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Thermal decomposition of copolymers of styrene and halogenated monomers
European Polymer Journal, Vol. 14, pp. 219 to 223. Pergamon Press 1978. Printed in Great Britain
THERMAL DECOMPOSITION OF COPOLYMERS OF STYRENE AND HALOGENATED MONOMERS F. HRAB~.K, J. MITERA*, V. KUBELKA* and M. BEZD~K Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6; *Institute of Chemical Technology, Department of Petroleum and Petrochemistry. 166 28 Prague 6, Czechoslovakia (Received 14 J u n e 1977)
Abstract--Copolymers of 1,2,2,2-tetrachloroethyl esters of unsaturated acids and halogenated N-phenyl maleimides with styrene were pyrolyzed; volatile products were analyzed with a mass spectrometer combined with a gas chromatograph. Hydrogen halide and carbon dioxide in the volatile products were determined during the thermal decomposition of copolymers in glass ampoules; the acyl chloride groups were determined in the residues. The thermal decomposition of copolymers of tetrachloroethyl esters with styrene sets in at ca. 230 ° by the release of chloral from the copolymer and splitting of some of the C--421 bonds in the copolymer. The decomposition of copolymers of styrene with halogenated N-phenyl maleimides starts above 300° by depolymerization of the polystyrene chain sections and by splitting of some of the carbon-halogen bonds. At 310 and 500 ° for copolymers of tetrachloroethyl esters and at 500° for halogenated N-phenyl maleimides, there is radical dehydrohalogenation of the copolymers, with depolymerization of polystyrene blocks and splitting of carbon carbon bonds in the main chain.
INTRODUCTION In an earlier paper [1] we investigated the thermal decomposition of copolymers of styrene with 1,2,2,2-tetrachloroethyl esters of acrylic and fumaric acids and with halogenated N-phenyl maleimides using thermogravimetric analysis; we determined the flash-ignition and self-ignition temperatures of these copolymers. An objective of this work is identification of the thermal decomposition products of the above copolymers and elucidation of the main chemical transformations in the polymers at temperatures from 200 to 500 °. EXPERIMENTAL
Copolymers of styrene and 1,2,2,2-tetrachloroethyl acrylate (TCEA), 1,2,2,2-tetrachloroethyl methacrylate (TCEMA), bis-(1,2,2,2-tetrachloroethyl) fumarate (TCEF), bis-(1,2,2,2-tetrachloroethyl) muconate (TCEM), N-(2,4,6tribromophenyl) maleimide (TBPMI) and N-(pentachlorophenyl) maleimide (PCPMI) were prepared by suspension copolymerization [1]. The formulae of halogenated comonomers and molar ratios styrene-comonomer (st-com) in the copolymers are given in Table 1. Methods
Flash pyrolysis of the copolymers was carried out at 310 and 500° in a pyrolyzer described earlier [2,3]. Products were led directly in to the chromatographic column of a gas chromatograph-mass spectrometer LKB 9000 apparatus. The flow rate of carrier helium was 40 ml/min; the amount of the polymer was about 0.5 rag. The decomposition products were separated chromatographically on a glass column, 5 m long, packed with 10~o Silikon Gummi Merck fixed on Chromaton NAW. The column temperature was linearly raised from 50 to 250° at a rate of 10°/min; the temperature of the carrier gas separator was 260 ° and that of the ion source was 280 °. The energy of ionizing electrons was 20eV during the chromatographic analysis, and 70 eV in recording the mass spectra. The pressure in the ion source was 10-4Pa. The
products of pyrolysis of the copolymers were identified by comparing their mass spectra with published spectra [4]. Hydrogen halide was determined in the thermal decomposition products of all copolymers arising in the heating of 0.2~3.5 of powder samples in a glass ampoule ca. 10 cm 3 in volume; for copolymers of tetrachoroethyl esters, carbon dioxide and acyl chloride groups also were determined. Pure nitrogen was introduced into heated ampoule during the determination of hydrogen halide and carbon dioxide. Volatile products carried by the nitrogen were cooled in a glass U-tube immersed in ice-water; uncondensed gases were collected in 40 cm 3 of distilled water, from which samples were taken for determination of dissolved hydrogen chloride and carbon dioxide. Hydrogen chloride was determined by titration with a standard solution Hg(CIO4)2 using diphenylcarbazone as indicator [5]. In control determinations, the analyzed solution was titrated with 0.1 N NaOH solution until phenolphthalein turned red. In order to verify the presence of carbon dioxide in the products of pyrolysis, 10cm 3 of clear 2~o aqueous Ba(OH)2 was added to 15 cm 3 of the solution. If any turbidity appeared, the sample was filtered; the residue was dried at 105°, and weighed. To check the formation of hydrogen chloride by the hydrolysis of chloral, a mixture of 1 g of freshly redistilled chloral and 3 cm 3 of acetone was shaken with 0.l g of .solid sodium acetate, and filtered. Then 25 cm 3 of water was added to the filtrate, the solution was left to stand for 45 rain at 20°, acidified with 5~o HNO3 until the Congored had turned blue, and titrated with a 0.1 N Hg(CIO4)2. The first drop of the titrating solution made the analyzed solution turn red-violet, indicating that under the given conditions chloral was not hydrolyzed to hydrogen chloride. The acyl chloride groups were determined in weighed residues obtained after heating 0.15-0.5 g of the copolymers for 40min in a stream of nitrogen in the vacuum of a water pump (1860 Pa). About 0.04g of the residue was taken for the determination of chlorine; the remainder was mixed with 0.17g of sodium acetate and 10cm 3 of ethanol. The mixture was boiled for 20 sec and diluted with water to ca. 30 cm 3. The fine sludge was removed by filtration, and the filtrate was made up with water to 50cm 3.
