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Direct pyrolysis mass spectrometry of diphenyl methyl allophanate
Polymer Degradation and Stability 18 (1987) 341-348
Direct Pyrolysis Mass Spectrometry of Diphenyl Methyl Allophanate Norimichi Yoshitake Department of Industrial Chemistry, Ariake National College of Technology, 150 Higashi Hagio-machi, Omuta, Fukuoka 836, Japan
Mutsuhisa Furukawa & Tetsuo Yokoyama Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852, Japan (Received 2 February 1987; accepted 20 February 1987)
A BSTRA CT The thermal decomposition mechanism of cc,y-diphenyl methyl allophanate as a model compound for the crosslinking site in polyurethane networks was investigated by direct pyrolysis in the mass spectrometer. The primary degradation reactions were dissociation of allophanate into isocyanate and urethane followed by dissociation of the urethane produced into isocyanate and alcohol. However, decarboxylation of the urethane fragment also took place slowly.
INTRODUCTION Thermal decomposition of polyurethanes has generally been investigated by pyrolysis gas chromatography, DSC, DTA, and TGA, etc. 1-6 However, thermal decomposition of the allophanate linkage, which is one of the crosslink sites in polyurethane networks, has not been extensively reported. Recently, several papers have been published which include thermal decomposition of polyurethanes by direct pyrolysis mass spectrometry, v'8 but nothing has been published about the thermal degradation of allophanates using this method. 341 Polymer Degradation and Stability 0141-3910/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
Norimichi Yoshitake, Mutsuhisa Furukawa, Tetsuo Yokoyama
342
H
O O II II --N--C--N--C--O-'
H O O /~"x I II II ~-N--C--N--C--O--CH
I
3
0I) The purpose of this paper is to elucidate the thermal decomposition mechanism of the allophanate linkage (I) by direct pyrolysis mass spectrometry of ~,7-diphenyl methyl allophanate (II) as a model compound. EXPERIMENTAL Methyl carbanilate (PMC) was prepared from phenyl isocyanate (PI) and absolute methanol. ~,7-Diphenyl methyl allophanate (DPMA) was prepared by the reaction of phenyl isocyanate with absolute methanol by Kogon's method. 9 The purity of these compounds was verified by elemental analyses, ir, and 13C-NMR spectra. ~° A double-focusing mass spectrometer EMD-05A (Electronic Science Co. Ltd, Japan) equipped with a direct insertion probe MD-D61 was used to obtain mass spectra. In this spectrometer the ion source was an electron impact type, in which the electron energy was 70 eV. A small amount of sample was placed in the direct insertion quartz probe. The probe was then introduced into the ionization chamber, preheated at 150°C, and was instantaneously heated to the degradation temperature which was 150, 250, or 350°C. RESULTS A N D DISCUSSION Figure 1 shows the direct pyrolysis mass spectra of DPMA at degradation temperatures of 150, 250, and 350°C. Figure 2 shows the direct pyrolysis mass spectra of PMC and PI, raw materials and the products of thermal dissociation of DPMA, at a degradation temperature of 250°C. In the mass spectrum of PMC the peak at m/z 151 was the base peak and was assigned to the molecular ion. It is generally accepted that carbamates degrade thermally according to the following scheme. 1' 11 HO R - - NIll CO--R
I
~ R--N=C=O + R--OH
(1)
~ R - - N H 2 + CO2 + Olefin products R - - N H - - R + CO2
(2) (3)
~
From these mechanisms, the peak at m/z 119 was assigned to phenyl isocyanate ion produced by thermal dissociation of PMC. The existence of
Direct Pyr-MS of diphenyl methyl allophanate
[
100
(A)
.-~
S •- "
119 3944 65 17 I, ,,,L ,
O-
0
I
(B)
50[
N
•--:~ VO 100
,
150 119
270 b
300
151
44 65 91106
,,ri,
o ,
V
11151
1
.
343
.
270 ii
l
150 119
(C)
44 65 77 91 106 : ~ i l v , l L, [, [
300
151
150
270 I.
300
mlz Fig. 1.
