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The free radical polymerization of vinyl monomers in the presence of carbon black
Cur/m.
IY75. \iol
13. pp 443448.
Pergamon Press.
Printed m Great Bntam
THE FREE RADICAL POLYMERIZATION OF VINYL MONOMERS IN THE PRESENCE OF CARBON BLACK K. OHKITA. N. TSUBOKAWA, E. SA~TOHand M. NODA Faculty of Engineering, Niigata University, Gakko-cho l-2 Nagaoka (940),Japan and N. TAKASHINA HighPolymerLaboratory,Mitsubishi
Gas ChemicalCompany,Inc., 1725, Kamikawahara,Yawata,Hiratsuka(254),
Japan (Received 18May 1975)
Abstract-The polymerizationof vinyl monomershas been carried out in the presence of furnace blacks using initiators such as 2,2’-azobisisobutyronitrile(AIBN)and benzoylperoxide(Bz20Z)in nitrogen or oxygen atmosphere. The results indicate that free radicals form by the decomposition of initiators reacting with carbon blacks to give active sites on their surface which then capture either the free radicals or the growing polymer radicals. Using the monomers with negative e values, such as styrene and vinyl acetate, a marked retardation was observed in Bz,02-initiated polymerization in the presence of furnace blacks, while a moderate inhibition was found in the polymerization initiated by AIBN. The polymerization reaction using Bz202was found to be very sensitive to oxygen in the presence of furnace blacks and the involvement of oxygen was found to promote grafting onto the surface of carbon black by the growing polymer radicals, consequently giving polymer-grafted particles while hindering the formation of homopolymers. Furthermore, the reaction of Bz,O> with the surface of furnace blacks in oxygen atmosphere has been studied in carbon tetrachloride at 45°C.The resulting carbon blacks show an increase in the number of surface quinone groups with an increase in reaction time.
1. INTRODUCTION The formation of a carbon gel in the milling process of rubber with carbon black drew the attention of a number of researchers. For instance, the basic study by Watson[l] was followed by an explanation by Kraus et al.[2] that the unpaired electrons present on surface of carbon black particles capture the rubber radicals formed by mechanical shearing of rubber molecules. The surface of carbon black has been known to react with free radicals readily; Donnet and Henrich[3] investigated the reaction of 2,2’-azobisisobutyronitrile (AIBN) with a variety of carbon blacks and concluded that the unpaired electrons on the surface of carbon blacks capture the free radicals, since the number of 2cyano-Zpropyl radicals captured on the surface was of the same order as the number of unpaired electrons reported by Kraus et al.[2]. On the contrary, in 1961 Spackman[4] found that the treatment of carbon black with AIBN or 2,2-diphenyl-lpicrylhydrazyl (DPPH) causes an increase in the number of the unpaired electrons in carbon black. Even below room temperature, benzoyl peroxide (Bz202)in a solution with carbon black added decomposes into benzoate radicals which react with the surface of carbon black or with solvent molecules without loss of carbon dioxide [5-71. The inhibition of free radical polymerization of vinyl monomers by carbon black is well known and Kraus et al. [8] interpreted it as a phenomenon due to the quinone groups on the surface. Donnet et al.[9] proposed an aroxylic structure having localized free radicals on the surface and explained the inhibition in vinyl polymerization by the so-called quinone radical theory. Vinyl
polymerization using initiators in the presence of carbon black has been variously reported[lO, Ill. In the present work, using initiators such as Bz202 and AIBN the polymerization reactions of vinyl monomers in the presence of furnace blacks in nitrogen or oxygen atmosphere were investigated. Moreover, the surface oxygen-containing groups of furnace blacks treated with Bzz02 in oxygen were examined and the reactivity of the groups toward the growing polymer radicals was discussed. 2. EXPERIMENTAL 2.1 Materials and reagents Carbon blacks used were mostly furnace blacks from Phillips Chemical Co., obtained through A. A. Chemical Co. They were kept in dark in a sealed glass container and dried at 110°C in vacua prior to use. The vinyl monomers and initiators were obtained from Wako Pure Chemical Ind., Ltd. and purified prior to use. For instance, styrene monomer containing 0.004% of t-butylcatechol as stabilizer was purified by washing with sodium hydroxide solution and water, drying over calcium chloride, and then by distillation in nitrogen under reduced pressure. Commercially available Bz20? with 98.9% purity was dissolved in warm acetone, and the solution was then added with the same volume of 80% ethyl alcohol, and precipitated crystals were collected. AIBN was recrystallized from methyl alcohol. 2.2 Polymerization in the presence of carbon black The polymerization of vinyl monomers in the presence of carbon blacks was carried out mostly using initiators in a 300ml tear-drop type flask attached with a reflux
443
444
K.
OHKITA et al.
