes 1a–d.

Table 1 Photochemical polymerization of 1-aminocarbonyl-1,2-diaza-1,3-butadienes 1a–d Substrate 1

R1

R2

Sn eq.

Irradiation time (h)

Temp. (°C)

Yield (%) of 2 (w/w)

Mn  102 (g/mol)

Mw  102 (g/mol)

Mw/Mn

1a 1a 1a 1a 1b 1c 1d

OMe OMe OMe OMe OEt OEt NMe2

H H H H H Ph H

2 2 2 1 2 2 2

50 50 50 50 50 50 50

8 4 r.t. r.t. r.t. r.t. r.t.

2a (75) 2a (76) 2a (75) 2a (25) 2b (80) 2c (78) –

1.73 2.03 25.7 0.93 6.58 33.4 –

8.37 4.40 558.8 4.47 14.62 550.47 –

4.84 2.17 21.7 4.81 2.22 16.5 –

O.A. Attanasi et al. / European Polymer Journal 44 (2008) 2545–2550

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Fig. 1. 1H NMR of polyazabutadiene 2a.

2.2. Photochemical polymerization – general procedure 1-Aminocarbonyl-1,2-diaza-1,3-butadienes 1a–c (0.6 mmol) and allyltributylstannane (0.39 ml, 1.2 mmol) were dissolved in benzene (70 ml). The mixture was outgassed with nitrogen for 20 min. The mixture was irradiated with a high pressure mercury arc (k > 300 nm) (Helios-Italquartz, Milan, Italy) surrounded by a Pyrex water jacket for 50 h. After evaporation of the solvent, the crude product was chromatographed on silica gel eluting with 95:5 CHCl3/MeOH. Poly{methyl 3-[(aminocarbonyl)hydrazono]butanoate} (2a): 1 H NMR (ppm, CDCl3) d: 4.804.60 (m, 2H), 3.78 (s, 1.5H),

3.75 (s, 1.5H), 3.06, 3.04, 2.40, 2.35 (m, 1H), 2.12 (s, 1.5H), and 1.98 ppm (s, 1.5H). 13C NMR (ppm, CDCl3) d: 172.3, 156.5, 142.3, 137.8, 109.3, 45.0, 39.2, 29.2, 27.4. Poly{ethyl 3-[(aminocarbonyl)hydrazono]butanoate} (2b): 1 H NMR (ppm, CDCl3) d: 4.804.75 (m, 1H), 4.684.62 (m, 1H), 4.21 (q, 1H, J = 7.0 Hz), 3.00, 2.98, 2.37, 2.34 (m, 1H), 2.11 (s, 1.5H), 1.96 (s, 1.5H), 0.90 (t, 3H, J = 7.0 Hz). 13C NMR (ppm, CDCl3) d: 171.5, 156.3, 141.9, 138.2, 109.1, 44.8, 39.1, 29.0, 27.3, 14.1, 9.08. Poly[ethyl 3-(benzoylhydrazono)butanoate] (2c): 1H NMR (ppm, CDCl3) d: 7.20–7.00 (m, 5H), 5.20–5.10 (m, 1H), 4.40–4.10 (m, 2H), 2.75 (s, 1.5H), 2.62–2.41 (m, 1H), 2.16 (s, 1.5H), 0.90 (m, 3H). 13C NMR (ppm, CDCl3) d: 172.0, 157.2, 142.4, 129.4, 127.9, 124.8, 122.5, 121.9, 121.6, 120.3, 110.7, 105.0, 40.3, 29.9, 29.4, 28.0, 27.6, 27.1, 14.9, 14.5, 14.0, 13.9, 13.4, 9.3. 2.3. Anionic polymerization – general procedure

Fig. 2. TGA of polyazabutadiene obtained from 1b.

