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Structure and stability of halogenated polymers: Part 1—Ring-chlorinated polystyrene
Polymer Degradation and Stability 17 (1987) 347-357
Structure and Stability of Halogenated Polymers: Part 1raRing-Chlorinated Polystyrene I. C. M c N e i l l & M. (~o~kun* Chemistry Department, University of Glasgow, Glasgow G12 8QQ, Great Britain (Received 27 November 1986; accepted 13 December 1986)
ABSTRACT Ring-chlorinated polystyrene has been prepared by reaction between polymer and chlorine at - 2 0 ° C in the presence of iodine, using a 1.2:1 molar ratio of chlorine to styrene units. Although the product has a composition corresponding precisely to 1 Cl atom per styrene unit and the predominant site of chlorination is the para position in the aromatic ring, some ortho chlorination, backbone chlorination and unchlorinated structures have been shown to be present by characterisation spectroscopically and from degradation experiments. The chlorinated polymer loses the backbone chlorine readily as HCl at temperatures from 200°C. The resulting unsaturation in the backbone appears to destabilise the polymer towards chain scission and the main breakdown process, which resembles that of polystyrene in consisting of depolymerisation and transfer reactions, occurs over a wider temperature range and at lower temperatures than the decomposition of polystyrene. Products have been identified and estimated quantitatively.
INTRODUCTION Halogenation has been used as a simple and convenient method of modifying polymer properties for many years. In some cases, industrially useful materials have been produced. For example, chlorinated polyethylene with 30% C1 is more elastomeric than the starting material and at 60% C1 the product is a resin. R u b b e r has been chlorinated to give a product such as * Present address: Firat Universitesi, Fen Fakultesi, Elazig, Turkey. 347 Polymer Degradation and Stabilio' 0141-3910/87/$03"50 r~ Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
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L C. McNeill, M. Coskun
Alloprene (ICI), with 65% C1, which is soluble in a wide variety of solvents and finds use in coatings, lacquers and adhesives. Chlorination of PVC leads to improved solubility and film-forming properties. The halogenation of polystyrene has received less attention, although both chlorinated and brominated polystyrene have attracted interest for use as non-inflammable expanded polymer, since the halogenated product is found to be self-extinguishing. ~-3 The mode of decomposition ofhalogenated polymers is a topic which has received little systematic study, although Dodson and McNeill 4 have looked in detail at the characteristics of the degradation of Alloprene. This series of papers seeks to extend such studies to other halogenated polymers in order to establish, first, the structure of the halogenated product and then to compare the mode of thermal breakdown with that of the parent polymer. The halogenated product may differ in structure according to the reaction conditions used. This is especially true in the case of polystyrene, in which halogenation may occur both in the aromatic ring and in the chain backbone. Reaction conditions have been studied, but not in depth. 5'6 This paper considers ring-chlorinated polystyrene. The chain-chlorinated material, at similar chlorine levels, behaves in a very different manner and is the subject of Part 2. Although it is possible to make chlorinated polystyrene which is almost exclusively chain-chlorinated, the ring-chlorination conditions lead to a product which also has a little chain chlorination. This feature has an important bearing on stability.
EXPERIMENTAL
Polystyrene starting material The polymer used was a standard polystyrene (PS) supplied by Polymer Supply and Characterisation Centre, with weight-average molecular weight, 300 000-350 000, and polydispersity, 2-4. The PSCC reference number for the sample is PS-2. The ir and nmr spectra and the degradation behaviour of this polymer were used to provide a reference basis for comparison with the chlorinated product.
Chlorination conditions Chlorination in an evacuated system, with or without free radical-forming additives and using a chlorinated solvent such as dichloromethane or tetrachloromethane, leads to chain chlorination, although Guseinov and his
Structure and stability of halogenated polymers: Part 1
349
co-workers claimed that the pure ring-chlorinated product was obtained by chlorination in the presence of hydrogen chloride and hydrogen peroxide. 6 Chlorination in the presence of iodine or Lewis acids such as A1C13 has been found to favour the ring-chlorinated material. The following procedure was found in the present study to give chlorinated PS with minimum indication of chain chlorination. A reaction tube of known volume was flushed with chlorine gas at atmospheric pressure, then closed and cooled in liquid nitrogen. A cold PS solution (1 g/100 ml dichloromethane, 0°C) was added in amount such that the chlorine was present at 20% above the molar ratio for monosubstitution; iodine was also added (0.3 g/g PS). The closed reaction tube was maintained at - 2 0 ° C for 5 h. An excess of potassium iodide solution was quickly added to react with unused chlorine. The liberated iodine was titrated with standard sodium thiosulphate solution and the hydrogen chloride formed was estimated with standard alkali. Both of these measurements indicated monosubstitution by chlorine.
