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Interactions between a telechelic polymer and a silica—I
European Polymer Journal, Vol. 12, pp. 79l to 794. Pergamon Press 1976. Printed in Great Britain.
INTERACTIONS BETWEEN A TELECHELIC POLYMER AND A SILICA--I INFLUENCE MEASURE
OF OF
THE THE
POLYMER SOLVOLYSIS
CONCENTRATIONTEMPERATURE
A. VIDAL, E. PAPmER a n d J. B. DONNET Centre de Recherches sur la Physico-Chimie des Surfaces Solides, C.N.R.S., 24, avenue du Pr6sident Kennedy, 68200 Mulhouse, France
(Received 16 February 1976)
Abstract--A study of the interactions between, a telechelic polybutadiene with isocyanate end-groups and a silica has shown the occurrence of grafting of the elastomer onto the solid surface through a urethane bond. The relationship between the grafting ratios, measured by pyrolysis, and the polymer concentration appeared to be linear but the solvolysis temperature was independent of it.
Considerable attention has been paid recently by the r u b b e r industry to new types of starting materials such as powder a n d liquid rubbers. So, it is not surprising to find an increasing n u m b e r of publications a b o u t these latter materials. W h e n liquid rubbers which are prepolymers, i.e. low molecular weight materials (10,000 or less) carry reactive groups, they are particularly interesting since they can enter into chemical reactions. These reactive groups can be either r a n d o m l y distributed along the b a c k b o n e or at the ends of molecules. In this last case, the materials are n a m e d telechelic polymers, as defined by U r a n e c k a n d co-workers [1]. The functional groups of telechelic polymers have been chosen to be related to surface chemical properties of silica. They are indeed k n o w n to be related to the presence of hydroxylic groups the neutrality of which confers a great homogeneity to the surface. F r o m a chemical point of view, these silanol groups behave as neutral hydroxyls. F o r example, it is possible to replace the protons by metallic or organic cations if a base is used; with an acid, the hydroxylic groups are replaced by halogenated ions or carboxylate groups. The isocyanate function is very reactive towards products having labile hydrogen atoms, such as alcohols or thiols I-2]. In such a case, carbamates or urethanes are obtained. M o r e o v e r recent work has been performed in our laboratory [3] on the adsorption of aliphatic isocyanates with short chains onto silica or alumina. It has been shown that in all cases isocyanates can react with isolated surface hydroxylic groups to yield a chemisorbed carbamate. Also we intended to use the potential reactivity of the isocyanate functions of a telechelic p o l y m e r towards the surface O H groups of silicas to perform grafting of a liquid r u b b e r onto this surface. For study, we selected a polybutadiene b a c k b o n e since liquid polybutadiene with reactive end-groups is a m o n g the most promising liquid prepolymers I-4].
1. E X P E R I M E N T A L
The "Soci~t~ Nationale des Poudres et Explosifs" supplied bifunctionnal liquid polybutadiene with isocyanate end-groups. It was prepared from a hydroxylic telechelic polybutadiene the characteristic of which are [4]: number average molecular weight = Mn = ~ 3750 intrinsic viscosity at 30 ° in toluene {r/} = 0.15 molecular weight distribution = bidisperse average functionality per molecule = 2.3 ratio of reactive groups = 0.79 eq/kg microstructure: 1-2 l i n k s = 20~; 1-4 t r a n s = 60Yo; 1-4 cis = 20Yo. All these quantities, except of course the average functionality per molecule and the ratio of reactive groups, cannot have been markedly affected by the conversion of the hydroxylic end-groups into isocyanate groups. The two modified values were respectively: ~ 2 and 0.67 equivalent/kg. To check this assumption, we have measured at 30 ° in toluene the intrinsic viscosity of the telechelic polybutadiene with isocyanate end-groups. The obtained value {r/} = 0.17 is close to that for the starting sample. The silica used was "Aerosil 130" (A130) prepared by Degussa Co. It was made in the gas phase by hydrolyzing silicon compounds. It is non-porous and its specific surface area is 130 m2/g. All other reactants or solvents (benzene, toluene, o.oxylene, o.dichlorobenzene, diphenylmethane) were pure grades and were used without further purification.
