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Thermal degradation of poly-N-methyl-N-tertbutyl-aminoethyl methacrylate
Polymer Degradation and Stability 55 (1997) 89-94 0 1996 Elsevier Science Limited PII:
ELSEVIER
SO141-3910(96)00113-9
Printed in Northern Ireland. All rights reserved 0141-3910/97/$17.00
Thermal degradation of poly-N-methyl-Ntertbutyl-aminoethyl methacrylate Shagufta Zulfiqar,“* M. Zafar-uz-Zaman,” Arshad Munif t & I. C. McNeillb *Department of Chemistry, Quiad-i-Axam University, Islamabad, Pakistan hPolymer Research, Chemistry Department, Joseph Black Building, University of Glasgow, Glasgow G12 SQQ? UK
(Received
15 March 1996: accepted 10 April 1996)
The thermal degradation behaviour of poly-N-methyl-N-tertbutyl-aminoethyl methacrylate (PMTBAEM) has been studied using thermogravimetry, differential thermal analysis and thermal volatilisation analysis, with programmed heating at lO”C/min. Subambient TVA was used to separate the volatile degradation products, which were characterised by mass spectrometry and IR spectroscopy. The main volatile products were found to be isobutene and carbon dioxide. The less volatile liquid components of this product fraction were further separated by gas chromatography and identified by mass spectrometry: the main liquid product was tertbutylaziridine, while methanol and traces of methyl-tertbutyl-aminoethyl methacrylate (MTBAEM) were also identified. The cold ring (tar/wax) fraction products, which formed the largest product fraction, were examined by IR spectroscopy and found to consist of modified oligomers of MTBAEM. It is concluded that the degradation mechanism involves two distinct types of side group decomposition together with fragmentation. mainlv at higher temperatures, of the modified chains. 0 1996 El&ier Science Limited -
acrylate, in which the vinyl bond is still intact, permitting subsequent polymerisation of this monomer with other vinyl monomers by a free radical route to produce a new series of polymers with pendant amine structures. By a similar process, N-methyl-N-tertbutylaminoethyl methacrylate (MTBAEM) was synthesised’ and polymerised, using benzoyl peroxide or the redox system FeSO,/H,O, for initiation of the free radical addition polymerisation (Scheme 1). The synthesis and thermal degradation of block”,’ ’ and graft12*” copolymers of poly(tertbutylaziridine) (PTBA) and poly(methy1 methacrylate) (PMMA) have been investigated previously. The monomer for the above polymerisation (Scheme 1) is synthesised by converting the TBA and MMA monomers into the corresponding salts and then reacting them to give MTBAEM. Studies of the thermal degradation behaviour of PMTBAEM provide further insight into the degradation behaviour of TBA and MMA derived structures in a different macromolecular environment to that in the
1 INTRODUCTION Functional macromolecules containing polymerisable vinyl groups have received considerable interest over the last few years as starting materials for the preparation of desired polymer structures. Such macromolecules have been synthesised by polyaddition reactions of divinyl compounds,’ by transformation of reactive end groups’ and more often by reacting anionic living polymers with electrophiles containing unsaturated functions.3” Cationic routes’ have also been used. In a previous study, N-methyl-N-tertbutylaziridinium triflate was synthesised and its reactions with a number of nucleophilic compounds reported.’ In all cases, the result was the opening of the highly reactive strained three-membered aziridinium ring. One of the nucleophilic reagents used was sodium acrylate. This led to the formation of N-methyl-N-tertbutyl-aminoethyl * To whom correspondence should be addressed. 7 Present address: Pakistan Atomic Energy Commission, Islamabad, Pakistan. 89
90
S. Zulfiqar
2.2 Degradation studies
CH3
CH3
-
CH2=C
CH2-C
-
//Y
bmoyl
peroxide . or FeSOdHzo2
CH3-
r;J
O 9 CH3-
id
CH3-
C-
I CH3
I
CH3- :: -cH3
CH3
CH3
et al.
Scheme 1.
only in the side groups, whereas in TBA-derived copolymers, it is present in the backbone. In both situations, however, the nitrogen appears as tertiary amino structures.
2 EXPERIMENTAL
The methods used included thermogravimetry (TG), differential thermal analysis (DTA) and thermal volatilisation analysis (TVA).“-” The products of degradation by TVA were separated first into the main fractions: non-condensable gases, condensable gases and liquids, cold ring fraction (products volatile at degradation temperature but not at ambient temperature) and involatile residue. The non-condensable gases were identified in the course of the decomposition using an on-line Leda-Mass Multiquad quadrupole mass spectrometer. The condensable gas/liquid fraction was further separated by subambient TVA (SATVA)‘X,‘” into several fractions according to volatility: the more volatile fractions were examined as gases by IR spectroscopy and mass spectrometry and the less volatile liquid products were further examined by GC-MS. The cold ring fraction was examined by IR spectroscopy.
