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Studies on the anionic oligomerization of methacrylonitrile by alkali metal alkoxides
European Polymer Journal, 1967, Vol. 3, pp. 253-304. F~,~/,kmon~
Ltd. Prints! in T~iland.
STUDIES ON THE ANIONIC OLIGOMERIZATION OF METHACRYLONITRILE BY ALKALI METAL ALKOXIDES GIORA T~CHMAN a n d A L n ~ T 7ALKHA Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel
(Received13 October1966) Abstract--The bulk anionic oligomerizition of methacrylonitrile (MAN) by RO-/ROH solutions was studied at constant temperature. The disappearance of monomer and of alcohol, and the formation of the different oligomers up to n ffi3 (trimers) were followed quantitatively by gas chromatography, and the change of D'P of the oligomers with conversion was determined. The effects of the type of alcohol, [MAN]/[ROHI ratio, initiator concentration, and the nature of the alkali metal counterion were investigated. ~ decreased in the order methanol>ethanol>isoprolmnol, and the rate of conversiunvicewrsa. Increase of the IMAN]/[ROH] ratio and ofinitiator concantration increased IYP. While the same bchaviour was observed with sodium and potassium alkoxides, with the corresponding lithium alkoxides practically no oligomeHzation occurred even under more drastic conditions. Under the investigated conditions, the formation of the addition product, R O - - M A N (nffi I) was reversible, contrary to the formation of the higher oligomers. T h e / ~ . of the revers/ble reaction, [MAN] [ROH]I[RO--MAN] was determined from reactions using excess ROH; for methanol it was 55 mole/L, and for ethanol 7.8 mole/l. Some of the rate constants of the various steps of the ofigomerizationreactions for methanol and ethanol were estimated taking into account the reversibifity of the initiation reaction, and assuming steady state conditions in the concentration of the various anions present in the system. The rate constant of the initiation reaction was greater than the rate constants of propagation; the rate constant of propagation of Mi" (formation of dimer anion) was greater than that of M~. The rate constants of termination of the various oligomer anions by ROH were only 2-4 times greater than the corresponding propagation rate constants. The steady state concentrations of Mi', M~ and M~ were intimated, and in some cases were relativtly high so that their sum was as much as 90 per cent of the initial concentration of the alkoxide initiator. The initiation rate comtant with C2HsO-/C2HsOH was about 4-5 times greater than that with CH30-/CH3OH, and the corrmponding termination rate constants were smaller by a factor of 4--5, which is in accordance with the acidity of the alcohols. INTRODUCTION WE HAVE p r e v i o u s l y r e p o r t e d ¢t,2) the o l i g o m e r i z a t i o n o f m e t h a c r y l o n i t r i l e ( M A N ) b y a l k a l i m e t a l a l k o x i d e s in the presence o f the p a r e n t alcohol. O l i g o m e r s h a v i n g D'P u p t o five were i s o l a t e d a n d characterized. T h e o l i g o m e r i z a t i o n was s t u d i e d b o t h in s o l u t i o n in a p r o t i c solvents, such as d i m e t h y l f o r m a m i d e a n d d i m e t h y l s u l f o x i d e , a n d in b u l k ¢2) ( m o n o m e r + a l c o h o l ) u n d e r reflux conditions. B o t h s o d i u m a n d p o t a s s i u m a l k o x i d e s were u s e d as initiators, a n d n o difference was f o u n d between t h e m . A n i m p o r t a n t aspect o f the w o r k was t h a t the degree o f o l i g o m e r i z a t i o n was g o v e r n e d b y the r a t i o [ M A N ] / [ R O H ] so t h a t ] 3 ~ P - - K [ M A N ] / [ R O H ] , K b e i n g a c o n s t a n t d e p e n d e n t o n the t y p e o f a l c o h o l a n d o n the solvent o f the r e a c t i o n (in bulk/~methtnot) ffi 1"4, Kceth~ob---- 1"85). T h i s e q u a t i o n was derived on the basis o f the simple r e l a t i o n s h i p existing in p o l y m e r systems t h a t ~ = Rp/Rt= (Kp/Kt)([MAN]/[ROH]) ( t e r m i n a t i o n b y transfer t o a l c o h o l ) . 283
284
GIORA TEICHMAN and ALBERT ZILKHA
W e have n o w studied i n m o r e detail the b u l k oligomerization of M A N by R O - / R O H at c o n s t a n t temperature. D u e to the very high boiling p o i n t s o f the higher oligomers, the systems studied were those yielding mostly the lower oligomers u p to trimers. Use was made o f q u a n t i t a t i v e gas c h r o m a t o g r a p h y techniques to follow the progress o f the reaction i n c l u d i n g the rates o f disappearance of the reactants, m o n o m e r a n d alcohol, a n d o f f o r m a t i o n o f the various oligomers; a picture was thus o b t a i n e d o f what was going o n all the time in the oligomerization mixture. F r o m these rates, a n estimate was m a d e of the rate constants of some o f the various steps o f the oligomerizations. It seems that such a detailed study has n o t been performed previously for similar a n i o n i c oligomerization systems. The technique was used to study various factors affecting the rate o f the oligomerization a n d the D--P o f the oligomers, such as the type o f alcohol, the ratio o f reactants, [ M A N ] / [ R O H ] , a n d the type o f the alkali metal alkoxide. The effect of catalyst c o n c e n t r a t i o n was also studied here for the first time, a n d it was f o u n d to affect considerably the rate o f conversion a n d the D-P. EXPERIMENTAL
Materials Methacrylonitrile (Fluka) was washed with 1 per cent sodium hydroxide, followed by water and dried over calcium chloride. It was distilled under argon and the fraction boiling at 86-87°/690 mm was collected and kept under argon in a cool dark place. Oxygen-free argon (Matheson) was dried by passage through concentrated sulphuric acid followed by sodium hydroxide pellets. Absolute methanol (BDH) containing 0-01 per cent water was used without further purification. Absolute ethanol was dried over magnesium and distilled. Isopropanol was dried over calcium sulphate, refluxed over calcium metal, and fractionally distilled. The alkali metal alkoxide solutions were prepared by dissolving the alkali metal in the required alcohol under reflux with exclusion of moisture. The solutions were standardized by titration with 0.I N hydrochloric acid using phenol phthalein as indicator. All the reagents were kept under argon and were transferred by syringes applying positive argon pressure.
Oligomerization procedure The olignmerizationwas carried out in a reaction vessel consistingof a three-necked flask, fitted with a reflux condenser and a calcium chloride guard tube, an adapter for introducingargon, and an opening fitted with a self-sealing rubber cap through which the reagents were added with syringes, or samples removed during the reaction. Magnetic stirring was employed. The reaction vessel was sealed in an external vessel having an opening fitted with a reflux condenser. The temperature inside the reaction vessel was kept constant by the vapours of boiling benzene in the external vessel. Alcohol and methacrylonitrile were added and heated to the required temperature, followed by the alkali metal alk0xide. Inallexperimentsthetotalvolumeofthereactionmixturewas70ml.Aliquots were removed from the homogeneous reaction mixture at various intervals, and were analysed by gas chromatography. Control experiments showed that it was possible to inject these solutions directly into the gas chromatograph without prior neutralization of the alkoxide. Experiments in which the oligomer mixture was neutralized, extracted by chloroform and injected, gave the same results within experimental error.
Gas chromatography analyses The concentrations of the monomer, alcohol, fl-alkoxy isobutyronitrile, dimer and trimer present in the oligomerization mixture were determined by gas chromatography using an Aerograph Autoprep Model A-700 instrument. Helium was used as the carrier gas and 2 m of standard 0.25 in. tubing was used to contain the packing of 20 per cent "DC-11" on Chromosorb W. The injector temperature was about 330°, and the flow rate was 40-60 ml/min in the determination of the higher oligomers, and 10-25 ml/min in that of methanol and monomer. The column temperature was held approximately as follows in the determinations of the various fractions: 40° (monomer and alcohol), 90° (n = 1), 140° (n=2) and 220° (n=3). The oligomers were determined quantitatively by comparison of the peak areas on the chromatograms, obtained by injection of known volumes of the reaction mixture, with those of known volumes of the pure materials, injected at about the same time and under the same conditions. Generally conditions were chosen such that sharp peaks were obtained, and then the height of the peaks was compared. In other cases, the peak areas were measured by a plani-
Studies in the Anionic Oligomerization of Methacrylonitrile by Alkali Metal Alkoxides
285
meter. Every analysis was repeated. It was found that samples taken from the reaction mixture could be stored at 0 ° for 36 hr before analysis, without any detectable change in the composition of the mixture.
