Photopolymerization of acrylonitrile in a homogeneous medium

Photopolymerization of acrylonitrile in a homogeneous medium

Photopolymerization of acrylonitrile in a homogeneous medium 1129 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. P. DOTY, ...

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Photopolymerization of acrylonitrile in a homogeneous medium

1129

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

P. DOTY, I. BRADBUR¥ and A. HOLTZER, J. Amer. Chem. Soc. 78: 947, 1956 I. VANG and P. DOTY, J. Amer. Chem. Soc. 78: 498, 1956; 79: 761, 1957 I. VANG, J. Amer. Chem. Soc. 80: 1783, 1958; 81: 3902, 1959 V. L UZZATI, J. Mol. Biol. 3: 566, 1961 C. O'KONSKI and A. HALTNER, J. Amer. Chem. Soc. 78: 3604, 1956; 79: 5634, 1957 C. O'KONSKI, K. VOSHIOKA and W. ORTTUNG, J. Phys. Chem. 63: 1558, 1959 A. WADA, J. Chem. Phys. 81: 495, 1959 M. W~,LI, ACH and H. BENOIT, J. Polymer Sci. 57: 41, 1962 I. VANG, J. Amer. Chem. Soc. 80: 5138, 1958 L. FREUD, Thesis, Strasburg, 1961 F. BLOUT and R. KARLSON, J. Amer. Chem. Soe. 78: 941, 1956 V. N. TSVETKOV and V. A. MARININ, Zh. eksperim, fiz. 18: 641, 1948 E. V. FRISMAN and V. N. TSVETKOV, Zh. tekh. fiz. 25: 447, 1955 I. TINOCCO, J. Amer. Chem. Soc. 79: 4336, 1957 A. PETERLIN, J. Polymer Sci. 12: 45, I954 A. PETERLIN and H. STUART, Hand- u. Jahr. Chem. Phys. 8/I Bd. 1 : 1943 V. N. TSVETKOV and A. I. PETROVA, Zh. tekh. fiz. 14: 289, 1944 V. N. TSVETKOV and N. N. B o r r s o v A , Vysokomol. soyed. 2: 1176, 1960 I. BURGERS, Sec. Rep. on Viscosity and Plasticity, Verh. Ak. Wet. 16: 113, 1938 P. DEBYE and G. ZAKK, Teoriya elektricheskikh svoistv molekul. (Theory of the Electrical Properties of Molecules.) Moscow, 1936 22. L. PAULING, R. COREY and H. BRANSON, Proc. Nat. Acad. Sci. U.S.A. 37: 205, 1951 23. W. KUHN, H. KUHN and P. BUCHNER, Ergebn. Exakt. l~aturwiss. 25: 1, 1951

PHOTOPOLYMERIZATION OF ACRYLONITRILE IN A HOMOGENEOUS MEDIUM* K . SIMIONESCU, D. FEL'DMAN and F. S H A N D R U P. Poni Institute of Chemistry and Physics, Jassy Branch, Academy of Sciences of the R u m a n i a n Peoples' Republic

(Received I1 September 1962) INTRODUCTION

At the present time great attention is being devoted to the polymerization of acrylonitrile (AN) in a medium which dissolves the monomer and the polymer. Under these conditions, concentrated solutions of polyacrylonitrile (PAN) with a molecular weight ensuring fibre formation are obtained. In this way, the industrial process for obtaining polyaerylonitrile fibres is simplified by the elimination of certain difficult and uneconomic operations such as, for example, precipitation, washing, drying, and dissolving the polymer in order to obtain spinning solutions. * Vysokomol. soyed. 5: No. 3, 460-466.

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al.

