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A study of the mechanism of action of aromatic thiols on carotenoid pigments
Polymer Degradation and Stability 19 (1987) 213-219
A Study of the Mechanism of Action of Aromatic Thiols on Carotenoid Pigments L. M. K. Tillekeratne," L. M. V. Tillekeratne b & P. A. D. T. Vimalasiri" Rubber Research Institute, Dartonfield, Agalawatta, Sri Lanka bUniversity of Colombo, Colombo 3, Sri Lanka (Received 29 April 1987; accepted 12 May 1987)
ABSTRACT Aromatic thiols are used widely to bleach carotenoid plant pigments present in natural rubber latex in trace quantities, in the manufacture o f white latex crepe rubber. I f this is not done these pigments impart a yellow colour to the crepe, thereby lowering its grade. The bleaching reaction between carotenoid pigments and aromatic thiols is known to occur in the presence of at least d~ffuse daylight. In this mechanistic stud), it was observed that the aromatic thiols have no action on unoxidised carotene. However, the pero.~v radicals .[brined on the carotene molecule undergoing autoxidation under the influence o f uv light initiate radical formation on the thiol, which is difficult to split by other means. These radicals then combine with the unsaturated chains of both carotene and cartenoids in the system, thereby disrupting the double bond con/ugation and making the compounds colourless.
INTRODUCTION Plant pigments such as carotene and oxidised forms of carotene of the xanthophyl type are bleached by aromatic thiols I (mercaptans). Aromatic thiols are widely used in the rubber industry to bleach carotenoid pigments present in natural rubber latex in the manufacture of water white latex crepe rubber, which would otherwise be yellowish in colour due to the presence of these pigments in trace quantities. This bleaching reaction requires at least diffuse daylight and there is n o 213 Polymer Degradation and Stabili O' 0141-3910/87/$03-50 (~ Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
214
L. M. K. Tillekeratne, L. M. V. Tillekeratne, P. A. D. T. Vimalasiri
reaction between the thiol and the carotenoid pigment in the dark. 2 Earlier workers have suggested that 3 in this reaction the carotene molecule undergoes photo-excitation and transfers energy to thiol molecules, thereby forming thiyl radicals which are capable of bleaching carotene. However, it is well known that it is very difficult to excite thiols to higher energy levels 9 by energy transfer and also the total excitation energy of carotene is not sufficient to excite a thiol molecule. Scott 4"s has reported that during the autoxidation of polyconjugated compounds such as carotene, peroxide formation is the predominant step and that this peroxide ultimately breaks down to form active radicals and carbonyl groups along the molecule thereby disrupting unsaturation. Carbonyl groups thus formed could also act as chromophores for the photooxidation of the residual unsaturation. Hence, the aim of this work was to determine whether the carotene molecule is undergoing excitation and transferring energy to the thiol molecule, thereby splitting it into radicals, or whether it is the radical formed on the carotene molecule as a result of the decomposition of the peroxide which is initiating the reaction between thiol and carotene.
EXPERIMENTAL The following sequence of reactions was carried out to throw light on the mechanism of the reaction. .
Pure unoxidised carotene (packed under argon) was dissolved in benzene under a nitrogen blanket and was treated with the thiol radical generating system azobisisobutyronitrile (AZBN) and mercaptobenzothiazole (MBT) in the presence of light. This system is known to produce thiyl radicals 1° of the type
2.
A solution of pure carotene in benzene was treated with a 0"5% w/w solution of p-toluene thiophenol in benzene in the presence of uv light. A solution of pure carotene in benzene was treated with a 0"5% w/w p-toluene thiophenol containing a trace amount of the photoactivator benzophenone, in the presence of uv radition. A solution of fresh carrot juice was treated with a 0-5% w/w emulsion of p-toluene thiophenol in low aromatic white spirit (LAWS) in the presence of uv radiation. The emulsifier was Nexol A.
3.
4.
Mechanism o[ action o/" aromatic thiols on carotenoid pigments
5. 6.
7.