Calculation of ~YPn. The average degree of oligomerization of mixtures was calculated using the equation:
No. of moles of monomer in the oligomers No. of oligomer molecules Since, No. of moles oligomer =
Weight of oligomer Molecular weight of oligomer'
and
No. of moles of monomer in trimer = 3(No. of moles of trimer), etc., it is possible by knowing the weight of the different oligomers present in the mixture (from n-- 1 to n = 3) to calculate the average ]3"15of the oligomers present in the reaction mixture at a certain reaction time. RESULTS T h e b u l k o l i g o m e r i z a t i o n o f M A N was p r e v i o u s l y c a r r i e d o u t u n d e r reflux c o n d i tions. (2) W e h a v e s t u d i e d such a reaction, using gas c h r o m a t o g r a p h y techniques, t o follow t h e p r o g r e s s o f t h e r e a c t i o n to find o u t the r e p r o d u c i b i l i t y o f t h e results ( T a b l e I). A d u p l i c a t e e x p e r i m e n t gave, after 90 rain, d e v i a t i o n s o f a b o u t 5 p e r cent in t h e v a r i o u s c o n c e n t r a t i o n s m e a s u r e d o f the c o m p o n e n t s in the o l i g o m e r mixture, which increased u p t o 2 0 - 2 5 p e r cent after 240 r a i n ; r e a c t i o n s c a r r i e d o u t u n d e r reflux c o n d i t i o n s a r e therefore poorly reproducible. TABLE I. OLIGOMERIZATIONUNDERREFLUXCONDmONS* Time
[MAN]
[C2HsOH]
In = 1 ]
In = 2]
In = 3]
(rain)
(mole/L)
(mole/L)
(mole/L)
(mole/L)
(mole/L)
0 10 20 30 50 70 90 130 180 240
7.26 5.22 4.83 4"56 4.40 4"32 4.31 4.21 4.12 3,00
6.24 4.04 3.81 3.57 3.47 3-42 3"40 3"40 3.37 3.32
0 2-04 2.43 2.62 2.70 2"70 2.66 2"63 2.60 2"55
0 --0"041 0"078 O"118 0"145 0-200 0"271 0-355
0" -n ---0.028
* Experimental conditions: the concentration of sodium ethoxide was 0-09 mole/l. The reaction was carried out in a flask fitted with a reflux condenser, and an electric mantle was used for heating. O n the o t h e r h a n d , o l i g o m e r i z a t i o n s c a r r i e d o u t a t c o n s t a n t t e m p e r a t u r e (78 °) as d e s c r i b e d in the E x p e r i m e n t a l section were m u c h m o r e r e p r o d u c i b l e , a n d the m a x i m u m d e v i a t i o n s o b s e r v e d in t h e m e a s u r e d c o n c e n t r a t i o n s were o f the o r d e r o f 5 p e r cent.
Effect of type of alcohol I n t h e p r e v i o u s w o r k o n the b u l k o l i g o m e r i z a t i o n , (2) r e a c t i o n c o n d i t i o n s were used so t h a t h i g h c o n v e r s i o n s (80-90 p e r cent) a n d relatively high degrees o f o l i g o m e r i z a t i o n ( ] ~ ) were o b t a i n e d . I n t h e p r e s e n t study, the r e a c t i o n c o n d i t i o n s were such t h a t the DlSs o b t a i n e d were n o t high, a n d u n d e r such c o n d i t i o n s c o n v e r s i o n s o f a b o u t 4 0 - 5 0 p e r cent were o b t a i n e d after 2 - 3 hr.
286
GIORA TEICHMAN and ALBERT ZILKHA
The effect of the type of alcohol was investigated using methanol, ethanol and isopropanol which differ appreciably in the pKa values; thus methanol is the strongest acid, while the methoxide initiator is the weakest derived base. Figure 1 shows that at a ratio [MAN]/[ROH], (R)= 1.5, the rate of conversion was highest with/so-propanol, as expected, although the catalyst concentration used was lower. 3
4(:
~
o,'~°
3G .,
"i 20 c~ ~o
0
40
Time,
rain
FIG. 1. Dependence of conversion on type of alcohol ([MAN]/[ROH]-- 1"5) [MAN]0 [RONa]0 (mole/L) (mole/L) I. Methanol; 9.04 0.120 2. Ethanol; 8.17 0.100 3. Iso-propanol; 7.43 0-095
The number-average degree of oligomerization, D-P., was dependent on the type of alcohol. Under comparable conditions, D-'Pnwas highest in the presence of methanol, and lowest in the presence of iso-propanol, both at (R)-- 1-5 (Fig. 2) and at (R) = 2 "0 (Fig. 3). At (R)= 1.0, the D-P, observed was approximately the same with the three alcohols, and after 2 hr it was not more than 1.1.
1.5-1.4--
/
,3-
/:/
/
+1.2- / . / . I II
I/
,.+-"
2f* 3 e ~ e a,,~..~0 ~
,io
+
Time,
rain
o
,+
FIG. 2. Dependence of ]~Pn on type of a]cohol ([MAN]/[ROHI-- 1.5).
1. Methanol; 2. Ethanol; 3. !so-propanol;
[MANI0
[RONal0
(mole/L) 9-04 8.]7 7.43
(mole/L)
0-100 0.100 0.095
Studies in the Anionic Oligomerization of Methacrylonitrile by Alkali Metal Alkoxides
287
1,5
,.,k// ....---" Time,
rain
FIG. 3. Dependence of ~iin on type of alcohol ([MAN]/[ROH]=2-0) [MAN]o [RONa]0 (mole/L) (mole/L) I. Methanol; 9.68 0.100 2. Ethanol; 8.84 0"100 3. Iso-propanol; 8.18 0.095
Effect of the ratio, [MAN]/[ROH], (R) Increasing (R) increased b o t h the rate o f conversion a n d the D-'P~ as f o u n d before Ct, 21 (Fig. 4). T h e increase in rate shows that the m o n o m e r is m o r e effective in increasing
8(
,oh
i/ ] '/
/
/
"t"
~ ~ . ~~' ' - ~ -~--~'---
o
0
40
80I Time,
,~o
,70
min
FIG. 4. D e p e n d e n c e o f per ce~ntag¢ c o n v e r s i o n o n initiator c o n c e n t r a t i o n .
1. 2. 3. 4.
[MAN]o (mole/L) 8.84 8-84 8"17 8"17
[ethanol]o (mole/L) 4.40 4'40 5"40 5-40
[Na-ethoxide]o (mole/L) 0.216 0.087 0"087 0"017
288
G I O R A T E I C H M A N and A L B E R T z I L K H A
'I 1.7--
1.6
1.5
c
I~
,4
1.2
15.
/2
i.i
I.c ~'/~'~1"-~' 40
"I'
80 Time,
I
17'0 rain
I
160
200
FIG. 5. Dependence of~-Pn on [MAN]/[lacohol] ratio (methanol) [Methanol]o [MAN]0 (mole/L) (mole/L) I. 4.82 9-60 2. 5.98 9.04 3. 8.05 8.06 [K-methoxide]0 = 0.120 mole/l, in all cases
1.5--
1.4~
/J
I /.1"
I.)l-
"~0
I
I
o'-° 2
40
3
80 Time,
I20 min
160
FIG. 6. Dependence of ]~Ein on [MAN]/[~Icohol] ratio (ethanol). [MAN]0 [ethanol]o (mole/L) (mole/L) 1. 8.84 4.40 2. 8.17 5.40 3. 7-16 6.86 [Na-ethoxide]0 = 0-087 mole/i, in all cases
Studies in the Anionic Oligomerization of Methacrylonitrile by Alkali Metal Alkoxides
289
the rate of the oligomerization than the alcohol. At (R)= 1.0, under the conditions investigated the main product obtained was ~-alkoxy iso-butyronitrile (n--1, where n is the degree of polymerization). Figures 5, 6, and 7 show the variation of D-P, with time in the case of methanol, ethanol, and/so-propanol respectively. In all cases, DP, increased continuously with the time of reaction. The effect of increase of (R) on 13-P, was greater in the case of methanol than in the case of/so-propanol. I.I~.
I
f
/"
2/"
~a
Time, rain FIO. 7. Dependence of ]51s. on [MAN]/[lacohol] ratio (isopropanol). [MAN]0 [iso-propanol]0 1. 8.18 4.10 2. 7.43 4.93 3. 6.24 6.23 [Na-iso-propoxide]o=O.095 mole/1, in all cases.
