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Synthesis and characterization of new telechelic poly(phenyleneoxide)s
EUROPEAN POLYMER JOURNAL
European Polymer Journal 40 (2004) 1169–1175
www.elsevier.com/locate/europolj
Synthesis and characterization of new telechelic poly(phenyleneoxide)s M. Jayakannan *, T.R. Smitha Polymer Science Division, Regional Research Laboratory, Thiruvananthapuram 695019, India Received 20 August 2003; received in revised form 3 November 2003; accepted 9 December 2003 Available online 4 February 2004
Abstract New well-defined telechelic poly(phenyleneoxide)s (PPO’s) were synthesized from 4-bromo-2,6-dimethylphenol and bi-phenolic compounds through phase transfer catalyzed aromatic nucleophilic substitution polymerization. Bisphenol–A (BPA), 4,40 -biphenol (BP), hydroquinone (HQ) and 2,6-dihydroxynaphthalene (DHN) were employed as telechelic units. The composition analysis by proton-nuclear magnetic resonance (1 H-NMR) spectroscopy revealed that DHN was highly reactive compared to BPA and HQ, whereas BP was un-reactive in the polymerization process. The number average repeating unit (n) in telechelic PPO was estimated as n ¼ 17–19 and n ¼ 17–20 for DHN and BPA (or HQ), respectively. The reactivity of the bi-phenolic in PPO synthesis are confirmed as DHN > HQ BPA P BP. The molecular weight determination by gel permeation chromatography (GPC) and viscosity method suggest that the molecular weight of PPO decreased drastically with increasing amount of bi-phenolic units in the feed. The GPC chromatogram of PPO showed a bi-modal distribution, clearly indicative of formation of two different types of molecular weight chains, whereas the telechelic polymers have a mono-modal distribution with a narrow polydispersity. Thermal analysis by differential scanning calorimetry revealed that telechelic polymers are highly amorphous, like PPO, and no crystallization or melting peaks were observed in the heating/cooling cycles. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Poly(phenyleneoxide)s; Telechelic polymers; Phase transfer catalyzed polymerization; Thermal analysis
1. Introduction Poly(phenyleneoxide) (PPO) is a well-known commercially important thermoplastic polyether. Its high glass transition temperature, impact and tensile strength make it an attractive candidate for automobile industry. PPO has outstanding resistance to most of the aqueous reagents being unaffected by acids, alkali, and detergents. PPO is commercially prepared from 2,6-dimethylphenol through oxidative coupling polymerization route in presence of copper/amine catalysts [1]. Ratio of amine to cuprous ion and reaction temperature are very
* Corresponding author. Tel.: +91-471-2515316; fax: +91471-2491712. E-mail addresses: [email protected], [email protected] (M. Jayakannan).
critical factors in the extent of C–O coupling (polymerization) relative to C–C coupling (dimerisation side reaction). Higher amine/cuprous ratio and lower reaction temperature favor polymerization while dimerisation is favored at higher temperature and lower amine/ cuprous ratio [2]. PPO prepared by this route has few limitations such as (a) promotion of branching via Ar– CH3 groups during high temperature processing by the un-reacted amine and copper salts (b) coloring due to presence of C–C coupled products (c) hazardous to health due to the evolution of unpleasant odor during the extrusion and injection molding. Hence, PPO free from the amine and copper salts would be a very attractive candidate for new applications and environmental safety. Aromatic nucleophilic substitution reaction (SN Ar) is a commonly used method for the preparation of poly(aryleneether)s such as poly(etherketone)s and
0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.12.019
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poly(ethersulfone)s [3,4]. The synthesis of PPO was also attempted by this method from 4-bromo-2,6-dimethylphenol (BDMP) using catalytic amounts of an oxidative initiator such as lead oxide and potassium ferricyanide [5–9]. These earlier attempts were found to lead to uncontrolled functionality and branching and failed in producing high molecular weight PPO [6]. The more recent phase-transfer catalyst (PTC) methodology developed by Percec et al. leads to well-defined PPO with controllable molecular weights and functionality [10,11]. This route has many advantages over oxidative coupling, because the polymer produced is of very good quality as they do not have any amine/Cu salt contaminations. However, the PTC route was not so far attempted in the manufacture of PPO and it is still in the research level. The requirement of additional brominating step in the monomer synthesis and the other manufacturing difficulties could be the reason for this limitations [8]. Nevertheless, the aromatic substitution reaction can tolerate a wide range of functionalities and is certainly a powerful tool for the PPO copolymer synthesis [12]. Some of the limitations that had restricted the direct application of PPO as such is its high glass transition temperature (Tg ¼ 220 °C), poor melt flow and the susceptibility of its methyl group to thermal oxidation. PPO crystallizes only with difficulty and behaves as an amorphous polymer during melt processing. To overcome the processing difficulty, usually PPO is blended with other polymers like polystyrene, polycarbonates and polyesters for commercial applications [2]. Attempts to improve the blending process and micro-phase separation of other PPO blends is a very active area of research for scientific communities both in industries and academic. Developing telechelic-PPO (telechelic––means more functionality) with reactive sites is one of the approach to produce stable morphology in PPO blends [13–15]. The reactive functional groups induce a type of cross-linking or grafting of PPO chains with other polymers [16]. In a typical A–B self polycondensation, telechelic architectures could be introduced by copolymerizing with monomers containing A0 n (or B0 n ) functional groups, where n P 1. Here, three cases can occur depending upon the Ôn’ values as shown in Scheme 1. In case-a, n ¼ 1, statistically the growth of the chain is disturbed and low molecular weight chains are produced with ends A0 and B, respectively; case-b, n ¼ 2, produces a functionalized linear telechelic with A0 –A0 as a middle point and A in both ends; and in case-c, n P 3, brush type or branched polymers having A0 n as a core and A in all ends are produced. In case-c, it is very important to stop the polymerization at low conversions otherwise the highly branched polymers produce cross-linked net work leading to gel [17]. 2,4,6-Trimethylphenol and 4-t-butyl-2,6-dimethylphenol capped PPO’s have been developed (example for
A B
B
A B
A n
A B + A'
A
A B
A'
End capping
n
Telechelic unit A B + A' A'
A B
A' A'
A B
B
A B
x
A' A B +
A
A'
A'
B
A B
A'
B
A B
A'
A B z
A'
A y
x
A A
y
Branching site
Scheme 1.
case Ôa’) and used for blending with other polymers [18]. Since the properties of linear polymers are different from the branched systems, not much report are seen in the literature for branched PPOs. The linear telechelic PPO’s (case b) are very attractive because they can be further used as macro-monomers for synthesizing various other polymers. Van Aert et al. reported the synthesis of low molecular weight telechelic PPO through precipitation polymerization-oxidative coupling route [19]. Further, the PPO-oligomers are functionalized at the ends to employ as compatibilizers for PPO-polyester blends [20]. Percec and coworkers explored the possibilities of linear telechelics using bis(3,5-dimethyl-4hydroxyphenyl)methane in PPO [21]. In this report, we have synthesized well-defined linear telechelic PPO’s via aromatic SN Ar polymerization using four commercially available bi-phenolic compounds. 2,6-dihydroxynaphthalene (DHN), bisphenol-A (BPA), hydroquinone (HQ) and 4,40 -biphenol (BP) are chosen for this purpose. The bi-phenolic unit act as a A0 –A0 type monomer and it reacts with the B functionality in the A–B monomer; in this case, BDMP.
