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On the synthesis of polyacetylene by the interphase dehydrochlorination of polyvinyl chloride
Synthesis of polyacetylene
1223
3. P. B. McALLISTER, T. J. CARTER and R. M. HINDE, J. Polymer Sci. Polymer Phys. Ed. 16: 1, 49, 1978. 4. J. GREMBOWICZ, J. F. ZAN and B. WUNDERLICH, J. Polymer Sci. Polymer Symp. No. 71, P. 19, 1984. 5. A. FICHERA and R. ZANETTI, Makromolek. Chem. 176: p. 1885, 1975. 6. R. ZANETTI, G. GELOTTI and A. FICHERA, Makromolek. Chem. 128: 11,137, 1969. 7. W. K. BUSFIELD and C. S. BLAKE, Polymer 21: p. 35, 1980. 8. V. VITTORIA and F. RIVA, Macromolecules 19: 7, 1935, 1986. 9. F. De CANDIA, R. RUSSO and V. VITTORIA, J. Appl. Polymer Sci. 34: 6, 689, 1987. 10. Ye. A. SHMATOK, O. V. KOZLOVA, L. M. YARYSHEVA, A. L. VOLYNSKII and N. F. BAKEYEV, Dokl. A . N . S . S . S . R 302: 6, 1428, 1988. 11. Ye. A. SHMATOK, L. M. YARYSHEVA, A. L. VOLYNSKII and N. F. BAKEYEV, Vysokomol. soyed. A31: 8, 1752, 1989 (translated in Polymer Sci. U.S.S.R. 31: 8, 1929, 1989). 12. S. Z. CANNON, G. B. McKENNA and W. O. STATTON, J. Polymer Sci. Macromolec. Rev. 11: p. 209, 1976. 13. V. V. BONDAREV, Dissertation submitted for the degree of candidate of Chemical Sciences, Moscow: Moscow State University pp. 151, 1983. 14. Ye. A. SHMATOK, O. V. ARZHAKOVA, L. M. YARYSHEVA, A. L. VOLYNSKII and N. F. BAKEYEV, Vysokomol. soyed. B31:9,691 (not translated in Polymer Sci. U.S.S.R.). 15. L. M. YARYSHEVA, N. B. GAL'PERINA, O. V. ARZHAKOVA, A. L. VOLYNSKII and N . F . BAKEYEV, Vysokomol. soyed. B31: 3,211, 1989 (not translated in Polymer Sci. U.S.S.R.).
PolymerScience U.S.S.R. Vol. 32, No. 6, pp. 1223-1229, 1990 Printed in Great Britain.
0032-3950/90 $10.00+ .00 ~) 1991 PergamonPressplc
ON THE SYNTHESIS OF POLYACETYLENE BY THE INTERPHASE DEHYDROCHLORINATION OF POLYVINYL CHLORIDE* G. V.
LEPLYANIN,V.
N. SALIMGAREEVA,N. S. SANNIKOVA,A. N. YU. A. LEBEDEV and P. G. VALYAMOVA
CHUVYROV,
Institute of Chemistry and the Physics Department of the Bashkir Science Centre, Urals Division of the U.S.S.R. Academy of Sciences (Received 18 April 1989)
The dehydrochlorination of PVC has been studied under interphase catalysis conditions, using various solvents and dehydrochiorinating agents. The predominating effect of the interface on the parameters of the process and on the properties of the polyacetylene prepared by the dehydrochlorination of PVC was observed. It is shown that a clearcut liquid-liquid interface (PVC solution-solution of the dehydrochiorinating agent) promotes the formation of crystalline polyacetylene structures. The formation of mainly amorphous polymer is promoted when the interface is a rather diffuse region as when dehydrochlorination takes place at interfaces such as: PVC film or PVC powder-solution of the dehydrochlorinating agent, or PVC solution-powder of the dehydrochlorinating agent, i.e. where a solid body-liquid type of interface is involved.
KEEN INTEREST shown by authors in polyacetylene type polyene structures has stimulated attempts to find novel methods for the synthesis of these structures. The present paper relates to one of these *Vysokomol. soyed. A32: No. 6, 1291-1296, 1990.
1224
G . V . LEPLYANINet al.