Effect of initiator concentration On increasing the alkoxide concentration the rate of conversion increased considerably (Fig. 4), as well as the ~Pn; the effect being similar to that of increasing (R). This effect was general and was found in all the systems investigated (Figs. 8-11). This important effect on ]~Pn was not noticed before. <1'2)
Effect of the alkali metal counterion~ In the previously studied oligomerizations, no difference in behaviour was noticed for sodium, potassium, and quaternary ammonium methoxides c2) in solution C1) or in bulk. <2) In the present work the same behaviour was found. Oligomerizations, performed using sodium or potassium methoxides in the range (R)-- 0.5-2 and initiator concentrations up to 0.2 mole/L, gave similar results for the rate of conversion and the formation of the different oligomers. 19
290
GIORA TEICHMAN and ALBERT ZILKI-IA
I~ 1.2-
LO0
o
o
• .~"-T'" 40
~
80
120
"
160
Time, rain
Fin. 8. Effect of initiator concentration on ]5~.([MAN]/[ethanol]-- 1.5). [Na-ethoxid¢]0, mole/l.: (1) 0.087; (2) 0-017
1.2
0
40
~ Time,
I
I
rain
FIG. 9. Effect of initiator concentration on ]3"~.([MAN]/[ethanol]=2.0). [Na-ethoxide]0, mol©/l.: (l) 0.216; (2) 0.100; (3) 0.071 The behaviour of lithium methoxide was, however, completely different. Thus a reaction carried out at ( R ) = 2 , ( M A N ) = 9 . 6 0 mole/1, and LiOCH3=0-175 mole/l. gave 2"5 per cent conversion to (n = l) after 4-5 hr, and no higher oligomers were formed. Increasing the initiator concentration to 0.206 mole/l, and (R) to 4, gave only 3 per cent conversion to (n = 1). Similar results were obtained with lithium ethoxide.
Studies in the Anionic Oligomerization of Methacrylonitril¢ by Alkali Metal Alkoxides
291
2.0 I.E
1.4 1.2 4to
t Time,
min
Flo. 10. Effect of initiator concentration on ZYPn([MAN]/[methanol]--2-0). [Na-methoxide]o,mole/l.: (1) 0.18; (2) 0-12
//
+
1.2 /
Time,
mln
FI~. 11. Effect of initiator concentration on ~Pn([MAN]l[methanol]-- 1-5 (1) [K-methoxide]0ffi0-18;(2) [Na-methoxide]0--0-10mole/l. T o find a way of increasing the yield with lithium, the alkoxide was f o r m e d / n situ. Lithium metal was added to a mixture of monomer and alcohol. Different experiments were carried out using ethanol,/so-propanol and tert-butanol at (R) = 2. Conversions of up to 10 per cent only were obtained, and the products were high oligomers (n >~3).
Relation between com,ersion and ~ , In most of the oligomerizations, ]3~P, increased slowly at first with conversion up to 20-30 per cent, and then the increase was faster (Figs. 12-14).
292
G I O R A T E I C H M A N and ALBERT Z I L K H A
I
I.I
o
I.C )
20 Conversion,
1.2
40 %
1.0
of
I.I
20 Conversion,
40 %
1.0
4[0 Conversion,
o/.
FIG. 12 (left). Dependence o f ] ~ n on percentage conversion. [Iso-propanol]o = 4.93; [MAN]o= 7-43; [Na-iso-propoxide]0 = 0.095 mole/l. Fie. 13 (centre). Dependence of 1S]5. on percentage conversion. [Ethanol]0=4.40; [MAN]o=8.84; [Na--ethoxide]o=0.10 mole/l. FIG. 14 (right). Dependence of ISI~. on percentage conversion. [Ethanol]0=5.40; [MAN]0=8.17; [Na-ethoxide]0=0.087 mole/l. TABLE 2. OUGOM~ZATION OF M A N BY C H 3 0 - / C H 3 O H Run* No. IA3
IA2
IC1
IA4
Time (rain) 0 30 60 90 120 150 180 0 2 15 45 75 105 135 165 0 15 40 75 0 2 8"5 20 37 58 81 105 134 174
[MAN] (mole/L)
[CH3OH] (mole/L)
[ n = 1] (mole/L)
[n--2] (mole/L)
[ n = 3] (mole/l.)
9"06 7-30 6"65 5"85 5"50 5"30 5"10 9"60 9"30 7"95 6"20 5.00 4-12 3"50 3"03 9-60 8"65 7"10 5"80 9"04 8"77 8"25 8.23 8"22 8.12 8"07 7.90 7.63 6.56
5.93 4.37 4"03 3"64 3"63 3"63 3"62 4"82 4"52 3"46 2"58 2" 12 1"78 1"59 1"46 4"82 4"01 3"08 2"36 5"98 5"71 5"21 5.22 5-25 5.25 5-25 5-20 5.10 4.59
0 1-35 1"41 1"49 1"27 1.06 0"91 0 0"30 1"08 1"16 1"03 0"99 0"93 0"88 0 0"66 1"00 1"17 0 0"26 0"74 0.70 0-61 0.56 0.46 0"41 0.36 0.30
0 0"21 0"46 0"69 0"83 1"00 1"14 0 0.0008 0"272 1"020 1'440 1-650 1.730 1"730 0 0"15 0"72 1"23 0 0"006 0"024 0.055 0-105 0.175 0.255 0-366 0"527 1.090
0 0 0"03 0" 11 0"21 0"24 0"26 0 0 0-007 0"065 0.234 0"397 0"572 0-751 0 0 0"017 0.057
* Experimental conditions: IA3 : [MAN]][CH3OH] = 1.5; IA2: [MAN]/[CH3OH]=2"0; ICI : [MAN]/[CH3OH]---2"0; IA4: [MAN]/[CH3OH]-- 1.5;
CH~ONa = 0.180 mole/l. CH3ONa--0"I80 mole/l. CH3OK=0-120 mole/l. CH3OK--0-120 mole/l.
Studies in the Anionic Oligomefization of Methacrylonitrile by Alkali Metal Alkoxides
293
TAm.E 3. Ouoo~a~IzT,'no~ oF M A N nY C2HsO-IC2HsOH Run* No. A
D
E
C
Time (rain)
[MAN] (mole/L)
[C2HsOH] (mole/L)
[n = 1] (mole/L)
[n = 2] (mole/L)
[n = 3] (mole/L)
0 2 10 20 28 38 56 73 81 107 149 0 5 20 35 55 71 90 106 130
8.17 8.00 7.17 6.70 6.36 6.16 5.85 5-62 5.57 5"46 5"26 8.84 7-64 6.54 6.20 5.84 5"58 5.27 4-95 4.66 8-84 6.50 5.60 5.00 4.71 4"27 4-03 8.17 8-12 8-05 7"91 7-82 7"67 7"43 7-33 7-26 7"24 7"07 6-95 6-74 6.74 6"72
5-40 5.23 4.43 4.00 3-66 3-53 3-29 3.13 3"12 3.08 3"05 4.40 3-22 2"27 2"12 1"96 1.87 1"76 1"65 1"54 4-40 2"29 1.74 1"47 1"43 1.29 1.25 5.40 5.35 5'29 5,14 5.05 4,90 4.66 4.56 4-49 4.47 4.30 4-18 3.97 3.97 3.95
0 0-17 0.95 1.37 1.67 1.77 1.93 2.04 2.02 1"98 1.96 0 1"17 2.00 2-00 2.00 1-97 1"95 1.92 1.90
0 0 0.013 0.023 0.046 0-072 0.135 0.183 0-212 0.260 0.380 0 0.01 0"10 0.20 0.31 0"38 0.46 0.52 0-60
0 0 0-008 0.012 0-015 0.032 0.042 0-046 0.050 0.071 0-097 0 0 0.03 0"08 0.13 0"18 0"23 0"31 t" 0"36~
0
30 60 90 120 150 180 0 1.5 9 19 27.5 37.5 55.5 72-5 80 93.5 106.5 148.5 244 267 287
0
0
0
1.91 2.17 2-13 2.04 1"97 1.90 0 0.05 0.11 0-25 0"35 0.50 0.73 0.84 0-91 0.93 1.10 1"21 1.41 1.41 1"41
0.17 0"40 0.55 0"65 0.80 0.84 0 0.00015 0.0011 0.0023 0.0033 0.0044 0.0061 0.0075 0.0081 0.0089 0.0097 0.0112 0.0129 0.0131 0"0133
0.031 0.090 0-170 0.235 0-328 0-410 0 0 0.0001 0.0005 0.0008 0.0012 0"0018 0-0023 0"0026 0"0029 0"0032 0.0041 0.0053 0"0055 0.0057
* Experimental conditions: A: [MAN]/[C,2HsOH]--- 1"5; C2HsONa-- 0.087 mole/l. D : [MAN]/[C2H~OH]=2.0; C2HsONa--0.100 mole/l. E: [MAN]I[C2HsOH]=2"0; C2HsONa=0"087 mole/l. C: [MAN]/[C2HsOH]= 1.5; C2HsONa=0"0172 mole/l. t [n~4]=0-047 mole/L; no tetramer was found at shorter reaction time. :l: [n~4]~0-117 mole/1.