Isolation and purification of chlorinated polymer The product was precipitated from the dichloromethane solution using methanol and reprecipitated from the same solvent, prior to drying under vacuum for two days.
Degradation procedure Thermogravimetry (TG) measurements were carried out using a Du Pont model 990 thermoanalyser, in dynamic nitrogen at 10°/min. Sample size was 2-3 mg. Thermal volatilisation analysis (TVA) experiments were carried out using a 4-line TVA system as described by McNeill. 7 The degradation conditions involved continuous pumping under vacuum and a heating rate of 10°/min. Sample size was 40-60 mg. Condensable volatile degradation products were trapped at - 196~'C and separated by subambient TVA (SATVA) s prior to investigation by ir spectroscopy. The cold ring fraction of products volatile under vacuum at degradation temperature but not at ambient temperature collected on the cooled upper part of the TVA tube, from which it was removed for investigation by means of a volatile solvent. Infra-red investigation of the products was undertaken using gas cells for volatile products, film between salt plates for liquid products and film deposited on a salt plate from solution by evaporation of solvent for the cold ring fraction. N m r spectra of cold ring fraction products were obtained using tetrachloromethane as solvent. Quantitative estimation of volatile products
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L C. McNeill. M. Coskun
was based upon titration with alkali for HC1, and for other products using SATVA calibration curves obtained by introducing known amounts of each substance and measuring the SATVA peak height. The amount of cold ring fraction was found by weight following evaporation of a known volume of solution; since no non-condensable gases were produced, the amount of volatile products could therefore be obtained by difference from the sample weight, less the 2% involatile residue indicated by TG.
RESULTS A N D DISCUSSION
Characterisation of the chlorinated polystyrene The chlorine content of chlorinated polystyrene was found by microanalysis to be 25"6% by weight. This is in agreement with the theoretical value for one chlorine atom per styrene unit (C8H7C1). The infra-red spectrum of the chlorinated polymer is compared in Fig. 1 with that of the original PS sample. The most significant differences are as follows: (a) disappearance of the characteristic aromatic monosubstitution pattern of PS in the region 2000-1650cm-1 and the appearance of a new absorption at 1890 c m - 1 attributed top-disubstituted aromatic ring, (b) new ring absorptions at 1410, 1470 and 1570cm-1 in the chlorinated polymer, the first of these being strong and clearly evident since PS does not absorb at 1410 c m - 1, (c) changes in C - - H in-plane bending absorptions between 1000
Q
u tO
m r-
3500 wovenumber,
3000
2500
2000
1600
1600
1400
1200
1000
800
cm -1
Fig. 1. Infra-red spectra of (a) polystyrene, (b) ring-chlorinated polystyrene.
Structure and stability of halogenated polymers: Part I
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and 1200cm -1, the band at 1090cm-1 being particularly strong in the chlorinated polymer, (d) a very strong absorption at 820 c m - 1, only in the chlorinated polymer, attributed to C - - H out-of-plane deformation for the p-disubstituted ring. Interpretation of the region below 800 c m - 1 is less easy and some absorptions could be due to unchlorinated or o-chlorinated aromatic rings. A weak absorption at 1235 c m - 1 in the chlorinated polymer may be due to C - - H bending in - - C H C I - - , i.e. main chain chlorination at the methylene group. The nmr spectrum of the chlorinated polymer shows signals due to aromatic protons at 6-6 and 7.15 ppm (6.49 and 6"95 ppm in PS) and aliphatic protons in a broad peak at 1-5 ppm (signals at 1"35 and 1.75 ppm in PS). There is also a weak, broad signal at 4-4 ppm, not found in the spectrum for PS, which may be attributed to --CHC1--. These results suggest that the chlorinated polymer is predominantly thepC1 product, but that some o-C1 substitution may also occur. There are also indications of a small amount of chain chlorination. Although the substitution is not 100% specific to the para position in the aromatic ring, there is no evidence for more than one chlorine in the ring. The interpretation was supported by comparison with spectra of poly(pchlorostyrene) and poly(chlorostyrene, o,p mixture). Effect of chlorination on molecular size This was assessed by comparison of viscosity measurements in benzene at 25°C for PS and ring-chlorinated PS. The intrinsic viscosity fell from 0.83 for the original polymer to 0"53 for the product, indicating that some chain scission occurs during the chlorination reaction. Stability studies by TG and TVA The TG behaviour of PS and ring-chlorinated PS is illustrated in Fig. 2. PS decomposes in a single stage to zero residue, with volatilisation commencing at about 300°C and reaching maximum rate around 420°C, for the heating rate of 10°/min employed. Chlorination reduces the stability and two stages of breakdown become evident, the first of these accounting for only about 10% of weight loss. The threshold for degradation is reduced to about 230°C and the maximum rate occurs at 390°C. There is a very small amount of residue at 500°C. The TVA curves are shown in Fig. 3. PS gives a single peak, the products (mainly styrene) being condensed at - 7 5 ° C or lower temperatures in the TVA system. No non-condensable products are formed: these would be indicated by a response from the Pirani pressure gauge on the TVA system
352
I . C . McNeill, M. Coskun
1.0
~b '\
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\
\
C:
.2 0.5
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0
I
I
I
I
100
200
300
400
',h,,
5O0
t e m p e r a t u r e , *C
Fig. 2. T G curves for (a) polystyrene, (b) ring-chlorinated polystyrene (dynamic nitrogen, 10°/min).