Handling proceduresfor the grafting reaction and the purification of the grafted materials A typical grafting reaction was performed as follows: to a solution of 5 g liquid rubber in 50 cm 3 benzene were added with stirring 2.5 g silica. The reaction mixture was refluxed for 4 hr and then allowed to stand overnight. A centrifugation, followed by three successive dispersions in benzene and centrifugations, allowed separation of the grafted silica from the polymer solution (centrifugation at 6000 r./mn). The grafted silica was then oven-dried at 80°, finely ground and extracted with solvent for 48 hr in a Kumagawa's extractor. The grafting ratios, defined as the quantity of polymer irreversibly bound to the solid surface, were measured on the silica extracted by boiling benzene. 791
792
A. VIDAL,E. PAPIRERand J. B. DONNE'r
In order to determine the solvolysis temperature, taken as the extrapolation to zero grafting ratio of the curve giving grafting ratio vs extraction temperature [5], five solvents were used viz. benzene (80°), toluene (110°), o.xylene (144°), o.dichlorobenzene (182°) and diphenylmethane (264.5°). The grafting ratios were deduced by pyrolysis of a known weight of grafted material under nitrogen at 750° for 1 hr and determining the loss in weight. All results are averages of several determinations. 2. R E S U L T S
AND
i
w
!
i
DISCUSSION
Control of the polybutadiene grafting onto the silica surface through a urethane bond It is well known that one of the main reactions of isocyanates is that with compounds having active hydrogens thus: FREQUENCY,CM"1
R-N-C-R"
H--R /
H--R t
@
®
~,
III H O Although this research area has remained nearly unexplored, it is known that isocyanate can react with the surface OH groups of mineral oxides. In this field there is a patent by Weldes I-61 and Lange I-7] by differential thermal analysis (DTA) showed that the adsorption of diisocyanate on silica occurs only through one of the isocyanate groups; these results have been confirmed by Gebura and Gradus I-8-1 using i.r. spectroscopy. A more fundamental study by i.r. spectroscopy on the adsorption of n-butyl-isocyanate on silica is due to Kulik and co-workers [9]. They pointed out the existence of two different adsorbed species: ---one is the physically adsorbed isocyanate with a band, characteristic of the isocyanate function at 2280 cm- 1; - - t h e other is a form from reaction between the isocyanate and the surface OH groups of the silica. This compound is characterized by the NH band at 3330cm-1; this band results from the formation of a urethane bond. Kulik observed that the chemisorption is slow but is markedly enhanced by rise of temperature unlike the physically adsorbed species. Recently, Guillet I-4-1tried to determine the bonding process for isocyanates on mineral oxides. He confirmed the formation of a chemical bond of a carbamate type (urethane) between n-butyl isocyanate and the surface OH groups of mineral oxides (silica or alumina). In order to check if the polymer fixed on the solid and not extracted by solvents after purification is grafted onto silica through urethane bonds, we compared the i.r. spectrum of the telechelic polymer alone with that recorded with grafted silica after solvent extraction. These spectra can be seen in Fig. 1. After reaction of the telechetic polybutadiene with silica, the isocyanate band at 2290-2300 cm- 1 and the 1750 cm -1 band attributed to the CN group nearly
Fig. 1. Infra-red spectra. I--Telechelic polybutadiene; II-Silica grafted with the prepolymer and extracted with benzene. disappear. Two new lines appear, one at 3350 cm -1 due to the stretching of the NH group in the urethane bond, and the other at 1680 cm-1 attributed by Guillet to the formation of the following species: V
R -- N -- C -- 0 -- ~ S i 0 2
I
H 0 By i.r. spectroscopy it is not only possible to confirm the results of Guillet but also to prove that the grafting of a telechelic polymer carrying isocyanate end-groups onto silica occurs through the formation of a urethane bond. With a grafting process now clearly established, kinetic study of the grafting reaction and measurement of the solvolysis temperafare have been undertaken. The only parameter considered here is the polymer concentration.
Influence of the polymer concentration on the grafting ratios The grafting ratios measured by pyrolysis have been found for grafted materials extracted with benzene. Typical results are plotted i n Fig. 2. They have been obtained using the following experimental conditions: total v o l u m e = 50cm 3, A 1 3 0 = 2.5g, temperature = 80 °, time = 4 hr, benzene extraction = 48 hr. They show that a linear relationship between the grafting ratio and the polymer content can be drawn. When the polymer content is low enough, the experimental points lie on a straight line through the origin. It is remarkable that when the polymer content of the solution is too high the experimental points are off the line. This can be explained by the high viscosity of the medium preventing the solid from being well dispersed in the liquid during the reaction.
