ISSN 1560-0904, Polymer Science, Ser. B, 2008, Vol. 50, Nos. 11–12, pp. 315–320. © Pleiades Publishing, Ltd., 2008. Original Russian Text © N.S. Shaglaeva, O.V. Lebedeva, Yu.N. Pozhidaev, R.G. Sultangareev, M.V. Markova, V.N. Salaurov, 2008, published in Vysokomolekulyarnye Soedineniya, Ser. B, 2008, Vol. 50, No. 11, pp. 2035–2041.
Copolymerization of Vinyl Chloride with 1-Vinyl-4,5,6,7-Tetrahydroindole and 2-Methyl-5-Vinylpyridine1 N. S. Shaglaevaa, O. V. Lebedevab, Yu. N. Pozhidaevb, R. G. Sultangareeva, M. V. Markovaa, and V. N. Salaurova a
Favorskii Institute of Chemistry, Siberian Branch, Russian Academy of Sciences, ul. Favorskogo 1, Irkutsk, 664033 Russia b Irkutsk State Technical University, ul. Lermontova 83, Irkutsk, 664074 Russia e-mail:
[email protected] Received August 30, 2007; Revised Manuscript Received April 8, 2008
Abstract—The radical copolymerization of vinyl chloride with 2-methyl-5-vinylpyridine and 1-vinyl-4,5,6,7tetrahydroindole is accompanied by dehydrochlorination. In the vinyl chloride–2-methyl-5-vinylpyridine system, the evolved hydrogen chloride interacts with a pyridine hydrogen atom to give charged units of a heterocycle. In the vinyl chloride–1-vinyl-4,5,6,7-tetrahydroindole system, the hydrogen chloride being formed initiates the cationic dimerization of a nitrogen-containing monomer. The synthesized copolymers based on vinyl chloride surpass the commercial poly(vinyl chloride) in terms of thermal stability and solubility in organic solvents. DOI: 10.1134/S1560090408110043
Poly(vinyl chloride) (PVC) is one of the most important high-volume thermoplastics. Its worldwide production is second only to polyolefins, and it may be processed by almost all known methods. The high content of chlorine in this polymer (up to 57%) determines both its basic advantages and shortcomings. PVC has a very high melt viscosity and is insufficiently stable at the processing temperature. Hydrogen chloride evolving upon heating of PVC catalyzes its further degradation. The softening temperature of PVC is higher than its degradation temperature, and it cannot be processed as a pure polymer. Therefore, stabilizers and plasticizers are introduced into PVC in order to substantially improve the ecological safety of PVC-based materials. Without question, the most promising way of modifying PVC is the copolymerization of vinyl chloride (VC) with different vinyl monomers. This method makes it possible to change the characteristics of the products. The areas of application of VC-based copolymers and their technical characteristics are determined in many respects by the chemical nature of a comonomer. Heterocyclic nitrogen-containing comonomers are used to enhance adhesion properties as well as photo1 This
work was supported by the Russian Foundation for Basic Research (project nos. 06-08-00317 and 06-08-00320).
and thermal stabilization of VC copolymers [1]. The high polymerization activity of nitrogen-containing monomers in copolymerization allows easy variation of the composition and hence of the properties of copolymers. It is expected that hydrogen chloride formed as a result of dehydrochlorination will be bound with the nitrogen atom of a heterocycle and, at a specific ratio of comonomer units, barely compatible characteristics, such as high performance characateristics and processability, may be combined. The aim of this work was to study the radical copolymerization of VC with vinyl derivatives of nitrogencontaining heterocycles and to examine the properties of the synthesized copolymers. 2-Methyl-5-vinylpyridine (MVP) and 1-vinyl4,5,6,7-tetrahydroindole (VTHI) were used as vinyl derivatives of nitrogen-containing heterocyclic compounds. The presence of a pyridine nitrogen atom capable of catching hydrogen chloride formed upon dehydrochlorination makes nitrogen-containing compounds very promising monomers, since the autocatalytic elimination can be excluded. It is also beyond any doubt that nitrogen-containing compounds will favor the evolution of hydrogen chloride.