Ct
F
b
0 > 0
I
200
10o temperature,
300
400
500
*C
Fig. 3. TVA curves for (a) polystyrene, (b) ring-chlorinated polystyrene (10°/min). Key to TVA traces: 0 °, . . . . . . . 45 °, . . . . . 75 °, . . . . 100°C. The - 196 ° trace follows the baseline in each case.
Structure and stability of halogenated polymers: Part 1
353
placed on the pump side of the liquid nitrogen trap. Ring-chlorinated PS shows a two-stage breakdown. The first stage is due to products which initially are volatile at - 1 0 0 ° C , but this stage overlaps with the main degradation process which results in products of lower volatility than styrene (indicated by the behaviour in the - 4 5 ° and - 75 ° tracesT). The rate m a x i m u m for the main degradation process occurs at 390°C, in agreement with the T G data. The breadth of the TVA peak, compared with that for PS, is notable and probably indicative of a more complex pattern of degradation. There are no non-condensable products.
Examination of condensable volatile products These were initially trapped at - 1 9 6 ° C . The SATVA curve indicates the successive volatilisation of products as the temperature is raised from - 1 9 6 ° to 0 ° in a controlled manner. The SATVA curve for products of degradation to 500°C of PS shows only one peak, due to styrene and traces of toluene. 9 In the case of ring-chlorinated PS degraded to 500 °, the products divide clearly into a single, very volatile, product (HC1) and a partially resolved mixture of much less volatile material, found to consist of pchlorostyrene plus some o-chlorostyrene and styrene (Fig. 4a). In a further experiment, the products evolved in TVA after 335 ° (the end of the initial stage as indicated by the - 1 0 0 ° response of the TVA curve) were also
b
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=o >
0 L_
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i
I
I
i
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-3
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-I
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A 0
-5
-t,
-3
-2
-1
0
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Fig. 4. S A T V A curves for volatilisation of condensable volatile products on w a r m up from - 196 ° to O°C, in d e g r a d a t i o n of ring-chlorinated polystyrene (a) from r o o m t e m p e r a t u r e to 500°C, (b) from 335" to 500°C.
354
I.C. McNeill, M. ~o~kun
@ U e"
E tlit.
2
I
I
I
3500
3000
2500
wavenumber,
2000
1800
1600 1/,00
1200 1000
800
cm-1
Fig. 5. Infra-red spectrum of cold ring fraction of degradation products from ringchlorinated polystyreneheated to 500°C under TVA conditions. collected and examined by TVA. As expected, if the first stage consists only of HC1 loss, the curve (Fig. 4b) is similar to that for total products except that the large HC1 peak has been almost completely eliminated. Quantitative measurements of the condensable volatile fraction, which amounted to 37% of the original sample weight, indicated the presence of 19% HC1, 11% styrene and 70% p- and o-chlorostyrene, by weight.
Examination of the partially degraded polymer The chlorinated polymer was heated for 6 h at 220°C in the TVA apparatus. About 6% of the sample was removed as HC1; there was no other volatile product. Investigation of the partially degraded polymer showed the disappearance of the small ir absorption band at 1235cm -1 and the 6 4.4 ppm peak in the nmr spectrum, compared with the undegraded polymer. A small additional amount of HC1 was obtained by raising the temperature above 220°C. Thus the programmed heating and isothermal experiments are in agreement that about 7% of the original sample weight is removable as HC1; this corresponds to about 26% of the available chlorine.
