- Email: [email protected]
styrene (bast) copolymers
2078
S . S . ZISLINA et al.
2. K. A. ANDRIANOV and N. N. M~AKAROVA, Izv. Akad. N a u k SSSR, Seriya khim., 625, 1969 3. V. N. TSVETKOV, V. Ye. ESKIN and S. Ya. FRENKEL, S t r u k t u r a makromolekul v rastvore (Macromolecular Structure in Solution). Izd. " N a u k a " , 1964 4. V. N. TSVETKOV, K. A. ANDRIANOV, M. G. VITOVSKAYA, N. N. MAKROVA, E. N. ZAKHAROVA, S. V. BUSH1N and P. N. LAVRENKO, Vysokomol. soyed. A14: 369, 1972 (Translated in Polymer U.S.S.R. 14: 2, 412, 1972) 5. J. E. HEARST, J. Chem. Phys. 40: 1906, 1964 6. H. KUHN, W. KUHN and A. SILBERBERG, J. Polymer Sci. 14: 193, 1953 7. V. N. TSVETKOV, Uspekhi khim. 38: 1674, 1969 8. J. F. BROWN, Jr., L. H. VOGT, Jr., A. KAT[~HMAN, J. W. EUSTANCE, K. M. KISER and K. W. KRANTZ, J. Am. Chem. Soc. 82: 6194, 1960 9. K. A. ANDRIANOV, G. L. SLONIMSKH~ Ya. V. GENIN, V. Yu. LEVIN, N. A. MAKAROVA and D. Ya. TSVANKIN, Dokl. Akad. N a u k SSSR 187: 1285, 1969 10. V. N. TSVETKOV, K. A. ANDRIANOV, L N. SHTENNIKOVA, G. I. OKHRIMENKO, L. N. ANDREYEVA, G. A. F O M ~ a n a V. I. PAKHO1KOV, Vysokomol. soyod. A I 0 : 547, 1968 (Translated in P o l y m e r Sci. U.S.S.R. 10: 3, 636, 1968) 11. V. N. TSVETKOV, K. A. ANDRIANOV, G. I. OKHRI1KENKO, I. N. SHTENNIKOVA, G. A. FOMIN, M. G. VlTOVSKAYA, V. I. PAKHOMOV, A. A. YAROSH and D. N. ANDREYEV, Vysokomol. soyed. A13: 1892, 1970 (Translated in Polymer Sci. U.S.S.R. 13: 8, 2128, 1970) 12. V. N. TSVETKOV, K. A. ANDRIANOV, M. G. VITOVSKAYA, N. N. Mfi.KROV, S. V. BUSHIN, E. N. ZAKHAROVA, P. N. LAVRENKO and A. A. GORBUNOV, Vysokomol. soyed. A15: 872, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 4, 980, 1973) 13. W. KUHN and H. KUHN, Helv. Chim. A c t a 26: 1394, 1943 14. V. N. TSVETKOV, Dokl. Akad. l~auk SSSR 192: 380, 1970 15. I. N. SHT IKOVA, V. N. TSVETKOV, T. V. PEKER, Ye. I. RUMTSEV and Yu. P. GERMANCHUK, Vysokomol. soyed. A16: 1086, 1974 (Translated in P o l y m e r Sci. U.S.S.R. :16: 5, 1974) 16. Yu. Ya. GOTLIB and Yu. Ye. SVETLOV, Dokl. Akad. N a u k SSSR 168: 621, 1966 17. J. NODA and J. E. HEARST, J. Chem. Phys. 54: 2342, 1971
THE THERMAL DECOMPOSITION OF RANDOM AND ALTERNATING BUTYL ACRYLATE/STYRENE (BAST) COPOLYMERS* S. S. ZISLINA, L. M. TERMAN, YU. D. SEMCHIKOV, Z. N. TIKHONOVA a n d R . YA. KHVILIVITSKII Chemistry Institute, Gor'ki, U.S.S.R. A c a d e m y of Sciences (Received 29 January 1973) The kinetics of thermal decomposition a n d the decomposition products at 300°C under vacuum from r a n d o m a n d alternating BAST copolymers were investigated. The lbrocess has a number of radical chain reactions amongst which the main * Vysokomol. soyed. A16: 1~o. 8, 1797-1802, 1974.
Thermal decomposition of random and alternating BAST eopolymers
2079
ones comprise the decomposition of acrylate chain units an d the liberation of COs, butylene, butanol, as well as a depolymerization. The rate for the decomposition of r a n d o m copolymer slows down as the ST content and the molecular weight (reel. wt.) of t h e copolymers increase. The alternating eopolymers decompose much more slowly an d the decomposition products contain less alcohol t h a n the B A S T r a n d o m eopolymer o f identical composition. The mol. wt. changes which occur during the decomposition o f the B A S T r a n d o m copolymers depend on their ST content. Those containing up to 60~o of ST are soluble and their mol.wt, quickly decreases during decomposition. The largest reel. wt. reduction takes place in copolymers with a 1 : 1 component ratio. The copolymers having ST content greater t h a n 60% become insoluble as a result of decomposition.