methods, i.e. the dehydrochlorination of PVC, whereby a great variety of structures, including crystalline ones, may be prepared. Elimination of HCI from PVC may be carried out under the action of heat, light, or chemical reagents. Both the thermal [1-4] and the photochemical processes [5-8] have low reaction rates and yield only partially dehydrochlorinated products. High degrees of dehydrochlorination (90--95%) are formed by reacting dissolved PVC with various bases. Here too, however, reaction rates are likewise low [9-13]. Kise and coworkers [14, 15] proposed a method of chemical dehydrochlorination involving interphase catalysis. In this way one may obtain an 80% dehydrochlorination in 3 h. PVC is introduced into the reaction in film form or powder form, or as a solution in THF. In the present instance we investigated the dehydrochlorination of PVC under conditions of interphase catalysis, using a variety of solvents and dehydrochlorinating agents. The dehydrochlorination reaction was carried out in the presence of triethylbenzylammonium chloride (TEBAC) as the phase transfer catalyst. TEBAC synthesized by the method described in [16] was twice recrystallized from a benzene-ethanol mixture (in the volume ratio of 10:1). A solution of PVC containing TEBAC was placed in a round bottomed flask fitted with a mechanical stirrer (9 rev/s) and the reaction was initiated by adding a dehydrochlorinating agent. Ethanol was added to stop the process. The reaction product was filtered, washed two or three times with ethyl alcohol and then with distilled water to get a negative reaction to the chlorine ion (in so doing the central reaction with respect to an alkali was also obtained), and with ethanol in a Soxhlet apparatus until the extracting agent had been dehydrated. The reaction product was vacuum dried at 370 K. (The results of special tests showed that heat treatment of the polymer at this temperature does not affect the structure or properties of the product.) The operations were all carried out in an inert gas atmosphere (argon); the solvents and solutions of the reagents were purged with argon for 15-20 minutes before use. Four samples of commercial PVC (grades S-63, S-65, S-70 and S-90 with degrees of polymerization of 1600, 1920, 2100 and 40 000) were investigated. The solvents used in the present work were benzene, chlorobenzene nitrobenzene, dichloroethane, THF, cyclohexanone, DMF, dioxan and methyl ethyl ketone. The solvents and ethyl alcohol underwent purification and distillation by standard methods. The dehydrochlorinating agents were sodium ethylate, lithium hydroxide, sodium hydroxide, ammonium hydroxide and potassium hydroxide. Sodium ethylate was prepared by the method outlined in [18]; the other hydroxides employed (without preliminary treatment) were in the form of aqueous solutions, alcoholic solutions, pellets and powder. Structural analysis of the crystals obtained was based on IR spectroscopy (Specord M-80 instrument), optical microscopy (microscope of type Amplival Pol. U, Zeiss, Jena) and electron microscopy (EMMA-4 type electron microscope) [19]. An RE-1306 type radio spectrometer was used to determine the amount of paramagnetic particles in the polymer. Evaluation of the degree of dehydrochlorination was based on the amount of residual chlorine determined by elemental analysis. The amounts of cis- and trans-structures in the polyacetylene were calculated by the equations [20] ac/~ % = 100 x 1.30 x Acifl(1.3OAas +Atrans) arrans % = 100 X A,rans/ (1.30 Aci~ + At,a,s )
where Acu and Atrans are the intensity of the bands for cis-C--H deformation vibrations at 740 cm -1
Synthesis of polyacetylene
1225
and the intensity of trans---C--H deformation vibrations at 1015 cm -1 in the spectra, respectively. In the case of interphase dehydrochlorination the phase interface has a determining influence on parameters of the process and on properties of the resulting polyacetylene. In a reaction at the solid substance (S)-liquid (L) interface (PVC film or powder, solution of the dehydrochlorinating agent or PVC solution-powder of the dehydrochlorinating agent) randomly disposed macromolecules in the surface-adjoining regions were subjected to dehydrochlorination; the resulting polyacetylene has an amorphous structure. In the L-L system (PVC solution-solution of the dehydrochlorinating agent) conditions may be such as will favour the formation of crystalline polyacetylene structures. PVC molecules in solution are oligomeric cooperative systems in which intramolecular interactions between chain units are stronger than the intermolecular interactions. In dehydrochlorination processes mobile segments of PVC macromolecules are transformed into oligomeric rigid polyconjugated structures whose orientation is parallel to the interface, which facilitates a "pulling" of subsequent chain segments out of solution onto the interface (into the reaction zone). Interchain interaction of the polyene transformation results in two-dimensional systems. The rigidity of the polyconjugated structures inhibits the folding of macromolecules whereby amorphous regions may be as it were "buried" in the mass of layers superposed on one another. This leads to the origination of a three-dimensional system accompanied by the formation of regular shaped "packet" crystals of the type observed in the polarization optical investigation (Fig. 1). The photomicrograph shows the outside faces of orthorhombic and hexagonal modifications of the crystals. Their sizes are in the region of 0.3-1 mm.
FIG. 1. Polarization-opticalphotomicrographs of the crystallinepolyacetylenesample.
