Study of the effect of the chloroallyl group content on the thermal dehydrochlorination of PVC

Eur. Polym. J. Vol. 21, No. 8, pp. 747-751, 1985 Printed in Great Britain. All rights reserved

0014-3057/85 $3.00 +0.00 Copyright © 1985 Pergamon Press Ltd

S T U D Y OF THE E F F E C T OF THE C H L O R O A L L Y L G R O U P C O N T E N T O N 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 PVC Z. VYMAZAL, L. MASTN'Y a n d Z. VYMAZALOV.~ Department of Polymers, Prague Institute of Chemical Technology, 166 28 Prague 6, Czechoslovakia (Received 30 August 1984; in revised form 19 March 1985)

Abstract--The chloroallyl group contents of PVC and its low molecular weight fractions, soluble in chloroform or acetone, have been determined on the assumption that the reaction of AgNO~ with labile chlorine atoms occurs only with the allylic chlorines. Their amount is highest in the low molecular weight fractions. The thermal stability of the polymer in inert atmosphere increases if these groups are removed. Neither the labile chlorine content nor the dehydrochlorination rate (in the subsequent degradation) of the initial polymer, or the fractions insoluble in the two solvents, is affected by heat treatment at 180° for 60 min, whereas the amount of labile chlorine in the low molecular weight fractions increases on heat treatment exceeding 30 min. No direct dependence of the dehydrochlorination rate on the amount of labile chlorine in the polymers under study has been established.

INTRODUCTION T h e chemical c o m p o s i t i o n o f P V C a n d structural anomalies in the polymer are receiving a t t e n t i o n [1-7] in view o f the generally accepted hypothesis t h a t structures different from the regular structural units can act as sites for initiation of d e h y d r o c h l o r i n a t i o n , a l t h o u g h there is no consensus as to their relative significance. It is assumed t h a t structural defects accumulate in the low molecular weight fractions o f the polymer. F r a c t i o n s o f P V C o f different molecular weight can be isolated f r o m the polymer by extraction fractiona t i o n using a series of solvents a n d mixtures [8]; for instance, two PVC fractions of molecular weight At, 1500 a n d 800, respectively ( 0 . 1 4 ~ wt o f total polymer), were o b t a i n e d from boiling m e t h a n o l [9]. In the polymer r e m a i n i n g after extraction with c h l o r o f o r m , the a m o u n t o f the split-off HC1 was lower t h a n in the initial sample [10]. Significant for the a m o u n t o f the material soluble in c h l o r o f o r m are the degree of conversion o f the m o n o m e r [10, 11], t e m p e r a t u r e o f polymerization a n d drying [5], type a n d a m o u n t of chain growth regulator [5, 10], etc. D e h y d r o c h l o r i n a t i o n m e a s u r e m e n t s a n d determ i n a t i o n s of some types of structural anomalies have also been p e r f o r m e d for P V C extracted with m e t h a n o l [12], c a r b o n tetrachloride [11], ether [1, 12], benzene [1], acetone [1,3, 10] a n d a c e t o n e - t e t r a h y d r o f u r a n mixture [3]. G u y o t f o u n d t h a t the total a m o u n t o f d o u b l e b o n d s a n d labile chlorines was highest in P V C fractions soluble in ether [1]. As the a m o u n t of this fraction increased (up to 1~o wt), so did the initial rate of d e h y d r o c h l o r i n a t i o n o f the original polymers d u r i n g d e g r a d a t i o n in solution. In this work, P V C a n d its fractions soluble in c h l o r o f o r m a n d acetone as well as the residues are studied f r o m the p o i n t o f view of the dehyd r o c h l o r i n a t i o n a n d the a m o u n t o f labile chlorine as m e a s u r e d by its reactivity to silver nitrate. The effects o f the time of d e g r a d a t i o n of the fractions at 180 ° in

air a n d in nitrogen o n the f o r m a t i o n of the chloroallyl groups a n d o n the a m o u n t of split-off HC1 are also examined. EXPERIMENTAL

