SOME FEATURES OF THE RADICAL COPOLYMERIZATION OF 3-METHYLHEPTATRIENE-1,4,6 WITH METHACRYLIC ACID * A. I. VOROB'EVA, M. A. GuaEvlc~r, G. V. LEPLWA~I~, S. 1%. RAFIKOV, 1%.G. TIMIROVAand V. N. ODI~OKOV C]mmistry Institute of the Bashkir Affiliated Branch of the U.S.S.R. Academy of Sciences
(Received 8 February 1973) A study was made of the eopolymerization Of 3-methylheptatriene-l,4,6 (MHPT) with methacrylic acid. I t is shown t h a t an increase in the M H P T concentration in the initial m i x t u r e exceeding a 1 : 2 molar ratio of M H P T to /VIAA does not lead to enrichment of the 3-methylheptatriene-l,4,6 copolymer; a eopolymer of constant composition ( M H P T : M A A = I :2) is formed. A copolymerization mechanism is proposed.
I~VESTIGATIONS of the copolymerization of some vinyl monomers with polyunsaturated compounds revealed the very peculiar behaviour exhibited by trans-3-methylheptatriene-l,4,6 (MHPT) in radical polymerization reactions. For instance, it proved impossible to carry out MHPT homopolymerization with thermal initiation (benzoyl peroxide, 0.1%, dinitrile of azoisobutyric acid (DAA) 0.1%, tertiary butyl peroxide, 0.1%, at 60, 80 and 120° respectively, nor could this be done with photoinitiation (benzyl--0.1%, 40 °, irradiation time 40 hr). It was similarly found impossible to eopolymerize MHPT with methyl methacrylate (MMA), styrene, aerylonitrfle (AN) and vinyl acetate. However, MHPT readily formed copolymers with acrylic (AA) and methacrylie (MAA) acids and copolymers of constant composition were formed with a wide range of ratios of MHPT to MAA in the initial mixtures. Moreover, MHPT-MAA-vinyl monomer (e.g. styrene, MMA, AN) triple copolymers were obtainable in presence of MAA. The kinetics and mechanism of the eopolymerization of MHPT with MAA, and some properties of the obtained copolymers are discussed in this paper. EXPERIMENTAL M H P T was synthesized by the m e t h o d described in [i], and purified on a column with a theoretical plate number of 21. A freshly collected fraction with b.p. 114-116 °, n~° 1.462, d~° 0-755 was used for the polymerization. The p u r i t y of the monomer was verified chromatographically. The Ii~ spectrum of the M]-I1)T had absorption bands at 965 (trans-C-~C); 910, 3090 (CH~=CHR); 1610, 1645 em -~ (G---~C--C~C). * Vysokomol. soyed. AI6: No. 8, 1826-1830, 1974. 2113
2114
A. I. ¥O~OB'EVA et al.
MAA was purified by double vacuum distillation, using for the polymerization a fraction with b.p. 44"5°/2 torr, n~° 1.4315; d,*° 1.015. Styrene, MMA and AN were purified by the usual methods, followed by drying and vacuum distillation. Fractions with the following characteristics were used for the polymerization: styrene--b.p. 42°/15 torr, n~)° 1.5465; MAA--b.p. 46°/100 torr, n~° 1.4130; AN--b.p. 79, n~° 1.3910. DAA was recrystallized repeatedly from methyl alcohol and vacuum dried to constant weight, m.p. 102 °. The copolymerization kinetics were investigated by dilatometry. The polymerization was carried out in thermostats in which the temperature was maintained to within -V0.05°. The copolymers were purified by triple reprecipitation from dioxan into petroleum ether, and were vacuum dried at 30-40 ° to constant weight. DISCUSSION OF RESUI~TS
I n all cases (unless s t a t e d to t h e c o n t r a r y ) t h e c o p o l y m e r i z a t i o n of M H P T w i t h M A A was i n v e s t i g a t e d in t h e p r e s e n c e of 0.05 wt. °/o of D A A a t 70 °. As m a y be seen f r o m Fig. la, it is o n l y u p to a 1 : 2 r a t i o of M H P T to M A A t h a t t h e c o p o l y m e r c o m p o s i t i o n is influenced b y t h e c o m p o s i t i o n of t h e initial m i x t u r e . T h e c o p o l y m e r c o m p o s i t i o n r e m a i n s u n a l t e r e d on f u r t h e r increasing t h e M H P T c o n t e n t in t h e initial m i x t u r e , a n d w i t h a n M H P T / M A A r a t i o of ~ 1 : 2 t h e r e is no c h a n g e in c o p o l y m e r c o m p o s i t i o n during p o l y m e r i z a t i o n in t h e t e m p e r a t u r e i n t e r v a l 60-90 ° . m
1"0
0.6
O'f
i
I
I
0.2
0.8
! 1"0M1
I 0.#
0.8 M~
FIG. 1. Copolymer composition (a) and rate of oopolymerization v of M H P T (M1) with MAA (Ms) (b) vs. composition of the initial mixture.