2.1 Polymer preparation 3 RESULTS Reagents and solvents were purified before use by standard procedures. The aziridinium salt was prepared by the method described previously,‘” by reacting equimolar quantities of TBA and sulphonate in tetmethyl trifluoromethane rahydrofuran. Equimolar quantities of the aziridinium salt and sodium methacrylate were then reacted in triply distilled water to form the monomer, MTBAEM, and the solvent was removed by freeze drying. The by-product of the reaction, sodium triflate (CF,SO,Na) was separated from the monomer by preparative TLC using silica gel 60 HF,,, (Merck) as the support and THF as eluant. The _monomer was finally distilled under vacuum (0.4 mmHg) at 46°C. The IR and NMR spectra confirmed the formation of MTBAEM. Free radical polymerisation of MTBAEM was carried out in glass ampoules. Monomer (4.0 g, 0.02 mole) in 2 ml acetone and 0.02 g benzoyl initiator was introduced into the peroxide ampoule, the solvent was removed under partial evacuation and the system was then thoroughly evacuated and sealed. The polymerisation was conducted at 80°C for 24 h. The polymer was purified by dissolving it in acetone and precipitating it using methanol. The structure was confirmedI by IR and NMR spectroscopy.
3.1 Thermal analysis experiments TG and DTA curves for PMTBAEM are shown in Fig. 1. The polymer is thermally stable up to about 180°C and 35% weight loss then occurs in a single step to about 260°C. Between 260 and 400°C a further 10% of weight loss takes place more gradually, at which point more rapid weight loss of about 37% leads to a relatively stable residue at 600°C. The DTA curve shows a small endotherm near 235°C corresponding to the first weight loss and a large exotherm a little below
-. \ ,
I
I
200
I
I
I
400
600
800
Temperature
Fig. 1. TG
I
1000
(“C)
and DTA curves for PMTBAEM. nitrogen, lO”C/min. - - - - TG, -- DTA.
Dynamic
Thermal degradation of poly-N-methyl-N-tertbutyl-aminoethyl
methacrylate
I
0
IO
Temperature (“C)
Fig. 2. TVA behaviour of PMTBAEM, degraded under continuous evacuation at 10”C/min to 600°C. Key to TVA -750, ---lOO”, curves: o”, . . . . . -450, -.-. - 196°C.
500°C corresponding to the second major weight loss. The TVA behaviour of the polymer is illustrated in Fig. 2, which shows the onset of volatilisation at about 210°C. The pattern of break-down shows main rate maxima at about 250 and 435°C the first agreeing well with the rate maximum in the first weight loss, but the second occurring at somewhat lower temperature than that for the maximum rate for the second major weight loss, which occurs at about 480°C. This difference suggests that the main products in the latter reaction are of low volatility, such as short chain fragments, and are more easily lost under the vacuum conditions of TVA. The TVA curve also indicates that the products at the first major volatilisation are of high volatility (not condensable at -100°C under TVA conditions) whereas those at the second cover a range of non-condensable including some volatilities gases. The latter behaviour is consistent with extensive backbone fragmentation. Gravimetric data were obtained for the main product fractions in degradation to 600°C by TVA. The residue, which was insoluble in common solvents, was 6% (less than in TG because of easier volatilisation of chain fragments under TVA conditions), cold ring fraction amounted to 45% and the liquid fraction of volatile products was 33%, the remaining 16% of the original sample weight being due to condensable and non-condensable gases. 3.2 Characterisation of volatile products The non-condensable materials produced during TVA were identified by on-line MS as methane and carbon monoxide. The SATVA trace for warm up from -196 to 0°C of the condensable
I
30 20 Time (min)
!
40
I
50
Fig. 3. SATVA curve for separation by controlled warm up from -196°C of condensable volatile products of degradation to 600°C at lO”C/min of PMTBAEM.
volatile products is shown in Fig. 3. The gas fractions corresponding to peaks 1 and 2 were collected separately and found by IR spectroscopy and MS to consist of carbon dioxide and a trace of dimethylketene at peak 1 and isobutene at peak 2. The SATVA trace clearly indicates the presence of a further component at peak 2: this could not be unambiguously identified but is probably a nitrogen-containing molecule of similar volatility to isobutene. 3.3 GC-MS investigation of liquid products The liquid fraction corresponding to peak 3 of the SATVA separation was examined by GC-MS. The main component was found to be tertbutylaziridine. Other materials found to be present in much smaller amounts included methanol and MTBAEM monomer. 3.4 Analysis of cold ring fraction The extensive cold ring fraction (45% of the initial sample weight) collected in the degradation by TVA was pale yellow in colour. The IR spectrum suggested the presence of oligomers of MTBAEM, while multiple absorptions in the region 1800-1840cm-’ could indicate the presence of some anhydride structures.