The direct pyrolysis mass spectra of ~,7-Diphenyl methyl allophanate (DPMA) Degradation temperature (A) 150°C. (B) 250°C, (C) 350°C.
peaks at m/z 106, 92 and 59 indicated thermal degradation by the decarboxylation of PMC.I'I 1.12 In the mass spectrum of PI, the peak at m/z 119 was the molecular ion peak. The peak at m/z 91 assigned to a fragment of PI was the most intense peak. Other peaks were assigned to fragments of the aromatic ring. The relative intensities and assignments of each peak in the pyrolysis mass spectra of D P M A are shown in Table 1. In the mass spectrum of DPMA, the peak at m/z 270 was the molecular ion peak. The peaks at m/z 151 and 119
(A) 65
>' 50 I'-
151
106 119
I-" I--I
G, 1 0 0 [
._> ,l,,a
CB)
64
]91
150
300
1 ~iO
300
119
mlz
Fig. 2. The direct pyrolysis mass spectra of methyl carbanilate (A) and phenyl isocyanate (B).
344
Norimichi Yoshitake, Mutsuhisa Furukawa, Tetsuo Yokoyama
TABLE 1 Assignment of Fragment Peaks in the Direct Pyrolysis Mass Spectra of D P M A at 150°C
Fragment m/z 151 119 106 91 65 92 270 120 77 39 59 51
Assignment
Relative intensity 100"0 56"1 36'9 20"4 17.2 14'6 14"0 10.8 10"8 9"6 8"3 7"0
C6H s- N H C O O - - C H 3"+ C6H 5--N~---C=O .+ C6Hs--NH~-CH2 +. or C 6 H s - - N - - C H 3 +C6Hs--N + Fragment of aromatic ring C6H 5 - - N H .+ C6H 5 - - H N C O - - N ( C 6 H s ) - - C O O - - C H 3 .+ C 6 H s - - N H C O +. Fragment of aromatic ring Fragment of aromatic ring COO--CH3 -+ Fragment of aromatic ring
TD ~ TD TD EB or TD EB EB (Parent peak) EB EB EB EB or TD EB
a TD, thermal degradation; EB, electron bombardment.
were assigned to methyl carbanilate and phenyl isocyanate which were produced by the thermal dissociation of DPMA.
~..
HO O I II II + NC--NCO--CH3"
~
*
~___~NCO--CH3"
(4)
+ @N-----C=0+"
The peak at m/z 119 was also formed by dissociation of the PMC ion according to eqn (1). Those at m/z 106 and 92 were assigned to fragments to which methyl carbanilate degraded by decarboxylation, while those at m/z 120 and 91 were assigned to the fragments produced by electron bombardment of PMC and PI ions. The peaks at m/z 77, 65 and 39 were due to fragments produced by electron bombardment of aromatic ring ions. From these mass spectra of D P M A at each degradation temperature, three peaks with strong intensity (m/z 151, 119, and 106) and the molecular ion peak (m/z 270) were selected to investigate the thermal degradation mechanism. The time dependences of the relative intensities of these peaks at each degradation temperature are shown in Figs 3-5. In these Figures, the ordinate indicates the relative intensity, which is the ratio of the peak
Direct Pyr-MS of diphenyl methyl allophanate
E oo
345
/•151
-~ 50ils
/'~
._> •
7o
O0
I
2 Time (rain)
3
Fig. 3. Time dependence of fragment peak intensity from DPMA decomposed at 150°C.
intensity at the measurement time to the maximum peak intensity at each degradation temperature. The abscissa indicates the time after insertion of the sample probe into the ionization chamber. Figure 3 shows that, on degradation of D P M A at 150°C, the P M C ion fragment peak at m/z 151 was the most intense. It was sharp with a maximum at 2 min. The peak due to the PI ion fragment at m/z 119 was broader with a maximum at 1 min, 50 s. The intensity of this peak was stronger initially than that of the PMC ion. The D P M A molecular ion peaked at 2 min while the secondary amine ion fragment at m/z 106 was a broad and weak peak with a maximum at 2 min. Figure 4 shows that, on degradation of D P M A at 250°C, all fragment ions peaked at 1 min. Fragments at m/z 151 and 119 appeared at 15-25 s and disappeared at about 1 min, 30 s. However, fragment m/z 106 appeared at 30-40s and disappeared at about 1 min, 50s. The spectrum of fragment m/z 106 was unsymmetrical and tailed off at the long time side. Figure 5 shows that, on degradation of D P M A at 350°C, all fragment ion peaks were broad with maxima at 1 min. The ions appeared more quickly at 250°C.