By electron diffraction investigations we were able to identify three forms of polyacetylene crystals whose parameters were as follows: the hexagonal form (a = b = 5.2 A, c = 2.52 A), the orthagonal form (a = 4.0/~, b = 7.9/~) and the monoclinic form (a = c = 3.9 ,A,, b = 2.52,A,). Figure 2 shows X-ray diffractograms typical for these three alternatives of the synthesized polyacetylene (Fig. 2a-c). Despite the limitations of the electron diffraction method that gives the diffraction pattern of only a thin surface layer, and is sensitive to surface impurities and defects, one may surmise in view of these findings along with the results of the polarization optical studies that the polyacetylene is a mixture of several crystalline forms (with one or another of these predominating) and an amorphous component, their ratio depending on the conditions under which the process is being conducted. The synthesized polyacetylene is characterized by a predominant amount of trans-polyene structures; the maximum content of c/s-structures amounts to 30%. The
G . V . LEPLYANIN et al,
1226
FIG. 2. X-ray diffraction patterns of polyacetylene samples in the hexagonal (a), rhombic (b) and monoclinic (c) forms. fact that the trans-polyacetylene also contains cis-structures and that there are several crystalline forms in a single sample m e a n s that X-ray diffraction methods cannot be used to determine the latter. This probably also explains why the stated e l e m e n t a r y cell p a r a m e t e r s differ f r o m those described in the literature for practically pure cis- and trans-forms of polyacetylene [21, 22]. It is seen f r o m T a b l e 1 that the nature of the P V C solvent has only a slight influence on degrees of dehydrochlorination obtained under identical conditions. The exceptions here are D M F and dichloroethane-solvents that are capable of interaction with a dehydrochlorinating agent (in the present case, with K O H ) . It appears that in such cases competing reactions of K O H with the solvent and P V C lead to a reduction in the concentration o f the dehydrochlorinating agent in the reaction zone, i.e. at the interface, such that the rate of the main process is very significantly reduced. If the reaction is run with a solution of P V C in solvents that are immiscible with water, the supermolecular structure (crystallinity) of the polyacetylene does not depend on the nature or the polarity of the solvents. A m o r p h o u s polyacetylene is f o r m e d in all cases, mixed (up to 20%) with TABLE1. EFFECTOFTHENATUREOFTHESOLVENTONTHEDEHYDROCHLORINATIONOF PVC ([TEBAC] = 5.1 x 10 - 2 mol/PVC unit; r = 7 h, 298 K)
Solvents
Miscibility of thesolvent with H 2 0
THF
Misc.
Cyclohexanone DMF Dioxan Methyl ethyl ketone Benzene Chlorobenzene Dichloroethane
Misc. Misc. Misc. Misc.
Nitrobenzene
e
7.39
Degree of dehydroPVC, KOH, chlorination, wt.% g/ml % 1.59
0.42
98
18.3 37.6 2.2 18.5
1.59 1.59 1.59 1.59
0.42 0.42 0.42 0.42
96 38 92 95
Immisc. Immisc. Immisc.
2.27 6.08 10.36
0.10 0.78 1.5
0.05 0.05 0.42
0 80 35
Immisc. Immisc.
34.75 --
1.5 0.78
0.42 0.05
98 80
Polymer structure Amorphous +20% of the hexagonal form Ditto ,, No dehydrochlorination takes place
Amorphous, no crystals Amorphous, +20% of the hexagonal form Perfect crystals Ditto
Synthesis of polyacetylene
1227
crystalline formations (hexagonal modification). If the dehydrochlorination is carried out with PVC dissolved in solvents that are immiscible with water the resulting polyacetylene will then have degrees of crystallinity proportional to the dielectric constants of the the solvents. The nature of the dehydrochlorinating agent and its concentration influence both the rate of dehydrochlorination and the crystallinity of the polyacetylene (Table 2). It is evident from these data that crystalline polyacetylene structures are formed in cases where there is a clear-cut L - L interface. In cases where the interface is a fairly diffuse region (when the PVC solvent is miscible with water, or where the solvent for the dehydrochlorinating agent is miscible with the organic phase, or where the reaction takes place at the S - L interface the polyacetylene structure is mainly of the amorphous type. A system of the type: solution of PVC in nitrobenzene-solution of potassium hydroxide in water was used to make a detailed study of the extent to which the method of synthesis affects the dehydrochlorination of PVC. The concentration of the PVC solution was 1.5 weight%. This is the highest polymer concentration at which a maximum degree of dehydrochlorination may be obtained. It can be seen from Figs 3 and 4 that the aqueous phase alkali concentration and the catalyst concentration influence the dehydrochlorination rate and the degree of completeness of the reaction only up to a definite limit. The process cannot be carried out up to 100% conversion. The dehydrochlorination temperature has scarcely any effect on the resulting degree of conversion, but it does have a significant influence on the crystallinity of the resultant polyacetylene, and on the amount of paramagnetic centres (PMC) it contains (Table 3). The most perfect crystalline structures are formed when the reaction conditions are as follows: concentration of PVC in nitrobenzene (1.5%), aqueous phase concentration of KOH 50%, KOH concentration in the system 0.41 g/ml organic phase, catalyst concentration 5 x 10-2 mol/PVC unit, temperature 283-309 K. The pycnometric density of the polyacetylene prepared in a solvent that is compatible with water is 0 . 7 6 g/cm 3, while the density of the polymer synthesized in nitrobenzene is 1.26-1.34 g / c m 3. A change in the degree of polymerization of PVC within the limits of 1600-2100 does not affect the properties of the polyacetylene (Table 4); the rhombic and hexagonal forms predominate in the crystals. A molecular mass increase of more than one order results in the synthesis of a polyacetylene with a crystalline structure enriched with the monoclinic form.
TABLE 2.