condenser. The reaction mixture was stirred by a magnetic stirrer. Comparisons were made between the runs in nitrogen or oxygen stream at the rate of 3-5 ml/min and the runs in sealed tube without bubbling gases. With an excess of carbon black stirring was difficult at the beginning, but once the reaction started, a sharp drop in thixotropy[l2] made the stirring easier. The conversion for a certain reaction time was obtained in a following fashion: Taking styrene for an example, the reaction mixture was dispersed in toluene and methyl alcohol was added to separate out the carbon black containing polymer; this was then washed with methyl alcohol and dried in vacua. The rate of conversion was obtained from the difference in the weights of this mix and the carbon black initially added [ 131.In case of methyl methacrylate, the dispersant was acetone and the precipitant methyl alcohol. Some of the experiments were carried out in sealed tubes. Carbon black was taken into a 60 ml test tube, a monomer and an initiator added, the mix chilled with crushed dry ice, and the tube sealed. The tubes were placed in a water bath set at a given temperature and tumbled at the rate of 30rpm to carry out the polymerization reaction. 2.3 Identification of polymers grafted onto the surface of carbon black
The polymerization of styrene in the presence of furnace black gives the latter a good dispersibility in tetrahydrofuran. In order to separate carbon black from tetrahydrofuran, a small amount of methyl alcohol was added, but not enough to cause homopolymer to precipitate, and then the mixture was centrifuged at the rate of lo4 rpm for 30 min. Precipitated carbon black was taken out, dispersed again in tetrahydrofuran, and then centrifuged again. After repeating the procedure, the separated carbon black was dried at 100°C.Upon drying, its dispersibility in organic media was lost. After this treatment, 1 g of the carbon black was subjected to the Soxhlet extraction with tetrahydrofuran at the flush-out rate of 10-15 min for 100hr. After extraction the carbon black was dried at 110°Cin vacua. The polystyrene grafted onto the surface of carbon black was analyzed by gas chromatograms of the thermal decomposition products [ 14, IS]. The gas chromato~aph used was Ohkura Rikagaku Model 6000. The operating conditions are given in Table 1. 2.4 TIte reaction of the carbon black surface with BzzOz Two grammes of carbon black, 2.Og of BZZOZ,and 150ml carbon tetrachloride as a solvent were placed into a 300 ml pear shape flask attached with a reflux condenser, and using a magnetic stirrer the mixture was reacted at 45°C. The evolution of carbon dioxide was negligible. Oxygen or nitrogen gas was introduced into the flask gradually at the rate of 5-10 ml/mm during the reaction. At completion of the reaction, the carbon black was separated on a centrifuge at 104rpm for 1 hr. It was air dried and then dried in vacua at room temperature, and finally subjected to Soxhlet extraction using carbon
Table 1. Operatingconditions columnlength
7% cm
Columnmateriels
10 wt8 silicone oilDC 550 and Diasolid M (60-80mesh)
Columntemperature
720%
Detector temperature
tze*c
Injection temperature
200%
grro1ysis tenperature
SOO'C
Samplequantity
3 to 5 mg
tetrachloride and flush-out intervals of lO-15min for 80 hr. During the reaction and purification steps a calcium chloride drying tube was used to remove the moisture. The carbon black so treated and dried at 80°C in vacua was used for quantitative analysis for quinone and hydroquinone groups. Analysis for surface quinone groups was carried out by determination of hydrogen uptake from sodium borohydride solution following a method proposed by Studebaker [ 161and later studied by Suzuki et al. [ 171in detail, and for surface hydroquinone groups by DPPH [ 131. Such treated carbon black was also used to examine the induction period observed during the thermal polymerization of styrene in comparison with the untreated one. 3. RESULTS AND DISCUSSION
3.1 The dependence of poly~e~zation on the su~~ce urea and adsorbed water in the capon
black
During the polymerization of vinyl monomers in the presence of furnace black the free radicals formed by the decomposition of initiators react with the carbon black to produce new unpaired electrons on the surface of carbon black particles, to which the free radicals and the growing polymer radicals compete to reactllg, 191. For such reactions, the nature of the surface of carbon black is the dominating factor, the conversion varying with the degree of agitation[ 191,this being a heterogeneous mixture. In our experiments therefore the stirring was kept always the same so as not to affect the rate of polymeri~tion. Table 2 shows the influence of adsorbed water on the conversion of methyl methacrylate in tetrahydrofuran for three types of furnace blacks used after the exposure to air for 48 hr. The initiator was AIBN and no nitrogen gas was passed through the polymerization vessel. The amounts of adsorbed water were determined beforehand, and exactly l-00 g of each carbon black was taken. As shown in Table 2, lower conversion is observed with dried carbon black and this tendency is larger for the carbon blacks having larger specific surface areas, this shows that the reaction taking place on the surface of carbon black is clearly affected by the water captured in mi~ropores in changing the amount of homopolymer formed. Figure 1 shows the relationship between the amount of
445
The free radical polymerization of vinyl monomers in the presenceof carbonblack Table 2. The influence of water adsorbed on the surface of carbon black on the conversion of methyl methacrylate (MMA) Conversion
(4)*
Specific surface area
Carbon black
Undried+
h'/g) (53
i3C°C
Dried at
hilblacb A (FEF) Philblack 0 (HAF) Philblack I (ISAF)
5
Bifteer.grames
of M4A were polymerized in the presence of
1.OOg of carbon blacks using 0.3Ag of AIBN in 1' ml of tetrahydrofuran at 65°C for 5 hr. Moisture adsorbed
t
Carbon MMA 45’C
black ,lO
0.02%/g
(II).
I
I 002
0.03ogig Cf,, a.mg/g
l.OOg 10.0 ml hr
1 0
$,
0.04
0.06 BzzOz
0.08
0
2
4
hr6
0.10
a9
Fig. 1. Polymerization of methyl methacrylate (MMA) using Bz202 in the presence of various carbon blacks. 0, Philblack A; q,PhilblackO;A,PhilblackI.
10
Fig. 2. Polymerization of styrene in the presence or absence of carbon black using BzzOzas an initiator under nitrogen. Philblack
(1000)
J
BzzOz used as an initiator and the conversion of methyl methacrylate in presence of the three different types of furnace black, the same as in Table 2. In this experiment, the polymerization reaction was carried out in sealed tubes. The results shown in Fig. 1 show that the larger the specific surface area of carbon black, the more BzzOzis needed before the initiation of homopolymer formation.
BZ,OZ Monomer
1
0 0 509 4.13x10~4md 0.10
md
/ (0 999)
(0 997)
(0 995)
3.2 The relationship between the e value for a monomer
(0993)
/_
and the retardation of polymerization
Figure 2 shows the relationship between reaction time and conversion for the polymerization of styrene, with a negative e value (based on Alfrey and Price’s Q and e scheme[20]), initiated with BzzOz in the presence of Philblack I. Even for a ten-fold increase in the amount of the initiator, the retardation in polymerization could not be eliminated, only the tangent of conversion-time curve changing slightly. In the experiments shown in Fig. 2, nitrogen gas was passed through the reaction vessel. Similar retardation as in Fig. 2 was also observed during the polymerization of vinyl acetate, with a negative e value, using BzzOz as an initiator in the presence of Philblack 0. The conversion in polymerization of methyl methacrylate was also examined using BzZOzin the presence of Philblack 0 and compared to that in copolymerization with styrene at 45 f O.l”C in a sealed tube. The results are shown in Fig. 3. The conversion-time curve for polymer-
8
Time,
0
2
4
6 Time,
8
l(
hr
Fig. 3. Copolymerization of methyl methacrylate with styrene.in the presence of carbon black at 45°C.The figures in brackets give mole fractions of methyl methacrylate in monomer mixtures. ization using methyl methacrylate, with a positive e value, is clearly different from that of styrene. Using acrylonitrile, with a positive e value, a similar behaviour was observed. It is interesting to note that a marked retardation is found in the copolymerization of methyl methacrylate with a small amount of styrene. Thus, for a monomer having a negative e value, a significant retardation in the polymerization or copolymerization initiated with BzzOz in the presence of furnace blacks indicates the capture of polymer radicals by the active sites formed during the reaction of benzoate radicals with the surface of carbon black.
