- Email: [email protected]
The electrochemical reductive degradation of polyvinyl chloride
1564
G.S. SHA~m,AL et aL
8. L. P. RAZUMOVSKII, V. S. MARKIN and G. Ye. ZAIKOV, Vysokomol. soyed. A21: 1671, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 7, 1843, 1979) 9. I. S. BELAYEV, V. D. GERASIMOV and L. B. SOKOLOV, Plast. massy, 8, 50, 1975 10. L. B. SOKOLOV, V. D. GERASIMOV, V. M. SAVINOV and V. K. BELYAKOV, Termostoikiye aromaticheskiye poliamidy (The Thermal Stability of Aromatic Polyamides). p. 253, Moscow, 1975 11. L. P. RAZUMOVSKH, V. S. MARKIN and G. Ye. ZAIKOV, Vysokomol. soyed. A27: 675, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 4, 751, 1985) 12. V. N. LEBEDEV, G. P. ANDRIANOVA and A. Ye. CHALYKH, Izv. vuzov. Khimiya i khim. teknologiya 23: 1286, 1980 13. H. FUJITA, J. Phys. Soc. Japan 8: 271, 1953 14. A. I. MAR'IN and Yu. A. SI-ILYAPNIKOV, Vysokomol. soyed. B23: 825, 1981 (Not translated in Polymer Sci. U.S.S.R.)
PolymerScienceU.S.S.R.Vol.29, No. 7, pp. 1564-1572,1987 Printedin Poland
0032-3950/87$10.00+.00 © 1988PergamonPresspie
THE ELECTROCHEMICAL REDUCTIVE DEGRADATION OF POLYVINYL CHLORIDE* G. S. SHAPOVAL,A. P. TOMILOV,A. A. PUD and K. V. BATSALOVA Petrochemical Department, Institute of Physical Organic Chemistry and Carbon Chemistry, Ukr.S.S.R. Academy of Sciences
(Received 3 February 1987) The reactions of solid PVC and PVC dissolved in DMF, occurring both by direct reduction and also through an electron transfer agent have been studied. At comparatively low negative potentials, PVC undergoes reductive electrochemical degradation, leading to dehalogenation of the macromolecules, radical decay and the formation of a polymer of 3-dimensional structure on the cathode surface. A mechanism has been proposed for these processes.
IT was shown in [1] that PVC, dissolved in fairly polar solvents, in distinction from solid PVC, readily reacts with alkaline reagents, each as caustic potash and sodium alkoxylates. This causes dehydrochlorination, forming a cinnamon coloured, insoluble powder, which is a crosslinked polymer with short conjugated bonds. At the same time, by reaction of PVC dissolved in carefully purified dioxane with zinc powder or metallic sodium, the same authors found a reduction in the extent of PVC polymerization without extensive chlorine elimination, which had been described earlier in [2]. Up to the present time, the degradation processes in PVC of various types have received * Vysokomol. soyed. A29: No. 7~ 1424-1430, !987,
Electrochemical reductive degradation of polyvinyl chloride
1565
insut~cient detailed study [3], but the effect of the transfer of the electrons, unrelated to ~ and p irradiation or' the action of alkalies, needs independent study. Electrochemical reduction, which is a means of changing the values of the cathode potential and simulates the action of electrons with varying reducing power, of PVC is of particular interest [4]. Prior to our work [5], no such studies had been carried out. This process has a theoretical and practical value since electron transfer from a metal to the functional groups of a macromolecule essentially affects the properties of the boundary*layers and accordingly the adhesion strength of the bonding of the polymeric protective covering to cathodicaHy polarized metalfic substrates. It was shown earlier [6] that the electrochemical reduction of both solid PVC and PVC dissolved in D M F causes the same changes at comparatively low negative potential at r o o m temperature, which were observed as an effect of temperatures of the order of 350 ° [3]. The present work is a continuation of these studies. PVC samples with M=- 5 x 10s and 4 x 104 with or without dioctyl phthalate (DOP) as plasticizer were used. Electrolysis was carried out at cathode potentials controlled in the range, Effi -(1.6-2.5) V (saturated calomel electrode) with a solution of PVC of M--4 x 10" or a finely dispersed suspension of PVC with M = 5 x l0 s in DMF with a 0.1 M tetrabutylammonium iodide background. Electrolysis occurred in a twin chambered sealed cell with the cathode and anode separated by a Schott No. 3 filter in an argon atmosphere. Each electrode was a 6 cm2 platinized plate; vitreous carbon ones were also used. Before and after electrolysis, voltametric curves of the catholyte were recorded, and the relative or intrinsic viscosity was determined. Polarographic studies were made on an LP-60 polarograph using a dropping mercury electrode with the capilliary characteristic, m2/Stxl6-~ 1.6 mg2/3sec - g2. Polarization curves were recorded on a platinum electrode and on vitreous carbon of area, 1.5 x 10-2 cm- 2 on a P-5827 potentiostat. The IR spectra of the PVC and its electrolysis products were recorded on UR-20 and Pye-Unicam SP200 spectrometers in the 600-4000 cm- ~ range. Samples for the IR spectra were prepared by pulverizing the PVC at liquid nitrogen temperature, further grinding with KBr, milling and tabletting. Viscometric measurements were made in an Ubbelohde viscometer at 25°. The electrical conductivity of the system was measured in a twin-chambered cell using a P5010 variable bridge. According to the potentiodynamic curves measured before electrolysis (Fig. 1) both dissolved and finely dispersed, unplasticized PVC are reduced on a platinum and vitreous carbon cathode, with E = - (2.0-2.2) V, respectively. The values of the potential and reduction current are determined by the concentration of dissolved or amount of suspended PVC. The electrolysits of a solution of unplasticized PVC at Effi - 2 . 0 V results causes a yellow-cinnamon colour to appear on the cathode surface which spreads into the solution. A friable polymer film forms on the cathode, which gradually thickens, its colour changes from yellow to cinnamon, then black. As the electrolysis is continued, the current strength falls (Fig. 2) due to the increasing resistance of the film forming and shielding the cathode surface. As the electrolysis goes further, with the thickening and increase in intensity of film colour, its resistance hardly changes (in the range, (8.4-12.7) x 10" ohms). The relative viscosity of the catholyte falls during the electrolysis by 20Yo due to the lower M M s of the PVC reduction products, The fall in concentration due to film
G. $. SHAPOV.~et aL
1566
f, mA I] 6O
z, 1o mA
~0 Iso
2
2-0
2"8 -E,V
6
....
