Thermal degradation of some transition metal (cobalt, nickel and copper) salts of poly(methacrylic acid)

PolymerDegradationand Stability45 ( 1994) 115-120 (~) 1994Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0141-3910/94/$07.00 ELSEVIER

Thermal degradation of some transition metal (cobalt, nickel and copper) salts of poly(methacrylic acid) The late Mohammad Zulfiqar, Rizwan Hussain, Shagufta Zulfiqar, Din Mohammad* Department of Chemistry, Quaid-i-Azam Unioersity, lslamabad, Pakistan

&

I. C. McNeill$ Department of Chemistry, University of Glasgow, Glasgow, UK, G12 8QQ (Received 3 February 1994; accepted 20 February 1994)

The thermal stabilities of cobalt, nickel and copper polymethacrylates, synthesized by free radical polymerization of methacrylic acid followed by the replacement of the carboxylic proton with the respective metal ion, have been examined. Various thermoanalytical techniques have been utilized. The results reveal that cobalt and nickel polymethacrylate are much more stable than copper polymethacrylate, although the products are similar. A general mechanism for their thermal degradation has been proposed.

INTRODUCTION

polymethacrylate salts of transition metals (cobalt, nickel and copper) belonging to the fourth period in the periodic table has been investigated. An attempt has been made to investigate the importance of the decarboxylation reaction which is dominant in the thermal degradation of alkali, alkaline earth and zinc metal salts of PMAA.I-3

The stability and degradation behaviour of various polymethacrylates has received considerable attention, although the degradation of polymethacrylate salts has not been extensively studied. The thermal breakdown of alkali and alkaline earth metal salts of poly(methacrylic acid) (PMAA) has been studied by McNeill and Zulfiqar, 1"2 who concluded that syndiotacticity is enhanced in polymethacrylate salts containing metal ions of larger ionic radii. It has also been confirmed that decarboxylation is dominant during the thermal breakdown of these polymers. Extensive studies have also been carried out on the thermal degradation of zinc polymethacrylates by McNeill et al.,3 and of barium and lead polymethacrylates by Zulfiquar et al. 4 In this paper the thermal degradation of some

EXPERIMENTAL Reagents and chemicals All the chemicals and the monomer were purified using methods described in the literautre. 5 Methacrylic acid (E. Merck) was vacuum distilled (14 mm) at 72°C to free it from the inhibitor. The middle fraction was collected. Azo bisisobutyronitrile (E. Merck) was recrystallized from absolute methanol (E. Merck), dried in a vacuum oven at room temperature and stored in an amber bottle. Methanol (E. Merck) was

* Present address: PO Box 1356, Pinstech, PO Nilore, Islamabad, Pakistan. ~:To whom correspondence should be addressed. 115

116

M. Zulfiqar et al.

purified by drying with calcium hydride (E. Merck) and calcium sulphate (E. Merck) for 24 h, followed by distillation. Diethyl ether (E. Merck) was treated with sodium wire for 24 h before distillation.

Synthesis Methacrylic acid (25% w/v) was polymerized at 60°C in methanol using azo-bisisobutyronitrile as initiator. The reaction was stopped after 120 min by pouring the mixture into stirred dried diethyl ether. The solution of PMAA was mixed with methanolic sodium hydroxide to obtain white gelatinous sodium polymethacrylate. The watersoluble sodium polymethacrylate was reacted with aqueous solutions of the respective metal acetates in 1:0-5 molar ratios to precipitate the transition metal polymethacrylates. The polymer samples were dried at 40°C in a vacuum oven for 24 h and stored in a desiccator.

Characterization The metal content in these samples was determined using atomic absorption spectrophotometry, inductively coupled plasma spectroscopy and potentiometry. The samples were digested in mixed perchloric acid and nitric acid before analysis. The average metal contents were found to be 25.06%, 25.39% and 26-64% for cobalt polymethacrylate (CoPMA), nickel polymethacrylate (NiPMA) and copper polymethacrylate (CuPMA), respectively. Those values are somewhat lower than theoretical values, indicating some residual carboxyl groups. The IR spectra were recorded on a Hitachi (Japan) 270-50 IR spectrophotometer using KBr discs. The symmetrical and antisymmetrical carbonyl peaks in the region 1600-1530cm -~ and the absence of a double bond peak at 1640cm -~ confirmed the formation of the polymers.

ture to 500°C in a dynamic nitrogen atmosphere at a flow rate of 20 ml/min. For pyrolysis-mass spectrometry (Py-MS), a Finnigan MAT (Germany) 271/45 mass spectrometer for highprecision analysis of gaseous mixtures was used for the identification of the volatile products evolved from the pyrolysing samples, which were heated in vacuum from 150°C to 500°C before mass spectrometric analysis. The instrument was controlled by a Hewlett-Packard (USA) (HPS) desktop computer.