K. OHKITAet al.
446
On the contrary, in Fig. 4, for example, the influence of the presence of a furnace black or of the different e value of monomers is not seen too clearly when AIBN was used as an initiator. Therefore, the radicals formed by the decomposition of AIBN have probably less reactivity toward the surface of carbon black than the benzoate radicals, and the concentration of newly formed unpaired electrons is much lower than in the case of Bz20Z. In our earlier work[6], it has shown that, when acted upon by BzzOzin nitrogen atmosphere, furnace blacks are attacked in their exposed benzenoidal rings at the edges of micrographitic platelets and benzoate radicals are substituted for hydrogen atoms like in the case of the reactions between a number of polycyclic aromatic hydrocarbons and Bzz02[21]. On the other hand, Donnet et al. [22] assumed that in nitrogen atmosphere no reaction takes place between AIBN and the benzenoidal rings at the edges of quasigraphitic crystallites on the surface of carbon black; the AIBN was considered to react only with quinonic oxygen atoms. In our earlier study of the polymerization of vinyl acetate containing a small amount of anthracene, pbenzoquinone, and hydroquinone respectively, we have obtained the following results [19] using AIBN and Bz202 as initiators: With BzzOz as an initiator, anthracene and hydroquinone cause retardation of the polymerization of vinyl acetate to a considerable extent like in the case of its polymerization in the presence of a furnace black. On the contrary, p-benzoquinone acts as an inhibitor giving a well defined induction period. With AIBN, all these compounds are inhibitors and no retardation is observed. Differently from benzoate radical, the 2-cyano-2-propyl radical does not abstract hydrogen atoms from alcohols, ketones, and aromatic hydrocarbons. The concentration of unpaired electrons of commercial carbon black[2] is already known to be lower than the concentration of surface quinone groups[23]. In case of Philblack I, the first is known to be 9.2 x 1019/g[2]and the latter about 3.2 x 10Zo/g[23]. Although the vinyl polymerization free-radical initiated in the presence of a furnace black is complex, free radicals of initiators, growing polymer radicals and free radicals formed by the interaction of solvent with benzoate radicals in case of solution polymerization using Bz202 are considered to compete in reacting with the suiface of carbon black[ll], and depending on the 20,
3.3 The influence of solvents on the polymerization Figure 5 shows the relationship between the concentration of AIBN used and the conversion for the polymerization of methyl methacrylate in tetrahydrofuran and in methyl isobutyl ketone in the presence of Philblack 0 at 65°C after 5 hr without bubbling nitrogen. Methyl isobutyl ketone gives a lower conversion than tetrahydrofuran under the same conditions. However, the remarkable difference in the rate of the polymerization in different solvents as shown in Fig. 5 is not observed in the absence of furnace black. Differing from BzzOz, AIBN in giving off 2-cyano-2propyl radicals does not react with solvents, and therefore, the difference in conversion appears to be caused by the inhibition by oxygen dissolved in the solvents during the polymerization. In order to clarify the above mentioned phenomenon, the experiments were carried out both in air and in nitrogen flow at the rate of 3-5ml/min at 65°C for 5 hr. The results show that in nitrogen the amount of AIBN required to start the formation of homopolymer is independent of the type of solvent used. Therefore, it is clear that the large difference in conversion in Fig. 5 depends on the amount of oxygen dissolved in the solvent. 3.4 The polymerization in the presence of oxygen In presence of oxygen different products in the reaction involving free radicals are obtained. Waters et a[.[211 have described the formation of quinone derivatives in the reaction of anthracene with Bzz02 in the presence of oxygen and Tokumaru et al. [24] the formation of phenol in the reaction of benzene with Bz202 in the presence of oxygen. On the other hand, Donnet et a[.,[221 reported that the action of AIBN upon carbon black surface in the presence of oxygen causes more the cyanopropyl radicals to be bound to the surface along with oxygen atoms. We carried out the reaction of Bz202 with carbon black in oxygen at 45°C in carbon tetrachloride and studied the variation in the concentration of quinone groups with an increase in reaction time by hydrogen
1
I A
0
I (g) (moi)
0.5 0.2
0.2
(10~%nol) (“C)
1.5 60
1.5 60
Philblack stvrene AI’BN
a
reactivity of the reactants, their interactions with carbon black surface cause either inhibition or retardation of polymerization.
s
Y
0.05 0
2 Time,
hr4
6
Fig. 4. Polymerization of styrene in the presence or absence of carbon black using AIBN as an initiator under nitrogen.
0.10
0.1 5
0.2c
AIBN,g
Fig. 5. Solution polymerization of methyl methacrylate in the presence of carbon black using AIBN. MIBK: Methyl isobutyl ketone, THF: Tetrahydrofuran.
447
The free radical polymerization of vinyl monomers in the presence of carbon black absorption measurement [ 16,171with sodium borohydride
solution. The results are given in Table 3. A fraction of hydroquinone groups on the carbon black surface may react with Bz202to give quinone radicals and benzoic acid[l8] like in the case of the reaction of BzZO? with the compounds having hydroquinone groups[25]. However, as it is known that hydroquinone groups represent only about a quarter of all quinones on the surface of commercial furnace black[23], the amount of quinone groups newly formed by the oxidation of hydroquinone groups with Bz202 is not large enough to upset the results shown in Table 3. The absorption of hydrogen from sodium borohydride solution often became two to three times as great for the carbon black treated with BzZ02 in nitrogen atmosphere as that for the untreated one. Since the carbon black surfaces are exposed to air after the treatment, as in a Soxhlet extraction, the unpaired electrons formed by the action of BzZOZupon the surfaces but not combined with benzoate radicals perhaps capture oxygen biradicals and turn into quinonic oxygen. This problem is under investigation and the results will be reported at a later opportunity. It is interesting to note that a carbon black treated with BzzOzgives more carbon gel in mixing with natural rubber than the untreated one[26]. In the work reported here, the polymerization of vinyl monomers in the presence of carbon black in oxygen atmosphere was also examined. Two grammes of Philblack 0 were added to 30 g of styrene monomer with 1 g of BzzOZand the polymerization was carried out at 45°C for 100hr in an oxygen stream at the rate of 5 ml/min. Conversion was less than 1% and no homopolymer was formed. The carbon black thus treated with styrene gave a stable colloidal dispersion in toluene. Tetrahydrofuran was used to wash the isolated carbon black, which was then pyrolyzed at 600°C after drying at 110°C in vacua. Its gas chromatogram was totally similar to that of polystyrene itself as shown in Fig. 6.
Time,
that of untreated blacks in nitrogen at 90°C. The results are given in Fig. 7. According to Deviney and Whittington[27], phenolic hydrogens on carbon black surface transfer to the growing polystyrene chains as checked by the use of tritium labelling technique.
