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On the chain transfer reaction during polymerization in presence of 4,4′-azo-bis-cyanovalerianic acid
ON THE CHAIN TRANSFER REACTION DURING POLYMERIZATION IN PRESENCE OF 4,4'-AZO-BIS-CYANOVALERIANIC ACID* V. P . KARTAVYKH, Y E . N . BARAIVTSEVICH a n d S. S. IVANCItEV S. V. Lebedev All-Union Synthetm R u b b e r Research I n s t i t u t e
(Received 23 July 1975) A s t u d y was m a d e of the reaction of the PS macroradical with 1,5,9-trans-, trans. cis-cyclododecatriene (a model of the butadmne chain) and with the recombination prod u c t of radicals of the initiator (model of the polymer endgroups). The results obtained m respect to 1,3-butadieno polymerizatmn in presence of 4,4'-azo-bm-4-cyanovalerianic acid suggest t h a t no significant role is played b y reactions of chain transfer to p o l y mer, or to initiator or initiator fragments in polymer, a n d this should be reflected in strmt blfunctmnality of the p o l y m e r m respect to endgroups.
I~ THE case of low molecular polymers with reactive end groups functionality (the average number of reactive groups per macromolecule) is a characteristic of major importance. As the curing of these polymers is done via endgroups of the latter it follows that the functionality of prepolymers, in the sense specified above, will largely determine the network structures of the vulcanizates and will have a corresponding influence on their mechanical properties. The two iactors determining the functionality of a polymer prepared by radical polymerization are: 1) the mode of chain termination (by recombination or disproportionation); 2) chain transfer reactions. It is now taken to have been established that chain termination takes place almost exclusively by recombination in the polymerization of diene hydrocarbons and styrene [1]. As regards chain transfer reactions it can be said that much less study has been made of their influence on functionality. The constant for chain transfer to polymer C~----1-1× 10-a (50°) obtained by Hayes [2] was in connection with an investigation of the graft eopolymerization of butadiene in an emulsion system. Reactions of chain transfer to polybutadiene or to models of the latter, e.g. to 1,5-hexadiene, in an emulsion system have likewise been investigated by other authors [3, 4]. The present paper relates to the results of our investigation of reactions of chain transfer to polymer, initiator, and initiator fragments in polymer during radical polymerization in presence of a earboxyl-containing initiator (4,4'-azobis-4-cyanovalerianic acid, (CVA)). Information regarding chain transfer to th~ solvent (acetone) is available in [5]. * Vysokomol. soyed. A18: No. 4, 868-871, 1976. 991
992
V.P. KARTAVYK~
et al.
Styrene containing not less t h a n 99% basic substance was freshly distilled, b.p. 143.5144 °. Acetone (of analytical purity) was not subjected to further purification, OVA and its decomposition product (recombinate) were prepared in accordance with [6]. The melting points of b o t h products were in line with d a t a in the hterature. The elemental composition a p p r o x i m a t e d to the calculated compomtion. 1,5,9-Trans-trans, eis-cyclododecatriene (CDT) of chromatographically pure grade was p r e p a r e d b y trimerization of butadiene m presence of AI(C2Hs)zC1 and TiCI~ [7], b.p. 93-94°/9 torr. The ampoules were charged m a v a c u u m a p p a r a t u s a t I0 -4 torr under conditions pre. cluding the e n t r y of oxygen into the system, using the normal procedure. The number average degree of polymerization of styrene was determined from the relation [8] P n = 1710 [~]~ 3, (1) T h e / ~ values calculated from formula (1) and from the n u m b e r of OOOH groups in polymer, calculating for bffunctionality, agreed within limits of 10 %. The discrepancy between P a values determined from (1) a n d from an expression relating number average with viscesity average degrees of polymerization for the case of t e r m i n a t i o n b y recombination: / ~ 1"03
--
-
•P~ [9] d i d not exceed 5-6 ~o.
1.5
The polymer was separated from concentrated acetone solution b y chilled methanol, a n d was then reprecipltated twice from benzene with chilled methanol.