3O
Time ~" 10-~ sec
FIG. 1 FIe. 2 FIO. 1. Potential curves for 0.75 (1) and 0.5 % (2) PVC solutions in DMF with a 0.1 MN(C4Hg)4I background, 3-background curve, 4-PVC plasticized with DOP. FIe. 2. Chromoamperogramsof PVC reduced by electrolysisin DMF at E= - 2.0 (1) and - 2.5 V (2).
deposition hardly affects it as shown by the concentrational dependence of PVC viscosity under the same conditions. The electrolysis at -2.5 V of dissolved unplasticized PVC is usually different with more intense colouring of the catholyte and a higher film deposition rate on the cathode. The film is black. A sharp drop in current occurs in this system. However, the magnitude of the current remains higher than with E = - 2 . 0 V (see Fig. 2). When solid finely dispersed PVC is electrolyzed under the same conditions, directly on the platinum cathode surface, a similar crosslinked polymer is formed as an insoluble film. At the same time, the relative viscosity of the catholyte, contrary to the above experiments with dissolved PVC, grows during the electrolysis by 20-25 % because the reduction products pass into the DMF solution. The data given prompt the conclusion that irrespective of whether PVC is dissolved or as solid suspended in DMF, reductive electrolysis causes certain irreversible changes in this polymer (an overall mass loss of the initial material, drop in MM, transfer of fragments into catholyte solution, formation of insoluble crosslinked product, changing its properties in the process of further reduction), which as a whole are defined as the electrochemical reductive degradation of PVC. The IR spectra of the yellow film (Fig. 3), differ from that of the original PVC by the appearance of absorption bands in the 1600 cm -1 range, due to the valency vibration of short carbon-carbon bonds. The intensity of this band in the 1660 cm-t region increases with deepening of film that, which indicates the formation of conjugated short bonds and growth of a conjugated chain [7].
Electrochemical reductive degradation of polyvinyl chloride
1567
This conclusion is confirmed by an EPR study of the dark cinnamon coloured PVC films which gives a signal represented by a singlet (Fig. 4) with a g factor of ca. 2.00 characteristic of centres in a polymer with a conjugated system. Similar EPR
"
3
~
3
2 2
! I 17
i
1 I5
~
I
I
7
G v , lO-e, cm -t
"~" ~'~~'~M~ ,~
FIG. 3 FIG. 4 FIG. 3. IR spectra of samples of PVC (1) and its degradation products-yellow (2), cinnamon (3) and black films (4). FIG. 4. NMR spectra of PVC electrolysis products immediately after the electrolysis at E = -2.0(•) and -2.5 V (2) and also after 14 days (3). signals were observed earlier in PTFE after the action of a plasma from a high frequency discharge with subsequent heat treatment [8]. The intensity of the signal from the PVC film, obtained by electrolysis at E = - 2.5 V, is significantly higher than with E = - 2.0 V. On keeping the film, the signal is reduced and the colour changes from dark to lightcinnamon, i.e. the so-called bleaching of degraded PVC [3] is observed. The bands from C - CI bond valency vibration (Fig. 3) are changed as follows during PVC electrolysis. The absorption band at 600 cm -1, typical of the crystalline part of the polymer is split and the one in the 700 c m - l region, typical of the amorphous part is broadened and then split. This is related to the occurrence of 2 processes- degradation (crystalline polymer region) and a secondary polymerization-erosslinking
1568
G.S.
SHAPOVAL et al.
of maerochains (amorphous region). A similar phenomenon is seen in thermal degradation of PVC [3]. A study of the black film formed during prolonged PVC electrolysis (ca. 10--12 h 0 showed the complete absence of the band in the bond vibration region for both C - C and C - C1. This fact and also the monotonic growth of the background lines from diffused IR-radiation is evidently due to the formation of a polydispersed phase, in particular of carbon. From this it may be assumed that on extended electrolysis, PVC graphitization occurs. Similar conclusions were made in [9], concerning graphitization to elemental carbon, in the case of PTFE treated with lithium amalgam as an electron donor. Suggestions on the mechanism of electrochemical reductive PVC degradation are based on the above results of electrochemical, IR, EPR and viscometfic studies of PVC and the electrolysis products and durations when compared with published data on the mechanism of electro-reduction of low MW chlorohydrocarbons and that of PVC dehalogenation by various energy sources. We could find no information on the electrochemical reduction of PVC in the literature. At the same time, during the polarographic reduction of low MW alkyl halides the potential-determining step is the transfer of an electron on to a vacant #-level of the carbon-halogen bond. From this and our results, it was assumed that during the corresponding orientation of the C - C1 bond in relation to the cathode, an additional polarization of this bond occurs, on account of the electrical field of the cathode, which has sufficiently high field strength (107--10s V/cm) in the double electrical layer. This facilitates the electron transfer on to the polarized section of the maeromolecule. Slow
.CH=-CH2~H--CH2 ~ "1" • ~I CI
F-~7 ~- "~ ICHJr-CH2--CH--CH2 , , , . ~
"-~dH--CHz--~H--CH2~ "l" (31(I) Cl The radical formed with the same potential* either adds a second electron, with subsequent protonation: I
N(~H-CH~-CH-CH2
~ +e+H
}
÷ --* ~ C H 2 - - C H 2
-- CH--
')
Cl
C H 2 ,,~,
(2)
Cl
or recombines with the same radical by intra- or intermolecular interaction Cl
Cl
I
I
~ C H - - C H 2 - - C H - - C H z ,v
',, C H - - C H ~ - - C H - - C H z ~ f
& s
-~ ,,, C H - - C H ~
--CH-CH2
~
I I N CH--CH2--CH--CH2
(3) ~
* It is known [10l that the radical possesses a higher electron affinity than the molecule from formed.