INFLUENCEOF THE NATURE OF THE DEHYDROCHLORINATINGAGENTS ON THE DEHYDROCHLORINATIONOF PVC
GRADE S-65 [PVC] = 1.5 w t . % , [TEBAC] = 5.1 x 10 -2 mol/PVC UNIT, CONCENTRATION OF THE DEHYDROCHLORINATING AGENT IN SOLUTION0.42 g/ml OF ORGANIC PHASE r = 7 h, 298 K)
Initial concentration, %
Degree of dehydrochlorination
100 100 100 Solid
34 67 90 98
Amorphous
NH4OH LiOH
30 60 g + 100 g
0 58
No dehydrochlorination takes place Fine crystalline + amorphous
NaOH KOH KOH
H20 50 30 50
96 84 97
Defective crystals
Dehydrochlorinating agents CzHsONa* C2HsONa KOH in ethanol KOH
* No catalyst was used.
Polymer structure
Amorphous +20% of the hexagonal form
Crystalline Perfect crystals, d = 1-0.3 mm
G . V . LEPLYANIN et aL
1228 Time, h zl i
o~, w t . %
# i
el, wt. %
LZ
80
6'0
#0
x
fx.T gO
~k~ 1
#0
I
I
a
8
[TEBAC] x 102 mol/PVC unit
Time, h
FIG. 3
FIG. 4
FIG. 3. Kinetic curves of dehydrochlorination of PVC, the dehydrochlorinating agent being KOH solutions at concentrations of 50(3), 20(2) and 10%(1), [KOH] =0.41g/ml organic phase, [TEBAC] = 2.8 x 10-2 mol/PVC unit. Here and in Fig. 4 the solvent was nitrobenzene, [PVC] = 1.5 wt.%, T = 301-303 K. a is the degree of dehydrochlorination. FIG. 4. Plots of the degree of dehydrochlorination versus the catalyst concentration, the concentrations of the dehydrochlorinating agent being 0.84 (1); 0.42 (2), and 0.05 g/ml organic phase (3).
TABLE 3.
DEHYDROCHLORINATION OF P V C AT VARIOUS TEMPERATURES
Dehydrochlorination, %
T, K 280 293 303 323
93 97 96 98.5
NpMCX 10-16
Polymer structure
0.8 1.2 1.8 2.7
Perfect crystalline structure ,, ,, Defective opaque crystals
TABLE4. DEHYDROCHLORINATIONOF DIFFERENTGRADESOF PVC IN NITROBENZENE ([PVC] = 1.5%; [KOH] = 50%; [KOH] = 0.41 g/ml, [TEBAC] = 5.1 x 10-2 mol/PVC unit, 280 K)
PVC grade
Degree of polymerization of PVC
Degree of dehydrochlorination, %
S-63
1 600
98
S-65 S-70 S-90
1 920 2 100 40 000
98 98 96
Polymer structure Perfect red crystals, predominantly rhombic and hexagonal syngonies
Tightly packed crystals; monoclinic syngony predominates
T o b r i n g t h e d e g r e e o f d e h y d r o c h l o r i n a t i o n o f P V C u p to 100 O%, s a m p l e s p r e p a r e d b y a p r o c e s s o f
Synthesis of polyacetylene
100 ~
[
1229
~
J
I
0 8 Time, h FxG. 5. Kineticcurves of thermal dehydrochlorination (493 K) of polymeric products obtained by catalytic dehydrochlorination of PVC with degrees of completeness of the reaction as follows: 34 (1), 42 (2), 78% (3) and 98% (4).
interphase interaction were subjected to heat treatment. Figure 5 shows that this is a rather slow process. Total dehydrochlorination is attained at 520-570 K.
Translated by R. J. A. HENDRY
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
B. BAUM and D. H. WARTMAN, J. Polymer Sci. 28: No. 118, p. 537, 1958. J. D. DARN-FORTH, Contemporary Topics in Polymer Science, vol. 4, New York-London, p. 163, 1983. B. IVAN and J. P. KENNEDY, J. Polymer Sci. Polymer Chem. Ed. 21: No. 8, 2187, 1983. R. LUKAS, O. PFADOVA, J. MICHALCOVA and V. PALASKOVA, J. Polymer Sci. Polymer Letters Ed. 23: p. 85, No. 2, 1983. G. J. ATCHISON, J. Polymer Sci. 49: No. 152, p. 385, 1961. B. R. LOY, J. Polymer Sci. 50: No. 153, p. 245, 1961. E. O. OWEN and K. MSAYIB, J. Polymer Sci., Polymer Chem. Ed. 23: No. 6, p. 1833, 1985. Yu. BENISH, M. A. ZHURAVLEVA and V. P. IVANOV, Vysokomol. soyed. B29: No. 10,787, 1987 (not translated in Polymer Sci. U.S.S.R.). Y. SHINDO and T. HIRAI, Makromolek Chem. 155: p. 1, 1972. B. IVAN, J. P. KENNEDY, T. KELEN and F. TUDOS, J. Polymer Sci. Polymer Chem. Ed. 19: No. 3,679, 1981. H. J. BOWLEY, D. L. GERRARD and W. F. MADDAMS, Makromolek. Chem. 186: No. 4, 695, 1985. H . J . BOWLEY, D. L. GERRARD and W. F. MADDAMS, Makromolek. Chem. 186: No. 4, p. 707, 1985. T. DANNO, H. KONDOH, K. J. FURUHATA