The concept of branching in the case of low molecular polymers with functional endgroups is normally identified with the polyfunctionality of a polymer, since the number of functional groups in branched molecules generally exceeds two. This factor, as was pointed out above, finds further reflection in the properties I ,i03 e
5"I 5"7
b
4.8
3"6~ 3"2
5"3
0,I
g'3
0"5
U'2
2 e
0"5
I'0
I i l.q I'8 [ A ] ~i0 z I:M]
FIG. 1. Plots of 1//3~ vs. [A]/[M]; a - - [ } I j / [ I ] ÷ = c o n s t , 70°; [A]--CDT concentrotion, mole/l; b - - 1 -- 70; 2 - - 124°; [A] --concentration of CH3.
Polymerization in presence of 4,4'-azo-bis-cyanovalerianic acid
993
of vuleanizates, which accounts for the interest shown b y authors in problems of the t y p e in question. Our aim in the present investigation was to determine how far the functionality of polymers is influenced b y chain transfer reactions. Styrene was selected as the monomer for purely practical reasons, as this makes it possible for the reaction to be run in glass ampoules over a wide range of temperature; degrees of polymerization can readily be determined from intrinsic viscosity values. It is known that the PS radical (benzyl radical) is very similar to the polybutadieno (allyl) radical [10] as regards reactivity. This means that results of chain transfer investigations obtained with styrene as the monomer can be transferred to cases
I/
7]]
Ig
2
I
0"05
~
1
I
0"15
I.
0"25 [ I ] 11z
Fia. 2. Plots of 1/[t/] vs. [I]* at 70 (1) and 124° (2). of polymerization of dieno monomers, without allowing for any considerable error in so doing. However, PS and polybutadiene m a y differ from one another as examples of chain transfer to polymer. In investigations of chain transfer to polymer we therefore took CDT as a model of the polybutadione chain, CDT being an individual compound and a cyclic trimer of butadieno. Like polybutadiene prepared b y radical polymerization, CDT contains mainly trans-units, b u t unlike the polymer it does not contain 1,2-units, which are found in the polymer in amounts equal to 15-20%. The constant for chain transfer to a n y substrato A (in our case, CDT) C A can be found from a relation similar to a Mayo formula, and can be determined from the slope of the curve of 1/fin against [A]/[M]. The transfer constant determined from Fig. la had a value of 2.59 × 10 -3, or, recalculating for the monomer unit, the corresponding value of C ~ 0 . 8 6 × 1 0 -3. Using the same method a s t u d y was made of chain transfer to the product CH3
I
of recombination of initiator (HOOCCH2CH2C--)2 radicals; the initiator is a good
I
CN
V. P. K A R T A ~ et cd.
994
model of the end section of the polymer chain. I t is seen from Fig. lb that the constant for transfer to the latter product at 70 ° is extremely low (3× 10-4), and the low number of these groups in the system means that no significant role is played b y the latter reaction. A marked rise in .temperature (124 °) brings the transfer constant up to 1.37× 10 -3 . Estimates of transfer to the initiator I were based on 1/Pn--[I] 1/2 plots. CAT.CULATEDD E G P ~ E S OF B ~ C H I ~ G OF P O L r S U T ~ U ~ N E (/5~----50, CVA initiator, 70°)
~,% 30 40 50 60
Each branch containing pX lOs ~r monomer monomer maeromoleunit units eules 0"17 0-24 0"33 0"46
5900 4160 3020 2070
118 83 60 41
Branched molecules, mole % 0.85 1.20 1.65 2.4
I t is clear from Fig. 2 where the quantity 1/[7] proportional to 1//~ was used instead of the latter, the dependence is a linear one at 70 °, while at 124 ° there is a deviation from linearity. This points to chain transfer to initiator taking place at high temperatures. An analysis of high temperature chain transfer reactions provided convincing evidence of the sensitivity of the adopted method of investigation, as the effect of a chain transfer reaction is very slight at 70 °. Selection of the temperature interval for determination of the transfer constants was governed b y conditions of synthesis of low molecular polybutadienes containing carboxyl endgroups (75-130 ° ) [1]]. In view of the constant obtained for chain transfer to polymer it appears possible to determine the proportions of branched (polyfunctional) chains in polybutadiene at different degrees of conversion of monomer. To do so one has the formula derived b y Flory and Stockmayer [1] which relates branching density p to C~ and to the conversion of monomer _ /2.3log(l-a)
_