w h i c h it is
Electrochemical reductive degradation of polyvinyl chloride
1569
Besides this, 2-electron reduction of a fragment of the PVC macromolecule is possible with splitting out of chlorine and formation of a short bond N C H - - C H 2 - - C H - - C H 2 ~ + 2e --* ~ C H = C H - - C H = C H
I
~ + 2 C I - + H2
(4)
I
C1
C1
and further reduction of conjugated short bonds to form macroanion-radicals ~CH=CH--CH=CH~
+ e --* [ ~ C H = C H - - C H = C H ~ I %
(5)
capable of intermolecular recombination which leads to formation of the crosslinked macromolccule of the insoluble PVC film. In the case when dehalogenation is accompanied by splitting out hydrogen, the latter either recombines, forming H2 or splits out HCI from a PVC molecule, forming a macroradical, capable of further electrochemical and chemical transformation ~CH2--CH--CH2--CH~
I
Cl
+ H " --* ~ ( ~ H 2 - - C H - - C H 2 -
I
CH~ +H + +CI-
(6)
I
CI
Cl
The catalytic action of HCI formed at the cathode by PVC reduction plays a definite part in the degradation, similar to the action of HCI in PVC thermodegradation [3]. Further, for crosslinking, the PVC macromolecules need the corresponding orientation and a sufficiently high concentration of radical centres. Just such conditions are directly obtained on the cathode surface when the insoluble PVC film is formed. Its reduction occurs both with the participation of lateral groups-chlorine elimination and double bond formation in the main chain, and with the participation of the latter. The first of these is manifested in an increase in the EPR signal and a slow increase in the film resistance and the second in its thickening, increase in rigidity and brittleness. Besides this, reduction of PVC is accompanied by elimination and transfer into solution of low MW degradation products, such as hydrocarbons being formed similarly to those from the electrochemical reduction of low MW alkyl halides [I 1] RX+2e ~ R- +X-
(7)
R- + H + ~ RH
(8)
for PVC ~ CHz--CH--CH2--CH--CH3
I
t
CI
C1
+2e) H + ) "~ CH~" + C H a - - - - - C H - - C H a -- C H 3
-2cl-
(9)
All the above refers to the direct reduction of PVC which does not contain plasticizer. In the electrolysis of PVC, plasticized with DOP, changes are observed which are also typical for the unplasticized polymer, but the electrolysis potential may be redu~cl to -(1.6-1.8) V. This is due to the electrolysis of the DOP which has a higher electron affinity, as can be judged both from the potentiodynamic curves (see Fig. 1) and from the classical polarogram of the solution of plasticized PVC which has 2 clear waves for the reduction of DOP at E = - (1"5-1"8) V (see Fig. 5).
1570
G.S. SHAPOVALet aL I,btA 1 q
2
t.4
t.a -~o,V
Fio. 5. Polarograms of catholytes of plasticized PVC before (1) and after electrolysis (2). The process of reducing the dialkyl phthalates in D M F on a background of tetraalkylammortium iodide was shown in [4] to proceed as follows: o
o
C --OR+e O
(t0)
~C--OR O
o
o (it)
o
o
o
o
o
o
c_oR+ o
-°R .2) o
The dianion formed by 2-electron reduction (reaction 11) or by disproportionation (reaction 12) of the products of reaction (10) is an active electron donor, capable of serving to accumulate and transfer electrons contrary to the potential gradient [13]. From this it may be supposed that in the presence of a plasticizer the PVC reduction reactions (1), (2), (4) and (5) take place by an indirect mechanism via electrochemical regeneration of dianions of DOP with subsequent electron transfer on to PVC macromolecules. The nucleophilic addition of dialkyl phthalate having an excess negative charge on the carbonyl [14] to PVC is not excluded, similar to that occurring in reaction of the latter with electrochemical regeneration of an oxygen anion-radical [15]. However, if in the case of 02- reaction with alkyl halides, peroxide formation occurs Jn 3 stages [16] RCI + O2- --, ROO" + C I -
(13)
ROO" + 0 2 - ~ R O O - + 0 2
(14)
R O O - +RC1 ~ R O O R + CI-,
(15)
Electrochemical reductive degradation of polyvinyl chloride
1571
where the superoxide ion has an electron donor role, then the reaction of the dialkyl phthalate dianion with PVC evidently may proceed by a route in which electron transfer is accompanied by fission of dialkyl phthalate, forming a catbonyl (PD = proton donor) A
o II._O
-- ~ C H 2 ' ~
~Cl+~.~_C_Onl--~aO+Cl
+ ~"
~ t0+atOU,
(t6)
o or peroxide derivative of PVC o
"o .RCI +R00---~ROOR+CI-
