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Features of styrene polymerization initiated by azo-bis-isobutyronitrile in the presence of Al(C2H5)3
Polymer Science U.S.S.R. Vol. 27, No. 10, pp. 2416-2423, 1985 Printed in Poland
0032-3950/85 $I0.00+.00 © Pergamon Journals Ltd.
FEATURES OF STYRENE POLYMERIZATION INITIATED BY AZO-BIS-ISOBUTYRONITRILE IN THE PRESENCE OF AI(C2Hs)3* Z. M . DZHABIYEVA, P. YE. MATKOVSKII, YE. L. PECHATNIKOV
and N. A. BYRIKnINA Department of the Institute of Chemical Physics, U.S.S.R. Academy of Sciences
(Received 12 March 1984) Polymerization of styrene initiated by A I B N in the presence of triethylaluminium was studied. It was found that at low ratios of triethylaluminium : A I B N ( < 1"0), the rate of initiation is increased by triethylaluminium. The dependence of the rate of polymerization on its concentration exhibits an extremum. For the first time it was observed that at the ratios triethylaluminium: A I B N > 0.5, polymerization of styrene in this system exhibits a prolonged induction period, and at the ratios>~ 1'5, polymerization does not take place at all. It is shown that the autoinhibition of styrene polymerization is caused by the aluminiumorganic radical (CH3)2C=t~-N{AI(C2Hs)2}2, which is formed in free-radical reactions of AIB N with triethylaluminium. It was also found that triethylaluminium is a very effective chain termination agent. A kinetic model of the process is proposed permitting explanation of the observed features of styrene polymerization initiated by AIBN in the presence of triethylaluminium.
IT is known [1-8] that the rate of polymerization of styrene, vinyl chloride and of some other vinyl monomers, initiated by AIBN, may be considerably increased by Lewis acids. In a number of cases, an effective interaction of primary and of growing radicals with the Lewis acid was observed [1, 5, 9-19]. An example of such type of initiator is the system AIBN-AI(C2Hs)3 (TEA), which has been relatively well, though not quantitatively, investigated. [1, 17-19]. It was found that TEA increases the rate of AIBN decomposition [1]; the ethyl groups of TEA are homolytically substituted by the isobutyric and the isomeric keteniminyl radicals formed by decomposition of AIBN, the generated ethyl radicals rapidly react further with aluminiumorganic compounds (AOC), leading to the condensation of TEA [17, 18]; as a result of the radical chain processes in the system AIBN-TEA, the aluminiumorganic radical (AOR) (CH3)2C = (~- N[AI(C2 H5)212 detected by EPR, is formed [19]. In order to elucidate the character of the effect of TEA on the process of the styrene polymerization initiated by AIBN, we have studied the effect of the mol. ratio TEA : : AIBN on the rate of styrene polymerization and on the intrinsic viscosity of the products formed. The obtained results (Fig. 1, Table 1) indicate that the rate of styrene polymerization in the presence of TEA is appreciably higher at the ratios TEA : AIBN < 1 * Vysokomol. soyed. A27: 1'4o. 10, 2150-2156, 1985. 2416
Styrene polymerization initiated by azo-bis-isobutyronitrile TABLE 1. EFFECT OF
2417"
T E A CONCENTRATION ON THE KINETIC CHARACTERISTICS OF STYRENE POLYMERIZA-TIoN INITIATED BY A I B N IN TOLUENE MEDIUM AT 353 K
(AIBN=0.0935, styrene=2.3 mole/1.)
(C2Hs)sAI, rnole/1.
A1 : AIBN
land, min
0 0"018 0'045 0"072 0.093 0"093 0.112 0"140
0 0.20 0"48 0.77 1"00 1 "00 1"20 1 "50
12 12 14 60 170 130 300 600
wp x 10", mole~1." sec
S,%
1 "30 2"74 6'85 1"52 1 '10 1'10 0"35
31 31 58 43 13 13 8
[ql, dl/g (benzene, 293 K) 0"35 0"10 0"09 0-14 0"14 0-11 0"08
Note. wD- Maximumrate of polymerization;S-conversion after2 hr of polymerization. a n d c o n s i d e r a b l y lower at the r a t i o s T E A : A I B N > 1, t h a n in the absence o f T E A . I n b l a n k e x p e r i m e n t s it was f o u n d t h a t in the presence o f T E A a n d absence o f A I B N , p o l y m e r i z a t i o n o f styrene does n o t t a k e place.
~,% SO
r
,
O0
20
;/~>,o
! , g
.sC 6 T i m e , hp
FIG. 1. Dependence of styrene conversion q on the concentration of TEA. Initiator - AIBN in toluene. at 353 K. Here and in Fig. 4: [AIBN]=0.0935; [styrene] =2.3; [AI(C2Hs)a]=0 (•); 0.045 (2); 0-072 (3); 0.093 (4); 0.112 mole/l. (5). A1 : A I B N = 0 (1); 0.48 (2); 0.77 (3); .10 (4); 1.20 (5).
The increased rate o f styrene p o l y m e r i z a t i o n in the presence o f T E A at l o w mol. r a t i o s o f T E A : A I B N m a y be e x p l a i n e d b y the f o r m a t i o n o f complexes o f T E A w i t h A I B N [1]; A I B N b o u n d in a c o m p l e x w i t h T E A u n d e r g o e s h o m o l y t i c d e c o m p o s i t i o n w i t h the f o r m a t i o n o f radicals i n i t i a t i n g a f r e e - r a d i c a l p o l y m e r i z a t i o n o f styrene; the r a t e c o n s t a n t o f the c a t a l y t i c d e c o m p o s i t i o n o f A I B N (i.e. b o u n d in a c o m p l e x w i t h T E A )
2418
Z.M. DZHABIYEVAet aL
is considerably higher than the rate constant of the thermal decomposition of uncomplexed AIBN. At the ratios TEA : AIBN> 0.5, styrene polymerization exhibits a well-pronounced induction period. By the EPR method we have found that during the induction period, paramagnetic particles are present in the reaction zone; their EPR spectrum (Fig. 2) is completely identical with the EPR spectrum of the AOR formed in this system [19]. This indicates that the highly effective autoinhibition in styrene polymerization initiated by the system AIBN-TEA is caused by the interaction of growth radicals with the stable AOR generated in the system. The duration of the induction period (Fig. 1, Table 1), at otherwise unchanged conditions, increases at first slowly, then very ~teeply with increasing concentration of TEA. At ratios TEA : AIBN~> 1.5, polym ~rization of styrene does not take place under our experimental conditions.