According to data in the literature, the first investigations in this field were carried out by H a m [1], who used dimethylformamide (DMF) as the polymerization medium. Thomas, Gleason and Pelion [2] showed that the relative reduction of the molecular weight of the PAN obtained from polymerization in DMF was due to the fact that the latter acts as a chain-carrier. Chain transfer by molecules of the solvent in the polymerization of AN in the presence of initiators in DMF, butyrolactone and ethylene glycol carbonate has been studied by Ulbricht [3, 4]. Onyon [5] has studied the photopolymerization of AN in DMF at 25 °, using secondary butyl peroxide as initiator. Srinivasan and Santappa [6] carried out a comparative study of the polymerization of AN in the presence of azoisobutyronitrile in a homogeneous medium (DMF), in a non-homogeneous medium (toluene), and in block. As Campbell [7] showed, AN is capable of polymerizing in DM_F solution under the action of BF a . His experiments, carried out in DMF, dimethylacetamide, N-methylpyrolidine, and other solvents, showed that it is possible to obtain polymers and copolymers of AN if the reaction is performed at a temperature above 100 ° in the presence of oxygen. Bamford, Jenkins, and Johnston [8], came to the same conclusions as Ulbricht, establishing that in the polymerization of AN in DMF in the presence of azoisobutyronitrile and FeC1a an important role in the chain-rupture reaction is played by recombination of the macroradicals. Numerous investigations are also known in which the polymerization of AN has been studied in a non-homogeneous medium using mixtures of DMF with various organic solvents (benzene, toluene, etc.) [9-14]. Copolymers of AN and vinyl acetate have also been obtained in DMF; the copolymerization was carried out under ordinary illumination [15]. The polymerization of AN in a homogeneous medium is carried out not only in organic solvents but also in concentrated solutions of certain inorganic salts. Some of them can be used alone, but most frequently mixtures of them are required. Solutions of PAN in inorganic salt solutions can be used for the direct formation of fibre, just like the solutions obtained in DMF. The first polymerizations in inorganic salt solutions were described by GrSbe [16], who heated a solution of the monomer in 80% ZnC12 solution in the presence of benzoyl peroxide. H u n y a r and Gr6be [16] studied the polymerization of AN in a solution of ZnC12~-CaC12 under the influence of UV radiation and in the presence of ferric chloride. Recently, Stoy [17] has carried out the oxidationreduction polymerization of AN in concentrated aqueous solutions of ZnC1e and CaCl~ (or MgC12). The high velocity of the reaction in the presence of Fe e~, Cu e+, and Mn ~+ ions makes it possible to carry out polymerization at relatively low temperatures. This author claims that the process can be used as a new method for obtaining acrylic fibres with a high molecular weight. Of particular interest are the investigations of Kargin, Kabanov, and Zubov [18], who obtained crystalline isotactic poly-(methyl methacrylate) by the radical

Photopolymerization of aerylonitrile in a homogeneous medium

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polymerization of methyl methacrylate in the presence of ZnC12 dissolved in the monomer. The polymer, which was obtained with the aid of UV irradiation, dissolved completely in excess of the monomer. The conversion was 20-30%. The resulting poly-(methyl methacrylate) dissolved in acetone from which it could be precipitated several times b y means of methanol in order to remove the zinc chloride completely. A similar type of investigation is also found in the work of Bovey [19]. In order to obtain PAN b y photopolymerization, solutions of magnesium perchlorate have also been used [20]. EXPERIMENTAL