Interactions between a telechelic polymer and a silica--I I
/
30
d
~2c I.L
I
I
10
5 POLYMER CONTENT, G.
Fig. 2. Grafting ratio vs polymer content of the reaction medium. In order to check that the selected time of 4 hr allows the system to reach equilibrium, a test (under conditions identical to those for results shown in Fig. 2 but stopped after 2 hr) has been undertaken. The grafting ratio did not show a substantial difference from those measured after 4 hr. Later, to establish the kinetics of the grafting process, a systematic study of the various parameters governing the reaction will be undertaken (silica concentration, silica specific surface area, temperature of reaction, reaction time etc.).
pounds [10]. They also showed that, when plotting the grafting ratio vs extraction temperature, the experimental points usually lie on straight lines. The solvolysis temperature is defined as the point where these experimental curves intersect the temperature axis. We tried to determine the solvolysis temperature for solids grafted with telechelic polybutadiene. Five solvents have been used. Results are plotted in Fig. 3. One can observe that, whatever the grafting ratio, the solvolysis temperature is nearly constant ranging from 250 to 280 °. It is particularly interesting to examine points obtained at the highest extraction temperature (diphenyl methane). The measured grafting ratios then have values higher than that obtained after o.dichlorobenzene extraction. This observation must be compared with that of Sircar and Voet [5]. They noticed similar behaviour when a HAF-HS calcined carbon black dispersed in a SBR-type elastomer is extracted with aliphatic solvents. We can apply the same interpretation as Sircar and Voet, i.e. at high temperature there is crosslinking of the elastomer which stops the extraction. Increase of the extraction temperature is related to the breaking of an increasing number of stronger bonds. So, more and more elastomer is removed by the solvent. Consequently, the solvolysis temperature is the temperature theoretically required to decompose all bonds between elastomer and solid and should be characteristic of the stability of the created bond. In order to relate the solvolysis temperature to a real physical dimension, we have studied the behaviour of one of our samples by DTA. However, no relationship could be established since at a temperature above 190 ° the grafted polybutadiene decomposes. At 120 and 180 ° the thermograms showed some transitions not yet interpreted. CONCLUSION
Measure of the solvolysis temperature Studying carbon blacks carrying chemisorbed elastomeric chains, Sircar and Voet [5] confirmed an observation by Rivin and co-workers for model corn!
793
Infra-red spectroscopy allowed confirmation of the grafting of a telechelic polybutadiene with isocyanate end-groups onto silica through urethane bonds. The
I
I
!
O
~2C _o OE
8
1
0
2
EXTRACTION TEMPERATURE , oC
Fig. 3. Grafting ratio vs extraction temperature. E P J . 12/1 l ~ - c
794
A. VIDAL,E. PAPmER and J. B. DONNET
relationship between the grafting ratio and the amount of polymer in the reaction medium is linear, but the solvolysis temperature is independent of it.
4. 5. 6.
REFERENCES
1. C. A. Uraneck, H. L. Hsieh and O. G. Buck, J. Polymer Sci. 46, 535 (1960). 2. D. H. Chadwick and E. E. Hardy, in Encyclopedia of Chemical Technolo#y, (Edited by K. Othmer) John Wiley, New York p. 45-64 (1967). 3. A. Guillet, Th6se 3e cycle, Universit6 Louis Pasteur,
7. 8. 9. 10.
juillet (1973); A. Guillet, M. Coudurier and J. B. Donnet, Bull. Soc. chim. Fr. (7-8), 1563 (1975). Compte-rendu de fin de contrat DGRST n ° 72-7-0883-00-221-71-01 Septembre (1974). A. K. Sircar and A. Voet, Rubb. Chem. Technol. 43, (5), 973 (1970). H. H. Weldes, Can. pat. 676-204 (1963); U.S. pat. 3208867 (1965). K. R. Lange, Chem. Ind. London 14, 441 (1968). S. E. Gebura and G. M. Gradus, Middle Atlantic Regional Meeting, New York 2/1967. N. V. Kulik, L. A. Negievich, N. P. Kurgan and A. A. Kachan, Ukr. Khim. Zh. 36 (9), 904 (1970). D. Rivin, J. Aron and A. I. Medalia, Rubb. Chem. Technol. 41, 330 (1968).