4 DISCUSSION degradation behaviour of The thermal PMTBAEM has not previously been reported. The thermal degradation of PMMA has been extensively studied,20,2’ and it is very well documented that PMMA degrades to monomer in almost quantitative yield. The degradation of the polymer of tertbutyl aziridine has also been
92
S. Zulfiqar
reported.22 The major proportion of the degradation products of PTBA consists of TBA oligomers, some monomer-related products were present in the liquid fraction, and various gaseous products were present in small amounts, of which isobutene and isobutane were the most abundant. In the present investigation, a variety of degradation products covering a range of volatilities have been observed. It is clear from the nature of these that as well as backbone scission, scission in the side group makes a major contribution to the degradation mechanism. Although the onset of degradation in PMTBAEM is at a lower temperature than that of block and graft copolymers of TBA and MMA, the rate of weight loss is initially very low. A further difference between PMTBAEM and these copolymers is that the former is still degrading to volatile products up to around 5Oo”C, whereas the degradation of the copolymers is complete by 450°C. It appears that the product of the first stage of degradation of PMTBAEM is relatively stable, only undergoing breakdown when the temperature reaches nearly 400°C. Since PMTBAEM is a modified methacrylate ester homopolymer, some unzipping to monomer could be considered as a possible degradation route, as with some other methacrylate ester polymers. Side group reactions in such polymers, if they can take place, may take precedence over the unzipping to monomer and, even if they occur only to a limited extent, have been found to interfere with the unzipping process by creating modifed chain units which do not readily split out the corresponding monomeric species.2” Monomer is indeed found as a liquid product in the degradation of PMTBAEM, but its very limited amount implies that decomposition in the side groups is much more important. The high volatility of the products released below 360°C as seen in Fig. 2, and the formation of products recognisable as originating from fragments of the side group, confirm this view. The major proportion of cold ring fraction products consisting of modified MTBAEM oligomers, including some anhydride structures, is consistent with fragmentation at higher temperatures of chains initially stabilised by modified structures left from side group scission in the lower temperature region, which effectively form a copolymer. Anhydride ring structures have been found in
et al. many cases in the degradation of MMA copolymers.*” There are various routes for their formation, including dehydration of adjacent methacrylic acid (MAA) units, whether present initially or formed by a low temperature side group ester decomposition, or interaction between adjacent ester and acid side groups, as in MMA-MAA copolymers (with elimination of methanol).25 Such MAA units are among the structures which could be formed from PMTBAEM, and would provide precursors for the observed anhydride structures in the cold ring products. The TVA data suggest that in the degradation below about 350°C the products reaching the cold traps are very volatile materials, and isobutene has been identified as predominant. Since the weight loss up to this point is over 30%, and most of the tertbutyl groups in the side groups are still available, the isobutene is not itself able to account for the weight loss observed. The possible formation of another very volatile product is considered below, but it appears that most of the weight loss at the first stage must be due to low-volatility tar fraction products, resulting from some chain scission to give oligomer size fragments. Any other route would require other by-products which would be less volatile than isobutene. Side group decomposition can occur in various methacrylate ester polymers: for it to take place, H-abstraction from the carbon in the beta position in the ester alkyl group must be possible. An analogous decomposition would be possible in the MTBAEM side group as indicated in Scheme 2. This would give a methacrylic acid unit in the chain and a volatile product (I) which is, however, of much lower volatility than isobutene and is in any case not observed at all in the total volatile degradation products over the full temperature range. It appears that the initial eneamine (I) readily splits to isobutene and a smaller eneamine (II) or the isomeric imine (III). These N-containing species are of the same volatility as isobutene and are not excluded by the observed IR spectrum of the volatile product fraction 2, in the SATVA product separation of Fig. 2, although positive identification is not possible. The reactions taking place above reach a maximum rate at a little over 300°C and then begin to subside. The reason for this is not clear. It may be that the powerful nucleophiles become
Thermal degradation of poly-N-methyl-N-tertbutyf-aminoethyl
methacrylate
93
CH3 T3 -
CH2--C
-
-
CH2--C
-
I Ho I
H,
w
k-
0
+
/CR
CH I CH3- N CH3-
A
CH2
CC I C%--- N I
CH3
bH3
CH3- :: -cH3 CH3
CH3,
CH3-NH-CH= C=CH2
+
CH3’
CH;?