0
2
4
6
Time,
carbon black
The thermal polymerization of styrene in the presence of BzzOZ-treatedblacks was examined in comparison with
Table 3. Hydrogen uptake from sodium borohydride solution Hydroger uptake
Carbon black
0
Untreated Treated
black with
Bz20,
c
at
L5’C
for
2C hr
under
O2
60 days * Treated hilblack ““treated Treated
with
Bz2C,
c
at
L5-C
for
‘C
hr under
at
ll5*C
for
20 hr
C2
I black with
Bz,O,
8
10
hr
Fig. 7. Thermal polymerization of styrene in the presence of Philblack I. 0, untreated black; A, treated with Bz,O, under nitrogen for 20 hr; 0, treated with Bz202 under air for 20 hr. The treatment of carbon black with BzIO. was carried out in carbon tetrachloride at 45°C.
3.5 Thermal polymerization of styrene in the presence of
hilblack
min
Fig. 6. Pyrolytic chromatograms (600°C) of the surface of Philblack 0 reacted with styrene using Bz202 in the presence of oxygen.
c.o:!, under
C,
C. 0””
6) 1
It was confirmed by the determination of hydroquinone groups on the surface of Bz,O,-treated carbon blacks using DPPH[ 131that the variations in the number of hydroquinone groups on the surface treated with Bz20Z in oxygen are very small in comparison with those on the original blacks. Accordingly, the long induction period of the inhibition of polymerization seen in Fig. 7 may be responsible for the new quinone groups formed. This is in accordance with the increase in the number of quinone groups.
4. SUMMARY
The interaction of free radicals such as benzoate radical with the surface of carbon black produces new unpaired electrons which capture growing polymer radicals in the presence of vinyl monomers. However, the number of newly born unpaired electrons should vary depending on the type of free radicals used or on the atmosphere in which the reactions are carried out. In the present paper, the vinyl ~lymerization in the presence of furnace blacks using Bz202 was examined. A marked retardation was observed in BzzO,-initiated polymerizations using the monomers with negative e values in the presence of a furnace black, while no such effect was found in the polymerization using the monomers with positive e values. The discrepancy seems to be due to predominant effect from the competition reaction between the free radicals formed by the decomposition of Bz,Oz and growing polymer radicals on the surface. The polymerization process is very sensitive to oxygen. This is due to the p~ticipation of oxygen molecules in the competitive reactions with free radicals present on the carbon black surface. The involvement of oxygen in the polymerization of vinyl monomers when using BZIOI in the presence of furnace blacks promotes the formation of polymer-grafted carbon blacks. The phenomenon is considered to be due to newly added surface quinone groups ur quinone radicals having unpaired electrons which are formed by the reaction of BzzOz with the carbon black surface in oxygen atmosphere.
REFERENCES 1.
Watson W. F., Ind. Engng Chem. 47, 1281(1955).
2. Kraus G. and Collins R. L., Rubber World 139, 219 (1958); Collins R. L., Bell M. D. and Kraus G., J. Appl. Phys. 30, 56 (1959). 3. Donnet J. B. and Henrich G., Compt. Rend. 2463230 (1958); Donnet J. B., Henrich G. and Geldreich L., Ibid. 249, 97 (1959). 4. Spackman J. W. C., &em. Itzd. 1532, London (1961). 5. Studebaker M. L., Rubber Chem. ~eck~o~. 30, 141X! (1957).
6. Ohkita K., Kasahara H., Ishizuki N. and Itagaki Y., Nippon GumaKyokaiski (J. Sot. Rubber Ind., Japan)-%, 361(1%3f. 7. Ohkita K.. Takehara R. and Katoh M.. Koevo Ka&u Zusshi (3. Chem.‘Soc., Japan, Ind. Chem. Sec.) 66, 13&(1%3). 8. Kraus G., Gruver J. T. and Rollmann K. W., J. Polymer Sci. 36, 564 (1959). 9. Donnet J. B., Henrich G. and Riess G., Rev. Gdn. &out. 38, 1803(1961);39, 583 (1962). 10. SociCt6anon. Crylor, Fr. Patent 1188128(1959). 11. Ohkita K. and Ishizuki N., Abstract of Conj on High Polymer held at Takada, p. 23. Hokuriku-shibu, Sot. of Polymer Sci., Japan (1960). 12. TakashinaN. andAidaT.,J. Appl. Polymer&i. 12,1109(1968). 13. Ohkita K. and Tsubokawa N.. Carbon 10.631 (1972). 14. Lehmann F. A. and Brauer G. M., Anal. Chem. 33,673(1961). IS. Ohkita K., Kitahara N., Saitoh H. and Aoyama M., Preprints of Scientific Papers IV-43, international Symposium on Macromolecular Chemistry, Tokyo, Kyoto (1966). 16. Studebaker M. L., Froc. 5th Conf. on Carbon, Vol. II, p. 189. Pergamon Press, New York (1962). 17. Suzuki S. and Takakuwa R., Nippon Kagaku Zasshi (J. Chem. Sot., Jauan. Pure Chem. Sec.) 88. 1271(19671. 18. Ohkita K. and Tajima T., Nidpon’ Gomu’ Kyokaishi (J. Sot. Rubber Ind., Japan) 43, 379 (1970); Ohkita K., Sekiyu Gakkai-shi (J. Japan Petroleum Inst.) 15, 909 (1972). 19. Ohkita K., Tsubokawa N., Kadoi H. and Suneya Y., Nippon Gomu Kyokaishi (J. Sot. Rubber Ind., Japan)45,1074 (1972). 20. Alfrey T., Jr., Bohrer J. J. and Mark H., Copolymerization,p. 64. Interscience, New York (1952). 21. Roitt I. M. and Waters W. A., J. Chem. Sot. 2695 (1952); Norman R. 0. C. and Waters W. A.. Ibid. 167 (1958). 22. Donnet J. B., Metzger J. and Riess G., ~einf~res, Pigments, Vernis 42 f2), 76 (1966). 23. Studebaker M. L. and Rinehart R. W. Sr., Rubber Chem. Technol. 45, 106 (1972).
24. Tokumaru K., Horie K. and Simamura 0.. Tetrahedron 21, 867 (1965).
2s. Cosgrove S. L. and Waters W. A., J. Chem. Sot. 3189(1949); 388 (1951). 26. Ohkita K. and Washizu T. Unpublished results. 27. Deviney M. L., Jr. and Whittington L. E., Rubber Chem. Technol. 41, 382 (1%8).