Kinetics o f the oligomerization system Experiments were performed to discover, on the one hand, the rates of disappearance of monomer and alcohol, and, on the other, the rate of formation of the different
294
GIORA TEICHMAN and ALBERT ZILKI-IA
oligomers. The results using methanol and ethanol are given in Tables 2 and 3. An important observation was that in most of the reactions the amount of (n = 1) increased to a m a x i m u m during the oligomerization and then decreased. The lowering in concentration of (n = 1) during the reaction seemed to be due to the reversibility of the reaction: (I)
R O - + CH2 = C - - C N ~-ROCH2--C----CN
I
I
CH3
CH3
(I) + ROH ~-RO- + ROCH2CH---CN
(II)
I
CH3 To confirm this point, experiments were carried out in which ethanolic sodium ethoxide was added to (II) ( R = C 2 H s ) under oligomerization conditions (Table 4). I t is seen that there is a rapid elimination of ethanol and formation of M A N . Even after 1 rain, there was about 15 per cent elimination. An equilibrium was established after about 2 hr and remained stable up to about 4 hr. When the reaction was continued up to 11.5 hr, a small amount of dimer (0.5 mole/L) was formed. It must be noted that the ratio of M A N to ethanol was less than unity, which is not in favour of the formation of oligomers higher than n = 1. A duplicate experiment gave similar results. TABLE 4. RI~VERSIBILITYOF THE CYANOETHYLATIONREACTION*
Time (rain)
[n ffi1] (mole/L)
[MAN] (mole/L)
0 1 11 20 35 55 82 100 128
7-09 5.99 5"19 4.54 4.19 3.82 3"49 3.27 3.03
0 1"I0 1.90 2-45 2.90 3-27 3.60 3.72 4.04
[C2HsOH] (mole/L) 1"46 2"56 3"36 3"91 4.36 4-73 5.06 5-18 5"50
* Sodium ethoxide (1-1 ml of 1.32 N solution) was added to fl-ethoxy iso-butyronitrile (7-09 mole/L) under the usual oligomerization conditions. To investigate further the equilibrium reaction, we carried out experiments in which (R) ~0.1, i.e. using a large excess o f alcohol. Various concentrations of initiator (0.05-0.15 mole/L) were used. In all experiments, the system reached equilibrium within 15 rain and no changes were observed even after several hours. The equilibrium concentrations were the same in all cases, and independent of the initiator concentration. The equilibrium concentrations with ethanol were: [ M A N ] = 0 - 5 2 mole/L, [C2HsOH]= 14-7 mole/L, and [ n = 1]=0.98 mole/1. With methanol using methoxide concentrations between 0.12-0.18 mole/L, the average values o f the equilibrium concentrations were [MAN] = 5.07 mole/L, [CH3OH] = 11.07 mole/l., and In = 1] = 1.02 mole/1.
Studies in the Anionic Oligomerization of Metba~_erylonitrileby Alkali Metal Alkoxides
295
To find out whether the formation of oligomers higher than (n = 1) was also reversible under the general conditions of the ofigomerization, experiments similar t o those carried out with (n= 1) were conducted. The dimer (RO-C2HsO) was heated with sodium ethoxide in ethanol, and even after 8 hr no MAN, no (n= 1) or a higher oligomer was found in the reaction mixture, indicating that in the ollgomerization system studied the reversibility of the formation of (n = 1) alone was important but not of the higher oligomers. DISCUSSION It was shown that the only significantly reversible step of the oligomerization was that of the decomposition of (n-- 1) to MAN and ROH. Therefore the oligomerization reaction may be represented by the scheme: R O - + CH2-, C(CH3)CN~.
kl "~RO--CH z---C(CH3)C"N (Mc) kl"
(i)
k2 MI-+ROH~.
>MI+RO-
(ii)
>M2-
(iii)
k2"
MI- + M A N " M2-+ROH M2- + MAN'
k3
k4 > M z + R O ks
(iv)
~Ms-
(v)
k6 Ms- + R O H - - : - + M s + RO-, etc.
(vi)
M~ = RO--I-CHr--C(C~,-13XC-~]o_i CH~-~(CI-13)CN Mn I RO--[-CHr--C(CH3XCN)]~--H
The oligomerization system as shown consists of a series of consecutive and competitive reactions, and their exact kinetic treatment is very complex. However we tried by using approximations to estimate some of the rate constants of the reactions.
Equilibrium constant of the addition reaction (steps i and ii) When reactions (i) and (ii) reach equilibrium there exist the following relationships:
kl/k1' = [MI-]/[MAN][RO-]
(I)
k2'/k2 = [MI-] [ROH]/[MI] [RO-]
(2)
Dividing (2) by (I) gives K~. = k1'k2'/klk2 = [ M A N ] [ROH]
[MI]
(3)
Substitution of the values of [MAN], [ROH] and [MI] in Eqn. 3 with the equilibrium concentrations determined before, gives the following equilibrium constants: K~. (methanol) = 55 _ 5 mole/l. K,q. (ethanol)= 7.8 mole/l.
296
GIORA TEICHMAN and ALBERT ZILKHA
Rate constants of initiation, propagations and terminations
In order to estimate these rate constants, two plausible assumptions were made. (a) The oligomerizations were carried out in bulk, and it was assumed that the changes in the composition of the reaction medium did not affect appreciably the rate constants. (b) A stationary state was assumed for the concentrations of the different anions present. This assumption is only true for the middle of the reaction (between about 15-150 mill). According to the kinetic scheme, d[M2] d----~ = k4[M2--] [ROH] and
d[M3] dt = k6[Ma-] [ROH]
In the assumption that [Mz-] and [M3-] are constant, it follows that for a certain reaction time, t, [M2] = k4[M2--] fo [ROH]dt
(4)
[M31 = k6[M3-] fo [ROH]dt.
(5)
The integrals, ~ [ROH]dt, were evaluated graphically for any time, t, at which measurements of the different species present in the reaction mixture were made. Using Eqns. 4 and 5, the values of k4[M2-] and k6[M3- ] were thus calculated for the different reaction times (in the range of about 15-150 rain) and were found to be constant for every run within 10-20 per cent error. Since the oligomerizations were carded out under conditions such that no significant amount of tetramer was formed, its concentration may be neglected, and therefore the stationary state concentration of [M3-] is given by ks[M2-] [MAN] = kdM3-] [ROH]
(6)
The stationary state concentration of [M2-] is given by k3[M1- ] [MAN] = k4[M2-] [ROH]+ks[M2-] [MAN] and
k3[M1-] = [ROH]/[MAN].k4[M2-]+ks[M2-]
(7)
The rate of disappearance of the monomer, based on the oligomerization steps, is given by -d[MAN] -- (kl [RO-] +k3[M1- l +ks[M2-]) [MAN]t-kI'[MI-] dt The solution of this differential equation is [MAN], -- [MAN]*+{[MAN]0- [MAN]*}e-pt
(8)
p = kl[RO-]+k3[Mt-]+ks[M2-].
(9)
where
It is based on the assumption that the different anions are present in stationary concentrations. Now [MAN]* should have been the value of [MAN] at infinite reaction
Studies in the Anionic Oligomerization o f Methacrylonitrile by Alkali Metal Alkoxides
297
time, but according to the oligomerization scheme, this value should be zero. Since at infinite reaction time the assumption of stationary state conditions is not legitimate, the value of [MAN]* cannot be the real value of [MAN]® but that obtained by extrapolation to t ® of the stationary state prevailing at the middle of the reaction (for which the equation was derived). Accordinglyp for each run was determined graphically from the slope of the graph ofln ([MAN]t- [MAN] *) vs, t. The value of [MAN]* was chosen to fit the equation so that the linear slope has an intercept at t = 0 equal to In {[MAN] 0 - [MAN]*}. From Eqns 1-9, kl[RO-], k3[M1-], ks[M2-], k4[MZ] and k6[M3-] were evaluated. The values were constant within 15-20 per cent. TABI.~ 5. PS~-DO-RAT~ CONSTANTSFOR THB SYSTEMM E r H A C R Y L O N I T R I I . ~ O L * Run No.
kl[RO-]xl0 s
ka[M1-]xl(P
ks[M2-]xl0 3
k4[M2-]×10 3
k6[Ms-]xl03
IA3 IA2 IC1
14"2 I 1.2 12"5
1"27 2.87 1.84
0"24 0.41 0-65
1-60 5-72 3.50
0"36 0.94 0.17
IA4
5.6
0"36
0
0-56
0
* The run numbers are as in Table 2. The rate constants are in 1. mole -1 min -1.