2. Experimental 2.1. Materials 4-Bromo-2,6-dimethylphenol (BDMP), bisphenol-A (BPA), hydroquinone (HQ), 4,40 -biphenol (BP), 2,6-dihydroxy naphthalene (DHN) and tetrabutylammoniumhydrogen sulphate (TBAHS) were purchased from Aldrich Chemicals and used without further purification. Sodium hydroxide, toluene and other solvents were purchased locally and purified using standard procedures.
M. Jayakannan, T.R. Smitha / European Polymer Journal 40 (2004) 1169–1175
2.2. Measurements
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3. Synthesis of poly(phenyleneoxide) (PPO)
1
H and 13 C-NMR spectra of the polymers were recorded using 300-MHz Brucker NMR spectrophotometer in CDCl3 containing small amount of TMS as internal standard. For 13 C-NMR experiments, the carbon atom in CDCl3 was taken as 77 ppm and all other peaks are assigned with respect to it. Thermal analysis of polymers were done using DSC 2920 Differential Scanning Calorimeter (TA Instruments). About 10 mg of the sample was taken in Aluminum (Al) pan and 10°/ min heating/cooling rate was applied to record the thermograms. The instrument was calibrated with Indium standards. The inherent viscosity (ginh ) of PPO and copolymers were measured using Ubbelohde viscometer at 25 °C. 0.5 wt.% of the polymer solutions were prepared by dissolving 125 mg of polymers in 25 ml of tetrahydrofuran (THF) and filtered through Whatmann No.1 filter paper prior to the viscosity measurement. An average of three readings (±1 s accuracy) was taken for each measurement. Gel permeation chromatography was also employed to determine the molecular weights of the polymers. Waters 510 Pump connected through three series of Styragel HR columns (HT-3, HT-4 and HT-5) and Waters 410 Differential Refractometer is used for analyzing the samples. The columns were calibrated using different molecular weight polystyrene standards. The flow rate of the THF was maintained as 1 ml throughout the experiments and 1 wt.% (10 mg in 1 ml) of polymer solution was filtered and injected for recording the GPC chromatograms.
Polyphenyleneoxide (PPO) was prepared via phase transfer catalyzed route. Sodium hydroxide (8.0 g, 200 mmol) was dissolved in distilled water (25 ml) and taken in a 100 ml two neck flask. It was provided with a mechanical stirrer at one neck and cooled to 0 °C using ice. BDMP (1.9 g, 20 mmol) was added and stirred for 15–20 min to enrich the formation of sodium phenolate ions. The phase-transfer catalyst TBAHS (0.34 g, 1 mmol) and the cosolvent toluene (25 ml) were added and the polymerization proceeded with vigorous stirring at 25 °C for 18 h. The aqueous layer was separated out; the toluene layer was washed with dilute HCl (50 ml, 1:1 v/v) and further with water to remove the inorganic impurities. The organic layer was precipitated into ethanol; the resultant white fiber-like polymer was dissolved in THF and re-precipitated again into methanol. The purification procedure was repeated few times to remove the monomer and oligomeric species. The white fiber polymer was dried well prior to further analysis. Yield ¼ 1.9 g (80%). 1 H-NMR (CDCl3 ): 6.47 ppm (s, 2H, Ar–H) and 2.17 ppm (s, 6H, Ar–CH3 ). 13 C-NMR (CDCl3 ): 154.75, 145.46, 132.55, 114.47, 110.54 (Ar–C), 16.77 ppm (Ar–CH3 ).
4. Synthesis of DHN and BPA containing telechelic PPO To an ice-cold NaOH solution (8 g, 200 mmol, 65 ml), 2,6-dihydroxy naphthalene (0.32 g, 2 mmol, for polymer PPO-2) was added and stirred well to obtain a
Table 1 Yield, viscosity and compositions of polymers Polymer
Monomer
Yielda (%)
ginh b (dl/g)
Monomer in feed (mol%)
Monomer in telechelic PPO (mol%)c
nd
PPO PPO-1 PPO-2 PPO-3 PPO-4 PPO-5 PPO-6 PPO-7 PPO-8 PPO-9 PPO-10
– DHN DHN BPA BPA HQ HQ HQ BP BP BP
80 33 10 31 37 49 38 37 50 48 42
0.46 0.21 0.14 0.28 0.28 0.40 0.17 0.24 0.35 0.31 0.35
–
– 1.8 5.2 – 1.2 0.8 1.4 2.0 – – –
>100 17 19 70 37 61 30 28 64 51 55
a
5 10 5 10 5 10 15 5 10 15
Calculated for isolated polymers. For 0.5 wt.% of polymer solution in THF at 30 °C. c Incorporation of comonomers are calculated from the 1 H-NMR spectra by comparing the peak intensities at 6.47 ppm (PPO main chain protons) with the bi-phenolic unit protons. d Number average repeating units calculated from the 1 H-NMR spectra by comparing the peak intensities at 6.47 (PPO main chain protons) and 6.36 ppm (PPO main end protons). b
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M. Jayakannan, T.R. Smitha / European Polymer Journal 40 (2004) 1169–1175
pale blue color sodium salt. Excess amount of water is required (instead of 25 ml as in PPO synthesis) in order to increase the solubility of the sodium salts. BDMP (3.6 g, 18 mmol) was added and the reaction mixture was stirred well before adding TBAHS (0.34 g, 1 mmol) and toluene (25 ml). The rest of the procedure is same as that described for PPO. The polymer was isolated as white powder and it weighed 0.3 g after drying (10% yield). The other DHN and BPA containing telechelic copolymers (PPO-1, PPO-3, and PPO-4) were prepared by following the similar procedures and their yields are reported in Table 1. 1 H-NMR (CDCl3 , for Polymer PPO-2): 7.25 ppm (s, 4H, Ar–H); 7.10 ppm (s, 4H, Ar– H); 6.47 ppm (s, 2H, Ar–H) and 2.17 ppm (s, 6H, Ar– CH3 ). 13 C-NMR (CDCl3 ): 154.75, 145.46, 132.55, 114.47, 110.54 (PPO–Ar–C), 133.79, 131.72, 129.02, 128.28, 144.03 ppm (Naph–Ar–C) and 6.77 ppm (Ar– CH3 ). 1 H-NMR (CDCl3 , for Polymer PPO-4): 7.24 ppm (b, 4H, Ar–H); 7.15 ppm (s, 2H, Ar–H); 6.47 ppm (s, 2H, Ar–H); 2.21 ppm (s, 6H, C-CH3 ) and 2.17 ppm (s, 6H, Ar–CH3 ).