215
A solution of pure carotene in benzene exposed to oxygen for 2 h was treated with p-toluene thiophenol in the presence of uv light. A solution mixture of pure carotene, p-toluene thiophenol, and benzophenone before and after uv irradiation was injected into a H P L C fitted with a Perkin-Elmer LC-85B spectrophotometric detector and a K n a u e r differential refractometer. The flow rate was 1 ml/min, and the wavelength used on the infra-red detector was 254nm. The column was PLgel 10/~m 103 A', 600-7.5 mm. Solutions of pure carotene freshly dissolved in benzene and carotene dissolved in benzene and exposed to air for 2 3 h were scanned on a Fourier Transform Perkin-Elmer model 1710 I R spectrometer using KBr plates. RESULTS
In reaction No. 1, in which pure carotene in benzene was treated with a mixture of A Z B N and M B T in the presence of light, the orange-yellow colour of the carotene disappeared instantly. In reaction No. 2, when pure carotene was treated with p-toluene thiophenol, there was no reaction between the two even in the presence of uv light. But in reaction No. 3, when a small a m o u n t of benzophenone was present in the mixture of carotene and the thiol, the yellow colour disappeared in the presence of uv radiation. Similarly, in reaction No. 4, when carrot juice was used as the source of carotene, and treated with p-toluene thiophenol in the presence of uv light, the yellow colour disappeared quickly when benzophenone was added. The same result was obtained in reaction No. 5, where, instead of carrot juice, an air oxidised sample of carotene in benzene was treated with p-toluene thiophenol in the presence of uv light. Results of the HPLC analyses carried out are reported in Tables 1 and 2. A calibration curve drawn for n-alkanes was used for the calculation of the probable molecular weights of each of the components indicated by the peaks in the spectrum. TABLE
I
Elution Times and the Corresponding Molecular Weights of the Principal Peaks obtained from the Refractive Index Plot of the Unirradiated Mixture of Carotene, p-Toluene Thiophenol and Benzophenone. Elution time (rain)
Average molecular weight (Mn)
32 1 34.5 36.4
536 182 125
Prohahh, compound
Carotene Benzophenone p-Toluene thiol
216
L. M. K. Tillekeratne, L. M. V. Tillekeratne, P. A. D. T. Vimalasiri TABLE 2 Elution Times and the Corresponding Molecular Weights of the Principal Peaks obtained from the Refractive Index Plot of the uv Irradiated Mixture of Carotene, p-Toluene Thiol and Benzophenone.
Elution time (min)
Average molecular weight (Mn)
Probable compound
3t-4
660
34.3 35.55
246 124
Carotene substituted with a molecule of thiol Dimerised thiol Excess thiol
The ir spectra of pure carotene and air oxidised carotene obtained in Experiment 7, are illustrated in Fig. 1. A new band appears in the air oxidised sample in the wavelength region 1660-1680 cm-1 due to the formation of a carbonyl group on the carotene molecule. The intensity of this band increases with the time of exposure of the carotene to air. Another new band was also detected in the region 3500-3600cm -1 which is due to the formation ofhydroperoxide (hindered) and hydroxyl groups on the carotene molecule undergoing oxidation. DISCUSSION The above results clearly indicate that pure carotene does not react with ptoluene thiophenol even in the presence of uv light. Hence the mechanism suggested by earlier workers 3 cannot be accepted as the mechanism of action of thiols on carotene under uv light. Further, the results of Experiment 1 indicate that aromatic thiol radicals of the type resulting from the reaction between A Z B N and MBT are quite capable of reacting with the carotene, thereby bleaching the yellow colour. Results of Experiments 3 and 4 indicate that carotene containing the oxidation products of carotene acts similarly to carotene mixed with a photo-activator. This is in agreement with the explanation given by earlier workers, 4'6"7 that carotene reacts with oxygen in air in the presence of uv light to form peroxy radicals and peroxides. These unstable peroxides decompose under uv light to form alkoxy radicals on the carotene molecule. These alkoxy radicals then undergo Norrish type 2 rearrangement to produce carbonyl groups, disrupting unsaturation of the carotene molecules. This is in agreement with the results obtained in Experiment 7, where there is clear evidence ofcarbonyl formation on carotene subjected to
Mechanism of action of aromatic thiols on carotenoid pigments
100
I
I
I
1
I
i
i
I
I
I
1
i
i
I
217
[
90
80
70
60
o o
v
50
u
40 I---
30
20
10
0
3200 Wave
Fig. 1.
