Thermal stability of poly(vinyl bromide) and copolymers of vinyl bromide with methyl vinyl ketone

Eur. Polym. J. Vol. 20, No. 6, pp. 599-603, 1984 Printed in Great Britain. All rights reserved

0014-3057/84 $3.00 + 0.00 Copyright c~ 1984 Pergamon Press Ltd

THERMAL STABILITY OF POLY(VINYL BROMIDE) A N D COPOLYMERS OF VINYL BROMIDE WITH METHYL VINYL KETONE M. AMIN DIAB Chemistry Department, Faculty of Education, University of Mansoura, Damietta, Egypt (Received 8 September 1983; in revised form 31 October 1983) Abstract--Thermal stability and degradation behaviour have been studied for PVB and VB MVK copolymers spanning the whole composition range, using thermogravimetric analysis. The reactivity ratios in the radical copolymerization were determined by using an NMR technique, leading to q(VB) = 3.6 + 0.2 and r2(MVK ) = 0.2 + 0.1. The introduction of MVK units into the VB chain leads to an interaction with release of methyl bromide. The stability of the copolymers increases with increasing MVK concentration.

INTRODUCTION

The p o o r thermal stability of poly(vinyl bromide) (PVB) a n d the release of H B r on heating have received m u c h a t t e n t i o n [1--3]. Blauer a n d G o l d s t e i n [4] reasoned t h a t interposing other m o n o m e r units in the PVB c h a i n would lend stability to the polymer. In a recent study Z h u b a n o v a n d co-worker [5] examined the possibility t h a t i n c o r p o r a t i o n o f VB units into the chains of polystyrene, polyacrylonitrile a n d poly(methyl methacrylate) m i g h t confer flame retardance. C o p o l y m e r i z a t i o n of methyl m e t h a c r y l a t e ( M M A ) with VB is also clearly an effective way of stabilizing the P M M A chain against the release of flammable p r o d u c t s [6]. T h e latter o b s e r v a t i o n is consistent with Z u t t y a n d Welch's inclusion of V B - M M A as one of several systems in which d e g r a d a t i o n can occur by i n t r a m o l e c u l a r lactonization involving adjacent vinyl halide a n d ester groups in the c o p o l y m e r chain a n d the release of alkyl halide as a volatile p r o d u c t [7]:

Purification of initiator 2,2'Azobisisobutyronitrile (AIBN) (Eastman Kodak), used as initiator (0.1% w/v), was purified by dissolving in hot ethanol and filtering [8]. Purification of the polymers PVB was prepared by bulk polymerization of a 25 ml sample at 60' with 30% hydrogen peroxide as initiator, as recommended by Guyer and Schutze [9]. The reaction was carried to 6% conversion. The polymer was precipitated in methanol, purified by reprecipitation from tetrahydrofuran (THF) solution and dried under vacuum. PMVK and VB-MVK copolymers were prepared using ABIN as radical initiator and methyl acetate (50/50% v/v) as solvent. Five copolymers were prepared having 87, 60, 47, 31 and 19°,o of VB units, so that the reactivity ratios might be determined. The polymerizations were performed at 6 0 to about 5% conversion. The copolymers were then precipitated in petroleum ether (b.p. 40-60'>), reprecipitated by methanol from benzene solution and dried under vacuum. The copolymers were white powders. Molecular weights

cm 3

CH 3

CH2 "--gV'- C H 2 " - -

C/

CH 2

~

CH--Mb- - W ' - C H 2 - -

c "/

~

cH-~-

I

4- CH3 Br

8r c

0

0//"C ~OCH3 The objective o f the present investigation was to examine the stability o f V B - M V K copolymers a n d to determine the n a t u r e a n d extent o f any interaction which m a y occur between the c o m o n o m e r units during degradation. EXPERIMENTAL

