RADIATION INITIATED POLYMERIZATION OF ACRYLONITRILE IN EMULSION* V. V. POLIKARPOV, V. I. LUKHOVITSKII, l~. M. POZDEYEVA and V. L. KARPOV L. Yu. Karpov Physico-Chemical Research Institute
(Received 17 July 1972) A study has been made of the emulsion polymerization of acrylonitrile initiated b y y-radiation, in the presence of cetylpyridinium chloride, K-30 and potassium laurato. The dependence of the n u m b e r of polymer particles and of the rate of polymerization on emulsifier concentration, dose rate and temperature was determined. I t is shown that at high concentrations of a cationic emulsifier the n u m b e r of particles increases during the course of polymerization, b u t in the presence of anionic emulsifiers or low concentrations of cationic emulsifiers, it remains constant. From the results of this work a qualitative picture of the emulsion polymerization of AN is proposed, based on the occlusion theory.
CgE~_tCAL initiation of the emulsion polymerization of acrylonitrile (AN) has been studied mainly within the limit of its solubility in water [1-3]. Only in reference  are results presented on polymerization of AN in an emulsion containing a b o u t 30% of AN. The kinetic results obtained b y various authors from emulsion polymerization of AN are not in accord with the "classical" theory of Smith and Ewart. Radiation initiation of emulsion polymerization of AN has received very little study. It was shown in references  and  that it is possible to carry out polymerization at high rates in systems with emulsifier concentrations above the critical mieelle concentration (CMC) and in reference  it was shown that it is possible to conduct emulsion polymerization at emulsifier concentrations below the CMC. Detailed kinetic data are not given in these papers. EXPERIMENTAL Acrylonitrile (specification GOST 11097-64) was redistilled at atmospheric pressure, the middle fraction being used for polymerization. Cetylpy,'idinium chloride (CPC) (MRTU 6-09-995-64), emulsifier K-30 (a mixture of sodium alkylsulpb.onates of the average composition Cl~H31SOaNa), lauric acid (MRTU 6-09-4211-67), potassium hydroxide (Chempol) and DMF (Soyuzkhimeksport) were used without further purification. The acrylonitrile was polymerized mainly in remote reading dilatometers, the apparatus, mgthod of filling and irradiation of which was described in reference . Sealed tubes were used in some experiments a n d the method of filling and irradiation of these was * Vysokomol. soyed. AI6: No. 10, 2207-2213, 1974. 2559
V. V. POLIY~ARPOV et al.
described in reference . The viscosity average molecular weight of the polyacrylonitrile was calculated from the formula [ff]= 2.33 X 10-4 × ~.Ts . The size of the latex particles "was measured by the light-scattering method [ll]. RESULTS AND DISCUSSION
F i g u r e 1 shows kinetic curves of the emulsion p o ] y m e r i z a t i o n of A N in the presence o f two emulsifiers, one being cationic (cetylpyridinium chloride ( C P C ) ) a n d t h e o t h e r anionic (potassium laurate (PL)). The curves are ~f the f o r m characteristic o f heterogeneous p o l y m e r i z a t i o n , i.e. t h e p o l y m e r i z a t i o n r a t e v increases as t h e degree of conversion increases. B e t w e e n 30 and 60% conversion the change
30 50 7"[me~m/n
I /~-] -2"0