219
F. HRAB~.K et al.
"220
Table 1. Copolymers of styrene (St) with halogenated comonomers (com) for pyrolysis Comonomer Formula
Designation TCEA
Mw
Copolymer ~o halog. St/corn
ct o I II CCL3--CH-O--C-CH=CH~
237.9
19.46
4.7
CCt3--CH-O-C--C=CH=
251.9
21.96
3.8
447.7
20.57
8.9
473.8
22.18
7.72
409.9
22.41
6.4
345.4
20.68
4.9
TCEMA
TCEF
CL
0
0
CL
I
LL
LL
I
CL
O
CCL3--CH--O-- C--CH=CH- C- O-CH-CCL 3
TCEM
I
0
II
IJ
ct I
CCL~-CH- O-C- CH=CH--CH=CH--C-O--CH--CCL 3
TBPMI
/C/ZCH
~r
8~-(\ /~N PCPMI
CL
CL
II O
/c--ell CL~ / ~ N ' c _ _ ~ H CL
CL
Jl 0
The sample of the solution was acidified with 5~o HNO3, and the C1- ions (corresponding to the content of the acyl chloride groups) were determined by titration with 0.1 N Hg(CIO4)2. RESULTS
The products of pyrolysis of the copolymers under investigation at 310 and 500°, identified with a gas chromatograph directly connected with the mass spectrometer, are summarized in Table 2. In this Table, nonhalogenated compounds are followed by the halogenated ones, according to the magnitude of the mass unit (re~e). The peak heights of the individual compounds read off from gas chromatograms have been divided into five groups, indicating only the ratio of the amounts of products. The most abundant nonhalogenated products from all copolymers were styrene, its dimers and trimers, and ~t-methylstyrene. Of the halogenated products, chloral and dichloroacetaldehyde were present in the greatest amount for the copolymers of tetrachloroethyl esters; the decomposition products of the copolymers of N-phenyl maleimides were their monomers. In the decomposition products of the copolymers of tetrachloroethyl esters, only TCEMA appears as monomer. The monomeric units of the other tetrachloroethyl esters probably decompose rather than depolymerize. The amounts of halogen transformed to HC1 during the thermal decomposition of copolymers, and the amount of ester groups transformed to carbon dioxide and --COC1 groups are shown in Tables 3 and 4. These amounts of HC1 were determined by the titration of an aqueous solution of volatile products with 0.1 N Hg(C104)2; the amount of hydrogen chloride determined in control titrations with 0.1 N NaOH differed from the above values by less than 0.6~o. By heating the copolymers of tetrachloroethyl
esters to 500°, more than half of the chlorine present was transformed to HC1; at 310°, the amount was about one-third. The presence of acyl chloride groups, which should have been formed in the copolymer by the splitting-off of chloral from the tetrachloroethyl ester monomeric units, could not be established in the residues of the above copolymers heated to 310 and 500° in the vacuum of the water pump. Carbon dioxide was indicated only at 500°; this insignificant amount does not suggest any perceptible decarboxylation process of the ester monomeric units. At 250 and 230 °, only one representative of the copolymers of tetrachloroethyl esters, viz. poly (TCEA-co-styrene), was pyrolyzed. The amount of chlorine transformed to HC1 dropped to 13.5 and 3.7%, but the copolymer residues contained bound acyl chloride groups (Table 4). Their formation is also indicated by thermogravimetric analysis [1] according to which a spontaneous decomposition of poly(TCEA-costyrene) sets in between 230 and 250°. Since the spontaneous decomposition of polystyrene starts at 280° [1], chloral can be regarded as the main volatile product of the pyrolysis of poly(TCEA-costyrene) at 230-250 °. At 310 and 500°, chloral is, with styrene, one of the main volatile products of pyrolysis and at the same time the starting compound for formation of dichloroacetaldehyde and chloroacetaldehyde (Table 2). Table 4 shows that, in the copolymer of TCEA heated to 230 or 250° for 40 min, some chlorine remains bound, while during the pyrolysis of the copolymers of TCEA and TCEF at 310 and 500° the chlorine is almost completely split off. On heating to 310°, only a little of the halogen present in the copolymers of styrene with halogenated N-phenyl maleimides was transformed to hydrogen halide (Table 3); no halogenated organic compounds were identified in the volatile products, apart from PCPMI (Table 2). The small amount of hydrogen chloride found corresponds to the results of thermo-
Thermal decomposition of copolymers
221
Table 2. Products of pyrolysis of the copolymers of styrene with halogenated comonomers; peak heights on the gas chromatogram h--high, l--low, m--medium, v--very: data refer to 500 °, those in brackets to 310"