(t7)
capable of further electrochemical reduction. In favour of the latter suggestion, there is a fall in the current of the threshold wave for DOP of the catholyte (see Fig. 5), after electrolysis, which is related to the reduction in the 1725 c m - 1 band in the spectrum of PVC which has been electrolyzed. Indirect electrochemical reduction is accompanied by intense electrochemical reductive degradation of PVC at the DOP reduction potential, its concentration in solution being an order lower than the PVC concentration and being incompletely depleted during the electrolysis time (Fig. 5), due to the reversibility of the electron transfer to PVC and regeneration of the original DOP. The PVC degradation mechanism needs further, deeper study but from the above results it may be considered as definite that independently of the M M and also of whether the PVC is present in the dissolved or solid state, PVC undergoes electrochemical reductive degradation due to electron transfer at comparatively low potentials, irrespective of whether the cathode is elect~on donating or there is electrochemical electron transfer or a chemical reducing agent. Translated by C. W. CAPl, REFERENCES
I. N. V. KORSHAK and V. A. ZAMYATINA, Zhnrn. prikl, khimii 14: 809, 1941 2. C.S. MARVEL and J. H. SAMPLE, J. Amer. Chem. Soc. 61: 3241, 1939 3. K. S. MINSKER and G. T. FEDOSEYEVA, Destruktsiya i stabilizatsiya polivinilkhlorida (Degradation and Stabilization of Polyvinylchloride). p. 272, Moscow, 1972 4. G. S. SHAPOVAL and T. E. LIPATOVA, Elektrokhimicheskoye initsirovaniye polimerizatsii (Electrochemical Initiation of Polymerization). 325 pp., Kiev, 1977 5. G. S. SHAPOVAL, A. V. GORODYSKH, A. P. TOMILOV and A. A. PUD, Dokl. Akad. Nauk SSSR 263: 1179, 1982 6. G. S. SHAPOVAL, A. P. TOMILOV and V. A. STERNIK, Vysokomol. soyed. B2S: 2319, 1983 (Not translated in Polymer Sci. U.S.S.R.) 7. L. Bk'~.LAMY, Infrakrasnye spcktry slozhnykh molekul (Infrared Spectra of Complex Molecules). 592 pp., Moscow, 1963
S . N . SAtT~zK~ et a/.
1572
8. V.A. VONSYATSKII, Ye. A. ROTER and V. A. TETERSKIi, Fiz.-khlm. mekhanika materlal0v, 5, 64, 1982 9. L. KAVEN, Z. BASTL and F. D. DOUSEK, Carbon 22: 71, 1984 10. G. HOITINK and J. VANSHOOTEN, Rec~eil tray. chim. 71: 1089, 1952 11. C. H. MANN an¢~ K. BARNES, Elektrokhimicheskiye reaktsii v nevodnykh sistemakh (Electrochemical Reactions in Non-aqueous Systems). 48 pp., Moscow, 1974 12. A. R. IL'YASOV, Yu. M. KAR(~IN, A. Ya. LEVIN, I. D. MOROZOVA, I. N. SOTNIKOVA, V. Kh. IVANOVA and R. T. SAFIN, Izv. Akad. Nauk SSSR, set. khim., 4, 736, 1968; 5, 1030, 1960 13. Z. OSAWA and J. UCHIDA, J. Polymer Sci., Polymer Chem. Ed. 20: 2259, 1982 14. I. B. AFANAS'EV, Uspekii khimii 48: 977, 1979
Polymer Science U.S.S.R. Vol. 29, No. 7, pp. 1572-1578) 1987 Printed in Poland
M O L E ~ MASS CHARACTERISTICS POLY(DIPHENYLENE PHTHALIDE)*
0032-3950/87 $10.00 + . 0 0 ~) 1988 Pergamon Press plc
OF
S. N. SALAZKIN,M. G. ZOLOTUKmN,V. A. KOVARDAKOV, S. R. RAHKOV,L. V. DUBROVINA, Ys. A. GLADKOVA and S.-S. A. PAVLOVA A. N. Nesmeyanov Institute for Elemento-Organic Compounds Chemical Institute of the Bashkir Associate, U.S.S.R. Academy of Sciences )
(Received 3 February 1986) The molecular mass characteristics of polydiphenylenephthalide, prepared by polycondensation of p-(3 chloro-3-phthalidyl) diphenyl, have been studied. The ~'w and ~rn of fractions were determined from studies of solution properties and of the spectra of the end groups. The constants of the Mark-Kuhn-Houwink equation were found for a good solvent (tetrachloroethane) and for 0 conditions. The equilibrium rigidity of the macromolecule was evaluated; the magnitude of the Kuhn segment is 24 A. The results obtained indicate the absence of appreciable branching in the polymer. It was shown that the polymer must have an -~'w> 3 x 104 to give strong films.
IN recent years a new type of polymer, the polyarylene phthalide, has been synthesized [1-4]. The study of these polymers, which have complex properties, and also of the features of the synthesis, achieved by a new method, is impossible without an investigation of the molecular mass characteristics and the properties of solutions of polyarylene phthalides. In the present work we studied one of the most promising polyarylene phthalides polydiphenylene" phthalide-prepared by polycondensation of the pseudomono-chloro* Vysokomol. soyed. A7,9: No. 7, 1431-1436, 1987.