~I00
c 50 ._>
00
I
2
Time (rain)
Fig. 4. Time dependence of fragment peak intensity from DPMA decomposed at 250°C.
346
Norimichi Yoshitake, Mutsuhisa Furukawa, Tetsuo Yokoyama
./,, 0
1
2 Time(rain)
3
Fig. 5, Time dependence of fragment peak intensity from DPMA decomposed at 350°C.
Figure 6 shows the relationship between the maximum relative intensity of each ion and the degradation temperature. The PMC ion peak was a base peak at degradation temperatures of 150-350°C. The relative intensity of the PI ion (m/z 119) increased with increasing degradation temperature. However, the relative intensities of the fragment ions at m/z 106 and 270 remained constant over the temperature range used. These results lead to the degradation mechanism of DPMA represented by the following scheme: O II ~--~N__C__N--C--O--CH ~-.P' I I II
3
•
H Oo
~,
H I
/='x
~.-~-o-~
H~ + ~ N - - - - ~
---0
~ / / ~ ~ ~/___Hi ~"'~ / N--q=o H"-'~~'sC'H~ ~ I~ \ ~' ' f f '\~× \O X /
N--c~o. ~-~.J),~_2 ~
/
H
~
-
.=c=o +
~
N - - C H 3"+
First, allophanate(DPMA) forms carbamate(PMC) and isocyanate(PI) by
Direct Pyr-MS of diphenyl methyl allophanate
,-.,100
~
~
151
~
~
>,
347
I19
(m/z)
so
.~_
••-•i06
®
er 0
I
I
270 !
150 250 350 Temperature(°c)
Fig. 6. Temperature dependence of the relative intensity at the peak maximum of each
fragment in Figs 2, 3 and 4. thermal dissociation. Then, the large amounts of carbamate produced also dissociate to form isocyanate(PI) and alcohol. However, a small a m o u n t of the carbamate forms secondary amine derivative by decarboxylation.
CONCLUSIONS F r o m direct pyrolysis mass spectrometry of diphenyl methyl allophanate, it was concluded that decomposition occurred in two steps. In the first step, allophanate is dissociated thermally to methyl carbanilate and phenyl isocyanate. In the second step, the carbanilate was also dissociated to phenyl isocyanate and methanol. A small a m o u n t of the carbanilate formed secondary amine derivatives by decarboxylation. The thermal degradation of other allophanates will be described in other papers in the near future.
REFERENCES 1. H. H. G. Jellinek and S. R. Dunkie, J. Polym. Chem., Polym. Chem., Ed., 21,487 (1983). 2. F. Gaboriaud and P. Vantelon, J. Polym. Chem., Polym. Chem. Ed., 20, 2063 (1978). 3. K.J. Voorhees, F. D. Hileman, I. N. Einhorn and J. H. Futrell, J. Polym. Sci., Polym. Chem. Ed., 16, 213 (1982). 4. A. Ballistreri, S. Foti, P. Maravigna, G. Montaudo and E. Scamporrino, J. Polym. Sci., Polym. Chem. Ed., 18, 1923 (1980). 5. J. E. Williamson, M. J. Cocksedge and N. Evans, J. Analytical and Applied Pyrolysis, 2, 195 (1980). 6. D. Joel, R. Kruger and R. Gnauk, Plaste und Kautsch, 27, 34 (1984). 7. D. Joel, R. Gnauk and P. Fritsch, Plaste und Kautsch, 27, 46 (1984).
348
Norimichi Yoshitake, Mutsuhisa Furukawa, Tetsuo Yokoyama
8. S. Foti, A. Liguori, P. Maravigana and G. Montaudo, Anal. Chem., 54, 671 (1982). 9. I. C. Kogon, J. Org. Chem., 24, 438 (1959). 10. M. Furukawa, M. Yatake and T. Yokoyama, Report of The Faculty of Engineering, Nagasaki University, 15, 105 (1985). 11. C. Wiinsche, Org. Mass Spectrom., 7, 1253 (1973). 12. C. P. Lewis, Anal. Chem., 36, 176 (1964).