Examination of the cold ring fraction products The ir spectrum of the C R F from the chlorinated PS is shown in Fig. 5. The most striking features are the indications of the presence of unsaturation at 1625, 965 and 895cm -x, the last of these absorptions, due to vinylidene, being particularly strong. The 965cm -~ band is attributed to transdisubstituted olefin. The weak 1235 cm - 1 absorption in the original polymer is not present. The nmr spectrum (Fig. 6), although complex, bears a striking resemblance to that of the C R F from degradation of PS under the same conditions. 9 The signal at 4.4 ppm in the chlorinated polymer, attributed to
Structure and stability of halogenated polymers: Part 1
355
____J I
I
I
I
I
I
I
I
I
9
8
7
6
5
4
3
2
1
0
~, p p m
Fig. 6. Nuclearmagneticresonancespectrum of cold ring fraction of degradationproducts from ring-chlorinatedpolystyreneheated to 500°C under TVA conditions. - - C H C I - - structures (backbone) is totally absent. The assignments are as follows: 6.5-7-5 ppm very complex pattern 5.2, 5"3 ppm 4.9 ppm 2.8 ppm 2.0 ppm 1.55 ppm
aromatic ring protons in various environments olefinic protons in ( - - C H z C H - - ) olefinic protons in ( C H 2 ~ C ( ) methylene protons in (C1)ArCH2-methine protons in (C1)ArCH~ methylene protons in normal chain backbone environment: --CH
CH2---CH--
I
I
Ar
Ar
tc1)
(Cl)
Essentially, therefore, the CRF consists of short chain fragments terminated by (C1)ArCHa--, CH2~C~ and --CH~CHAr(C1) structures.
Structure of the chlorinated polystyrene Taking into account both the spectroscopic and degradation evidence, it appears that the structural features of the chain of 'ring-chlorinated' PS containing 25.6% C1 (1 CI per styrene unit) may be represented as follows: C1
I
NCH 2 - - C H - - C H 2 - - C H - - C H 2--CH--CH---CH
C1
CI
C1
CH2--CH--CH2--CH~
CI
356
L C. McNeill, M. Co~kun
There is no evidence for disubstitution in the aromatic ring. From the appearance of styrene as a significant degradation product, it appears that some of the aromatic rings are not chlorinated, as must be the situation for a product with an average chlorine content of 1 C1 per styrene unit, if some of the chlorine is in the backbone.
Degradation mechanism The degradation results indicate an early loss of one-quarter of the chlorine content as HC1 on heating the chlorinated PS up to temperatures typical of dehydrochlorination reactions in polymers containing C1 attached to the chain backbone. This elimination of HC1 is associated with the simultaneous disappearance of the band in the ir spectrum of the polymer at 1235 cm-1, attributed to --CHC1--. It is concluded that the sites for HC1 production are the parts of the polymer backbone which have been subject to chlorine substitution at the same time as the ring substitution reaction. The dehydrochlorination reaction is separated from the main degradation process when the polymer is heated isothermally at 220°C, but under programmed heating conditions the latter reaction has a threshold at about 250°C, at which stage dehydrochlorination is not complete. The lower threshold than for PS for production of aromatic volatile products and CRF, and the greater breadth of the TVA peak, are probably a consequence of a destabilising effect of chain unsaturation resulting from dehydrochlorination: I
~CH2--CH--CH2--C=CH--CH-I-CH 2--CH~
CI
C1
C1
C1
leading to chain scission at the point indicated. This reaction provides an additional initiation route in the degradation, with slightly lower activation energy than the normal chain scission process. Apart from this additional mode of initiation, the degradation process appears to be rather similar to that of PS, giving a little more than 30% of monomers, the remaining products (apart from the HC1) being short chain fragments. Thus, the depolymerisation and transfer reactions found in the thermal degradation of PS are also occurring in the decomposition of the chlorinated material after it has lost HC1.
Structure and stabifity of halogenated polymers: Part 1
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REFERENCES 1. K. Wintersberger, F. Meyer and H. E. Knobloch, BASF, Ger. Pat. 1,169,122 (Chem. Abs., 61, 2027c, 1964). 2. G. Maura, Chem. Ind. (Milan), 60, 643 (1978). 3. R. Grundman, Chemische Werke Huels A.-G., Ger. Pat. 2,800,013 (Chem. Abs., 91, 92280w, 1979). 4. B. iDodson and I. C. McNeill, J. Polym. Sci., Polym. Chem. Ed., 12, 2305 (1974). 5. G. B. Bachman, H. Hellman, K. R. Robinson, R. W. Finholt, E. J. Kahler, L. J. Filar, L. V. Heisy, L. L. Lewis and D. D. Micucci, J. Org. Chem., 12, 108 (1947). 6. M. M. Guseinov, M. S. Salakhov, A. Kh. Rzaev and Ch. A. Chalabiev, Plast. Massy, 9 (1982). 7. I. C. McNeill, Europ. Polym. J., 6, 373 (1970). 8. I. C. McNeill, L. Ackerman, S. N. Gupta, M. Zulfiqar and S. Zulfiqar, J. Polym. Sci., Polym. Chem. Ed., 15, 2381 (1977). 9. I. C. McNeill and W. T. K. Stevenson, Poly. Deg. and Stab., 10, 247 (1985).