or CH3-N=CH-CH3
Scheme 2.
retained by the polymer matrix, or that crosslinking slows down release of volatile material, until the temperature becomes high enough to break other bonds. The degradation taking place at above 360°C is clearly different in character: a range of products with very different volatilities is present, including some non-condensable gases. At this stage, further side group scission by a different route appears to occur, giving TBA as a significant product, and there is also extensive fragmentation of the modified polymer backbone. Carbon dioxide and carbon monoxide are formed and traces of dimethylketene and methane are
CH3
CH3
CH2=C
CH2=C I
0 CH2
.
cG2 I CH3-- N
HC\OCH3
REFERENCES
t CHz-CH2 ’ N’
I
CH37-cH3
CH3- 7 -cH3
CH3
CH3
I-tertbu&zztidine
Scheme 3.
observed: these are commonly found at such temperatures from polymer systems with MAA anhydride rings. Small groups or derived amounts of methanol are also evolved. The new side group decomposition process appears to involve C-C rather than C-O scission, as shown in Scheme 3, giving TBA and a MMA unit in the backbone. The latter probably interacts with an available MAA unit from the previous decomposition stage to give methanol and an anhydride ring.25 The degradation behaviour of PMTBAEM is therefore very different from that of PTBA. The MTBAEM monomer unit is closely related in structure to the branch point in PMMA grafted with PTBA. The results of a study of the thermal degradation behaviour of this graft copolymer will be reported subsequently.
(TBA)
1. Tsuruta, T., in Polymeric Amines and Ammonium Salts, ed. E. J. Goethals. Pergamon Press, Oxford, 1980. 2. Hudeck, S., Spevasec, J., Hudeckova, I. & Nikesova, J., Polym. Bull., 3 (1980) 143. 3. Milkovich, R., Polymer Preprints, 21 (1980) 40. 4. Milkovich, R. & Chiang, M. T., US Pat. 3786116 (1974). 5. Milkovich, R. & Chiang, M. T., US Pat. 3862267 (1975). 6. Milkovich, R. & Chiang, M. T., US Pat. 4085168 (1978). 7. Vargas, S. J., Zilliox, J. G., Rempp, P. & Franta, E., Polym. Bull., 3 (1980) 83.
94 8. Munir,
A. & Daly, W. H., Macromol.
Chem.
S. Zuffiqar
et al.
Rapid
18. McNeil],
Commun., 4 (1983) 589. 9. Zafar-uz-Zaman,
10. 11. 12. 13. 14. 15. 16. 17.
M., Munir, A., Zulfiqar, M., Zulfiqar, S. & Farhat, S., J. Chem. Sot. Pak., 14 (1992) 230. Zafar-ur-Zarman, M., Zulfiqar, S. & Munir, A., submitted for publication. Zulliqar. S., Zafar-ur-Zarman, M., Munir, A. & McNeil], I. C., Polym. Degrad. Stab., 50 (1995) 33. Munir, A. & Goethals, E. J., J. Polym. Sci. Polym. Chem. Ed., 19 (1981) 1985. Zulfiqar, S., Zafar-uz-Zaman, M., Munir, A. & McNeil], I. C., Polym. Degrad. Stab., in press. Tam, M. & Ozgun, H. B., Eur. Polym. J., 26 (1990) 5. McNeil], I. C. & Stevenson, W. T. K., Polym. Degrad. Stab., 10 (1985) 247. McNeill, I. C., Eur. Polym. J., 3 (1967) 409. McNeil], 1. C., Eur. Polym. J., 6 (1970) 373.
C., in Developments in Polymer Vol l., ed. N. Grassie. Elsevier Applied Science, London, 1977, p. 43. McNeil], I. C., Ackerman, L., Gupta, S. N., Zulfiqar, M. & Zulfiqar, S., J. Polym. Sci. Polym. Chem. Ed., 15 (1977) 2381. Grassie, N. & Melville, H. W., Proc. R. Sot. (Land.), A199 (1949) 1. McNeil], I. C., Eur. Polym. J., 4 (1968) 21. Zulfiqar, S., Zafar-uz-Zaman, M., Munir, A. & McNeil], K., Polym. Degrad. Stab., 47 (1995) 59. McNeil], 1. C., in Comprehensive Polymer Science, Vol. 6, Chapter 15, ed. G. Eastmond, A. Ledwith, S. Russo & R. Sigwalt. Pergamon Press, London, 1989. McNeil], I. C., Int. J. Polymeric Mater., 214 (1993) 31. Jamieson, A. & McNeil]. I. C., Eur. Polym. J., 10 (1974) 217. I.
