Chinese Journal of Polymer Science Vol. 25, No. 3, (2007), 297−302
Chinese Journal of Polymer Science ©2007 World Scientific
PYRIDINATION OF POLY(VINYL CHLORIDE) via A HOMOLYTIC PATHWAY Saâd Moulay* and Zakia Zeffouni
Chinese J. Polym. Sci. 2007.25:297-302. Downloaded from www.worldscientific.com by NATIONAL UNIVERSITY OF SINGAPORE on 11/28/15. For personal use only.
Laboratoire de Chimie-Physique Moléculaire et Macromoléculaire Faculté des Sciences de l’Ingénieur, Département de Chimie Industrielle, Université Saâd Dahlab de Blida, Route de Soumâa, B. P. 270, Blida 09000, Algeria
Abstract PVC was subjected to a chemical modification aiming at replacing the chlorine atoms by pyridine groups via a homolytic route. Pyridine groups were peculiarly affixed to the PVC matrix via a carbon-carbon bond rather than a carbonnitrogen one. PVC was first iodinated using the Conant-Finkelstein reaction, followed by the application of the homolytic conditions of Minisci to generate pyridinated PVC with a varying degree of substitution. The extent of substitution was not high; the reaction on 30% iodinated PVC afforded a degree of modification no greater than 3%.
Keywords: Conditions of Minisci; Iodinated PVC; Pyridinated PVC.
INTRODUCTION Chemical reactions on polymers have proved to be fascinating alternatives to make new materials for specific uses. However, the facility of chemically modifying a polymer is contingent upon the chemical reactivity of the atom, the group, or the functionality present in the polymer. Poly(vinyl chloride) (PVC), a world-wide used plastic, has undergone many chemical transformations via substitution, elimination, reduction, grafting, crosslinking, and degradation reactions[1, 2]. The main reason for modifying PVC has been always the remedy of its well-known instability that stems from its structure defects in the PVC manufacture. Substitution reaction on PVC was carried out with a number of nucleophiles, to name a few, azide[3], thiophenate[4], mercaptobenzothiazolate[5], and others[2, 6−11]. Interesting is the PVC modification with pyridine via 4-mercaptopyridine, giving a pyridinated PVC which was subsequently transformed by quaternization into an ionomer[1]. The binding of transition metal ions was assessed by terpyridine moiety pendant from a PVC backbone via a side chain containing thiophenyl group[12]. We wish to report the results of our research on the substitution of chlorine atoms of PVC by pyridine moieties using the homolytic conditions of Minisci[13]. As far as we know, this is the first report on the chemical modification of PVC via a radical process. In addition, the modification resulted in pyridinated PVC in which the pyridine groups are linked to PVC through a carbon-carbon bond and not a carbon-nitrogen one as generally reported. Recently, we have reported the application of Minisci’s conditions to the halomethylated polystyrene to afford pyridinated polystyrene[14]. EXPERIMENTAL Materials and Measurements The chemicals and solvents were purchased from Fluka, Prolabo and Aldrich. Poly(vinyl chloride) (PVC), poly2-vinylpyridine-co-styrene) containing 70% of 2-vinylpyridine, trifluoroacetic acid, and sodium iodide were
*
Corresponding author: Saâd Moulay, E-mail:
[email protected] Received June 8, 2006; Revised June 25, 2006; Accepted July 6, 2006
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used without further purification. Benzoyl peroxide (BPO) was purified by recrystallization from methanol. Pyridine, tetrahydrofuran (THF), methanol, and acetone were distilled before use. Infrared spectra were recorded using a Genesis II FTIRTM; the samples were in the form of either cast films or KBr pellets. UV-Visible spectra were recorded using a UV–Visible spectrophotometer Shimadzu 1201; analytical grade THF was employed for UV analyses. The viscometric measurements were performed in THF at 25°C with a Cannon Ubbelohde capillary viscometer 532 10 / I, Schott-Gerate CT 1650; The average-viscosity molecular weights Μv were estimated by the relation of Mark-Houwink-Sakurada, [η] = KΜva where [η] is the intrinsic viscosity, K and a are the Mark-Houwink constants; the latter ones were taken as 16.3 × 10−5 dL/g and 0.766, respectively, for these conditions[15]. Differential scanning calorimetry thermograms were recorded on Setaram Labsys DSC 16; samples in the range of 12−18 mg were heated up to 100°C under N2 at a heating rate of 20 K/min, and quenched to 25°C. The Tg values were taken from the second run at a rate of 5 K/min, and estimated as the midpoints of the first transition curves. Reaction of PVC with NaI Into a 250 mL three-necked round-bottomed flask fitted with a condenser, a thermometer, a magnetic bar and a nitrogen inlet, 1 g of PVC (16 × 10−3 mol) was charged and dissolved in 50 mL of THF. Sodium iodide (2.4 g, 16 × 10−3 mol) was added as a solution in 100 mL of acetone. The mixture was then stirred for half an hour before