Thus, the results of a study of the foregoing model reactions show that the main reaction loading to the formation of branched polyfunctional molecules is chain transfer to polymer. I f the polymerization reaction is run at 70 °, chain transfer to polymer is practically the only reaction that takes place, b u t at temlaeratures above 100 ° reactions of chain transfer to initiator and to initiator fragments in polymer play a considerable role. As can be seen from the Table, polybutadiene containing carboxyl endgroups which was prepared at 70 ° ought
Polymerization m presence of 4,4'-azo-bls-cyanovalerlanic acid
995
t o have a f u n c t i o n a l i t y a p p r o x i m a t i n g to 2, a n d a d m i x t u r e s of mono- a n d trif u n c t i o n a l macromolecules should a m o u n t t o n o m o r e t h a n 3 - 5 ~ . This conclusion is b o r n e out b y painstaking investigations based on f r a c t i o n a t i o n o f the p o l y m e r of interest, as well as on analysis o5 the f u n c t i o n a l i t y of the p o l y m e r in a c c o r d a n c e w i t h t h e gel p o i n t m e t h o d [12]. After e x a m i n i n g a large n u m b e r of p o l y b u t a d i e n e samples p r e p a r e d u n d e r c o m p a r a b l e conditions V a l u y e v a n d coworkers [12] came to the conclusion t h a t the p o l y m e r s in question were strictly bifunctional. I n view of this it is h a r d t o u n d e r s t a n d t h e results r e p o r t e d b y R e e d [13] who investigated the f u n c t i o n a l i t y of p o l y b u t a d i e n e s of a similar t y p e . According t o R e e d [13] average f u n c t i o n a l i t y changes within limits of 1.62 a n d 2.63 on v a r y i n g the concentrations o f m o n o m e r a n d initiator, a n d the time of polymerization, a n d no regularities of a n y sort are detectable. This is all the more surprising as one is referring n o t to the f u n c t i o n a l i t y of individual fractions, b u t t o t h e a v e r a g e f u n c t i o n a l i t y of the p o l y m e r as a whole. Our investigation of chain t r a n s f e r reactions show t h a t t h e role o f these reactions is insignificant w h e n t h e p o l y m e r i z a t i o n is r u n in presence of anizonitrile initiators, a n d t h e r e is therefore n o reason w h y polymers with m a r k e d variability in f u n c t i o n a l i t y should result. This bring us to the main difference between azonitrile initiators a n d those o f t h e p e r o x i d e t y p e w i t h which one associates reactions of chain t r a n s f e r to p o l y m e r b y p r i m a r y radicals leading to t h e a p p e a r a n c e of macromolecules of v a r y i n g f u n c t i o n a l i t y [14, 15]. Translated by R. J. A. HEI~DRY REFERENCES
1. P. FLORY, Principles of Polymer Chemistry, N. Y., 1953; Khimicheskie reaktsii polimerov (Chemical Reactions of Polymers). vol. 1, p. 275, Izd. "Mir", 1973 2. R. A. HAYES, J. Polymer Sci. 13: 583, 1954 3. M. MORTON and P. SALATIELLO, J. Polymer Sci. 6: 225, 1951 4. J. DRYSDALL and C. MARVEL, J. Polymer Sci. 6: 225, 1951 5. J. N. SEN, U. NANDI and S. R. PALIT, Indian Chem. Soc. 4@: 729, 1963 6. R. M. HAINES and W. WATERS, J. Chem. Soc., 4256, 1955 7. {~. WILKE, Germ. Pat. 1050333, 1959, Chem. Abstrs 55: 4393d, 1961 8. F. MAYO, R. GREGG and M. MATHESON, J. Amer. Chem. Soc. 73: 1961, 1951 9. S. Ya. FRENKEL, Vysokomol. soyed. 2: 731, 1960 (Translated in Polymer Sci. U.S.S.R. 2: 4, 427, 1961) 10. Kh. S. BAGDASAR'¥AN, Teoriya radikal'noi polimerizatsii (Theory of Radical Polymerization), p. 204, Izd. "Nauka", 1966 11. Germ. Pat. 1259575, 1965; U.S.A. Pat. 3235589, 1961 12. V. I. VALUYEV, R. A. SHLYAKHTER, Ye. G. ERENBURG and I. Ya. PODDUBNYI, Vysokomol. soyed. A14: 2291, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 11, 2674, 1972) 13. S. F. REED, J. Polymer Sei. 9, A - l : 2147, 1971 14. Ye. N. BARANTSEVICH, V. P. KARTAVYKH, V. I. VALUYEV and R. A. SHLYAKHo TER, Dokl. AN SSSR 225: 101, 1975 15. Yu. N. ANISIMOV, S. S. IVANCHEV and A. I. YURZHENKO, Vysokomol. soyed. A9: 692, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 3, 773, 1967)
(Received 23 July 1975) A s t u d y was m a d e of the reaction of the PS macroradical with 1,5,9-trans-, trans. cis-cyclododecatriene (a model of the butadmne chain) and with the recombination prod u c t of radicals of the initiator (model of the polymer endgroups). The results obtained m respect to 1,3-butadieno polymerizatmn in presence of 4,4'-azo-bm-4-cyanovalerianic acid suggest t h a t no significant role is played b y reactions of chain transfer to p o l y mer, or to initiator or initiator fragments in polymer, a n d this should be reflected in strmt blfunctmnality of the p o l y m e r m respect to endgroups.