Purification of monomers Vinyl bromide (VB) (BDH Chemical Ltd) boils at 15.5' but can be handled conveniently, provided it is chilled below 0 . Methyl vinyl ketone (MVK) (BDH Chemical Ltd). The two monomers were degassed and twice distilled on a vacuum line, dried over calcium hydride and kept below -18. 599

were determined by osmometry with toluene or cyctohexanone as solvent; PVB and PMVK had ,~o values of 25,000 and 140,000 respectively and those of the copolymers were 34,000, 40,000, 55,000, 95,000 and 120,000 for 87, 60, 47, 31 and 19% of VB units respectively. Analytical techniques InJra-red spectroscopy (i.r.). Spectra were recorded on Unicam S.P.200 i.r. spectrometry, for either qualitative or quantitative measurements of the degradation products. Liquid fractions were run in solution using CHCI 3 as solvent. Condensable products were recorded in the gas phase using a small gass cell with NaCI windows (15 mm). Nuclear magnetic resonance spectroscopy ( N M R ). spectra were obtained using Varian A-60 spectrometer with integration, using 20 mg sample of copolymers. The polymers were

600

M. AMIN DIAB

(B)

I O=C--CH 3

4 / I - 70

I

I

-60

I

-5o

-40

I

-3o

I

/""

-2o/-~o

2

/

-CHBr -

I

4

j I 3

I 2

r

I 1

p

3 ppm

-

Fig. 1. NMR spectrum of VB-MVK copolymers. dissolved in 10 ml of CDC13. Ten integrals were obtained for each sample and the average used for determination of polymer compositions.

Thermal methods of analysis Thermogravimetry (TG). TG measurements were made with a Du Pont 950 thermobalance. Finely powdered 10 mg samples were heated at 10°/min in a dynamic N2 atmosphere (70 ml/min); the sample holder was boat-shaped, 10 x 5 x 2.5 mm deep; the temperature-measuring thermocouple was placed 1 mm from the sample holder. Thermal degradation of the polymer. Samples (50 mg) were heated under vacuum from ambient temperature to 500° at 10°/min. The volatile products were collected for either qualitative or quantitative measurements in a small gas cell.

-14

Fig. 2. Graph of [f~(F, - 1)1/[(1 - f 0 2 F,] vs ~(1 - 2F0]/ [(1 - f 0 F t ] for VB--MVK copolymers. and a plot of f2(1 - 2F2) f ~ ( F 2 - 1) (1 - f 2 ) F ~ <5> vs (1 -f2)2F2 where

F2

M2/M, M2/M1 + 1 (mole fraction of M V K (M2) in polymer)

and f2 =

n2

(mole fraction of (M2) in feed).

nl + n2 RESULTS AND DISCUSSION

Determination o f reactivity ratios o f V B - M V K copolymers The average of ten integrals of N M R spectrum from each sample was used for calculation of copolymer composition. The compositions of the copolymers may be calculated from the ratios of these integrals which are proportional to the n u m b e r of protons contributing to the peaks. This method has previously been used to determine the reactivity ratios for styrene M M A [10], methacrylate-acrylate copolymers[ll] and dibrominated methacrylateM M A copolymers [12]. Figure 1 show the N M R spectrum of a V B - M V K copolymer; peak A at 4.35 is due to - - C H B r - - protons of VB, and peak B at 2.32 to O==C--CH3 protons of M V K units. By knowing the n u m b e r of moles of the monomer mixture and the molar ratio in the copolymer, reactivity ratios were calculated by using equation [13]: f~(1 -- 2F,) f~(F, -- l) (1 - - f l ) F I -- (1 --fl)ZFi r, + r 2 where

F,-

Mr~M2 MI/M2 + 1 (mole fraction of VB (M0 in polymer) nl

fl = ~ n z n l (mole fraction of Mi in feed).

From the slope and intercepts, reactivity ratios for V B - M V K are r , ( V B ) = 3.6 _+ 0.2 and r 2 ( M V K ) = 0.2_+0.1.