G.S. SHA~m,AL et aL
8. L. P. RAZUMOVSKII, V. S. MARKIN and G. Ye. ZAIKOV, Vysokomol. soyed. A21: 1671, 1979 (Translated in Polymer Sci. U.S.S.R. 21: 7, 1843, 1979) 9. I. S. BELAYEV, V. D. GERASIMOV and L. B. SOKOLOV, Plast. massy, 8, 50, 1975 10. L. B. SOKOLOV, V. D. GERASIMOV, V. M. SAVINOV and V. K. BELYAKOV, Termostoikiye aromaticheskiye poliamidy (The Thermal Stability of Aromatic Polyamides). p. 253, Moscow, 1975 11. L. P. RAZUMOVSKH, V. S. MARKIN and G. Ye. ZAIKOV, Vysokomol. soyed. A27: 675, 1985 (Translated in Polymer Sci. U.S.S.R. 27: 4, 751, 1985) 12. V. N. LEBEDEV, G. P. ANDRIANOVA and A. Ye. CHALYKH, Izv. vuzov. Khimiya i khim. teknologiya 23: 1286, 1980 13. H. FUJITA, J. Phys. Soc. Japan 8: 271, 1953 14. A. I. MAR'IN and Yu. A. SI-ILYAPNIKOV, Vysokomol. soyed. B23: 825, 1981 (Not translated in Polymer Sci. U.S.S.R.)
PolymerScienceU.S.S.R.Vol.29, No. 7, pp. 1564-1572,1987 Printedin Poland
0032-3950/87$10.00+.00 © 1988PergamonPresspie
THE ELECTROCHEMICAL REDUCTIVE DEGRADATION OF POLYVINYL CHLORIDE* G. S. SHAPOVAL,A. P. TOMILOV,A. A. PUD and K. V. BATSALOVA Petrochemical Department, Institute of Physical Organic Chemistry and Carbon Chemistry, Ukr.S.S.R. Academy of Sciences
(Received 3 February 1987) The reactions of solid PVC and PVC dissolved in DMF, occurring both by direct reduction and also through an electron transfer agent have been studied. At comparatively low negative potentials, PVC undergoes reductive electrochemical degradation, leading to dehalogenation of the macromolecules, radical decay and the formation of a polymer of 3-dimensional structure on the cathode surface. A mechanism has been proposed for these processes.
IT was shown in [1] that PVC, dissolved in fairly polar solvents, in distinction from solid PVC, readily reacts with alkaline reagents, each as caustic potash and sodium alkoxylates. This causes dehydrochlorination, forming a cinnamon coloured, insoluble powder, which is a crosslinked polymer with short conjugated bonds. At the same time, by reaction of PVC dissolved in carefully purified dioxane with zinc powder or metallic sodium, the same authors found a reduction in the extent of PVC polymerization without extensive chlorine elimination, which had been described earlier in [2]. Up to the present time, the degradation processes in PVC of various types have received * Vysokomol. soyed. A29: No. 7~ 1424-1430, !987,
Electrochemical reductive degradation of polyvinyl chloride
1565
insut~cient detailed study [3], but the effect of the transfer of the electrons, unrelated to ~ and p irradiation or' the action of alkalies, needs independent study. Electrochemical reduction, which is a means of changing the values of the cathode potential and simulates the action of electrons with varying reducing power, of PVC is of particular interest [4]. Prior to our work [5], no such studies had been carried out. This process has a theoretical and practical value since electron transfer from a metal to the functional groups of a macromolecule essentially affects the properties of the boundary*layers and accordingly the adhesion strength of the bonding of the polymeric protective covering to cathodicaHy polarized metalfic substrates. It was shown earlier [6] that the electrochemical reduction of both solid PVC and PVC dissolved in D M F causes the same changes at comparatively low negative potential at r o o m temperature, which were observed as an effect of temperatures of the order of 350 ° [3]. The present work is a continuation of these studies. PVC samples with M=- 5 x 10s and 4 x 104 with or without dioctyl phthalate (DOP) as plasticizer were used. Electrolysis was carried out at cathode potentials controlled in the range, Effi -(1.6-2.5) V (saturated calomel electrode) with a solution of PVC of M--4 x 10" or a finely dispersed suspension of PVC with M = 5 x l0 s in DMF with a 0.1 M tetrabutylammonium iodide background. Electrolysis occurred in a twin chambered sealed cell with the cathode and anode separated by a Schott No. 3 filter in an argon atmosphere. Each electrode was a 6 cm2 platinized plate; vitreous carbon ones were also used. Before and after electrolysis, voltametric curves of the catholyte were recorded, and the relative or intrinsic viscosity was determined. Polarographic studies were made on an LP-60 polarograph using a dropping mercury electrode with the capilliary characteristic, m2/Stxl6-~ 1.6 mg2/3sec - g2. Polarization curves were recorded on a platinum electrode and on vitreous carbon of area, 1.5 x 10-2 cm- 2 on a P-5827 potentiostat. The IR spectra of the PVC and its electrolysis products were recorded on UR-20 and Pye-Unicam SP200 spectrometers in the 600-4000 cm- ~ range. Samples for the IR spectra were prepared by pulverizing the PVC at liquid nitrogen temperature, further grinding with KBr, milling and tabletting. Viscometric measurements were made in an Ubbelohde viscometer at 25°. The electrical conductivity of the system was measured in a twin-chambered cell using a P5010 variable bridge. According to the potentiodynamic curves measured before electrolysis (Fig. 1) both dissolved and finely dispersed, unplasticized PVC are reduced on a platinum and vitreous carbon cathode, with E = - (2.0-2.2) V, respectively. The values of the potential and reduction current are determined by the concentration of dissolved or amount of suspended PVC. The electrolysits of a solution of unplasticized PVC at Effi - 2 . 0 V results causes a yellow-cinnamon colour to appear on the cathode surface which spreads into the solution. A friable polymer film forms on the cathode, which gradually thickens, its colour changes from yellow to cinnamon, then black. As the electrolysis is continued, the current strength falls (Fig. 2) due to the increasing resistance of the film forming and shielding the cathode surface. As the electrolysis goes further, with the thickening and increase in intensity of film colour, its resistance hardly changes (in the range, (8.4-12.7) x 10" ohms). The relative viscosity of the catholyte falls during the electrolysis by 20Yo due to the lower M M s of the PVC reduction products, The fall in concentration due to film
G. $. SHAPOV.~et aL
1566
f, mA I] 6O
z, 1o mA
~0 Iso
2
2-0
2"8 -E,V
6
....