We have studied the photopolymerization of AN in DMF in a concentrated solution of ZnC12 or a mixture of CaCl~ and ZnCl~. The experiments were carried out in a quartz vessel the dimensions of which are shown in Fig. 1 at a constant temperature under the influence of the UV radiation of a Tel't lamp (300 W) with a 2260-5790 A spectrum. An acidified aqueous solution of cerium sulphate was used as initiator. We have used this initiator, which is mentioned in the literature [21, 22], for other investigations also [23-25]. The mechanism of the action of cerium salts as polymerization initiators has not been fully explained at the present time. It has not been established whether it consists in the simple transfer of one electron from the monomer to the cerium ion and the formation of a free radical or whether the cerium ion oxidizes the monomer, forming intermediate radicals which initiate the polymerization. Since the cerium salt is reduced, the mechanism can be represented schematically in the following way: Ce4+~ M-->C1s+~-M'; M"÷ M->M~ . . . -->M~ where M is the monomer, M" its radical, M~ the radical of the dimer, and M~ the macroradical; chain rupture can take place by recombination and disproportionnation of the radicals, b y chain transfer b y the solvent, etc. Bamford and White [26] have shown that during the polymerization of AN in DMF chain transfer b y the solvent takes place which leads to the appearance of HCON(CHa)CH~. Other investigators [2, 3, 6] explain the cessation of the growth of the macroradicals by chain transfer b y the solvent with the participation of the hydrogen atom connected with the CO-group, which leads to the radical "CON(CHa) ~. Bevington and Eaves' experimental results [27] confirm that during the polymerization of AN in a homogeneous medium recombination of the macroradicals takes place. In this case, AN behaves abnormally, since it is assumed that in the polymerization of vinyl compounds rupture takes place by disproportionation of the macroradicals. What has been said above shows that the photopolymerization of AN in the presence of cerium salts takes place b y a radical mechanism.

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We have studied the conversion of AN as a function of the time of irradiation in an atmosphere of purified COs at a volume ratio between the monomer, the initiator (Ce(SO4)~.4H~O , 1.537 g/l., acidified with hydrochloric acid 26.82 g/1.), and the solvent of 1 : 3 : 15. The solvent consisted of 31.25% by weight of CaC12 , 31.25% by weight of ZnCl~, and 37-50% by weight of I-I20, or 62.50% by weight of ZnC12 and 37.50~ by weight of H20. ¢2.4ern

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FIO. 2. Influence of the time of irradiation on tho conversion in the polymerization of AN in solutions of ZnClI and ZnCli-}-CaC]I. Concentration of cerium salt 1.537 g/1.; 1--ZnC] 8, 2-- ZnClI ~-CaC]I . I t can be seen from Fig. 2 t h a t in ZnCl~ solution after 2 hours' irradiation the conversion is greater than in the mixed salts, while after 6 hours conversion into polymer exceeds 8 0 ~ . In the course of the first hour's irradiation, because of an induction period, the conversion is lower (by approximately 10%). The polymer formed in the salt solutions mentioned imparts to them the necessary viscosity for spinning solutions. When the solution is diluted with water, the polymer precipitates and the salts are washed out. In spite of this, the polymer obtained in a solution of ZnC1s retains it partially even after repeated washing, which leads to a distortion of the experimental results. The molecular weights determined viscosimetrically at 20-4-0.1 ° in DMF solution are shown in Fig. 3. From the Figure it can be seen t h a t the highest molecular weight (above 400,000) is possessed by a polymer obtained in a solution of the mixed salts, and the lowest (below 400,000) in ZnCl~ solution. I t must be noted t h a t in the latter case the change in the molecular weight depends on the time of irradiation over a wide range. Thus, at the beginning of the polymerization the molecular weight is less than 100,000, and after 3.5 hours' irradiation the molecular weight reaches maximum figures. This shows t h a t when polymerization is carried out in ZnCl~

Photopolymerization of acrylonitrile in a homogeneous medium .

1133

solution it is possible to v a r y the properties of the polymer considerably, while the molecular weight of the PA N obtained in a solution of mixed salts remains almost constant with sufficiently prolonged irradiation.