The PVC samples were prepared by suspension polymerization at 59° up to 86~ conversion of the monomer. A combination of organic peroxides was used for initiation. The extraction of the polymer with chloroform or acetone was carried out at room temperature. The polymer was precipitated in methanol. The extracted fractions and the residues were dried in vacuum at 40 °. The /I/w and /13n values were determined by GPC on a Waters Model 150 C-ALC/GPC instrument in tetrahydrofuran at 30°. The dehydrochlorination at 180° in air and in N2 was monitored by continuous potentiometric titration [15]. Powders of the fractions and the precipitated polymers were subjected to heat treatment at 180° in air and in N 2 for periods up to 60 rain. The labile chlorine contents in the polymer fractions were determined by an adapted method after de Hoog [13, 14]; the underlying principle is the reaction of the labile chlorine atoms in the polymer chain with silver nitrate, the excess of which is potentiometrically titrated with sodium chloride. It is assumed that this technique measures the allylic chlorine content but it should be mentioned that AgNO 3 does not react with terminal groups of the type --CH2--CH=CH--CH2C1 [14]. About 1 g of polymer was weighed, with accuracy of 1 nag, and dissolved in 25 ml of cyclohexanone. One millilitre of 0.01 M aqueous AgNO 3 was added and the whole was heated at 80° for 4 hr under reflux. The reflux condenser then was flushed with 25 ml of methyl ethyl ketone; the contents of the flask were transferred to a beaker, and the flask was rinsed. The system was diluted with methyl ethyl ketone to 80 ml, 4 drops of concentrated HNO 3 were added, and the unreacted AgNO 3 was titrated with 0.001 M aqueous NaC1 using potentiometric indication; solutions 10 times more concentrated were used for higher labile chlorine contents of the polymer. The amount of AgNO 3 reacted was determined from the difference between the take-up of NaC1 in a blank experiment and in the actual analysis. 747

748

Z. VYMAZALet al. Table I. Reproducibilityof determination of labile chlorines in PVC (take-up of 0.001 M NaCI in the blank experimentv0, in the actual determination vl)

Polymer

Commercial sample

Laboratory sample

Sample* A 40

Essay

Weight of PVC (g)

I 2 3 4 5 6 7 1 2 3 4 5 1 2 3 4 5

1.0068 1.0046 0.9915 1.0072 1.0016 1.0079 1.0059 1.0183 0.5151 1.0236 0.9963 1.0063 0.9995 0.9977 1.0033 0.5235 0.3032

0.001 M NaCI consumption (ml) v0

vI

Labile chlorine (mol/lO00VC)

7.65 7.7 7.7 7.7 7.75 7.8 8.65 9.1 9.1 9.1 9.6 9.5 9.7 9.75 9.3 9.3 9.3

4.8 4.8 5.0 4.8 4.9 5.1 5.7 6.5 7.6 6.3 7.0 6.7 5.2 5.2 4.85 7.0 8.0

0.18 0.18 0.17 0.18 0.18 0.17 0.18 0.16 0.18 0.17 0.16 0.17 0.28 0.29 0.28 0.27 0.27

*Sample A 40 is a laboratory sample of PVC after thermal degradation during 40min at 180° in air.

In the original procedure [14] the blank was made up free of the polymer. Here the procedure for the blank was exactly the same as for the main procedure except that the 4 hr reflux at 80° was replaced by 4 hr contact at room temperature. Blank experiments involving the polymer are of particular importance for commercial samples of PVC and for stabilized PVC. The reproducibility of the method was tested on a series of laboratory and commercial PVC samples. The data for some measurements are given in Table 1.

RESULTS AND DISCUSSION

Batch extraction o f the initial polymer gave 13.6% wt o f a fraction soluble in c h l o r o f o r m (CHE) or 17.4% wt o f a fraction soluble in acetone (AE). The molecular characteristics o f the initial polymer, the extracted polymer, and the extraction residues ( C H R and AR) are given in Table 2. The polydispersities as well as the shapes o f the curves o f differential molecular weight distribution for the soluble fractions show that their molecular weight distributions are narrower than for the initial polymer. G o o d extraction selectivity o f the solvents was verified for several industrial and laboratory PVC samples. The fractions were degraded in air and under N2 at 180 °. The results for the dehydrochlorination in air (Fig. 1) do not indicate any marked differences between the time dependences o f evolution o f HC1 for the initial polymer, the extracted fractions, and the residues. F o r degradation under N2 (Fig. 2), the a m o u n t o f HCI evolved from the residues after extraction with c h l o r o f o r m and acetone was lower than for the whole polymer. This was particularly marked for the residue after extraction with chloroform; C H E appeared to be the least stable. Removal o f the low molecular weight fractions was also effective for decreasing the development o f colour in the residues during thermal degradation in the two atmospheres.