Degradation,
19.
20. 21. 22. 23.
24. 25.
ELSEVIER
SO141-3910(96)00113-9
Printed in Northern Ireland. All rights reserved 0141-3910/97/$17.00
Thermal degradation of poly-N-methyl-Ntertbutyl-aminoethyl methacrylate Shagufta Zulfiqar,“* M. Zafar-uz-Zaman,” Arshad Munif t & I. C. McNeillb *Department of Chemistry, Quiad-i-Axam University, Islamabad, Pakistan hPolymer Research, Chemistry Department, Joseph Black Building, University of Glasgow, Glasgow G12 SQQ? UK
(Received
15 March 1996: accepted 10 April 1996)
The thermal degradation behaviour of poly-N-methyl-N-tertbutyl-aminoethyl methacrylate (PMTBAEM) has been studied using thermogravimetry, differential thermal analysis and thermal volatilisation analysis, with programmed heating at lO”C/min. Subambient TVA was used to separate the volatile degradation products, which were characterised by mass spectrometry and IR spectroscopy. The main volatile products were found to be isobutene and carbon dioxide. The less volatile liquid components of this product fraction were further separated by gas chromatography and identified by mass spectrometry: the main liquid product was tertbutylaziridine, while methanol and traces of methyl-tertbutyl-aminoethyl methacrylate (MTBAEM) were also identified. The cold ring (tar/wax) fraction products, which formed the largest product fraction, were examined by IR spectroscopy and found to consist of modified oligomers of MTBAEM. It is concluded that the degradation mechanism involves two distinct types of side group decomposition together with fragmentation. mainlv at higher temperatures, of the modified chains. 0 1996 El&ier Science Limited -
acrylate, in which the vinyl bond is still intact, permitting subsequent polymerisation of this monomer with other vinyl monomers by a free radical route to produce a new series of polymers with pendant amine structures. By a similar process, N-methyl-N-tertbutylaminoethyl methacrylate (MTBAEM) was synthesised’ and polymerised, using benzoyl peroxide or the redox system FeSO,/H,O, for initiation of the free radical addition polymerisation (Scheme 1). The synthesis and thermal degradation of block”,’ ’ and graft12*” copolymers of poly(tertbutylaziridine) (PTBA) and poly(methy1 methacrylate) (PMMA) have been investigated previously. The monomer for the above polymerisation (Scheme 1) is synthesised by converting the TBA and MMA monomers into the corresponding salts and then reacting them to give MTBAEM. Studies of the thermal degradation behaviour of PMTBAEM provide further insight into the degradation behaviour of TBA and MMA derived structures in a different macromolecular environment to that in the
1 INTRODUCTION Functional macromolecules containing polymerisable vinyl groups have received considerable interest over the last few years as starting materials for the preparation of desired polymer structures. Such macromolecules have been synthesised by polyaddition reactions of divinyl compounds,’ by transformation of reactive end groups’ and more often by reacting anionic living polymers with electrophiles containing unsaturated functions.3” Cationic routes’ have also been used. In a previous study, N-methyl-N-tertbutylaziridinium triflate was synthesised and its reactions with a number of nucleophilic compounds reported.’ In all cases, the result was the opening of the highly reactive strained three-membered aziridinium ring. One of the nucleophilic reagents used was sodium acrylate. This led to the formation of N-methyl-N-tertbutyl-aminoethyl * To whom correspondence should be addressed. 7 Present address: Pakistan Atomic Energy Commission, Islamabad, Pakistan. 89
90
S. Zulfiqar
2.2 Degradation studies
CH3
CH3
-
CH2=C
CH2-C
-
//Y
bmoyl
peroxide . or FeSOdHzo2
CH3-
r;J
O 9 CH3-
id
CH3-
C-
I CH3
I
CH3- :: -cH3
CH3
CH3
et al.
Scheme 1.
only in the side groups, whereas in TBA-derived copolymers, it is present in the backbone. In both situations, however, the nitrogen appears as tertiary amino structures.
2 EXPERIMENTAL
The methods used included thermogravimetry (TG), differential thermal analysis (DTA) and thermal volatilisation analysis (TVA).“-” The products of degradation by TVA were separated first into the main fractions: non-condensable gases, condensable gases and liquids, cold ring fraction (products volatile at degradation temperature but not at ambient temperature) and involatile residue. The non-condensable gases were identified in the course of the decomposition using an on-line Leda-Mass Multiquad quadrupole mass spectrometer. The condensable gas/liquid fraction was further separated by subambient TVA (SATVA)‘X,‘” into several fractions according to volatility: the more volatile fractions were examined as gases by IR spectroscopy and mass spectrometry and the less volatile liquid products were further examined by GC-MS. The cold ring fraction was examined by IR spectroscopy.