To a solution of 1.25 mmol of the 1-alkoxycarbonyl-1,2diaza-1,3-butadienes 1e–h in anhydrous toluene (5 ml), a solution (2.5 M in hexane) of n-butyllithium (0.06 ml, 0.15 mmol) was added. The polymerization under inert atmosphere was allowed to stand at room temperature under stirring until the disappearance of 1e–h (Table 3). After the evaporation of the solvent, the crude mixture was solved in CH2Cl2, washed with aqueous 1% H2SO4, dried over Na2SO4, filtered and concentrated under reduced pressure. Poly{methyl 3-[(methoxycarbonyl)hydrazono]butanoate} (2d): 1H NMR (ppm, CDCl3) d: 4.003.40 (m, 6H), 3.22 (s, 1H), 2.401.60 (m, 3H). 13C NMR (ppm, CDCl3) d: 167.0, 160.8, 158.5, 156.7, 154.2, 101.3, 67.9, 54.3, 53.1, 51.4. Poly{methyl 3-[(ethoxycarbonyl)hydrazono]butanoate} (2e): 1 H NMR (ppm, CDCl3) d: 4.204.00 (m, 2H), 3.803.40 (m, 3H), 3.22 (s, 1H), 2.201.80 (m, 3H), 1.151.05 (m, 3H).

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Fig. 3. DSC of polyazabutadiene obtained from 1b.

Table 2 DSC and TGA properties of synthesized polyazabutadienes 2a–c Polyazabutadiene

DSC Tg (°C)

TGA Tonset (°C)

2a 2b 2c

34 34 33

205 200 200

O R1

N

N

OR2

O 1e−h

t -BuLi N COR1

COOR2 N n

2d−g

Scheme 2. Anionic polymerization of 1-alkoxycarbonyl-1,2-diaza-1,3butadienes 1e–h.

13

C NMR (ppm, CDCl3) d: 168.0, 167.0, 160.8, 157.8, 128.9, 128.1, 125.2, 101.6, 63.0, 61.6, 52.8, 51.2, 49.3, 48.2, 14.3. Poly{ethyl 3-[(tert-butoxycarbonyl)hydrazono]butanoate} (2f): 1H NMR (ppm, CDCl3) d: 4.303.90 (m, 2H), 2.401.80 (m, 4H), 1.401.00 (m, 9H). 13C NMR (ppm, CDCl3) d: 167.0, 165.1, 156.4, 155.8, 154.0, 134.4, 128.9, 128.0, 110.3, 81.3, 61.6, 59.9, 27.9, 14.1. Poly{3-[(tert-butoxycarbonyl)hydrazono]-N,N-dimethylbutanamide} (2g): 1H NMR (ppm, CDCl3) d: 3.002.70 (m, 6H), 2.29 (s, 1H), 2.001.80 (m, 3H), 1.501.30 (m, 9H). 13C NMR (ppm, CDCl3) d: 167.2, 152.3, 128.9, 128.1, 125.1, 81.4, 36.7, 35.6, 28.0. 3. Result and discussion 1-Aminocarbonyl-1,2-diaza-1,3-butadiene 1a (R1 = OMe, R2 = H) showed an intense absorption at k = 253 nm (e = 7500 l cm1 mol1) and a very small absorption at k = 453 nm. The absorption at k = 253 nm

can be attributed to the p,p transition of the substrate. The irradiation of this compound in benzene for 18 h with high pressure mercury arc (k P 300 nm) showed no photochemical reactivity. In order to tentatively induce the photochemical polymerization of this compound, we tested a range of initiators. Acetophenone, benzophenone, 4,40 -dichlorobenzophenone, benzoin, benzoin ethyl ether, and 2-hydroxy-2methylpropiophenone failed and only decomposition products were observed. So, we tested the reactivity of 1a towards photochemically generated radical species by using allyl radical generated as described by Hasegawa [28]. Under these conditions, the reaction of 1a with allyl radical smoothly gave a polymer (Scheme 1, Table 1). The 1 H NMR spectrum of 1a showed the olefinic proton as a singlet at d 7.02, the amidic protons at d 6.18, the methyl ester group at d 3.90, and the methyl group on the double bond at d 2.32 ppm. After the irradiation with high pressure mercury arc (k = 300 nm), the product showed the amidic protons at d 4.70 (multiplet), while at d 3.78 and 3.75 we observed two peaks in 1:1 ratio due to methyl ester protons (Fig. 1). At d 2.12 and 1.98 we observed two singlets in 1:1 ratio due to the methyl group. At d 3.06 and 3.04 we observed the presence of two multiplets, while other two multiplets were present at d 2.40 and 2.35: the groups of signals were in the 3:4 ratio and all the signals were integrated for one proton. The IR spectrum was in agreement with the presence of the amide (3481 and 1685 cm1) and the ester (1739 cm1). The average molecular mass was found by using gel permeation chromatography giving Mn = 1.73  102 and Mw = 8.37  102. All these data are in agreement with the formation of a polymer, deriving from a selective 1,4 polymerization where the polymer is a 1:1 mixture of cis–trans isomers. Furthermore, the polymer is a 3:4 mixture of units where there is a NAC bond [@NAN(CONH2)A CH(CO2CH3)AC(CH3)@] between two monomers and units