5. Synthesis of HQ and BP containing telechelic PPO The solubility of di-sodium salts of HQ and BP are very high similar to BDMP and needs only minimum amount of water (25 ml) to carry out the polymerization. The telechelic copolymers PPO-5 to PPO-10 were synthesized by varying the amount of comonomers (5, 10 and 15 mol%) in the feed with respect to BDMP (95, 90 and 85 mol%, respectively). The yields of the polymers are reported in Table 1. 1 H-NMR (CDCl3 , for Polymer PPO-6): 7.24 ppm (s, 4H, Ar–H); 6.47 ppm (s, 2H, Ar–H) and 2.17 ppm (s, 6H, Ar–CH3 ). 1 H-NMR (CDCl3 , for Polymer PPO-9): 6.47 ppm (s, 2H, Ar–H) and 2.17 ppm (s, 6H, Ar–CH3 ). No other new peaks.
CH3 Br
OH CH3
O
TBAHS 25 C, Toluene/H2O
n
o
CH3 PPO
Scheme 2. Synthesis of PPO.
polymers and produces very high molecular weight chains. The chain growth in this case takes place by the addition of the phenoxy-radical to a phenolate-anion and therefore, PTC is also referred to as an anionradical reaction [22]. In order to synthesize linear telechelic PPO’s, we have chosen four bi-phenolic compounds such as 2,6dihydroxynaphthalene (DHN), bisphenol-A (BPA), hydroquinone (HQ) and 4,40 -biphenol (BP). The biphenolic unit act as a A0 –A0 type monomer and reacts with the B functionality in the A–B monomer, in this case, BDMP (see Scheme 3). The reactivity of A0 and A (phenol groups) towards B (aromatic bromide) in the polymerization condition is a critical factor for producing the telechelic PPO. Four series of telechelic PPO’s are synthesized by copolymerizing bi-phenolic compound with BDMP using the identical polymerization conditions as described for PPO. The amount of telechelic units in the PPO was varied by using different amounts (5, 10 and 15 mol%) of DHN, BPA, HQ and BP in the feed. The polymerization of 5 and 10 mol% of DHN with BDMP resulted in telechelics PPO-1 and PPO-2, respectively. The attempt to use more than 15% of DHN in the feed was not successful.
CH3 Br
+
OH
HO
OH
CH3
6. Results and discussion Poly(phenyleneoxide), PPO, was synthesized from 4-bromo-2,6-dimethylphenol (BDMP) by aromatic nucleophilic substitution reaction using a phase-transfer catalyst (see Scheme 2). Percec et al. had developed this method and now it has been extensively practiced for synthesizing various PPO copolymers [10]. Typically, the polymerization was carried out by stirring the monomer, base and PTC (TBAHS) in water/toluene solvent mixture at room temperature for 18 h. The advantages in the PTC route is that the selective solubility of the polymers in toluene and the polycondensation bi-product, NaBr in water, shifts the equilibrium towards
CH3
NaOH
CH3 HO
O
O
O CH3
CH3
CH3
CH3
CH3
O y CH3
x
OH CH3
Telechelic PPO
HO
OH
= HO HO
OH HO
OH
OH
HO
Scheme 3. Synthesis of telechelic PPO.
OH
M. Jayakannan, T.R. Smitha / European Polymer Journal 40 (2004) 1169–1175
The structures of the PPO and telechelics PPO-1 and PPO-2 were confirmed by 1 H and 13 C-NMR spectroscopy and their corresponding spectra are shown in Fig. 1 (for simplicity only the aromatic region is shown). The peak at 6.47 and 6.36 are corresponding to the aromatic protons of PPO in the repeating units and chain ends, respectively [18]. In Fig. 1b, the appearance of two new peaks at 7.25 ppm (two protons next to the ether linkage) and 7.17 ppm are corresponding to the aromatic protons in naphthalene units in PPO-2. The comparison of peak intensities at 6.47 and 7.17 ppm gave the actual incorporation of DHN in PPO-1 and PPO-2 as 1.8 and 5.2 mol%, respectively (see Table 1). The peak at 7.25
Fig. 1. 1 H-NMR [(a) PPO and (b) PPO-2] and 13 C-NMR Spectra [(c) PPO and (d) PPO-2] of polymers. The peak in asterisk is corresponding to the chloroform.
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ppm was not used for the composition analysis since it overlaps with the chloroform solvent peak at 7.26 ppm. The incorporation of DHN is also confirmed from the appearance of new peaks at 133.79, 131.72, 129.02 and 128.28 ppm (in Fig. 1d) in addition to the aromatic carbon peaks for PPO chains (in Fig. 1c). The molecular weights of the PPO, PPO-1 and PPO2 were determined by GPC in THF using Polystyrene standards for the calibration. The GPC chromatograms of the polymer are shown in Fig. 2 and the molecular weights of the polymers are reported in Table 2. The formation of high molecular weight polymers suggest that the PTC conditions employed for the synthesis is highly effective. The molecular weights of telechelic polymers (PPO-1 and PPO-2) are much lower compared to that of PPO. The chromatogram of PPO is showing a bi-modal distribution instead of the expected mono-modal for solution polymerizations. This indicates the formation of two different types of polymer chains in the PPO synthesis. It may occur followed by the precipitation of high molecular weight PPO chains from the toluene layer during the polymerization. Jayakannan et al. has reported a similar bi-modal observation in the bis-benzyl polyether synthesis [23,24]. Percec et al. also reported similar bi-modal distribution in their high molecular weight PPO synthesis [18]. Here, interestingly, the telechelic polymers PPO-1 and PPO-2 showed a mono-modal distribution, unlike PPO. The incorporation of DHN limits the formation of very high
Fig. 2. GPC chromatograms of polymers in THF. (a) PPO, (b) PPO-1 and (c) PPO-2.
Table 2 GPC molecular weights and Tg of PPO and copolymers Polymer
Mp (g/mol)
Mn (g/mol)
Mw (g/mol)
Mw /Mn
Tg (°C)
PPO PPO-1 PPO-2
121,000 50,650 41,900
87,400 48,200 41,300
108,800 53,300 44,100
1.25 1.17 1.07
220.5 194.7 186.8
(a) Determined by GPC using Polystyrene standards as references in THF. (b) Determined by DSC at 10°/min heating rate.