2.1 Polymer preparation 3 RESULTS Reagents and solvents were purified before use by standard procedures. The aziridinium salt was prepared by the method described previously,‘” by reacting equimolar quantities of TBA and sulphonate in tetmethyl trifluoromethane rahydrofuran. Equimolar quantities of the aziridinium salt and sodium methacrylate were then reacted in triply distilled water to form the monomer, MTBAEM, and the solvent was removed by freeze drying. The by-product of the reaction, sodium triflate (CF,SO,Na) was separated from the monomer by preparative TLC using silica gel 60 HF,,, (Merck) as the support and THF as eluant. The _monomer was finally distilled under vacuum (0.4 mmHg) at 46°C. The IR and NMR spectra confirmed the formation of MTBAEM. Free radical polymerisation of MTBAEM was carried out in glass ampoules. Monomer (4.0 g, 0.02 mole) in 2 ml acetone and 0.02 g benzoyl initiator was introduced into the peroxide ampoule, the solvent was removed under partial evacuation and the system was then thoroughly evacuated and sealed. The polymerisation was conducted at 80°C for 24 h. The polymer was purified by dissolving it in acetone and precipitating it using methanol. The structure was confirmedI by IR and NMR spectroscopy.
3.1 Thermal analysis experiments TG and DTA curves for PMTBAEM are shown in Fig. 1. The polymer is thermally stable up to about 180°C and 35% weight loss then occurs in a single step to about 260°C. Between 260 and 400°C a further 10% of weight loss takes place more gradually, at which point more rapid weight loss of about 37% leads to a relatively stable residue at 600°C. The DTA curve shows a small endotherm near 235°C corresponding to the first weight loss and a large exotherm a little below
-. \ ,
I
I
200
I
I
I
400
600
800
Temperature
Fig. 1. TG
I
1000
(“C)
and DTA curves for PMTBAEM. nitrogen, lO”C/min. - - - - TG, -- DTA.
Dynamic
Thermal degradation of poly-N-methyl-N-tertbutyl-aminoethyl
methacrylate
I
0
IO
Temperature (“C)
Fig. 2. TVA behaviour of PMTBAEM, degraded under continuous evacuation at 10”C/min to 600°C. Key to TVA -750, ---lOO”, curves: o”, . . . . . -450, -.-. - 196°C.
500°C corresponding to the second major weight loss. The TVA behaviour of the polymer is illustrated in Fig. 2, which shows the onset of volatilisation at about 210°C. The pattern of break-down shows main rate maxima at about 250 and 435°C the first agreeing well with the rate maximum in the first weight loss, but the second occurring at somewhat lower temperature than that for the maximum rate for the second major weight loss, which occurs at about 480°C. This difference suggests that the main products in the latter reaction are of low volatility, such as short chain fragments, and are more easily lost under the vacuum conditions of TVA. The TVA curve also indicates that the products at the first major volatilisation are of high volatility (not condensable at -100°C under TVA conditions) whereas those at the second cover a range of non-condensable including some volatilities gases. The latter behaviour is consistent with extensive backbone fragmentation. Gravimetric data were obtained for the main product fractions in degradation to 600°C by TVA. The residue, which was insoluble in common solvents, was 6% (less than in TG because of easier volatilisation of chain fragments under TVA conditions), cold ring fraction amounted to 45% and the liquid fraction of volatile products was 33%, the remaining 16% of the original sample weight being due to condensable and non-condensable gases. 3.2 Characterisation of volatile products The non-condensable materials produced during TVA were identified by on-line MS as methane and carbon monoxide. The SATVA trace for warm up from -196 to 0°C of the condensable
I
30 20 Time (min)
!
40
I
50
Fig. 3. SATVA curve for separation by controlled warm up from -196°C of condensable volatile products of degradation to 600°C at lO”C/min of PMTBAEM.
volatile products is shown in Fig. 3. The gas fractions corresponding to peaks 1 and 2 were collected separately and found by IR spectroscopy and MS to consist of carbon dioxide and a trace of dimethylketene at peak 1 and isobutene at peak 2. The SATVA trace clearly indicates the presence of a further component at peak 2: this could not be unambiguously identified but is probably a nitrogen-containing molecule of similar volatility to isobutene. 3.3 GC-MS investigation of liquid products The liquid fraction corresponding to peak 3 of the SATVA separation was examined by GC-MS. The main component was found to be tertbutylaziridine. Other materials found to be present in much smaller amounts included methanol and MTBAEM monomer. 3.4 Analysis of cold ring fraction The extensive cold ring fraction (45% of the initial sample weight) collected in the degradation by TVA was pale yellow in colour. The IR spectrum suggested the presence of oligomers of MTBAEM, while multiple absorptions in the region 1800-1840cm-’ could indicate the presence of some anhydride structures.
