The mechanism and kinetics of the dehydrochlorination of polyvinyl chloride

The mechanism and kinetics of the dehydrochlorination of polyvinyl chloride

974 K . ' S . MI~S~R et al. REFERENCES 1. Brit. Pat. 1~o. 925433, 1963 2. A. UBBELOHDE, Plavlenie i kristallicheskaya struktura (Melting and the Cry...

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974

K . ' S . MI~S~R et al. REFERENCES

1. Brit. Pat. 1~o. 925433, 1963 2. A. UBBELOHDE, Plavlenie i kristallicheskaya struktura (Melting and the Crystal Structure). Izd. "Mir", 1969 3. A. V. SIDOROVICH, V. V. RAGLIS and Ye. V. KUVSHINSKII, Mekhanika polimerov, 746, 1968 4. A. V. SIDOROVICH and Ye. V. KUVSH1NSKII, Zavod. lab. 9: 1124, 1959 5. Yu. P. BARSKII. II. Soveshclmnie po teplo- i massoohmenu (Second Heat- and MassExchange Conference). Minsk, 1965 6. M. I. YAGFAROV, Z. G. GIZATULLINA and V. S. IONKIN, Vysokomol. soyed. B I I : 815, 1969 (Not translated in Polymer Sci. U.S.S.R.) 7. T. L. MAGILA and D. C. Le GRAND, Polymer Enging. and Sci. 10: 349, 1970 8. A. V. SIDOROVICH and Ye. V. KUVSHINSKII, Vysokomol. soyed. 2: 778, 1960 (Not translated in Polymer Sci. U.S.S.R.) 9. B. KI, Noveishie metody issledovaniya polimerov (Mcdern Study Methods of Polymers). Izd. "Mir", 1966

THE MECHANISM AND KINETICS OF THE DEHYDROCHLORINATION OF POLYVINYL CHLORIDE* :K. S. MINSKER, D. V. KAZACHENKO, R. G. ABDULLINA, R. B. I~OVLER

and A. A. BERLIN 40th Anniversary of October State University, Bashkiria (Received 28 M a y 1971)

PRESENT-DAY ideas about the thermal degradation of polyvinyl chloride (PVC) at temperatures b~low 200°C, especially early in the process, are that it consists chiefly of hydrogen chloride liberation (de-HC1). This reaction is characterized by a single constant kl, although the process consists of two parallel-consecutive reactions, i.e. the statistical de-HC1 (random process) and the so-called "zipper" reaction [1], which is due to a de-HC1 activated by the adjacent ~ C = C ~ bond and results in the formation of conjugated systems. Earlier statistical calculations [2, 3] showed the rate of the conjugated double bond formation to be probably much larger than that of an individual double bond, but so far there is no quantitative experimental evidence for this theory. Ozonization of organic compounds, followed by oxidative hydrolysis is being used in determining the position and number of double bonds present [4, 5]. The ~ C ~ - C ~ bond is broken as a result of this treatment. Rupture of the internal double bonds was expected in the ease of PVC to result in a considerable molecular weight (mol.wt.) change of the polymer molecules, * Vysokomol. soyed. A15: No. 4, 866-871, 1973.

Dehydroehlorination of PVC

975

b u t n o t w h e n these, or s y s t e m s of c o n j u g a t e d b o n d s , are p r e s e n t o n t h e c h a i n ends. F o r instance, a P V C s a m p l e h a v i n g 111w----74,900, w h e n s u b j e c t e d to 30 m i n t h e r m a l d e g r a d a t i o n a t 175°C ( a v e r a g e degree of p o l y m e r i z a t i o n P = 1 3 0 0 ) , s h o w e d a 12.9% mol.wt, change (Sample No. 4 in Table). T h e theoretical change of t h e mol.wt, due to t h e r u p t u r e of t h e c o n j u g a t e d t e r m i n a l double b o n d s w i t h n = 3 should h a v e b e e n n o t m o r e t h a n b y 0.3O/o . This m e a n s t h a t a b o u t one q u a r t e r of t h e existing m a c r o m o l e c u l e s c o n t a i n s i n t e r n a l ~ C = C ~ bonds. T h e n u m b e r of r u p t u r e s ~ w h i c h r e s u l t in a mol.wt, r e d u c t i o n d u e to ozonization, followed b y a n o x i d a t i v e h y d r o l y s i s of t h e p a r t l y de-HC1 p o l y m e r , is g i v e n b y t h e following equation: 0 2 X 62.5 ~=62.5(2/Jl~w--2/Mw°): 2~o ([~]]o/[~]]--1), (1) in which tt~° a n d /14w--number a v e r a g e mol.wt, of P V C a n d o f c h l o r i n a t e d P V C (CPVC) before a n d a f t e r ozonization, [0]0 a n d [ ~ ] - - t h e r e s p e c t i v e intrinsic viscosities of these. I f t h e n u m b e r of r u p t u r e s p e r m a c r o m o l e c u l e is n o t g r e a t e r t h a n 2, t h e b r e a k i n g o f t h e r a n d o m l y f o r m e d d o u b l e b o n d s will give a c o n s t a n t Afw/iqn r a t i o of 2, b e c a u s e a n y chain s h o r t e n i n g will be c o m p e n s a t e d b y a r e d u c t i o n of t h e n u m b e r o f chains. Dissociation o f t e r m i n a l c o n j u g a t e d g r o u p s f r o m t h e p o l y m e r w i t h a n e x p o n e n t i a l mol.wt, d i s t r i b u t i o n (MWD) should n o t result in a v i s c o s i t y decrease or in a r e d u c t i o n of Mw, or of Mn [6]. THE