IY75. \iol
13. pp 443448.
Pergamon Press.
Printed m Great Bntam
THE FREE RADICAL POLYMERIZATION OF VINYL MONOMERS IN THE PRESENCE OF CARBON BLACK K. OHKITA. N. TSUBOKAWA, E. SA~TOHand M. NODA Faculty of Engineering, Niigata University, Gakko-cho l-2 Nagaoka (940),Japan and N. TAKASHINA HighPolymerLaboratory,Mitsubishi
Gas ChemicalCompany,Inc., 1725, Kamikawahara,Yawata,Hiratsuka(254),
Japan (Received 18May 1975)
Abstract-The polymerizationof vinyl monomershas been carried out in the presence of furnace blacks using initiators such as 2,2’-azobisisobutyronitrile(AIBN)and benzoylperoxide(Bz20Z)in nitrogen or oxygen atmosphere. The results indicate that free radicals form by the decomposition of initiators reacting with carbon blacks to give active sites on their surface which then capture either the free radicals or the growing polymer radicals. Using the monomers with negative e values, such as styrene and vinyl acetate, a marked retardation was observed in Bz,02-initiated polymerization in the presence of furnace blacks, while a moderate inhibition was found in the polymerization initiated by AIBN. The polymerization reaction using Bz202was found to be very sensitive to oxygen in the presence of furnace blacks and the involvement of oxygen was found to promote grafting onto the surface of carbon black by the growing polymer radicals, consequently giving polymer-grafted particles while hindering the formation of homopolymers. Furthermore, the reaction of Bz,O> with the surface of furnace blacks in oxygen atmosphere has been studied in carbon tetrachloride at 45°C.The resulting carbon blacks show an increase in the number of surface quinone groups with an increase in reaction time.
1. INTRODUCTION The formation of a carbon gel in the milling process of rubber with carbon black drew the attention of a number of researchers. For instance, the basic study by Watson[l] was followed by an explanation by Kraus et al.[2] that the unpaired electrons present on surface of carbon black particles capture the rubber radicals formed by mechanical shearing of rubber molecules. The surface of carbon black has been known to react with free radicals readily; Donnet and Henrich[3] investigated the reaction of 2,2’-azobisisobutyronitrile (AIBN) with a variety of carbon blacks and concluded that the unpaired electrons on the surface of carbon blacks capture the free radicals, since the number of 2cyano-Zpropyl radicals captured on the surface was of the same order as the number of unpaired electrons reported by Kraus et al.[2]. On the contrary, in 1961 Spackman[4] found that the treatment of carbon black with AIBN or 2,2-diphenyl-lpicrylhydrazyl (DPPH) causes an increase in the number of the unpaired electrons in carbon black. Even below room temperature, benzoyl peroxide (Bz202)in a solution with carbon black added decomposes into benzoate radicals which react with the surface of carbon black or with solvent molecules without loss of carbon dioxide [5-71. The inhibition of free radical polymerization of vinyl monomers by carbon black is well known and Kraus et al. [8] interpreted it as a phenomenon due to the quinone groups on the surface. Donnet et al.[9] proposed an aroxylic structure having localized free radicals on the surface and explained the inhibition in vinyl polymerization by the so-called quinone radical theory. Vinyl
polymerization using initiators in the presence of carbon black has been variously reported[lO, Ill. In the present work, using initiators such as Bz202 and AIBN the polymerization reactions of vinyl monomers in the presence of furnace blacks in nitrogen or oxygen atmosphere were investigated. Moreover, the surface oxygen-containing groups of furnace blacks treated with Bzz02 in oxygen were examined and the reactivity of the groups toward the growing polymer radicals was discussed. 2. EXPERIMENTAL 2.1 Materials and reagents Carbon blacks used were mostly furnace blacks from Phillips Chemical Co., obtained through A. A. Chemical Co. They were kept in dark in a sealed glass container and dried at 110°C in vacua prior to use. The vinyl monomers and initiators were obtained from Wako Pure Chemical Ind., Ltd. and purified prior to use. For instance, styrene monomer containing 0.004% of t-butylcatechol as stabilizer was purified by washing with sodium hydroxide solution and water, drying over calcium chloride, and then by distillation in nitrogen under reduced pressure. Commercially available Bz20? with 98.9% purity was dissolved in warm acetone, and the solution was then added with the same volume of 80% ethyl alcohol, and precipitated crystals were collected. AIBN was recrystallized from methyl alcohol. 2.2 Polymerization in the presence of carbon black The polymerization of vinyl monomers in the presence of carbon blacks was carried out mostly using initiators in a 300ml tear-drop type flask attached with a reflux
443
444
K.
OHKITA et al.