Structure and Stability of Halogenated Polymers: Part 1raRing-Chlorinated Polystyrene I. C. M c N e i l l & M. (~o~kun* Chemistry Department, University of Glasgow, Glasgow G12 8QQ, Great Britain (Received 27 November 1986; accepted 13 December 1986)
ABSTRACT Ring-chlorinated polystyrene has been prepared by reaction between polymer and chlorine at - 2 0 ° C in the presence of iodine, using a 1.2:1 molar ratio of chlorine to styrene units. Although the product has a composition corresponding precisely to 1 Cl atom per styrene unit and the predominant site of chlorination is the para position in the aromatic ring, some ortho chlorination, backbone chlorination and unchlorinated structures have been shown to be present by characterisation spectroscopically and from degradation experiments. The chlorinated polymer loses the backbone chlorine readily as HCl at temperatures from 200°C. The resulting unsaturation in the backbone appears to destabilise the polymer towards chain scission and the main breakdown process, which resembles that of polystyrene in consisting of depolymerisation and transfer reactions, occurs over a wider temperature range and at lower temperatures than the decomposition of polystyrene. Products have been identified and estimated quantitatively.
INTRODUCTION Halogenation has been used as a simple and convenient method of modifying polymer properties for many years. In some cases, industrially useful materials have been produced. For example, chlorinated polyethylene with 30% C1 is more elastomeric than the starting material and at 60% C1 the product is a resin. R u b b e r has been chlorinated to give a product such as * Present address: Firat Universitesi, Fen Fakultesi, Elazig, Turkey. 347 Polymer Degradation and Stabilio' 0141-3910/87/$03"50 r~ Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
348
L C. McNeill, M. Coskun
Alloprene (ICI), with 65% C1, which is soluble in a wide variety of solvents and finds use in coatings, lacquers and adhesives. Chlorination of PVC leads to improved solubility and film-forming properties. The halogenation of polystyrene has received less attention, although both chlorinated and brominated polystyrene have attracted interest for use as non-inflammable expanded polymer, since the halogenated product is found to be self-extinguishing. ~-3 The mode of decomposition ofhalogenated polymers is a topic which has received little systematic study, although Dodson and McNeill 4 have looked in detail at the characteristics of the degradation of Alloprene. This series of papers seeks to extend such studies to other halogenated polymers in order to establish, first, the structure of the halogenated product and then to compare the mode of thermal breakdown with that of the parent polymer. The halogenated product may differ in structure according to the reaction conditions used. This is especially true in the case of polystyrene, in which halogenation may occur both in the aromatic ring and in the chain backbone. Reaction conditions have been studied, but not in depth. 5'6 This paper considers ring-chlorinated polystyrene. The chain-chlorinated material, at similar chlorine levels, behaves in a very different manner and is the subject of Part 2. Although it is possible to make chlorinated polystyrene which is almost exclusively chain-chlorinated, the ring-chlorination conditions lead to a product which also has a little chain chlorination. This feature has an important bearing on stability.
EXPERIMENTAL
Polystyrene starting material The polymer used was a standard polystyrene (PS) supplied by Polymer Supply and Characterisation Centre, with weight-average molecular weight, 300 000-350 000, and polydispersity, 2-4. The PSCC reference number for the sample is PS-2. The ir and nmr spectra and the degradation behaviour of this polymer were used to provide a reference basis for comparison with the chlorinated product.
Chlorination conditions Chlorination in an evacuated system, with or without free radical-forming additives and using a chlorinated solvent such as dichloromethane or tetrachloromethane, leads to chain chlorination, although Guseinov and his
Structure and stability of halogenated polymers: Part 1
349
co-workers claimed that the pure ring-chlorinated product was obtained by chlorination in the presence of hydrogen chloride and hydrogen peroxide. 6 Chlorination in the presence of iodine or Lewis acids such as A1C13 has been found to favour the ring-chlorinated material. The following procedure was found in the present study to give chlorinated PS with minimum indication of chain chlorination. A reaction tube of known volume was flushed with chlorine gas at atmospheric pressure, then closed and cooled in liquid nitrogen. A cold PS solution (1 g/100 ml dichloromethane, 0°C) was added in amount such that the chlorine was present at 20% above the molar ratio for monosubstitution; iodine was also added (0.3 g/g PS). The closed reaction tube was maintained at - 2 0 ° C for 5 h. An excess of potassium iodide solution was quickly added to react with unused chlorine. The liberated iodine was titrated with standard sodium thiosulphate solution and the hydrogen chloride formed was estimated with standard alkali. Both of these measurements indicated monosubstitution by chlorine.
Isolation and purification of chlorinated polymer The product was precipitated from the dichloromethane solution using methanol and reprecipitated from the same solvent, prior to drying under vacuum for two days.