Sample ~o.+

THERMAL DEHYDROCHLORINATIOlq

De-HC1 duration, min

[HC1] X 104, mole/inole PVC

0 5 15 30 0 5 15 30 45

m

1.70 8'56 17.90 0.0 1.53 4.25 6.80

[,l] §, dl/g

0.88 0.87 0.84 0.81 0.93 0.93 0.93 0.86 0.83

OF PVC*

AND CPVC

? AT

175°C

X t = (ao-~-~)X X 103, × 103, Change moleof frac°fo~]' /o

tures/mole PVC

5.4 6.5 9.7 12.9 0.0 0.0 3.0 7.1 10.9

9.4 11.9 17.1 24.9 0.0 0.0 5.3 13.9 20.6

mole of fractures/mole PVC 1-09

1.12 1-17 1-25 0.0 0.0 0.05 0.13 0.20

1.1 1.7 2.3 0 0 2.9 3.1 3.4

* d~w= 7-49× 10~; xo = 1.09x lO-3 moleof fractures/molePVC. t Mw=7"49×10~; Xo=O. Y;Samples1-4 were PVC,5-9 CPVC. § The intrinsic viscositybeforeozonlzatlonwas 0.93dug. T h e P V C m a c r o m o l e c u l e s will contain a n u m b e r of i n t e r n a l double bonds; t h c original P V C s a m p l e u s e d b y us s h o w e d a mol.wt, r e d u c t i o n of 5 . 4 % a f t e r o z o n i z a t i o n a n d o x i d a t i v e h y d r o l y s i s (sample No. 1 in Table), so t h a t t h e a v e r a g e

K. S. MINSKER e~ al.

976

number of double bonds per 1 mole PVC was 9.4 × 10 -5. Mild chlorination of the original PVC samples produced saturation of the double bonds. There was no detectable mol.wt, change as a result of ozonization and subsequent oxidative hydrolysis (sample No. 5). I m p o r t a n t is that the decay of the double bonds present in the macromoleeules produced a marked reduction of the de-HC1 rate of the polymer (Fig. 1) due to there being no fl-elimination of HC1 caused by the presence of unstable allyl chloride groups. The reduction of the overall rate of decomposition of chlorinated PVC samples is a well-known fact [4, 7]. The exposure to heat of PVC and CPVC caused the number of randomly formed double bonds to increase (Fig. 2), and this process gave rise to reactive eentres (unstable allyl chloride groups); this process was independent of the sample type and had a constant rate (kc~8.6 × 10 -s see-i). JO

8O

T/me, mz'n O0 IzO

/~0

~r"

2

s. t

2O

lO

~'me ,

~

T/me, mz'n

rnin

FIG. 1

I

FIG. 2

FIe. 1. The de-HC1 of: a - - P V C , b--CPVC; full lines--experimental, dashes--theoretical dependence calculated from eqn. (5). FIG. 2. Functions characterizing the rate of the statistical de-HCI of: 1--PVC, 2 - - C P V C

I f one assumes the thermal degradation of PVC to be a combination of random de-HC1 with formation of allyl chloride in the macromolecules (having constant kc), as well as the more rapid, specific formation of polyene sequences from the ~C~---C~ conjugated double bonds (having rate constant kp), one can describe the total de-HC1 of PVC, neglecting the probable effects of any interfering factors [8], by the system of equations: dX/dt~vc--ktX

;

d [t-IC1]/dt~vc+kpX,

(2)

Dehydrochlorination of PVC

977

in which kt--termination constant of conjugated double bond development, X - - n u m b e r of reactive contres, %--rate of random double bond formation. One gets from this that ~c

X----~- [1--exp ( - - k t t ) ] + X o exp (--ktt) tot

(3)

while the number of HC1 molecules eliminated from PVC will be obtained from:

[HCl]=kd+k~/~,t ÷ -kvXo [1 --exp (--ktt) k;~[l_exp (_ktt)] Rt

kt

/Ct

(4)

Experiments have shown that the kinetic chains responsible for the formation of polyene sequences do not decay for at least 2-2.5 hr. The system of equations for the initial stage of thermal degradation will therefore change to d [HC1]~dr= vc÷ k pX = vc÷ (Vpvc-- vcPvc) X/Xo ,

(5)

because kp is easily determined from the initial rate of de-HC1 of the original polymer:

k, = (vpvc- crvc)/Xo

(6)

The number of double bonds present in the original PVC, activating conjugation during thermal degradation, is given by the sum of terminal (60% of the PVC macromolecules contain terminal > C = C ~ bonds) [9] and internal bonds in: X o _ 0"6 × 2 × 62"5 /[q]0 + ---1

"~2 × 62.5 -~

The experimental results listed in the Table give a k p = 9 . 3 × 1 0 -4 sac -1. The k¢- and k,-values calculated according to eqn. (5) yielded kinetic equations characterizing the de-HC1 of PVC and CPVC which agreed well with the experimental (Fig. 1). This is evidence of good agreement between the mathematical model and the actual de-HC1 of PVC. Although k¢ differed by four orders of magnitude from k,, the rates of both the processes, i.e. the de-HC1 of PVC (random), having a kc=8-6× 10 -s sac -1, and the conjugated double bond formation having k , = 9 . 3 × 1 0 -4 see -t, are comparable because the number of double bonds present in PVC due to polyene sequence formation is quite smM1 ( X = 10 -3 mole fractures/mole PVC). The average length of the conjugation block ~ of the partly dehydrochlorihated PVC and CPVC can be assessed as a function of time t from:

~Pvc--([HCl]÷Xo)/(~÷a) ZcPvc= [HC1]/7,

(8)

in which [HC1]--number of all the double bonds formed during the de-HC1 of the polymer up to time t, a - - n u m b e r of double bonds at the chain ends, which equals

978

K. S. M~SK~R et al.

0.6 × 2 × 6 2 - 5 ] ~ ° = 7 5 / ~ °. The values ~, Xo and a are given in moles of fractures/ /mole PVC. The average length of the polyene sequences will increase continuously during the thermal degradation, but will not exceed ~ = 3 - 5 in one hour (see Table). As conjugation blocks of ~ = 10 or more are normally the result of prolonged thermal degradation per macromolecule [10-13], the fi-value found confirmed the conclusion that kt does not play a decisive part during the initial stages of thermal degradation, and that the polyene sequences form in a time interval commensurate with that of the thermal degradation. Some idea about the mechanism of the de-HC1 of PVC can be got on the basis of our results. The random de-HC1 has a monomolecular mechanism. The specific reaction leading to double bond formation, however, is activated by the allyl chloride groups [14] and is characterized by a relatively large kp. It is incorrect to presume that the thermal degradation of PVC consists predominantly of an HC1 elimination which starts from the chain ends, i.e. the unstable double bonds [15], The random double bond formation proceeds parallel to the development of conjugated > C = C < bonds and at comparable rate.

EXPERIMENTAL The PVC used in this investigation was produced by suspension polymerization an d had ~ w = 7 . 4 9 × 104 ( d = l . 4 1 , bulk weight 0.48 cm3/g). Thermal degradation was carried o u t in a nitrogen atmosphere in t h e presence of an HC1 trap filled with barium stearate [16]. F i v e per cent solutions of the polymer in eyclohexane were ozonized at 25°C in the presence of acetic acid, using 08 with a 5-6yo ozone content; the rate of ozonization was 25-30 1./hr for 2.5 hr, which was a sufficient time to stop the mol.wt, of the macromoleeules from changing a n y further. This t r e a t m e n t was followed by oxidative hydrolysis with an excess of a 30~/o HaOz solutions. Viscometrie determinations were made in eyclohexane at 255= 0"01°C in an Ubbelohde viscometer. The flow time of the solvent exceeded 100 s, so t h a t the correction for kinetic viscosity was less t h a n 2 Yo and could be neglected. The intrinsic viscosity was established from the extrapolated Huggins and Fuoss-Mead lines [17] at l Yo determination error. The linearity of the [t/I-concentration function was evidence of there being no associations over a wide range of concentrations; this was also confirmed by the temperature independence of the Huggins constant. The 11~2 of the original PVC was determined from the MWD curves obtained b y fractional precipitation of a 2 ~ PVC solution in a solvent-precipitant system (eyelohexanone and ethanol respectively). The ratio 71~w/~7/n~2 [18], [19], while M~/Mn equalled [~]0/[t/] according to Ciampa and Sehwindt [20]. The solvents used in this study were purified by established methods [21, 22]. Their constants were those given in the literature. The CPVC was produced by chlorinating PVC powder with gaseous chlorine at 25°C for 20-40 hr, so t h a t the double bonds present in the original polymer became saturated (checked by the ozonization method). The determination error of k c and kp was calculated by the least squares method. I t was about 10 -~0 for kc, 10 -6 for kp; the statistical error was about 10 -a for k o and 10 -5 se¢ -1 for kp.