I~ THE case of low molecular polymers with reactive end groups functionality (the average number of reactive groups per macromolecule) is a characteristic of major importance. As the curing of these polymers is done via endgroups of the latter it follows that the functionality of prepolymers, in the sense specified above, will largely determine the network structures of the vulcanizates and will have a corresponding influence on their mechanical properties. The two iactors determining the functionality of a polymer prepared by radical polymerization are: 1) the mode of chain termination (by recombination or disproportionation); 2) chain transfer reactions. It is now taken to have been established that chain termination takes place almost exclusively by recombination in the polymerization of diene hydrocarbons and styrene [1]. As regards chain transfer reactions it can be said that much less study has been made of their influence on functionality. The constant for chain transfer to polymer C~----1-1× 10-a (50°) obtained by Hayes [2] was in connection with an investigation of the graft eopolymerization of butadiene in an emulsion system. Reactions of chain transfer to polybutadiene or to models of the latter, e.g. to 1,5-hexadiene, in an emulsion system have likewise been investigated by other authors [3, 4]. The present paper relates to the results of our investigation of reactions of chain transfer to polymer, initiator, and initiator fragments in polymer during radical polymerization in presence of a earboxyl-containing initiator (4,4'-azobis-4-cyanovalerianic acid, (CVA)). Information regarding chain transfer to th~ solvent (acetone) is available in [5]. * Vysokomol. soyed. A18: No. 4, 868-871, 1976. 991
992
V.P. KARTAVYK~
et al.
Styrene containing not less t h a n 99% basic substance was freshly distilled, b.p. 143.5144 °. Acetone (of analytical purity) was not subjected to further purification, OVA and its decomposition product (recombinate) were prepared in accordance with [6]. The melting points of b o t h products were in line with d a t a in the hterature. The elemental composition a p p r o x i m a t e d to the calculated compomtion. 1,5,9-Trans-trans, eis-cyclododecatriene (CDT) of chromatographically pure grade was p r e p a r e d b y trimerization of butadiene m presence of AI(C2Hs)zC1 and TiCI~ [7], b.p. 93-94°/9 torr. The ampoules were charged m a v a c u u m a p p a r a t u s a t I0 -4 torr under conditions pre. cluding the e n t r y of oxygen into the system, using the normal procedure. The number average degree of polymerization of styrene was determined from the relation [8] P n = 1710 [~]~ 3, (1) T h e / ~ values calculated from formula (1) and from the n u m b e r of OOOH groups in polymer, calculating for bffunctionality, agreed within limits of 10 %. The discrepancy between P a values determined from (1) a n d from an expression relating number average with viscesity average degrees of polymerization for the case of t e r m i n a t i o n b y recombination: / ~ 1"03
--
-
•P~ [9] d i d not exceed 5-6 ~o.
1.5
The polymer was separated from concentrated acetone solution b y chilled methanol, a n d was then reprecipltated twice from benzene with chilled methanol.
The concept of branching in the case of low molecular polymers with functional endgroups is normally identified with the polyfunctionality of a polymer, since the number of functional groups in branched molecules generally exceeds two. This factor, as was pointed out above, finds further reflection in the properties I ,i03 e
5"I 5"7
b
4.8
3"6~ 3"2
5"3
0,I
g'3
0"5
U'2
2 e
0"5
I'0
I i l.q I'8 [ A ] ~i0 z I:M]
FIG. 1. Plots of 1//3~ vs. [A]/[M]; a - - [ } I j / [ I ] ÷ = c o n s t , 70°; [A]--CDT concentrotion, mole/l; b - - 1 -- 70; 2 - - 124°; [A] --concentration of CH3.