T h e I R s p e c t r a of t h e c o p o l y m e r s h a v e a b s o r p t i o n b a n d s c o r r e s p o n d i n g t o t h e c i s - C = C b o n d (675, 725, 1655 cm-1). As M H P T was i n t r o d u c e d in t h e trans f o r m into t h e c o p o l y m e r i z a t i o n , t h e a p p e a r a n c e of cis-C----C b o n d s in t h e c o p o l y m e r could be a t t r i b u t e d t o t h e f o r m a t i o n o f c y c l o h e x e n e rings as a result of a D i e l s - A l d e r reaction. This e x p l a n a t i o n is in a c c o r d w i t h t h e results of i n v e s t i g a -
l~adical oopolymerization of 3-methylheptatriene-1,4,6 with methaorylic acid
2115
tions [2, 3] relating to the interaction of diene hydrocarbons with maleic anhydride and AN in the presence of free radicals. The presence of fragments formed as a result of a Diels-Alder reaction in the M H P T - M A A copolymers was confirmed by comparison of the mass spectra of degradation products of the copolymers and of a specially synthesized adduct. The adduct was synthesized in an autoclave at 170 °, the MAA/MHPT ratio being 1 : 1-5, reaction time 6 to 7 hr, using hydroquinone (0.5% on total weight of the mixture) as the polymerization inhibitor. A mixture of 1-methyl-2-(!'-methylpropene-2')zt3-cyclohexenecarbonic acid and 1-methyl-3-(l'-methylpropene-2')z/4-cyclohexenecarbonic acid, having the following characteristics was obtained: b.p. 163°/8 torr, n~° 1-4950, d~° 1-018; MRfound~-56.0; MRca~e=56"0. Found, °/o: C 74.1; H 9"3. C12H1sO2. Ca]~culated, ~o:C 74-2; I-I 9.3. IR spectrum, cm-l: 675, 740, 1652--cis-C~C; .915, 3030, 3 0 8 0 - - C H 2 : C H R ; 1705--COOH. Lines corresponding to masses 79, 93 and 105 appear in the mass spectra of the Diels-Alder adduct in the products of thermal degradation of the copolymer with M H P T / M A A = I : 2; this means that the copolymer includes structures formed from M H P T and MAA by diene synthesis. Finally, absorption bands corresponding to a cyclic five membered anhydride (1785 and 1865 cm -1) were detected in the I R spectra of the dehydrated products of ozonolitical splitting of the copolymer of MHPT and MAA. An analysis of every one of the structures that could be formed through MHPT-MAA interaction shows t h a t five membered cyclic anhydride units cannot appear in the products of ozonolytie splitting unless the copolymer composition includes cyclohexenecarbonic chain fragments, i.e. fragments formed through the addition of two MAA molecules ("head-to-head") or fragments formed through MAA addition in a conjugated system of MHPT bonds in the 1,4-position. However, if the copolymer were formed by the latter mechanism, ozonolysis would lead to complete destruction of the molecule, whereas in the case of "headto-head" addition five membered anhydride rings would be present in the deh y d r a t e d initial copolymer also. Since ozonolysis is not accompanied by breakdown of the copolymer, and since the I R spectra of the dehydrated initial copol:/mer has bands relating solely to a cyclic six membered anhydride (1770, 1820 cm -1) the fact t h a t diene synthesis does take place during the copolymerization of M H P T with MAA is beyond doubt. In the light of these results the mechanism of MHPT-MAA copolymerization involving the formation of a copolymer of constant composition may be represented as follows. Adduct I formed through a Diels reaction /\--CH3
oo. ( \1
+\
,,/
-*
; \I I
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A.I.
VOROB'~.VA et al.
enters into copolymerization with MAA and forms alternating copdlymers (one would naturally conclude t h a t the copolymerization takes place through CH2----CH-- bonds)
x] .CH3
/COOH
(./ICOOH +
-I ()<::o. I
\[I
f\
However, attempts to copolymerize adduct I with MAA showed t h a t alternating copolymers are not formed. Thus, the copolymerization of a mixture with a 1 : 2 ratio of adduct I and MAA, for 6 hr, resulted in a product with an elementary composition corresponding to a 98 mole °/o content of MAA. Moreover, it is improbable t h a t the Diels-Alders reaction, under the conditions of the experiment, took place through the interaction of MAA with a polymer molecule formed as a result of alternating copolymerization of MHPT through the isolated double bond with MAA (even if this copolymer were actually formed). In all probability, therefore, the mechanism underlying the formation of the copolymer with a 1 : 2 ratio of M H P T to MAA must include simultaneous occurrence of both a Diels-Alders reaction and alternating eopolymerization. This process could occur only through the formation of an intermediate molecular compound such as a donor-acceptor complex [2] including in its composition one molecule of MHPT and two molecules of MAA. The interaction of this intermediate compound with a free radical leads to the excitation of all three double bonds of MHPT. Consequently a conjugated system of u-bonds of MHPT participates in the Diels-Alder reaction, and the isolated double bond enters into copolymerization with MAA
+ 2 \
/COOH
IL '
R"
I
\
11 COOH I
R~CH~--C--CH--CH~
I'
CH--CHa
ell8 I
'\/COOH
An excess of M H P T in the initial mixture over and above a 1 : 2 ratio of MHPT to MAA has almost no effect on copolymer composition, but in the case of an excess of MAA the copolymer is enriched with the acid.