4 DISCUSSION degradation behaviour of The thermal PMTBAEM has not previously been reported. The thermal degradation of PMMA has been extensively studied,20,2’ and it is very well documented that PMMA degrades to monomer in almost quantitative yield. The degradation of the polymer of tertbutyl aziridine has also been
92
S. Zulfiqar
reported.22 The major proportion of the degradation products of PTBA consists of TBA oligomers, some monomer-related products were present in the liquid fraction, and various gaseous products were present in small amounts, of which isobutene and isobutane were the most abundant. In the present investigation, a variety of degradation products covering a range of volatilities have been observed. It is clear from the nature of these that as well as backbone scission, scission in the side group makes a major contribution to the degradation mechanism. Although the onset of degradation in PMTBAEM is at a lower temperature than that of block and graft copolymers of TBA and MMA, the rate of weight loss is initially very low. A further difference between PMTBAEM and these copolymers is that the former is still degrading to volatile products up to around 5Oo”C, whereas the degradation of the copolymers is complete by 450°C. It appears that the product of the first stage of degradation of PMTBAEM is relatively stable, only undergoing breakdown when the temperature reaches nearly 400°C. Since PMTBAEM is a modified methacrylate ester homopolymer, some unzipping to monomer could be considered as a possible degradation route, as with some other methacrylate ester polymers. Side group reactions in such polymers, if they can take place, may take precedence over the unzipping to monomer and, even if they occur only to a limited extent, have been found to interfere with the unzipping process by creating modifed chain units which do not readily split out the corresponding monomeric species.2” Monomer is indeed found as a liquid product in the degradation of PMTBAEM, but its very limited amount implies that decomposition in the side groups is much more important. The high volatility of the products released below 360°C as seen in Fig. 2, and the formation of products recognisable as originating from fragments of the side group, confirm this view. The major proportion of cold ring fraction products consisting of modified MTBAEM oligomers, including some anhydride structures, is consistent with fragmentation at higher temperatures of chains initially stabilised by modified structures left from side group scission in the lower temperature region, which effectively form a copolymer. Anhydride ring structures have been found in
et al. many cases in the degradation of MMA copolymers.*” There are various routes for their formation, including dehydration of adjacent methacrylic acid (MAA) units, whether present initially or formed by a low temperature side group ester decomposition, or interaction between adjacent ester and acid side groups, as in MMA-MAA copolymers (with elimination of methanol).25 Such MAA units are among the structures which could be formed from PMTBAEM, and would provide precursors for the observed anhydride structures in the cold ring products. The TVA data suggest that in the degradation below about 350°C the products reaching the cold traps are very volatile materials, and isobutene has been identified as predominant. Since the weight loss up to this point is over 30%, and most of the tertbutyl groups in the side groups are still available, the isobutene is not itself able to account for the weight loss observed. The possible formation of another very volatile product is considered below, but it appears that most of the weight loss at the first stage must be due to low-volatility tar fraction products, resulting from some chain scission to give oligomer size fragments. Any other route would require other by-products which would be less volatile than isobutene. Side group decomposition can occur in various methacrylate ester polymers: for it to take place, H-abstraction from the carbon in the beta position in the ester alkyl group must be possible. An analogous decomposition would be possible in the MTBAEM side group as indicated in Scheme 2. This would give a methacrylic acid unit in the chain and a volatile product (I) which is, however, of much lower volatility than isobutene and is in any case not observed at all in the total volatile degradation products over the full temperature range. It appears that the initial eneamine (I) readily splits to isobutene and a smaller eneamine (II) or the isomeric imine (III). These N-containing species are of the same volatility as isobutene and are not excluded by the observed IR spectrum of the volatile product fraction 2, in the SATVA product separation of Fig. 2, although positive identification is not possible. The reactions taking place above reach a maximum rate at a little over 300°C and then begin to subside. The reason for this is not clear. It may be that the powerful nucleophiles become
Thermal degradation of poly-N-methyl-N-tertbutyf-aminoethyl
methacrylate
93
CH3 T3 -
CH2--C
-
-
CH2--C
-
I Ho I
H,
w
k-
0
+
/CR
CH I CH3- N CH3-
A
CH2
CC I C%--- N I
CH3
bH3
CH3- :: -cH3 CH3
CH3,
CH3-NH-CH= C=CH2
+
CH3’
CH;?