THE sum of reactions resulting in the thermal degradation of a polymer can be arbitrarily divided into two groups: 1) the reactions not involving the functional groups of the monomer units, e.g. depolymerization, random chain fracture and 2) the reactions by functional groups, such as their dissociation in polyvinyl chloride, polyvinyl acetate, ether bond fracture in polyacrylates. If a functional group reacts by any other than a monomolecular mechanism, the rate of such a reaction will depend on the surroundings, and primarily on the adjacent chain units. B y alternating the nature of these one can change the reaction rate of the functional groups within a wide range. This study deals with the effect of the ST chain unit on the heat resistance o f the ester groups in random and alternating BAST copolymers. EXPERIMENTAL Industrial grade monomers were used. The purification and analytical methods used w i t h BA have already been described [1]. The ST was purified by fraction distillation under vaouttm. Chromatographic analysis on a " T s v e t - l " instrument showed the purity of ST t o be at least 99.9~/o, n~° 1-5462. The random copolymers were produced by copolymerizing BA(M1) with ST(M2) in bulk to 15% conversion at 25°C. The initiator was a 0.5% dicyeloh ex y l p ero x y diearbonate solution (DCPC). The composition was calculated on the basis of relative reactivities given in the literature (ret-= 0"15, r~t:0"43 [2]) and was checked by elemental analysis. The alternating copolymers were produced in the presence of differing amounts of ZnCl~ at 22°C (sequential polymers). Values r ef and r,et were used to estimate the order o f the sequences. The polymerization was taken to 10~o conversion and 0"2~o of D~PC was used as initiator. The random eopolymers were purified by a double reprecipitation from acetone with methanol, the sequential eopolymers by a six fold reprceipitation. The compositions of the latter were determined by elemental analysis. The absence of ZnCls was checked by analysis of the polymer for Zn [3]. The kinetic measurements were m a d e on a high temperature MoBain balance fitted with a quartz coil. The method used and the chromatographic analysis of the thermal degradation products have been described [1]. The deoomposition kinetics were investigated in some experiments as a function of pressnre in the system. The sample contained in an ampoule was fused to a H g oapillary m a n o m e t e r and the system was e v a c u a te d to 10 -4 m m H g . The viscosity of the polymer . ~olutions was determined in an Ubbelohde viseometer at 25°C and acetone as solvent. The infrared (IR) speotra of the residues were recorded on instrument IKS-14.
2080
S.S.
ZISLINA et al.
RESULTS
T h e B A S T c o p o l y m e r d e c o m p o s e d a t 300°0 a n d l i b e r a t e d volatile fractions w h i c h c o n d e n s e d in t h e side a r m of t h e a m p o u l e cooled w i t h liquid nitrogen, a l o n g w i t h t h o s e of low volatility, which were condensed in a h o r i z o n t a l t u b e a t r o o m t e m p e r a t u r e . T h e c o n t e n t of t h e l a t t e r f r a c t i o n in t h e t h e r m a l d e g r a d a t i o n p r o d u c t s of t h e B A S T c o p o l y m e r ' o b t a i n e d u n d e r v a c u u m (300°C, 4.5 hr) is g i v e n below: ST in copolymer 20 30 50 (random) 50 (sequential) 65 90 100 mole% Fraction of low 52 68 60 58 60.5 52 59 volatility, ~o w/w One can see t h a t t h e r e l a t i v e a m o u n t s o f t h e fractions d e p e n d e d o n l y slightly on t h e c o p o l y m e r composition. T h e ~R s p e c t r a of t h e low v o l a t i l i t y fraction were identical w i t h those o f t h e r e s p e c t i v e c o p o l y m e r s , so t h a t one can s a y t h a t it is m a d e u p of t h e m a i n chain f r a g m e n t s . T h e c o m p o s i t i o n s of t h e volatile f r a c t i o n s are s h o w n in T a b l e r. TABLE 1.
ml
mole % 0 20 30.5 37 44 50 69 80 90 100
THE
MAIN VOLATILE FRACTION PRODUCTS OF THE THERMAL DECOMPOSITION OF BAST UNDER VACUUM(300°C, 4"5 hr)
Decomposition, % 4.6 9.4 9.4 7"1 9.3 12.1 23'5 26.8 23.6 20"3
Products in ~o w/w of all volatile fraction products butylene butanol toluene CO2 BA I ST 1.0 4-6 5.5 3.2 9.2 8"0 23.1 25.1 14.8 19.2
6.2 15.5 32.0 28.5 40.0 48.5 53.8 69-0 66-5
7"4 7'2 7"1 28"8 24'0 18'0 13"8 15.2 13-5
1.2 0.7 2.0 1'8 0.9 1.0 0:3 0.1 0-1
-
3.6 5.8 5.7 4.6 6.2 2-0 1.0
0.5 0.5
97.0 69.5 60.0 48.0 23.3 17.0 4.8 2.0 0.3
T h e m a i n t h e r m a l d e g r a d a t i o n p r o d u c t s of t h e c o p o l y m e r s were CO2, b u t y l e n e , b u t a n o l , a n d t h e m o n o m e r s . H y d r o g e n was discovered in a d d i t i o n b y c h r o m a t o g r a p h y (at 300°C, 4.5 hr): ml, mole % H2, ~o w/w
0 0.003
25 0.008
37 0.005
50 0.022
69 0.008
95 0.007,
