The mechanism and kinetics of dehydrochlorination of polyvinylchloride

Dehydrochlorination of polyvinylchloride

35

5. K. A. BOGDANOVA, M. A. MA1EKEVICH, A. A. BERLIN, G. V. RAKOVA and N. S. YENIKOLOPYAN, I)okl. Akad, Nauk SSSR 211: 874, 1973 6. K. A. BOGDANOVA, G. V. RAKOVA, A. A. BERLIN and N.S. YENIKOLOPYAN, Vysoko. inol. soyed. A14: 1976, 1972 (Translated in Polymer Sei. U.S.S.R. 14: 9, 2216, 1972) 7. G. P. SAVUSH]KINA, V. V. IVANOV and N. S. YENIKOLOPYAN, Vysokomol. soyed. A17: 865, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 4, 995, 1975) 8. K. A. BOGDANOVA, A. K. BONETSKAYA, A. A. BERLIN, G. V. RAKOVA and N. S. YENIKOLOPYAN, Dokl. Akad. Nauk SSSR 197: 618, 1971 9. W. K. BUSFIELD and D. MERIGOLD, Makromolek. Chem. 138: 65, 1970 10. T. MIKI, L HIGASHIMURA and S. OKAMURA, J. Polymer Sci. A - l , 8: 157, 1970 11. T. F. ORESHENKOVA, A. G. GRUZNOV and L. M. ROMANOV, Vysokomol. soyed. A17: 1927, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 9, 2220, 1975) 12. T. F. ORESHENKOVA, A. Kh. BULAI, A. G. GRUZNOV, I. Ya. SLONIM, L. M. ROMANOV and Ya. G. URMAN, Vysokomol. soyed. A18: 1371, 1976 (Translated in Polymer Sei. U.S.S.R. 18: 6, 1572, 1976) 13. A. V. TOBOLSKY and A. EISENBERG, J. Amer. Chem. Soc. 82: 289, 1960 14. A. WEISSBERGER, E. S. PROSKAUER, J. A. RIDDICK and E. E. TOOPS, Organicheskie rastvoriteli (Organic Solvents). p. 285, Foreign Literature Publishing House, 1958 (Russian translation) 15. K. A. BOGDANOVA, A. A. BERLIN, V. Z. KOMPANIETS, G. V. RAKOVA, Ye. A. MIROSHNICHENKO, Yu. A. LEBEDEV and N. S. YENIKOLOPYAN, Vysokomol. soyed. A17: 658, 1975 (Translated in Polymer Sci. U.S.S.R. 17: 3, 759, 1975) 16. T. F. ORESHENKOVA, A. G. GRUZNOV and L. M. ROMANOV, Proizvodstvo i pererabotka plastieheskikh mass i sinteticheskikh stool, No. 10, 32, 1974 17. M. INDUE, J. Polymer Sci. A I : 2697, 1963 18. H. W. STARKWEATHER and R. H. BOYD, J. Phys. Chem. 64: 410, 1960 19. Industria 3: 99, 1974 (Commercial publication) 20. W. H. LINTON and H. H. GOODMAN, J. Appl. Polymer Sci. 1: 170, 1959

THE MECHANISM AND KINETICS OF DEHYDROCHLORINATION OF POLYVINYLCHLORIDE* K . S. MINSKER, A. A. BERLIN, V. V. LISITSKII

and S. V. KOLESOV "40th October Anniversary" State University, Bashkir

(Received 9 February 1976) Several chemical reactions are involved in dehydrochlorination of PVC. These are: 1) random elimination of HCI from normal PVC units; 2) growth of polyene sequences initiated by C O - - C H ~ C H structures; 3) slow growth of polyene sequences * Vysokomol. soyed. A19: No. 1, 32-36, 1977.