315
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Tb , °C/Pa
nD
–13.7/101.3 × 103
–
VTHI
85–86/400
1.5562
as a relaxant. The molar ratio of the copolymer components was determined by the usual method: the fraction of one carbon atom of nitrogen-containing monomer qN was taken as equal to one carbon atom. This corresponds to 1 mol of the monomer (åN). The amount of moles of other components was calculated by the formula
MVP
46–48/266
1.5454
åX = qX /qN,
Table 1. Characteristics of monomers Monomer VC
20
EXPERIMENTAL VTHI was synthesized via the Trofimov reaction with a yield of 93% [2, 3]: + HC CH NOH
KOH/DMSO
N
where qX is the fraction of the carbon atom of component X, qX = IX /Itot (IX and Itot are integral intensity of the signal due to the X atom and the total integral intensity, respectively). The relative error of integration is 3%. The relative error of the quantitative determination of comonomers (wt %) and of the elemental composition of copolymers does not exceed 6.7%. 1H
The commercial MVP was vacuum distilled before experiments. The purity of monomers was controlled by measuring constants given in Table 1. The copolymerization of VC–VTHI and VC–MVP systems was performed at 60°ë under free-radical initiation conditions using AIBN as an initiator. Ampoules were filled by the gravimetric method. Once the copolymerization was completed, the reaction mass was dissolved in DMF and the copolymer was precipitated into ice water. The copolymers were reprecipitated from DMF solution into ice water and dried under vacuum to a constant weight. The copolymers were reprecipitated two times. The turbidimetric titration was performed with a KFK-2 photometer using polymer solutions in DMSO or DMF and hexane or acetone as precipitants, respectively. All measurements were carried out at a wavelength of 670 nm. The viscosity of copolymer dilute solutions was measured on an Ubbelohde capillary viscometer at 25°ë. The elemental analysis of reaction products was performed with a Thermo Finnigan gas analyzer. The IR spectra of the copolymers were recorded on a Specord IR-75 spectrometer using samples prepared as KBr pellets or nujol mulls, and on a Bruker IFS-25 spectrometer. The 13C NMR spectra of copolymer samples were recorded on a Varian VXR-500S spectrometer operating at a frequency of 125.5 MHz. Measurements were performed in DMSO-d6 solution under the following condtions: a relaxation delay of 2.5 s and a pulse of 90°. Tris(chromium acetylacetonate) (0.02 mol/l) was used
NMR spectra were recorded on a Bruker DPX-400 spectrometer operating at a frequency of 400.13 MHz in C6ç6–d6 and DMSO-d6. The polymer solution concentration was 1%. The molecular characteristics of copolymer solutions were determined by elastic light scattering on a Milton Roy KMX-6/DC small-angle scattered light laser photometer (United States). A helium–neon laser with a power of 2 W and λ = 630 nm was used as a light source, and the measurements were carried out at a scattering angle of 6.5°. Before measurements, dust was removed from copolymer solutions via filtration through Millipore membrane filters with an average pore diameter of 0.45 µm. The measurement results were