I
2000 number
I
1200 (cm -1)
Infra-red spectra of carotene before and after exposure to air. - - - Before oxidation. . . . . After oxidation.
o x i d a t i o n . T h e a l k o x y radical t h e n reacts with a thiol m o l e c u l e t h e r e b y f o r m i n g a thiyl radical, l o T h e thiyl radical t h u s f o r m e d t h e n acts as i n i t i a t o r f o r the s u b s t i t u t i o n r e a c t i o n b e t w e e n the thiol a n d c a r o t e n e . As a result m o r e a n d m o r e radicals will be f o r m e d a n d the yellow c o l o u r o f c a r o t e n e d i s a p p e a r s , because, o n s u b s t i t u t i o n o f a thiol radical o n the c a r o t e n e , the c o n j u g a t i o n is d i s r u p t e d .
218
L. M. K. Tillekeratne, L. M. V. Tillekeratne, P. A. D. T. Vimalasiri
l
CH 3 ~ . -
-I
Cn 3
/
CH3
CH3
CH3
J2
//-Carotene CH3
CH 3
I --C=CH
I
CH=CH--C
CH-I OO.
I CH 3
CH 3 I I --C=CH--CH--CH~C--CH~
CH 3
CH 3
I --C=CH
I CH--CH=C--CH-I OO.
I O I O I ~-,CH--C=CH--CH--CH=C~ I I [ OO. CH 3 CH 3
~roxide ~rmed
CH 3 CH 3 I I -----C=CH--CH--CH=C--CH~
I O. CH 3
R--SH
+-----CH C = C H ] I OO. CH 3
N ~ ......
I [ - - - , - , C = C H - - C H - - C H = C - - C H - - + R--S" [ (thiyl radical) OH where R - - S H -- H 3 C - - ~ - ~ S H
O
pig....
CH .CH
'F CH 3
CH--CH:C~ f CH 3 Carbonyl compound H3C CH3
CIH CH
CH ~ C H
,-.1, CH 3
+
[]
S
CH3cH
g.....
--CH--C=CH z [ I OO. CH 3
Any Carotenoid
3
i CH3
I CH 3 ~ N(Norrishty~ 2
CH 3
~CH3
O" I CH--CH=C~
CH3
Thiol substituted fl-Carotene ~ H3C-Scheme 1
CH ~ C H " ~
T ,. CH3 -f~ S--S-Disulphide formed
CH3 CH 3
Mechanism of action of aromatic thiols on carotenoid p~gments
219
These results support the results obtained in Experiment 7 according to which there was obvious growth of the absorption band due to the - - O H group when carotene was oxidised. Also, the results of the H P L C experiment clearly indicate that on uv exposure in the presence of a photo-activator, substitution of one thiyl radical on carotene occurs. However, there was no evidence found in the H P L C analysis to show that more than one thiyl group is substituted on a molecule of carotene, as was believed earlier. H P L C work also indicated the possibility of combining two thiyl radicals to form the disulphide. Hence, it could be concluded from those experiments that the mechanism of action of thiols on carotenoid pigments in the presence of uv light is represented by reaction Scheme 1.
ACKNOWLEDGEMENT The authors are indebted to Prof. G. Scott of Aston University, Birmingham, U K , for helping to sort out the mechanism of the reaction of thiol with carotene under ultraviolet radiation, and also for editing this paper.
REFERENCES I. Handbook ~f Rubber Processing and Culture. Rubber Research Institute of Sri
Lanka. Caves Ltd, Colombo (1969). 2. L.M.K. Tillekeratne and W. S. E. Fernando, Sri Lanka Assn. for the Advanc. of Science. Proc. Annual Session, Colombo (1980). 3. W. S. E. Fernando, Annual Report Rubber Res. Inst. Sri Lanka, 65 (1979). 4. G. Scott, Atmospheric oxidation and antioxidants. Elsevier Pub. Co., London, p. 358 (1965). 5. A.M.U. Amin, G. Scott and L. M. K. Tillkeratne, Europ. Polym. J., 11,85 (1975). 6. A. W. Pross and R. M. Black, J. Soc. Chem. Ind., 69, 113 (1950). 7. C. H. Bamford and R. G. W. Norrish, J. Chem. Sot., 1505 (1935). 8. L. M. K. Tillekeratne, Chem. in Sri Lanka, 2 (2) (1985). 9. G. Caspari and A. Granzow, J. Phys. Chem., 74, 836 (1970). 10. W. Hofmann, Vulcanization and Vulcanizing Agents, Maclaren & Sons, London, p. 153 (1967).