or CH3-N=CH-CH3
Scheme 2.
retained by the polymer matrix, or that crosslinking slows down release of volatile material, until the temperature becomes high enough to break other bonds. The degradation taking place at above 360°C is clearly different in character: a range of products with very different volatilities is present, including some non-condensable gases. At this stage, further side group scission by a different route appears to occur, giving TBA as a significant product, and there is also extensive fragmentation of the modified polymer backbone. Carbon dioxide and carbon monoxide are formed and traces of dimethylketene and methane are
CH3
CH3
CH2=C
CH2=C I
0 CH2
.
cG2 I CH3-- N
HC\OCH3
REFERENCES
t CHz-CH2 ’ N’
I
CH37-cH3
CH3- 7 -cH3
CH3
CH3
I-tertbu&zztidine
Scheme 3.
observed: these are commonly found at such temperatures from polymer systems with MAA anhydride rings. Small groups or derived amounts of methanol are also evolved. The new side group decomposition process appears to involve C-C rather than C-O scission, as shown in Scheme 3, giving TBA and a MMA unit in the backbone. The latter probably interacts with an available MAA unit from the previous decomposition stage to give methanol and an anhydride ring.25 The degradation behaviour of PMTBAEM is therefore very different from that of PTBA. The MTBAEM monomer unit is closely related in structure to the branch point in PMMA grafted with PTBA. The results of a study of the thermal degradation behaviour of this graft copolymer will be reported subsequently.
(TBA)
1. Tsuruta, T., in Polymeric Amines and Ammonium Salts, ed. E. J. Goethals. Pergamon Press, Oxford, 1980. 2. Hudeck, S., Spevasec, J., Hudeckova, I. & Nikesova, J., Polym. Bull., 3 (1980) 143. 3. Milkovich, R., Polymer Preprints, 21 (1980) 40. 4. Milkovich, R. & Chiang, M. T., US Pat. 3786116 (1974). 5. Milkovich, R. & Chiang, M. T., US Pat. 3862267 (1975). 6. Milkovich, R. & Chiang, M. T., US Pat. 4085168 (1978). 7. Vargas, S. J., Zilliox, J. G., Rempp, P. & Franta, E., Polym. Bull., 3 (1980) 83.
94 8. Munir,
A. & Daly, W. H., Macromol.
Chem.
S. Zuffiqar
et al.
Rapid
18. McNeil],
Commun., 4 (1983) 589. 9. Zafar-uz-Zaman,
10. 11. 12. 13. 14. 15. 16. 17.
M., Munir, A., Zulfiqar, M., Zulfiqar, S. & Farhat, S., J. Chem. Sot. Pak., 14 (1992) 230. Zafar-ur-Zarman, M., Zulfiqar, S. & Munir, A., submitted for publication. Zulliqar. S., Zafar-ur-Zarman, M., Munir, A. & McNeil], I. C., Polym. Degrad. Stab., 50 (1995) 33. Munir, A. & Goethals, E. J., J. Polym. Sci. Polym. Chem. Ed., 19 (1981) 1985. Zulfiqar, S., Zafar-uz-Zaman, M., Munir, A. & McNeil], I. C., Polym. Degrad. Stab., in press. Tam, M. & Ozgun, H. B., Eur. Polym. J., 26 (1990) 5. McNeil], I. C. & Stevenson, W. T. K., Polym. Degrad. Stab., 10 (1985) 247. McNeill, I. C., Eur. Polym. J., 3 (1967) 409. McNeil], 1. C., Eur. Polym. J., 6 (1970) 373.
C., in Developments in Polymer Vol l., ed. N. Grassie. Elsevier Applied Science, London, 1977, p. 43. McNeil], I. C., Ackerman, L., Gupta, S. N., Zulfiqar, M. & Zulfiqar, S., J. Polym. Sci. Polym. Chem. Ed., 15 (1977) 2381. Grassie, N. & Melville, H. W., Proc. R. Sot. (Land.), A199 (1949) 1. McNeil], I. C., Eur. Polym. J., 4 (1968) 21. Zulfiqar, S., Zafar-uz-Zaman, M., Munir, A. & McNeil], K., Polym. Degrad. Stab., 47 (1995) 59. McNeil], 1. C., in Comprehensive Polymer Science, Vol. 6, Chapter 15, ed. G. Eastmond, A. Ledwith, S. Russo & R. Sigwalt. Pergamon Press, London, 1989. McNeil], I. C., Int. J. Polymeric Mater., 214 (1993) 31. Jamieson, A. & McNeil]. I. C., Eur. Polym. J., 10 (1974) 217. I.
Degradation,
19.
20. 21. 22. 23.
24. 25.