condenser. The reaction mixture was stirred by a magnetic stirrer. Comparisons were made between the runs in nitrogen or oxygen stream at the rate of 3-5 ml/min and the runs in sealed tube without bubbling gases. With an excess of carbon black stirring was difficult at the beginning, but once the reaction started, a sharp drop in thixotropy[l2] made the stirring easier. The conversion for a certain reaction time was obtained in a following fashion: Taking styrene for an example, the reaction mixture was dispersed in toluene and methyl alcohol was added to separate out the carbon black containing polymer; this was then washed with methyl alcohol and dried in vacua. The rate of conversion was obtained from the difference in the weights of this mix and the carbon black initially added [ 131.In case of methyl methacrylate, the dispersant was acetone and the precipitant methyl alcohol. Some of the experiments were carried out in sealed tubes. Carbon black was taken into a 60 ml test tube, a monomer and an initiator added, the mix chilled with crushed dry ice, and the tube sealed. The tubes were placed in a water bath set at a given temperature and tumbled at the rate of 30rpm to carry out the polymerization reaction. 2.3 Identification of polymers grafted onto the surface of carbon black
The polymerization of styrene in the presence of furnace black gives the latter a good dispersibility in tetrahydrofuran. In order to separate carbon black from tetrahydrofuran, a small amount of methyl alcohol was added, but not enough to cause homopolymer to precipitate, and then the mixture was centrifuged at the rate of lo4 rpm for 30 min. Precipitated carbon black was taken out, dispersed again in tetrahydrofuran, and then centrifuged again. After repeating the procedure, the separated carbon black was dried at 100°C.Upon drying, its dispersibility in organic media was lost. After this treatment, 1 g of the carbon black was subjected to the Soxhlet extraction with tetrahydrofuran at the flush-out rate of 10-15 min for 100hr. After extraction the carbon black was dried at 110°Cin vacua. The polystyrene grafted onto the surface of carbon black was analyzed by gas chromatograms of the thermal decomposition products [ 14, IS]. The gas chromato~aph used was Ohkura Rikagaku Model 6000. The operating conditions are given in Table 1. 2.4 TIte reaction of the carbon black surface with BzzOz Two grammes of carbon black, 2.Og of BZZOZ,and 150ml carbon tetrachloride as a solvent were placed into a 300 ml pear shape flask attached with a reflux condenser, and using a magnetic stirrer the mixture was reacted at 45°C. The evolution of carbon dioxide was negligible. Oxygen or nitrogen gas was introduced into the flask gradually at the rate of 5-10 ml/mm during the reaction. At completion of the reaction, the carbon black was separated on a centrifuge at 104rpm for 1 hr. It was air dried and then dried in vacua at room temperature, and finally subjected to Soxhlet extraction using carbon
Table 1. Operatingconditions columnlength
7% cm
Columnmateriels
10 wt8 silicone oilDC 550 and Diasolid M (60-80mesh)
Columntemperature
720%
Detector temperature
tze*c
Injection temperature
200%
grro1ysis tenperature
SOO'C
Samplequantity
3 to 5 mg
tetrachloride and flush-out intervals of lO-15min for 80 hr. During the reaction and purification steps a calcium chloride drying tube was used to remove the moisture. The carbon black so treated and dried at 80°C in vacua was used for quantitative analysis for quinone and hydroquinone groups. Analysis for surface quinone groups was carried out by determination of hydrogen uptake from sodium borohydride solution following a method proposed by Studebaker [ 161and later studied by Suzuki et al. [ 171in detail, and for surface hydroquinone groups by DPPH [ 131. Such treated carbon black was also used to examine the induction period observed during the thermal polymerization of styrene in comparison with the untreated one. 3. RESULTS AND DISCUSSION
3.1 The dependence of poly~e~zation on the su~~ce urea and adsorbed water in the capon
black
During the polymerization of vinyl monomers in the presence of furnace black the free radicals formed by the decomposition of initiators react with the carbon black to produce new unpaired electrons on the surface of carbon black particles, to which the free radicals and the growing polymer radicals compete to reactllg, 191. For such reactions, the nature of the surface of carbon black is the dominating factor, the conversion varying with the degree of agitation[ 191,this being a heterogeneous mixture. In our experiments therefore the stirring was kept always the same so as not to affect the rate of polymeri~tion. Table 2 shows the influence of adsorbed water on the conversion of methyl methacrylate in tetrahydrofuran for three types of furnace blacks used after the exposure to air for 48 hr. The initiator was AIBN and no nitrogen gas was passed through the polymerization vessel. The amounts of adsorbed water were determined beforehand, and exactly l-00 g of each carbon black was taken. As shown in Table 2, lower conversion is observed with dried carbon black and this tendency is larger for the carbon blacks having larger specific surface areas, this shows that the reaction taking place on the surface of carbon black is clearly affected by the water captured in mi~ropores in changing the amount of homopolymer formed. Figure 1 shows the relationship between the amount of
445
The free radical polymerization of vinyl monomers in the presenceof carbonblack Table 2. The influence of water adsorbed on the surface of carbon black on the conversion of methyl methacrylate (MMA) Conversion
(4)*
Specific surface area
Carbon black
Undried+
h'/g) (53
i3C°C
Dried at
hilblacb A (FEF) Philblack 0 (HAF) Philblack I (ISAF)
5
Bifteer.grames
of M4A were polymerized in the presence of
1.OOg of carbon blacks using 0.3Ag of AIBN in 1' ml of tetrahydrofuran at 65°C for 5 hr. Moisture adsorbed
t
Carbon MMA 45’C
black ,lO
0.02%/g
(II).
I
I 002
0.03ogig Cf,, a.mg/g
l.OOg 10.0 ml hr
1 0
$,
0.04
0.06 BzzOz
0.08
0
2
4
hr6
0.10
a9
Fig. 1. Polymerization of methyl methacrylate (MMA) using Bz202 in the presence of various carbon blacks. 0, Philblack A; q,PhilblackO;A,PhilblackI.
10
Fig. 2. Polymerization of styrene in the presence or absence of carbon black using BzzOzas an initiator under nitrogen. Philblack
(1000)
J
BzzOz used as an initiator and the conversion of methyl methacrylate in presence of the three different types of furnace black, the same as in Table 2. In this experiment, the polymerization reaction was carried out in sealed tubes. The results shown in Fig. 1 show that the larger the specific surface area of carbon black, the more BzzOzis needed before the initiation of homopolymer formation.
BZ,OZ Monomer
1
0 0 509 4.13x10~4md 0.10
md
/ (0 999)
(0 997)
(0 995)
3.2 The relationship between the e value for a monomer
(0993)
/_
and the retardation of polymerization
Figure 2 shows the relationship between reaction time and conversion for the polymerization of styrene, with a negative e value (based on Alfrey and Price’s Q and e scheme[20]), initiated with BzzOz in the presence of Philblack I. Even for a ten-fold increase in the amount of the initiator, the retardation in polymerization could not be eliminated, only the tangent of conversion-time curve changing slightly. In the experiments shown in Fig. 2, nitrogen gas was passed through the reaction vessel. Similar retardation as in Fig. 2 was also observed during the polymerization of vinyl acetate, with a negative e value, using BzzOz as an initiator in the presence of Philblack 0. The conversion in polymerization of methyl methacrylate was also examined using BzZOzin the presence of Philblack 0 and compared to that in copolymerization with styrene at 45 f O.l”C in a sealed tube. The results are shown in Fig. 3. The conversion-time curve for polymer-
8
Time,
0
2
4
6 Time,
8
l(
hr
Fig. 3. Copolymerization of methyl methacrylate with styrene.in the presence of carbon black at 45°C.The figures in brackets give mole fractions of methyl methacrylate in monomer mixtures. ization using methyl methacrylate, with a positive e value, is clearly different from that of styrene. Using acrylonitrile, with a positive e value, a similar behaviour was observed. It is interesting to note that a marked retardation is found in the copolymerization of methyl methacrylate with a small amount of styrene. Thus, for a monomer having a negative e value, a significant retardation in the polymerization or copolymerization initiated with BzzOz in the presence of furnace blacks indicates the capture of polymer radicals by the active sites formed during the reaction of benzoate radicals with the surface of carbon black.