A Study of the Mechanism of Action of Aromatic Thiols on Carotenoid Pigments L. M. K. Tillekeratne," L. M. V. Tillekeratne b & P. A. D. T. Vimalasiri" Rubber Research Institute, Dartonfield, Agalawatta, Sri Lanka bUniversity of Colombo, Colombo 3, Sri Lanka (Received 29 April 1987; accepted 12 May 1987)
ABSTRACT Aromatic thiols are used widely to bleach carotenoid plant pigments present in natural rubber latex in trace quantities, in the manufacture o f white latex crepe rubber. I f this is not done these pigments impart a yellow colour to the crepe, thereby lowering its grade. The bleaching reaction between carotenoid pigments and aromatic thiols is known to occur in the presence of at least d~ffuse daylight. In this mechanistic stud), it was observed that the aromatic thiols have no action on unoxidised carotene. However, the pero.~v radicals .[brined on the carotene molecule undergoing autoxidation under the influence o f uv light initiate radical formation on the thiol, which is difficult to split by other means. These radicals then combine with the unsaturated chains of both carotene and cartenoids in the system, thereby disrupting the double bond con/ugation and making the compounds colourless.
INTRODUCTION Plant pigments such as carotene and oxidised forms of carotene of the xanthophyl type are bleached by aromatic thiols I (mercaptans). Aromatic thiols are widely used in the rubber industry to bleach carotenoid pigments present in natural rubber latex in the manufacture of water white latex crepe rubber, which would otherwise be yellowish in colour due to the presence of these pigments in trace quantities. This bleaching reaction requires at least diffuse daylight and there is n o 213 Polymer Degradation and Stabili O' 0141-3910/87/$03-50 (~ Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
214
L. M. K. Tillekeratne, L. M. V. Tillekeratne, P. A. D. T. Vimalasiri
reaction between the thiol and the carotenoid pigment in the dark. 2 Earlier workers have suggested that 3 in this reaction the carotene molecule undergoes photo-excitation and transfers energy to thiol molecules, thereby forming thiyl radicals which are capable of bleaching carotene. However, it is well known that it is very difficult to excite thiols to higher energy levels 9 by energy transfer and also the total excitation energy of carotene is not sufficient to excite a thiol molecule. Scott 4"s has reported that during the autoxidation of polyconjugated compounds such as carotene, peroxide formation is the predominant step and that this peroxide ultimately breaks down to form active radicals and carbonyl groups along the molecule thereby disrupting unsaturation. Carbonyl groups thus formed could also act as chromophores for the photooxidation of the residual unsaturation. Hence, the aim of this work was to determine whether the carotene molecule is undergoing excitation and transferring energy to the thiol molecule, thereby splitting it into radicals, or whether it is the radical formed on the carotene molecule as a result of the decomposition of the peroxide which is initiating the reaction between thiol and carotene.
EXPERIMENTAL The following sequence of reactions was carried out to throw light on the mechanism of the reaction. .
Pure unoxidised carotene (packed under argon) was dissolved in benzene under a nitrogen blanket and was treated with the thiol radical generating system azobisisobutyronitrile (AZBN) and mercaptobenzothiazole (MBT) in the presence of light. This system is known to produce thiyl radicals 1° of the type
2.
A solution of pure carotene in benzene was treated with a 0"5% w/w solution of p-toluene thiophenol in benzene in the presence of uv light. A solution of pure carotene in benzene was treated with a 0"5% w/w p-toluene thiophenol containing a trace amount of the photoactivator benzophenone, in the presence of uv radition. A solution of fresh carrot juice was treated with a 0-5% w/w emulsion of p-toluene thiophenol in low aromatic white spirit (LAWS) in the presence of uv radiation. The emulsifier was Nexol A.
3.
4.
Mechanism o[ action o/" aromatic thiols on carotenoid pigments
5. 6.
7.