M. Jayakannan, T.R. Smitha / European Polymer Journal 40 (2004) 1169–1175
The inherent viscosity of all the polymers were measured in THF at 25 °C for 0.5 wt.% polymer solution and the values are reported in Table 1. The ginh values of the copolymers were plotted against the comonomer incorporation in the feed and shown in Fig. 3. It is very clear from the plot that in all the cases, the increase in the comonomer amount in the feed decreases the molecular weight of the resultant polymer. The plot suggests that the DHN produces low molecular weight polymers when compared to other phenolics. This is in good agreement with the NMR results. Thermal properties of the PPO and telechelic PPO were analyzed by differential scanning calorimeter. The sample was taken in an Al pan and heated to 280 °C at 10°/min and then quenched to 30 °C at 10°/min. The quenched samples were reheated to 280 °C at 10°/min to record the thermal transitions. The DSC thermograms of the PPO, PPO-1 and PPO-2 are shown in Fig. 4. It is very clear from the plot that, the polymers showed only
0.50
Inherent Viscosity (dl/g)
molecular weight polymers and hence all the polymer chain lengths produced are almost of the same length. BPA, HQ and BP are also employed as bi-phenolic compounds to synthesize telechelics PPO-3 to PPO-10. The extent of incorporation and yields are listed in Table 1. The limited solubility of BPA sodium salts in water hampered its incorporation more than 10 mol% in PPO. On the other hand, the high solubility of HQ and BP sodium salts allowed up to 15 mol% sample to be taken in the feed. The 1 H-NMR spectra of PPO-4 showed two new peaks at 7.24 and 7.10 ppm, which are assigned to the aromatic protons in BPA. Similarly, a new peak at 7.24 ppm is also noticed in the 1 H-NMR spectrum of PPO-6 corresponding to the HQ protons. Surprisingly, no new peaks have been observed for PPO-9 and the spectrum is almost identical to that of PPO (Fig. 1a), which suggests that BP is not incorporated in PPO. It may be that the non-coplanarity of the bi-phenyl rings makes the phenol less acidic in BP compared to that of BDMP in the SN Ar reaction. The compositions of telechelics were determined by comparing the intensities of aromatic protons in the PPO chain (6.47 ppm) with that of comonomers. In PPO-4, the peak at 7.24 ppm is not used for composition analysis, since they are very close to the chloroform solvent peak. However, in the case of PPO-6, the peak at 7.24 ppm is used because no other peak is available for finding the HQ incorporation in the telechelic PPOs. The amount of incorporation of BPA and HQ in PPO are calculated and reported in Table 1. It is very clear from the table that the DHN is more reactive than BPA and HQ. From the current investigation, the reactivity of the bi-phenolic in PPO synthesis could be written as DHN > HQ BPA P BP. In general, the incorporation of bi-phenols (DHN BPA and HQ) in the PPO chain is relatively low. It may be due to the poor nucleophilic substitution reactions of aryl bromides towards the electron rich bi-phenols [3]. The comparison of the aromatic peak intensities at 6.47 and 6.34 ppm give the average number of repeating units (n) in the polymer chain (in Fig. 1) [18]. The values are reported for all the polymers in Table 1. PPO has more than 100 repeating units in the polymer chain indicating the formation of high molecular weight polymers. The incorporation of DHN produces short polymer chains with total n ¼ 17–19. It indicates that DHN produces small well-defined block copolymers having average of 8–10 PPO chains on either side of the naphthalene units. At 10 mol% incorporation, both BPA and HQ produce a much longer block telechelic with 15–18 PPO units on either side. The GPC molecular weights of the PPO, PPO-1 and PPO-2 reported are much higher than that determined by NMR. It may be attributed to the difference in the hydrodynamic volume for the polystyrene standards and PPO chains, which could overestimate the molecular weights of PPO, PPO-1 and PPO-2.
0.45 0.40 0.35
BP
0.30 BPA
0.25
HQ
0.20 0.15
DHN
0.10 0
2
4
6
8
10
12
14
16
Comonomer in Feed ( mol %) Fig. 3. Plot of inherent viscosity versus the amount of comonomer in the feed.
Heat Flow (mCal/mg) (a.u)
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PPO
Tg = 220.5 oC PPO-1 Tg = 194.7 oC
PPO-2 o Tg = 186.8 C
170
180
190
200
210
220
230
240
o
Temperature ( C) Fig. 4. DSC thermograms of polymers at 10°/min heating rate.
M. Jayakannan, T.R. Smitha / European Polymer Journal 40 (2004) 1169–1175
a glass transition (Tg ) temperature and no melting or crystallization peaks were observed. In order to investigate the quenching effect on the thermal transition, the polymers were heated to 280 °C and the sample pan was rapidly quenched on a metal block that was kept in contact with liq. N2 . The rapidly quenched samples were heated/cooled at 10°/min to record the thermal transitions. The plots were found identical to that reported in Fig. 4 and no melting/crystallization peaks were observed. It suggests that PPO and the telechelic copolymers are highly amorphous. It is interesting to note that the Tg of the PPO decreased with the increase in the naphthalene contents in the copolymers. Since the incorporation of BPA and HQ in PPO is not very significant (<2%) compared to DHN, their thermal properties and GPC are not investigated.
7. Conclusion New class of telechelic PPOs were synthesized through A–B and A0 –A0 copolymerization route. The composition analysis by NMR reveals that DHN is highly reactive in the polymerization conditions compared to BPA and HQ; but BP was found to be unreactive. The inherent viscosity of the polymers decreases with increase in the bi-phenolic units and the decrease is very drastic for DHN containing telechelic polymers. GPC plot showed a bi-modal distribution for PPO corresponding to the formation of two ranges of molecular weight species during the polymerization. Interestingly, the incorporation of telechelic units produces a mono-modal molecular weight distribution. The polymers were highly amorphous and they have only glass transitions, no melting/crystallization peaks are observed in the heating/cooling cycles. The Tg of the PPO decreased with increase in the amount of DHN in the feed. We are further continuing the work to functionalize DHN-containing PPO block to be used as
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compatibilizers for PPO blends as well as to produce other PPO block copolymers.