K. OHKITAet al.
446
On the contrary, in Fig. 4, for example, the influence of the presence of a furnace black or of the different e value of monomers is not seen too clearly when AIBN was used as an initiator. Therefore, the radicals formed by the decomposition of AIBN have probably less reactivity toward the surface of carbon black than the benzoate radicals, and the concentration of newly formed unpaired electrons is much lower than in the case of Bz20Z. In our earlier work[6], it has shown that, when acted upon by BzzOzin nitrogen atmosphere, furnace blacks are attacked in their exposed benzenoidal rings at the edges of micrographitic platelets and benzoate radicals are substituted for hydrogen atoms like in the case of the reactions between a number of polycyclic aromatic hydrocarbons and Bzz02[21]. On the other hand, Donnet et al. [22] assumed that in nitrogen atmosphere no reaction takes place between AIBN and the benzenoidal rings at the edges of quasigraphitic crystallites on the surface of carbon black; the AIBN was considered to react only with quinonic oxygen atoms. In our earlier study of the polymerization of vinyl acetate containing a small amount of anthracene, pbenzoquinone, and hydroquinone respectively, we have obtained the following results [19] using AIBN and Bz202 as initiators: With BzzOz as an initiator, anthracene and hydroquinone cause retardation of the polymerization of vinyl acetate to a considerable extent like in the case of its polymerization in the presence of a furnace black. On the contrary, p-benzoquinone acts as an inhibitor giving a well defined induction period. With AIBN, all these compounds are inhibitors and no retardation is observed. Differently from benzoate radical, the 2-cyano-2-propyl radical does not abstract hydrogen atoms from alcohols, ketones, and aromatic hydrocarbons. The concentration of unpaired electrons of commercial carbon black[2] is already known to be lower than the concentration of surface quinone groups[23]. In case of Philblack I, the first is known to be 9.2 x 1019/g[2]and the latter about 3.2 x 10Zo/g[23]. Although the vinyl polymerization free-radical initiated in the presence of a furnace black is complex, free radicals of initiators, growing polymer radicals and free radicals formed by the interaction of solvent with benzoate radicals in case of solution polymerization using Bz202 are considered to compete in reacting with the suiface of carbon black[ll], and depending on the 20,
3.3 The influence of solvents on the polymerization Figure 5 shows the relationship between the concentration of AIBN used and the conversion for the polymerization of methyl methacrylate in tetrahydrofuran and in methyl isobutyl ketone in the presence of Philblack 0 at 65°C after 5 hr without bubbling nitrogen. Methyl isobutyl ketone gives a lower conversion than tetrahydrofuran under the same conditions. However, the remarkable difference in the rate of the polymerization in different solvents as shown in Fig. 5 is not observed in the absence of furnace black. Differing from BzzOz, AIBN in giving off 2-cyano-2propyl radicals does not react with solvents, and therefore, the difference in conversion appears to be caused by the inhibition by oxygen dissolved in the solvents during the polymerization. In order to clarify the above mentioned phenomenon, the experiments were carried out both in air and in nitrogen flow at the rate of 3-5ml/min at 65°C for 5 hr. The results show that in nitrogen the amount of AIBN required to start the formation of homopolymer is independent of the type of solvent used. Therefore, it is clear that the large difference in conversion in Fig. 5 depends on the amount of oxygen dissolved in the solvent. 3.4 The polymerization in the presence of oxygen In presence of oxygen different products in the reaction involving free radicals are obtained. Waters et a[.[211 have described the formation of quinone derivatives in the reaction of anthracene with Bzz02 in the presence of oxygen and Tokumaru et al. [24] the formation of phenol in the reaction of benzene with Bz202 in the presence of oxygen. On the other hand, Donnet et a[.,[221 reported that the action of AIBN upon carbon black surface in the presence of oxygen causes more the cyanopropyl radicals to be bound to the surface along with oxygen atoms. We carried out the reaction of Bz202 with carbon black in oxygen at 45°C in carbon tetrachloride and studied the variation in the concentration of quinone groups with an increase in reaction time by hydrogen
1
I A
0
I (g) (moi)
0.5 0.2
0.2
(10~%nol) (“C)
1.5 60
1.5 60
Philblack stvrene AI’BN
a
reactivity of the reactants, their interactions with carbon black surface cause either inhibition or retardation of polymerization.
s
Y
0.05 0
2 Time,
hr4
6
Fig. 4. Polymerization of styrene in the presence or absence of carbon black using AIBN as an initiator under nitrogen.
0.10
0.1 5
0.2c
AIBN,g
Fig. 5. Solution polymerization of methyl methacrylate in the presence of carbon black using AIBN. MIBK: Methyl isobutyl ketone, THF: Tetrahydrofuran.
447
The free radical polymerization of vinyl monomers in the presence of carbon black absorption measurement [ 16,171with sodium borohydride
solution. The results are given in Table 3. A fraction of hydroquinone groups on the carbon black surface may react with Bz202to give quinone radicals and benzoic acid[l8] like in the case of the reaction of BzZO? with the compounds having hydroquinone groups[25]. However, as it is known that hydroquinone groups represent only about a quarter of all quinones on the surface of commercial furnace black[23], the amount of quinone groups newly formed by the oxidation of hydroquinone groups with Bz202 is not large enough to upset the results shown in Table 3. The absorption of hydrogen from sodium borohydride solution often became two to three times as great for the carbon black treated with BzZ02 in nitrogen atmosphere as that for the untreated one. Since the carbon black surfaces are exposed to air after the treatment, as in a Soxhlet extraction, the unpaired electrons formed by the action of BzZOZupon the surfaces but not combined with benzoate radicals perhaps capture oxygen biradicals and turn into quinonic oxygen. This problem is under investigation and the results will be reported at a later opportunity. It is interesting to note that a carbon black treated with BzzOzgives more carbon gel in mixing with natural rubber than the untreated one[26]. In the work reported here, the polymerization of vinyl monomers in the presence of carbon black in oxygen atmosphere was also examined. Two grammes of Philblack 0 were added to 30 g of styrene monomer with 1 g of BzzOZand the polymerization was carried out at 45°C for 100hr in an oxygen stream at the rate of 5 ml/min. Conversion was less than 1% and no homopolymer was formed. The carbon black thus treated with styrene gave a stable colloidal dispersion in toluene. Tetrahydrofuran was used to wash the isolated carbon black, which was then pyrolyzed at 600°C after drying at 110°C in vacua. Its gas chromatogram was totally similar to that of polystyrene itself as shown in Fig. 6.
Time,
that of untreated blacks in nitrogen at 90°C. The results are given in Fig. 7. According to Deviney and Whittington[27], phenolic hydrogens on carbon black surface transfer to the growing polystyrene chains as checked by the use of tritium labelling technique.
