Polymer Journal, Vol. 5, No. 3, pp 243-247 (1973)

The Chemical-Composition Distribution of - Synthesized by Radiation-Induced Copolymerization at Low Temperature

Shinya TERAMAcm* and Hisashi UCHIYAMA

Department of Applied Chemistry, Faculty of Engineering, Nagoya University, Nagoya, Japan. (Received February 19, 1973)

ABSTRACT: A styrene-acrylonitrile copolymer was synthesized by radiation-induced copolymerization in ethyl bromide at - 78°C. The chemical-composition distribution curve of the sample obtained from the results of compositional fractionation is not continuous, but shows two distinctly different peaks. The acrylonitrile contents of the two peaks are close to those of polymerized by anionic and free-radical mechanisms, respectively. From this result, it becomes clear that the copolymer sample is a mixture of two kinds of copolymers polymerized by both mechanisms, and it is also clear that both anionic and free-radical mechanisms coexist in this copolymerization process. Moreover, it is shown that the copolymer may have a chemical composition corre­ sponding to an anionic mechanism only if the reaction mixture is highly dried under vacuum. KEY WORDS Radiation-Induced Copolymerization / Styrene-Ac- rylonitrile Copolymer / Chemical-Composition Distribution / Frac­ tionation /

In polymerizations induced by radiation, the radical initiator. Tsuda6 first tried to solve the reaction generally proceeds either by a free­ problem by fractionating a styrene-acrylonitrile radical mechanism or by an ionic mechanism, copolymer synthesized by radiation-induced co­ depending on the experimental conditions. polymerization. · The copolymers initiated by Sometimes however, both free-radical and ionic free-radical and by anionic species must have polymerization may proceed simultaneously. In different chemical compositions, as shown later. that case, it has been reported1 - 6 that the Hence, it is possible to answer the above ques­ monomer-copolymer composition curve is often tion by carefully fractionating the copolymer as situated at an intermediate position between the function of the chemical composition. From curves related to the free-radical and the ionic Tsuda's results, it was suggested that the co­ mechanisms. It is likely that the relative im­ polymerization is initiated from an anion-radical portance of the contributions from both mecha­ such as nisms is affected by suppression or acceleration of each mechanism due to the effects of tem­ -CH=CH-CH=N8 perature, impurities, additions of or and, consequently, that the copolymer produced inhibitors, etc. is in fact a block copolymer, one block consisting However, it is not yet clear whether those of the anionic copolymer and the other of the polymerizations are initiated by two different free-radical copolymer. free-radical and ionic initiators, or by an ion- However, considering recent progress in co­ fractionation as function of chemical * Present address: Department of Industrial Chem­ composition, it may now be appropriate to istry, Kogakuin University, Nakqno-Cho 2665-1, Hachi­ reexamine the above conclusion. It is the pur­ oji, Japan. pose of the present work to determine whether

243 s. TERAMACHI and H. UCHIYAMA

the radiation-induced copolymerization of styrene thus precipitated were washed with and acrylonitrile is initiated from two different methanol and dried in vacuo at room tempera­ initiators or from an anion-radical initiator by ture. The acrylonitrile contents of the samples determining the chemical composition distribu­ were determined by micro-Dumas method of tion of the copolymer by compositional frac­ nitrogen analysis. tionation. Fractionation In order to carry out chemical-composition EXPERIMENT AL fractionation, it is necessary to find a combina­ Polymerization tion of and nonsolvent such that the Ethyl bromide was used as the solvent. It is difference in the interaction parameters with monomer units A and B is as large as possible.7 known from Tsuda's data6 that a copolymer having an intermediate composition between that The solubility of the copolymer sample I, having of the free-radical and the anionic is obtained an intermediate composition between the free­ using ethyl bromide as the solvent. Research radical and the anionic ones, was tested in various solvents. The sample was only partially grade ethyl bromide was washed with concen­ trated sulfuric acid and distilled water, dried soluble in benzene, toluene, methyl ethyl ketone, and others, which are solvents for over calcium chloride and distilled. Styrene and chloroform, acrylonitrile were washed several times with polystyrene but nonsolvents for polyacrylonitrile, and also in and ethylene cyano­ dilute aqueous solution of sodium hydroxide and distilled water, and then dried over calcium hydrin, which are solvents for polyacrylonitrile chloride. Just before use, the monomers were but nonsolvents for polystyrene. All solutions distilled under reduced nitrogen atmosphere. In were turbid even when the temperature was the polymerization of sample I, the monomers raised to 80°C or close to the boiling points of and the solvent thus purified were sealed into those solvents. It was observed that the un­ a polymerization ampoule. On the contrary, in soluble part of the copolymer precipitated at the the polymerization of sample II, solvent and bottom of the test tube after a day. monomers were mixed together with calcium The sample was thoroughly soluble in di­ chloride into the glass bulb attached to the high methylformamide which is a common solvent vacuum system, dried by calcium chloride in for polystyrene and polyacrylonitrile. However, the bulb, and then transferred into the polym­ precipitation was not observed when the solvents erization ampoule by vacuum distillation. In mentioned above (the solvents for either poly­ this case, therefore, the monomers and the sol­ styrene or polyacrylonitrile) were added to di­ vent were highly dried. Solutions containing methylformamide solutions of the sample. From 25% weight of the monomer mixture with a the above results, the conclusion was reached composition shown in Table I were subject to that it is difficult to fractionate the sample by irradiation by r-rays from Co60 source at -78°C. the conventional method, i.e., by varying the The dose rates and the total doses were 1.6 x mixing ratio of two appropriate solvents. That 105 rad/hr and 3.6 X 107 rad for sample I, and is, in order to fractionate this sample as a function 4.0 X 105 rad/hr and 1.9 X 107 rad for sample II, of the chemical composition, it is necessary to respectively. After the irradiation, the solutions employ various solvents covering a wide range of were poured into excess methanol. The co- solvent power. The solvents employed in this work are therefore cyclohexane, toluene, benzene, Table I. methyl ethyl ketone and , and AN content, mole fract. are shown in Table II. Sample Monomer Conversion, % The fractionation was carried .out by an ordi­ mixture Copolymer nary column-elution method. The sample was deposited on glass beads, having a size between I 0.179 0.439 1.0 100 and 200 mesh by evaporating the solvent, II 0.329 0.840 0.4 1 dimethylformamide, slowly. The glass beads