a n d t h e m a x i m u m q u a n t i t y was o b t a i n e d f r o m a 50 : 50 r a t i o c o p o l y m e r . T a b l e 2 c o n t a i n s t h e c o m p a r a t i v e c o m p o s i t i o n s of t h e d e g r a d a t i o n p r o d u c t s o f r a n d o m a n d s e q u e n t i a l e q u i m o l a r c o p o l y m e r s . I t shows t h a t one of t h e m a i n differences is t h e larger a m o u n t o f alcohol in t h e d e g r a d a t i o n p r o d u c t s of t h e r a n d o m e o p o l y m e r . T h e m o r e severe t h e d e g r a d a t i o n , t h e smaller will be t h e r e l a t i v e q u a n t i t y of alcohol p r e s e n t in t h e d e g r a d a t i o n p r o d u c t s of the r a n d o m c o p o l y m e r ,
Thermal decomposition of random and alternating BAST copolymers
2081
a n d t h e m o r e will it a p p r o a c h t h a t t y p i c a l for t h e sequential. T h e s u m o f all t h e d a t a e x a m i n e d a b o v e f o r m s t h e basis o f t h e conclusion t h a t t h e g r o u p i n g s m o s t f a v o u r a b l e for l i b e r a t i o n of alcohol are t h e B A diades w h e r e are n o t p r e s e n t in TABLE
2. T H E I~AII~ P R O D U C T S I N T H E V O L A T I L E FRACTIOI~ OF T H E 5 0 : 5 0
BAST
~A~DOM
A N D S E Q U E N T I A L COPOLYI~IER D U R I N G T H E R M A L V A C U U M D E C O M P O S I T I O N ( 3 0 0 ° C )
Products in ~o w/w of all volatile fraction products
Copolymer
Sequential Statistical
8 CO~
4.5 4.5 9.5 14.5
9"9 12.1 19.8 30.3
8-2 8.0 8-9 9.2
36.3 24.0 21.0 15.3
25.5 40.0 23.6 22.2
0.8 1.0 0.8 1.7
unidentified products
BA
ST
10.6 6.2 8.8 8.1
15.4 I 17.0 33-2 34.5 I
0.9 1.4 3.7 7.8
the sequential p o l y m e r , a n d r a p i d l y d i s a p p e a r during t h e d e c o m p o s i t i o n of t h e r a n d o m c o p o l y m e r . T h e r e l a t i v e q u a n t i t i e s of COs h a r d l y differ for t h e t w o c o p o l y m e r t y p e s , b u t t h e r e is m o r e b u t y l e n e p r e s e n t in t h e p r o d u c t s coming f r o m t h e sequential copolymer. p, rn nH,.q 150 U, % / m / n
! 2
0"8
3 90
II
O'G
30
O.Z
ziO
8O rnz , mo/e
FIG. 1
[
/
I
I
3 Time, hr
I
I
5
FIG. 2
FIG. 1. The initial rate of vacuum degradation of BAST copolymers at 300°C as a function of composition. FIG. 2. The dependence of pressure on the duration of heating at 300°C in a closed system under vacuum for 50:50 copolymers at [~]-values of (dl/g): 1--0-06, 2--0.47, 3--0.57, 4--1.37.
2082
S. S, ZISLINA et al.
Some corrections can be introduced into the probable scheme of CO S production [4] and of butanol [5] o n t h e basis of our findings; these are based on the theory of a radical chain decomposition of the ester bonds in the polyacrylates. The majority of the schemes [5, 6] assumed the alcohol to be produced from the diades or triades of acrylic chain units (bonds) as a result of the acrylic radical of a central group attacking those in chain units in front of or behind it. I t follows from our results t h a t the b u t a n o l is produced fastest from the BA diades. The fact t h a t the degradation products from the sequential copolymer contain a considerable amount of alcohol show however t h a t the latter can also form more slowly, but at a commensurate rate, from ~ , ~ ] ~ A - B A ~ diades and - - B A - S T - B A - - triades [15]: CH2
/\
/\ ~CH~--C
CH~
II
I
R0--C
C=0 0"
~CH2--C II RO--C 0
0 I
/\
I
ctt
CH~
Re/
CIt~ I + Oil" C=O
N/
OR
<) /
CH2
0
\
CH
CH~
CH~
CH~ --~ C--C C H - - C H ~ + OR" I / l\ / CH--CH~,-, olt C II I c o
-I0I/ o The most probable of the reactions for production of 002 is regarded to be t h a t of diades of BA units [5-7]. The considerable quantities found by us in the degradation product of t h e s e q u e n t i a l copolymer and the absence of any dependence of the relative 00~ content on the extent of degradation, leads to the assumption t h a t there is still another mechanism of CO~ formation which has at least the same rate as t h a t of formation from diades. The most probable amongst these is a monomolecular reaction, which we had examined before [1], and which envisages a simultaneous formation of CO~ and butylene
tt ---[- -
~C-=O , I
/o
--, H/
\C-~O-*R'-2Ctt=CH2-t- CO~ q- C 1
o
H--C--C
]t--¢---d/--
a,/H/~H
a,/H/~H
t~
Thermal decomposition of random and alternating BAST copolymers
2083
Butylene is the product of two reactions [11]; one of these has been examined above, the other is connected with the formation of carboxyl groups in the copolymer. BAST degradation, in which the copolymer contains BA chain units as isolated ones, will give rise to the formation of butylene in larger quantities than of CO~. Such copolymers were found to show a 3100-3600 cm -z absoption line after decomposition, which belongs to the carboxyl group. These spectra were found to be identical with those of BA-acrylic acid copolymet models in this range. Table 2 makes it clear t h a t the yield of butylene is larger in the degradation of the sequential copolymer. The explanation is the lower probability of a bimolecular radical reaction of the acrylic unit diades; this naturally leads to an increase in the probability of a monomolecular reaction.
~-q]dfq] 5g dP[po, %
@2
10
45
G
/5
!
1
2 !