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O.A. Attanasi et al. / European Polymer Journal 44 (2008) 2545–2550 Table 3 Anionic polymerization of 1-alkoxycarbonyl-1,2-diaza-1,3-butadienes 1e–h Substrate 1

R1

R2

Yield (%) of 2 (w/w)

Time (h)

Mn  102 (g/mol)

Mw  102 (g/mol)

Mw/Mn

1e 1f 1g 1h

OMe OMe OEt NMe2

Me Et t-Bu t-Bu

2d (82) 2e (80) 2f (87) 2g (83)

3.5 4 10 8

1.27 1.85 25.7 2.42

25.35 33.53 241.83 8.44

19.96 18.12 9.40 3.49

where there is a CAC bond [AN(CONH2)AN@C(CH3)A CH(CO2CH3)ACH(CO2CH3)AC(CH3)@NAN(CONH2)A] between two monomers. Probably, the reaction occurs through a hydrogen abstraction by the allyl radical to give a diazabutadiene radical that is able to react with another monomer. The generation of allyl radical by using the procedure described by Hasegawa [28] required that the reaction had to be performed at 8 °C. In order to optimize the reaction conditions, we carried out the reaction both at 4 °C and at room temperature. The best molecular weight (Mw = 55,880) was obtained performing the reaction at room temperature. The most important limitation was found performing the reaction with an equimolar amount of allyltributylstannane; in this case, we recovered a lower amount of polymer (Table 1). However, in all cases, the spectroscopic properties were the same. The polymerization of the 1-aminocarbonyl-1,2-diaza-1,3-butadienes 1b (R1 = OEt, R2 = H) and 1c (R1 = OEt, R2 = Ph) also successfully occurred, while the use of the 1-aminocarbonyl-1,2diaza-1,3-butadiene 1d (R1 = NMe2, R2 = H) did not allow any reaction (Scheme 1, Table 1). Considering the thermal properties of the polymer obtained from the 1-aminocarbonyl-1,2-diaza-1,3-butadiene 1b (Fig. 2), the TGA curve reveals a first weight loss starting at about 200 °C of some 85%, and a second starting at about 300 °C. At 500 °C, the complete degradation of the sample takes place. In Fig. 3, the DSC scan is reported where the presence of the glass transition at about 34 °C is evident. The same trend was observed with the other compounds 2a, c (Table 2). We studied also the anionic polymerization. As known, anionic polymerization takes place with monomers containing electron-withdrawing groups such as nitrile, carboxyl, phenyl, and vinyl. These polymerizations are initiated by nucleophilic addition to the double bond of the monomer originate from a carbanion alkyl lithium species. In this work, 1-alkoxycarbonyl-1,2-diaza-1,3-butadienes 1e–h have been polymerized anionically by using nbutyllithium (2.5 M hexane solution) as the initiator. Polymerization/termination-free system of the 1-alkoxycarbonyl-1,2-diaza-1,3-butadienes 1e–h with n-butyllithium gave the polymers 2d–g with excellent efficiency (Scheme 2, Table 3). It is not surprising that the polymerization reaction failed by using 1-aminocarbonyl-1,2-diaza-1,3butadienes 1a–c, likely because of the n-butyllithium quench due to the protons of the terminal ureidic residue of 1a–c. The 1H NMR spectrum of 2d showed the methoxy group at d 3.76 and a complex multiplet in the range 1.602.40 ppm in agreement with the presence of a methyl group on a double bond.

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