treated as described in [4]. The refractive index increment of copolymer solutions (dn/dc)µ was measured with a Milton Roy KMX-16 differential refractometer (United States). A solvent set to dialysis equilibrium with the copolymer solution was used as a reference solution. The thermographic analysis curves were recorded in air on a MOM derivatograph (Hungary); the heating rate was 5 K/min. RESULTS AND DISCUSSION The radical copolymerization of VC with VTHI at different ratios of monomers makes it possible to synthesize copolymers in the form of powders varying in color from yellow to dark brown that are soluble in DMSO, DMF, DMAA, and N-methylpyrrolidone-2 (MP). An increase in the VTHI content of the initial mixture results in a higher yield of the copolymer and a decrease in its intrinsic viscosity (Table 2). The IR spectra of the samples show absorption bands due to the pyrrole ring (690, 912, 1200, 1300, 1380, 1490, and 1540 cm–1) and a C–Cl bond (around 660 cm–1). The 1H
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Table 2. Copolymerization of VTHI (M1) with VC (M2) ([AIBN] = 1.5 wt %, 60°C, 6 h) Composition of copolymer, molar fractions
Composition of monomer mixture, molar fractions
calculation by N
calculation by Cl
[η], dl/g
Yield of copolymer, %
M1
M2
m1
m2
m1
m2
0.23
0.77
0.49
0.51
0.51
0.49
0.18
7
0.39
0.61
0.64
0.36
0.68
0.32
0.09
12
0.52
0.48
0.73
0.27
0.82
0.18
0.08
18
0.72
0.28
0.85
0.15
0.91
0.09
0.07
23
0.88
0.12
0.90
0.10
0.98
0.02
0.05
31
and 13C NMR spectra of the samples exhibit resonance signals due to the following dimer: 8
The above results can be explained assuming that the radical copolymerization of VC and VTHI is accompanied by dehydrochlorination. The eliminated hydrogen chloride reacts with tetrahydroindole to yield an iminium cation. The electrophilic attack of the iminium cation at the pyrrole ring of the VC–VTHI copolymer macromolecule results in VTHI dimers [3, 5, 8].
9
7
10 4
N 13 14
CH
3
5
2
N
CH3
11 12
from [3, 5–7], where VTHI dimers and oligomers were first synthesized and described.
CH2
CH3
NMR: δ 1.77 and 2.66 (cyclohexane fragment), 5.84 and 6.19 (pyrrole ring), 5.17 and 1.54 ( ëç– ëç3) ppm. 1H
NMR: δ 128.99 (ë2, 7), 116.69 (ë3, 8), 106.99 (ë4), 129.04 (C5), 108.67 (C9), 117.50 (C10), 49.87 (C11), 24.23 (C12), 47.94 (C13), 22.64 (C14) ppm. 13C
These values of chemical shifts in 1H and 13C NMR spectra are in good agreement with the similar data
In addition to the VTHI dimer, a VC–VTHI copolymer was isolated, in which a molecule of the pyrrole monomer is added to a VTHI unit in the macromolecular backbone. The formation of copolymers is confirmed by the turbidimetric titration curves, which are of the smooth pattern typical of the single-component system. The results suggest that the combined reaction of VC with VTHI in the presence of AIBN proceeds according to the following scheme:
x
+ y N
x' – n
–HCl
N
y' – z
Cl
n
z
N
Cl
N Me
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Table 3. Copolymerization of MVP (M1) with VC (M2) ([AIBN] = 0.5 wt %, 60°C, 0.5 h) Composition of copolymer, molar fractions
Composition of initial mixture, molar fractions
calculation by N
calculation by Cl
[η], dl/g