heating. After 20 h of reaction, the polymer was precipitated by pouring the solution into methanol:water (2׃1, V׃V ) and being purified by dissolution/ precipitation cycles. The purified products were dried at 40°C in vacuo until constant weight. The degree of substitution was ascertained by gravimetric titration of the unreacted iodide with PbNO3 resulting in the yellow precipitate, PbI2[14, 16]. The iodine-modified PVC’s are herein designated as PVC-I30, PVC-I27, and PVC-I11, corresponding to the degrees of substitution 30%, 27%, and 11%, respectively. Reaction of Iodinated PVC with Protonated Pyridine Into a 250 mL three-necked round-bottomed flask fitted with a condenser, a thermometer, a magnetic bar and a nitrogen inlet, a 50 mL THF solution of pyridine, trifluoroacetic acid and benzyl peroxide was charged. This solution was stirred for 15 min before the addition of a 50 mL THF solution of PVC-I. The system was then allowed to stir for 24 h at 60°C. The molar ratio of [PVC-I]/[Pyridine]/[CF3COOH]/[BPO] was 1׃1׃1׃1. The modified PVC-I was isolated by precipitation into a 10% water-methanolic solution of NaOH, purified by dissolution/precipitation cycles, and finally dried at 40°C in vacuo until constant weight. The modified PVC-I was designated by PVC-Py and the degree of substitution was monitored by UV-Visible spectroscopy, providing a calibration curve with poly(styrene-co-2-vinylpyridine) at λmax = 256 nm. RESULTS AND DISCUSSION Iodination of PVC PVC was first iodinated using the Conant-Finkelstein reaction as illustrated in Eq. (1). The reaction was carried out in THF/acetone (1׃2, V׃V) solvent at a temperature of 50°C, and for a time as long as 20 h. These reaction conditions were set because the chlorides are mostly secondary. It is worth recalling that, under ConantFinkelstein’s conditions, the tertiary alkyl halides do not react, but the primary and secondary ones do, with the former more readily. As it can be seen in Table 1, an extent of substitution of 30% was obtained at a temperature of 50°C. Lower temperatures led to lower degrees of substitution but the molecular weights remained unchanged. It can also be remarked that the concomitant creation of unsaturation via an elimination during the substitution reaction occurred, as the number of conjugated double bonds (CDB), experimentally determined, may suggest; the higher the temperature, the greater the CDB.
(1)
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Table 1. Results of the modification of PVC* T (°C) DS (%) CDB Mv Tg (°C) PVC 0.20 32900 81 − − PVC-I30 50 30.5 8.14 26700 111 PVC-I27 40 27 2.45 25800 83 PVC-I11 30 11 1.45 35900 85 60 2.79 25000 106 PVC-Py3 − PVC-Py2 60 1.78 23200 79 − PVC-Py1/2 60 0.45 32000 75 − * DS is the degree of substitution, CDB the number of conjugated double bonds, Mv the average-viscosity molecular weight, and Tg the glass transition temperature. PVC-Py3, PVC-Py2, and PVC-Py1/2 are the corresponding pyridinated PVC-I, with degrees of substitution 2.79%, 1.78%, and 0.45%, respectively; they are the products of the pyridination of PVC-I50, PVCI 27 , and PVC-I 11 , respectively, under the following conditions: temperature of 60°C, time of 24 h, and [PVC-I]/ [Pyridine]/[CF3COOH]/[BPO]: 1׃1׃1׃1.
At 40 and 50°C, the polymer was substituted with a concomitant degradation via, probably, a chain scission and/or a dehydrochlorination as the molecular weights were found lower than that of the virgin PVC. However, at 30°C, a higher molecular weight was obtained implying the occurrence of substitution with no degradation. In all cases, the obtained polymer was white to slightly yellow and readily soluble in THF. A significant decrease in the degree of substitution, DS, can be noted at a temperature of 30°C. At 40 and 50°C, a relatively substantial substitution seemed to be always accompanied with an appreciable extent of elimination. The PVC-I’s were characterized by IR and UV-Vis. The UV spectra of all modified PVC-I’s showed two bands at λmax = 295 and 366 nm, with the first band being larger. For an illustration, the UV spectrum for the modified PVC-I30 is given in Fig. 1. Those absorption bands at wavelengths higher than 280 nm (for the starting PVC) would owe to the creation of highly conjugated systems: a set of trienes (λmax = 295 nm) and pentaene sequences(λmax = 366 nm). Simionescu et al. reported that the UV spectrum of phenated PVC produced at 50°C showed a series of bands between 275 ad 450 nm that were assigned to polyenic domains composed of 2 to 6 conjugated double bonds[17]. In the IR spectrum of PVC-I30 (Fig. 2), new bands of weak intensities appeared at 1640, 1720, and 1770 cm−1. Conspicuously, the ratio A1064/A964 (the ratio of absorbances at 1064 and 964 cm−1, corresponding to the absorption of C C stretching and CH2 rocking, respectively), which was 1.05 for raw PVC, dropped to lower values, 0.53, 0.83, and 0.95 for PVC-I30, PVC-I27, and PVC-I11, respectively.