-f'2 log Co
FIG. 1. Kinetic curves of emulsion polymerization of AN initiated by 7-radiation; I = 16 rad/ /sec, 25 °, [AN]=20~/o, [CPC]=0'5% (1), 0.02% (2~, [PL]----1% (3) and 0.02% (4). FIG. 2. Dependence of the number of particles (m1-1) on the concentration (~o) of emulsifier; CPC (I) and K-30 (II), 25°; I = 9 ( 0 , [~) and 2 rad/sec (©, ×); [AN]=7"5~o by volume. in r a t e is small however. T h e r a t e of p o l y m e r i z a t i o n t a k e n below is the a v e r a g e r a t e in this section of the curve. I t is seen f r o m the curves t h a t p o l y m e r i z a t i o n in t h e presence of the cationic emulsifier is faster t h a n w h e n the anionic emulsifier is used. W h e n 7-irradiation is used initiation a n d p o l y m e r i z a t i o n can occur in b o t h t h e aqueous (saturated aqueous solution of AN) a n d organic phases, if the concentration of A N in the s y s t e m is higher t h a n its solubility in water. I t is seen f r o m T a b l e 1 t h a t the composition o f the p o l y m e r i z a t i o n p r o d u c t is d e p e n d e n t on the t y p e a n d c o n c e n t r a t i o n of the emulsifier (Ce) a n d on the m o n o m e r c o n c e n t r a t i o n [M] in t h e s y s t e m initially. W h e n P L is used a dispersion is f o r m e d (at a conversion
Radiation initiated polymerization of acrylonitrile in emulsion
of 8 0 - 9 0 % ) . W i t h CPC (0.02-1%) a n d K-30 (0.02-0.5%) latices are f o r m e d if t h e original m i x t u r e does n o t contain m o n o m e r droplets. I n m i x t u r e s c o n t a i n i n g an organic phase a c o a g u l u m is f o r m e d in a d d i t i o n to the latex, the q u a n t i t y of eoag u l u m increasing as the emulsifier c o n c e n t r a t i o n is decreased a n d the phase ratio is increased. I t should be n o t e d h o w e v e r t h a t a t c o n c e n t r a t i o n s of K-30 of 1% a n d above, p o l y m e r is f o r m e d only in the f o r m of a suspension. TABLE 1. COMPOSITIONOF THE POLYMERIZATIONPRODUCTAT VARIOUSINITIALCONCENTRATIONS OF AN AND EMULSIFIER (25 °, I = 9 rad/sec)
Ratio of organic to aqueous phase by weight 1
3-1 0 1 3-1
Degree of conversion, %
Quantity of polymer in latex form, % I
inQUantity of polymer coagulum, % I
92.0 80-0 90.0
0 0 0
100-0 100-0 lOO.O
1.0 0.02 1.0
20-0 10.0 10.0
6.4 1 6.4 0 1 6.4 0 1 6.4 0 0 1 6.4 1 6.4 0
I n order to eliminate the effect of the coagulum, s t u d y of the effect of v a r i o u s p a r a m e t e r s on the size a n d n u m b e r o f the p o l y m e r particles was carried out a t A N c o n c e n t r a t i o n s below the solubility limit. Figure 2 shows the effect of the conc e n t r a t i o n of CPC a n d K-30 on the n u m b e r 9 f particles at two dose rates (9 a n d 2 rad/see). The n u m b e r of particles is i n d e p e n d e n t of the dose rate (I) a n d is prop o r t i o n a l to the c o n c e n t r a t i o n of CPC to the p o w e r of 0.6. W h e n K-30 is used as t h e emulsifier the n u m b e r of particles increases as the emulsifier c o n c e n t r a t i o n is
V . V . P O L I K A R P O V et al.
increased between 0.02 and 0.2%. In the region of emulsifier concentrations of 0-2-0-5% (the CMC of K-30 is 0.2%) a limiting number of particles is reached and if the concentration of K-30 is further increased coagulation occurs.