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FIG. 3. Change in the molecular weight of PAN as a function of the time o f irradiation: 1--ZnCI=; 2--ZnCI=+CaC]=. FIG. 4. Specific viscosity of solutions of PAN in ZnCI= (1) and ZnCI=+CaCla(2) as a function of the time of irradiation. Figure 4 shows the change in viscosity (in centipoises) of a solution of the polymer as a function of the time or irradiation. The measurements were carried out using a Vogel-Ossag viscometer at 50 ° for solutions in mixtures of ZnC] 2 and CaCl 2 and at 70 ° for solutions in ZnCl=. I t was established t h a t the viscosity of these solutions rises considerably as the time of irradiation is increased. Experiments were carried out simultaneously on the polymerization of AN in DMF with the same initiator and with the same ratio of monomer to initiator to solvent in order to elucidate the influence of the t em perat ure and the time of photopolymerization on the yield of polymer and its properties. I t can be seen from Fig. 5 t h a t when the reaction was carried out in DMF the conversion was considerably lower t h a n in the salt solutions mentioned above and t h a t the conversion depended on the temperature; the highest yield of polymer was found at 0 °. For the experiments carried out at 0 °, ice was used for cooling. The molecular weight of polymers obtained in DMF (Fig. 6) in the presence of cerium perchlorate as initiator was considerably less t h a n t h a t of polymers obtained in the salt solutions. Thus, for example, in DMF at 0 ° a PA N was obtained with a molecular weight of 26,000-34,000, while at 30 ° it was approximately 20,000. The low molecular weight is due to a chain transfer reaction by the solvent, as in cont~rmed by Thomas, Gleason, and Pellon [2] and other investigators [3, 6, 26].

1134

K. SIMIONESCU et al. f 25

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Polymerization carried out in ZnC1s + CaC1s ZnCl~ DMF DMF DMF DMF

Polymerization temperature, °C

Velocity of polymerization, mole. 1.-1 × s e e - l ,

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K. SIMIONESCUet al.

Properties of P A N . The PAN obtained in DMF has the form of geometrically regular plates, while t h a t obtained in inoroganic solvents is in the form of a powder. The polymer precipitated by water from solution in DMF turned yellow if it was heated to 190* and red in the range of temperature from 250 to 280 °. The PAN did not decompose even at 300 °, in spite of the fact t h a t on cooling it darkened; however, its geometrical form was retained. The I R spectrum of the PAN (Fig. 7), obtained with a UR-10 spectrophotometer, is practically identical with the spectrum of PAN obtained by the usual methods. The I R spectrum is characterized by the following bands: 2930 and 2875 era-t--bands corresponding to the --CH 2 - groups; 2247 c m - l - - a band corresponding to the --CN group; a band at 1720 cm -1 probably corresponds to the vibrations of the carbonyl group of the amide derivative which m a y be formed as the result of the hydrolysis of some of the nitrile groups; and the band at 1460 cm -t corresponds to the deformation vibrations of the methylene groups. The origin of the bands corresponding to frequencies of 1600-1700 cm -1 and appearing about the bands of normal spectrograms is still unknown to us. The Table gives the rates of the photopolymerization of AN under the conditions which we used. CONCLUSIONS

(1) The possibility of obtaining spinning solutions of PAN by polymerizing aerylonitrile by a radical mechanism photoinitiated in the presence of cerium salts has been shown. (2) The polymerization was carried out either in salt solutions (ZnCl~, ZnCI~ -}-CaC]~) or in DMF and polymers were obtained with molecular weights of ~ 400,000 (in ZnC12, ZnC1e +CaCl~) and 26.000-34,000 (in DMF). The conversion of acrylonitrile into polymer is higher when the polymerization is carried out in salt solutions than when it is carried out in DMF. (3) The properties of the polymers do not differ from the properties of polymers obtained by other methods. Translated by B. J. HAZZARD REFERENCES

1. 2. 3. 4. 5. 6. 7. 8.