The data o f Table 3 indicate that the dehydrochlorination rates in air and in N 2 are not directly d e p e n d e n t on the a m o u n t o f labile chlorine over the region o f 0.1-0.6 mol/1000 VC units. Minsker [6] claims that the polymer contains ketoallylchloride groups - - C O - - C H = C H - - C H C I - - , the a m o u n t o f which does not change with time o f degradation o f the polymer in vacuum at 175 ° , and that the a m o u n t o f fl-chloroallyl groups, - - C H 2 - - C H ~ - - - C H - - C H C I - - , varies because o f rand o m d e t a c h m e n t o f HCI. Therefore we expected that polymers subjected to heat treatment in air and in N2 for various times would contain markedly different amounts of chloroallyl groups, - - C H = C H - - C H C I - - , so that the rate o f liberation o f HCI during the subsequent thermal degradation would be influenced. Intensely coloured polymers containing long sequences o f conjugated double b o n d s were actually obtained by the heat treatment. Unexpectedly, no

Table 2. Molecular characteristicsof the initial polymer and of the fractions after extraction with chloroform or acetone -~w h4. h4w/.~n

PVC 117,500 53,100 2.2

CHE 34,800 19,800 1.8

CHR AE AR 138,800 46,700 126,800 60,000 26,900 57,000 2.3 1.7 2.2

Table 3. Labilechlorinecontent and dehydrochlorinationrates in air and in N2 of the initial polymer and of the fractions after extraction with chloroform or acetone Rate of dehydrochlorination (109mol HCI/0.2 g sec) Fraction PVC CHE CHR AE AR

Air 5.9 5.5 5.8 5.8 6.0

Nitrogen 4.4 5.2 3.0 4.8 3.6

Labile chlorine (tool/1000 VC) 0.16 0.64 0.16 0.24 0.16

Effect of the chloroallyl group content on PVC

749

20 o~

1

3

5

10

0 0

10

20

30

40

50 t [mini

Fig. 1. Dependence of the amount of eliminated HCI on the time of degradation at 180° in air. (1) Initial polymer (PVC). (2) Fraction soluble in chloroform (CHE). (3) Residue from the extraction with chloroform (CHR). (4) Fraction soluble in acetone (AE). (5) Residue from the extraction with acetone (AR). appreciable increase in the chloroallyl group content of the initial polymer or the CHR or AR residues appeared (Table 4). This implies that the heat treatment resulted in the formation of sequences of conjugated double bonds, on the ends of which chloroallyl groups were present in amounts only slightly higher than in the initial polymer (Table 3). It can be deduced that new chloroallyl groups do not form or they form only in small amounts and in longer times of degradation. Our hypotheses are borne out by dehydrochlorination measurements in air and in N 2 for samples of PVC, CHR and AR subjected to heat treatment for different times. The rates of HC1 de-

tachment (Table 5) were virtually identical with those for the initial polymers (Table 3) and did not change with the heat treatment period. For the low molecular weight extracts (CHE and AE), an increased amount of chloroallyl groups was found after the heat treatment. The dehydrochlorination rate in the two atmospheres used did not change appreciably for CHE. For AE, the amount of chloroallyl groups (as found by the AgNO3 method) increased considerably during extended heat treatment (Table 4); for instance, thermally treated for 60 min in air, the sample contained 10 times more chloroallyl groups but the rate of dehydrochlorination was only doubled.

2°t

t'N

2

4

10

1

0 0

10

20

30

40

50 t [rnir~

Fig. 2. Dependence of the amount of eliminated HCI on the time of degradation at 180° under N 2 (curve labelling as in Fig. l).

Z. VYMAZALet al.

750

Table 4. Labile chlorine content in the initial polymer and its fractions after extraction with chloroform or acetone after various periods of heat treatment at 180° in air and in N 2 Labile chlorine (mol/1000VC) Thermal treatment (rain) Air Polymer PVC CHE CHR AE AR

5 0.25 0.56 0.18 0.26 0.19

10 0.21 0.54 0.19 0.32 0.20

Nitrogen

20 0.23 0.59 0.27 0.53 0.23

40 0.25 0.65 0.32 1.81 0.28

60 0.26 0.84 0.37 2.42 0.36

5 0.22 0.73 0.20 0.28 0.16

10 0.19 0.96 0.16 0.37 0.16

20 0.19 0.97 0.20 0.44 0.17

40 0.22 1.02 0.23 1.55 0.20

60 0.24 0.99 0.26 1.98 0.22

Table 5. Rate of dehydroehlorination at 180° in air and in N2 for the initial polymer and its fractions after extraction with chloroform or acetone, subjected to heat treatment for various periods Rate of dehydrochlorination (109mol HCI/0.2 g sec) Thermal* Air Nitrogen treatment (min) PVC CHE CHR AE AR PVC CHE CHR AE AR 5 6,4 6.2 6.1 8.4 6.2 4.9 5.3 3.3 6.5 4.0 10 6.2 6.3 5.8 8.5 6.2 5.1 5.3 3.3 7.5 4.1 20 6,5 6.3 5.8 8.8 6.2 4.8 5.8 3.3 7.9 4.1 40 6.3 6.6 6.1 10.0 6.6 5.0 5.8 3.3 9.3 4.0 60 6.6 6.8 5.8 12.0 6.4 5.1 5.8 3.3 12.0 4.4 *Polymer and fractions thermally treated in air or in nitrogen were subsequently degraded in air or in nitrogen, respectively.