Polymerization in presence of 4,4'-azo-bis-cyanovalerianic acid
993
of vuleanizates, which accounts for the interest shown b y authors in problems of the t y p e in question. Our aim in the present investigation was to determine how far the functionality of polymers is influenced b y chain transfer reactions. Styrene was selected as the monomer for purely practical reasons, as this makes it possible for the reaction to be run in glass ampoules over a wide range of temperature; degrees of polymerization can readily be determined from intrinsic viscosity values. It is known that the PS radical (benzyl radical) is very similar to the polybutadieno (allyl) radical [10] as regards reactivity. This means that results of chain transfer investigations obtained with styrene as the monomer can be transferred to cases
I/
7]]
Ig
2
I
0"05
~
1
I
0"15
I.
0"25 [ I ] 11z
Fia. 2. Plots of 1/[t/] vs. [I]* at 70 (1) and 124° (2). of polymerization of dieno monomers, without allowing for any considerable error in so doing. However, PS and polybutadiene m a y differ from one another as examples of chain transfer to polymer. In investigations of chain transfer to polymer we therefore took CDT as a model of the polybutadione chain, CDT being an individual compound and a cyclic trimer of butadieno. Like polybutadiene prepared b y radical polymerization, CDT contains mainly trans-units, b u t unlike the polymer it does not contain 1,2-units, which are found in the polymer in amounts equal to 15-20%. The constant for chain transfer to a n y substrato A (in our case, CDT) C A can be found from a relation similar to a Mayo formula, and can be determined from the slope of the curve of 1/fin against [A]/[M]. The transfer constant determined from Fig. la had a value of 2.59 × 10 -3, or, recalculating for the monomer unit, the corresponding value of C ~ 0 . 8 6 × 1 0 -3. Using the same method a s t u d y was made of chain transfer to the product CH3
I
of recombination of initiator (HOOCCH2CH2C--)2 radicals; the initiator is a good
I
CN
V. P. K A R T A ~ et cd.
994
model of the end section of the polymer chain. I t is seen from Fig. lb that the constant for transfer to the latter product at 70 ° is extremely low (3× 10-4), and the low number of these groups in the system means that no significant role is played b y the latter reaction. A marked rise in .temperature (124 °) brings the transfer constant up to 1.37× 10 -3 . Estimates of transfer to the initiator I were based on 1/Pn--[I] 1/2 plots. CAT.CULATEDD E G P ~ E S OF B ~ C H I ~ G OF P O L r S U T ~ U ~ N E (/5~----50, CVA initiator, 70°)
~,% 30 40 50 60
Each branch containing pX lOs ~r monomer monomer maeromoleunit units eules 0"17 0-24 0"33 0"46
5900 4160 3020 2070
118 83 60 41
Branched molecules, mole % 0.85 1.20 1.65 2.4
I t is clear from Fig. 2 where the quantity 1/[7] proportional to 1//~ was used instead of the latter, the dependence is a linear one at 70 °, while at 124 ° there is a deviation from linearity. This points to chain transfer to initiator taking place at high temperatures. An analysis of high temperature chain transfer reactions provided convincing evidence of the sensitivity of the adopted method of investigation, as the effect of a chain transfer reaction is very slight at 70 °. Selection of the temperature interval for determination of the transfer constants was governed b y conditions of synthesis of low molecular polybutadienes containing carboxyl endgroups (75-130 ° ) [1]]. In view of the constant obtained for chain transfer to polymer it appears possible to determine the proportions of branched (polyfunctional) chains in polybutadiene at different degrees of conversion of monomer. To do so one has the formula derived b y Flory and Stockmayer [1] which relates branching density p to C~ and to the conversion of monomer _ /2.3log(l-a)
_