8 Time, h p
:FIG. 3
5
20
60
100 mz , mole %
Fio. 4
FIG. 3. The weight loss curves of 50 : 50 BAST copolymers under vacuum at 300°C: 1 random copolymer, 2--with a ZnC12 molar content of (per mole of BA): 2--0.17, 3--0.44, 4--0-70. FIo. 4. The intrinsic viscosity changes off I--random, 2--sequential BAST eopolymers during thermal degradation under vacuum at 300°C. The eopolymer is insoluble in the shaded area. The type of adjacent chain unit has a significant effect also on the rate of decomposition of copolymers with different compositions. The dependence of the starting rates of BAST copolymer degradation under vacuum at 300°C on its composition (Fig. 1) shows the styrene units to have some inhibiting activity and the explanation could be the following: a) the ST unit has a more reactive a-hydrogen atom t h a n BA (QsT~QBA) due to the respective radical being more stable. The ST, although added for eopolymer production at smaller than BA quantities, will strongly compete with BA in the a-hydrogen atom termination reactions; b) the "central" aeryl radical units forming when the ST content is large m a y be surrounded b y ST units and thus be unable to form diades.
2084
S.S.
ZISLINA et al.
The influence of the terminal groups present in the copolymer can be traced from the rate of thermal degradation as a function of the mol.wt. As Fig. 2 shows, an increase of the mol.wt, results in a decrease of the rate of thermal degradation of the copolymer. The kinetic curves reflect in this case t h a t only very volatile products are being produced, such as CO2, alcohol, butylene, and monomers, but not main chain fragments. As the volatile products contain only 250//0 of the monomers (see above), the rate of thermal degradation as a function of mol.wt. will mainly be due to ester group decomposition reactions. TABLE
3.
SOME
OF
THE
SEQUENTIAL
COPOLYMER
CHARACTERISTICS
[ZnC12]/ /[BA]
~7
ef r1
2O
[~]acetone
(initial)
0 0-17 0-44 0.55 0.70
0.03
0"15 0"06 0
0
0
0"48 0"11
0'57 0-85 1 "40 2-20 2"20
We believe this dependence to be due to the rate of initiation of the radical chain decomposition of ester units to be the same in all the copolymers, regardless of mol.wt*., because of the unequal concentration of initiating groups present in them (weak bonds). The terminations taking place during copolymer synthesis are the result of disproportionation as well as of recombinations of macroradicals. Weak allyl bonds form in the first case, and --CH(CsH~)--CH(CsHs)--, --CH(CeHs--CH(COOR)--, - - C H ( C O O R ) - - C H ( C 0 0 R ) groups in the second. All these groups have bonds with lower dissociation energies and can therefore become the initiators of radical processes at high temperatures. This bond concentration per unit weight of the copolymer gets larger when the mol.wt, decreases and will result in an increase of the degradation rate. Figure 3 shows t h a t sequential decompose at a lower rate than random copolymers. One can gather from Table 3 t h a t an increase in ZnC12 concentration will increase the mol.wt, and the extent of alternation of monomer chain units in the copolymer. A lower initial degradation rate of sequential, compared with random copolymers, can be correlated with the greater mol.wt, of the former, as well as a decrease in the number of BA-BA diades, and thus of the rate of butanol formation. The changes in mol.wt, resulting from the copolymer degradation are a complex function of its composition (Fig. 4). The poly-BA becomes partly crosslinked during thermal degradation as indicated by the formation of CO2 and alcohol. The BAST copolymer containing up to 60% ST is soluble; its mol.wt, rapidly drops during degradation. Figure 4 also shows t h a t the largest mol.wt, drop occurs in
Thermal decomposition of random and alternating BAST eopolymers
2085
t h e 50 : 50 copolymer. This can be explained as due to two causes: 1) the B A - S T bonds are the weakest; 2) the fl±decomposition of the central radicals (intramolecular disproportionation) is energetically more advantageous in ~ , ~ B A - S T ~ or ~ B A - S T ~ diades t han in those of ~ B A - B A ~ or ~ S T - S T ~ . The most favourable is the B A - S T diade fracture as the less stable BA radical is replaced b y the more stable ST radical. At 60~o and larger ST contents, and up to poly-ST, the copolymers again become insoluble as a result of degradation. The main t y p e of reaction in the latter case is the decomposition of the ester chain units and the formation of main-chain carboxyl groups. At such high temperatures (300°C) the carboxyl groups of poly-acids are unstable and give the anhydride with elimination of water. This reaction normally takes place in the adjacent groups, but the carboxyl groups are well isolated from each other by ST chain units where the ST content of the copolymer is large, so t h a t t h e y cannot react by an intramolecular reaction. A single possibility remain for them, i.e. an intermolecular reaction, which leads to the polymer becoming insoluble. The thermal degradation of the BAST copolymer thus consists of a num ber of radical chain reactions amongst which the main is the decomposition of the central acrylic chain unit radical forming after elimination of ~-hydrogen, and resulting in the formation of C02, butanol, butylene, and in depolymerization. Judging from the yield of the monomers, one can conclude t h a t the depolymerization has a limited significance for the system due to its decay in the intraand intermolecular chain transfer reactions. Translated by K. A. ALLEh" REFERENCES
1. Yu. D. SFJvICHIKOV, L. iV[. TERMAN, N. A. SENINA and S. S. ZISL[NA, Vysokomol.
soyed. A14: 238, 1972 (Translated in Polymer Sci. U.S.S.R. I4: 1, 267, 1972) 2. L. J. YOUNG, v. Polymer Sci. 54: 411, 1961 3. R. P~IBYL, Kompleksony v khimieheskoi analize (Complexones in Chemical Analysis). Izd. inostr, lit., 1960 4. R. B. FOX, L. G. ISAACS, S. STOKES and P. E. KAGARISE, J. Polymer Sci. A2: 2085, 1964 5. G. G. CAMERON and D. R. KANE, Makromol. Chemic 113: 75, 1968 6. N. GRASSIE and B. J. D. FORRANCE, J. Polymer Sci. 9: A-l: 931, 1971 7. N. GRASSIE and B. J. D. F()I~I~ANCE, J. Polymer Sei. 6, A-I: 3303, 3315, 1968
S . S . ZISLINA et al.