Ft~. 2. EPR spectrum of reaction products of AIBN with AI(C2Hs)3 in a mixture of styrene with toluene ([styrene]=2"3 mole/k) at 353 K. [AIBN]--0.0935; [Al(C2Hs)3]=0.187 mole/l.; AI: AIBN =2.0. Using the known relation ItX= w i t i , d [20] (It-'stoichiometric coefficient of autoinhibition (set equal to one); X - concentration ofAOR; wi - total rate of thermal and catalytic initiation; tl.d--length of the induction period) we have found that at [TEA]=0.072 and 0.093 and [AIBN]=0.0935 mole/1., the quasistationary concentration of the inhibitor (AOR) is ~0.00067 and 0.0019 mole/l., respectively (i.e. ~0.7 and ,-~2.0~ of the iuitial concentration of AIBN, respectively). This estimate of the concentration of the inhibitor is in satisfactory agreement with the order of our previous estimate of the concentration of AOR based on EPR measurements [19]. At tool. ratios TEA : AIBN ~<0.77, beyond the induction period polymerization proceeds at a constant rate up to -,~25 ~ conversion, as is typical of strong inhibitors [21]. From the dependence of the maximum rate of polymerization on the concentration of TEA, shown in Table 1,
Styrene polymerization initiated by azo-bis-isobutyronitrile
2419
it is evident that this dependence is of extremal character. The decreasing part of this dependence indicates the presence of a secondary inhibition which is evidently caused by the products of the primary effective autoinhibition. From Table 1 it is also evident that the intrinsic viscosity of PS obtained in the absence of TEA is higher than the intrinsic viscosity of PS samples obtained under the same conditions, but in the presence of TEA. The lowering of the intrinsic viscosity of polystyrene by TEA is observed even at .ratios TEA : AIBN=0.2-0.77. The observed lowering of the intrinsic viscosity of SP is caused by transfer to AOC, and not by chain termination with participation of TEA, as at the applied experimental conditions the rate of polymerization is 1.5-5 times higher than the rate of polymerization in the absence of TEA. In styrene polymerization, chain transfer to TEA, representing a homolytic Sh2 substitution of the ethyl group on aluminium by styryl radical, may be represented by the following scheme: CHz--CH" -}- AI(CzH~)s---, ~ CH2--CIt--AI(C~Hs)~ -{- Cztts" I 1 Cell5 CeH~ Based on our results we may assume that the growing radicals react very rapidly with AOC. In order to substantiate this conclusion we have studied the reactions of A1BN with TEA in octadeuterotoluene (ODT) at 353 K. In Table 2 it is shown that the gaseous products of the studied reactions consist of ethylene, ethane, monodeuteroethane, dideuteroethane and nitrogen. Butane is present in vcry small amounts in the gaseous products. This indicates that recombination of ethyl radicals proceeds at a very low rate under our experimental conditions. As the rate constant of ethyl radical recombination in liquid phase at 353-413 K is 6-7 times higher than the rate constant of their disproportionation [22], it may be concluded that the contribution of the latter reaction to the formation of ethane and ethylene under the studied conditions is negligibly small. Considering this we have to conclude that ethylene is evidently formed during decomposition of the intermediate radicals (C2Hs)2A1CH2CH2 or by intrasphere disproportionation of the ethyl radical with an eth3.1 group on aluminium. The isotopic composition of the ethanes produced in the course of the reaction in ODT medium indicates produced in the course of the reaction in O D T medium indicates that the reaction of C_,H5 with ODT proceeds at a low rate, TABLE 2. CHEMICAt. ,AND ISOTOPIC COMPOSITION OF GASEOUS PRODUCTS OF THE REACTION OF A I B N
WITH AI(CzHs)3 IN ODT MEDIUMAT 353 K AI(C2H5)3, mole/l.
AI : AIBN
0"24 0"28 2"10 2"81
2"3 9"6 37"0 13"8
Composition of gaseous products, C2H4 N2 C2H6 3-0 73"4 23"6 15"0 75"5 9"5 83"0 8.0 9"0 57"9 14.7 27"4
Isotopic composition of ethanes, C2H6 C2HsD C2H4D2 96"1 3.9 0"4 95.9 2"8 1"3 97.2 1.1 0"7 98"9 0.8 0"3
2420
Z.M. DZHABIYEVAet
al.