0
2
4
6
Time,
carbon black
The thermal polymerization of styrene in the presence of BzzOZ-treatedblacks was examined in comparison with
Table 3. Hydrogen uptake from sodium borohydride solution Hydroger uptake
Carbon black
0
Untreated Treated
black with
Bz20,
c
at
L5’C
for
2C hr
under
O2
60 days * Treated hilblack ““treated Treated
with
Bz2C,
c
at
L5-C
for
‘C
hr under
at
ll5*C
for
20 hr
C2
I black with
Bz,O,
8
10
hr
Fig. 7. Thermal polymerization of styrene in the presence of Philblack I. 0, untreated black; A, treated with Bz,O, under nitrogen for 20 hr; 0, treated with Bz202 under air for 20 hr. The treatment of carbon black with BzIO. was carried out in carbon tetrachloride at 45°C.
3.5 Thermal polymerization of styrene in the presence of
hilblack
min
Fig. 6. Pyrolytic chromatograms (600°C) of the surface of Philblack 0 reacted with styrene using Bz202 in the presence of oxygen.
c.o:!, under
C,
C. 0””
6) 1
It was confirmed by the determination of hydroquinone groups on the surface of Bz,O,-treated carbon blacks using DPPH[ 131that the variations in the number of hydroquinone groups on the surface treated with Bz20Z in oxygen are very small in comparison with those on the original blacks. Accordingly, the long induction period of the inhibition of polymerization seen in Fig. 7 may be responsible for the new quinone groups formed. This is in accordance with the increase in the number of quinone groups.
4. SUMMARY
The interaction of free radicals such as benzoate radical with the surface of carbon black produces new unpaired electrons which capture growing polymer radicals in the presence of vinyl monomers. However, the number of newly born unpaired electrons should vary depending on the type of free radicals used or on the atmosphere in which the reactions are carried out. In the present paper, the vinyl ~lymerization in the presence of furnace blacks using Bz202 was examined. A marked retardation was observed in BzzO,-initiated polymerizations using the monomers with negative e values in the presence of a furnace black, while no such effect was found in the polymerization using the monomers with positive e values. The discrepancy seems to be due to predominant effect from the competition reaction between the free radicals formed by the decomposition of Bz,Oz and growing polymer radicals on the surface. The polymerization process is very sensitive to oxygen. This is due to the p~ticipation of oxygen molecules in the competitive reactions with free radicals present on the carbon black surface. The involvement of oxygen in the polymerization of vinyl monomers when using BZIOI in the presence of furnace blacks promotes the formation of polymer-grafted carbon blacks. The phenomenon is considered to be due to newly added surface quinone groups ur quinone radicals having unpaired electrons which are formed by the reaction of BzzOz with the carbon black surface in oxygen atmosphere.
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Watson W. F., Ind. Engng Chem. 47, 1281(1955).
2. Kraus G. and Collins R. L., Rubber World 139, 219 (1958); Collins R. L., Bell M. D. and Kraus G., J. Appl. Phys. 30, 56 (1959). 3. Donnet J. B. and Henrich G., Compt. Rend. 2463230 (1958); Donnet J. B., Henrich G. and Geldreich L., Ibid. 249, 97 (1959). 4. Spackman J. W. C., &em. Itzd. 1532, London (1961). 5. Studebaker M. L., Rubber Chem. ~eck~o~. 30, 141X! (1957).
6. Ohkita K., Kasahara H., Ishizuki N. and Itagaki Y., Nippon GumaKyokaiski (J. Sot. Rubber Ind., Japan)-%, 361(1%3f. 7. Ohkita K.. Takehara R. and Katoh M.. Koevo Ka&u Zusshi (3. Chem.‘Soc., Japan, Ind. Chem. Sec.) 66, 13&(1%3). 8. Kraus G., Gruver J. T. and Rollmann K. W., J. Polymer Sci. 36, 564 (1959). 9. Donnet J. B., Henrich G. and Riess G., Rev. Gdn. &out. 38, 1803(1961);39, 583 (1962). 10. SociCt6anon. Crylor, Fr. Patent 1188128(1959). 11. Ohkita K. and Ishizuki N., Abstract of Conj on High Polymer held at Takada, p. 23. Hokuriku-shibu, Sot. of Polymer Sci., Japan (1960). 12. TakashinaN. andAidaT.,J. Appl. Polymer&i. 12,1109(1968). 13. Ohkita K. and Tsubokawa N.. Carbon 10.631 (1972). 14. Lehmann F. A. and Brauer G. M., Anal. Chem. 33,673(1961). IS. Ohkita K., Kitahara N., Saitoh H. and Aoyama M., Preprints of Scientific Papers IV-43, international Symposium on Macromolecular Chemistry, Tokyo, Kyoto (1966). 16. Studebaker M. L., Froc. 5th Conf. on Carbon, Vol. II, p. 189. Pergamon Press, New York (1962). 17. Suzuki S. and Takakuwa R., Nippon Kagaku Zasshi (J. Chem. Sot., Jauan. Pure Chem. Sec.) 88. 1271(19671. 18. Ohkita K. and Tajima T., Nidpon’ Gomu’ Kyokaishi (J. Sot. Rubber Ind., Japan) 43, 379 (1970); Ohkita K., Sekiyu Gakkai-shi (J. Japan Petroleum Inst.) 15, 909 (1972). 19. Ohkita K., Tsubokawa N., Kadoi H. and Suneya Y., Nippon Gomu Kyokaishi (J. Sot. Rubber Ind., Japan)45,1074 (1972). 20. Alfrey T., Jr., Bohrer J. J. and Mark H., Copolymerization,p. 64. Interscience, New York (1952). 21. Roitt I. M. and Waters W. A., J. Chem. Sot. 2695 (1952); Norman R. 0. C. and Waters W. A.. Ibid. 167 (1958). 22. Donnet J. B., Metzger J. and Riess G., ~einf~res, Pigments, Vernis 42 f2), 76 (1966). 23. Studebaker M. L. and Rinehart R. W. Sr., Rubber Chem. Technol. 45, 106 (1972).
24. Tokumaru K., Horie K. and Simamura 0.. Tetrahedron 21, 867 (1965).
2s. Cosgrove S. L. and Waters W. A., J. Chem. Sot. 3189(1949); 388 (1951). 26. Ohkita K. and Washizu T. Unpublished results. 27. Deviney M. L., Jr. and Whittington L. E., Rubber Chem. Technol. 41, 382 (1%8).