O Z .~ z'
FIG. 3. Dependence of the number of particles on the degree of conversion at 25°; I = 9 rad/sec; [AN]=7.5% by volume; 1--[CPC]----0.5%; 2--[K-30]----0.5~o, 3--[0P0]--0.02% (A), [K-30] -----0.05% (B). I~G. 4. Dependence of polymerization rate (mng/ml of emulsion/min) oh emulsifier concentration at 25°, I----16rad/sec; [AN]-----20~o; 1--CPC, 2--K-30, 3--PL. The variation in the number of particles with increase in conversion is dependent both on the type of emulsifier and on its concentration in the aqueous phase (Fig. 3). At high concentrations of CPC the number of particles is proportional to the degree of conversion, the diameter of the particles remaining paractically constant, as is shown b y the figures presented below ([AN]----7.5%, ce-~0.5%, I ~ 9 rad/sec, 25°). Degree of conversion, ~o Diameter of particles,/~
19.0 34.4 54.2 69.7 78.1 82.1 89.5 367 347 363 362 364 362 371
At low concentrations of CPC the particle diameter increases as the'conversion increases, b u t the number of particles remaining constant at conversions above 20%. When K-30 is used, both at low (0.05%) and high (0.5 %) concentrations the number of particles is practically constant at the degrees of conversion studied (above 20%). The polymerization rate is much higher in the presence of small quantities of an emulsifier than when no emulsifier is used. For example in a mixture consisting of 20% of AN 80% of water, in the absence of an emulsifier v : 0 . 9 mg/ml of emulsion/min and in the presence of 0.01% of CPC v z 2 . 6 mg/ml of emulsion/min. The polymerization rate varies very little when the emulsifier concentration is increased beyond this. The dependence of the rate on emulsifier concentration is 6 practically the same for the anionic and cationic types i.e. Y- N r 9_0-13 for CPC and v ~Ce°'15 for K-30 and P L (Fig. 4).
Radiation initiated polymerization of acrylonitrile in eTnttlsiort
The dependence of the polymerization rate on dose rate over the range from 2.5 to 55 rad/sec was studied at [CPC]=0.02 and [AN]=20%. The log-log graph o f this r e l a t i o n s h i p is n o t linear. I n t h e dose r a t e i n t e r v a l s f r o m 2.5 to 16 a n d f r o m 16 to 55 rad/see t h e r e l a t i o n s h i p can be e x p r e s s e d as v ~ I °'s-l"° a n d v , , , I °'3-°'4 respectively. T h u s as t h e dose r a t e is increased its effect on t h e p o l y m e r i z a t i o n r a t e decreases. log u 0.8 O.6 O.Zl 0.2 0 -0.
FIG. 5. Dependence of polymerization rate on dose rate at 25 °, [CPC]=0"02~o, [AN]=20%.
A similar relationship has been found in polymerization of AN in bulk and in solution . H o w e v e r t h e p o w e r i n d e x o f t h e dose r a t e in reference  begins to fall a t 5 rad/sec. T h e p o l y m e r i z a t i o n r a t e increases s u b s t a n t i a l l y as t h e t e m p e r a t u r e is increased. As t h e t e m p e r a t u r e is raised from. 25 to 55 ° t h e p o l y m e r i z a t i o n r a t e is m o r e t h a n
3"2# /03/7 -
[A N], vo/. % FIG. 7
FIG. 6. Dependence of polymerization rate on temperature at I = 16 rad/sec; [CPC]= 0"02~o; EAN] = 20 % FIG. 7. Dependence of polymerization rate on the concentration of AN at 25°; I = 12 rad/sec; [CPC]=0-5~o (1), 0-02% (2).
V.V. POLIKaaU'OVet al.