in spite of the high concentration of ODT (8.7 mole/1.). This analysis together with the results in Table 2 indicate that ethane is formed mostly by reactions of ethyl radicals with ethyl groups of TEA. By the method of competing reactions using the ratio C2HsD : C2H6, the value of the rate constant of the reaction of the ethyl radical with the ethyl group of T EA was estimated. At 353 K, k(C2H5 + TEA) = (0.6 + 0.5) x 106 l./mole.sec. The accuracy of this estimate is low because of the low content of C2HsD in the gaseous products of the studied reaction. The obtained value probably represents the minimum value, as according to Ingold [23] for the gas phase reaction k(C2H5 + TEA)~ 108 1./mole.sec. As the radicals C2H5, "CH2CH2AI(C2Hs)2 and "AI(C2Hs)2 formed in the series of reactions of the styryl radical with TEA can react with the monomer yielding a growing chain, these estimates confirm our assumption that TEA is a very effective chain transfer agent. Based on the present data on the mechanism of reactions in the system AIBN-TEA [1, 17-19], the following kinetic model was adopted for the interpretation of the observed features of styrene polymerization initiated by AIBN and autoinhibited by the presence of TEA: 1. AIBNo2(CHs)2CCN(R)+ N2 2. AIBN + AI(C2Hs)3 ~AiBN-AI(C2Hs)a(It ) 3. AIBN + AI(C2 Hs)3Al(C2Hs)a ~AIBN'A12(C2Hs)6(I2) 4. A12(C2H5)6~AI(C2Hs)3 + Al(C2H5)3 5. Ix-~R + C2H~ + N2 + (CH3)2C = C-- N - Al(C2 Hs)2(A') 6. I21~-~+ C2H4 + C2H6 + N2 + (CHa)2C = C - N{AI(C2Hs)2}2(AOR) 7. 1~+ A12(C2Hs)6 o A O R + C2H6 + C2H4 8. C2H~ + AI(C2Hs)3-~C2H6 + "CH2 - CH2AI(C2Hs)2(A') 9. C2H~ +A12(C2Hs)6-~C2H6 +Al(CEHs)a +A" 10. A" +Al(CEHs)a ~C2H~ + (C2Hs)2A1CH2 - CH2AI(C2Hs)2(A") 11. A ' + AI2(C2Hs)6--*C2H~ + AI(C2Hs)3 + A" 12. C2H5 +A"~C2H6 +A" 13. R" + AI(C2Hs)a-~A' + C2H5 14. A' +AI(C2Hs)a-~C2H5 + AOR 15. A ' + R'(C2H~)~) 16. R" + R'(C2H~)~fn°nradical- products inhibition and chain growth 17. R', C2H~, A'+M-~,,s~M" ,-~M" + M-~,,sMM. Chain termination 18. R', C2H5, A'+AOR-.nonradical products , 19. ,,,M,~+ ~ M ~ } 20. R', C2H5, A ' + "~ M'--* products of recombination and disproportionation d
This model is confirmed by the results of the numerical solution of a system of differential equations corresponding to the reaction scheme (1)-(20). The following values were used in the preliminary estimates: k~=0.1 x 10 -4 see -1 [22]; k2=k3=0.1 x 104
Styrene polymerization initiated by azo-bis-isobutyronitrile
2421
1./mole.see; k4=0.1 x 103 sec-1; k _ 4 = 0 " 3 x 103 1./mole.sec; ks = ( 0 . 1 - 0 . 5 ) x 10 -3 sec-1; k 6 = ( 0 . 1 - 1 ) s e c - 1; k7 = 106_ 10 a l./mole.sec; k16 =0.1 x 101°; k18=0.5 x 109 l./mole.sec. Figure 3 (curves 1, 2) shows that the introduction of TEA into the system at the ratios AI : A I B N = 0 - 0 . 5 leads to an increase of the concentration of active radicals, and an induction period does not appear on the curves of cumulation of active radicals. At the ratio A1 : A I B N = 0 . 7 5 , an induction period appears (Fig. 3, curve 3). In this case, the postinduction concentration of active radicals is higher than their quasi-stationary concentration in cases of reactions without an induction period. A further increase of the mol. ratio A1 : A I B N from 0.75 to 1.2 is accompanied by a considerable increase of the duration of the induction period and decrease of the postinduction concentration o f active radicals (Fig. 3, curve 4).
[R;/,lO~,ole/l.
.J/la~,,.de/L.sec
I ~9 l~'J ~rmoJe/l.J
1"£
0.25 9
1~
10
0"15
0.5
-12
0"05 I
3
Time~ h , FIG. 3
6
I
I
0.8
12
[(CeHs)aAI]: [ArBN] FIG. 4
F16. 3. Effect of concentration of Al(C2Hs)3 on the kinetics of cumulation and consumption of active radicals R" in the course of AIBN decomposition in the. presence of AI(C2Hs)a (results of the numerical solution of the system of differential equations corresponding to the reaction schemes (1)--(7), (18). The values of the constants for the corresponding stages are given in text. [AIBN] =0"1 mole/l.; [AI(C2Hs)a] =0 (•); 0.05 (2); 0.075 (3); 0.12 mole/l. (4); 353 K. FIc. 4. Effect of mol. ratio Al(C2Hs)3 : AIBN on the rate of styrene polymerization (1) and on the calculated value of the quasi-stationary concentration of active radicals (2). Calculations indicate that with an increase of the mol. ratio A1 : AIBN, the quasistationary concentration of the radicals [R']i.d established after the completion of the induction period decreases considerably (Fig. 4). However, a comparison of the rates of decrease of [R']i,a and of the initial rate of polymerization after the induction period w~,"d at an increase of the ratio A1 : A I B N indicates that the decrease of wpind cannot be explained only by the consumption of A I B N f r o m Io to Ii,a (Fig. 4). The observed steeper decrease of the rate of polymerization evidently indicates the presence of secondary inhibition which is not accounted for in the reaction scheme (1)-(20).
2422
Z . M . DZHABIYEVAet al.