3O
Time ~" 10-~ sec
FIG. 1 FIe. 2 FIO. 1. Potential curves for 0.75 (1) and 0.5 % (2) PVC solutions in DMF with a 0.1 MN(C4Hg)4I background, 3-background curve, 4-PVC plasticized with DOP. FIe. 2. Chromoamperogramsof PVC reduced by electrolysisin DMF at E= - 2.0 (1) and - 2.5 V (2).
deposition hardly affects it as shown by the concentrational dependence of PVC viscosity under the same conditions. The electrolysis at -2.5 V of dissolved unplasticized PVC is usually different with more intense colouring of the catholyte and a higher film deposition rate on the cathode. The film is black. A sharp drop in current occurs in this system. However, the magnitude of the current remains higher than with E = - 2 . 0 V (see Fig. 2). When solid finely dispersed PVC is electrolyzed under the same conditions, directly on the platinum cathode surface, a similar crosslinked polymer is formed as an insoluble film. At the same time, the relative viscosity of the catholyte, contrary to the above experiments with dissolved PVC, grows during the electrolysis by 20-25 % because the reduction products pass into the DMF solution. The data given prompt the conclusion that irrespective of whether PVC is dissolved or as solid suspended in DMF, reductive electrolysis causes certain irreversible changes in this polymer (an overall mass loss of the initial material, drop in MM, transfer of fragments into catholyte solution, formation of insoluble crosslinked product, changing its properties in the process of further reduction), which as a whole are defined as the electrochemical reductive degradation of PVC. The IR spectra of the yellow film (Fig. 3), differ from that of the original PVC by the appearance of absorption bands in the 1600 cm -1 range, due to the valency vibration of short carbon-carbon bonds. The intensity of this band in the 1660 cm-t region increases with deepening of film that, which indicates the formation of conjugated short bonds and growth of a conjugated chain [7].
Electrochemical reductive degradation of polyvinyl chloride
1567
This conclusion is confirmed by an EPR study of the dark cinnamon coloured PVC films which gives a signal represented by a singlet (Fig. 4) with a g factor of ca. 2.00 characteristic of centres in a polymer with a conjugated system. Similar EPR
"
3
~
3
2 2
! I 17
i
1 I5
~
I
I
7
G v , lO-e, cm -t
"~" ~'~~'~M~ ,~
FIG. 3 FIG. 4 FIG. 3. IR spectra of samples of PVC (1) and its degradation products-yellow (2), cinnamon (3) and black films (4). FIG. 4. NMR spectra of PVC electrolysis products immediately after the electrolysis at E = -2.0(•) and -2.5 V (2) and also after 14 days (3). signals were observed earlier in PTFE after the action of a plasma from a high frequency discharge with subsequent heat treatment [8]. The intensity of the signal from the PVC film, obtained by electrolysis at E = - 2.5 V, is significantly higher than with E = - 2.0 V. On keeping the film, the signal is reduced and the colour changes from dark to lightcinnamon, i.e. the so-called bleaching of degraded PVC [3] is observed. The bands from C - CI bond valency vibration (Fig. 3) are changed as follows during PVC electrolysis. The absorption band at 600 cm -1, typical of the crystalline part of the polymer is split and the one in the 700 c m - l region, typical of the amorphous part is broadened and then split. This is related to the occurrence of 2 processes- degradation (crystalline polymer region) and a secondary polymerization-erosslinking
1568
G.S.
SHAPOVAL et al.
of maerochains (amorphous region). A similar phenomenon is seen in thermal degradation of PVC [3]. A study of the black film formed during prolonged PVC electrolysis (ca. 10--12 h 0 showed the complete absence of the band in the bond vibration region for both C - C and C - C1. This fact and also the monotonic growth of the background lines from diffused IR-radiation is evidently due to the formation of a polydispersed phase, in particular of carbon. From this it may be assumed that on extended electrolysis, PVC graphitization occurs. Similar conclusions were made in [9], concerning graphitization to elemental carbon, in the case of PTFE treated with lithium amalgam as an electron donor. Suggestions on the mechanism of electrochemical reductive PVC degradation are based on the above results of electrochemical, IR, EPR and viscometfic studies of PVC and the electrolysis products and durations when compared with published data on the mechanism of electro-reduction of low MW chlorohydrocarbons and that of PVC dehalogenation by various energy sources. We could find no information on the electrochemical reduction of PVC in the literature. At the same time, during the polarographic reduction of low MW alkyl halides the potential-determining step is the transfer of an electron on to a vacant #-level of the carbon-halogen bond. From this and our results, it was assumed that during the corresponding orientation of the C - C1 bond in relation to the cathode, an additional polarization of this bond occurs, on account of the electrical field of the cathode, which has sufficiently high field strength (107--10s V/cm) in the double electrical layer. This facilitates the electron transfer on to the polarized section of the maeromolecule. Slow
.CH=-CH2~H--CH2 ~ "1" • ~I CI
F-~7 ~- "~ ICHJr-CH2--CH--CH2 , , , . ~
"-~dH--CHz--~H--CH2~ "l" (31(I) Cl The radical formed with the same potential* either adds a second electron, with subsequent protonation: I
N(~H-CH~-CH-CH2
~ +e+H
}
÷ --* ~ C H 2 - - C H 2
-- CH--
')
Cl
C H 2 ,,~,
(2)
Cl
or recombines with the same radical by intra- or intermolecular interaction Cl
Cl
I
I
~ C H - - C H 2 - - C H - - C H z ,v
',, C H - - C H ~ - - C H - - C H z ~ f
& s
-~ ,,, C H - - C H ~
--CH-CH2
~
I I N CH--CH2--CH--CH2
(3) ~
* It is known [10l that the radical possesses a higher electron affinity than the molecule from formed.