References [1] Hay AS. Prog Polym Sci 1999;24:45. [2] Hay AS. J Polym Sci Polym Chem Ed 1998;36:505. [3] Parodi F. In: Allen G, Bevington JC, editors. Comprehensive polymer science, vol. 5. New York: Pergamon Press; 1989. p. 157. [4] Jayakannan M, Ramakrishnan S. Macromol Rapid Commun 2001;22:1463. [5] Staffin GD, Price CC. J Am Chem Soc 1960;82:3632. [6] Price CC, Chu NS. J Polym Sci 1962;61:135. [7] Blanchard HS, Finkbeiner HL, Russell GA. J Polym Sci 1962;58:469. [8] Stammatoff GS. US Pat 3257358, 1996. [9] Price CC. Acc Chem Res 1974;7:294. [10] Percec V, Shaffer TD. J Polym Sci Polym Lett Ed 1986; 24:439. [11] Muhlbach K, Percec V. J Polym Sci Polym Chem Ed 1987;25:2605. [12] Wang JH, Percec V. Polym Bull 1991;25:25. [13] Hay AS. US Pat 3402144, 1968. [14] Hay AS, Chalk AJ. Polym Lett 1968;6:105. [15] Chalk AJ, Hay AS. J Polym Sci A-1 1969;7:691. [16] Botick EE, Hay AS, Chalk AJ. US Pat 3522326, 1970. [17] Flory PJ. Principles of polymer chemistry. New York: Cornell University Press; 1953. [18] Percec V, Wang JH. J Polym Sci Polym Chem 1991;29:63. [19] Van Aert HAM, Burkard MEM, Jansen JFGA, van Genderen MHP, Meijer EW, Oevering H, et al. Macromolecules 1995;28:7967. [20] Van Aert HAM. Ph.D. Thesis, Eindhoven University of Technology, The Netherlands, 1997. [21] Percec V, Wang JH. J Polym Bull 1990;24:493. [22] Percec V, Wang JH, Clough RS. Macromol Chem Macromol Symp 1992;54/55:258. [23] Jayakannan M, Ramakrishnan S. J Polym Sci Polym Chem 2001;39:1615. [24] Jayakannan M, Ramakrishnan S. Macromol Chem Phys 2000;201:759.
European Polymer Journal 40 (2004) 1169–1175
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Synthesis and characterization of new telechelic poly(phenyleneoxide)s M. Jayakannan *, T.R. Smitha Polymer Science Division, Regional Research Laboratory, Thiruvananthapuram 695019, India Received 20 August 2003; received in revised form 3 November 2003; accepted 9 December 2003 Available online 4 February 2004
Abstract New well-defined telechelic poly(phenyleneoxide)s (PPO’s) were synthesized from 4-bromo-2,6-dimethylphenol and bi-phenolic compounds through phase transfer catalyzed aromatic nucleophilic substitution polymerization. Bisphenol–A (BPA), 4,40 -biphenol (BP), hydroquinone (HQ) and 2,6-dihydroxynaphthalene (DHN) were employed as telechelic units. The composition analysis by proton-nuclear magnetic resonance (1 H-NMR) spectroscopy revealed that DHN was highly reactive compared to BPA and HQ, whereas BP was un-reactive in the polymerization process. The number average repeating unit (n) in telechelic PPO was estimated as n ¼ 17–19 and n ¼ 17–20 for DHN and BPA (or HQ), respectively. The reactivity of the bi-phenolic in PPO synthesis are confirmed as DHN > HQ BPA P BP. The molecular weight determination by gel permeation chromatography (GPC) and viscosity method suggest that the molecular weight of PPO decreased drastically with increasing amount of bi-phenolic units in the feed. The GPC chromatogram of PPO showed a bi-modal distribution, clearly indicative of formation of two different types of molecular weight chains, whereas the telechelic polymers have a mono-modal distribution with a narrow polydispersity. Thermal analysis by differential scanning calorimetry revealed that telechelic polymers are highly amorphous, like PPO, and no crystallization or melting peaks were observed in the heating/cooling cycles. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Poly(phenyleneoxide)s; Telechelic polymers; Phase transfer catalyzed polymerization; Thermal analysis
1. Introduction Poly(phenyleneoxide) (PPO) is a well-known commercially important thermoplastic polyether. Its high glass transition temperature, impact and tensile strength make it an attractive candidate for automobile industry. PPO has outstanding resistance to most of the aqueous reagents being unaffected by acids, alkali, and detergents. PPO is commercially prepared from 2,6-dimethylphenol through oxidative coupling polymerization route in presence of copper/amine catalysts [1]. Ratio of amine to cuprous ion and reaction temperature are very
* Corresponding author. Tel.: +91-471-2515316; fax: +91471-2491712. E-mail addresses: [email protected], [email protected] (M. Jayakannan).
critical factors in the extent of C–O coupling (polymerization) relative to C–C coupling (dimerisation side reaction). Higher amine/cuprous ratio and lower reaction temperature favor polymerization while dimerisation is favored at higher temperature and lower amine/ cuprous ratio [2]. PPO prepared by this route has few limitations such as (a) promotion of branching via Ar– CH3 groups during high temperature processing by the un-reacted amine and copper salts (b) coloring due to presence of C–C coupled products (c) hazardous to health due to the evolution of unpleasant odor during the extrusion and injection molding. Hence, PPO free from the amine and copper salts would be a very attractive candidate for new applications and environmental safety. Aromatic nucleophilic substitution reaction (SN Ar) is a commonly used method for the preparation of poly(aryleneether)s such as poly(etherketone)s and
0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2003.12.019
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poly(ethersulfone)s [3,4]. The synthesis of PPO was also attempted by this method from 4-bromo-2,6-dimethylphenol (BDMP) using catalytic amounts of an oxidative initiator such as lead oxide and potassium ferricyanide [5–9]. These earlier attempts were found to lead to uncontrolled functionality and branching and failed in producing high molecular weight PPO [6]. The more recent phase-transfer catalyst (PTC) methodology developed by Percec et al. leads to well-defined PPO with controllable molecular weights and functionality [10,11]. This route has many advantages over oxidative coupling, because the polymer produced is of very good quality as they do not have any amine/Cu salt contaminations. However, the PTC route was not so far attempted in the manufacture of PPO and it is still in the research level. The requirement of additional brominating step in the monomer synthesis and the other manufacturing difficulties could be the reason for this limitations [8]. Nevertheless, the aromatic substitution reaction can tolerate a wide range of functionalities and is certainly a powerful tool for the PPO copolymer synthesis [12]. Some of the limitations that had restricted the direct application of PPO as such is its high glass transition temperature (Tg ¼ 220 °C), poor melt flow and the susceptibility of its methyl group to thermal oxidation. PPO crystallizes only with difficulty and behaves as an amorphous polymer during melt processing. To overcome the processing difficulty, usually PPO is blended with other polymers like polystyrene, polycarbonates and polyesters for commercial applications [2]. Attempts to improve the blending process and micro-phase separation of other PPO blends is a very active area of research for scientific communities both in industries and academic. Developing telechelic-PPO (telechelic––means more functionality) with reactive sites is one of the approach to produce stable morphology in PPO blends [13–15]. The reactive functional groups induce a type of cross-linking or grafting of PPO chains with other polymers [16]. In a typical A–B self polycondensation, telechelic architectures could be introduced by copolymerizing with monomers containing A0 n (or B0 n ) functional groups, where n P 1. Here, three cases can occur depending upon the Ôn’ values as shown in Scheme 1. In case-a, n ¼ 1, statistically the growth of the chain is disturbed and low molecular weight chains are produced with ends A0 and B, respectively; case-b, n ¼ 2, produces a functionalized linear telechelic with A0 –A0 as a middle point and A in both ends; and in case-c, n P 3, brush type or branched polymers having A0 n as a core and A in all ends are produced. In case-c, it is very important to stop the polymerization at low conversions otherwise the highly branched polymers produce cross-linked net work leading to gel [17]. 2,4,6-Trimethylphenol and 4-t-butyl-2,6-dimethylphenol capped PPO’s have been developed (example for
A B
B
A B
A n
A B + A'
A
A B
A'
End capping
n
Telechelic unit A B + A' A'
A B
A' A'
A B
B
A B
x
A' A B +
A
A'
A'
B
A B
A'
B
A B
A'
A B z
A'
A y
x
A A
y
Branching site
Scheme 1.