RESULTS AND DISCUSSION TVA traces for CoPMA, NiPMA and CuPMA are shown in Figs 1-3. CoPMA and NiPMA show only one peak with a maximum at 396°C and 347°C, respectively, whereas CuPMA yields three incompletely resolved peaks with maxima at 170°C, 246°C and 345°C. Figures 1-3 also show that degradation starts at 294°C, 265°C and 130°C for CoPMA, NiPMA and CuPMA, respectively. The stability of CoPMA and NiPMA is broadly comparable with that of other polymethacrylate salts previously studied, but the stability of CuPMA is unusually low. The reason for this is not at present understood.

,

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Analytical techniques mr

Thermal volatilization analysis (TVA) was employed using the equipment described by McNeill.6 Powdered samples (50 mg) were heated in vacuum from ambient temperature to 500°C at a heating rate of 10°C/min. A Stanton Redcroft (UK) 750 B thermobalance was used to record the thermogravimetric (TG) curves at a heating rate of 10°C/min from ambient tempera-

177

232

279

323

373

417

469

Temperature (°C)

Fig.

1.

TVA

curves

for CoPMA,

- 75*C; ...... , - 100°C;

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0°C;

.... , -45°C;

- 196°C.

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Thermal degradation of PMAA transition metal salts

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126

177

232

279

323

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Temperature (°C)

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curves -75°C;

for

NiPMA.

...... , - 100°C;

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0°C;

.... , -45°C;

.... ,

- 196°C.

In the T V A traces of C o P M A and NiPMA, the 0°C, - 4 5 ° C , - 7 5 ° C and -100°C curves almost coincide, whereas the -196°C curve is separate. This suggests that a large proportion of the volatile products are volatile at -100°C but condensable in liquid N2, but that noncondensable gases such as carbon monoxide or, methane are also evolved. The 0°C and - 4 5 ° C T V A curves for C u P M A coincide throughout the course of the experiment, whereas - 7 5 ° C and -100°Ccurves are separate, suggesting the

117

condensation of some of the volatile products at - 7 5 ° C and -100°C. The evolution of permanent gases is also evident, from the -196°C trace. The IR spectra of the yellow-brown cold ring fractions showed peaks at 1650-1620cm -~, indicating the presence of unsaturation. In each case, there is a considerable amount of residue at 500°C. The T G curves for C o P M A , C u P M A and NiPMA are shown in Fig. 4. All the polymer samples were preheated to 125°C before recording the TG curves from ambient temperature to 500°C. The weight loss observed up to 125°C was due to the loss of moisture and solvents. The single-stage thermal breakdown of these transition metal polymethacrylates suggests main-chain scission. It is discernible from Fig. 4 that C u P M A starts to degrade around 145°C, whereas the onset degradation temperatures for C o P M A (300°C) and NiPMA (274°C) are much higher. Residues at 500°C were 32%, 31% and 34% for C o P M A , NiPMA and C u P M A , respectively. The residues are in fair agreement with the calculated amounts of the respective oxides, as the carbonates decompose at temperatures much lower than 500°C, 16 i.e. 220°C, 200°C and 402°C for COCO3, NiCO~ and CuCO3, respectively. From the theoretical structure, CoO would be approximately 38% of polymer weight, and the others would be almost the same. With the actual compositions, residues of about 35% should be expected. The volatile products were identified by the use of subambient thermal volatilization analysis ( S A T V A ) and P y - M S . The S A T V A traces for

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Temperature (°C)

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M. Zulfiqar et al.

118 CoPMA 1.0

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Temperature (°C) Fig. 4. TG curves for the transition metal polymethacrylates.

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Time (min)

Fig. 5. SATVA curves from transition metal polymethacrylates. (a) CoPMA; (b) NiPMA: (c) CuPMA.

these polymers are shown in Fig. 5. The materials from the individual SATVA peaks were collected in miniature gas cells and subjected to IR spectroscopy. The spectra were compared with those of reference compounds. 8'9 Four peaks were observed in the SATVA trace for separation of condensable volatile degradation products from CoPMA. Well-defined and large peaks appeared at 13 and 30min, and a small peak was recorded at 19 min and a minor deflection in Pirani response at 37 min. Carbon dioxide (3720 and 3700, 3620 and 3600, and 2380-2300cm -~) along with butene (918cm -~) and isobutene (890cm -~) were identified at the first peak. The second SATVA peak showed the presence of compounds with ketenic (2120 cm -~) and ketonic (1740cm -~) functional groups. Ketones were observed in the SATVA collections for peaks 3 and 4. Three fractions appearing at 13, 19 and 29 min were collected from the SATVA separation of NiPMA products. The first peak revealed the presence of carbon dioxide. Ketonic and acetylenic (732 and 722cm -~) groups were identified in the second fraction, and the third was mainly moisture (3400 cm-~). The SATVA fractions from separation of CuPMA products were recorded at 13, 18, 20, 29 and 31min. The fractions appearing at 18 and 20min were not large enough to give any conclusive evidence about the evolved compounds. Carbon dioxide (3720, 3700, 3620 and 3600, and 2380-2300 cm- ~) and ketenes (2120cm -~) were identified in the first fraction. The IR spectra of the fourth and fifth fractions