and K. MIYASOKA, J. Appl. Polymer Sci. 29: No. 10, p. 3171, 1984. H. KISE, J. Polymer Sci. Polymer Chem. Ed. 20: No. 11, 3189, 1982. K . T . HOWANG, K. IWAMOTO, M. SERO and H. KISE, Makromolek. Chem. 187: No. 3, p. 3189, 1986. E. V. DEHMLOW and S. S. DEHMLOW, Transfer Catalysis, Florida-Basel, p. 46, 1980. A. WEISSBERGER, E. PROSKAUER, J. RIDDICK and E. TOPPS, Organicheskie rastvoriteli (Organic Solvents), Moscow, p. 276, 285, 1958. Preparativnaya organicheskaya khimiya (Preparative Organic Chemistry), ed. by N. S. VUL'FSON, Moscow, p. 157,599, 1959. G. THOMAS and M. GORRINGE, Prosvechibayushchaya elektronnaya microskopiya materialov (Transmission Electron Microscopy of Materials), Moscow, p. 100, 1983. T. ITO, H. SHIRAKAWA and S. IKEDA, J. Polymer Sci., Polymer Chem. Ed. 12: No. 1, p. 11, 1974. R. H. IIAUGHMAN, S. L. HSU, Y. P. REZ and A. Y. SIGNORELLI, J. Chem. Phys., 68: No. 12, p. 5405, 1978. SHIMAMURA, F. E. KARASZ, J. A. HIRSCH and J. C. CHIEN, Makromolek. Chem. Rapid Commun. 2: No. 8, p. 2473, 1981.
PS 32,6-L
1223
3. P. B. McALLISTER, T. J. CARTER and R. M. HINDE, J. Polymer Sci. Polymer Phys. Ed. 16: 1, 49, 1978. 4. J. GREMBOWICZ, J. F. ZAN and B. WUNDERLICH, J. Polymer Sci. Polymer Symp. No. 71, P. 19, 1984. 5. A. FICHERA and R. ZANETTI, Makromolek. Chem. 176: p. 1885, 1975. 6. R. ZANETTI, G. GELOTTI and A. FICHERA, Makromolek. Chem. 128: 11,137, 1969. 7. W. K. BUSFIELD and C. S. BLAKE, Polymer 21: p. 35, 1980. 8. V. VITTORIA and F. RIVA, Macromolecules 19: 7, 1935, 1986. 9. F. De CANDIA, R. RUSSO and V. VITTORIA, J. Appl. Polymer Sci. 34: 6, 689, 1987. 10. Ye. A. SHMATOK, O. V. KOZLOVA, L. M. YARYSHEVA, A. L. VOLYNSKII and N. F. BAKEYEV, Dokl. A . N . S . S . S . R 302: 6, 1428, 1988. 11. Ye. A. SHMATOK, L. M. YARYSHEVA, A. L. VOLYNSKII and N. F. BAKEYEV, Vysokomol. soyed. A31: 8, 1752, 1989 (translated in Polymer Sci. U.S.S.R. 31: 8, 1929, 1989). 12. S. Z. CANNON, G. B. McKENNA and W. O. STATTON, J. Polymer Sci. Macromolec. Rev. 11: p. 209, 1976. 13. V. V. BONDAREV, Dissertation submitted for the degree of candidate of Chemical Sciences, Moscow: Moscow State University pp. 151, 1983. 14. Ye. A. SHMATOK, O. V. ARZHAKOVA, L. M. YARYSHEVA, A. L. VOLYNSKII and N. F. BAKEYEV, Vysokomol. soyed. B31:9,691 (not translated in Polymer Sci. U.S.S.R.). 15. L. M. YARYSHEVA, N. B. GAL'PERINA, O. V. ARZHAKOVA, A. L. VOLYNSKII and N . F . BAKEYEV, Vysokomol. soyed. B31: 3,211, 1989 (not translated in Polymer Sci. U.S.S.R.).
PolymerScience U.S.S.R. Vol. 32, No. 6, pp. 1223-1229, 1990 Printed in Great Britain.
0032-3950/90 $10.00+ .00 ~) 1991 PergamonPressplc
ON THE SYNTHESIS OF POLYACETYLENE BY THE INTERPHASE DEHYDROCHLORINATION OF POLYVINYL CHLORIDE* G. V.
LEPLYANIN,V.
N. SALIMGAREEVA,N. S. SANNIKOVA,A. N. YU. A. LEBEDEV and P. G. VALYAMOVA
CHUVYROV,
Institute of Chemistry and the Physics Department of the Bashkir Science Centre, Urals Division of the U.S.S.R. Academy of Sciences (Received 18 April 1989)
The dehydrochlorination of PVC has been studied under interphase catalysis conditions, using various solvents and dehydrochiorinating agents. The predominating effect of the interface on the parameters of the process and on the properties of the polyacetylene prepared by the dehydrochlorination of PVC was observed. It is shown that a clearcut liquid-liquid interface (PVC solution-solution of the dehydrochiorinating agent) promotes the formation of crystalline polyacetylene structures. The formation of mainly amorphous polymer is promoted when the interface is a rather diffuse region as when dehydrochlorination takes place at interfaces such as: PVC film or PVC powder-solution of the dehydrochlorinating agent, or PVC solution-powder of the dehydrochlorinating agent, i.e. where a solid body-liquid type of interface is involved.