K. S. ~INSKER e~ a~.

86

initiated by isolated )CH~---CH( linkages; 4) growth of polyene systems initiated by conjugated )CH~-~CH( linkages. The relatively high rate of the over-an process of dehydrochlorination of PVC is attributable mainly to the conjugated ~ CO--(CH~ -~CH)n~ structures (n > 1) originally present in the polymer molecules. THE major process in degradation of PVC is dehydrochlorination, which is a complex combination of reactions, occurring simultaneously or successively in various directions. The widely held .view t h a t breakdown of PVC occurs by a single reaction is therefore clearly inadequate for solution of important problems connected with the mechanism and kinetics of elimination of HC1, and also of the effect of stabilizing and reactive additives on dehydrochlorination of PVC, and in m a n y instances is unsuitable. Up ~/0 s,mole lOCI//molePl/#. aec-t 0

/'2 0"8 O'd .

0"5

/

I

/'5

.

l

I

2 2.5 .I0 ~ mole/mole of PVC ,

Fro. 1. Dependence of vp on the concentration T0 for different samples of PVC at 175%

10-4 torr." 1--ozonization, 2--hydrolysis, 3--hydrolysis of a sample (~) after previous degradation and oxidation (70°, 40 hr). More deep seated and more complete information is obtained if two reactions are Tecognized in dehydrochlorination of PVC. These are: a) formation of isolated double bonds as a result of random scission of HC1 from normal PVC units (with the chlorine atoms in the 1,3-position) at the rate vr, and b) growth of polyconjugated systems as a result of activation of HC1 elimination by adjacent ~ C = C ~ bonds (fl-chloroallyl activation) at the rate vp [1, 2]. By combining the results of oxidative scission of ozonized samples b y H20~ with the kinetics of dehydrochlorination of PVC, it was possible to separate these stages and determine the apparent kinetic parameters of the dehydrochlorination reactions. The results so obtained were log Ar~3.4, E ~ 2 1 . 5 ~ 1 . 5 kcal/mole, log A p ~ l d . 9 , E~a~3$11 kcal/mole [3]. Even this division of the dehydrochlorination of PVC is, however, not in accord with a number of experimental facts. I n particular the rate of elimination of HCI is directly proportional to the concentration, T0, of internal unsaturated groupings, at which rupture of the chain occurs in oxidative break-

Dehydroehlorination of polyvinylehloride

37

down of ozonized PVC (Fig. 1). These structures are generally accepted as being ~ C H = C H - - C H C 1 - - C H 2 ~ fl-chloroalkyl groups. I t is seen that, when extrapolated, the straight liile in Fig. 1 passes through $he origin. This shows t h a t unsaturated end-groups of PVC macromolecules are not active with respect to the specific reaction of formation of polyene sequences [3]. Meanwhile, during the course of degradation of PVC the concentration of newly formed, isolated CO~C~ATrO~ Or m~g~AL > C = C ( ~o~¢Ds Iz¢ THE ~ACROMOLECVL~S~TER OZOmZAT I O N AND H Y D R O L Y S I S OF ~ V ~

Ozonization /t/J0, dl/g

[r/], dl/g

0.725 1.070 1.180 0.970 1.170 1-140 0.850 0.770 1.210 0.690 1.280 1.320

0.675 0.975 1.060 0.895 1.060 1.040 0.800 0.730 1.110 0-665 1.190 1.250

Hydrolysis acid alkaline ~0× 10% ~* × 10 4, ~'o"x 10% ~r ° × 10-* mole/mole [r/], dl/g mole/mole [t/I, dl/g mole/mole of PVC of PVC of PVC 2.0 1.5 1.5 1.5 1.4 1-4 1.3 1-3 1.2 1-0 0.9 0-8