2. K. A. ANDRIANOV and N. N. M~AKAROVA, Izv. Akad. N a u k SSSR, Seriya khim., 625, 1969 3. V. N. TSVETKOV, V. Ye. ESKIN and S. Ya. FRENKEL, S t r u k t u r a makromolekul v rastvore (Macromolecular Structure in Solution). Izd. " N a u k a " , 1964 4. V. N. TSVETKOV, K. A. ANDRIANOV, M. G. VITOVSKAYA, N. N. MAKROVA, E. N. ZAKHAROVA, S. V. BUSH1N and P. N. LAVRENKO, Vysokomol. soyed. A14: 369, 1972 (Translated in Polymer U.S.S.R. 14: 2, 412, 1972) 5. J. E. HEARST, J. Chem. Phys. 40: 1906, 1964 6. H. KUHN, W. KUHN and A. SILBERBERG, J. Polymer Sci. 14: 193, 1953 7. V. N. TSVETKOV, Uspekhi khim. 38: 1674, 1969 8. J. F. BROWN, Jr., L. H. VOGT, Jr., A. KAT[~HMAN, J. W. EUSTANCE, K. M. KISER and K. W. KRANTZ, J. Am. Chem. Soc. 82: 6194, 1960 9. K. A. ANDRIANOV, G. L. SLONIMSKH~ Ya. V. GENIN, V. Yu. LEVIN, N. A. MAKAROVA and D. Ya. TSVANKIN, Dokl. Akad. N a u k SSSR 187: 1285, 1969 10. V. N. TSVETKOV, K. A. ANDRIANOV, L N. SHTENNIKOVA, G. I. OKHRIMENKO, L. N. ANDREYEVA, G. A. F O M ~ a n a V. I. PAKHO1KOV, Vysokomol. soyod. A I 0 : 547, 1968 (Translated in P o l y m e r Sci. U.S.S.R. 10: 3, 636, 1968) 11. V. N. TSVETKOV, K. A. ANDRIANOV, G. I. OKHRI1KENKO, I. N. SHTENNIKOVA, G. A. FOMIN, M. G. VlTOVSKAYA, V. I. PAKHOMOV, A. A. YAROSH and D. N. ANDREYEV, Vysokomol. soyed. A13: 1892, 1970 (Translated in Polymer Sci. U.S.S.R. 13: 8, 2128, 1970) 12. V. N. TSVETKOV, K. A. ANDRIANOV, M. G. VITOVSKAYA, N. N. Mfi.KROV, S. V. BUSHIN, E. N. ZAKHAROVA, P. N. LAVRENKO and A. A. GORBUNOV, Vysokomol. soyed. A15: 872, 1973 (Translated in Polymer Sci. U.S.S.R. 15: 4, 980, 1973) 13. W. KUHN and H. KUHN, Helv. Chim. A c t a 26: 1394, 1943 14. V. N. TSVETKOV, Dokl. Akad. l~auk SSSR 192: 380, 1970 15. I. N. SHT IKOVA, V. N. TSVETKOV, T. V. PEKER, Ye. I. RUMTSEV and Yu. P. GERMANCHUK, Vysokomol. soyed. A16: 1086, 1974 (Translated in P o l y m e r Sci. U.S.S.R. :16: 5, 1974) 16. Yu. Ya. GOTLIB and Yu. Ye. SVETLOV, Dokl. Akad. N a u k SSSR 168: 621, 1966 17. J. NODA and J. E. HEARST, J. Chem. Phys. 54: 2342, 1971
THE THERMAL DECOMPOSITION OF RANDOM AND ALTERNATING BUTYL ACRYLATE/STYRENE (BAST) COPOLYMERS* S. S. ZISLINA, L. M. TERMAN, YU. D. SEMCHIKOV, Z. N. TIKHONOVA a n d R . YA. KHVILIVITSKII Chemistry Institute, Gor'ki, U.S.S.R. A c a d e m y of Sciences (Received 29 January 1973) The kinetics of thermal decomposition a n d the decomposition products at 300°C under vacuum from r a n d o m a n d alternating BAST copolymers were investigated. The lbrocess has a number of radical chain reactions amongst which the main * Vysokomol. soyed. A16: 1~o. 8, 1797-1802, 1974.
Thermal decomposition of random and alternating BAST eopolymers
2079
ones comprise the decomposition of acrylate chain units an d the liberation of COs, butylene, butanol, as well as a depolymerization. The rate for the decomposition of r a n d o m copolymer slows down as the ST content and the molecular weight (reel. wt.) of t h e copolymers increase. The alternating eopolymers decompose much more slowly an d the decomposition products contain less alcohol t h a n the B A S T r a n d o m eopolymer o f identical composition. The mol. wt. changes which occur during the decomposition o f the B A S T r a n d o m copolymers depend on their ST content. Those containing up to 60~o of ST are soluble and their mol.wt, quickly decreases during decomposition. The largest reel. wt. reduction takes place in copolymers with a 1 : 1 component ratio. The copolymers having ST content greater t h a n 60% become insoluble as a result of decomposition.