KEEN INTEREST shown by authors in polyacetylene type polyene structures has stimulated attempts to find novel methods for the synthesis of these structures. The present paper relates to one of these *Vysokomol. soyed. A32: No. 6, 1291-1296, 1990.
1224
G . V . LEPLYANINet al.
methods, i.e. the dehydrochlorination of PVC, whereby a great variety of structures, including crystalline ones, may be prepared. Elimination of HCI from PVC may be carried out under the action of heat, light, or chemical reagents. Both the thermal [1-4] and the photochemical processes [5-8] have low reaction rates and yield only partially dehydrochlorinated products. High degrees of dehydrochlorination (90--95%) are formed by reacting dissolved PVC with various bases. Here too, however, reaction rates are likewise low [9-13]. Kise and coworkers [14, 15] proposed a method of chemical dehydrochlorination involving interphase catalysis. In this way one may obtain an 80% dehydrochlorination in 3 h. PVC is introduced into the reaction in film form or powder form, or as a solution in THF. In the present instance we investigated the dehydrochlorination of PVC under conditions of interphase catalysis, using a variety of solvents and dehydrochlorinating agents. The dehydrochlorination reaction was carried out in the presence of triethylbenzylammonium chloride (TEBAC) as the phase transfer catalyst. TEBAC synthesized by the method described in [16] was twice recrystallized from a benzene-ethanol mixture (in the volume ratio of 10:1). A solution of PVC containing TEBAC was placed in a round bottomed flask fitted with a mechanical stirrer (9 rev/s) and the reaction was initiated by adding a dehydrochlorinating agent. Ethanol was added to stop the process. The reaction product was filtered, washed two or three times with ethyl alcohol and then with distilled water to get a negative reaction to the chlorine ion (in so doing the central reaction with respect to an alkali was also obtained), and with ethanol in a Soxhlet apparatus until the extracting agent had been dehydrated. The reaction product was vacuum dried at 370 K. (The results of special tests showed that heat treatment of the polymer at this temperature does not affect the structure or properties of the product.) The operations were all carried out in an inert gas atmosphere (argon); the solvents and solutions of the reagents were purged with argon for 15-20 minutes before use. Four samples of commercial PVC (grades S-63, S-65, S-70 and S-90 with degrees of polymerization of 1600, 1920, 2100 and 40 000) were investigated. The solvents used in the present work were benzene, chlorobenzene nitrobenzene, dichloroethane, THF, cyclohexanone, DMF, dioxan and methyl ethyl ketone. The solvents and ethyl alcohol underwent purification and distillation by standard methods. The dehydrochlorinating agents were sodium ethylate, lithium hydroxide, sodium hydroxide, ammonium hydroxide and potassium hydroxide. Sodium ethylate was prepared by the method outlined in [18]; the other hydroxides employed (without preliminary treatment) were in the form of aqueous solutions, alcoholic solutions, pellets and powder. Structural analysis of the crystals obtained was based on IR spectroscopy (Specord M-80 instrument), optical microscopy (microscope of type Amplival Pol. U, Zeiss, Jena) and electron microscopy (EMMA-4 type electron microscope) [19]. An RE-1306 type radio spectrometer was used to determine the amount of paramagnetic particles in the polymer. Evaluation of the degree of dehydrochlorination was based on the amount of residual chlorine determined by elemental analysis. The amounts of cis- and trans-structures in the polyacetylene were calculated by the equations [20] ac/~ % = 100 x 1.30 x Acifl(1.3OAas +Atrans) arrans % = 100 X A,rans/ (1.30 Aci~ + At,a,s )
where Acu and Atrans are the intensity of the bands for cis-C--H deformation vibrations at 740 cm -1
Synthesis of polyacetylene
1225
and the intensity of trans---C--H deformation vibrations at 1015 cm -1 in the spectra, respectively. In the case of interphase dehydrochlorination the phase interface has a determining influence on parameters of the process and on properties of the resulting polyacetylene. In a reaction at the solid substance (S)-liquid (L) interface (PVC film or powder, solution of the dehydrochlorinating agent or PVC solution-powder of the dehydrochlorinating agent) randomly disposed macromolecules in the surface-adjoining regions were subjected to dehydrochlorination; the resulting polyacetylene has an amorphous structure. In the L-L system (PVC solution-solution of the dehydrochlorinating agent) conditions may be such as will favour the formation of crystalline polyacetylene structures. PVC molecules in solution are oligomeric cooperative systems in which intramolecular interactions between chain units are stronger than the intermolecular interactions. In dehydrochlorination processes mobile segments of PVC macromolecules are transformed into oligomeric rigid polyconjugated structures whose orientation is parallel to the interface, which facilitates a "pulling" of subsequent chain segments out of solution onto the interface (into the reaction zone). Interchain interaction of the polyene transformation results in two-dimensional systems. The rigidity of the polyconjugated structures inhibits the folding of macromolecules whereby amorphous regions may be as it were "buried" in the mass of layers superposed on one another. This leads to the origination of a three-dimensional system accompanied by the formation of regular shaped "packet" crystals of the type observed in the polarization optical investigation (Fig. 1). The photomicrograph shows the outside faces of orthorhombic and hexagonal modifications of the crystals. Their sizes are in the region of 0.3-1 mm.