TABLE 6. STATIONARY C O N ~ T I O N
OF THE ANIONS IN THE SYSTEM METHACRYLONITRILE-METHANOL*
Run No.
[RO-]o x I0 a
[RO-] x I0 a
[ M c ] x 10 3
[M2- ] x 10 3
[Ma-] x 10 3
IA3 IA2 IC1 IA4
180 180 120 120
142 112 85 113
26 40 22 5
10 23 12 2
2 5 1 0
* The run numbers are as in Table 2. The concentrations are in mole/1.
TAeI~ 7. RATZ CONS'rANTSVOlt THE svsrmd METHACRYLONITRILE--METHANOL* Run
No.
kl
k3
k5
k4
k6
IA3 IA2 IC1 IA4
0.100 0.100 0"147 0"050
0-049 0-072 0-084 0"072
0"024 0.018 0-054 --
0.160 0.249 0"292 0"280
0"180 0.190 0" 170 --
Average Standard
0.120
(~060
0.036
0.250
0.180
deviation
(0-031)
(0-016)
(0.016)
(0-042)
(0.008)
* The Run Nos. are as in Table 2. The rate constants are in 1. mole -1 rain -1.
G I O R A T E I C H M A N and ALBERT Z I L K H A
298
TAm.E 8. PSEUDO-RATECONSTANTSFOR THE SYSTEMMETHACRYLONITRILE--ETHANOL* Run NO.
kl[RO-]xl03
k3[M1-]x103
ks[M2-]xl03
k4[M2-]x103
k6[M3-]x103
A D
14"5 6"5
0.493 1.220
0.103 0.450
0.684 2-300
0"181 1-300
E C
11-8 9"1
0.387 0-017
0.102 0.004
0-930 0.021
0.330 0.006
* The run numbers are as in Table 3. The rate constants are in 1. mole -1 min-L TABLE 9. STATIONARY CONCENTRATION OF THE ANIONS IN THE SYSTEM METHACRYLONITRII.EETHANOL* Run
No.
[gO-J0 × 103
A D E C
87 100 87 17"2
[RO-] x 103 30 12 25 15"7
[Ml-] x 103 28 38 24 0"8
[ M 2 - I x 103
[Ms-] x 103
19 31 22 0"5
10 19 16 0.2
* The run numbers are as in Table 3. The concentrations are in mole/litre.
TABLE 10. R A n CONSTANTS FOR THE SYSTEM MErHACRYLON1TRILE-LrrHANOL * Run
No.
kl
ks
ks
k4
ke
A D E C
0-485 0.530 0.472 0-576
0.0176 0.0320 0.0161 0.0225
0-0054 0.0145 0-0064 0-0076
0-036 0-074 0.042 0.032
0.018 0.068 0.021 0.025
0.520
0.023
0.0093
0-049
0.039
(0.031)
(0-006)
(0.0036)
(0-017)
(0.021)
Average Standard deviation
* The run numbers are as in Table 3. The rate constants are in L mole -x min-l.
To estimate the rate constants alone, we assumed the f o l l o w i n g plausible assumption, [M~-] I> [M2-] i> [M3-']
(I0)
[RO=]0 = [RO-] + [MI-] + [M2-] + M3-].
(II)
Stoichiometrically,
From these last two relationshipsusing least square analyses, we sought numerical values for the stationarystateconcentrationsof the differentanions which would give approximately constant values for the rate constants of the differentreactions. The resultsof these calculationsare given in Tables 5-7 for C H 3 0 - / C H 3 O H and in Tables 8-I0 for CzHsO-/CzHsOH oligomerization.
Studies in the Anionic Oligomerization of MethacrylonRrileby Alkali Metal Alkoxides
299
Comparison of the rate constants of the oligomerizations From Tables 7 and 10 it is seen that for oligomerizations in both methanol and ethanol, the initiation rate constant kl is greater than both the rate constants of propagation. In the first case kl/k3=2, and in the second case kl/k3--23. This is because k3 (methanol)> k3 (ethanol) while kx (ethanol)> kl (methanol). The former may be due, at least in part, to the greater basicity of the anion M1- with methoxy end group, as compared to that with an ethoxy end group (compare the pKo of methoxy acetic acid=3.53 with that of ethoxy acetic acid=3.60). °) The larger value of kx (ethanol) may be due to the greater nucleophilic activity of the initiator, C2H50-. Such differences between the rate constants of initiation and propagation were observed in a similar system, namely that of the polymerization of acrylonitrile in dimethylformamide by CH30-/CH3OH, where the initiation rate constant (as measured from cyanoethylation) was about seventeen times greater than the propagation rate constant. (4) These differences between the rates of initiation and propagation may explain the observed changes in ] ~ , with conversion (Figs. 12-14). Thus up to about 20-30 per cent conversion there was only a slow increase of D-P~, which was followed by a faster increase at higher conversions. Due to the higher values of kt, most of the monomer is initially consumed in the formation of (n--1) in a relatively fast reaction. The equilibrium of the addition reaction is achieved rapidly, and after this stage, the monomer consumed is used in the formation of the higher oligomers at a slower rate. The observed reversibility of the addition reaction, as compared to the further oligomerization reactions, also predicts that, on lengthening the reaction time, it would be possible to increase the ]SIsn by converting the (n = 1) to higher oligomers. In the previous oligomerization of MAN in dimethylformamide by CH30-/CH3OH, an induction period was observed before the formation of high oligomers; it was dependent on the variables of the polymerization system. (1) It seems that the cause for this is the fast addition reaction, and the induction period is the time required to achieve the equilibrium of the addition reaction. Differences between the rates of the propagation reactions were also observed. Thus k3/ks=l'7 (methanol) and k3/ks--2"5 (ethanol). This means that the anion CH30,-CHz--C-(CH3)(CN) is more active than the dimer anion C H 3 0 - - C H 2 C(CH3)(CN)---CH2--C-(CH3)(CN ). The former anion seems to be actually the more basic (compare the pKa of methoxy acetic acid---3-53 with that of/3-cyano propionic acid = 3.99). (3) An interesting observation is that the rate constants for termination, k4 and k 6 (protonation by alcohol of the growing anions) are only 2-5 times greater than the propagation rate constants. In the case of ethanol, they are even much smaller than the initiation rate constant, kx. A partial explanation for this behaviour may be found in the fact that the oligomerization was carried out in bulk, not in the presence of excess alcohol or alcohol as solvent. The effective concentration of the alcohol may be reduced by such factors as solvation of the growing anions, of RO-, and of the polar nitrile groups by alcohol, in addition to association and hydrogen bonding between the alcohol molecules. This reduction of the effective concentration of the alcohol may be, at least in part, the cause for the relatively low values of the observed termination rate constants.