Yield, %
M1
M2
m1
m2
m1
m2
0.09
0.91
0.65
0.35
0.65
0.35
0.09
5.7
0.12
0.88
0.70
0.30
0.70
0.30
0.10
6.0
0.20
0.80
0.80
0.20
0.80
0.20
0.11
5.1
0.40
0.60
0.90
0.10
0.91
0.09
0.12
5.9
0.52
0.48
0.94
0.06
0.95
0.05
0.08
7.9
0.60
0.40
0.96
0.04
0.97
0.03
0.19
8.0
0.88
0.12
0.98
0.02
0.99
0.01
0.24
9.1
M × 10–3
Yield, %
Table 4. Copolymerization of MVP (M1) with VC (M2) ([AIBN] = 0.5 wt %, 60°C, 6 h) Composition of monomer mixture, molar fractions
Composition of copolymer calculated by 13C NMR spectra, molar fractions CH=CH
charged heterocycle unit
M1
M2
m1
m2
0.80
0.20
0.9767
0.2333
No data
316
84
0.60
0.40
0.9612
0.0388
No data
189
71
0.50
0.50
0.8225
0.1411
0.0182
0.0182
129
62
0.40
0.60
0.7944
0.1907
0.0149
0.0149
134
53
The radical copolymerization of VC with MVP was carried out in bulk and in the DMF solution. Unlike the insoluble copolymer of N-vinylimidazole with VC obtained in bulk [9], the products of copolymerization of MVP with VC are soluble in organic solvents (C6H6, DMF, DMSO, MP) regardless of the synthesis procedure. In the IR spectra of the copolymers, the absorption bands characteristic of the double bond of MVP (1630 cm–1) are absent, but the spectrum exhibits the bands due to pyridine (1600, 1580, 1490, 1020 cm–1) and a C–Cl bond (around 680 cm–1). The 13C NMR spectra of the VC–MVP copolymers show broadened signals at 156.7, 149.0, 136.2, 135.1, 123.2 (carbon atoms of the pyridine cycle), 38.1 ( ëç2 groups of MVP), 24.1 (the ëç3 group of MVP), and 55.0–58.2 (the ëçël fragment of VC) ppm. The signals due to the ëç2 group of VC and the ëç group of MVP overlap in the region of 40.0–48.2 ppm. The copolymers of VC with MVP (Table 3) form at any comonomer ratio in the studied range of initial mixture compositions and they are always enriched in MVP units. The increase in the heterocycle content in the ini-
tial mixture leads to an increase in the yield and viscosity of the copolymers. Based on the dependence of the compositions of the initial mixture and copolymers at low conversions, the reactivity ratios of VC and MVP were calculated: rVC = 0.04 and rMVP = 14.13. These values indicate that the rate constant for the interaction of a VC radical with MVP is much higher than the rate constant of its interaction with its own monomer and is higher than the analogous value for vinylazoles [10]. The 13C NMR spectra of the MVP–VC copolymers prepared at high conversions and at a VC content in the initial mixture higher than 50 mol % display a set of signals at 128.03–127.58 (~ëç=ëç~ fragment) ppm. This directly indicates the occurrence of the dehydrochlorination reaction (Table 4). The quantitative analysis of the 13C NMR spectrum of the VC–MVP copolymer (the content of the nitrogen-containing monomer in the initial mixture is 50 molar % between MVP and VC units, the vinyl group, and the MVP charged unit is 90 : 15 : 2 : 2. This implies that the number of ~ëç=ëç~ fragments in the MVP–VC macromolecule is equal to the number of
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Weight loss, % 0
40 2
1
3
80
0
200
400
600 T, °C
Curves of weight loss for (1) PVC, (2) VC–MVP copolymer, and (3) VC–VTHI copolymer (equimolar ratio of comonomer units).