m/e Products 78 92 104 106 118 118 118 144 154 158 160 172 182 196 208 312 78 112 146 160 140 166 180 252 329 407 289 291 309 343 357
TCEA
Benzene Toluene Styrene Ethylbenzene Allylbenzene ct-Methylstyrene Indane Isopropenylstyrene Diphenyl 5,6-Benzo-2-methyl-3,5cyclohexadienone Propylpropenylbenzene Unidentified Diphenylethane L3-Diphenylpropane Dimers of styrene
TBPMI
PCPMI
vl
1
1 (1)
1
1 (1)
vl (vl)
1
m
m
h
h
m
vh (m)
vh (1)
vh (h)
vh (h)
vh (vh)
vh (vh)
m (vl)
m
m
I
(t)
1 vh (1)
1 h (1)
1
1
1
vl
r0 h'(vl)
1 (m) 1 (I)
1 1
vl
vl vl
Trimers of styrene Chloroacetaldehyde Dichloroacetaldehyde Trichloroacetaldehyde Trichloropropanal Benzyl chloride 2-Chloroethylstyrene 2,2-Dichloroethenyl methacrylate TCEMA N-Dibromophenyl maleinimide TBPMI Pentachlorophenyl isocyanate N-Pentachlorophenyl formamide N-Tetrachlorophenyl maleinimide PCPMI Methyl maleic acid pentachlorophenylimide
Comonomers TCEF TCEM
TCEMA
vl
vl
vl
1
vl
m h (1) m vl
vl (1)
1
1 (m)
m (1)
m
m (m)
1 1 (1)
1 1 (1)
h,2m(1) 31 (vl) vh,m(2m)
2h 3m vh,h (1)
vh 2m,l (vh) vh,m(vh)
vh,h m,1 vh,h(vh,h)
vh 21 (1) vh,h(h,m)
vh 21 vh,h(vhl
1 (vl)
1
1 (m)
m
m (1) vh (vh vl
vh (1) vh (vh) m
h (vh) h (vh)
h (h) vh (vh) 1
1 (1)
1
1
vl m 1 (11 1
vh (m) 1 1 m
vh (h) m
The figure before the symbol gives the number of peaks of given intensity. gravimetric analysis [1] a c c o r d i n g to which the s p o n t a n e o u s d e c o m p o s i t i o n of the c o p o l y m e r s T B P M I a n d P C P M I sets in a b o v e 320 °. By h e a t i n g the cop o l y m e r T B P M I to 500 °, 27.4~o of b r o m i n e was transformed to HBr, a n d m o r e b r o m i n e (31.07~) was
f o u n d in the residue t h a n in the starting c o p o l y m e r (22.41~o Br). U n d e r the same conditions, 51.6~/o of h a l o g e n was t r a n s f o r m e d to HC1 in the c o p o l y m e r P C P M I (20.68~o CI); the residue c o n t a i n e d only 13.05% of chloride. D e h a l o g e n a t e d derivatives of
Table 3. Amount of hydrogen halide, CO2 and COCI groups in the thermal decomposition products of styrene and halogenated comonomers; X = h a l o g e n ; H X = h y d r o g e n halide Decomposition temperature (°C) 500
310
250 230
E.P.J. 14/3
D
Comonomer TCEA TCEMA TCEF TCEM TBPMI PCPMI TCEA TCEMA TCEF TCEM TBPMI PCPMI TCEA TCEA
°J~ - - C - - O il ~o X of the compolymer O transformed to HX transformed to CO2 51.4 51.3 64.4 48.2 27.4 51.6 28.8 28.6 44.5 33.8 4.5 8.7 13.5 3.7
2.2 -1.1 ----. -----
222
F. HRABAKet al. Table 4. Weight loss of the copolymers of styrene and halogenated monomers heated in ampoules evacuated with a water pump % ~C--O--
Comonomer
qt
Decomposition temperature (°C)
Residue after pyrolysis, %
230 250 310 500 310 500 310 500 310 500
79.76 76.65 33.9 14.7 39.2 17.3 69.7 38.4 71.59 39.0
TCEA
TCEF TBPMI PCPMI
T B P M I and PCPMI, N-dibromophenyl maleimide and N-tetrachlorophenyl maleimide respectively, were identified in the volatile products of pyrolysis. N o chlorine could be found iodometrically in an aqueous solution of volatile products. Residues of copolymers after pyrolysis between 230 and 500 ° could not be dissolved in common solvents for polymers.
%N % of halogen in the residue ------1.85 2.85 1.82 3.80
:H3 C6Hs-CH-CHz--CH2--C ~ COCL
The fact that the majority of identified nonhalogenated compounds appear in the decomposition products of all copolymers under investigation indicates that these compounds were predominantly formed by the decomposition of the polystyrene sections of copolymers. The participation of a halogenated comonomer unit in the formation of a nonhalogenated product of pyrolysis seems likely only in the case of compounds with weight units 144, 158 and 172. The first and third units appear only in the decomposition products of the copolymer TCEMA, while the second is present in the products of the copolymers T C E A and TCEMA. In the pyrogram of T C E M A , the compound with Mw 172 is not perfectly separated from the dimer of styrene (concentration ratio m/e2oa:m/e172 = 2:1), and its structure cannot be determined unambiguously from the mass spectrum. The ways of transformation of the copolymer sections to nonhalogenated compounds with mass units 144, 158 and 172 may be indicated by the following schemes; the structures of final products suggested on the basis of the mass spectra are not to be regarded as proved:
~H3
~ H3
C6Hs--~H-CHz--(~--CH2~ -N~CO -HCL > C6Hs-CH=CH-C=CH2 ~ /CH2 COCt or l ~ ~ICIH_.
~ C11HI2;
role =
144
c..