doubled (Fig. 6). The apparent energy of activation calculated from Fig. 6 is 7 kcal/ /mole, which is fairly close to the total energy of activation for photochemical polymerization of AN in solution in DMF (5 kcal/mole), given in reference . In "classical" emulsion polymerization the rate is independent of the ratio of the phases, i.e. polymerization does not occur in the organic phase. In emulsion polymerization of AN initiated by y-radiation polymerization takes place in both phases, as a consequence of which the rate is dependent on the concentration of free monomer in the original emulsion. I t is seen from Fig. 7 t h a t the emulsifier concentration affects this relationship. At a low emulsifier concentration (0.02%) as the concentration of AN is increased from 10 to 15% the rate increases by a factor of 1.4 and then remains practically constant. When the concentration of CPC is high (0.5%), over the range of concentration of AN from 10 to 20% the rate increases by a factor of 1.8 and then the variation becomes much less. I t is seen from Fig. 2 t h a t the molecular weight falls a little as the emulsifier concentration, dose rate and degree of conversion are increased and increases with increase in monomer concentration and temperature. TABLE
Emulsi tier CPC
2. D E P E N D E N C E
OF ~ v
ON THE POLYMERIZATION ,
0"05 0"05 0"5 0-5 0"5 0"5 0"5 0"5 0"5 0"5 0"5 0"05 0"5
7.5 7.5 7"5 7"5 7.5 10.0 15.0 20.0 30.0 20.0 20.0 7'5 7-5
I, rad/sec I 9.0 2.0 9'0 9"0 9.0 12.0 12-0 12.0 12.0 16.0 16.0 9.0 9.0
T, °C 25 25 25 25 25 25 25 25 25 35 55 25 25
~Degree of Iconversion, % 87.0 70.0 19.0 54.2 82"0 78.5 82.0 81.5 70"5 93.0 86.0 70.0 72.0
2~v × 10-~
7.9 10-0 9.3 7.8 7-0 9.0 13.0 16.0 20.0 15.0 19.0 8.5 7-9
The relationships found here differ considerably from those given in references [1-4]. The dependence of the rate of polymerization on the initiation rate is not constant and varies with dose rate. In contrast to reference  the number of particles increases during the course of polymerization at high concentrations (0.5%) of the cationic emulsifier (Fig. 3), while their size remains practically unchanged. At low concentrations of CPC and over the entire range of concentrations of K-30 studied, after a low degree of conversion the number of particles remains practically constant. In contrast to references  and  the number of particles does not reach a limit in fully polymerized latices with variation in the concentration of CPC over a wide range (0.01-1%), the number of particles being
Radiation initiated polymerization of acrylonitrile in emulsion
proportional to the concentration of CPC to the power of 0.6. Approximately the same relationship is found for K-30 up to Ce----0"2-0"5~oat higher concentrations, in contrast to references  and , where a limit is reached, coagulation occurs. A possible cause of the difference from the results in the literature is the method of initiation. In references [1-4] the initiator was ammonium persulphate, which generates free radicals in the aqueous phase or on the surface of the latex particles. ]n our work free radicals are produced under the influence of ~,-radiation in both the aqueous and organic phases. Moreover the persulphate affects the stability of the latex particles since it increases the ionic strength of the solution. Our results are not in accord with the theory of "ideal" emulsion polymerization, evidently because of the high solubility of AN in water and the insolubility of the monomer in the polymer. A possible approach to an explanation of the present results is to consider the emulsion polymerization of AN from the point of view of the occlusion theory of heterogeneous polymerization of monomers that are insoluble in their own polymer . According to this theory a polymeric radical formed in the aqueous phase separates from that phase in the form of a tight coil. The average reactivity of the radicals will fall as the size of the coils increase by agglomeration with others or with polymer particles. In the presence of surface active agents (emulsifiers) the pattern of the polymerization can be represented qualitatively in the following way. Chain propagation begins in the aqueous phase and the quantity of polymer chains formed is proportional to the rate of initiation. The polymeric radicals (coils) are stabilized b y the emulsifier present in the system (in polymerization without an emulsifier the macroradicals agglomerate and separate as a precipitate at an early stage). Since the activity of a radical in a coil, which is determined by the accessibility of monomer to the active end, is higher than in the precipitate, the polymerization rate is higher in the presence of emulsifier. As the size of the coil increases the rate of polymerization in the coil decreases, as a result of hindrance to diffusion of monomer to the radical, and it becomes low enough to be equivalent to first order termination. Polymer particles are formed b y agglomeration of these coils b y the mechanism described in reference . I f agglomeration occurs at the stage of propagation in which the polymerization rate in the coil is rather low, it will have no appreciable effect on the togal polymerization rate. When the concentration of CPC is high (0.5%) after a particle has attained a certain size the degree of saturation of its surface with emulsifier becomes sufficient to prevent its agglomeration with new polymer coils and other particles, which obviously explains the constant particle size. At low concentrations of CPC (0.05%) the quantity of emulsifier is insufficient to prevent agglomeration of newly formed coils with latex particles already present. As a consequence of this, after a certain concentration of latex particles is reached no new particles are formed.
V.V. POLIK~POV et al.