215
A solution of pure carotene in benzene exposed to oxygen for 2 h was treated with p-toluene thiophenol in the presence of uv light. A solution mixture of pure carotene, p-toluene thiophenol, and benzophenone before and after uv irradiation was injected into a H P L C fitted with a Perkin-Elmer LC-85B spectrophotometric detector and a K n a u e r differential refractometer. The flow rate was 1 ml/min, and the wavelength used on the infra-red detector was 254nm. The column was PLgel 10/~m 103 A', 600-7.5 mm. Solutions of pure carotene freshly dissolved in benzene and carotene dissolved in benzene and exposed to air for 2 3 h were scanned on a Fourier Transform Perkin-Elmer model 1710 I R spectrometer using KBr plates. RESULTS
In reaction No. 1, in which pure carotene in benzene was treated with a mixture of A Z B N and M B T in the presence of light, the orange-yellow colour of the carotene disappeared instantly. In reaction No. 2, when pure carotene was treated with p-toluene thiophenol, there was no reaction between the two even in the presence of uv light. But in reaction No. 3, when a small a m o u n t of benzophenone was present in the mixture of carotene and the thiol, the yellow colour disappeared in the presence of uv radiation. Similarly, in reaction No. 4, when carrot juice was used as the source of carotene, and treated with p-toluene thiophenol in the presence of uv light, the yellow colour disappeared quickly when benzophenone was added. The same result was obtained in reaction No. 5, where, instead of carrot juice, an air oxidised sample of carotene in benzene was treated with p-toluene thiophenol in the presence of uv light. Results of the HPLC analyses carried out are reported in Tables 1 and 2. A calibration curve drawn for n-alkanes was used for the calculation of the probable molecular weights of each of the components indicated by the peaks in the spectrum. TABLE
I
Elution Times and the Corresponding Molecular Weights of the Principal Peaks obtained from the Refractive Index Plot of the Unirradiated Mixture of Carotene, p-Toluene Thiophenol and Benzophenone. Elution time (rain)
Average molecular weight (Mn)
32 1 34.5 36.4
536 182 125
Prohahh, compound
Carotene Benzophenone p-Toluene thiol
216
L. M. K. Tillekeratne, L. M. V. Tillekeratne, P. A. D. T. Vimalasiri TABLE 2 Elution Times and the Corresponding Molecular Weights of the Principal Peaks obtained from the Refractive Index Plot of the uv Irradiated Mixture of Carotene, p-Toluene Thiol and Benzophenone.
Elution time (min)
Average molecular weight (Mn)
Probable compound
3t-4
660
34.3 35.55
246 124
Carotene substituted with a molecule of thiol Dimerised thiol Excess thiol
The ir spectra of pure carotene and air oxidised carotene obtained in Experiment 7, are illustrated in Fig. 1. A new band appears in the air oxidised sample in the wavelength region 1660-1680 cm-1 due to the formation of a carbonyl group on the carotene molecule. The intensity of this band increases with the time of exposure of the carotene to air. Another new band was also detected in the region 3500-3600cm -1 which is due to the formation ofhydroperoxide (hindered) and hydroxyl groups on the carotene molecule undergoing oxidation. DISCUSSION The above results clearly indicate that pure carotene does not react with ptoluene thiophenol even in the presence of uv light. Hence the mechanism suggested by earlier workers 3 cannot be accepted as the mechanism of action of thiols on carotene under uv light. Further, the results of Experiment 1 indicate that aromatic thiol radicals of the type resulting from the reaction between A Z B N and MBT are quite capable of reacting with the carotene, thereby bleaching the yellow colour. Results of Experiments 3 and 4 indicate that carotene containing the oxidation products of carotene acts similarly to carotene mixed with a photo-activator. This is in agreement with the explanation given by earlier workers, 4'6"7 that carotene reacts with oxygen in air in the presence of uv light to form peroxy radicals and peroxides. These unstable peroxides decompose under uv light to form alkoxy radicals on the carotene molecule. These alkoxy radicals then undergo Norrish type 2 rearrangement to produce carbonyl groups, disrupting unsaturation of the carotene molecules. This is in agreement with the results obtained in Experiment 7, where there is clear evidence ofcarbonyl formation on carotene subjected to
Mechanism of action of aromatic thiols on carotenoid pigments
100
I
I
I
1
I
i
i
I
I
I
1
i
i
I
217
[
90
80
70
60
o o
v
50
u
40 I---
30
20
10
0
3200 Wave
Fig. 1.