Thermogravimetry ( TG ) T G curves of PVB, P M V K homopolymers and V B - M V K copolymers are shown in Fig. 3. The degradation starts at ~ 150 ° for PVB and at ~ 2 7 0 ° for PMVK. The T G curves show that the stabilities of the copolymers are intermediate between those of the homopolymers. Thermal degradation o f the polymers The major products of degradation of PVB and P M V K are HBr and water respectively, as confirmed by i.r. McNeili [6] suggested that the degradation of PVB occurs in two stages. The first stage reaches its maximum rate, for a heating rate of 10°/min, at nearly 100 ° [14]. The weight loss in this stage corresponds to quantitative loss of HBr from the polymer. The second stage of decomposition occurs with a rate maximum near 500 ° and involves fragmentation and crosslinking of the conjugated polyene structure which results from loss of HBr. During thermal degradation, P M V K is known [15-17] to lose water by a cyclization resulting in a structure of the following type: "-9¢'-- C H 2

HC

and r~ and r2 are the reactivity ratios of VB and M V K respectively. Figure 2 represents a plot of f~(1 -- 2F,) - (1 - - f , ) r I

f~(F, -- 1) (1 -f~)2F'

CH 2

C

VS

o

CH 2 --'#---

CH

C

Thermal stability of PVB and VB MVK

601 PVB

100

--- 87 % VB ......... 6 0 % VB - - o - - 47% VB

--

/ "".\o\(\

90

\

--

--

--o--

":

/

80

/

:(:. o \"\ \

------

/

31% VB 19%

VB

PMVK

70 "

It

60

u~ 50

mo

/

40

\°\2,\

o ,\

~o\~ \\.~\

30

20

10

I

I

l

I

1O0

200

300

400

500

Temp (°C)

Fig. 3. TO curves for PVB, PMVK and VB MVK copolymers.

,°°t 90 8

0

~

o~ 70

CH38r

60 50

HBr

CH3Br

~ 40

F-

3O 20 10 I

3000

I

J---_

I

2800

2400

2300

1400

1200

1000

Wave number (cm -1)

Fig. 4. The 900-3100cm

~ region of the 1.r. spectra of degradation products of V B - M V K copolymers.

900

602

M. AMINDIAB

Table 1. Mol% of HBr and mol% of HBr per VB monomer unit produced in the degradation of VB-MVK copolymers at various temperatures Polymer

Polymer

Mol% of HBr per VB unit in the copolymers

Mol% of HBr

%VB

400 °

500 °

400

PVB 87 60 47 31 19 PMVK

65.0 54.3 39.8 21.9 11.1 8.6 --

65.5 55.3 40.3 22.3 12.7 9.3 --

100 83.54 61.23 33.69 17.08 13.23

-

-

500 100 84.43 61.53 34.66 19.39 14.20

The formation of methyl bromide in the thermal degradation of VB-MVK, not formed in the decomposition of either the homopolymers, demonstrates that interaction takes place between the brominated monomer and MVK during decomposition of the copolymers. Figure 4 shows the i.r. spectrum of the degradation products of VB-MVK copolymers. The results of quantitative measurements of HBr at 400 and 500 ° using i.r. spectroscopy for the characteristic band at 2500 cm t are shown in Table 1. It is evident that the quantity of HBr is proportional to the VB content where this is above 50%, but far short of what would be expected at lower VB contents. The temperature ranges over which CH3Br evolved were identified as follows. A sample of the polymer was heated from ambient temperature to 40if' under vacuum and under programmed conditions of 10°/min. The sample was held at this temperature and the volatile fraction was subjected to quantitative analysis for CH3Br using i.r. spectroscopy for the characteristic band at 1325cm ~. Another sample --4N,--CH 2

Table 2. Mole percentage of CH3Br in the degradation of VB-MVK copolymers at various temperatures

CH 2

HC

M o l % o f CH3Br

% VB

400 °

PVB 87

--

60 47 31 19 PMVK

---

420 °

440 °

460 °

480 °

500 °

0.08 0.22 0.55 0.32 0.20

0.10 0.34 0.83 0.44 0.20

0.12 0.57 1.21 0.71 0.33

0.17 0.79 1.65 0.99 0.39

0.20 1.21 2.72 1.77 0.69

formation of CH3Br is a direct function of the proportion of adjacent VB-MVK units in the copolymers. Table 3 represents data on the sequence distribution of monomer units in the copolymers, obtained using a method developed by Harwood [18] which requires reactivity ratio values, monomer mixture and copolymer composition data. This statistical method shows that the maximum number of VB-MVK junctions occurs for the copolymer with 47% of VB units; it gives more CH3Br than the other copolymers. From the results in Tables 1, 2 and 3 there are two mechanisms which might contribute to the formation of CH3Br. Firstly, the ring structures formed during the degradation of PMVK at earlier stages breaks down [19] to give small amounts of materials not condensable at - 1 0 0 ° such as CO, C H 4 , propene etc. HBr attacks some of these materials with evolution of methyl bromide. Secondly, it is possible that HBr is reacting with cyclized MVK units by direct addition to the double bond, giving a bromine substituent on the rings. - w CH 2