INTERACTIONS BETWEEN A TELECHELIC POLYMER AND A SILICA--I INFLUENCE MEASURE
OF OF
THE THE
POLYMER SOLVOLYSIS
CONCENTRATIONTEMPERATURE
A. VIDAL, E. PAPmER a n d J. B. DONNET Centre de Recherches sur la Physico-Chimie des Surfaces Solides, C.N.R.S., 24, avenue du Pr6sident Kennedy, 68200 Mulhouse, France
(Received 16 February 1976)
Abstract--A study of the interactions between, a telechelic polybutadiene with isocyanate end-groups and a silica has shown the occurrence of grafting of the elastomer onto the solid surface through a urethane bond. The relationship between the grafting ratios, measured by pyrolysis, and the polymer concentration appeared to be linear but the solvolysis temperature was independent of it.
Considerable attention has been paid recently by the r u b b e r industry to new types of starting materials such as powder a n d liquid rubbers. So, it is not surprising to find an increasing n u m b e r of publications a b o u t these latter materials. W h e n liquid rubbers which are prepolymers, i.e. low molecular weight materials (10,000 or less) carry reactive groups, they are particularly interesting since they can enter into chemical reactions. These reactive groups can be either r a n d o m l y distributed along the b a c k b o n e or at the ends of molecules. In this last case, the materials are n a m e d telechelic polymers, as defined by U r a n e c k a n d co-workers [1]. The functional groups of telechelic polymers have been chosen to be related to surface chemical properties of silica. They are indeed k n o w n to be related to the presence of hydroxylic groups the neutrality of which confers a great homogeneity to the surface. F r o m a chemical point of view, these silanol groups behave as neutral hydroxyls. F o r example, it is possible to replace the protons by metallic or organic cations if a base is used; with an acid, the hydroxylic groups are replaced by halogenated ions or carboxylate groups. The isocyanate function is very reactive towards products having labile hydrogen atoms, such as alcohols or thiols I-2]. In such a case, carbamates or urethanes are obtained. M o r e o v e r recent work has been performed in our laboratory [3] on the adsorption of aliphatic isocyanates with short chains onto silica or alumina. It has been shown that in all cases isocyanates can react with isolated surface hydroxylic groups to yield a chemisorbed carbamate. Also we intended to use the potential reactivity of the isocyanate functions of a telechelic p o l y m e r towards the surface O H groups of silicas to perform grafting of a liquid r u b b e r onto this surface. For study, we selected a polybutadiene b a c k b o n e since liquid polybutadiene with reactive end-groups is a m o n g the most promising liquid prepolymers I-4].
1. E X P E R I M E N T A L
The "Soci~t~ Nationale des Poudres et Explosifs" supplied bifunctionnal liquid polybutadiene with isocyanate end-groups. It was prepared from a hydroxylic telechelic polybutadiene the characteristic of which are [4]: number average molecular weight = Mn = ~ 3750 intrinsic viscosity at 30 ° in toluene {r/} = 0.15 molecular weight distribution = bidisperse average functionality per molecule = 2.3 ratio of reactive groups = 0.79 eq/kg microstructure: 1-2 l i n k s = 20~; 1-4 t r a n s = 60Yo; 1-4 cis = 20Yo. All these quantities, except of course the average functionality per molecule and the ratio of reactive groups, cannot have been markedly affected by the conversion of the hydroxylic end-groups into isocyanate groups. The two modified values were respectively: ~ 2 and 0.67 equivalent/kg. To check this assumption, we have measured at 30 ° in toluene the intrinsic viscosity of the telechelic polybutadiene with isocyanate end-groups. The obtained value {r/} = 0.17 is close to that for the starting sample. The silica used was "Aerosil 130" (A130) prepared by Degussa Co. It was made in the gas phase by hydrolyzing silicon compounds. It is non-porous and its specific surface area is 130 m2/g. All other reactants or solvents (benzene, toluene, o.oxylene, o.dichlorobenzene, diphenylmethane) were pure grades and were used without further purification.