Direct Pyrolysis Mass Spectrometry of Diphenyl Methyl Allophanate Norimichi Yoshitake Department of Industrial Chemistry, Ariake National College of Technology, 150 Higashi Hagio-machi, Omuta, Fukuoka 836, Japan
Mutsuhisa Furukawa & Tetsuo Yokoyama Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852, Japan (Received 2 February 1987; accepted 20 February 1987)
A BSTRA CT The thermal decomposition mechanism of cc,y-diphenyl methyl allophanate as a model compound for the crosslinking site in polyurethane networks was investigated by direct pyrolysis in the mass spectrometer. The primary degradation reactions were dissociation of allophanate into isocyanate and urethane followed by dissociation of the urethane produced into isocyanate and alcohol. However, decarboxylation of the urethane fragment also took place slowly.
INTRODUCTION Thermal decomposition of polyurethanes has generally been investigated by pyrolysis gas chromatography, DSC, DTA, and TGA, etc. 1-6 However, thermal decomposition of the allophanate linkage, which is one of the crosslink sites in polyurethane networks, has not been extensively reported. Recently, several papers have been published which include thermal decomposition of polyurethanes by direct pyrolysis mass spectrometry, v'8 but nothing has been published about the thermal degradation of allophanates using this method. 341 Polymer Degradation and Stability 0141-3910/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
Norimichi Yoshitake, Mutsuhisa Furukawa, Tetsuo Yokoyama
342
H
O O II II --N--C--N--C--O-'
H O O /~"x I II II ~-N--C--N--C--O--CH
I
3
0I) The purpose of this paper is to elucidate the thermal decomposition mechanism of the allophanate linkage (I) by direct pyrolysis mass spectrometry of ~,7-diphenyl methyl allophanate (II) as a model compound. EXPERIMENTAL Methyl carbanilate (PMC) was prepared from phenyl isocyanate (PI) and absolute methanol. ~,7-Diphenyl methyl allophanate (DPMA) was prepared by the reaction of phenyl isocyanate with absolute methanol by Kogon's method. 9 The purity of these compounds was verified by elemental analyses, ir, and 13C-NMR spectra. ~° A double-focusing mass spectrometer EMD-05A (Electronic Science Co. Ltd, Japan) equipped with a direct insertion probe MD-D61 was used to obtain mass spectra. In this spectrometer the ion source was an electron impact type, in which the electron energy was 70 eV. A small amount of sample was placed in the direct insertion quartz probe. The probe was then introduced into the ionization chamber, preheated at 150°C, and was instantaneously heated to the degradation temperature which was 150, 250, or 350°C. RESULTS A N D DISCUSSION Figure 1 shows the direct pyrolysis mass spectra of DPMA at degradation temperatures of 150, 250, and 350°C. Figure 2 shows the direct pyrolysis mass spectra of PMC and PI, raw materials and the products of thermal dissociation of DPMA, at a degradation temperature of 250°C. In the mass spectrum of PMC the peak at m/z 151 was the base peak and was assigned to the molecular ion. It is generally accepted that carbamates degrade thermally according to the following scheme. 1' 11 HO R - - NIll CO--R
I
~ R--N=C=O + R--OH
(1)
~ R - - N H 2 + CO2 + Olefin products R - - N H - - R + CO2
(2) (3)
~
From these mechanisms, the peak at m/z 119 was assigned to phenyl isocyanate ion produced by thermal dissociation of PMC. The existence of
Direct Pyr-MS of diphenyl methyl allophanate
[
100
(A)
.-~
S •- "
119 3944 65 17 I, ,,,L ,
O-
0
I
(B)
50[
N
•--:~ VO 100
,
150 119
270 b
300
151
44 65 91106
,,ri,
o ,
V
11151
1
.
343
.
270 ii
l
150 119
(C)
44 65 77 91 106 : ~ i l v , l L, [, [
300
151
150
270 I.
300
mlz Fig. 1.