G. E. HAM, Textile Res. d. 24: 597, 1954 W. M. THOMAS, E. H. GLEASON and J. J. PELLON, J. Polymer Sci. 17: 275, 1955 J. ULBRICHT,Fa~erforsch. 19: 115, 1959 J. ULBRICHT, Fa~erforsch. 10: 166, 1959. P. E. ONYON, Trans. Faraday See. 52: 80, 1956 N. T. SRINIVASAN and M. SANTAPPA, Makromolek. Chem. 26: 80, 1959 T. W. CAMPBELL,J. Polymer Sci. 28: 87, 1958 C. H. BAMFORD, A. D. JENKINS and R. JOHNSTON, Trans. Faraday Soc. 55: 179, 1959 9. I. MINORU and T. HIROSHI, J. Polymer SoL 31: 195, 208, 1959 10. M. IMOTO and H. TAKATSUGI, Makromolek. Chem. 23: 119, 1957 11. M. IMOTO and H. TAKATSUGI, Chem. of High Polymers (Japan) 15: 70, 1958

Long-lived radicals formed in },-irradiation of cellulose

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12. K. NAKATSUGA, S. WADA and M. KOIZUMI, Chem. of High Polymers (Japan) 14: 613, 1957 13. K. NAKATSUGA, Chem. of High Polymers (Japan) 14: 619, 1957 14. K. NAKATSUGA, Chem. of High Polymers (Japan) 15: 48, 1958 15. M. TANYAMA and G. eSTER, Bull. Chem. Soc. (Japan) 30: 856, 1957 16. A. HUNYAR and V. GREBE, Faserforsch. 6: 548, 1955 17. A. STOY, Sbornik v~deck~ch pr~ci, 231, 1959 18. V. A. KARGIN, V. A. KABANOV and V. A. ZUBOV, Vysokomol. soyed. 2: 765, 1960 19. F. A. BOVEY, J. Polymer Sci. 47: 480, 1960 20. V. GR~JBE and A. SPODE, Naturwiss. 44: 506, 1957 21. J. SALDICK, J. Polymer. Sci. 19: 73, 1956 22. S. VENKATAKRISHNAN and M. SANTAPPA, Makromolek. Chem. 27: 51, 1958 23. C. SIMIONESCU and D. FELDMAN, Buletinul Inst. Politehnic Iasi, seria noun, VI, (XI), 3-4, 161, 1960 24. C. SIMIONESCU, D. FELDMAN and C. VASILIU, Rev. de Chimie, Bucuresti, 12: 525, 1961 25. C. SIMIONESCU, D. FELDMAN and C. VASILIU, Faserforsch. 12: 70, 1962 26. C. H. BAMFORD and E. F. T. WHITE, J. Chem. Soc. 1860, 1959 27. J. C. BEVINGTON and D. E. EAVES, Trans. F a r a d a y Soc. 55: 1777, 1959

Letter to the Editor LONG-LIVED RADICALS FORMED IN THE v-IRRADIATION OF CELLULOSE * V. N. MAKATUN,

A. K. I)OTAPOVICH

and I. N. YERMOLENKO

(Received 13 June 1962) D E A R SIR, The radiation of cellulosic materials is known to give rise to free radicals since E P R spectra have been reported [I]. Since the evidence is very scanty the nature a n d property of the radicals cannot be established. This note describes a study of E P R spectra of the products of a radiation of cellulosic materials (in air, in vacuum a n d in the presence of additives) using e0Co },-radiation (intensity 36 roentgens/sec, dose 1.2× 107 roentgens). Spectra have been obtained by means of a spectroscope with a high-frequency modulation of the magnetic field working at a wavelength of 3.1 cm b y the method of Semenov a n d B u b n o v [2]. The irradiation product gives a n intense E P R spectrum (5 x l0 ~° particles per 162 g of cellulose). The breadth of the spectrum is 76 oersted. When the irradiated material is stored at room temperature, the intensity of the signal falls slowly--after 45 days the signal is still recorded at a signal/noise ratio of the order of 3 : 1. The spectrum of the irradiated cellulose consists of a n intense central line with a g-factor close to that of diphenyl* Vysokomol. soyed. 5: No. 3, 467-468, 1963.