Handling proceduresfor the grafting reaction and the purification of the grafted materials A typical grafting reaction was performed as follows: to a solution of 5 g liquid rubber in 50 cm 3 benzene were added with stirring 2.5 g silica. The reaction mixture was refluxed for 4 hr and then allowed to stand overnight. A centrifugation, followed by three successive dispersions in benzene and centrifugations, allowed separation of the grafted silica from the polymer solution (centrifugation at 6000 r./mn). The grafted silica was then oven-dried at 80°, finely ground and extracted with solvent for 48 hr in a Kumagawa's extractor. The grafting ratios, defined as the quantity of polymer irreversibly bound to the solid surface, were measured on the silica extracted by boiling benzene. 791
792
A. VIDAL,E. PAPIRERand J. B. DONNE'r
In order to determine the solvolysis temperature, taken as the extrapolation to zero grafting ratio of the curve giving grafting ratio vs extraction temperature [5], five solvents were used viz. benzene (80°), toluene (110°), o.xylene (144°), o.dichlorobenzene (182°) and diphenylmethane (264.5°). The grafting ratios were deduced by pyrolysis of a known weight of grafted material under nitrogen at 750° for 1 hr and determining the loss in weight. All results are averages of several determinations. 2. R E S U L T S
AND
i
w
!
i
DISCUSSION
Control of the polybutadiene grafting onto the silica surface through a urethane bond It is well known that one of the main reactions of isocyanates is that with compounds having active hydrogens thus: FREQUENCY,CM"1
R-N-C-R"
H--R /
H--R t
@
®
~,
III H O Although this research area has remained nearly unexplored, it is known that isocyanate can react with the surface OH groups of mineral oxides. In this field there is a patent by Weldes I-61 and Lange I-7] by differential thermal analysis (DTA) showed that the adsorption of diisocyanate on silica occurs only through one of the isocyanate groups; these results have been confirmed by Gebura and Gradus I-8-1 using i.r. spectroscopy. A more fundamental study by i.r. spectroscopy on the adsorption of n-butyl-isocyanate on silica is due to Kulik and co-workers [9]. They pointed out the existence of two different adsorbed species: ---one is the physically adsorbed isocyanate with a band, characteristic of the isocyanate function at 2280 cm- 1; - - t h e other is a form from reaction between the isocyanate and the surface OH groups of the silica. This compound is characterized by the NH band at 3330cm-1; this band results from the formation of a urethane bond. Kulik observed that the chemisorption is slow but is markedly enhanced by rise of temperature unlike the physically adsorbed species. Recently, Guillet I-4-1tried to determine the bonding process for isocyanates on mineral oxides. He confirmed the formation of a chemical bond of a carbamate type (urethane) between n-butyl isocyanate and the surface OH groups of mineral oxides (silica or alumina). In order to check if the polymer fixed on the solid and not extracted by solvents after purification is grafted onto silica through urethane bonds, we compared the i.r. spectrum of the telechelic polymer alone with that recorded with grafted silica after solvent extraction. These spectra can be seen in Fig. 1. After reaction of the telechetic polybutadiene with silica, the isocyanate band at 2290-2300 cm- 1 and the 1750 cm -1 band attributed to the CN group nearly
Fig. 1. Infra-red spectra. I--Telechelic polybutadiene; II-Silica grafted with the prepolymer and extracted with benzene. disappear. Two new lines appear, one at 3350 cm -1 due to the stretching of the NH group in the urethane bond, and the other at 1680 cm-1 attributed by Guillet to the formation of the following species: V
R -- N -- C -- 0 -- ~ S i 0 2
I
H 0 By i.r. spectroscopy it is not only possible to confirm the results of Guillet but also to prove that the grafting of a telechelic polymer carrying isocyanate end-groups onto silica occurs through the formation of a urethane bond. With a grafting process now clearly established, kinetic study of the grafting reaction and measurement of the solvolysis temperafare have been undertaken. The only parameter considered here is the polymer concentration.
Influence of the polymer concentration on the grafting ratios The grafting ratios measured by pyrolysis have been found for grafted materials extracted with benzene. Typical results are plotted i n Fig. 2. They have been obtained using the following experimental conditions: total v o l u m e = 50cm 3, A 1 3 0 = 2.5g, temperature = 80 °, time = 4 hr, benzene extraction = 48 hr. They show that a linear relationship between the grafting ratio and the polymer content can be drawn. When the polymer content is low enough, the experimental points lie on a straight line through the origin. It is remarkable that when the polymer content of the solution is too high the experimental points are off the line. This can be explained by the high viscosity of the medium preventing the solid from being well dispersed in the liquid during the reaction.