Degradation procedure Thermogravimetry (TG) measurements were carried out using a Du Pont model 990 thermoanalyser, in dynamic nitrogen at 10°/min. Sample size was 2-3 mg. Thermal volatilisation analysis (TVA) experiments were carried out using a 4-line TVA system as described by McNeill. 7 The degradation conditions involved continuous pumping under vacuum and a heating rate of 10°/min. Sample size was 40-60 mg. Condensable volatile degradation products were trapped at - 196~'C and separated by subambient TVA (SATVA) s prior to investigation by ir spectroscopy. The cold ring fraction of products volatile under vacuum at degradation temperature but not at ambient temperature collected on the cooled upper part of the TVA tube, from which it was removed for investigation by means of a volatile solvent. Infra-red investigation of the products was undertaken using gas cells for volatile products, film between salt plates for liquid products and film deposited on a salt plate from solution by evaporation of solvent for the cold ring fraction. N m r spectra of cold ring fraction products were obtained using tetrachloromethane as solvent. Quantitative estimation of volatile products
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L C. McNeill. M. Coskun
was based upon titration with alkali for HC1, and for other products using SATVA calibration curves obtained by introducing known amounts of each substance and measuring the SATVA peak height. The amount of cold ring fraction was found by weight following evaporation of a known volume of solution; since no non-condensable gases were produced, the amount of volatile products could therefore be obtained by difference from the sample weight, less the 2% involatile residue indicated by TG.
RESULTS A N D DISCUSSION
Characterisation of the chlorinated polystyrene The chlorine content of chlorinated polystyrene was found by microanalysis to be 25"6% by weight. This is in agreement with the theoretical value for one chlorine atom per styrene unit (C8H7C1). The infra-red spectrum of the chlorinated polymer is compared in Fig. 1 with that of the original PS sample. The most significant differences are as follows: (a) disappearance of the characteristic aromatic monosubstitution pattern of PS in the region 2000-1650cm-1 and the appearance of a new absorption at 1890 c m - 1 attributed top-disubstituted aromatic ring, (b) new ring absorptions at 1410, 1470 and 1570cm-1 in the chlorinated polymer, the first of these being strong and clearly evident since PS does not absorb at 1410 c m - 1, (c) changes in C - - H in-plane bending absorptions between 1000
Q
u tO
m r-
3500 wovenumber,
3000
2500
2000
1600
1600
1400
1200
1000
800
cm -1
Fig. 1. Infra-red spectra of (a) polystyrene, (b) ring-chlorinated polystyrene.
Structure and stability of halogenated polymers: Part I
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and 1200cm -1, the band at 1090cm-1 being particularly strong in the chlorinated polymer, (d) a very strong absorption at 820 c m - 1, only in the chlorinated polymer, attributed to C - - H out-of-plane deformation for the p-disubstituted ring. Interpretation of the region below 800 c m - 1 is less easy and some absorptions could be due to unchlorinated or o-chlorinated aromatic rings. A weak absorption at 1235 c m - 1 in the chlorinated polymer may be due to C - - H bending in - - C H C I - - , i.e. main chain chlorination at the methylene group. The nmr spectrum of the chlorinated polymer shows signals due to aromatic protons at 6-6 and 7.15 ppm (6.49 and 6"95 ppm in PS) and aliphatic protons in a broad peak at 1-5 ppm (signals at 1"35 and 1.75 ppm in PS). There is also a weak, broad signal at 4-4 ppm, not found in the spectrum for PS, which may be attributed to --CHC1--. These results suggest that the chlorinated polymer is predominantly thepC1 product, but that some o-C1 substitution may also occur. There are also indications of a small amount of chain chlorination. Although the substitution is not 100% specific to the para position in the aromatic ring, there is no evidence for more than one chlorine in the ring. The interpretation was supported by comparison with spectra of poly(pchlorostyrene) and poly(chlorostyrene, o,p mixture). Effect of chlorination on molecular size This was assessed by comparison of viscosity measurements in benzene at 25°C for PS and ring-chlorinated PS. The intrinsic viscosity fell from 0.83 for the original polymer to 0"53 for the product, indicating that some chain scission occurs during the chlorination reaction. Stability studies by TG and TVA The TG behaviour of PS and ring-chlorinated PS is illustrated in Fig. 2. PS decomposes in a single stage to zero residue, with volatilisation commencing at about 300°C and reaching maximum rate around 420°C, for the heating rate of 10°/min employed. Chlorination reduces the stability and two stages of breakdown become evident, the first of these accounting for only about 10% of weight loss. The threshold for degradation is reduced to about 230°C and the maximum rate occurs at 390°C. There is a very small amount of residue at 500°C. The TVA curves are shown in Fig. 3. PS gives a single peak, the products (mainly styrene) being condensed at - 7 5 ° C or lower temperatures in the TVA system. No non-condensable products are formed: these would be indicated by a response from the Pirani pressure gauge on the TVA system
352
I . C . McNeill, M. Coskun
1.0
~b '\
0
\
\
C:
.2 0.5
I ]= ':3 o--
0
I
I
I
I
100
200
300
400
',h,,
5O0
t e m p e r a t u r e , *C
Fig. 2. T G curves for (a) polystyrene, (b) ring-chlorinated polystyrene (dynamic nitrogen, 10°/min).