Thus it is evident that the combination of A I B N with TEA can lead both to a catalytic increase of the rate of initiation, or to inhibition of the polymerization process. In addition, T E A also takes part in chain transfer reactions. Calculations indicate that the proposed kinetic model of the process permits a satisfactory interpretation of all the observed effects. Styrene polymerization was followed by the dilatometric method. The inhibitor was removed f r o m styrene by the known method [24], styrene was dried by calcium hydride a n d distilled twice over calcium hydride: b.p. 418.5 K, p=906.3 and 871.2 kg/m 3 at 293 and 353 K, respectively. F o r the determination of conversion, the mean value p (353 K ) = 1057 kg/m 3 was adpoted for PS. Contraction at 353 K was 17 ml/mole. The i n i t i a t o r - A I B N was recrystallized twice from solution in absolute ethanol and vacuum dried to constant weight. Toluene of "scintillation" grade was purified according to known recipees, dried by calcium hydride, distilled over calcium hydride into a 293 flask with a freshly prepared sodium wire; b.p. 383-3 K, p (293 K ) = 866 kg/m 3, n o 1-4961. T E A was vacuum-distilled (,-,1.33 Pa) at 322 K before use, p (293 K ) = 8 4 0 kg/m a. O D T was dried by sodium wire and vacuum freezing. The content of CTDTH in O D T did not exceed 2.0 mole %. E P R spectra of the paramagnetic products of interaction of A I B N with TEA in styrene medium were recorded by means of the spectrometer EPR-2 ( I K h F A N SSSR). The yield of gaseous products was determined volumetrically, and the composition by chromatographic methods (column 4 x 1000 ram, 5 % apiezon on. A1203, 293 K). The isotopic composition of the ethanes was determined by means of the mass spectrometer MI-1305. The numerical integration of the system of differential equations was performed on the computer BESM-6 using the programme of G. A. Furman. Translated by D. DOSKO~ILOV.~,
REFERENCES
1. S. T. HIRANO, T. MIKI and T. TSURUTA, Makromol. Chem. 104: 230, 1967 2. G. S. HAMMOND, S. Wu. CHIN-HUA, O. D. TRAPP, J. WARKENTIN and R. T. KEYS, J. Amer. Chem. Soc. 82: 5394, 1960 3. M. B. LACHINOV, V. P. ZUBOV and V. A. KABANOV, J. Polymer Sci. Polymer Chem. Ed. 15: 1777, 1977 4. A. HAERING and G. RIESS, Europ. Polymer J. 14: 117, 1978 5. N. G. GAYLORD and S. MAITI, Makromol. Chem. 142: 101, 1971 6. N. G. GAYLORD and S. MAITI, Polymer Letters 10: 35, 1972 7. N. J. BENGOUGH and T. O. NEILL, Trans. Farady Soc. 64: 4, 1014, 1968 8. J. KUMAMOTO, H. E. DE LA MARE and F. F. RUST, J. Amer. Chem. Soc. 82: 1935, 1960 9. J. BARTON and M. LASAR, Maki'omol. Chem. 124: 38, 1969 10. N. MOSZNER and M. HARTMANN, J. Prakt. Chem. 322: 963, 1980 11. T. HUFF and E. PERRY, J. Amer. Chem. Soc. 82: 4277, 1960 12. T. HUFF and E. PERRY, J. Polymer Sci. AI: 1553, 1963 13. J. GROTEWOLD and M. M. HIRSCHLER, J. Polymer Sci. Polymer Chem. Ed. 15: 383, 1977 14. V. S. RAIDA, G. P. VINARSKAYA and Yu. G. KRYAZHEV, Vysokomol. soyed. 1320: 593, 1978 (Not translated in Polymer Sci. U.S.S.R.)
Regeneration of macromoleculax matrix in matrix polymerization
2423
15. L , A. S M I R N O V A , Yu. D. S E M C H I K O V , L. I. K A M Y S H E N K O V A , T. G. S V E S H N I K O V A , A. N. Y E G O R O C H K I N , G. S. KALININA and B. A. YEGOROV, Vysokomol. soyed. 24: 999, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 5, 1127, 1982) 16. R. KOSTER, G. B E N E D I C T and H. W. S C H R O T T E R , Angew. Chem. 76: 649, 1964 17. N. S. YENIKOLOPYAN, F. S. DYACHKOVSKII, Z. M. D Z H A B I Y E V A and P. Ye. MATKOVSKH, Kinetika i kataliz (Kinetics and Catalysis). 21: 294, 1980 18. Z. M. D Z H A B I Y E V A , P. Ye. MATKOVSKII, L. I. CHERNAYA, F. C. DYACHKOVSKI[ and N. S. YENIKOLOPYAN, In: Mater. lII Vsesoyuz. konf. " M e k h a n i z m kataliticheskikh reaktsii" (Materials of the IIi all-union conference " M e c h a n i s m of Catalytic Reactions"). part. 2. p. 113, In-t kataliza CO A N SSSR, Novosibirsk, 1982 19. N. S. YENIKOLOPYAN, F. S. DYACHKOVSKII, Z. M. D Z H A B I E V A , P. Ye. MATKOVSKII and A. F. SHESTAKOV, Dokl. A N SSSR 261: 1145, 1981 20. A. V. TRUBN1KOV, M. D. GOLDFEIN, N. V. K O Z H E V N I K O V , E. A. RAFIKOV,A. D. S T E P U K H O V I C H and V. I. T O M A S H C H U K , Vysokomol. soyed. 20: 2448, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 11, 2751, 1978) 21. Y. M I U R A , S. M A S U T D A and i . KINOSHITA, Makromol. Chem. 160: 243, 1972 22. Ye. T. DENISOV, Konstanty skorosti gomoliticheskikh zhidkofaznykh reaktsii (Rate Constants of Homolytical Reactions in the Liquid Phase). p. 376, Nauka, Moscow, 1971 23. K. INGOLD and B. ROBERTS, Reaktsii svobodnoradikalnovo zameshcheniya (Reactions of Free-radical Substitution). p. 270, Mir, Moscow, 1974 24. P. VACULIK, Khimiya m o n o m e r o v (Chemistry of Monom-rs). Izdatelstvo inostr, lit., Moscow, 1960
Polymer Science U.S.S.R. Vol. 27, No. I0, pp. 2423-2427, 1985 Printed in Poland
0032-3950/85 $10.00+.00 © Pergamon Journals Ltd.