case Ôa’) and used for blending with other polymers [18]. Since the properties of linear polymers are different from the branched systems, not much report are seen in the literature for branched PPOs. The linear telechelic PPO’s (case b) are very attractive because they can be further used as macro-monomers for synthesizing various other polymers. Van Aert et al. reported the synthesis of low molecular weight telechelic PPO through precipitation polymerization-oxidative coupling route [19]. Further, the PPO-oligomers are functionalized at the ends to employ as compatibilizers for PPO-polyester blends [20]. Percec and coworkers explored the possibilities of linear telechelics using bis(3,5-dimethyl-4hydroxyphenyl)methane in PPO [21]. In this report, we have synthesized well-defined linear telechelic PPO’s via aromatic SN Ar polymerization using four commercially available bi-phenolic compounds. 2,6-dihydroxynaphthalene (DHN), bisphenol-A (BPA), hydroquinone (HQ) and 4,40 -biphenol (BP) are chosen for this purpose. The bi-phenolic unit act as a A0 –A0 type monomer and it reacts with the B functionality in the A–B monomer; in this case, BDMP.
2. Experimental 2.1. Materials 4-Bromo-2,6-dimethylphenol (BDMP), bisphenol-A (BPA), hydroquinone (HQ), 4,40 -biphenol (BP), 2,6-dihydroxy naphthalene (DHN) and tetrabutylammoniumhydrogen sulphate (TBAHS) were purchased from Aldrich Chemicals and used without further purification. Sodium hydroxide, toluene and other solvents were purchased locally and purified using standard procedures.
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2.2. Measurements
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3. Synthesis of poly(phenyleneoxide) (PPO)
1
H and 13 C-NMR spectra of the polymers were recorded using 300-MHz Brucker NMR spectrophotometer in CDCl3 containing small amount of TMS as internal standard. For 13 C-NMR experiments, the carbon atom in CDCl3 was taken as 77 ppm and all other peaks are assigned with respect to it. Thermal analysis of polymers were done using DSC 2920 Differential Scanning Calorimeter (TA Instruments). About 10 mg of the sample was taken in Aluminum (Al) pan and 10°/ min heating/cooling rate was applied to record the thermograms. The instrument was calibrated with Indium standards. The inherent viscosity (ginh ) of PPO and copolymers were measured using Ubbelohde viscometer at 25 °C. 0.5 wt.% of the polymer solutions were prepared by dissolving 125 mg of polymers in 25 ml of tetrahydrofuran (THF) and filtered through Whatmann No.1 filter paper prior to the viscosity measurement. An average of three readings (±1 s accuracy) was taken for each measurement. Gel permeation chromatography was also employed to determine the molecular weights of the polymers. Waters 510 Pump connected through three series of Styragel HR columns (HT-3, HT-4 and HT-5) and Waters 410 Differential Refractometer is used for analyzing the samples. The columns were calibrated using different molecular weight polystyrene standards. The flow rate of the THF was maintained as 1 ml throughout the experiments and 1 wt.% (10 mg in 1 ml) of polymer solution was filtered and injected for recording the GPC chromatograms.
Polyphenyleneoxide (PPO) was prepared via phase transfer catalyzed route. Sodium hydroxide (8.0 g, 200 mmol) was dissolved in distilled water (25 ml) and taken in a 100 ml two neck flask. It was provided with a mechanical stirrer at one neck and cooled to 0 °C using ice. BDMP (1.9 g, 20 mmol) was added and stirred for 15–20 min to enrich the formation of sodium phenolate ions. The phase-transfer catalyst TBAHS (0.34 g, 1 mmol) and the cosolvent toluene (25 ml) were added and the polymerization proceeded with vigorous stirring at 25 °C for 18 h. The aqueous layer was separated out; the toluene layer was washed with dilute HCl (50 ml, 1:1 v/v) and further with water to remove the inorganic impurities. The organic layer was precipitated into ethanol; the resultant white fiber-like polymer was dissolved in THF and re-precipitated again into methanol. The purification procedure was repeated few times to remove the monomer and oligomeric species. The white fiber polymer was dried well prior to further analysis. Yield ¼ 1.9 g (80%). 1 H-NMR (CDCl3 ): 6.47 ppm (s, 2H, Ar–H) and 2.17 ppm (s, 6H, Ar–CH3 ). 13 C-NMR (CDCl3 ): 154.75, 145.46, 132.55, 114.47, 110.54 (Ar–C), 16.77 ppm (Ar–CH3 ).
4. Synthesis of DHN and BPA containing telechelic PPO To an ice-cold NaOH solution (8 g, 200 mmol, 65 ml), 2,6-dihydroxy naphthalene (0.32 g, 2 mmol, for polymer PPO-2) was added and stirred well to obtain a
Table 1 Yield, viscosity and compositions of polymers Polymer
Monomer
Yielda (%)
ginh b (dl/g)
Monomer in feed (mol%)
Monomer in telechelic PPO (mol%)c
nd
PPO PPO-1 PPO-2 PPO-3 PPO-4 PPO-5 PPO-6 PPO-7 PPO-8 PPO-9 PPO-10
– DHN DHN BPA BPA HQ HQ HQ BP BP BP
80 33 10 31 37 49 38 37 50 48 42
0.46 0.21 0.14 0.28 0.28 0.40 0.17 0.24 0.35 0.31 0.35
–
– 1.8 5.2 – 1.2 0.8 1.4 2.0 – – –
>100 17 19 70 37 61 30 28 64 51 55
a
5 10 5 10 5 10 15 5 10 15
Calculated for isolated polymers. For 0.5 wt.% of polymer solution in THF at 30 °C. c Incorporation of comonomers are calculated from the 1 H-NMR spectra by comparing the peak intensities at 6.47 ppm (PPO main chain protons) with the bi-phenolic unit protons. d Number average repeating units calculated from the 1 H-NMR spectra by comparing the peak intensities at 6.47 (PPO main chain protons) and 6.36 ppm (PPO main end protons). b
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pale blue color sodium salt. Excess amount of water is required (instead of 25 ml as in PPO synthesis) in order to increase the solubility of the sodium salts. BDMP (3.6 g, 18 mmol) was added and the reaction mixture was stirred well before adding TBAHS (0.34 g, 1 mmol) and toluene (25 ml). The rest of the procedure is same as that described for PPO. The polymer was isolated as white powder and it weighed 0.3 g after drying (10% yield). The other DHN and BPA containing telechelic copolymers (PPO-1, PPO-3, and PPO-4) were prepared by following the similar procedures and their yields are reported in Table 1. 1 H-NMR (CDCl3 , for Polymer PPO-2): 7.25 ppm (s, 4H, Ar–H); 7.10 ppm (s, 4H, Ar– H); 6.47 ppm (s, 2H, Ar–H) and 2.17 ppm (s, 6H, Ar– CH3 ). 13 C-NMR (CDCl3 ): 154.75, 145.46, 132.55, 114.47, 110.54 (PPO–Ar–C), 133.79, 131.72, 129.02, 128.28, 144.03 ppm (Naph–Ar–C) and 6.77 ppm (Ar– CH3 ). 1 H-NMR (CDCl3 , for Polymer PPO-4): 7.24 ppm (b, 4H, Ar–H); 7.15 ppm (s, 2H, Ar–H); 6.47 ppm (s, 2H, Ar–H); 2.21 ppm (s, 6H, C-CH3 ) and 2.17 ppm (s, 6H, Ar–CH3 ).