-.c~ \ ~ , . \ ; c . 2 II 0 c~120 ; m/w
= 172
37.7 16.5 --------
/CH----CH Cs Hs--C.H I "~CO--CH--CH 3 Cl2HI20 ; role =172
CH3 C6Hs--CH-CH2--C~ J
DISCUSSION
9.17 3.21 1.05 2.04 0.98 0.58 27.80 31.07 20.34 13.05
O transformed to COC1
I
COCL
Cs Hs-CH--CH2--~:H--CH2 ~ COCt
0 C11HIoO; role =
158
The condition for formation of COC1 groups bound to polymer is the splitting-off of the chloral molecule from the monomer unit of tetrachloroethyl ester. This is confirmed by the presence of chloral in large amounts among the volatile products of pyrolysis of all copolymers of tetrachloroethyl esters at temperatures from 230 to 500 ° . However, the presence of the COCI groups in the residues of the copolymers of tetrachloroethyl esters was proved only after pyrolysis at 230 and 250 ° . Their absence in the residues after pyrolysis at 310 and 500 ° and at the same time the high content of HC1 in volatile decomposition products corroborate the above equations describing transformations of the acyl chloride groups. These transformations probably involve not only the formation of HC1 and identified volatile compounds (C11H12, C l l H t 0 0 and C12H120) but also the formation of similar structures bound to the polymer, poorer in hydrogen content than the starting sections of the copolymer. The presence of T C E M A and 2,2-dichloroethenyl methacrylate in the volatile products from poly(TCEMA-costyrene) shows that chloral is not completely split-off from tetrachloroethyl esters even at 500 °, and that dehalogenation of the tetrachloroethyl groups, probably even those bound on the polymer, is the competing reaction in this case. Also the finding that at 500 ° more than 50% of chlorine present in the copolymers "of tetrachloroethyl esters is transformed to HCI indicates formation of HC1 at the expense of chlorine from the trichloromethyl group of chloral and tetrachloroethyl ester. Obviously, the dehalo-
Thermal decomposition of copolymers genation of the latter does not occur by the splittingoff of the HCI molecule, because saturated chloro derivatives of acetaldehyde--monochloroacetaldehyde and dichl'oroacetaldehyde--are formed from chloral, and 2,2-dichloroethenyl methacrylate from TCEMA (Table 2). It may be inferred therefore that the dehalogenation of the trichloromethyl group proceeds by a radical mechanism. First, the weaker C--C1 bond is split [6,7]. Then, the generated C1 atom removes a hydrogen atom from the hydrocarbon group with formation of HC1 and of the radical in the polymer chain or in the volatile organic compound. (?-Radicals formed by the removal of CI" from chloral or dichloroacetaldehyde (CClzCHO and CHC1CHO) probably react with the hydrocarbon groups similarly, while being transformed to saturated molecules of di- and monochloroacetaldehyde. The presence of benzyl chloride and 2-chloroethylstyrene in the volatile products of pyrolysis (Table 2) also indicates dehalogenation of the tetrachloroethyl and trichloromethyl groups by the carbon radicals arising by the thermal splitting of the polymer chain. The existing data do not allow an answer to the question whether the dehalogenation of the tetrachloroethyl ester groups in the polymer is the primary process and the splitting-off of di- and monochloroacetaldehyde or of 2,2-dichloroethenyl methacrylate is a consecutive one, or whether the chloral and TCEMA are dehalogenated in the gaseous phase. The insolubility of the copolymer residues after pyrolysis, already starting from 230~, indicates that the 12-radicals also terminate by combination. The presence of TBPM1 and PCP\MI in the decomposition products of their copolymers with styrene indicates depolymerization of the maleinimide monomeric units at 310'. Since in thermogravimetric analysis [1] the onset of spontaneous decomposition of polystyrene was recorded at 280~', that of its copolymers with TBPMI and PCPMI respectively at 352 and 326', and that of the homopolymers of TBPMI and PCPMI respectively at 372 and 375°, it is quite probable that the depolymerization at 310° starts in the polystyrene sections of the chain but is retarded by the imide monomeric units. The latter may also be stripped and released from the chain by the decomposition of the adjacent polystyrene blocks.
223
The small quantities of released hydrogen halide at 310° indicate that even at this temperature the thermal splitting of the C~,r--halogen bond took place, followed by the formation of hydrogen halide and dehalogenated derivatives of TBPMI and PCPMI, i.e. dibromophenyl maleimide and tetrachlorophenyl maleimide, found in the volatile products only at 500'~. The finding that, in spite of the higher energy of the C--CI bond compared with the C--Br bond, a larger fraction of halogen is transformed to hydrogen halide in the pyrolysis of the copolymer of PCPMI than in the pyrolysis of the copolymer of TBPMI, can be assigned to the higher number of halogen atoms in the molecule of PCPMI. This is also suggested by the approximate balancing of the number of halogen atoms in the monomeric units of PCPMI and TBPMI during pyrolysis. Data in Tables 3 and 4 indicate that residues after the heating of the two copolymers to 500 for 40min contain 1.4 CI atoms or 1.9 Br atoms for each N atom. At 500°, the decomposition of the maleinimide ring bound on the polymer chain occurs along with the fasl dehalogenation of the benzene ring, giving rise to pentachlorophenyl isocyanate and pentachlorophenyl formamide. At the same time, bonds are split between the carbon atoms of the polymer chain belonging to a styrene monomer unit, as suggested by the presence of methylmaleic acid pentachlorophenylimide in the decomposition products: ~ C H -- CH--CH2--CH ~ 1 t C~
CO
N~C6Cl5
~-
CH t
CH--CH 2 I
CO
CO
S
\N/--C6C15
CH ~ C - - C H 3 I ~
CO
CO
\N~C6Cl ~
REFERENCES
1. F. Hrabfik. 1. Masafik, K. Bouchal and M. Slavi~ek, Europ. Polym. J. 13, 509 (1977). 2. J. Mitera and J. Michal, Chem. Prfim. 26, 417 (1976). 3. J. Mitera, J. Michal, J. Kubfit and V. Kubelka, Z. analyt. Chem. 281, 23 (1976). 4. Eight Peak Index of Mass Spectral Data MSDC, AWRE, Aldermaston, Reading, U.K. (1974). 5. M. Jure~ek and M. Ve~e~a, Chem. Listy 46, 620 (1952). 6. M. W. Wolkenstein, Swuktur und Physikalische Eigenschaften der Molekfile, p. 586. Teubner, Leipzig (1960). 7. J. K. Syrkin, Zh. fiz. Khim 17, 347 (1953).