REGENERATION OF THE MACROMOLECULAR MATRIX IN MATRIX POLYMERIZATION* I. M . PAPISOV a n d A . A . LITMANOVICH Moscow Institute of Road-Building
(Received 12 March 1984) In matrix polymerization it is in principle possible that the macromolecular matrix is regenerated (i.e., the polycomplex m a t r i x - d a u g h t e r polymer dissociates) provided two different macromolecular matrices are present in the system: a strong matrix with short chains and a weak matrix with long ones. Conditions have been found for the chain that has at first grown to a certain degree of polymerization on the strong matrix to go over to the weak one as a result of a substitution reaction; the strong matrix set free in this manner can subsequently, control the growth of new macromolecules. * Vysokomol. soyed. A27: No. 10, 2157-2160, 1985.
0032-3950/85 $I0.00+.00 © Pergamon Journals Ltd.
FEATURES OF STYRENE POLYMERIZATION INITIATED BY AZO-BIS-ISOBUTYRONITRILE IN THE PRESENCE OF AI(C2Hs)3* Z. M . DZHABIYEVA, P. YE. MATKOVSKII, YE. L. PECHATNIKOV
and N. A. BYRIKnINA Department of the Institute of Chemical Physics, U.S.S.R. Academy of Sciences
(Received 12 March 1984) Polymerization of styrene initiated by A I B N in the presence of triethylaluminium was studied. It was found that at low ratios of triethylaluminium : A I B N ( < 1"0), the rate of initiation is increased by triethylaluminium. The dependence of the rate of polymerization on its concentration exhibits an extremum. For the first time it was observed that at the ratios triethylaluminium: A I B N > 0.5, polymerization of styrene in this system exhibits a prolonged induction period, and at the ratios>~ 1'5, polymerization does not take place at all. It is shown that the autoinhibition of styrene polymerization is caused by the aluminiumorganic radical (CH3)2C=t~-N{AI(C2Hs)2}2, which is formed in free-radical reactions of AIB N with triethylaluminium. It was also found that triethylaluminium is a very effective chain termination agent. A kinetic model of the process is proposed permitting explanation of the observed features of styrene polymerization initiated by AIBN in the presence of triethylaluminium.
IT is known [1-8] that the rate of polymerization of styrene, vinyl chloride and of some other vinyl monomers, initiated by AIBN, may be considerably increased by Lewis acids. In a number of cases, an effective interaction of primary and of growing radicals with the Lewis acid was observed [1, 5, 9-19]. An example of such type of initiator is the system AIBN-AI(C2Hs)3 (TEA), which has been relatively well, though not quantitatively, investigated. [1, 17-19]. It was found that TEA increases the rate of AIBN decomposition [1]; the ethyl groups of TEA are homolytically substituted by the isobutyric and the isomeric keteniminyl radicals formed by decomposition of AIBN, the generated ethyl radicals rapidly react further with aluminiumorganic compounds (AOC), leading to the condensation of TEA [17, 18]; as a result of the radical chain processes in the system AIBN-TEA, the aluminiumorganic radical (AOR) (CH3)2C = (~- N[AI(C2 H5)212 detected by EPR, is formed [19]. In order to elucidate the character of the effect of TEA on the process of the styrene polymerization initiated by AIBN, we have studied the effect of the mol. ratio TEA : : AIBN on the rate of styrene polymerization and on the intrinsic viscosity of the products formed. The obtained results (Fig. 1, Table 1) indicate that the rate of styrene polymerization in the presence of TEA is appreciably higher at the ratios TEA : AIBN < 1 * Vysokomol. soyed. A27: 1'4o. 10, 2150-2156, 1985. 2416
Styrene polymerization initiated by azo-bis-isobutyronitrile TABLE 1. EFFECT OF
2417"
T E A CONCENTRATION ON THE KINETIC CHARACTERISTICS OF STYRENE POLYMERIZA-TIoN INITIATED BY A I B N IN TOLUENE MEDIUM AT 353 K
(AIBN=0.0935, styrene=2.3 mole/1.)
(C2Hs)sAI, rnole/1.
A1 : AIBN
land, min
0 0"018 0'045 0"072 0.093 0"093 0.112 0"140
0 0.20 0"48 0.77 1"00 1 "00 1"20 1 "50
12 12 14 60 170 130 300 600
wp x 10", mole~1." sec
S,%
1 "30 2"74 6'85 1"52 1 '10 1'10 0"35
31 31 58 43 13 13 8
[ql, dl/g (benzene, 293 K) 0"35 0"10 0"09 0-14 0"14 0-11 0"08
Note. wD- Maximumrate of polymerization;S-conversion after2 hr of polymerization. a n d c o n s i d e r a b l y lower at the r a t i o s T E A : A I B N > 1, t h a n in the absence o f T E A . I n b l a n k e x p e r i m e n t s it was f o u n d t h a t in the presence o f T E A a n d absence o f A I B N , p o l y m e r i z a t i o n o f styrene does n o t t a k e place.
~,% SO
r
,
O0
20
;/~>,o
! , g
.sC 6 T i m e , hp
FIG. 1. Dependence of styrene conversion q on the concentration of TEA. Initiator - AIBN in toluene. at 353 K. Here and in Fig. 4: [AIBN]=0.0935; [styrene] =2.3; [AI(C2Hs)a]=0 (•); 0.045 (2); 0-072 (3); 0.093 (4); 0.112 mole/l. (5). A1 : A I B N = 0 (1); 0.48 (2); 0.77 (3); .10 (4); 1.20 (5).