THE sum of reactions resulting in the thermal degradation of a polymer can be arbitrarily divided into two groups: 1) the reactions not involving the functional groups of the monomer units, e.g. depolymerization, random chain fracture and 2) the reactions by functional groups, such as their dissociation in polyvinyl chloride, polyvinyl acetate, ether bond fracture in polyacrylates. If a functional group reacts by any other than a monomolecular mechanism, the rate of such a reaction will depend on the surroundings, and primarily on the adjacent chain units. B y alternating the nature of these one can change the reaction rate of the functional groups within a wide range. This study deals with the effect of the ST chain unit on the heat resistance o f the ester groups in random and alternating BAST copolymers. EXPERIMENTAL Industrial grade monomers were used. The purification and analytical methods used w i t h BA have already been described [1]. The ST was purified by fraction distillation under vaouttm. Chromatographic analysis on a " T s v e t - l " instrument showed the purity of ST t o be at least 99.9~/o, n~° 1-5462. The random copolymers were produced by copolymerizing BA(M1) with ST(M2) in bulk to 15% conversion at 25°C. The initiator was a 0.5% dicyeloh ex y l p ero x y diearbonate solution (DCPC). The composition was calculated on the basis of relative reactivities given in the literature (ret-= 0"15, r~t:0"43 [2]) and was checked by elemental analysis. The alternating copolymers were produced in the presence of differing amounts of ZnCl~ at 22°C (sequential polymers). Values r ef and r,et were used to estimate the order o f the sequences. The polymerization was taken to 10~o conversion and 0"2~o of D~PC was used as initiator. The random eopolymers were purified by a double reprecipitation from acetone with methanol, the sequential eopolymers by a six fold reprceipitation. The compositions of the latter were determined by elemental analysis. The absence of ZnCls was checked by analysis of the polymer for Zn [3]. The kinetic measurements were m a d e on a high temperature MoBain balance fitted with a quartz coil. The method used and the chromatographic analysis of the thermal degradation products have been described [1]. The deoomposition kinetics were investigated in some experiments as a function of pressnre in the system. The sample contained in an ampoule was fused to a H g oapillary m a n o m e t e r and the system was e v a c u a te d to 10 -4 m m H g . The viscosity of the polymer . ~olutions was determined in an Ubbelohde viseometer at 25°C and acetone as solvent. The infrared (IR) speotra of the residues were recorded on instrument IKS-14.
2080
S.S.
ZISLINA et al.
RESULTS
T h e B A S T c o p o l y m e r d e c o m p o s e d a t 300°0 a n d l i b e r a t e d volatile fractions w h i c h c o n d e n s e d in t h e side a r m of t h e a m p o u l e cooled w i t h liquid nitrogen, a l o n g w i t h t h o s e of low volatility, which were condensed in a h o r i z o n t a l t u b e a t r o o m t e m p e r a t u r e . T h e c o n t e n t of t h e l a t t e r f r a c t i o n in t h e t h e r m a l d e g r a d a t i o n p r o d u c t s of t h e B A S T c o p o l y m e r ' o b t a i n e d u n d e r v a c u u m (300°C, 4.5 hr) is g i v e n below: ST in copolymer 20 30 50 (random) 50 (sequential) 65 90 100 mole% Fraction of low 52 68 60 58 60.5 52 59 volatility, ~o w/w One can see t h a t t h e r e l a t i v e a m o u n t s o f t h e fractions d e p e n d e d o n l y slightly on t h e c o p o l y m e r composition. T h e ~R s p e c t r a of t h e low v o l a t i l i t y fraction were identical w i t h those o f t h e r e s p e c t i v e c o p o l y m e r s , so t h a t one can s a y t h a t it is m a d e u p of t h e m a i n chain f r a g m e n t s . T h e c o m p o s i t i o n s of t h e volatile f r a c t i o n s are s h o w n in T a b l e r. TABLE 1.
ml
mole % 0 20 30.5 37 44 50 69 80 90 100
THE
MAIN VOLATILE FRACTION PRODUCTS OF THE THERMAL DECOMPOSITION OF BAST UNDER VACUUM(300°C, 4"5 hr)
Decomposition, % 4.6 9.4 9.4 7"1 9.3 12.1 23'5 26.8 23.6 20"3
Products in ~o w/w of all volatile fraction products butylene butanol toluene CO2 BA I ST 1.0 4-6 5.5 3.2 9.2 8"0 23.1 25.1 14.8 19.2
6.2 15.5 32.0 28.5 40.0 48.5 53.8 69-0 66-5
7"4 7'2 7"1 28"8 24'0 18'0 13"8 15.2 13-5
1.2 0.7 2.0 1'8 0.9 1.0 0:3 0.1 0-1
-
3.6 5.8 5.7 4.6 6.2 2-0 1.0
0.5 0.5
97.0 69.5 60.0 48.0 23.3 17.0 4.8 2.0 0.3
T h e m a i n t h e r m a l d e g r a d a t i o n p r o d u c t s of t h e c o p o l y m e r s were CO2, b u t y l e n e , b u t a n o l , a n d t h e m o n o m e r s . H y d r o g e n was discovered in a d d i t i o n b y c h r o m a t o g r a p h y (at 300°C, 4.5 hr): ml, mole % H2, ~o w/w
0 0.003
25 0.008
37 0.005
50 0.022
69 0.008
95 0.007,