Thermal degradation of PMAA transition metalsalts revealed the presence of acetylenic (732 and 722 cm -1) and ketonic (1730 cm -1) linkages. The volatile products obtained from the thermal breakdown of each polymer were further identified by MS. The mass spectra of CoPMA, NiPMA and CuPMA products are presented in Fig. 6. The compounds identified by MS include carbon dioxide (m/z 43-9898; e in Fig. 6), carbon monoxide (m/z 27-9949; c in Fig. 6); methane (m/z 16.0313; a in Fig. 6), acetylene (m/z

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26.0157; b in Fig. 6), ketene (m/z 42.0106, d in fig. 6), methyl ketene (m/z 56.0626; g in Fig. 6), dimethyl ketene (m/z 70.0419; h in Fig. 6) and butene (m/z 56.0262; f in Fig. 6). There were also various ketones, namely methyl cyclpentanone (m/z 98-0732; m in Fig. 6), dimethyl cyclopentanone (m/z 112.08888; o in Fig. 6), butan-2-one (m/z 72.091); i in Fig. 6) methyl isopropyl ketone (m/z 86-0732; 1 in Fig. 6), methyl n-propenyl ketone (m/z 84-0575; k in Fig. 6) and di-isopropyl ketone (m/z 114.1045; p in Fig. 6). The amount of carbon dioxide was similar for all three polymers, whereas slightly larger amounts of carbon dioxide were evolved for NiPMA. Larger amounts of aliphatic and cyclic ketones were detected in the volatiles from CoPMA and CuPMA compared with those from NiPMA. This can be attributed to the lower stability 6 of COCO3 (d. 220°C) and CuCO3 (d. 200°C) compared with NiCO3 (d. 402°C), which facilitates decarboxylation and hence the formation of ketones. It is discernible from the results that slightly smaller amounts of hydrocarbons (acetylene and butenes) were evolved during the thermal degradation of NiPMA, but a larger amount of carbon monoxide.

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MECHANISM OF D E G R A D A T I O N

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6. M a s s s p e c t r a f r o m t r a n s i t i o n m e t a l p o l y m e t h a c r y lates. (a) C o P M A ; ( b ) N i P M A ; (c) C u P M A .

The volatile products identified suggest the following general mechanism for the thermal breakdown of these polymers. Alkali metal salts of PMAA ~ are known to degrade to monomer and isobutyrate via reaction I. These compounds are sufficiently volatile to escape from the degradation zone. In the case of the alkaline earth and zinc salts of PMAA, however, these degradation products are not volatile and undergo decomposition. 2.3 The formation of dimethyl ketene from the isobutyrate ~° by reactions II and III has been proposed for divalent metal salts at higher temperatures by McNeill and Zulfiqar, z resulting in a metal oxide residue. The formation of cyclic and acyclic ketones can be envisaged via the cyclic intermediates A and B, respectively (reactions IV, VI and VII). Side-group elimination produces carbon monoxide via reaction VIII, with the possibility of formation of some hydrocarbon products from the backbone struc-

M. Zulfiqar et al.

120

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ture so produced. The metal carbonate, formed either from the degradation of the polymer backbone or side-group elimination, decomposes to carbon dioxide and metal oxide (reaction V), as the metal carbonates involved are unstable below the maximum temperature reached in these experiments. This general reaction scheme is the same as that previously proposed for metal polymethacrylates, t-3 and the behaviour of these transition metal polymethacrylates is essentially similar to that of the other divalent metal methacrylates studied, as the monomer and isobutyrate are involatile. These resemble zinc polymethacrylate more closely than the alkaline earth salt polymers, however, in giving metal oxide rather than metal carbonate, because of the relative instability of the carbonates.

REFERENCES 1. McNeill, I. C. & Zulfiqar, M., J. Polym. Sci., Polym. Chem. Ed., 16 (1978) 2465. 2. McNeill, I. C. & Zulfiqar, M., Polym. Deg. Stab., 1 (1979) 89. 3. McNeill, I. C., Zulfiqar, M. & Vrie, C., Polym. Dig. Stab., 9 (1984) 239. 4. Zulfiqar, M., Hussain, R., Zulfiqar, S., Mohammad, D. & McNeill, I. C., Polym. Deg. Stab. (in press). 5. Riddick, J. A., Bunger, W. B. & Sakano, T. K.,

Organic Solvents--Physical and Methods of Purification. John Wiley, New York, 1986. 6. McNeill, I. C., Eur. Polym. J., 6 (1970) 373. 7. Dean, J. A. (ed.), Lange's Handbook of Chemistry. McGraw-Hill, 1985, p. 4. 8. Schrader, B., Raman /Infrared Atlas of Organic Compounds. VCH, Weinheim, 1989. 9. Welti, D., Infrared Vapour Spectra. Heyden, London, 1970. 10. Pregaglia, G. F. & Binaghi, M., Macromolecular Syntheses, 3 (1968) 154.