5. Synthesis of HQ and BP containing telechelic PPO The solubility of di-sodium salts of HQ and BP are very high similar to BDMP and needs only minimum amount of water (25 ml) to carry out the polymerization. The telechelic copolymers PPO-5 to PPO-10 were synthesized by varying the amount of comonomers (5, 10 and 15 mol%) in the feed with respect to BDMP (95, 90 and 85 mol%, respectively). The yields of the polymers are reported in Table 1. 1 H-NMR (CDCl3 , for Polymer PPO-6): 7.24 ppm (s, 4H, Ar–H); 6.47 ppm (s, 2H, Ar–H) and 2.17 ppm (s, 6H, Ar–CH3 ). 1 H-NMR (CDCl3 , for Polymer PPO-9): 6.47 ppm (s, 2H, Ar–H) and 2.17 ppm (s, 6H, Ar–CH3 ). No other new peaks.
CH3 Br
OH CH3
O
TBAHS 25 C, Toluene/H2O
n
o
CH3 PPO
Scheme 2. Synthesis of PPO.
polymers and produces very high molecular weight chains. The chain growth in this case takes place by the addition of the phenoxy-radical to a phenolate-anion and therefore, PTC is also referred to as an anionradical reaction [22]. In order to synthesize linear telechelic PPO’s, we have chosen four bi-phenolic compounds such as 2,6dihydroxynaphthalene (DHN), bisphenol-A (BPA), hydroquinone (HQ) and 4,40 -biphenol (BP). The biphenolic unit act as a A0 –A0 type monomer and reacts with the B functionality in the A–B monomer, in this case, BDMP (see Scheme 3). The reactivity of A0 and A (phenol groups) towards B (aromatic bromide) in the polymerization condition is a critical factor for producing the telechelic PPO. Four series of telechelic PPO’s are synthesized by copolymerizing bi-phenolic compound with BDMP using the identical polymerization conditions as described for PPO. The amount of telechelic units in the PPO was varied by using different amounts (5, 10 and 15 mol%) of DHN, BPA, HQ and BP in the feed. The polymerization of 5 and 10 mol% of DHN with BDMP resulted in telechelics PPO-1 and PPO-2, respectively. The attempt to use more than 15% of DHN in the feed was not successful.
CH3 Br
+
OH
HO
OH
CH3
6. Results and discussion Poly(phenyleneoxide), PPO, was synthesized from 4-bromo-2,6-dimethylphenol (BDMP) by aromatic nucleophilic substitution reaction using a phase-transfer catalyst (see Scheme 2). Percec et al. had developed this method and now it has been extensively practiced for synthesizing various PPO copolymers [10]. Typically, the polymerization was carried out by stirring the monomer, base and PTC (TBAHS) in water/toluene solvent mixture at room temperature for 18 h. The advantages in the PTC route is that the selective solubility of the polymers in toluene and the polycondensation bi-product, NaBr in water, shifts the equilibrium towards
CH3
NaOH
CH3 HO
O
O
O CH3
CH3
CH3
CH3
CH3
O y CH3
x
OH CH3
Telechelic PPO
HO
OH
= HO HO
OH HO
OH
OH
HO
Scheme 3. Synthesis of telechelic PPO.
OH
M. Jayakannan, T.R. Smitha / European Polymer Journal 40 (2004) 1169–1175
The structures of the PPO and telechelics PPO-1 and PPO-2 were confirmed by 1 H and 13 C-NMR spectroscopy and their corresponding spectra are shown in Fig. 1 (for simplicity only the aromatic region is shown). The peak at 6.47 and 6.36 are corresponding to the aromatic protons of PPO in the repeating units and chain ends, respectively [18]. In Fig. 1b, the appearance of two new peaks at 7.25 ppm (two protons next to the ether linkage) and 7.17 ppm are corresponding to the aromatic protons in naphthalene units in PPO-2. The comparison of peak intensities at 6.47 and 7.17 ppm gave the actual incorporation of DHN in PPO-1 and PPO-2 as 1.8 and 5.2 mol%, respectively (see Table 1). The peak at 7.25
Fig. 1. 1 H-NMR [(a) PPO and (b) PPO-2] and 13 C-NMR Spectra [(c) PPO and (d) PPO-2] of polymers. The peak in asterisk is corresponding to the chloroform.
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ppm was not used for the composition analysis since it overlaps with the chloroform solvent peak at 7.26 ppm. The incorporation of DHN is also confirmed from the appearance of new peaks at 133.79, 131.72, 129.02 and 128.28 ppm (in Fig. 1d) in addition to the aromatic carbon peaks for PPO chains (in Fig. 1c). The molecular weights of the PPO, PPO-1 and PPO2 were determined by GPC in THF using Polystyrene standards for the calibration. The GPC chromatograms of the polymer are shown in Fig. 2 and the molecular weights of the polymers are reported in Table 2. The formation of high molecular weight polymers suggest that the PTC conditions employed for the synthesis is highly effective. The molecular weights of telechelic polymers (PPO-1 and PPO-2) are much lower compared to that of PPO. The chromatogram of PPO is showing a bi-modal distribution instead of the expected mono-modal for solution polymerizations. This indicates the formation of two different types of polymer chains in the PPO synthesis. It may occur followed by the precipitation of high molecular weight PPO chains from the toluene layer during the polymerization. Jayakannan et al. has reported a similar bi-modal observation in the bis-benzyl polyether synthesis [23,24]. Percec et al. also reported similar bi-modal distribution in their high molecular weight PPO synthesis [18]. Here, interestingly, the telechelic polymers PPO-1 and PPO-2 showed a mono-modal distribution, unlike PPO. The incorporation of DHN limits the formation of very high
Fig. 2. GPC chromatograms of polymers in THF. (a) PPO, (b) PPO-1 and (c) PPO-2.