The direct pyrolysis mass spectra of ~,7-Diphenyl methyl allophanate (DPMA) Degradation temperature (A) 150°C. (B) 250°C, (C) 350°C.
peaks at m/z 106, 92 and 59 indicated thermal degradation by the decarboxylation of PMC.I'I 1.12 In the mass spectrum of PI, the peak at m/z 119 was the molecular ion peak. The peak at m/z 91 assigned to a fragment of PI was the most intense peak. Other peaks were assigned to fragments of the aromatic ring. The relative intensities and assignments of each peak in the pyrolysis mass spectra of D P M A are shown in Table 1. In the mass spectrum of DPMA, the peak at m/z 270 was the molecular ion peak. The peaks at m/z 151 and 119
(A) 65
>' 50 I'-
151
106 119
I-" I--I
G, 1 0 0 [
._> ,l,,a
CB)
64
]91
150
300
1 ~iO
300
119
mlz
Fig. 2. The direct pyrolysis mass spectra of methyl carbanilate (A) and phenyl isocyanate (B).
344
Norimichi Yoshitake, Mutsuhisa Furukawa, Tetsuo Yokoyama
TABLE 1 Assignment of Fragment Peaks in the Direct Pyrolysis Mass Spectra of D P M A at 150°C
Fragment m/z 151 119 106 91 65 92 270 120 77 39 59 51
Assignment
Relative intensity 100"0 56"1 36'9 20"4 17.2 14'6 14"0 10.8 10"8 9"6 8"3 7"0
C6H s- N H C O O - - C H 3"+ C6H 5--N~---C=O .+ C6Hs--NH~-CH2 +. or C 6 H s - - N - - C H 3 +C6Hs--N + Fragment of aromatic ring C6H 5 - - N H .+ C6H 5 - - H N C O - - N ( C 6 H s ) - - C O O - - C H 3 .+ C 6 H s - - N H C O +. Fragment of aromatic ring Fragment of aromatic ring COO--CH3 -+ Fragment of aromatic ring
TD ~ TD TD EB or TD EB EB (Parent peak) EB EB EB EB or TD EB
a TD, thermal degradation; EB, electron bombardment.
were assigned to methyl carbanilate and phenyl isocyanate which were produced by the thermal dissociation of DPMA.
~..
HO O I II II + NC--NCO--CH3"
~
*
~___~NCO--CH3"
(4)
+ @N-----C=0+"
The peak at m/z 119 was also formed by dissociation of the PMC ion according to eqn (1). Those at m/z 106 and 92 were assigned to fragments to which methyl carbanilate degraded by decarboxylation, while those at m/z 120 and 91 were assigned to the fragments produced by electron bombardment of PMC and PI ions. The peaks at m/z 77, 65 and 39 were due to fragments produced by electron bombardment of aromatic ring ions. From these mass spectra of D P M A at each degradation temperature, three peaks with strong intensity (m/z 151, 119, and 106) and the molecular ion peak (m/z 270) were selected to investigate the thermal degradation mechanism. The time dependences of the relative intensities of these peaks at each degradation temperature are shown in Figs 3-5. In these Figures, the ordinate indicates the relative intensity, which is the ratio of the peak
Direct Pyr-MS of diphenyl methyl allophanate
E oo
345
/•151
-~ 50ils
/'~
._> •
7o
O0
I
2 Time (rain)
3
Fig. 3. Time dependence of fragment peak intensity from DPMA decomposed at 150°C.
intensity at the measurement time to the maximum peak intensity at each degradation temperature. The abscissa indicates the time after insertion of the sample probe into the ionization chamber. Figure 3 shows that, on degradation of D P M A at 150°C, the P M C ion fragment peak at m/z 151 was the most intense. It was sharp with a maximum at 2 min. The peak due to the PI ion fragment at m/z 119 was broader with a maximum at 1 min, 50 s. The intensity of this peak was stronger initially than that of the PMC ion. The D P M A molecular ion peaked at 2 min while the secondary amine ion fragment at m/z 106 was a broad and weak peak with a maximum at 2 min. Figure 4 shows that, on degradation of D P M A at 250°C, all fragment ions peaked at 1 min. Fragments at m/z 151 and 119 appeared at 15-25 s and disappeared at about 1 min, 30 s. However, fragment m/z 106 appeared at 30-40s and disappeared at about 1 min, 50s. The spectrum of fragment m/z 106 was unsymmetrical and tailed off at the long time side. Figure 5 shows that, on degradation of D P M A at 350°C, all fragment ion peaks were broad with maxima at 1 min. The ions appeared more quickly at 250°C.