0.670

2.2

0.725 1.095

1.4 1.4

1-180

1.0

0.680 0.975 1.065 0-890 1.070 1.040 0.805

1.8 1.5 1.5 1.7 1.3 1.4 1.2

1.105 0.660

1.3 1.3

1.240

0.8

61 105 120 97 119 115 76 67 124 57 134 140

CH-~ CH-- CHC1-- CH2 ~ structures resulting from random scission of HC1, increases continuously and this should bring about acceleration of the combined process of dehydrochlorination of PVC. However it is found experimentally t h a t the rate remains constant for a long time, up to 30% by weight conversion or more [1, 4]. The natural assumption of the possibility of counterbalancing of t h e increased concentration of internal, \/ C ---- C \/ groups by a restriction of t h e growth of polyene sequences, is not in accord with experimental evidence for different samples of PVC with ~0=(0.5-2-5)x10 -4 mole/mole of PVC.* The calculated values of the rate constant of decay, kd, differ substantially for samples with different values of 70. Moreover the kinetic parameters of elimination of HC1 from low molecular analogues of PVC, modelling both normal and "irregular" structures, are not in agreement with the values of the rate constants and' E a of overall dehydrochlorination of PVC. This has led to the opinion being expressed t h a t it is not possible to simulate the reactions of decomposition o f

PVC [1]. Separation of dehydroehlorination of PVC into two stages [1, 2] has however shown satisfactory agreement between the rate constants of random scissiorl * The error in determination of ~ was not above 13~o in any experiment.

38

K . S. MrNsx~g e~ al.

of HC1 from PVC (kr~0"8× 10 -7 sec -1 at 115°) and its low molecular analogues in the liquid phase (10-~-10 -a sec -1 [5]). At the same time the rate constant of growth of polyconjugated systems (kp~0.75 × 10-~ sec -1) is higher by two or more orders of magnitude than the constants obtained for elimination of HC1 from model compounds containing C1 atoms in the ~-position with respect to isolated ) C ~ C ~ bonds (10-5-10 -4 sec-1), but is in good agreement with the rate constant of decomposition of low molecular models containing conjugated ~C----C( bonds (10 -s sec -1) [5]. The original PVC, which contains an appreciable amount of internal ) C = C ~ bonds (~0) does not contain conjugated - - C H = ~ C H - - C H ~ C H - - groupings, because if there were, reaction with organic phosphites (0Ps) should lead either to preservation of ~C-~C(, bonds or to degradation of the molecules [6] I

i~ I

I

II

I

I

I

~C--C=C--CR ~ t (RO)2P-~O

--*C----C--C~C~ ~ P(OR)a--

I

I

I

I

~C--C----C~ ~ --C--R (nO)21p=O 1 I t was found experimentally however in the case of PVC (80-100 °, 0.5-16 hr) that after ozonization and oxidative decomposition no change occurs in the molecular weight of the PVC [7]. Reactions of OPs that lead to disappearance of unsaturated structures in the main chain detectable by ozonization are known

[6] HCl 0 ~C--CH~--CIt~ -F RC[

0 IL

~C--CH~-CH~ -F- P(OR)3--

0-----P(0R)~

-

~C----CH--CH~ I

i

0 ~P(OR)a Since in practice 0Ps do not react with isolated ~C-~C~ b o n d s in fl-cMoroalkyl groupings (in our experiments at 50-175 ° with 3-chloropent-l-ene, 4-chloropent-2-ene and according to reference [8] 4-chlorohex-2-ene), it was natural to suppose that it is ~C--CH----CH--CH--CH2~ groupings that are II

[

0 C1 present in the PVC originally. These structures could easily be formed during production and storage of PVC by oxidation at hydrogen atoms in the fl-position with respect to ~C----C~ bonds [9] formed by random elimination of HC1 from PVC.