Thus, the results of a study of the foregoing model reactions show that the main reaction loading to the formation of branched polyfunctional molecules is chain transfer to polymer. I f the polymerization reaction is run at 70 °, chain transfer to polymer is practically the only reaction that takes place, b u t at temlaeratures above 100 ° reactions of chain transfer to initiator and to initiator fragments in polymer play a considerable role. As can be seen from the Table, polybutadiene containing carboxyl endgroups which was prepared at 70 ° ought
Polymerization m presence of 4,4'-azo-bls-cyanovalerlanic acid
995
t o have a f u n c t i o n a l i t y a p p r o x i m a t i n g to 2, a n d a d m i x t u r e s of mono- a n d trif u n c t i o n a l macromolecules should a m o u n t t o n o m o r e t h a n 3 - 5 ~ . This conclusion is b o r n e out b y painstaking investigations based on f r a c t i o n a t i o n o f the p o l y m e r of interest, as well as on analysis o5 the f u n c t i o n a l i t y of the p o l y m e r in a c c o r d a n c e w i t h t h e gel p o i n t m e t h o d [12]. After e x a m i n i n g a large n u m b e r of p o l y b u t a d i e n e samples p r e p a r e d u n d e r c o m p a r a b l e conditions V a l u y e v a n d coworkers [12] came to the conclusion t h a t the p o l y m e r s in question were strictly bifunctional. I n view of this it is h a r d t o u n d e r s t a n d t h e results r e p o r t e d b y R e e d [13] who investigated the f u n c t i o n a l i t y of p o l y b u t a d i e n e s of a similar t y p e . According t o R e e d [13] average f u n c t i o n a l i t y changes within limits of 1.62 a n d 2.63 on v a r y i n g the concentrations o f m o n o m e r a n d initiator, a n d the time of polymerization, a n d no regularities of a n y sort are detectable. This is all the more surprising as one is referring n o t to the f u n c t i o n a l i t y of individual fractions, b u t t o t h e a v e r a g e f u n c t i o n a l i t y of the p o l y m e r as a whole. Our investigation of chain t r a n s f e r reactions show t h a t t h e role o f these reactions is insignificant w h e n t h e p o l y m e r i z a t i o n is r u n in presence of anizonitrile initiators, a n d t h e r e is therefore n o reason w h y polymers with m a r k e d variability in f u n c t i o n a l i t y should result. This bring us to the main difference between azonitrile initiators a n d those o f t h e p e r o x i d e t y p e w i t h which one associates reactions of chain t r a n s f e r to p o l y m e r b y p r i m a r y radicals leading to t h e a p p e a r a n c e of macromolecules of v a r y i n g f u n c t i o n a l i t y [14, 15]. Translated by R. J. A. HEI~DRY REFERENCES
1. P. FLORY, Principles of Polymer Chemistry, N. Y., 1953; Khimicheskie reaktsii polimerov (Chemical Reactions of Polymers). vol. 1, p. 275, Izd. "Mir", 1973 2. R. A. HAYES, J. Polymer Sci. 13: 583, 1954 3. M. MORTON and P. SALATIELLO, J. Polymer Sci. 6: 225, 1951 4. J. DRYSDALL and C. MARVEL, J. Polymer Sci. 6: 225, 1951 5. J. N. SEN, U. NANDI and S. R. PALIT, Indian Chem. Soc. 4@: 729, 1963 6. R. M. HAINES and W. WATERS, J. Chem. Soc., 4256, 1955 7. {~. WILKE, Germ. Pat. 1050333, 1959, Chem. Abstrs 55: 4393d, 1961 8. F. MAYO, R. GREGG and M. MATHESON, J. Amer. Chem. Soc. 73: 1961, 1951 9. S. Ya. FRENKEL, Vysokomol. soyed. 2: 731, 1960 (Translated in Polymer Sci. U.S.S.R. 2: 4, 427, 1961) 10. Kh. S. BAGDASAR'¥AN, Teoriya radikal'noi polimerizatsii (Theory of Radical Polymerization), p. 204, Izd. "Nauka", 1966 11. Germ. Pat. 1259575, 1965; U.S.A. Pat. 3235589, 1961 12. V. I. VALUYEV, R. A. SHLYAKHTER, Ye. G. ERENBURG and I. Ya. PODDUBNYI, Vysokomol. soyed. A14: 2291, 1972 (Translated in Polymer Sci. U.S.S.R. 14: 11, 2674, 1972) 13. S. F. REED, J. Polymer Sei. 9, A - l : 2147, 1971 14. Ye. N. BARANTSEVICH, V. P. KARTAVYKH, V. I. VALUYEV and R. A. SHLYAKHo TER, Dokl. AN SSSR 225: 101, 1975 15. Yu. N. ANISIMOV, S. S. IVANCHEV and A. I. YURZHENKO, Vysokomol. soyed. A9: 692, 1967 (Translated in Polymer Sci. U.S.S.R. 9: 3, 773, 1967)