The differences observed when anionic emulsifiers are used (formation of a suspension in the presence of PL, coagulation at high concentrations of K-30 and a constant number of particles at concentrations of 0.01-5%) are probably due to the lower stabilization effect of anionic emulsifiers. It is possible also that t h e differences are a s s o c i a t e d w i t h a specific effect of r a d i a t i o n on t h e s t a b i l i t y o f p o l y m e r i z i n g l a t e x s y s t e m s [9, 16]. The p r o p o s e d q u a l i t a t i v e outline p r o v i d e s an e x p l a n a t i o n of t h e f a c t t h a t t h e n u m b e r of particles is i n d e p e n d e n t of t h e initiating dosage (since t h e n u m b e r of p a r t i c l e s is d e t e r m i n e d o n l y b y t h e c o n c e n t r a t i o n a n d t y p e of emulsifier) a n d t h e a l m o s t linear r e l a t i o n s h i p b e t w e e n p o l y m e r i z a t i o n r a t e a n d dose r a t e (since firsto r d e r t e r m i n a t i o n is assumed). Translated by E. O. PHrr.LIPS REFERENCES
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W. M. THOMAS, E. GLEASON and G. M]~I0, J. Polymer Sci. 24: 43, 1957 Z. IZUMI. H. KIUCHI and M. WATANABE, J. Polymer Sei. A-I, 5: 455, 1967 Z. IZUMI, J. Polymer Sei. A-l, 5: 469, 1967 H. CHEItDROIq, Kunststoffe 50: 568, 1960 G. LEY, C. SgHNEIDER and D. O. HUMIYIEL, Polymer Preprints 7: 725, 1966 G. LEY, D. O. HUMMEL and C. SCHNEIDER, Advances Chem. 66: 184, 1967 V. I. LUKHOVITSKH~ A. M. SMXRNOV, A. M. LEBEDEVA, V. V. POLIKARPOV, K. G. GLAZKOVA, R. M. POZDEYEVA and V. L. KARPOV, Vysokomol. soyed. B13: 173, 1971 V. I. LUKI~OVITSKII, V. V. POLIZKAItPOV, A. 1K. LEBEDEVA, It. M. LAGUCHEVA and V. L. KARPOV, Khimiya vysokikh energii 4: 173, 1970 V. I. LUKI~OVITSKH, V. V. POLIKAItPOV, A. 1K. LEBEDEVA, R. M. LAGUCHEVA and V. L. KARPOV, Vysokomol. soyed. A10: 835, 1968 (Translated in Polymer Sci. U.S.S.R. 1O: 4, 969, 1968) S. R. RAFIKOV, S. A. PAVLOVA and I. I. TVERDOKLEBOVA, Metody opredeleniya molekulyarnykh vesov i polidispersnosti vysokomolekulyarnykh §oyedinenii (Methods of Determination of the Molecular Weights and Polydispersity of Maeromoleeular Compounds). p. 326, Izd. Akad. Nauk SSSR, 1963 I. Ya. SLONIM, Optika i spektroskopiya 8: 98, 1960 A. CI~.PIIt0, Radiation Chemistry of Polymeric Systems, p. 205, Interscieneo Publishers, 1963 It. H O U W and A. STAVERMAN, Khimiya i tekhnologiya polimerov, (Chemistry and Technology of Polymers). Vol. 1, p. 261, Izd. "Khimiya", 1965 (Russian translation) C. H. BEMFORD, W. BARB, A. D. JENKINS and P. F. ON-YON, Kinetika radikal'noi polimerizatsii vinilovykh monomerov (Kinetics of Vinyl Polymerization by Radical l~Iechanisms), p. 129, Foreign Literature Publishing House, 1961 (l%Bssian translation) V. I. YELIsE~rEvA (Ed.), Polymerizatsionnye plonkoobrazovateli (Film-forming Polymers). p. 16, Izd. "Khimiya", 1971 A. M. SMIRlgOV, V. I. LUKHOVITSKII and V. L. KAItPOV, Khimiya vysokikh energii 5: 470, 1971