Dohydrochlorination of polyvinylchloride

39

Although the concentration of ~C--Ctt----CH--0H--CH2~ I1

groupings in

I

O C1 PVC is small (in the region of 10 -4 mole/mole of PVC), the infrared spectra of sufficiently thick films (0.5-1 mm) prepared from 5% solution in tetrachloroethane i n vacuo (10-z-10 -3 torr), contain weak bands at 1600 and 1675 em -z, characteristic of the valency'vibrations of ) C = C ( linkages and of ) C = O groups conjugated with the latter [10]. ~,I0 ~,mole/mole of PVC 5 1 3

Z I 10

zx

I 2O

~. l

zs---2 J

30

40

Time, mfn Fzo. 2. Formation of ) C = C ( unsaturated linkages during I dehydroohlorination o f PVC (175 °, 10' tort):/---oxidative scission by H20, of ozonizod PVC, 2--alkaline hydrolysis.

I t has been shown independently that alkaline or acid hydrolysis of PVO (5% solution of PVC in cyclohexanone, 2.5-3% aqueous solution of KOH or HC1, 25 °, 1-10 hr) brings about a fall in the molecular weight of PVC, like oxidative breakdown of the ozonized polymer (Table), and at the same time appearance in the IR spectra (films of PVC from THF, 50 p) of strong bands in the 1715 cm -z region (valency vibrations of ) C = O end groups) [11]. This is characteristic of unsaturated ketones [12] 0

II

~Ctt=CH--C--CH3

H

mo +

H2SO, or KOH

--C

/

O

II

+ CH3-- C--CH2~

o It is an important fact that in thermal dehydrochlorination of PVC (175 °, 10 -4 torr) a continuous increase occurs in the concentration of internal ) C = C / bonds, as shown by the change in the MW of PVC using the ozonolytic method of analysis (i.e. in fl-chloroallyl groups), and by the number of breaks in the polymer chain in hydrolysis of the polymer (ketoallyl structures)but the degree of polymerization does not change because hydrolytic splitting of single or conjugated ) C = C ~ bonds in fl-chloroallyl groupings does not occur (Fig. 2). At the same time the overall rate of dehydrochlorination does not change. Mild oxidation of the same PVC samples (60-70 °, 40 hr when degradation

40

K . S. MII~SX_E~ et a/.

of the polymer chain does not occur -- [t/]0=[t/]ox=0.725) results in increase in the concentration of ~ G H = C H - - C ~ structures ( ~ = 2 . 6 x 10 -4 mole/mole II

O of PVC in comparison with 2.0 × 10 -4 mole/mole of PVC before oxidation), while the total number of > C = C < bonds ( ~ = 4 × 10 -4 mole/mole of PVC) remains unchanged. As a consequence the overall rate of dehydrochlorination increases uniformly from 1.52 × 10 -e mole HC1/mole PVC/sec to 1.98 × 10 -e mole HC1/molo PVC/see (Fig. 1). Thus the following series of chemical reactions occurs in dehydrochlorination of PVC. 1. Random elimination of HC1 from normal PVC units, with kr=0"8 × × 10 -7 sec -~ (175°). 2. Growth of polyene sequences, initiated by ~ C - - C H = C H ~ II

O structures, with kr = 0.75 × 10 -B sec -x (175°). 3. Slow growth of polyene sequences, initiated b y isolated > C = C < bonds. Formation of diene groupings from > C = C < bonds (kr =10-5-10-4 sec -1 at 175 ° [6]) becomes noticeable on the total level of dehydrochlorination of PVC only after exposure for 3-5 hr. 4. Growth of polyene systems, initiated b y conjugated > C = C < bonds, with kp, in the region of 10 -8 sec -x at 175 ° (according to reference [6]). Thus the generally accepted view that the low stability of PVC is due to the presence of internal fl-chloroallyl groups [1, 13-16] is in all probability incorrect. The relatively high overall rate of dehydrochlorination of PVC is mainly attributable to its content of conjugated ~ C0--(HC=CH)~--CHC1--CH2 ~ structures (n >t 1). To confirm: with the complete reaction system, the equation describing dehydrochlorination of PVC will take the form [HC1]oo=

2kr+

kp~[_

kh ]t-- k--~l" kh--kpl kr

+-~h~7O-- ~hh-[

kr

kh--kpl)'(1-e-~ht)