Dehydroehlorination of PVC

979

CONCLUSIONS (1) T h e possibility of dividing t h e t o t a l dehydroclflorination of P V C into t w o parallel-consecutive reactions, one being a r a n d o m process w i t h a r a t e c o n s t a n t k c = 1 0 - v - 1 0 -s, the o t h e r being t h a t o f polyene sequence f o r m a t i o n , k p = 10-3-10 -4, was shown b y experiment. (2) A n e q u a t i o n describing the t h e r m a l d e h y d r o c h l o r i n a t i o n of P V C d u r i n g its early phase, w h i c h takes into a c c o u n t the two established routes of t h e process, is proposed. (3) The f o r m a t i o n of r a n d o m double bonds takes place at the same time as the d e v e l o p m e n t of the c o n j u g a t e d > C = C < bonds a t c o m p a r a b l e rates during the t h e r m a l d e g r a d a t i o n of PVC.

Translated by K. A. ALLEN REFERENCES l. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20. 2 I. 22.

V. V. KORSHAK and V. A. ZAMYATINA, Zh. priklad, khim. 14: 809, 1941 R. F. BOYER, J. Phys. Chem. 51: 80, 1947 C. S. MARVEL, J. H. SAMPLE and M. F. ROY, J. Am. Chem. See. 61: 3241, 1939 B. BAUM and L. H. WARTMAN, J. Polymer Sci. 28: 537, 1958 A. T. MENYAILO and M. V. POSPELOV, Uspekhi khim. 36: 662, 1967 A. A. BERLIN and N. S. YENIKOLOPYAN, Vysokomol. soyed. A1O: 1475, 1968 (Translated in Polymer Sei. U.S.S.R. 10: 7, 1706, 1968) Ye. N. ZIL'BERMAN, P. S. PYRYALOVA and D. A. EKSTRIN, Plast. massy, No. 8, 10, 1968 L. VAL'KO, Mezhdunarod. simpozium po makromolekulyarnoi khimii, Praga (Internat. Symposium on Maeromoleeular Chemistry, Prague) 1965 V. A. DODONOV, G. G. PETUKHOV and G. A. RAZUVAYEV, Izv. Akad. Nauk SSSR, Seriya khim., 1009, 1965 W. C. GEDDES, Europ. Polymer J. 3: 747, 1967 W. KUHN, Helv. Chim. Aeta 31: 1780, 1948 H. KUHN, Z. Elektrochemie 53: 165, 1949 L. V. SMIRNOV and K. R. POPOV, Vysokomol. soyed. AI3: 1204, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 5, 1356, 1971) R. R. STROMBERG, S. STRAUS and B. G. ACHHA1VIMER,J. Polymer Sci. 35: 355, 1959 Ye. FETTES, (Ed.), Khimicheskie reaktsii polimerov (The Chemical Reactions of Polymers), p. 84, Izd. "Mir", 1967 K. S. MINSKER, V. P. MALINSKAYA and A. A. PANASENKO, Vysokomol. soyed. A12: 1151, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 5, 1304, 1970) A.I. SHATENStITEIN, Yu.P. VYRSKII etal.,Prakticheskoye rukovodstvo poopredeleniyu molekulyarnykh vesov i molekulyarnovesovogo raspredeleniya polimerov (Practical Guide to Determinations of molecular Weigths and their Distribution in Pol~ners). pp. 144, 166, Izd. "KbJmiya", 1964 P. V. McKINNEY, J. Appl. Polymer Sci. 9: 583, 1965 G. A. R. MATTHEWS and R. B. PEARSON, Plastics 28: 99, 1963 G. CIAMPA and H. SCI-IWINDT, Makromol. Chemie 21: 169, 1956 B. KEIL (Ed.), Laboratornaya tektmika po organicheskoi khimii (Laboratory Techniques in Organic Chemistry). p. 604, Izd. "Mir", 1966 O. F. GINZBURG and A. A. PETROVA (Eds.), Laboratornye raboty po organicheskoi khimii (Organic Chemistry Laboratory Work). p. 56, Izd. "Vyssehaya shkola", 1970

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