~I00
c 50 ._>
00
I
2
Time (rain)
Fig. 4. Time dependence of fragment peak intensity from DPMA decomposed at 250°C.
346
Norimichi Yoshitake, Mutsuhisa Furukawa, Tetsuo Yokoyama
./,, 0
1
2 Time(rain)
3
Fig. 5, Time dependence of fragment peak intensity from DPMA decomposed at 350°C.
Figure 6 shows the relationship between the maximum relative intensity of each ion and the degradation temperature. The PMC ion peak was a base peak at degradation temperatures of 150-350°C. The relative intensity of the PI ion (m/z 119) increased with increasing degradation temperature. However, the relative intensities of the fragment ions at m/z 106 and 270 remained constant over the temperature range used. These results lead to the degradation mechanism of DPMA represented by the following scheme: O II ~--~N__C__N--C--O--CH ~-.P' I I II
3
•
H Oo
~,
H I
/='x
~.-~-o-~
H~ + ~ N - - - - ~
---0
~ / / ~ ~ ~/___Hi ~"'~ / N--q=o H"-'~~'sC'H~ ~ I~ \ ~' ' f f '\~× \O X /
N--c~o. ~-~.J),~_2 ~
/
H
~
-
.=c=o +
~
N - - C H 3"+
First, allophanate(DPMA) forms carbamate(PMC) and isocyanate(PI) by
Direct Pyr-MS of diphenyl methyl allophanate
,-.,100
~
~
151
~
~
>,
347
I19
(m/z)
so
.~_
••-•i06
®
er 0
I
I
270 !
150 250 350 Temperature(°c)
Fig. 6. Temperature dependence of the relative intensity at the peak maximum of each
fragment in Figs 2, 3 and 4. thermal dissociation. Then, the large amounts of carbamate produced also dissociate to form isocyanate(PI) and alcohol. However, a small a m o u n t of the carbamate forms secondary amine derivative by decarboxylation.
CONCLUSIONS F r o m direct pyrolysis mass spectrometry of diphenyl methyl allophanate, it was concluded that decomposition occurred in two steps. In the first step, allophanate is dissociated thermally to methyl carbanilate and phenyl isocyanate. In the second step, the carbanilate was also dissociated to phenyl isocyanate and methanol. A small a m o u n t of the carbanilate formed secondary amine derivatives by decarboxylation. The thermal degradation of other allophanates will be described in other papers in the near future.
REFERENCES 1. H. H. G. Jellinek and S. R. Dunkie, J. Polym. Chem., Polym. Chem., Ed., 21,487 (1983). 2. F. Gaboriaud and P. Vantelon, J. Polym. Chem., Polym. Chem. Ed., 20, 2063 (1978). 3. K.J. Voorhees, F. D. Hileman, I. N. Einhorn and J. H. Futrell, J. Polym. Sci., Polym. Chem. Ed., 16, 213 (1982). 4. A. Ballistreri, S. Foti, P. Maravigna, G. Montaudo and E. Scamporrino, J. Polym. Sci., Polym. Chem. Ed., 18, 1923 (1980). 5. J. E. Williamson, M. J. Cocksedge and N. Evans, J. Analytical and Applied Pyrolysis, 2, 195 (1980). 6. D. Joel, R. Kruger and R. Gnauk, Plaste und Kautsch, 27, 34 (1984). 7. D. Joel, R. Gnauk and P. Fritsch, Plaste und Kautsch, 27, 46 (1984).
348
Norimichi Yoshitake, Mutsuhisa Furukawa, Tetsuo Yokoyama
8. S. Foti, A. Liguori, P. Maravigana and G. Montaudo, Anal. Chem., 54, 671 (1982). 9. I. C. Kogon, J. Org. Chem., 24, 438 (1959). 10. M. Furukawa, M. Yatake and T. Yokoyama, Report of The Faculty of Engineering, Nagasaki University, 15, 105 (1985). 11. C. Wiinsche, Org. Mass Spectrom., 7, 1253 (1973). 12. C. P. Lewis, Anal. Chem., 36, 176 (1964).