The increased rate o f styrene p o l y m e r i z a t i o n in the presence o f T E A at l o w mol. r a t i o s o f T E A : A I B N m a y be e x p l a i n e d b y the f o r m a t i o n o f complexes o f T E A w i t h A I B N [1]; A I B N b o u n d in a c o m p l e x w i t h T E A u n d e r g o e s h o m o l y t i c d e c o m p o s i t i o n w i t h the f o r m a t i o n o f radicals i n i t i a t i n g a f r e e - r a d i c a l p o l y m e r i z a t i o n o f styrene; the r a t e c o n s t a n t o f the c a t a l y t i c d e c o m p o s i t i o n o f A I B N (i.e. b o u n d in a c o m p l e x w i t h T E A )
2418
Z.M. DZHABIYEVAet aL
is considerably higher than the rate constant of the thermal decomposition of uncomplexed AIBN. At the ratios TEA : AIBN> 0.5, styrene polymerization exhibits a well-pronounced induction period. By the EPR method we have found that during the induction period, paramagnetic particles are present in the reaction zone; their EPR spectrum (Fig. 2) is completely identical with the EPR spectrum of the AOR formed in this system [19]. This indicates that the highly effective autoinhibition in styrene polymerization initiated by the system AIBN-TEA is caused by the interaction of growth radicals with the stable AOR generated in the system. The duration of the induction period (Fig. 1, Table 1), at otherwise unchanged conditions, increases at first slowly, then very ~teeply with increasing concentration of TEA. At ratios TEA : AIBN~> 1.5, polym ~rization of styrene does not take place under our experimental conditions.
Ft~. 2. EPR spectrum of reaction products of AIBN with AI(C2Hs)3 in a mixture of styrene with toluene ([styrene]=2"3 mole/k) at 353 K. [AIBN]--0.0935; [Al(C2Hs)3]=0.187 mole/l.; AI: AIBN =2.0. Using the known relation ItX= w i t i , d [20] (It-'stoichiometric coefficient of autoinhibition (set equal to one); X - concentration ofAOR; wi - total rate of thermal and catalytic initiation; tl.d--length of the induction period) we have found that at [TEA]=0.072 and 0.093 and [AIBN]=0.0935 mole/1., the quasistationary concentration of the inhibitor (AOR) is ~0.00067 and 0.0019 mole/l., respectively (i.e. ~0.7 and ,-~2.0~ of the iuitial concentration of AIBN, respectively). This estimate of the concentration of the inhibitor is in satisfactory agreement with the order of our previous estimate of the concentration of AOR based on EPR measurements [19]. At tool. ratios TEA : AIBN ~<0.77, beyond the induction period polymerization proceeds at a constant rate up to -,~25 ~ conversion, as is typical of strong inhibitors [21]. From the dependence of the maximum rate of polymerization on the concentration of TEA, shown in Table 1,
Styrene polymerization initiated by azo-bis-isobutyronitrile
2419
it is evident that this dependence is of extremal character. The decreasing part of this dependence indicates the presence of a secondary inhibition which is evidently caused by the products of the primary effective autoinhibition. From Table 1 it is also evident that the intrinsic viscosity of PS obtained in the absence of TEA is higher than the intrinsic viscosity of PS samples obtained under the same conditions, but in the presence of TEA. The lowering of the intrinsic viscosity of polystyrene by TEA is observed even at .ratios TEA : AIBN=0.2-0.77. The observed lowering of the intrinsic viscosity of SP is caused by transfer to AOC, and not by chain termination with participation of TEA, as at the applied experimental conditions the rate of polymerization is 1.5-5 times higher than the rate of polymerization in the absence of TEA. In styrene polymerization, chain transfer to TEA, representing a homolytic Sh2 substitution of the ethyl group on aluminium by styryl radical, may be represented by the following scheme: CHz--CH" -}- AI(CzH~)s---, ~ CH2--CIt--AI(C~Hs)~ -{- Cztts" I 1 Cell5 CeH~ Based on our results we may assume that the growing radicals react very rapidly with AOC. In order to substantiate this conclusion we have studied the reactions of A1BN with TEA in octadeuterotoluene (ODT) at 353 K. In Table 2 it is shown that the gaseous products of the studied reactions consist of ethylene, ethane, monodeuteroethane, dideuteroethane and nitrogen. Butane is present in vcry small amounts in the gaseous products. This indicates that recombination of ethyl radicals proceeds at a very low rate under our experimental conditions. As the rate constant of ethyl radical recombination in liquid phase at 353-413 K is 6-7 times higher than the rate constant of their disproportionation [22], it may be concluded that the contribution of the latter reaction to the formation of ethane and ethylene under the studied conditions is negligibly small. Considering this we have to conclude that ethylene is evidently formed during decomposition of the intermediate radicals (C2Hs)2A1CH2CH2 or by intrasphere disproportionation of the ethyl radical with an eth3.1 group on aluminium. The isotopic composition of the ethanes produced in the course of the reaction in ODT medium indicates produced in the course of the reaction in O D T medium indicates that the reaction of C_,H5 with ODT proceeds at a low rate, TABLE 2. CHEMICAt. ,AND ISOTOPIC COMPOSITION OF GASEOUS PRODUCTS OF THE REACTION OF A I B N
WITH AI(CzHs)3 IN ODT MEDIUMAT 353 K AI(C2H5)3, mole/l.