Polymers o b t a i n e d at high conversions (more t h a n 80%) o f m o n o m e r exhibited the lowest t h e r m a l stability a n d greatest chloroform-soluble fraction [5, 11]. T h e degree o f b r a n c h i n g is also supposed to be directly related to the conversion [11, 16]. Recent results o f Hjertberg a n d S f r v i k [2--4] showed t h a t tertiary chlorine is m o r e effective t h a n internal d o u b l e b o n d s at initiating d e h y d r o c h l o r i n a t i o n . In view o f the fact t h a t the polymer u n d e r study came from a high conversion polymerization, we suppose t h a t the e n h a n c e d a m o u n t o f chloroallyl groups, f o u n d particularly in A E exposed to heat for more t h a n 30 min, is associated primarily with the elimination o f chlorine a t o m s b o n d e d at tertiary c a r b o n atoms,

REFERENCES

I

CHCI

I CH2 [

--HCI

-CH2--CH--CH:--C~CH2mCH--CH2--CH~

I

I

C1

1

C1

I

CI

CI

t

CHCI

I

CH:

I

--CH2--CH--CH:C--CH2--CH--CH2--CH

J

Cl

W i t h respect to the polymer p r e p a r a t i o n , it can be concluded t h a t the structural defects t h a t are reactive towards AgNO3 a n d the a m o u n t o f these defects in the polymer will be principally affected by the conditions o f polymerization a n d will n o t be modified to a decisive extent by subsequent heat treatment. The defects believed to be choroallyl groups, in the a m o u n t s f o u n d in the polymer studied a n d for the times a n d temperatures used d u r i n g P V C processing, d o n o t seem to have a great effect u p o n the thermal stability o f the polymer.

J

Cl

m.

f

Cl

This a s s u m p t i o n is in agreement with results [17] o b t a i n e d for c o m p o u n d s modelling structural defects of the chloroallyl type a n d branching, with the Cl a t o m b o n d e d at the tertiary c a r b o n a t o m ; the decomposition temperatures o f these models were 160 a n d 180 °, respectively.

1. A. Guyot, M. Bert, P. Burille, M.-F. Llauro and A. Michel, Pure appL Chem. 53, 401 (1981). 2. T. Hjertberg and E. M. S6rvik, Third International Symposium of PVC, August 1980, Cleveland, U.S.A., preprints, p. 60. 3. T. Hjertberg and E. M. S6rvik, Polymer 24, 673 (1983). 4. T. Hjertberg and E. M. S6rvik, Polymer 24, 685 (1983). 5. K. S. Minsker, S. V. Kolesov and G. E. Zaikov, Starenije i Stabilizatsiya Polimerov na Osnove Vinilkhlorida. Nauka, Moscow (1982). 6. K. S. Minsker, M. I. Abdullin, S. V. Kolesov and G. E. Zaikov, Developments in Polymer Stabilization---6 (Edited by G. Scott), p. 173. Applied Science, London (1983). 7. C. J. M. van der Heuvel and A. J. M. Weber, Makromolek. Chem. 184, 2261 (1983). 8. H. L. Pedersen, J. Polym. Sci. B5, 239 (1967). 9. V. Schwenk, F. Cavagna, F. L6mker, I. K6nig and H. Streiberger, J. appL Polym. Sci. 23, 1589 (1979). 10. V. I. Zegelman, S. V. Svetozarskij and J. B. Kotljar, Plast. Massy 2, 16 (1969). 11. V. I. Zegelman, V. A. Titova, D. N. Bort, V. A. Popov, I. K. Pakhomova, Ya.A. Konkhin, V. V. Lisitsky and K. S. Minsker, Plast. Massy 8, 8 (1980).

Effect of the chloroatlyl group content on PVC 12. W. C. Geddes, Eur. Polym. J. 3, 267 (1967). 13. A. J. de Hoog, Report, IUPAC Working Party on PVC, Cardiff (1978). 14. T. Hjertberg and E. M. S6rvik, Report, IUPAC Working Party on PVC, Cleveland (1980). 15. Z. Vymazal, E. Czak6, B. Meissner and J. gt~pek, Plast.

751

Kaue. 11, 260 (1974); J. appl. Polym. Sci. 18, 2861 (1974). 16. T. Suzuki, M. Nakamura, M. Yasuda and J. Tatsumi, J. Polym. Sci. C33, 281 (1971). 17. M. Asahina and M. Onozuka, J. Polym. Sci. A2, 3505, 3515 (1964).