Ct
F
b
0 > 0
I
200
10o temperature,
300
400
500
*C
Fig. 3. TVA curves for (a) polystyrene, (b) ring-chlorinated polystyrene (10°/min). Key to TVA traces: 0 °, . . . . . . . 45 °, . . . . . 75 °, . . . . 100°C. The - 196 ° trace follows the baseline in each case.
Structure and stability of halogenated polymers: Part 1
353
placed on the pump side of the liquid nitrogen trap. Ring-chlorinated PS shows a two-stage breakdown. The first stage is due to products which initially are volatile at - 1 0 0 ° C , but this stage overlaps with the main degradation process which results in products of lower volatility than styrene (indicated by the behaviour in the - 4 5 ° and - 75 ° tracesT). The rate m a x i m u m for the main degradation process occurs at 390°C, in agreement with the T G data. The breadth of the TVA peak, compared with that for PS, is notable and probably indicative of a more complex pattern of degradation. There are no non-condensable products.
Examination of condensable volatile products These were initially trapped at - 1 9 6 ° C . The SATVA curve indicates the successive volatilisation of products as the temperature is raised from - 1 9 6 ° to 0 ° in a controlled manner. The SATVA curve for products of degradation to 500°C of PS shows only one peak, due to styrene and traces of toluene. 9 In the case of ring-chlorinated PS degraded to 500 °, the products divide clearly into a single, very volatile, product (HC1) and a partially resolved mixture of much less volatile material, found to consist of pchlorostyrene plus some o-chlorostyrene and styrene (Fig. 4a). In a further experiment, the products evolved in TVA after 335 ° (the end of the initial stage as indicated by the - 1 0 0 ° response of the TVA curve) were also
b
Q
=o >
0 L_
-5
i
I
I
i
-4
-3
-2
-I
subambient thermocoupte o u t p u t ,
A 0
-5
-t,
-3
-2
-1
0
mV (-196"--~0°C)
Fig. 4. S A T V A curves for volatilisation of condensable volatile products on w a r m up from - 196 ° to O°C, in d e g r a d a t i o n of ring-chlorinated polystyrene (a) from r o o m t e m p e r a t u r e to 500°C, (b) from 335" to 500°C.
354
I.C. McNeill, M. ~o~kun
@ U e"
E tlit.
2
I
I
I
3500
3000
2500
wavenumber,
2000
1800
1600 1/,00
1200 1000
800
cm-1
Fig. 5. Infra-red spectrum of cold ring fraction of degradation products from ringchlorinated polystyreneheated to 500°C under TVA conditions. collected and examined by TVA. As expected, if the first stage consists only of HC1 loss, the curve (Fig. 4b) is similar to that for total products except that the large HC1 peak has been almost completely eliminated. Quantitative measurements of the condensable volatile fraction, which amounted to 37% of the original sample weight, indicated the presence of 19% HC1, 11% styrene and 70% p- and o-chlorostyrene, by weight.
Examination of the partially degraded polymer The chlorinated polymer was heated for 6 h at 220°C in the TVA apparatus. About 6% of the sample was removed as HC1; there was no other volatile product. Investigation of the partially degraded polymer showed the disappearance of the small ir absorption band at 1235cm -1 and the 6 4.4 ppm peak in the nmr spectrum, compared with the undegraded polymer. A small additional amount of HC1 was obtained by raising the temperature above 220°C. Thus the programmed heating and isothermal experiments are in agreement that about 7% of the original sample weight is removable as HC1; this corresponds to about 26% of the available chlorine.