THERMAL DECOMPOSITION OF COPOLYMERS OF STYRENE AND HALOGENATED MONOMERS F. HRAB~.K, J. MITERA*, V. KUBELKA* and M. BEZD~K Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6; *Institute of Chemical Technology, Department of Petroleum and Petrochemistry. 166 28 Prague 6, Czechoslovakia (Received 14 J u n e 1977)
Abstract--Copolymers of 1,2,2,2-tetrachloroethyl esters of unsaturated acids and halogenated N-phenyl maleimides with styrene were pyrolyzed; volatile products were analyzed with a mass spectrometer combined with a gas chromatograph. Hydrogen halide and carbon dioxide in the volatile products were determined during the thermal decomposition of copolymers in glass ampoules; the acyl chloride groups were determined in the residues. The thermal decomposition of copolymers of tetrachloroethyl esters with styrene sets in at ca. 230 ° by the release of chloral from the copolymer and splitting of some of the C--421 bonds in the copolymer. The decomposition of copolymers of styrene with halogenated N-phenyl maleimides starts above 300° by depolymerization of the polystyrene chain sections and by splitting of some of the carbon-halogen bonds. At 310 and 500 ° for copolymers of tetrachloroethyl esters and at 500° for halogenated N-phenyl maleimides, there is radical dehydrohalogenation of the copolymers, with depolymerization of polystyrene blocks and splitting of carbon carbon bonds in the main chain.
INTRODUCTION In an earlier paper [1] we investigated the thermal decomposition of copolymers of styrene with 1,2,2,2-tetrachloroethyl esters of acrylic and fumaric acids and with halogenated N-phenyl maleimides using thermogravimetric analysis; we determined the flash-ignition and self-ignition temperatures of these copolymers. An objective of this work is identification of the thermal decomposition products of the above copolymers and elucidation of the main chemical transformations in the polymers at temperatures from 200 to 500 °. EXPERIMENTAL
Copolymers of styrene and 1,2,2,2-tetrachloroethyl acrylate (TCEA), 1,2,2,2-tetrachloroethyl methacrylate (TCEMA), bis-(1,2,2,2-tetrachloroethyl) fumarate (TCEF), bis-(1,2,2,2-tetrachloroethyl) muconate (TCEM), N-(2,4,6tribromophenyl) maleimide (TBPMI) and N-(pentachlorophenyl) maleimide (PCPMI) were prepared by suspension copolymerization [1]. The formulae of halogenated comonomers and molar ratios styrene-comonomer (st-com) in the copolymers are given in Table 1. Methods
Flash pyrolysis of the copolymers was carried out at 310 and 500° in a pyrolyzer described earlier [2,3]. Products were led directly in to the chromatographic column of a gas chromatograph-mass spectrometer LKB 9000 apparatus. The flow rate of carrier helium was 40 ml/min; the amount of the polymer was about 0.5 rag. The decomposition products were separated chromatographically on a glass column, 5 m long, packed with 10~o Silikon Gummi Merck fixed on Chromaton NAW. The column temperature was linearly raised from 50 to 250° at a rate of 10°/min; the temperature of the carrier gas separator was 260 ° and that of the ion source was 280 °. The energy of ionizing electrons was 20eV during the chromatographic analysis, and 70 eV in recording the mass spectra. The pressure in the ion source was 10-4Pa. The
products of pyrolysis of the copolymers were identified by comparing their mass spectra with published spectra [4]. Hydrogen halide was determined in the thermal decomposition products of all copolymers arising in the heating of 0.2~3.5 of powder samples in a glass ampoule ca. 10 cm 3 in volume; for copolymers of tetrachoroethyl esters, carbon dioxide and acyl chloride groups also were determined. Pure nitrogen was introduced into heated ampoule during the determination of hydrogen halide and carbon dioxide. Volatile products carried by the nitrogen were cooled in a glass U-tube immersed in ice-water; uncondensed gases were collected in 40 cm 3 of distilled water, from which samples were taken for determination of dissolved hydrogen chloride and carbon dioxide. Hydrogen chloride was determined by titration with a standard solution Hg(CIO4)2 using diphenylcarbazone as indicator [5]. In control determinations, the analyzed solution was titrated with 0.1 N NaOH solution until phenolphthalein turned red. In order to verify the presence of carbon dioxide in the products of pyrolysis, 10cm 3 of clear 2~o aqueous Ba(OH)2 was added to 15 cm 3 of the solution. If any turbidity appeared, the sample was filtered; the residue was dried at 105°, and weighed. To check the formation of hydrogen chloride by the hydrolysis of chloral, a mixture of 1 g of freshly redistilled chloral and 3 cm 3 of acetone was shaken with 0.l g of .solid sodium acetate, and filtered. Then 25 cm 3 of water was added to the filtrate, the solution was left to stand for 45 rain at 20°, acidified with 5~o HNO3 until the Congored had turned blue, and titrated with a 0.1 N Hg(CIO4)2. The first drop of the titrating solution made the analyzed solution turn red-violet, indicating that under the given conditions chloral was not hydrolyzed to hydrogen chloride. The acyl chloride groups were determined in weighed residues obtained after heating 0.15-0.5 g of the copolymers for 40min in a stream of nitrogen in the vacuum of a water pump (1860 Pa). About 0.04g of the residue was taken for the determination of chlorine; the remainder was mixed with 0.17g of sodium acetate and 10cm 3 of ethanol. The mixture was boiled for 20 sec and diluted with water to ca. 30 cm 3. The fine sludge was removed by filtration, and the filtrate was made up with water to 50cm 3.