AI : AIBN
0"24 0"28 2"10 2"81
2"3 9"6 37"0 13"8
Composition of gaseous products, C2H4 N2 C2H6 3-0 73"4 23"6 15"0 75"5 9"5 83"0 8.0 9"0 57"9 14.7 27"4
Isotopic composition of ethanes, C2H6 C2HsD C2H4D2 96"1 3.9 0"4 95.9 2"8 1"3 97.2 1.1 0"7 98"9 0.8 0"3
2420
Z.M. DZHABIYEVAet
al.
in spite of the high concentration of ODT (8.7 mole/1.). This analysis together with the results in Table 2 indicate that ethane is formed mostly by reactions of ethyl radicals with ethyl groups of TEA. By the method of competing reactions using the ratio C2HsD : C2H6, the value of the rate constant of the reaction of the ethyl radical with the ethyl group of T EA was estimated. At 353 K, k(C2H5 + TEA) = (0.6 + 0.5) x 106 l./mole.sec. The accuracy of this estimate is low because of the low content of C2HsD in the gaseous products of the studied reaction. The obtained value probably represents the minimum value, as according to Ingold [23] for the gas phase reaction k(C2H5 + TEA)~ 108 1./mole.sec. As the radicals C2H5, "CH2CH2AI(C2Hs)2 and "AI(C2Hs)2 formed in the series of reactions of the styryl radical with TEA can react with the monomer yielding a growing chain, these estimates confirm our assumption that TEA is a very effective chain transfer agent. Based on the present data on the mechanism of reactions in the system AIBN-TEA [1, 17-19], the following kinetic model was adopted for the interpretation of the observed features of styrene polymerization initiated by AIBN and autoinhibited by the presence of TEA: 1. AIBNo2(CHs)2CCN(R)+ N2 2. AIBN + AI(C2Hs)3 ~AiBN-AI(C2Hs)a(It ) 3. AIBN + AI(C2 Hs)3Al(C2Hs)a ~AIBN'A12(C2Hs)6(I2) 4. A12(C2H5)6~AI(C2Hs)3 + Al(C2H5)3 5. Ix-~R + C2H~ + N2 + (CH3)2C = C-- N - Al(C2 Hs)2(A') 6. I21~-~+ C2H4 + C2H6 + N2 + (CHa)2C = C - N{AI(C2Hs)2}2(AOR) 7. 1~+ A12(C2Hs)6 o A O R + C2H6 + C2H4 8. C2H~ + AI(C2Hs)3-~C2H6 + "CH2 - CH2AI(C2Hs)2(A') 9. C2H~ +A12(C2Hs)6-~C2H6 +Al(CEHs)a +A" 10. A" +Al(CEHs)a ~C2H~ + (C2Hs)2A1CH2 - CH2AI(C2Hs)2(A") 11. A ' + AI2(C2Hs)6--*C2H~ + AI(C2Hs)3 + A" 12. C2H5 +A"~C2H6 +A" 13. R" + AI(C2Hs)a-~A' + C2H5 14. A' +AI(C2Hs)a-~C2H5 + AOR 15. A ' + R'(C2H~)~) 16. R" + R'(C2H~)~fn°nradical- products inhibition and chain growth 17. R', C2H~, A'+M-~,,s~M" ,-~M" + M-~,,sMM. Chain termination 18. R', C2H5, A'+AOR-.nonradical products , 19. ,,,M,~+ ~ M ~ } 20. R', C2H5, A ' + "~ M'--* products of recombination and disproportionation d
This model is confirmed by the results of the numerical solution of a system of differential equations corresponding to the reaction scheme (1)-(20). The following values were used in the preliminary estimates: k~=0.1 x 10 -4 see -1 [22]; k2=k3=0.1 x 104
Styrene polymerization initiated by azo-bis-isobutyronitrile
2421
1./mole.see; k4=0.1 x 103 sec-1; k _ 4 = 0 " 3 x 103 1./mole.sec; ks = ( 0 . 1 - 0 . 5 ) x 10 -3 sec-1; k 6 = ( 0 . 1 - 1 ) s e c - 1; k7 = 106_ 10 a l./mole.sec; k16 =0.1 x 101°; k18=0.5 x 109 l./mole.sec. Figure 3 (curves 1, 2) shows that the introduction of TEA into the system at the ratios AI : A I B N = 0 - 0 . 5 leads to an increase of the concentration of active radicals, and an induction period does not appear on the curves of cumulation of active radicals. At the ratio A1 : A I B N = 0 . 7 5 , an induction period appears (Fig. 3, curve 3). In this case, the postinduction concentration of active radicals is higher than their quasi-stationary concentration in cases of reactions without an induction period. A further increase of the mol. ratio A1 : A I B N from 0.75 to 1.2 is accompanied by a considerable increase of the duration of the induction period and decrease of the postinduction concentration o f active radicals (Fig. 3, curve 4).
[R;/,lO~,ole/l.
.J/la~,,.de/L.sec
I ~9 l~'J ~rmoJe/l.J
1"£
0.25 9
1~
10
0"15
0.5
-12
0"05 I
3
Time~ h , FIG. 3
6
I
I
0.8
12
[(CeHs)aAI]: [ArBN] FIG. 4
F16. 3. Effect of concentration of Al(C2Hs)3 on the kinetics of cumulation and consumption of active radicals R" in the course of AIBN decomposition in the. presence of AI(C2Hs)a (results of the numerical solution of the system of differential equations corresponding to the reaction schemes (1)--(7), (18). The values of the constants for the corresponding stages are given in text. [AIBN] =0"1 mole/l.; [AI(C2Hs)a] =0 (•); 0.05 (2); 0.075 (3); 0.12 mole/l. (4); 353 K. FIc. 4. Effect of mol. ratio Al(C2Hs)3 : AIBN on the rate of styrene polymerization (1) and on the calculated value of the quasi-stationary concentration of active radicals (2). Calculations indicate that with an increase of the mol. ratio A1 : AIBN, the quasistationary concentration of the radicals [R']i.d established after the completion of the induction period decreases considerably (Fig. 4). However, a comparison of the rates of decrease of [R']i,a and of the initial rate of polymerization after the induction period w~,"d at an increase of the ratio A1 : A I B N indicates that the decrease of wpind cannot be explained only by the consumption of A I B N f r o m Io to Ii,a (Fig. 4). The observed steeper decrease of the rate of polymerization evidently indicates the presence of secondary inhibition which is not accounted for in the reaction scheme (1)-(20).