I
2000 number
I
1200 (cm -1)
Infra-red spectra of carotene before and after exposure to air. - - - Before oxidation. . . . . After oxidation.
o x i d a t i o n . T h e a l k o x y radical t h e n reacts with a thiol m o l e c u l e t h e r e b y f o r m i n g a thiyl radical, l o T h e thiyl radical t h u s f o r m e d t h e n acts as i n i t i a t o r f o r the s u b s t i t u t i o n r e a c t i o n b e t w e e n the thiol a n d c a r o t e n e . As a result m o r e a n d m o r e radicals will be f o r m e d a n d the yellow c o l o u r o f c a r o t e n e d i s a p p e a r s , because, o n s u b s t i t u t i o n o f a thiol radical o n the c a r o t e n e , the c o n j u g a t i o n is d i s r u p t e d .
218
L. M. K. Tillekeratne, L. M. V. Tillekeratne, P. A. D. T. Vimalasiri
l
CH 3 ~ . -
-I
Cn 3
/
CH3
CH3
CH3
J2
//-Carotene CH3
CH 3
I --C=CH
I
CH=CH--C
CH-I OO.
I CH 3
CH 3 I I --C=CH--CH--CH~C--CH~
CH 3
CH 3
I --C=CH
I CH--CH=C--CH-I OO.
I O I O I ~-,CH--C=CH--CH--CH=C~ I I [ OO. CH 3 CH 3
~roxide ~rmed
CH 3 CH 3 I I -----C=CH--CH--CH=C--CH~
I O. CH 3
R--SH
+-----CH C = C H ] I OO. CH 3
N ~ ......
I [ - - - , - , C = C H - - C H - - C H = C - - C H - - + R--S" [ (thiyl radical) OH where R - - S H -- H 3 C - - ~ - ~ S H
O
pig....
CH .CH
'F CH 3
CH--CH:C~ f CH 3 Carbonyl compound H3C CH3
CIH CH
CH ~ C H
,-.1, CH 3
+
[]
S
CH3cH
g.....
--CH--C=CH z [ I OO. CH 3
Any Carotenoid
3
i CH3
I CH 3 ~ N(Norrishty~ 2
CH 3
~CH3
O" I CH--CH=C~
CH3
Thiol substituted fl-Carotene ~ H3C-Scheme 1
CH ~ C H " ~
T ,. CH3 -f~ S--S-Disulphide formed
CH3 CH 3
Mechanism of action of aromatic thiols on carotenoid p~gments
219
These results support the results obtained in Experiment 7 according to which there was obvious growth of the absorption band due to the - - O H group when carotene was oxidised. Also, the results of the H P L C experiment clearly indicate that on uv exposure in the presence of a photo-activator, substitution of one thiyl radical on carotene occurs. However, there was no evidence found in the H P L C analysis to show that more than one thiyl group is substituted on a molecule of carotene, as was believed earlier. H P L C work also indicated the possibility of combining two thiyl radicals to form the disulphide. Hence, it could be concluded from those experiments that the mechanism of action of thiols on carotenoid pigments in the presence of uv light is represented by reaction Scheme 1.
ACKNOWLEDGEMENT The authors are indebted to Prof. G. Scott of Aston University, Birmingham, U K , for helping to sort out the mechanism of the reaction of thiol with carotene under ultraviolet radiation, and also for editing this paper.
REFERENCES I. Handbook ~f Rubber Processing and Culture. Rubber Research Institute of Sri
Lanka. Caves Ltd, Colombo (1969). 2. L.M.K. Tillekeratne and W. S. E. Fernando, Sri Lanka Assn. for the Advanc. of Science. Proc. Annual Session, Colombo (1980). 3. W. S. E. Fernando, Annual Report Rubber Res. Inst. Sri Lanka, 65 (1979). 4. G. Scott, Atmospheric oxidation and antioxidants. Elsevier Pub. Co., London, p. 358 (1965). 5. A.M.U. Amin, G. Scott and L. M. K. Tillkeratne, Europ. Polym. J., 11,85 (1975). 6. A. W. Pross and R. M. Black, J. Soc. Chem. Ind., 69, 113 (1950). 7. C. H. Bamford and R. G. W. Norrish, J. Chem. Sot., 1505 (1935). 8. L. M. K. Tillekeratne, Chem. in Sri Lanka, 2 (2) (1985). 9. G. Caspari and A. Granzow, J. Phys. Chem., 74, 836 (1970). 10. W. Hofmann, Vulcanization and Vulcanizing Agents, Maclaren & Sons, London, p. 153 (1967).