219
F. HRAB~.K et al.
"220
Table 1. Copolymers of styrene (St) with halogenated comonomers (com) for pyrolysis Comonomer Formula
Designation TCEA
Mw
Copolymer ~o halog. St/corn
ct o I II CCL3--CH-O--C-CH=CH~
237.9
19.46
4.7
CCt3--CH-O-C--C=CH=
251.9
21.96
3.8
447.7
20.57
8.9
473.8
22.18
7.72
409.9
22.41
6.4
345.4
20.68
4.9
TCEMA
TCEF
CL
0
0
CL
I
LL
LL
I
CL
O
CCL3--CH--O-- C--CH=CH- C- O-CH-CCL 3
TCEM
I
0
II
IJ
ct I
CCL~-CH- O-C- CH=CH--CH=CH--C-O--CH--CCL 3
TBPMI
/C/ZCH
~r
8~-(\ /~N PCPMI
CL
CL
II O
/c--ell CL~ / ~ N ' c _ _ ~ H CL
CL
Jl 0
The sample of the solution was acidified with 5~o HNO3, and the C1- ions (corresponding to the content of the acyl chloride groups) were determined by titration with 0.1 N Hg(CIO4)2. RESULTS
The products of pyrolysis of the copolymers under investigation at 310 and 500°, identified with a gas chromatograph directly connected with the mass spectrometer, are summarized in Table 2. In this Table, nonhalogenated compounds are followed by the halogenated ones, according to the magnitude of the mass unit (re~e). The peak heights of the individual compounds read off from gas chromatograms have been divided into five groups, indicating only the ratio of the amounts of products. The most abundant nonhalogenated products from all copolymers were styrene, its dimers and trimers, and ~t-methylstyrene. Of the halogenated products, chloral and dichloroacetaldehyde were present in the greatest amount for the copolymers of tetrachloroethyl esters; the decomposition products of the copolymers of N-phenyl maleimides were their monomers. In the decomposition products of the copolymers of tetrachloroethyl esters, only TCEMA appears as monomer. The monomeric units of the other tetrachloroethyl esters probably decompose rather than depolymerize. The amounts of halogen transformed to HC1 during the thermal decomposition of copolymers, and the amount of ester groups transformed to carbon dioxide and --COC1 groups are shown in Tables 3 and 4. These amounts of HC1 were determined by the titration of an aqueous solution of volatile products with 0.1 N Hg(C104)2; the amount of hydrogen chloride determined in control titrations with 0.1 N NaOH differed from the above values by less than 0.6~o. By heating the copolymers of tetrachloroethyl
esters to 500°, more than half of the chlorine present was transformed to HC1; at 310°, the amount was about one-third. The presence of acyl chloride groups, which should have been formed in the copolymer by the splitting-off of chloral from the tetrachloroethyl ester monomeric units, could not be established in the residues of the above copolymers heated to 310 and 500° in the vacuum of the water pump. Carbon dioxide was indicated only at 500°; this insignificant amount does not suggest any perceptible decarboxylation process of the ester monomeric units. At 250 and 230 °, only one representative of the copolymers of tetrachloroethyl esters, viz. poly (TCEA-co-styrene), was pyrolyzed. The amount of chlorine transformed to HC1 dropped to 13.5 and 3.7%, but the copolymer residues contained bound acyl chloride groups (Table 4). Their formation is also indicated by thermogravimetric analysis [1] according to which a spontaneous decomposition of poly(TCEA-costyrene) sets in between 230 and 250°. Since the spontaneous decomposition of polystyrene starts at 280° [1], chloral can be regarded as the main volatile product of the pyrolysis of poly(TCEA-costyrene) at 230-250 °. At 310 and 500°, chloral is, with styrene, one of the main volatile products of pyrolysis and at the same time the starting compound for formation of dichloroacetaldehyde and chloroacetaldehyde (Table 2). Table 4 shows that, in the copolymer of TCEA heated to 230 or 250° for 40 min, some chlorine remains bound, while during the pyrolysis of the copolymers of TCEA and TCEF at 310 and 500° the chlorine is almost completely split off. On heating to 310°, only a little of the halogen present in the copolymers of styrene with halogenated N-phenyl maleimides was transformed to hydrogen halide (Table 3); no halogenated organic compounds were identified in the volatile products, apart from PCPMI (Table 2). The small amount of hydrogen chloride found corresponds to the results of thermo-
Thermal decomposition of copolymers
221
Table 2. Products of pyrolysis of the copolymers of styrene with halogenated comonomers; peak heights on the gas chromatogram h--high, l--low, m--medium, v--very: data refer to 500 °, those in brackets to 310"