CH2w-

CH 2

CH 2 w -

HC

CH

CH

+ HBr ~

Br

C o

CH 3

HO

was then programmed up to 420 ° and the process repeated. Table 2 shows the mole percentage of Ch3Br at different temperatures. It is clear that CH3Br is formed at more elevated temperature (420 °) (i.e. in the later stages of degradation) and the mole percentage of CH3Br increases by increasing the temperature; the highest value of CH3Br is 2.72% for the copolymer with 47% of VB. It seems that the

C

CH

CH3

Subsequent fragmentation of the cyclized structures would provide the source of CH3Br at >400 °. This explains the surprising result of the comparative shortage of HBr at lower VB contents. REFERENCES 1. E. K . Boylston a n d L. L. Muller, 19, 1079 (1975).

J. appl. Polym. Sci.

Table 3. Data on sequence distribution in VB-MVK copolymers Copolymer composition A--B%

A--A bond in copolymer %

A--B and B--A bond in copolymer %

B--B bond in copolymer %

A--A--A triads 0CAAA)

B--A--A and A - - A - - B triads triads 0CRAA)

B--A--B triads

87-13 60-40 47-53 31 69 19-81

67.5 48.54 23.98 5.86 2.71

31.01 42.32 54.04 44.28 30.58

1.5 8.54 21.98 49.86 66.71

0.661 0.481 0.221 0.044 0.023

0.304 0.425 0.498 0.331 0.0256

0.035 0.094 0.28 l 0.625 0.722

~fBAB)

A: VB & B: MVK. The symbol f denotes the fraction of a monomer to be found in the middle of the triads AAA, BAA and (AAB) and BAB.

Thermal stability of PVB and VB-MVK 2. J. P. Neumeyer, J. I. Wadsworth, N. B. Knoepfler and C. H. Mack, Thermochim. Acta 16, 133, 149 (1976). 3. I. C. McNeill, M. J. Drews and R. H. Barker, J. Fire Ret. Chem. 4, 222 (1977). 4. G. Blauer and L. Goldstein, J. Polym. Sci. 25, 19 (1957). 5. B. A. Zhubanov, S. A. Nazarova, R. G. Karzhanhaeva and K. M. Gibov, Vysokomolek. Soedin. Ser. B, 18, 150 (1976). 6. I. C. McNeill, T. Straiton and P. Anderson, J. Polym. Sci. 18, 2085-2101 (1980). 7. N. L. Zutty and F. J. Welch, J. Polym. Sci. A-I, 1, 2289 (1963). 8. D. M. Grant and N. Grassie, J. Polym. Sei. 42, 587 (1969). 9. A. Guyer and H. Schutze, Helv. ehim. Acta 17, 1544 (1934).

603

10. Y. Kato, N. Ashikari and A. Nishioka, Bull. chem. Soc. Japan 37, 163 (1964). 11. N. Grassie, B. J. D. Torrance, J. D. Fortune and J. D. Gemmell, Polymer 6, 653 (1965). 12. M. A. Diab, Ph.D. Thesis. University of Glasgow (1981). 13. Fred W. Billmeyer Jr, Textbook o f Polymer Science, Chap. 11, pp. 328-354. Wiley, New York (1971). 14. S. Crawely and I. C. McNeill, J. Polym. Sci., Polym. Chem. Ed. 16, 2593 (1978). 15. C. S. Marvel and C. L. L. Levesque, J. Am. chem. Soc. 60, 280 (1938). 16. J. N. Hay, Makromolek. Chem. 67, 31 (1963). 17. K. Matsuzaki and T. C. Lay, Makromolek. Chem. 110, 185 (1967). 18. H. J. Harwood, Angew. Chem. 4, 394 (1965). 19. I. C. McNeill, Eur. Polym. J. 6, 373 (1970).