Interactions between a telechelic polymer and a silica--I I
/
30
d
~2c I.L
I
I
10
5 POLYMER CONTENT, G.
Fig. 2. Grafting ratio vs polymer content of the reaction medium. In order to check that the selected time of 4 hr allows the system to reach equilibrium, a test (under conditions identical to those for results shown in Fig. 2 but stopped after 2 hr) has been undertaken. The grafting ratio did not show a substantial difference from those measured after 4 hr. Later, to establish the kinetics of the grafting process, a systematic study of the various parameters governing the reaction will be undertaken (silica concentration, silica specific surface area, temperature of reaction, reaction time etc.).
pounds [10]. They also showed that, when plotting the grafting ratio vs extraction temperature, the experimental points usually lie on straight lines. The solvolysis temperature is defined as the point where these experimental curves intersect the temperature axis. We tried to determine the solvolysis temperature for solids grafted with telechelic polybutadiene. Five solvents have been used. Results are plotted in Fig. 3. One can observe that, whatever the grafting ratio, the solvolysis temperature is nearly constant ranging from 250 to 280 °. It is particularly interesting to examine points obtained at the highest extraction temperature (diphenyl methane). The measured grafting ratios then have values higher than that obtained after o.dichlorobenzene extraction. This observation must be compared with that of Sircar and Voet [5]. They noticed similar behaviour when a HAF-HS calcined carbon black dispersed in a SBR-type elastomer is extracted with aliphatic solvents. We can apply the same interpretation as Sircar and Voet, i.e. at high temperature there is crosslinking of the elastomer which stops the extraction. Increase of the extraction temperature is related to the breaking of an increasing number of stronger bonds. So, more and more elastomer is removed by the solvent. Consequently, the solvolysis temperature is the temperature theoretically required to decompose all bonds between elastomer and solid and should be characteristic of the stability of the created bond. In order to relate the solvolysis temperature to a real physical dimension, we have studied the behaviour of one of our samples by DTA. However, no relationship could be established since at a temperature above 190 ° the grafted polybutadiene decomposes. At 120 and 180 ° the thermograms showed some transitions not yet interpreted. CONCLUSION
Measure of the solvolysis temperature Studying carbon blacks carrying chemisorbed elastomeric chains, Sircar and Voet [5] confirmed an observation by Rivin and co-workers for model corn!
793
Infra-red spectroscopy allowed confirmation of the grafting of a telechelic polybutadiene with isocyanate end-groups onto silica through urethane bonds. The
I
I
!
O
~2C _o OE
8
1
0
2
EXTRACTION TEMPERATURE , oC
Fig. 3. Grafting ratio vs extraction temperature. E P J . 12/1 l ~ - c
794
A. VIDAL,E. PAPmER and J. B. DONNET
relationship between the grafting ratio and the amount of polymer in the reaction medium is linear, but the solvolysis temperature is independent of it.
4. 5. 6.
REFERENCES
1. C. A. Uraneck, H. L. Hsieh and O. G. Buck, J. Polymer Sci. 46, 535 (1960). 2. D. H. Chadwick and E. E. Hardy, in Encyclopedia of Chemical Technolo#y, (Edited by K. Othmer) John Wiley, New York p. 45-64 (1967). 3. A. Guillet, Th6se 3e cycle, Universit6 Louis Pasteur,
7. 8. 9. 10.
juillet (1973); A. Guillet, M. Coudurier and J. B. Donnet, Bull. Soc. chim. Fr. (7-8), 1563 (1975). Compte-rendu de fin de contrat DGRST n ° 72-7-0883-00-221-71-01 Septembre (1974). A. K. Sircar and A. Voet, Rubb. Chem. Technol. 43, (5), 973 (1970). H. H. Weldes, Can. pat. 676-204 (1963); U.S. pat. 3208867 (1965). K. R. Lange, Chem. Ind. London 14, 441 (1968). S. E. Gebura and G. M. Gradus, Middle Atlantic Regional Meeting, New York 2/1967. N. V. Kulik, L. A. Negievich, N. P. Kurgan and A. A. Kachan, Ukr. Khim. Zh. 36 (9), 904 (1970). D. Rivin, J. Aron and A. I. Medalia, Rubb. Chem. Technol. 41, 330 (1968).