300
GIORA TEICHMAN and ALBERT ZILKHA
Another interesting observation is that the estimated stationary concentrations of the different anions, MT, M~- and M~ vary with the reaction conditions (Tables 6 and 9). Under conditions where the addition reaction alone is of importance and almost only (n = 1) is formed, the stationary state concentrations of all these anions are low (Run C, Table 9). On the other hand, under reaction conditions leading to the formation of a high concentration of oligomers with D-P, > 1"5-2, the sum of the concentrations of these anions may reach almost 90 per cent of the total initial concentration of initiator, and are much higher than the stationary concentration of the RO- present in the oligomerization system (Run D, Table 9). This seems to result from the small differences found between the propagation and termination rate constants. Effect of the type of alcohol. When considering the differences in behaviour observed between the various alcohols, it is important to remember that the reactions were carried out under bulk conditions, and there are differences between the dielectric constants of the reaction media. Further, there may be differences in the extent of solvation of the anions by alcohol molecules and in the degree of association of the different alcohols. These factors may themselves lead to quantitative differences in the rate constants, and the differences found must be considered with reservation. However, it can be seen that the initiation rate constant, kl, for C2HsO-/C2HsOH is 4-5 times greater than that for CH30-/CH3OH, and yet the termination rate constants, k4 and k6, differ in the opposite sense. This observation is compatible with the order of acidity of the alcohols, and the order of basicity of the alkoxides. It follows that if the formation of (n = 1) was not an equilibrium reaction, then under the same conditions higher D-'Ps should have been attained in the case of C2HsO-/C2HsOH than in the case of CH30-/CH3OH, even though the propagation rate constants in the former case are about one-third those of the latter. However, the results show that under comparable conditions, lower D--Ps were obtained with ethanol. This may be understood as follows. If the equilibrium constants of the addition reaction, Kin. =
[ROH] [MAN] [MI] '
are compared (55 mole/l, for methanol and 7.8 mole/l, for ethanol) it can be seen that in the oligomerization with methanol, the equilibrium is shifted more to the reactants than in the case of ethanol. This means that after the fast step of the oligomerization in which the addition product, M1, is formed and dynamic equilibrium is established, more monomer will be available for propagation in the case of methanol, due to the high value of K~. This leads to the higher D-Ps found in methanol as compared with ethanol. From Tables 2 and 3, it was seen that the maximum concentration attained of (n = 1) was lower in the case of methanol and was only about half that for ethanol. This also shows that the elimination of a proton from (n = 1) in the reversible reaction, is easier in the case of methanol than in that of ethanol, leading to more availability of M1- (or monomer) for further propagation. These considerations apply equally well to the case of isopropanol in comparison with the more acidic alcohols. It was found that the rate of conversion was in the order isopropanol > ethanol >
Studies in the AnionicOligomerizationof Methacryionitrileby Alkali MetalAlkoxides
301
methanol. The conversion rate is expected to be higher firstlywhen the basic strength of the initiatoris greater leading to higher initiationrates, and secondly when the K~q. value of the addition reaction,which is relativelyfast,is smaller. Thus it can be seen that the experimental results are in accord with the higher initiationrates and lower equilibrium constants of the addition reaction observed with the less acidic alcohols. In the earlier preliminary study of the bulk oligomcrization of M A N , (e) higher D--Pswere obtained in the case of ethanol than methanol, contrary to the findingsof the present study. However, closerexamination of the previous reaction conditions shows first,that the reactions were carried out to high conversions, contrary to the present study. Thus higher I~-Psmay be obtained in converting more of the (n = I) oligomcr, via the reversiblereaction, to higher oligomers. Secondly, the previous oligomcrizations with the two alcohols were not carried out under the same conditions. A higher initiatorconcentration was used in the case of ethanol as compared with methanol (0.272 mole/l, instead of 0-187 mole/L) and as seen from the results of the present study, the concentration of the initiatorisunexpectedly an important factorcontrolling the ]5"i5, higher initiatorconcentrations leading to higher molecular weights. Previously(e)the ~ of the oligomcrization was connected with the concentrations of reactants thus: ..[MAN] Derivation of this equation was based on the simple relationship existing in polymer systems where transfer is dominant, namely [MAN] = Rp/Rt = Kp/Kt[ ROH ]" From the results of the present work, it is seen that this equation is oversimplified. It does not take into consideration the reversibility of the formation of (n = 1), and the differences, in the early stages, of the propagation and termination rate constants. Also it does not show the effect of the initiator concentration on ]515. Therefore the previously derived equation may be regarded as only a rough approximation and the K-values observed ce) are a function of initiator concentration.
Effect of initiator concentration This effect was found to be considerable, and it should be considered when planning oligomerization conditions for attaining a desirable 13"P. N o simple relationship between ~ and initiator concentration could be derived, but ]3-P increased always with increase of the initiator concentration. Increase of the initiator concentration also leads to higher conversion rates, as expected. Although Keq. of the addition reaction is not influenced by the catalyst concentration, the rate of its attainment is expected to rise with increasing initiator concentration. This means that more monomer (or M1-) may be available for oligomerization, thus leading to increase in ]3-15 with increasing initiator concentration. However, many other factors also may be the cause for the dependence of ]515 on initiator concentration. In an ordinary anionic polymerization, where transfer to solvent is dominant, 15P does not depend on initiator concentration. But in the present case, some of the
302
GIORA TEICHMAN and ALBERT ZILKHA
assumptions on which the derivation of the dependence of D-P on the reaction variables are based, such as constancy of the propagation or termination rate constants from the first stages o f the polymerization, are not justified. Further, an important factor in the system is the reversibility of the addition reaction. Thus the observed effect of initiator on ~ in oligomerization reactions m a y be expected.
Effect of the counter-ion While no difference was observed between sodium and potassium alkoxides in the oligomerization, with lithium alkoxides practically no oligomerization occurred. This is interesting since in the previously studied (2) cyanoethylation of methanol in methano[ as solvent, no difference in the rate constants was found between potassium, sodium and lithium methoxides. In that case it is obvious that the various alkoxides are dissociated completely to ions, and thus there was no effect o f the electropositivity of the alkali metal counter-ion. Under the oligomerization conditions investigated, the results obtained indicate that the alkoxide is dissociated to ions in the case of potassium and sodium but is in the form of ion-pairs, or more highly associated, in the case o f lithium, so lowering considerablythe catalyticactivity. Comparison of the behaviour of methacrylonitrile and acrylonitrile under oligomerization conditions Unlike methacrylonitrile, acrylonitrile was not found to yield oligomers under comparable conditions using R O - / R O H solutions. ~) fl-Alkoxy propionitrile was solely formed with all the alcohol present, and excess m o n o m e r was polymerized to relatively high molecular weight polymer. This may be due to a rapid cyanoethylation reaction, which is much faster than propagation, thus leading to complete consumption of the alcohol before the start of the polymerization. Subsequent polymerization will occur in the absence of a chain terminator (alcohol). Further, it seems that the reversibility of the formation o f fl-alkoxy propionitrile is negligible under these conditions, so that no alcohol is released later in the reaction to cause termination o f very short polymer chains and formation of oligomers. REFERENCES (1) B. A. Feit, J. Wallach and A. Zilkha, J. Polym. ScL A2, 4743 (1964). (2) B. A. Fcit, E. Heller and A. Zilkha, J. Polym. Sci..4,4, 1499 (1966). (3) E. A. Braude and F. C. Nachod, editors. Determination of Organic Structures by Phy$ica Methods, p. 578, Academic Press, New York (1955). (4) B. A. Feit, Ph.D. Thesis, Th© Hebrew University, Jerusalem (1962). (5) B. A. Feit and A. Zilkha, J. org. Chem.28, 406 (1963). (6) Y. Barzilai, B. A. Feit and A. Zilkha, unpublished results. ROann6---L'oligom(~risation anionique en vrac de m6thacrylonitrile (MAN) par des solutions R O - / R O H a ~t6 ~tudi~e h temp6rature constante. La disparition des monom/~reset des alcools, e~ la formation des diff~rents oligom6res jusqu'/L n ffi3 (trim/~res) furent suivies quantitativement par chromatographie des gazet le changement de ]~YPdes oligom~res avec conversion fut d6terrnin(~. Les effets du type d'alcool, du rapport [MAN]/[ROH], de la concentration de rinitiateur, et la nature de l'ion d'opposition de m6tal alcalin furent examine. Z[3J~ diminua dans l'ordre m(~thanol> 6thanol > isopropanol, et le taux de conversion vice versa. L,augmentation du rapport [MAN]/[ROH] et de la concentration de l'initiateur augmenta ]3~. Tandis que le meme comportement fut observ~ clans le cas des oxydes alcooliques de sodium et de potassium, pour les oxydes alcooliques dc lithium pratiquementaucune oligorn~'risation n'a lieu meme darts des conditions plus rigoureuses.