charged heterocycle units like in the case of VC–vinylazoles systems [11, 12]. Molecular masses of the samples isolated at high conversions were estimated by laser light scattering as (12.9–31.6) × 104 (Table 4). As was shown in our earlier works, the radical copolymerization of VC with 1-vinylimidazole (VIM), 1-vinylbenzimidazole (VBI) or 1-vinyl-1,2,4-triazole is accompanied by dehydrochlorination only at high conversions (the copolymer yield is 53–92%) [11, 12] and the degree of dehydrochlorination depends on the content of the nitrogen-containing heterocycle in the initial mixture and on the basicity of the mixture. In spite of the similar basicities of VBI and MVP +
( pK BH 5.78 and 5.23, respectively) [11, 12], dehydrochlorination occurs during the copolymerization of VC and VBI in DMF, but it does not take place when the copolymerization of VC with MVP occurs under analogous conditions. This can be explained on the assumption that the reactivity of MVP, unlike VBI [10], is several times higher than the reactivity of VC. Thus, it has been suggested that the copolymer is composed of very short, perhaps even individual, VC units separated by MVP units. Such a distribution of VC units in a macromolecule does not favor dehydrochlorination processes. The results show that, in radical copolymerization with VC, MVP behaves like VIM, VBI, and 1-vinyl1,2,4-triazole [13, 14]. A somewhat different copolymerization pathway is observed for the VC–VTHI system: the copolymerization of these monomers is accompanied by the fast cationic oligomerization of VTHI. POLYMER SCIENCE
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The thermal stability of the synthesized VC copolymers with MVP and VTHI is better than that of PVC obtained by suspension polymerization (figure). An increase in the onset degradation temperature of the copolymers under investigation can be explained by the absence of long VC blocks. At the stage of basic weight loss, the rate of degradation of the copolymers is significantly lower than that of PVC. In the case of the VC– VTHI copolymer, the formation of polyene units as a result of dehydrochlorination in the course of synthesis is responsible for the very low rate of its degradation in the range 200–450°ë and a high temperature of full degradation (600°ë). VC copolymers with ionogenic groups, unlike PVC, are soluble in a wider range of solvents and can form concentrated solutions (up to 50%). Expansion of the assortment of solvents for vinyl chloride copolymers and the possibility of obtaining concentrated solutions based on VC are of great importance for producers and consumers of these polymers. REFERENCES 1. N. A. Bichun, T. G. Ganyukhina, and A. G. Kronman, Plast. Massy, No. 9, 31 (2001). 2. B. A. Trofimov, T. T. Minakova, T. A. Tandura, et al., Vysokomol. Soedin., Ser. B 22, 103 (1980). 3. B. A. Trofimov and A. I. Mikhaleva, N-Vinylpyrroles (Nauka, Novosibirsk, 1984) [in Russian]. 4. A. Kh. Ibragimova, E. M. Ivleva, N. V. Pavlova, et al., Polymer Science, Ser. A 34, (1992) [Vysokomol. Soedin., Ser. A 34, 139 (1992)]. 5. B. A. Trofimov, L. V. Morozova, M. V. Sigalov, et al., Makromol. Chem. 188, 2251 (1987). 2008
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6. L. V. Morozova, A. I. Mikhaleva, M. V. Markova, et al., Izv. Akad. Nauk, Ser. Khim., No. 2, 423 (1996). 7. M. G. Voronkov, V. G. Kozyrev, M. V. Sigalov, et al., Khim. Geterotsikl. Soedin., No. 3, 420 (1984). 8. N. S. Shaglaeva, A. I. Mikhaleva, G. I. Sarapulova, et al., Izv. Akad. Nauk, Ser. Khim., No. 12, 2267 (1997). 9. V. A. Kruglova, G. A. Izykenova, A. V. Kalabina, et al., Vysokomol. Soedin., Ser. B 24, 691 (1982). 10. N. S. Shaglaeva, O. V. Lebedeva, L. V. Kanitskaya, et al., Polymer Science, Ser. B 45, (2003) [Vysokomol. Soedin., Ser. B 45, 827 (2003)].
11. L. V. Baikalova, E. S. Domnina, T. V. Kashik, et al., Zh. Obshch. Khim. 68, 842 (1998). 12. A. I. Kipper, L. V. Dmitrienko, O. B. Ptitsyn, and Zh. S. Sogomonyants, Mol. Biol. (Moscow) 4, 175 (1970). 13. N. S. Shaglaeva, S. V. Fedorov, O. V. Lebedeva, et al., Polymer Science, Ser. B 46, (2004) [Vysokomol. Soedin., Ser. B 46, 1434 (2004)]. 14. N. S. Shaglaeva, O. V. Lebedeva, G. A. Pirogova, et al., Polymer Science, Ser. B 45, (2003) [Vysokomol. Soedin., Ser. B 45, 1769 (2003)].
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