2422
Z . M . DZHABIYEVAet al.
Thus it is evident that the combination of A I B N with TEA can lead both to a catalytic increase of the rate of initiation, or to inhibition of the polymerization process. In addition, T E A also takes part in chain transfer reactions. Calculations indicate that the proposed kinetic model of the process permits a satisfactory interpretation of all the observed effects. Styrene polymerization was followed by the dilatometric method. The inhibitor was removed f r o m styrene by the known method [24], styrene was dried by calcium hydride a n d distilled twice over calcium hydride: b.p. 418.5 K, p=906.3 and 871.2 kg/m 3 at 293 and 353 K, respectively. F o r the determination of conversion, the mean value p (353 K ) = 1057 kg/m 3 was adpoted for PS. Contraction at 353 K was 17 ml/mole. The i n i t i a t o r - A I B N was recrystallized twice from solution in absolute ethanol and vacuum dried to constant weight. Toluene of "scintillation" grade was purified according to known recipees, dried by calcium hydride, distilled over calcium hydride into a 293 flask with a freshly prepared sodium wire; b.p. 383-3 K, p (293 K ) = 866 kg/m 3, n o 1-4961. T E A was vacuum-distilled (,-,1.33 Pa) at 322 K before use, p (293 K ) = 8 4 0 kg/m a. O D T was dried by sodium wire and vacuum freezing. The content of CTDTH in O D T did not exceed 2.0 mole %. E P R spectra of the paramagnetic products of interaction of A I B N with TEA in styrene medium were recorded by means of the spectrometer EPR-2 ( I K h F A N SSSR). The yield of gaseous products was determined volumetrically, and the composition by chromatographic methods (column 4 x 1000 ram, 5 % apiezon on. A1203, 293 K). The isotopic composition of the ethanes was determined by means of the mass spectrometer MI-1305. The numerical integration of the system of differential equations was performed on the computer BESM-6 using the programme of G. A. Furman. Translated by D. DOSKO~ILOV.~,
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Regeneration of macromoleculax matrix in matrix polymerization
2423
15. L , A. S M I R N O V A , Yu. D. S E M C H I K O V , L. I. K A M Y S H E N K O V A , T. G. S V E S H N I K O V A , A. N. Y E G O R O C H K I N , G. S. KALININA and B. A. YEGOROV, Vysokomol. soyed. 24: 999, 1982 (Translated in Polymer Sci. U.S.S.R. 24: 5, 1127, 1982) 16. R. KOSTER, G. B E N E D I C T and H. W. S C H R O T T E R , Angew. Chem. 76: 649, 1964 17. N. S. YENIKOLOPYAN, F. S. DYACHKOVSKII, Z. M. D Z H A B I Y E V A and P. Ye. MATKOVSKH, Kinetika i kataliz (Kinetics and Catalysis). 21: 294, 1980 18. Z. M. D Z H A B I Y E V A , P. Ye. MATKOVSKII, L. I. CHERNAYA, F. C. DYACHKOVSKI[ and N. S. YENIKOLOPYAN, In: Mater. lII Vsesoyuz. konf. " M e k h a n i z m kataliticheskikh reaktsii" (Materials of the IIi all-union conference " M e c h a n i s m of Catalytic Reactions"). part. 2. p. 113, In-t kataliza CO A N SSSR, Novosibirsk, 1982 19. N. S. YENIKOLOPYAN, F. S. DYACHKOVSKII, Z. M. D Z H A B I E V A , P. Ye. MATKOVSKII and A. F. SHESTAKOV, Dokl. A N SSSR 261: 1145, 1981 20. A. V. TRUBN1KOV, M. D. GOLDFEIN, N. V. K O Z H E V N I K O V , E. A. RAFIKOV,A. D. S T E P U K H O V I C H and V. I. T O M A S H C H U K , Vysokomol. soyed. 20: 2448, 1978 (Translated in Polymer Sci. U.S.S.R. 20: 11, 2751, 1978) 21. Y. M I U R A , S. M A S U T D A and i . KINOSHITA, Makromol. Chem. 160: 243, 1972 22. Ye. T. DENISOV, Konstanty skorosti gomoliticheskikh zhidkofaznykh reaktsii (Rate Constants of Homolytical Reactions in the Liquid Phase). p. 376, Nauka, Moscow, 1971 23. K. INGOLD and B. ROBERTS, Reaktsii svobodnoradikalnovo zameshcheniya (Reactions of Free-radical Substitution). p. 270, Mir, Moscow, 1974 24. P. VACULIK, Khimiya m o n o m e r o v (Chemistry of Monom-rs). Izdatelstvo inostr, lit., Moscow, 1960
Polymer Science U.S.S.R. Vol. 27, No. I0, pp. 2423-2427, 1985 Printed in Poland
0032-3950/85 $10.00+.00 © Pergamon Journals Ltd.
REGENERATION OF THE MACROMOLECULAR MATRIX IN MATRIX POLYMERIZATION* I. M . PAPISOV a n d A . A . LITMANOVICH Moscow Institute of Road-Building
(Received 12 March 1984) In matrix polymerization it is in principle possible that the macromolecular matrix is regenerated (i.e., the polycomplex m a t r i x - d a u g h t e r polymer dissociates) provided two different macromolecular matrices are present in the system: a strong matrix with short chains and a weak matrix with long ones. Conditions have been found for the chain that has at first grown to a certain degree of polymerization on the strong matrix to go over to the weak one as a result of a substitution reaction; the strong matrix set free in this manner can subsequently, control the growth of new macromolecules. * Vysokomol. soyed. A27: No. 10, 2157-2160, 1985.