244 Polymer J., Vol. 5, No. 3, 1973 Composition Distribution of Radiation-Polymerized Copolymer

Table II.

Fraction no. Wi, mg Wi AN content, mole fract. Solvent mixture, volume. ratio 411.9 0.4134 0.0772 cyclohexane 2 38.8 0.0389 0.157 cyclohexane/toluene (7 : 3) 3 cyclohexane/toluene (3 : 7) 4 11.0 toluene 0.0557 0.177 5 20.0 toluene/benzene (1 : 1) 6 "'l8.9. benzene 7 86.2 0.0865 0.719 benzene/methyl ethyl ketone (1 : l) 8 methyl ethyl ketone 9 8.2 dimethylformamide/benzene (5 : 995) 0.0143 10 0.8'-'l dimethylformamide/benzene (2 : 98) 11 3.5 dimethylformamide/benzene (4: 96) 12 72.8 0.0731 0.839 dimethylformamide/benzene (1 : 9) 13 65.8 0.0660 0.779 dimethylformamide/benzene (3 : 7) 14 49.2 0.0494 0.852 dimethylformamide/benzene (l : 1) 15 158.9 0.1595 0.841 dimethylformami

Polymer J., Vol. 5, No. 3, 1973 245 s. TERAMACHI and H. UCHIYAMA

1.0.-----~-~-~­ ing in chemical composition a, a2 1 therefore, it is 0 possible that the sample is fractionated into a multitude of fractions having continuously vari­ able compositions. This could happen with a

::: 0.5 system of low fractionation efficiency and Tsuda's results may be due to such an effect. Thus, it is clear that both anionic and free­ , radical mechanisms coexist in this copolymeri­ 0 ~'-'-~-m~----~ 0 0.5 1.0 zation process. In the radiation-induced co­ AN (mole fr.) polymerization of styrene and p-chlorostyrene Figure 2. Integral-chemical-composition curve of at -4O°C, the coexistence of cationic and free­ sample I: m, average composition of the sample; radical mechanisms was confirmed by using dieth­ a1, anionic composition calculated using r1 =14 and ylamine and 1, l-diphenyl-2-picryhydrazyl which r2=O.2O; 9 a2, anionic composition calculated using are inhibitors for the corresponding mechanisms r1=33 and r2 =O.OO5; 10 r, radical composition calcu­ of co polymerization. 1 The present results agree lated using r1=O.O3 and r2 =O.52. 8 with the above conclusion in the sense that anionic and free-radical mechanisms coexist, and 85.4%. The average acrylonitrile conterit calcu­ in addition, it was confirmed that the copolym­ lated from the fractionation results agreed with erization processes are not initiated from an an­ that of the original sample with an error of ion-radical. However, the present results do not 0.1%. necessarily deny the existence of the anion­ As shown in Figure 2, the chemical-composi­ radical of acrylonitrile. tion distribution curve of sample I obtained The chemical composition of sample II ob­ from the fractionation results is not continuous tained from a highly dried system agrees with but shows two distinctly different peaks. The the value calculated using a pair of reactivity