w h i c h it is
Electrochemical reductive degradation of polyvinyl chloride
1569
Besides this, 2-electron reduction of a fragment of the PVC macromolecule is possible with splitting out of chlorine and formation of a short bond N C H - - C H 2 - - C H - - C H 2 ~ + 2e --* ~ C H = C H - - C H = C H
I
~ + 2 C I - + H2
(4)
I
C1
C1
and further reduction of conjugated short bonds to form macroanion-radicals ~CH=CH--CH=CH~
+ e --* [ ~ C H = C H - - C H = C H ~ I %
(5)
capable of intermolecular recombination which leads to formation of the crosslinked macromolccule of the insoluble PVC film. In the case when dehalogenation is accompanied by splitting out hydrogen, the latter either recombines, forming H2 or splits out HCI from a PVC molecule, forming a macroradical, capable of further electrochemical and chemical transformation ~CH2--CH--CH2--CH~
I
Cl
+ H " --* ~ ( ~ H 2 - - C H - - C H 2 -
I
CH~ +H + +CI-
(6)
I
CI
Cl
The catalytic action of HCI formed at the cathode by PVC reduction plays a definite part in the degradation, similar to the action of HCI in PVC thermodegradation [3]. Further, for crosslinking, the PVC macromolecules need the corresponding orientation and a sufficiently high concentration of radical centres. Just such conditions are directly obtained on the cathode surface when the insoluble PVC film is formed. Its reduction occurs both with the participation of lateral groups-chlorine elimination and double bond formation in the main chain, and with the participation of the latter. The first of these is manifested in an increase in the EPR signal and a slow increase in the film resistance and the second in its thickening, increase in rigidity and brittleness. Besides this, reduction of PVC is accompanied by elimination and transfer into solution of low MW degradation products, such as hydrocarbons being formed similarly to those from the electrochemical reduction of low MW alkyl halides [I 1] RX+2e ~ R- +X-
(7)
R- + H + ~ RH
(8)
for PVC ~ CHz--CH--CH2--CH--CH3
I
t
CI
C1
+2e) H + ) "~ CH~" + C H a - - - - - C H - - C H a -- C H 3
-2cl-
(9)
All the above refers to the direct reduction of PVC which does not contain plasticizer. In the electrolysis of PVC, plasticized with DOP, changes are observed which are also typical for the unplasticized polymer, but the electrolysis potential may be redu~cl to -(1.6-1.8) V. This is due to the electrolysis of the DOP which has a higher electron affinity, as can be judged both from the potentiodynamic curves (see Fig. 1) and from the classical polarogram of the solution of plasticized PVC which has 2 clear waves for the reduction of DOP at E = - (1"5-1"8) V (see Fig. 5).
1570
G.S. SHAPOVALet aL I,btA 1 q
2
t.4
t.a -~o,V
Fio. 5. Polarograms of catholytes of plasticized PVC before (1) and after electrolysis (2). The process of reducing the dialkyl phthalates in D M F on a background of tetraalkylammortium iodide was shown in [4] to proceed as follows: o
o
C --OR+e O
(t0)
~C--OR O
o
o (it)
o
o
o
o
o
o
c_oR+ o
-°R .2) o
The dianion formed by 2-electron reduction (reaction 11) or by disproportionation (reaction 12) of the products of reaction (10) is an active electron donor, capable of serving to accumulate and transfer electrons contrary to the potential gradient [13]. From this it may be supposed that in the presence of a plasticizer the PVC reduction reactions (1), (2), (4) and (5) take place by an indirect mechanism via electrochemical regeneration of dianions of DOP with subsequent electron transfer on to PVC macromolecules. The nucleophilic addition of dialkyl phthalate having an excess negative charge on the carbonyl [14] to PVC is not excluded, similar to that occurring in reaction of the latter with electrochemical regeneration of an oxygen anion-radical [15]. However, if in the case of 02- reaction with alkyl halides, peroxide formation occurs Jn 3 stages [16] RCI + O2- --, ROO" + C I -
(13)
ROO" + 0 2 - ~ R O O - + 0 2
(14)
R O O - +RC1 ~ R O O R + CI-,
(15)
Electrochemical reductive degradation of polyvinyl chloride
1571
where the superoxide ion has an electron donor role, then the reaction of the dialkyl phthalate dianion with PVC evidently may proceed by a route in which electron transfer is accompanied by fission of dialkyl phthalate, forming a catbonyl (PD = proton donor) A
o II._O
-- ~ C H 2 ' ~
~Cl+~.~_C_Onl--~aO+Cl
+ ~"
~ t0+atOU,
(t6)
o or peroxide derivative of PVC o
"o .RCI +R00---~ROOR+CI-