Table 2 GPC molecular weights and Tg of PPO and copolymers Polymer
Mp (g/mol)
Mn (g/mol)
Mw (g/mol)
Mw /Mn
Tg (°C)
PPO PPO-1 PPO-2
121,000 50,650 41,900
87,400 48,200 41,300
108,800 53,300 44,100
1.25 1.17 1.07
220.5 194.7 186.8
(a) Determined by GPC using Polystyrene standards as references in THF. (b) Determined by DSC at 10°/min heating rate.
M. Jayakannan, T.R. Smitha / European Polymer Journal 40 (2004) 1169–1175
The inherent viscosity of all the polymers were measured in THF at 25 °C for 0.5 wt.% polymer solution and the values are reported in Table 1. The ginh values of the copolymers were plotted against the comonomer incorporation in the feed and shown in Fig. 3. It is very clear from the plot that in all the cases, the increase in the comonomer amount in the feed decreases the molecular weight of the resultant polymer. The plot suggests that the DHN produces low molecular weight polymers when compared to other phenolics. This is in good agreement with the NMR results. Thermal properties of the PPO and telechelic PPO were analyzed by differential scanning calorimeter. The sample was taken in an Al pan and heated to 280 °C at 10°/min and then quenched to 30 °C at 10°/min. The quenched samples were reheated to 280 °C at 10°/min to record the thermal transitions. The DSC thermograms of the PPO, PPO-1 and PPO-2 are shown in Fig. 4. It is very clear from the plot that, the polymers showed only
0.50
Inherent Viscosity (dl/g)
molecular weight polymers and hence all the polymer chain lengths produced are almost of the same length. BPA, HQ and BP are also employed as bi-phenolic compounds to synthesize telechelics PPO-3 to PPO-10. The extent of incorporation and yields are listed in Table 1. The limited solubility of BPA sodium salts in water hampered its incorporation more than 10 mol% in PPO. On the other hand, the high solubility of HQ and BP sodium salts allowed up to 15 mol% sample to be taken in the feed. The 1 H-NMR spectra of PPO-4 showed two new peaks at 7.24 and 7.10 ppm, which are assigned to the aromatic protons in BPA. Similarly, a new peak at 7.24 ppm is also noticed in the 1 H-NMR spectrum of PPO-6 corresponding to the HQ protons. Surprisingly, no new peaks have been observed for PPO-9 and the spectrum is almost identical to that of PPO (Fig. 1a), which suggests that BP is not incorporated in PPO. It may be that the non-coplanarity of the bi-phenyl rings makes the phenol less acidic in BP compared to that of BDMP in the SN Ar reaction. The compositions of telechelics were determined by comparing the intensities of aromatic protons in the PPO chain (6.47 ppm) with that of comonomers. In PPO-4, the peak at 7.24 ppm is not used for composition analysis, since they are very close to the chloroform solvent peak. However, in the case of PPO-6, the peak at 7.24 ppm is used because no other peak is available for finding the HQ incorporation in the telechelic PPOs. The amount of incorporation of BPA and HQ in PPO are calculated and reported in Table 1. It is very clear from the table that the DHN is more reactive than BPA and HQ. From the current investigation, the reactivity of the bi-phenolic in PPO synthesis could be written as DHN > HQ BPA P BP. In general, the incorporation of bi-phenols (DHN BPA and HQ) in the PPO chain is relatively low. It may be due to the poor nucleophilic substitution reactions of aryl bromides towards the electron rich bi-phenols [3]. The comparison of the aromatic peak intensities at 6.47 and 6.34 ppm give the average number of repeating units (n) in the polymer chain (in Fig. 1) [18]. The values are reported for all the polymers in Table 1. PPO has more than 100 repeating units in the polymer chain indicating the formation of high molecular weight polymers. The incorporation of DHN produces short polymer chains with total n ¼ 17–19. It indicates that DHN produces small well-defined block copolymers having average of 8–10 PPO chains on either side of the naphthalene units. At 10 mol% incorporation, both BPA and HQ produce a much longer block telechelic with 15–18 PPO units on either side. The GPC molecular weights of the PPO, PPO-1 and PPO-2 reported are much higher than that determined by NMR. It may be attributed to the difference in the hydrodynamic volume for the polystyrene standards and PPO chains, which could overestimate the molecular weights of PPO, PPO-1 and PPO-2.
0.45 0.40 0.35
BP
0.30 BPA
0.25
HQ
0.20 0.15
DHN
0.10 0
2
4
6
8
10
12
14
16
Comonomer in Feed ( mol %) Fig. 3. Plot of inherent viscosity versus the amount of comonomer in the feed.
Heat Flow (mCal/mg) (a.u)
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PPO
Tg = 220.5 oC PPO-1 Tg = 194.7 oC
PPO-2 o Tg = 186.8 C
170
180
190
200
210
220
230
240
o
Temperature ( C) Fig. 4. DSC thermograms of polymers at 10°/min heating rate.
M. Jayakannan, T.R. Smitha / European Polymer Journal 40 (2004) 1169–1175
a glass transition (Tg ) temperature and no melting or crystallization peaks were observed. In order to investigate the quenching effect on the thermal transition, the polymers were heated to 280 °C and the sample pan was rapidly quenched on a metal block that was kept in contact with liq. N2 . The rapidly quenched samples were heated/cooled at 10°/min to record the thermal transitions. The plots were found identical to that reported in Fig. 4 and no melting/crystallization peaks were observed. It suggests that PPO and the telechelic copolymers are highly amorphous. It is interesting to note that the Tg of the PPO decreased with the increase in the naphthalene contents in the copolymers. Since the incorporation of BPA and HQ in PPO is not very significant (<2%) compared to DHN, their thermal properties and GPC are not investigated.
7. Conclusion New class of telechelic PPOs were synthesized through A–B and A0 –A0 copolymerization route. The composition analysis by NMR reveals that DHN is highly reactive in the polymerization conditions compared to BPA and HQ; but BP was found to be unreactive. The inherent viscosity of the polymers decreases with increase in the bi-phenolic units and the decrease is very drastic for DHN containing telechelic polymers. GPC plot showed a bi-modal distribution for PPO corresponding to the formation of two ranges of molecular weight species during the polymerization. Interestingly, the incorporation of telechelic units produces a mono-modal molecular weight distribution. The polymers were highly amorphous and they have only glass transitions, no melting/crystallization peaks are observed in the heating/cooling cycles. The Tg of the PPO decreased with increase in the amount of DHN in the feed. We are further continuing the work to functionalize DHN-containing PPO block to be used as
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compatibilizers for PPO blends as well as to produce other PPO block copolymers.
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