Radical copolymerization of 3-methylheptatriene- 1,4,6 with methacrylic acid CHARACTERISTICS
Experiment, No. 1
2 3 4 6
OF
MI'~T
Composition of initial mixture, mole % MHPT MAA M* 24.9 44.7 76.2 35.1 33.5 25.1
23.8
20.9 23.7 23.9
[~], dl/g
.,=
25-4 26.9 23.7 11.8 30-6 52.8
55.3
40.0 42.8 51-0
COPOLYMERS
Copolymer composition, mole % MHPT MAA
75-1
2117
74-6 73-1 76.3 12.6 59-3 27.6
75.6 10.2 19-5
,.,
0.77 0.34 0.10 0.30t 0-42 0.28
• M,=MMA,styrene and ANin experiments4-6 respectively. The solvent was DMFA. Doubtless the p r o p o s e d scheme can in no w a y be r e g a r d e d as complete. Along w i t h this main process M H P T enters into the copolymerization process b y o t h e r mechanisms as well, w i t h t h e result t h a t the c o p o l y m e r composition diverges from an e x a c t 1 • 2 ratio of M H P T to MAA. The p a r t of these processes is inconsiderable, however. tO0
!
2
C
"~ 60
20 2
6
10
T/me, hr FIG. 2. Polymerization of MMA in the presence of 0-05 wt.% of 70° without the addition of MHPT (1) and with the addition of 1.07 wt.% of MHPT (2). I n the case of the f o r m a t i o n of triple copolymers, of course, there will be only partial realization of the proposed m e c h a n i s m of M H P T - M A A copolymerization. I t is seen from Fig. lb t h a t as the M H P T c o n t e n t in the initial m i x t u r e rises, the r a t e of f o r m a t i o n of t h e M H P T - M A c o p o l y m e r is reduced, a n d becomes practically i n d e t e r m i n a b l e in the case of an M H P T c o n t e n t of ~ 80 mole %. The inhibiting effect of M H P T is also confirmed b y t h e kinetic investigations of m e t h y l m e t h a c r y l a t e p o l y m e r i z a t i o n in presence of M H P T (Fig. 2). The energy of a c t i v a t i o n for t h e copolymerization process w i t h a 1 : 2 m o l a r ratio of M H P T to MAA in the initial m i x t u r e , d e t e r m i n e d from t h e slope of
A. I. VOROB'EVA d al.
2118
the curve of the Arrhenius equation in the temperature interval 60-90 ° amounts to 23-7 kcal/mole, which agrees with normal values for radical polymerization. The copolymers dissolve in dioxan, DMFA and DMSO, but after thorough drying at temperatures above 40 ° the copolymers suffer loss of solubility in organic solvents, a n d only swell. 4
~o
I
180
30O
I
~ygr,°c
FIG. 3. Derivatograms of the copolymers. The figures b y the curves are the numbers of experiments appearing in the Table.
M H P T - - M A A copolymers have fairly high molecular weights ([~] = 0.3-0.8 dl/g, with dioxan as solvent, 20°). However, an increase in the MHPT content in the initial mixture not only lowers the rate of copolymerization, but also leads to considerably lower molecular weights, as may be seen from the values given for the copolymers in the Table. The thermooxidative degradatioa was carried out with the aid of the PaulikPaulik-Erden derivatograph, (produced by MOM), in air, with a heating rate of 5 deg/min. Samples weighing 50 mg were used m all cases. As may be seen from Fig. 3, the onset of rapid decomlJosition is observed both for the MHPT-MAA double copolymers, and for the triple copolymers, at 300 °. Translated by R. J. A. I~END:RY REFERENCES
]. G. A. TOLSTIKOV, V. P. YUR'EV, I. M. SALIMGAREYEVA and S. R. RAFIKOV, U.S.S.R. Pat. No. 374263, 1973; Byull. izob., No. 15, 1973 2. N. G. GAYLORD, 1K. STOLKA, A. TAKAKASHI and S. MAITI, J. Macromolec. Sci. A5: 867, 1971 3. F. $UNJI, K. F,IICHI, H. KAZUO and J. YUTAKA, Polymer J. 2: 475, 1971