In the initial stages of decomposition of PVC when following equation will apply

kp~_kp,, and kpt

and

kht ~<1 the

[HC1] [HCl]. =(kr+k60)t, which is the same as the equation proposed i n reference [3], but it has a different chemical meaning (~0 represents the concentration of ~ C O - - C H = C H - - C H C 1 - --CH~ ~ and not ~ C H = C H - - C H C 1 - - C H 2 ~ groups). At the present time the part played by end groups and the value of the rate constant at high degrees of dehydrochlorination, kh, remain open questions.

Dehydrochlorination of polyvinylchloride

41

I n this work we used commercial a n d l a b o r a t o r y samples of PVC, produced in t h e U.S.S.R., Czech.S.S.R., I t a l y , Federal Republic of Germany etc., differing with respect t o the m e t h o d of preparation (suspension, bulk) and molecular weight (50,000-250,000). The characteristics of the samples a n d the experimental m e t h o d are described in references [3]. and [16]. Translated b y E. O. PH~I~IrS

REFERENCES 1. K. S. MINSKER and G. T. FEDOSEYEVA, Destruktsiya i stabilizatsiya polivinilkhorid~ (Degradation and Stabilization of Polyvinylchloride). pp. 25, 30, 125, 126, " K h i m i y a " , 1972 2. K. S. MINSKER, A. A. BERLIN, D. V. KAZACHENKO and R. G. ABDULLINA, Dokl. Akad. l~auk SSSR 203: 881, 1972 3. K. S. MINSKER, A. A. BERLIN and V. V. LISITSKH~ Vysokomol. soyed. BI8: 54, 1976 (Not t r a n s l a t e d in P o l y m e r Sei. U.S.S.R.) 4. Z. VYMAZAL, E. CZAK0, B. MEISSNER and J. STEPEK, J. Appl. Polymer Sei. 18: 2861, 1974 5. Z. MAYER, B. OBEREIGNER and D. LIM, J. P o l y m e r Sei. C33: 283, 1971 6. B. Ye. IVANOV and V. F. ZHELTUKHIN, Uspekhi khimii 39: 773, 1970 7. K. S. MINSI~ER, N. A. MUKMENOVA, A. A. BERLIN, D. V. KAZAUItENKO, M. Ya. YANBERDINA, S. I. AGADZHANIYAN and P. A. KIRPICHNIKOV, Dokl. Akad. N a u k SSSR 226: 1088, 1976 8. T. V. HOANG, A. MICHEL, Q. T. PHAM and A. GUYOT, European Polymer J. 11: 475, 1975 9. M. ONOZUKA and M. ASAHINA, J. Macromolec. Sci. C3: 235, 1969 10. L. T. BELLAMY, N o v y dannye po I K - s p e k t r a m slozl~lykh molekul (New D a t a on the Infrared Spectra of Complex Molecules). p. 42, "Mir", 1971 (Russian translation) 11. G. SCOTT and M. TAHAN, E u r o p e a n Polymer J. 11: 535, 1975 12. M. M. SHEMYAKIN and L. A. SHCHUKINA, Uspekhi khimii 26: 528, 1957 13. D. BRAUN and W. QUARG, Angew. Makromolek. Chem. 29/30: 163, 1973 14. L. VALKO, J. TVAROSKA a n d P. KOVARIK, European Polymer J. 11: 411, 1975 15. B. B. TROITSKII, V. A. DOZOROV, F. F. MINCHUK and L. S. TROITSKAYA, European P o l y m e r J. 11: 277, 1975 16. K. S. MINSKER, V. V. LISITSKII, Z. VYMAZAL, M. KOLINSKI, Ya. K A Z A L , Ye. N. SHVAREV, I . V . KOTI.YAR, I. I. GORBACHESKAYA and I. G. SAMOILOVA, Plast. massy, No. 1, 19, 1976