FIG. 1. Polarization-opticalphotomicrographs of the crystallinepolyacetylenesample.
By electron diffraction investigations we were able to identify three forms of polyacetylene crystals whose parameters were as follows: the hexagonal form (a = b = 5.2 A, c = 2.52 A), the orthagonal form (a = 4.0/~, b = 7.9/~) and the monoclinic form (a = c = 3.9 ,A,, b = 2.52,A,). Figure 2 shows X-ray diffractograms typical for these three alternatives of the synthesized polyacetylene (Fig. 2a-c). Despite the limitations of the electron diffraction method that gives the diffraction pattern of only a thin surface layer, and is sensitive to surface impurities and defects, one may surmise in view of these findings along with the results of the polarization optical studies that the polyacetylene is a mixture of several crystalline forms (with one or another of these predominating) and an amorphous component, their ratio depending on the conditions under which the process is being conducted. The synthesized polyacetylene is characterized by a predominant amount of trans-polyene structures; the maximum content of c/s-structures amounts to 30%. The
G . V . LEPLYANIN et al,
1226
FIG. 2. X-ray diffraction patterns of polyacetylene samples in the hexagonal (a), rhombic (b) and monoclinic (c) forms. fact that the trans-polyacetylene also contains cis-structures and that there are several crystalline forms in a single sample m e a n s that X-ray diffraction methods cannot be used to determine the latter. This probably also explains why the stated e l e m e n t a r y cell p a r a m e t e r s differ f r o m those described in the literature for practically pure cis- and trans-forms of polyacetylene [21, 22]. It is seen f r o m T a b l e 1 that the nature of the P V C solvent has only a slight influence on degrees of dehydrochlorination obtained under identical conditions. The exceptions here are D M F and dichloroethane-solvents that are capable of interaction with a dehydrochlorinating agent (in the present case, with K O H ) . It appears that in such cases competing reactions of K O H with the solvent and P V C lead to a reduction in the concentration o f the dehydrochlorinating agent in the reaction zone, i.e. at the interface, such that the rate of the main process is very significantly reduced. If the reaction is run with a solution of P V C in solvents that are immiscible with water, the supermolecular structure (crystallinity) of the polyacetylene does not depend on the nature or the polarity of the solvents. A m o r p h o u s polyacetylene is f o r m e d in all cases, mixed (up to 20%) with TABLE1. EFFECTOFTHENATUREOFTHESOLVENTONTHEDEHYDROCHLORINATIONOF PVC ([TEBAC] = 5.1 x 10 - 2 mol/PVC unit; r = 7 h, 298 K)
Solvents
Miscibility of thesolvent with H 2 0
THF
Misc.
Cyclohexanone DMF Dioxan Methyl ethyl ketone Benzene Chlorobenzene Dichloroethane
Misc. Misc. Misc. Misc.
Nitrobenzene
e
7.39
Degree of dehydroPVC, KOH, chlorination, wt.% g/ml % 1.59
0.42
98
18.3 37.6 2.2 18.5
1.59 1.59 1.59 1.59
0.42 0.42 0.42 0.42
96 38 92 95
Immisc. Immisc. Immisc.
2.27 6.08 10.36
0.10 0.78 1.5
0.05 0.05 0.42
0 80 35
Immisc. Immisc.