a n d t h e m a x i m u m q u a n t i t y was o b t a i n e d f r o m a 50 : 50 r a t i o c o p o l y m e r . T a b l e 2 c o n t a i n s t h e c o m p a r a t i v e c o m p o s i t i o n s of t h e d e g r a d a t i o n p r o d u c t s o f r a n d o m a n d s e q u e n t i a l e q u i m o l a r c o p o l y m e r s . I t shows t h a t one of t h e m a i n differences is t h e larger a m o u n t o f alcohol in t h e d e g r a d a t i o n p r o d u c t s of t h e r a n d o m e o p o l y m e r . T h e m o r e severe t h e d e g r a d a t i o n , t h e smaller will be t h e r e l a t i v e q u a n t i t y of alcohol p r e s e n t in t h e d e g r a d a t i o n p r o d u c t s of the r a n d o m c o p o l y m e r ,
Thermal decomposition of random and alternating BAST copolymers
2081
a n d t h e m o r e will it a p p r o a c h t h a t t y p i c a l for t h e sequential. T h e s u m o f all t h e d a t a e x a m i n e d a b o v e f o r m s t h e basis o f t h e conclusion t h a t t h e g r o u p i n g s m o s t f a v o u r a b l e for l i b e r a t i o n of alcohol are t h e B A diades w h e r e are n o t p r e s e n t in TABLE
2. T H E I~AII~ P R O D U C T S I N T H E V O L A T I L E FRACTIOI~ OF T H E 5 0 : 5 0
BAST
~A~DOM
A N D S E Q U E N T I A L COPOLYI~IER D U R I N G T H E R M A L V A C U U M D E C O M P O S I T I O N ( 3 0 0 ° C )
Products in ~o w/w of all volatile fraction products
Copolymer
Sequential Statistical
8 CO~
4.5 4.5 9.5 14.5
9"9 12.1 19.8 30.3
8-2 8.0 8-9 9.2
36.3 24.0 21.0 15.3
25.5 40.0 23.6 22.2
0.8 1.0 0.8 1.7
unidentified products
BA
ST
10.6 6.2 8.8 8.1
15.4 I 17.0 33-2 34.5 I
0.9 1.4 3.7 7.8
the sequential p o l y m e r , a n d r a p i d l y d i s a p p e a r during t h e d e c o m p o s i t i o n of t h e r a n d o m c o p o l y m e r . T h e r e l a t i v e q u a n t i t i e s of COs h a r d l y differ for t h e t w o c o p o l y m e r t y p e s , b u t t h e r e is m o r e b u t y l e n e p r e s e n t in t h e p r o d u c t s coming f r o m t h e sequential copolymer. p, rn nH,.q 150 U, % / m / n
! 2
0"8
3 90
II
O'G
30
O.Z
ziO
8O rnz , mo/e
FIG. 1
[
/
I
I
3 Time, hr
I
I
5
FIG. 2
FIG. 1. The initial rate of vacuum degradation of BAST copolymers at 300°C as a function of composition. FIG. 2. The dependence of pressure on the duration of heating at 300°C in a closed system under vacuum for 50:50 copolymers at [~]-values of (dl/g): 1--0-06, 2--0.47, 3--0.57, 4--1.37.
2082
S. S, ZISLINA et al.
Some corrections can be introduced into the probable scheme of CO S production [4] and of butanol [5] o n t h e basis of our findings; these are based on the theory of a radical chain decomposition of the ester bonds in the polyacrylates. The majority of the schemes [5, 6] assumed the alcohol to be produced from the diades or triades of acrylic chain units (bonds) as a result of the acrylic radical of a central group attacking those in chain units in front of or behind it. I t follows from our results t h a t the b u t a n o l is produced fastest from the BA diades. The fact t h a t the degradation products from the sequential copolymer contain a considerable amount of alcohol show however t h a t the latter can also form more slowly, but at a commensurate rate, from ~ , ~ ] ~ A - B A ~ diades and - - B A - S T - B A - - triades [15]: CH2
/\
/\ ~CH~--C
CH~
II
I
R0--C
C=0 0"
~CH2--C II RO--C 0
0 I
/\
I
ctt
CH~
Re/
CIt~ I + Oil" C=O
N/
OR
<) /
CH2
0
\
CH
CH~
CH~
CH~ --~ C--C C H - - C H ~ + OR" I / l\ / CH--CH~,-, olt C II I c o
-I0I/ o The most probable of the reactions for production of 002 is regarded to be t h a t of diades of BA units [5-7]. The considerable quantities found by us in the degradation product of t h e s e q u e n t i a l copolymer and the absence of any dependence of the relative 00~ content on the extent of degradation, leads to the assumption t h a t there is still another mechanism of CO~ formation which has at least the same rate as t h a t of formation from diades. The most probable amongst these is a monomolecular reaction, which we had examined before [1], and which envisages a simultaneous formation of CO~ and butylene
tt ---[- -
~C-=O , I
/o
--, H/
\C-~O-*R'-2Ctt=CH2-t- CO~ q- C 1
o
H--C--C
]t--¢---d/--
a,/H/~H
a,/H/~H
t~
Thermal decomposition of random and alternating BAST copolymers
2083
Butylene is the product of two reactions [11]; one of these has been examined above, the other is connected with the formation of carboxyl groups in the copolymer. BAST degradation, in which the copolymer contains BA chain units as isolated ones, will give rise to the formation of butylene in larger quantities than of CO~. Such copolymers were found to show a 3100-3600 cm -z absoption line after decomposition, which belongs to the carboxyl group. These spectra were found to be identical with those of BA-acrylic acid copolymet models in this range. Table 2 makes it clear t h a t the yield of butylene is larger in the degradation of the sequential copolymer. The explanation is the lower probability of a bimolecular radical reaction of the acrylic unit diades; this naturally leads to an increase in the probability of a monomolecular reaction.
~-q]dfq] 5g dP[po, %
@2
10
45
G
/5
!
1
2 !