Studies in the Anionic Oligomerization of Methacrylonitrile by Alkali Metal Alkoxides
303
Dans les conditions ~tudi~es, la formation du produit d'addition RO---MAN (n ffi 1) ~tait r6versible, ~t l'oppos6 de la formation d'oligom~res plus 6iev~. Le/~q. de la r~action r6versible [MAN] [ROH]/ [RO--MAN] fur ~tabli ~t partir de r6~ctions utilisant ROH en exc~; pour du rn~qhanol il ~tait de 55 mole/L, et pour l'~thanol de 7.8 mole/l. Quelques unes des constantes du rapport des diff~rents stades des r~actions d'oligon~isation pour le m~thanol et l'6thanol furent ~ t l u / ~ s en tenant compte de la r~versibilit6 de la r~tction d'initiation, et en supposant des conditions d'~tat stable dam la concentration des diff&ents anions presents dans le syst~me. La constante du taux de la r ~ ' t i o n d'initiation ~tait plus ~lev~e que les constantes du taux de propagation; la constante du taux de propagation de M l - (formation d'anion dim~re) ~tait plus grande clue celle de M2-. Les constantes de taux de terminaison des diffh~*nts anions oligom~res par ROH n'~taient que de 2-4 fois plus grandes qu¢ ies constantes des taux de propagation correspondants. Les concentrations ~ ~tat stable M~-, M2- et M3furent ~valu~s, et darts certains cas ~taient relativement ~ de telle sorte que leur total ~tait d'autant que 90 ~ de la concentration initiale de l'initiateur d'oxyde d'alcool. La constante du taux d'initiation avec C2H~O-/C2H~OH ~tait d'environ 4-5 fois plus grande que celie avec CH30-/CH3OH, et les constantes de taux de terminaison correspondames ~qaient plus petites clans ie rapport d'un facteur de 4-5, c¢ qui est conforme ~ l'acidit6 des alcools. Sommsrlo---l~ stata studiata la oligomer~r,'~=ione anionica del metacrilonitrile (MAN) in presenza di soluzioni RO-IROH a temperatura costant¢. La scomparsa del monomero • dell'alcol • la formazione dei vari oligomeri fino a nm 3 (trimeri) 6 stata seguita quantitativamente per via gas-cromatografica ed ~ stata determinata la variazione del ]3'P degli oligomeri con la conversione. Si ~ studiata l'influenza del tipo di aicol, del rapporto (MAN)/(ROH), delia concentrazione dell'iniziatore e della natura del metallo alcalino che fa da controione. ]3]s diminuisce in questo ordine: metanolo > etanolo > isopropanolo, mentre l'inverso si verifica per la velocitb di conversione. Un aumento del rapporto (MAN)/(ROH) e della concentrazione dell'iniziatore porta ad un aumento di ]515. Con gli alcolati di sodio • potassio si osserva 1o sUmo comportamento, mentre col corrispondente alcoiato di litio non awiene, anche in condizioni molto pi~ drastiche, alcuna ofigomerJ-~ione. Nelle condizioni di esperienza, la formazione del prodotto di addizione R O - - M A N (n--l) reversibile, contrariamente alia formazione di oligomeri con n maggiore. La /~q. della reazione reversibile, (MAN) (ROI-I)/(RO--MAN), t stata determinata daUe reazioni nsando un ~ di ROH; nel caso del metanolo risulta 55 moli/l., • per l'etanolo 7.8 moli/l. Alcune delle costanti di veloci~ dei vari stadi delle reazioni di oligomeri per il metanolo • per l'etanolo sono state determinate tenendo conto della reversibiliUt della reazione di iniziazione • assumendo condizioni di stato stazionario per le concentrazioni dei vari anioni prescnti nel sistema. La costante di velociflt della reazione di iniziazione ~ maggiore delie costanti di velocit~ di propa~aTione; la costante di velocit~ della propagazione di M l - (formazione dell'anione dimero) ~ maggiore di quella di M2-. Le constanti di velocit~ della terminazione dei vari anioni oligomeri da parte di ROH sono solo 2-4 V pi/~ grandi delle corrispondenti costanti di propagazione. Le concantrazioni stazionarie di M~-, M2- e M3- sono state valutate e in alcuni casi risultavano relativamente elevate tanto chela loro somma risultava pari al 90 por cente della concentrazione iniziale dell'iniziatore alcolato. Le costanti di velociUt di iniziazione con C2HsO-/C2HsOH risultavano 4-5 V pi~ grandi di quelie con CH30-/CHsOH • le corrispondenti costanti di velocit~ di terminazione erano pit~ piccole di un fattore 4-5: cib ~ in accordo con l'acidit~ degli alcoli. Zussameafassag--Die anionische Oligomerisation in Substanz yon Methacrylnitril (MAN) durch RO-/ROH LOsungen wurde bei konstanter Temperatur untersucht. Die Abnahme von Monomerund Alkohol, sowie die Bildong der verschiedenen Oligomeren bis zu n = 3 (Trirnere) wurde durch Gaschromatographie quantitativ verfolgt, und die Verinderung des [YP der Oligomeren mit dem Urnmtz bestimmt. Der EinfluB tier Art des Aikohols, des (MAN)/(ROH) VerhiUtnisses, der Initiatorkonzentration und der Natur des Alkalimetall Gegenions wurde untersucht. Der D'P nahm ab in der Reihenfolge Methanol > ~thanol > Isopropanol, und im umgekehrten Sinne mit dem Umsatz. Eine Zunahme im Verhaltnis (MAN)/(ROH) und in der Initiatorkonzentration ergab eine Zunahme des DI s. Wfihrend/'fir Natrium- und Kaliumalkoxyde gieiches Verhalten beobachtet wurde, fand mit den entsprechenden Lithium-alkoxyden praktisch keine Oligomerisation start, auch unter energischeren Reaktionsbedingungen. Unter den untersuchten Bedingungen war die Bildung des Additionsproduktes, RO-MAN (nffi 1) reversibel, im Gegensatz zur Bildung tier hOheren Oligomeren. Das Kc¢. der reversiblen Reaktion, (MAN)(ROH)/(RO-MAN) wurde dutch Reaktionen mit iiberschiissigem ROH bestimmt. Fiir Methanol ergab sich ein Wert yon 55 Mol/l. und fth",~thanol von 7.8 Mol/L Einige der Geschwindigkeitskonstanten der verschiedenen Stufen der Oligomerisationsreaktionen wurden for Methanol und ,~thanol abgeschatzt unter Beriicksichtigung der Reversibili~t der Startreaktion und unter der Annahme von steady-state Bedingongen fOr die Konzentration tier verschiedenen im System vor-
304
GIORA TEICHMAN ann ALBERT ZILKHA
handenen Anionen. Die Geschwindigkeitskomtante der Startreaktion war gr6Ber als die Geschwindigkeitskonstanten des Wachstumsschrittes. Die Geschwindigkeitskonstame des Wachstums von M l - (Bildung des Dimer-Anions) war gr6Ber als die f'tir M2-. Die Geschwindigkeitskonstanten f'ur den Abbruch der verschiedenen Oligomer-Anionen durch ROH waren nur 2-4 mal gr6Ber also die entsprechenden Wachstumskonstanten. Die steady state Konzentrationen yon MI-, M2- und M3wurden bestimmt und waren in einigen Fallen relativ hoch, so dab ihre Summe bis zu 9 0 ~ der anf~mglichen Konzentration an Alkoxyd-Initiator ausmachte. Die Geschwindigkeitskonstante der Startreaktion mit C2HsO-/C2HsOH war etwa 4-5 mal gr6Ber als die mit CH30-/CH3OH, und die entsprechenden Abbruchskonstanten waren um den Faktor 4-5 kleiner, was mit der Azidit~t der Alkohole in Einldang steht.
Ltd. Prints! in T~iland.