34.75 --
1.5 0.78
0.42 0.05
98 80
Polymer structure Amorphous +20% of the hexagonal form Ditto ,, No dehydrochlorination takes place
Amorphous, no crystals Amorphous, +20% of the hexagonal form Perfect crystals Ditto
Synthesis of polyacetylene
1227
crystalline formations (hexagonal modification). If the dehydrochlorination is carried out with PVC dissolved in solvents that are immiscible with water the resulting polyacetylene will then have degrees of crystallinity proportional to the dielectric constants of the the solvents. The nature of the dehydrochlorinating agent and its concentration influence both the rate of dehydrochlorination and the crystallinity of the polyacetylene (Table 2). It is evident from these data that crystalline polyacetylene structures are formed in cases where there is a clear-cut L - L interface. In cases where the interface is a fairly diffuse region (when the PVC solvent is miscible with water, or where the solvent for the dehydrochlorinating agent is miscible with the organic phase, or where the reaction takes place at the S - L interface the polyacetylene structure is mainly of the amorphous type. A system of the type: solution of PVC in nitrobenzene-solution of potassium hydroxide in water was used to make a detailed study of the extent to which the method of synthesis affects the dehydrochlorination of PVC. The concentration of the PVC solution was 1.5 weight%. This is the highest polymer concentration at which a maximum degree of dehydrochlorination may be obtained. It can be seen from Figs 3 and 4 that the aqueous phase alkali concentration and the catalyst concentration influence the dehydrochlorination rate and the degree of completeness of the reaction only up to a definite limit. The process cannot be carried out up to 100% conversion. The dehydrochlorination temperature has scarcely any effect on the resulting degree of conversion, but it does have a significant influence on the crystallinity of the resultant polyacetylene, and on the amount of paramagnetic centres (PMC) it contains (Table 3). The most perfect crystalline structures are formed when the reaction conditions are as follows: concentration of PVC in nitrobenzene (1.5%), aqueous phase concentration of KOH 50%, KOH concentration in the system 0.41 g/ml organic phase, catalyst concentration 5 x 10-2 mol/PVC unit, temperature 283-309 K. The pycnometric density of the polyacetylene prepared in a solvent that is compatible with water is 0 . 7 6 g/cm 3, while the density of the polymer synthesized in nitrobenzene is 1.26-1.34 g / c m 3. A change in the degree of polymerization of PVC within the limits of 1600-2100 does not affect the properties of the polyacetylene (Table 4); the rhombic and hexagonal forms predominate in the crystals. A molecular mass increase of more than one order results in the synthesis of a polyacetylene with a crystalline structure enriched with the monoclinic form.
TABLE 2.
INFLUENCEOF THE NATURE OF THE DEHYDROCHLORINATINGAGENTS ON THE DEHYDROCHLORINATIONOF PVC
GRADE S-65 [PVC] = 1.5 w t . % , [TEBAC] = 5.1 x 10 -2 mol/PVC UNIT, CONCENTRATION OF THE DEHYDROCHLORINATING AGENT IN SOLUTION0.42 g/ml OF ORGANIC PHASE r = 7 h, 298 K)
Initial concentration, %
Degree of dehydrochlorination
100 100 100 Solid
34 67 90 98
Amorphous
NH4OH LiOH
30 60 g + 100 g
0 58
No dehydrochlorination takes place Fine crystalline + amorphous
NaOH KOH KOH
H20 50 30 50
96 84 97
Defective crystals
Dehydrochlorinating agents CzHsONa* C2HsONa KOH in ethanol KOH
* No catalyst was used.
Polymer structure
Amorphous +20% of the hexagonal form
Crystalline Perfect crystals, d = 1-0.3 mm
G . V . LEPLYANIN et aL
1228 Time, h zl i
o~, w t . %
# i
el, wt. %
LZ
80
6'0
#0
x
fx.T gO
~k~ 1
#0
I
I
a
8
[TEBAC] x 102 mol/PVC unit
Time, h
FIG. 3
FIG. 4
FIG. 3. Kinetic curves of dehydrochlorination of PVC, the dehydrochlorinating agent being KOH solutions at concentrations of 50(3), 20(2) and 10%(1), [KOH] =0.41g/ml organic phase, [TEBAC] = 2.8 x 10-2 mol/PVC unit. Here and in Fig. 4 the solvent was nitrobenzene, [PVC] = 1.5 wt.%, T = 301-303 K. a is the degree of dehydrochlorination. FIG. 4. Plots of the degree of dehydrochlorination versus the catalyst concentration, the concentrations of the dehydrochlorinating agent being 0.84 (1); 0.42 (2), and 0.05 g/ml organic phase (3).
TABLE 3.
DEHYDROCHLORINATION OF P V C AT VARIOUS TEMPERATURES
Dehydrochlorination, %
T, K 280 293 303 323
93 97 96 98.5
NpMCX 10-16
Polymer structure
0.8 1.2 1.8 2.7
Perfect crystalline structure ,, ,, Defective opaque crystals
TABLE4. DEHYDROCHLORINATIONOF DIFFERENTGRADESOF PVC IN NITROBENZENE ([PVC] = 1.5%; [KOH] = 50%; [KOH] = 0.41 g/ml, [TEBAC] = 5.1 x 10-2 mol/PVC unit, 280 K)
PVC grade
Degree of polymerization of PVC
Degree of dehydrochlorination, %
S-63
1 600
98
S-65 S-70 S-90
1 920 2 100 40 000
98 98 96
Polymer structure Perfect red crystals, predominantly rhombic and hexagonal syngonies
Tightly packed crystals; monoclinic syngony predominates
T o b r i n g t h e d e g r e e o f d e h y d r o c h l o r i n a t i o n o f P V C u p to 100 O%, s a m p l e s p r e p a r e d b y a p r o c e s s o f
Synthesis of polyacetylene
100 ~
[
1229
~
J
I
0 8 Time, h FxG. 5. Kineticcurves of thermal dehydrochlorination (493 K) of polymeric products obtained by catalytic dehydrochlorination of PVC with degrees of completeness of the reaction as follows: 34 (1), 42 (2), 78% (3) and 98% (4).
interphase interaction were subjected to heat treatment. Figure 5 shows that this is a rather slow process. Total dehydrochlorination is attained at 520-570 K.
Translated by R. J. A. HENDRY
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PS 32,6-L