8 Time, h p
:FIG. 3
5
20
60
100 mz , mole %
Fio. 4
FIG. 3. The weight loss curves of 50 : 50 BAST copolymers under vacuum at 300°C: 1 random copolymer, 2--with a ZnC12 molar content of (per mole of BA): 2--0.17, 3--0.44, 4--0-70. FIo. 4. The intrinsic viscosity changes off I--random, 2--sequential BAST eopolymers during thermal degradation under vacuum at 300°C. The eopolymer is insoluble in the shaded area. The type of adjacent chain unit has a significant effect also on the rate of decomposition of copolymers with different compositions. The dependence of the starting rates of BAST copolymer degradation under vacuum at 300°C on its composition (Fig. 1) shows the styrene units to have some inhibiting activity and the explanation could be the following: a) the ST unit has a more reactive a-hydrogen atom t h a n BA (QsT~QBA) due to the respective radical being more stable. The ST, although added for eopolymer production at smaller than BA quantities, will strongly compete with BA in the a-hydrogen atom termination reactions; b) the "central" aeryl radical units forming when the ST content is large m a y be surrounded b y ST units and thus be unable to form diades.
2084
S.S.
ZISLINA et al.
The influence of the terminal groups present in the copolymer can be traced from the rate of thermal degradation as a function of the mol.wt. As Fig. 2 shows, an increase of the mol.wt, results in a decrease of the rate of thermal degradation of the copolymer. The kinetic curves reflect in this case t h a t only very volatile products are being produced, such as CO2, alcohol, butylene, and monomers, but not main chain fragments. As the volatile products contain only 250//0 of the monomers (see above), the rate of thermal degradation as a function of mol.wt. will mainly be due to ester group decomposition reactions. TABLE
3.
SOME
OF
THE
SEQUENTIAL
COPOLYMER
CHARACTERISTICS
[ZnC12]/ /[BA]
~7
ef r1
2O
[~]acetone
(initial)
0 0-17 0-44 0.55 0.70
0.03
0"15 0"06 0
0
0
0"48 0"11
0'57 0-85 1 "40 2-20 2"20
We believe this dependence to be due to the rate of initiation of the radical chain decomposition of ester units to be the same in all the copolymers, regardless of mol.wt*., because of the unequal concentration of initiating groups present in them (weak bonds). The terminations taking place during copolymer synthesis are the result of disproportionation as well as of recombinations of macroradicals. Weak allyl bonds form in the first case, and --CH(CsH~)--CH(CsHs)--, --CH(CeHs--CH(COOR)--, - - C H ( C O O R ) - - C H ( C 0 0 R ) groups in the second. All these groups have bonds with lower dissociation energies and can therefore become the initiators of radical processes at high temperatures. This bond concentration per unit weight of the copolymer gets larger when the mol.wt, decreases and will result in an increase of the degradation rate. Figure 3 shows t h a t sequential decompose at a lower rate than random copolymers. One can gather from Table 3 t h a t an increase in ZnC12 concentration will increase the mol.wt, and the extent of alternation of monomer chain units in the copolymer. A lower initial degradation rate of sequential, compared with random copolymers, can be correlated with the greater mol.wt, of the former, as well as a decrease in the number of BA-BA diades, and thus of the rate of butanol formation. The changes in mol.wt, resulting from the copolymer degradation are a complex function of its composition (Fig. 4). The poly-BA becomes partly crosslinked during thermal degradation as indicated by the formation of CO2 and alcohol. The BAST copolymer containing up to 60% ST is soluble; its mol.wt, rapidly drops during degradation. Figure 4 also shows t h a t the largest mol.wt, drop occurs in
Thermal decomposition of random and alternating BAST eopolymers
2085
t h e 50 : 50 copolymer. This can be explained as due to two causes: 1) the B A - S T bonds are the weakest; 2) the fl±decomposition of the central radicals (intramolecular disproportionation) is energetically more advantageous in ~ , ~ B A - S T ~ or ~ B A - S T ~ diades t han in those of ~ B A - B A ~ or ~ S T - S T ~ . The most favourable is the B A - S T diade fracture as the less stable BA radical is replaced b y the more stable ST radical. At 60~o and larger ST contents, and up to poly-ST, the copolymers again become insoluble as a result of degradation. The main t y p e of reaction in the latter case is the decomposition of the ester chain units and the formation of main-chain carboxyl groups. At such high temperatures (300°C) the carboxyl groups of poly-acids are unstable and give the anhydride with elimination of water. This reaction normally takes place in the adjacent groups, but the carboxyl groups are well isolated from each other by ST chain units where the ST content of the copolymer is large, so t h a t t h e y cannot react by an intramolecular reaction. A single possibility remain for them, i.e. an intermolecular reaction, which leads to the polymer becoming insoluble. The thermal degradation of the BAST copolymer thus consists of a num ber of radical chain reactions amongst which the main is the decomposition of the central acrylic chain unit radical forming after elimination of ~-hydrogen, and resulting in the formation of C02, butanol, butylene, and in depolymerization. Judging from the yield of the monomers, one can conclude t h a t the depolymerization has a limited significance for the system due to its decay in the intraand intermolecular chain transfer reactions. Translated by K. A. ALLEh" REFERENCES
1. Yu. D. SFJvICHIKOV, L. iV[. TERMAN, N. A. SENINA and S. S. ZISL[NA, Vysokomol.
soyed. A14: 238, 1972 (Translated in Polymer Sci. U.S.S.R. I4: 1, 267, 1972) 2. L. J. YOUNG, v. Polymer Sci. 54: 411, 1961 3. R. P~IBYL, Kompleksony v khimieheskoi analize (Complexones in Chemical Analysis). Izd. inostr, lit., 1960 4. R. B. FOX, L. G. ISAACS, S. STOKES and P. E. KAGARISE, J. Polymer Sci. A2: 2085, 1964 5. G. G. CAMERON and D. R. KANE, Makromol. Chemic 113: 75, 1968 6. N. GRASSIE and B. J. D. FORRANCE, J. Polymer Sci. 9: A-l: 931, 1971 7. N. GRASSIE and B. J. D. F()I~I~ANCE, J. Polymer Sei. 6, A-I: 3303, 3315, 1968