acrylonitrile contents of the two peaks are close ratios, r1= 14 (for acrylonitrile; AN) and r2 = to those of copolymers polymerized by anionic 0.20 (for styrene; St). 9 However, this agreement and free-radical mechanisms, respectively. (The may be rather fortuitous since the values of components with higher and lower AN content the reactivity ratios were obtained under quite in sample I are indicated as H and L, different experimental conditions, i.e., at -12°C respectively.) with n-buthyl lithium. In fact, the chemical com­ position of the component H in sample I, which DISCUSSION should be obtained only by an anionic mechanism, does not agree with the value calculated using the From the fractionation results shown in Figure above pair of reactivity ratios, but it is close to 2, it is clear that the copolymer sample is not a the value cakulated using r 1=33 and r 2 =O.OO5 block copolymer as stated by Tsuda, but a mix­ obtained by Mezhirova, Sheinker, and Abkin10 ture of two kinds of copolymers polymerized by as shown in Figure 2. These values of reactivity anionic and free-radical mechanisms, respectively. ratios were obtained by radiation~induced co­ If the sample were such a block copolymer as polymerization at - 78°C in dimethyl stated by Tsuda, the chemical composition of the solution and dried more carefully than in the sample should be broadly distributed between the present experiments. Taking into account the anionic and- the free-radical compositions. difference between the solvents used by Abkin, It was pointed out that the true distribution et al., and by the present authors, it may be curve of chemical compositions cannot generally concluded that the reactivity ratios for an an­ be obtained by one-direction fractionation, i.e., ionic copolymerization 'or styrene and acrylo­ by a fractionation carried out in one fractiona­ nitrile at - 78°C are given by the values of tion system because of the effect of molecular Abkin, et al., and the component H was obtained weight on fractionation. 7 •11 Even when the by an anionic mechanism only. This may mean sample is a mixture of two components differ- that there was some contribution from a radical

246 Polymer J., Vol. 5, No. 3, 1973 Composition Distribution of Radiation Polymerized Copolymer

mechanism even in sample II. activation energies reported by them to estimate

Thus, it is also possible that the component L r1 and r2 at - 78°C, the AN content calculated

may be due to a radical mechanism. Moreover, using values of r 1 and r2 does not agree with it was shown by Tsuda6 that if the reaction the authors' experimental value at all. mixtures of St and AN contain fairly large Acknowledgments. The authors wish to thank amounts of water, the chemical compositions of Professor Mitsuru Nagasawa and Professor the samples obtained by radiation-induced polym­ Zen-ichiro Kuri for their helpful discussions. erization are situated in the vicinity of the curve calculated using the reactivity ratios for the radi­ REFERENCES cal mechanism. Considering that anionic polym­ erization is almost stopped by the presence of I. H. Yamaoka, K. Nishiyama, K. Hayashi, and water, the polymerization in this wet mixture S. Okamura, Kobunshi Kagaku (Chem. High may be mainly by a radical mechanism. Polymers), 24, 649 (1967). In fact, the component L has the chemical 2. A. P. Sheinker, M. K. Yakovlieva, E. V. composition close to the value calculated using Kristalinii, and A. D. Abkin, Dok!. Acad. Nauk SSSR, 124, 632 (1959). r 1 =0.03 and r2 =0.52 reported for a radical co­ 3. Y. Tabata, Y. Hashizume, and H. Sobue, J. polymerization. 8 Therefore, it may be concluded Polym. Sci., Part A, 2, 2647 (1964). that the component L in sample I is synthesized 4. Y. Tabata, Y. l-Iashizume, and H. Sobue, ibid., by a radical mechanism_ only. Part A, 2, 3649 (1964). Unfortunately, the reactivity ratios for radical 5. Y. Tsuda, ibid., 54, 193 (1961). copolymerizations of St and AN were all ob­ 6. Y. Tsuda, ibid., 58, 289 (1962). tained at 40 to 80°C. Strictly speaking, it is 7. S. Teramachi and M. Nagasawa, J. Macromol. meaningless to compare the present data obtained Sci.-Chem.; A2, 1169 (1968). at - 78°C with those values. Nevertheless, it is 8. R. G. Fordyce, J. Amer. Chem. Soc., 69, 1903 interesting to see that the chemical composition (1947). 9. N. L. Zutty and F. J. Welch, J. Polym. Sci., calculated using the values of r1 and r2 obtained at higher temperatures are situated in the vicinity 43, 445 (1960). 10. L. P. Mezhirova, A. P. Sheinker, and A. of the composition of the component L. This D. Abkin, Vysokomol. Soedin., 3, 99 (1961). may mean that the temperature dependence of 11. S. Teramachi and Y. Kato, J. Macromol. Sci.­ r1 and r2 in the radical copolymerization of St Chem., A4, 1985 (1970). and AN may not be so high as reported by 12. G. Goldfinger and M. Steidlitz, J. Polym. Sci., Goldfinger and Steidlitz. 13 If one employs the 59, 163 (1963).

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