m/e Products 78 92 104 106 118 118 118 144 154 158 160 172 182 196 208 312 78 112 146 160 140 166 180 252 329 407 289 291 309 343 357
TCEA
Benzene Toluene Styrene Ethylbenzene Allylbenzene ct-Methylstyrene Indane Isopropenylstyrene Diphenyl 5,6-Benzo-2-methyl-3,5cyclohexadienone Propylpropenylbenzene Unidentified Diphenylethane L3-Diphenylpropane Dimers of styrene
TBPMI
PCPMI
vl
1
1 (1)
1
1 (1)
vl (vl)
1
m
m
h
h
m
vh (m)
vh (1)
vh (h)
vh (h)
vh (vh)
vh (vh)
m (vl)
m
m
I
(t)
1 vh (1)
1 h (1)
1
1
1
vl
r0 h'(vl)
1 (m) 1 (I)
1 1
vl
vl vl
Trimers of styrene Chloroacetaldehyde Dichloroacetaldehyde Trichloroacetaldehyde Trichloropropanal Benzyl chloride 2-Chloroethylstyrene 2,2-Dichloroethenyl methacrylate TCEMA N-Dibromophenyl maleinimide TBPMI Pentachlorophenyl isocyanate N-Pentachlorophenyl formamide N-Tetrachlorophenyl maleinimide PCPMI Methyl maleic acid pentachlorophenylimide
Comonomers TCEF TCEM
TCEMA
vl
vl
vl
1
vl
m h (1) m vl
vl (1)
1
1 (m)
m (1)
m
m (m)
1 1 (1)
1 1 (1)
h,2m(1) 31 (vl) vh,m(2m)
2h 3m vh,h (1)
vh 2m,l (vh) vh,m(vh)
vh,h m,1 vh,h(vh,h)
vh 21 (1) vh,h(h,m)
vh 21 vh,h(vhl
1 (vl)
1
1 (m)
m
m (1) vh (vh vl
vh (1) vh (vh) m
h (vh) h (vh)
h (h) vh (vh) 1
1 (1)
1
1
vl m 1 (11 1
vh (m) 1 1 m
vh (h) m
The figure before the symbol gives the number of peaks of given intensity. gravimetric analysis [1] a c c o r d i n g to which the s p o n t a n e o u s d e c o m p o s i t i o n of the c o p o l y m e r s T B P M I a n d P C P M I sets in a b o v e 320 °. By h e a t i n g the cop o l y m e r T B P M I to 500 °, 27.4~o of b r o m i n e was transformed to HBr, a n d m o r e b r o m i n e (31.07~) was
f o u n d in the residue t h a n in the starting c o p o l y m e r (22.41~o Br). U n d e r the same conditions, 51.6~/o of h a l o g e n was t r a n s f o r m e d to HC1 in the c o p o l y m e r P C P M I (20.68~o CI); the residue c o n t a i n e d only 13.05% of chloride. D e h a l o g e n a t e d derivatives of
Table 3. Amount of hydrogen halide, CO2 and COCI groups in the thermal decomposition products of styrene and halogenated comonomers; X = h a l o g e n ; H X = h y d r o g e n halide Decomposition temperature (°C) 500
310
250 230
E.P.J. 14/3
D
Comonomer TCEA TCEMA TCEF TCEM TBPMI PCPMI TCEA TCEMA TCEF TCEM TBPMI PCPMI TCEA TCEA
°J~ - - C - - O il ~o X of the compolymer O transformed to HX transformed to CO2 51.4 51.3 64.4 48.2 27.4 51.6 28.8 28.6 44.5 33.8 4.5 8.7 13.5 3.7
2.2 -1.1 ----. -----
222
F. HRABAKet al. Table 4. Weight loss of the copolymers of styrene and halogenated monomers heated in ampoules evacuated with a water pump % ~C--O--
Comonomer
qt
Decomposition temperature (°C)
Residue after pyrolysis, %
230 250 310 500 310 500 310 500 310 500
79.76 76.65 33.9 14.7 39.2 17.3 69.7 38.4 71.59 39.0
TCEA
TCEF TBPMI PCPMI
T B P M I and PCPMI, N-dibromophenyl maleimide and N-tetrachlorophenyl maleimide respectively, were identified in the volatile products of pyrolysis. N o chlorine could be found iodometrically in an aqueous solution of volatile products. Residues of copolymers after pyrolysis between 230 and 500 ° could not be dissolved in common solvents for polymers.
%N % of halogen in the residue ------1.85 2.85 1.82 3.80
:H3 C6Hs-CH-CHz--CH2--C ~ COCL
The fact that the majority of identified nonhalogenated compounds appear in the decomposition products of all copolymers under investigation indicates that these compounds were predominantly formed by the decomposition of the polystyrene sections of copolymers. The participation of a halogenated comonomer unit in the formation of a nonhalogenated product of pyrolysis seems likely only in the case of compounds with weight units 144, 158 and 172. The first and third units appear only in the decomposition products of the copolymer TCEMA, while the second is present in the products of the copolymers T C E A and TCEMA. In the pyrogram of T C E M A , the compound with Mw 172 is not perfectly separated from the dimer of styrene (concentration ratio m/e2oa:m/e172 = 2:1), and its structure cannot be determined unambiguously from the mass spectrum. The ways of transformation of the copolymer sections to nonhalogenated compounds with mass units 144, 158 and 172 may be indicated by the following schemes; the structures of final products suggested on the basis of the mass spectra are not to be regarded as proved:
~H3
~ H3
C6Hs--~H-CHz--(~--CH2~ -N~CO -HCL > C6Hs-CH=CH-C=CH2 ~ /CH2 COCt or l ~ ~ICIH_.
~ C11HI2;
role =
144
c..