Examination of the cold ring fraction products The ir spectrum of the C R F from the chlorinated PS is shown in Fig. 5. The most striking features are the indications of the presence of unsaturation at 1625, 965 and 895cm -x, the last of these absorptions, due to vinylidene, being particularly strong. The 965cm -~ band is attributed to transdisubstituted olefin. The weak 1235 cm - 1 absorption in the original polymer is not present. The nmr spectrum (Fig. 6), although complex, bears a striking resemblance to that of the C R F from degradation of PS under the same conditions. 9 The signal at 4.4 ppm in the chlorinated polymer, attributed to
Structure and stability of halogenated polymers: Part 1
355
____J I
I
I
I
I
I
I
I
I
9
8
7
6
5
4
3
2
1
0
~, p p m
Fig. 6. Nuclearmagneticresonancespectrum of cold ring fraction of degradationproducts from ring-chlorinatedpolystyreneheated to 500°C under TVA conditions. - - C H C I - - structures (backbone) is totally absent. The assignments are as follows: 6.5-7-5 ppm very complex pattern 5.2, 5"3 ppm 4.9 ppm 2.8 ppm 2.0 ppm 1.55 ppm
aromatic ring protons in various environments olefinic protons in ( - - C H z C H - - ) olefinic protons in ( C H 2 ~ C ( ) methylene protons in (C1)ArCH2-methine protons in (C1)ArCH~ methylene protons in normal chain backbone environment: --CH
CH2---CH--
I
I
Ar
Ar
tc1)
(Cl)
Essentially, therefore, the CRF consists of short chain fragments terminated by (C1)ArCHa--, CH2~C~ and --CH~CHAr(C1) structures.
Structure of the chlorinated polystyrene Taking into account both the spectroscopic and degradation evidence, it appears that the structural features of the chain of 'ring-chlorinated' PS containing 25.6% C1 (1 CI per styrene unit) may be represented as follows: C1
I
NCH 2 - - C H - - C H 2 - - C H - - C H 2--CH--CH---CH
C1
CI
C1
CH2--CH--CH2--CH~
CI
356
L C. McNeill, M. Co~kun
There is no evidence for disubstitution in the aromatic ring. From the appearance of styrene as a significant degradation product, it appears that some of the aromatic rings are not chlorinated, as must be the situation for a product with an average chlorine content of 1 C1 per styrene unit, if some of the chlorine is in the backbone.
Degradation mechanism The degradation results indicate an early loss of one-quarter of the chlorine content as HC1 on heating the chlorinated PS up to temperatures typical of dehydrochlorination reactions in polymers containing C1 attached to the chain backbone. This elimination of HC1 is associated with the simultaneous disappearance of the band in the ir spectrum of the polymer at 1235 cm-1, attributed to --CHC1--. It is concluded that the sites for HC1 production are the parts of the polymer backbone which have been subject to chlorine substitution at the same time as the ring substitution reaction. The dehydrochlorination reaction is separated from the main degradation process when the polymer is heated isothermally at 220°C, but under programmed heating conditions the latter reaction has a threshold at about 250°C, at which stage dehydrochlorination is not complete. The lower threshold than for PS for production of aromatic volatile products and CRF, and the greater breadth of the TVA peak, are probably a consequence of a destabilising effect of chain unsaturation resulting from dehydrochlorination: I
~CH2--CH--CH2--C=CH--CH-I-CH 2--CH~
CI
C1
C1
C1
leading to chain scission at the point indicated. This reaction provides an additional initiation route in the degradation, with slightly lower activation energy than the normal chain scission process. Apart from this additional mode of initiation, the degradation process appears to be rather similar to that of PS, giving a little more than 30% of monomers, the remaining products (apart from the HC1) being short chain fragments. Thus, the depolymerisation and transfer reactions found in the thermal degradation of PS are also occurring in the decomposition of the chlorinated material after it has lost HC1.
Structure and stabifity of halogenated polymers: Part 1
357
REFERENCES 1. K. Wintersberger, F. Meyer and H. E. Knobloch, BASF, Ger. Pat. 1,169,122 (Chem. Abs., 61, 2027c, 1964). 2. G. Maura, Chem. Ind. (Milan), 60, 643 (1978). 3. R. Grundman, Chemische Werke Huels A.-G., Ger. Pat. 2,800,013 (Chem. Abs., 91, 92280w, 1979). 4. B. iDodson and I. C. McNeill, J. Polym. Sci., Polym. Chem. Ed., 12, 2305 (1974). 5. G. B. Bachman, H. Hellman, K. R. Robinson, R. W. Finholt, E. J. Kahler, L. J. Filar, L. V. Heisy, L. L. Lewis and D. D. Micucci, J. Org. Chem., 12, 108 (1947). 6. M. M. Guseinov, M. S. Salakhov, A. Kh. Rzaev and Ch. A. Chalabiev, Plast. Massy, 9 (1982). 7. I. C. McNeill, Europ. Polym. J., 6, 373 (1970). 8. I. C. McNeill, L. Ackerman, S. N. Gupta, M. Zulfiqar and S. Zulfiqar, J. Polym. Sci., Polym. Chem. Ed., 15, 2381 (1977). 9. I. C. McNeill and W. T. K. Stevenson, Poly. Deg. and Stab., 10, 247 (1985).