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Fig. 1 UV-Vis spectrum of PVC-I30 THF as solvent, 0.25 g/L
Fig. 2 FT-IR spectrum of PVC-I30
The Tg’s were found to be 85, 83, and 111°C for the PVC-I11, PVC-I27, and PVC-I30, respectively. The Tg changes seem not to vary systematically with the molecular weight, the degree of substitution, and the extent of unsaturation, probably because the composition of the polymeric chain is heterogeneous with varying degrees of different microstructures.
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Pyridination of PVC The homolytic alkylation of heteroaromatic bases, such as the pyridine derivatives, is best carried out with alkyl iodide (R-I) because the C I bond dissociation energy (BDE) is in the range of 50−56 kcal/mol[18], values lower than BDE’s of other alkyl halides, meaning the iodine is the most labile halogen atom. Hence, with alkyl iodides, the alkyl radical can be readily formed. Therefore, to apply the Minisci’s conditions to the PVC modification with pyridine, the replacement of chlorine atoms by iodine ones is imperative, as studied above. In fact, when the conditions shown in Eq. (2) were applied to the raw PVC, no substitution occurred as confirmed by UV-Vis and FT-IR spectroscopy analyses. In this equation, pyridine is homolytically alkylated by a polymeric backbone, PVC-I conceived as the alkylating polymer.
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The mechanism proposed for this modification can be the one advanced by Minisci[13, 19], extended to the polymeric analogue as depicted in Scheme 1. Benzoyl peroxide is first decomposed into phenyl radicals which abstract the labile iodine atoms, giving PVC macroradicals, carbon-centered radicals. The latter radicals act as nucleophilic alkylating species on the protonated pyridine, considered electron-deficient entity, at the carbon C-2 (α to the nitrogen atom). Upon such a homolytic alkylation, the pyridine moiety loses its aromaticity which can be restored (rearomatization) by oxidation of the PVC-pyridinyl radical with benzyl peroxide. Indeed, the rearomatization is actually promoted through the induced decomposition of BPO by the strongly nucleophilic pyridinyl radical, being a potent reducing agent.
Scheme 1 Proposed mechanism for the modification of PVC-I by pyridine
The reaction of PVC-I with protonated pyridine in the presence of BPO at 60°C and at a time of 24 h generated a slightly yellow polymer. The modified polymer PVC-Py was soluble in THF. The degrees of modification were low: 2.79%, 1.78%, and 0.45% for PVC-Py3, PVC-Py2, and PVC-Py1/2, respectively. In
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addition, the molecular weights of these resins were somewhat lower than those of PVC-I’s, indicating that this transformation did not alter the polymeric chain length by any kind of phenomenon because the heavy iodine atom is substituted with a lighter pyridine group. The UV-Vis spectra of different PVC-Py’s (Fig. 3) exhibit two distinct absorption bands at λmax = 295 and 256 nm. The first band corresponds to the triene domains as detected in PVC-I’s, and the second one is attributable to the absorption of the pyridine group. Surprisingly, the absorption band at λmax = 366 nm of the pentaene sequences disappeared and no explanation as to this result can be advanced at the present time.
Fig. 3 UV-Vis spectrum of PVC-Py3 THF as solvent, 10 g/L
Although not quite discernible, the band at 3060 cm−1 in the FT-IR spectrum (Fig. 4) appeared and is attributed to the C H stretching of the pyridine. However, a distinguished band, assigned to the C H deformation of pyridine is clearly seen at 800 cm−1. On the other hand, the very weak absorption bands at 1480, 1540, 1575, and 1597 cm−1 of the aromatic C C and C N bonds of the pyridine, confirm the low extent of modification. These IR bands were also observed by Reinecke[1].
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Fig. 4 FT-IR spectrum of PVC-Py3
As compiled in Table 1, the Tg value of the pyridinated PVC materials seem to be dependent on the degree of substitution. The Tg of PVC-Py3 was higher than that of PVC, but those of PVC-Py2 and PVC-Py1/2 were rather lower. It might be of no surprise that the Tg changes do not follow a certain reasoning because the modified PVC, apart from the structure defects and the microstructures, is actually randomly and heterogeneously composed of several units which are polyenics, vinyl choride, vinyl iodide, and vinylpyridine. Figure 5 shows the effect of the molar ratio [pyridine]/[PVC-I] on the degree of modification. The results were that the increase in pyridine concentration does not improve substantially the substitution, and indeed the molar ratio 3׃2 leads to an optimal degree of substitution of 3%.
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Fig. 5 Plot of the variation of the degree of substitution, as a function of the molar ratio [Pyridine]/[PVC-I]
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CONCLUSIONS This study has shown that the conditions of Minisci can be applied to the modification of PVC to a certain extent. Also, the low degree of modification can throw some light on the complex heterogeneous composition and structure of the raw PVC. Moreover, this study might indicate that the homolytic conditions are not favorable for a quantitative substitution; however, they provide the modification of PVC by pyridine through a C―C direct bonding.
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