-.c~ \ ~ , . \ ; c . 2 II 0 c~120 ; m/w
= 172
37.7 16.5 --------
/CH----CH Cs Hs--C.H I "~CO--CH--CH 3 Cl2HI20 ; role =172
CH3 C6Hs--CH-CH2--C~ J
DISCUSSION
9.17 3.21 1.05 2.04 0.98 0.58 27.80 31.07 20.34 13.05
O transformed to COC1
I
COCL
Cs Hs-CH--CH2--~:H--CH2 ~ COCt
0 C11HIoO; role =
158
The condition for formation of COC1 groups bound to polymer is the splitting-off of the chloral molecule from the monomer unit of tetrachloroethyl ester. This is confirmed by the presence of chloral in large amounts among the volatile products of pyrolysis of all copolymers of tetrachloroethyl esters at temperatures from 230 to 500 ° . However, the presence of the COCI groups in the residues of the copolymers of tetrachloroethyl esters was proved only after pyrolysis at 230 and 250 ° . Their absence in the residues after pyrolysis at 310 and 500 ° and at the same time the high content of HC1 in volatile decomposition products corroborate the above equations describing transformations of the acyl chloride groups. These transformations probably involve not only the formation of HC1 and identified volatile compounds (C11H12, C l l H t 0 0 and C12H120) but also the formation of similar structures bound to the polymer, poorer in hydrogen content than the starting sections of the copolymer. The presence of T C E M A and 2,2-dichloroethenyl methacrylate in the volatile products from poly(TCEMA-costyrene) shows that chloral is not completely split-off from tetrachloroethyl esters even at 500 °, and that dehalogenation of the tetrachloroethyl groups, probably even those bound on the polymer, is the competing reaction in this case. Also the finding that at 500 ° more than 50% of chlorine present in the copolymers "of tetrachloroethyl esters is transformed to HCI indicates formation of HC1 at the expense of chlorine from the trichloromethyl group of chloral and tetrachloroethyl ester. Obviously, the dehalo-
Thermal decomposition of copolymers genation of the latter does not occur by the splittingoff of the HCI molecule, because saturated chloro derivatives of acetaldehyde--monochloroacetaldehyde and dichl'oroacetaldehyde--are formed from chloral, and 2,2-dichloroethenyl methacrylate from TCEMA (Table 2). It may be inferred therefore that the dehalogenation of the trichloromethyl group proceeds by a radical mechanism. First, the weaker C--C1 bond is split [6,7]. Then, the generated C1 atom removes a hydrogen atom from the hydrocarbon group with formation of HC1 and of the radical in the polymer chain or in the volatile organic compound. (?-Radicals formed by the removal of CI" from chloral or dichloroacetaldehyde (CClzCHO and CHC1CHO) probably react with the hydrocarbon groups similarly, while being transformed to saturated molecules of di- and monochloroacetaldehyde. The presence of benzyl chloride and 2-chloroethylstyrene in the volatile products of pyrolysis (Table 2) also indicates dehalogenation of the tetrachloroethyl and trichloromethyl groups by the carbon radicals arising by the thermal splitting of the polymer chain. The existing data do not allow an answer to the question whether the dehalogenation of the tetrachloroethyl ester groups in the polymer is the primary process and the splitting-off of di- and monochloroacetaldehyde or of 2,2-dichloroethenyl methacrylate is a consecutive one, or whether the chloral and TCEMA are dehalogenated in the gaseous phase. The insolubility of the copolymer residues after pyrolysis, already starting from 230~, indicates that the 12-radicals also terminate by combination. The presence of TBPM1 and PCP\MI in the decomposition products of their copolymers with styrene indicates depolymerization of the maleinimide monomeric units at 310'. Since in thermogravimetric analysis [1] the onset of spontaneous decomposition of polystyrene was recorded at 280~', that of its copolymers with TBPMI and PCPMI respectively at 352 and 326', and that of the homopolymers of TBPMI and PCPMI respectively at 372 and 375°, it is quite probable that the depolymerization at 310° starts in the polystyrene sections of the chain but is retarded by the imide monomeric units. The latter may also be stripped and released from the chain by the decomposition of the adjacent polystyrene blocks.
223
The small quantities of released hydrogen halide at 310° indicate that even at this temperature the thermal splitting of the C~,r--halogen bond took place, followed by the formation of hydrogen halide and dehalogenated derivatives of TBPMI and PCPMI, i.e. dibromophenyl maleimide and tetrachlorophenyl maleimide, found in the volatile products only at 500'~. The finding that, in spite of the higher energy of the C--CI bond compared with the C--Br bond, a larger fraction of halogen is transformed to hydrogen halide in the pyrolysis of the copolymer of PCPMI than in the pyrolysis of the copolymer of TBPMI, can be assigned to the higher number of halogen atoms in the molecule of PCPMI. This is also suggested by the approximate balancing of the number of halogen atoms in the monomeric units of PCPMI and TBPMI during pyrolysis. Data in Tables 3 and 4 indicate that residues after the heating of the two copolymers to 500 for 40min contain 1.4 CI atoms or 1.9 Br atoms for each N atom. At 500°, the decomposition of the maleinimide ring bound on the polymer chain occurs along with the fasl dehalogenation of the benzene ring, giving rise to pentachlorophenyl isocyanate and pentachlorophenyl formamide. At the same time, bonds are split between the carbon atoms of the polymer chain belonging to a styrene monomer unit, as suggested by the presence of methylmaleic acid pentachlorophenylimide in the decomposition products: ~ C H -- CH--CH2--CH ~ 1 t C~
CO
N~C6Cl5
~-
CH t
CH--CH 2 I
CO
CO
S
\N/--C6C15
CH ~ C - - C H 3 I ~
CO
CO
\N~C6Cl ~
REFERENCES
1. F. Hrabfik. 1. Masafik, K. Bouchal and M. Slavi~ek, Europ. Polym. J. 13, 509 (1977). 2. J. Mitera and J. Michal, Chem. Prfim. 26, 417 (1976). 3. J. Mitera, J. Michal, J. Kubfit and V. Kubelka, Z. analyt. Chem. 281, 23 (1976). 4. Eight Peak Index of Mass Spectral Data MSDC, AWRE, Aldermaston, Reading, U.K. (1974). 5. M. Jure~ek and M. Ve~e~a, Chem. Listy 46, 620 (1952). 6. M. W. Wolkenstein, Swuktur und Physikalische Eigenschaften der Molekfile, p. 586. Teubner, Leipzig (1960). 7. J. K. Syrkin, Zh. fiz. Khim 17, 347 (1953).