(t7)
capable of further electrochemical reduction. In favour of the latter suggestion, there is a fall in the current of the threshold wave for DOP of the catholyte (see Fig. 5), after electrolysis, which is related to the reduction in the 1725 c m - 1 band in the spectrum of PVC which has been electrolyzed. Indirect electrochemical reduction is accompanied by intense electrochemical reductive degradation of PVC at the DOP reduction potential, its concentration in solution being an order lower than the PVC concentration and being incompletely depleted during the electrolysis time (Fig. 5), due to the reversibility of the electron transfer to PVC and regeneration of the original DOP. The PVC degradation mechanism needs further, deeper study but from the above results it may be considered as definite that independently of the M M and also of whether the PVC is present in the dissolved or solid state, PVC undergoes electrochemical reductive degradation due to electron transfer at comparatively low potentials, irrespective of whether the cathode is elect~on donating or there is electrochemical electron transfer or a chemical reducing agent. Translated by C. W. CAPl, REFERENCES
I. N. V. KORSHAK and V. A. ZAMYATINA, Zhnrn. prikl, khimii 14: 809, 1941 2. C.S. MARVEL and J. H. SAMPLE, J. Amer. Chem. Soc. 61: 3241, 1939 3. K. S. MINSKER and G. T. FEDOSEYEVA, Destruktsiya i stabilizatsiya polivinilkhlorida (Degradation and Stabilization of Polyvinylchloride). p. 272, Moscow, 1972 4. G. S. SHAPOVAL and T. E. LIPATOVA, Elektrokhimicheskoye initsirovaniye polimerizatsii (Electrochemical Initiation of Polymerization). 325 pp., Kiev, 1977 5. G. S. SHAPOVAL, A. V. GORODYSKH, A. P. TOMILOV and A. A. PUD, Dokl. Akad. Nauk SSSR 263: 1179, 1982 6. G. S. SHAPOVAL, A. P. TOMILOV and V. A. STERNIK, Vysokomol. soyed. B2S: 2319, 1983 (Not translated in Polymer Sci. U.S.S.R.) 7. L. Bk'~.LAMY, Infrakrasnye spcktry slozhnykh molekul (Infrared Spectra of Complex Molecules). 592 pp., Moscow, 1963
S . N . SAtT~zK~ et a/.
1572
8. V.A. VONSYATSKII, Ye. A. ROTER and V. A. TETERSKIi, Fiz.-khlm. mekhanika materlal0v, 5, 64, 1982 9. L. KAVEN, Z. BASTL and F. D. DOUSEK, Carbon 22: 71, 1984 10. G. HOITINK and J. VANSHOOTEN, Rec~eil tray. chim. 71: 1089, 1952 11. C. H. MANN an¢~ K. BARNES, Elektrokhimicheskiye reaktsii v nevodnykh sistemakh (Electrochemical Reactions in Non-aqueous Systems). 48 pp., Moscow, 1974 12. A. R. IL'YASOV, Yu. M. KAR(~IN, A. Ya. LEVIN, I. D. MOROZOVA, I. N. SOTNIKOVA, V. Kh. IVANOVA and R. T. SAFIN, Izv. Akad. Nauk SSSR, set. khim., 4, 736, 1968; 5, 1030, 1960 13. Z. OSAWA and J. UCHIDA, J. Polymer Sci., Polymer Chem. Ed. 20: 2259, 1982 14. I. B. AFANAS'EV, Uspekii khimii 48: 977, 1979
Polymer Science U.S.S.R. Vol. 29, No. 7, pp. 1572-1578) 1987 Printed in Poland
M O L E ~ MASS CHARACTERISTICS POLY(DIPHENYLENE PHTHALIDE)*
0032-3950/87 $10.00 + . 0 0 ~) 1988 Pergamon Press plc
OF
S. N. SALAZKIN,M. G. ZOLOTUKmN,V. A. KOVARDAKOV, S. R. RAHKOV,L. V. DUBROVINA, Ys. A. GLADKOVA and S.-S. A. PAVLOVA A. N. Nesmeyanov Institute for Elemento-Organic Compounds Chemical Institute of the Bashkir Associate, U.S.S.R. Academy of Sciences )
(Received 3 February 1986) The molecular mass characteristics of polydiphenylenephthalide, prepared by polycondensation of p-(3 chloro-3-phthalidyl) diphenyl, have been studied. The ~'w and ~rn of fractions were determined from studies of solution properties and of the spectra of the end groups. The constants of the Mark-Kuhn-Houwink equation were found for a good solvent (tetrachloroethane) and for 0 conditions. The equilibrium rigidity of the macromolecule was evaluated; the magnitude of the Kuhn segment is 24 A. The results obtained indicate the absence of appreciable branching in the polymer. It was shown that the polymer must have an -~'w> 3 x 104 to give strong films.
IN recent years a new type of polymer, the polyarylene phthalide, has been synthesized [1-4]. The study of these polymers, which have complex properties, and also of the features of the synthesis, achieved by a new method, is impossible without an investigation of the molecular mass characteristics and the properties of solutions of polyarylene phthalides. In the present work we studied one of the most promising polyarylene phthalides polydiphenylene" phthalide-prepared by polycondensation of the pseudomono-chloro* Vysokomol. soyed. A7,9: No. 7, 1431-1436, 1987.