STUDIES ON THE ANIONIC OLIGOMERIZATION OF METHACRYLONITRILE BY ALKALI METAL ALKOXIDES GIORA T~CHMAN a n d A L n ~ T 7ALKHA Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel
(Received13 October1966) Abstract--The bulk anionic oligomerizition of methacrylonitrile (MAN) by RO-/ROH solutions was studied at constant temperature. The disappearance of monomer and of alcohol, and the formation of the different oligomers up to n ffi3 (trimers) were followed quantitatively by gas chromatography, and the change of D'P of the oligomers with conversion was determined. The effects of the type of alcohol, [MAN]/[ROHI ratio, initiator concentration, and the nature of the alkali metal counterion were investigated. ~ decreased in the order methanol>ethanol>isoprolmnol, and the rate of conversiunvicewrsa. Increase of the IMAN]/[ROH] ratio and ofinitiator concantration increased IYP. While the same bchaviour was observed with sodium and potassium alkoxides, with the corresponding lithium alkoxides practically no oligomeHzation occurred even under more drastic conditions. Under the investigated conditions, the formation of the addition product, R O - - M A N (nffi I) was reversible, contrary to the formation of the higher oligomers. T h e / ~ . of the revers/ble reaction, [MAN] [ROH]I[RO--MAN] was determined from reactions using excess ROH; for methanol it was 55 mole/L, and for ethanol 7.8 mole/l. Some of the rate constants of the various steps of the ofigomerizationreactions for methanol and ethanol were estimated taking into account the reversibifity of the initiation reaction, and assuming steady state conditions in the concentration of the various anions present in the system. The rate constant of the initiation reaction was greater than the rate constants of propagation; the rate constant of propagation of Mi" (formation of dimer anion) was greater than that of M~. The rate constants of termination of the various oligomer anions by ROH were only 2-4 times greater than the corresponding propagation rate constants. The steady state concentrations of Mi', M~ and M~ were intimated, and in some cases were relativtly high so that their sum was as much as 90 per cent of the initial concentration of the alkoxide initiator. The initiation rate comtant with C2HsO-/C2HsOH was about 4-5 times greater than that with CH30-/CH3OH, and the corrmponding termination rate constants were smaller by a factor of 4--5, which is in accordance with the acidity of the alcohols. INTRODUCTION WE HAVE p r e v i o u s l y r e p o r t e d ¢t,2) the o l i g o m e r i z a t i o n o f m e t h a c r y l o n i t r i l e ( M A N ) b y a l k a l i m e t a l a l k o x i d e s in the presence o f the p a r e n t alcohol. O l i g o m e r s h a v i n g D'P u p t o five were i s o l a t e d a n d characterized. T h e o l i g o m e r i z a t i o n was s t u d i e d b o t h in s o l u t i o n in a p r o t i c solvents, such as d i m e t h y l f o r m a m i d e a n d d i m e t h y l s u l f o x i d e , a n d in b u l k ¢2) ( m o n o m e r + a l c o h o l ) u n d e r reflux conditions. B o t h s o d i u m a n d p o t a s s i u m a l k o x i d e s were u s e d as initiators, a n d n o difference was f o u n d between t h e m . A n i m p o r t a n t aspect o f the w o r k was t h a t the degree o f o l i g o m e r i z a t i o n was g o v e r n e d b y the r a t i o [ M A N ] / [ R O H ] so t h a t ] 3 ~ P - - K [ M A N ] / [ R O H ] , K b e i n g a c o n s t a n t d e p e n d e n t o n the t y p e o f a l c o h o l a n d o n the solvent o f the r e a c t i o n (in bulk/~methtnot) ffi 1"4, Kceth~ob---- 1"85). T h i s e q u a t i o n was derived on the basis o f the simple r e l a t i o n s h i p existing in p o l y m e r systems t h a t ~ = Rp/Rt= (Kp/Kt)([MAN]/[ROH]) ( t e r m i n a t i o n b y transfer t o a l c o h o l ) . 283
284
GIORA TEICHMAN and ALBERT ZILKHA
W e have n o w studied i n m o r e detail the b u l k oligomerization of M A N by R O - / R O H at c o n s t a n t temperature. D u e to the very high boiling p o i n t s o f the higher oligomers, the systems studied were those yielding mostly the lower oligomers u p to trimers. Use was made o f q u a n t i t a t i v e gas c h r o m a t o g r a p h y techniques to follow the progress o f the reaction i n c l u d i n g the rates o f disappearance of the reactants, m o n o m e r a n d alcohol, a n d o f f o r m a t i o n o f the various oligomers; a picture was thus o b t a i n e d o f what was going o n all the time in the oligomerization mixture. F r o m these rates, a n estimate was m a d e of the rate constants of some o f the various steps o f the oligomerizations. It seems that such a detailed study has n o t been performed previously for similar a n i o n i c oligomerization systems. The technique was used to study various factors affecting the rate o f the oligomerization a n d the D--P o f the oligomers, such as the type o f alcohol, the ratio o f reactants, [ M A N ] / [ R O H ] , a n d the type o f the alkali metal alkoxide. The effect of catalyst c o n c e n t r a t i o n was also studied here for the first time, a n d it was f o u n d to affect considerably the rate o f conversion a n d the D-P. EXPERIMENTAL
Materials Methacrylonitrile (Fluka) was washed with 1 per cent sodium hydroxide, followed by water and dried over calcium chloride. It was distilled under argon and the fraction boiling at 86-87°/690 mm was collected and kept under argon in a cool dark place. Oxygen-free argon (Matheson) was dried by passage through concentrated sulphuric acid followed by sodium hydroxide pellets. Absolute methanol (BDH) containing 0-01 per cent water was used without further purification. Absolute ethanol was dried over magnesium and distilled. Isopropanol was dried over calcium sulphate, refluxed over calcium metal, and fractionally distilled. The alkali metal alkoxide solutions were prepared by dissolving the alkali metal in the required alcohol under reflux with exclusion of moisture. The solutions were standardized by titration with 0.I N hydrochloric acid using phenol phthalein as indicator. All the reagents were kept under argon and were transferred by syringes applying positive argon pressure.
Oligomerization procedure The olignmerizationwas carried out in a reaction vessel consistingof a three-necked flask, fitted with a reflux condenser and a calcium chloride guard tube, an adapter for introducingargon, and an opening fitted with a self-sealing rubber cap through which the reagents were added with syringes, or samples removed during the reaction. Magnetic stirring was employed. The reaction vessel was sealed in an external vessel having an opening fitted with a reflux condenser. The temperature inside the reaction vessel was kept constant by the vapours of boiling benzene in the external vessel. Alcohol and methacrylonitrile were added and heated to the required temperature, followed by the alkali metal alk0xide. Inallexperimentsthetotalvolumeofthereactionmixturewas70ml.Aliquots were removed from the homogeneous reaction mixture at various intervals, and were analysed by gas chromatography. Control experiments showed that it was possible to inject these solutions directly into the gas chromatograph without prior neutralization of the alkoxide. Experiments in which the oligomer mixture was neutralized, extracted by chloroform and injected, gave the same results within experimental error.
Gas chromatography analyses The concentrations of the monomer, alcohol, fl-alkoxy isobutyronitrile, dimer and trimer present in the oligomerization mixture were determined by gas chromatography using an Aerograph Autoprep Model A-700 instrument. Helium was used as the carrier gas and 2 m of standard 0.25 in. tubing was used to contain the packing of 20 per cent "DC-11" on Chromosorb W. The injector temperature was about 330°, and the flow rate was 40-60 ml/min in the determination of the higher oligomers, and 10-25 ml/min in that of methanol and monomer. The column temperature was held approximately as follows in the determinations of the various fractions: 40° (monomer and alcohol), 90° (n = 1), 140° (n=2) and 220° (n=3). The oligomers were determined quantitatively by comparison of the peak areas on the chromatograms, obtained by injection of known volumes of the reaction mixture, with those of known volumes of the pure materials, injected at about the same time and under the same conditions. Generally conditions were chosen such that sharp peaks were obtained, and then the height of the peaks was compared. In other cases, the peak areas were measured by a plani-
Studies in the Anionic Oligomerization of Methacrylonitrile by Alkali Metal Alkoxides
285
meter. Every analysis was repeated. It was found that samples taken from the reaction mixture could be stored at 0 ° for 36 hr before analysis, without any detectable change in the composition of the mixture.
Calculation of ~YPn. The average degree of oligomerization of mixtures was calculated using the equation:
No. of moles of monomer in the oligomers No. of oligomer molecules Since, No. of moles oligomer =
Weight of oligomer Molecular weight of oligomer'
and
No. of moles of monomer in trimer = 3(No. of moles of trimer), etc., it is possible by knowing the weight of the different oligomers present in the mixture (from n-- 1 to n = 3) to calculate the average ]3"15of the oligomers present in the reaction mixture at a certain reaction time. RESULTS T h e b u l k o l i g o m e r i z a t i o n o f M A N was p r e v i o u s l y c a r r i e d o u t u n d e r reflux c o n d i tions. (2) W e h a v e s t u d i e d such a reaction, using gas c h r o m a t o g r a p h y techniques, t o follow t h e p r o g r e s s o f t h e r e a c t i o n to find o u t the r e p r o d u c i b i l i t y o f t h e results ( T a b l e I). A d u p l i c a t e e x p e r i m e n t gave, after 90 rain, d e v i a t i o n s o f a b o u t 5 p e r cent in t h e v a r i o u s c o n c e n t r a t i o n s m e a s u r e d o f the c o m p o n e n t s in the o l i g o m e r mixture, which increased u p t o 2 0 - 2 5 p e r cent after 240 r a i n ; r e a c t i o n s c a r r i e d o u t u n d e r reflux c o n d i t i o n s a r e therefore poorly reproducible. TABLE I. OLIGOMERIZATIONUNDERREFLUXCONDmONS* Time
[MAN]
[C2HsOH]
In = 1 ]
In = 2]
In = 3]
(rain)
(mole/L)
(mole/L)
(mole/L)
(mole/L)
(mole/L)
0 10 20 30 50 70 90 130 180 240
7.26 5.22 4.83 4"56 4.40 4"32 4.31 4.21 4.12 3,00
6.24 4.04 3.81 3.57 3.47 3-42 3"40 3"40 3.37 3.32
0 2-04 2.43 2.62 2.70 2"70 2.66 2"63 2.60 2"55
0 --0"041 0"078 O"118 0"145 0-200 0"271 0-355
0" -n ---0.028
* Experimental conditions: the concentration of sodium ethoxide was 0-09 mole/l. The reaction was carried out in a flask fitted with a reflux condenser, and an electric mantle was used for heating. O n the o t h e r h a n d , o l i g o m e r i z a t i o n s c a r r i e d o u t a t c o n s t a n t t e m p e r a t u r e (78 °) as d e s c r i b e d in the E x p e r i m e n t a l section were m u c h m o r e r e p r o d u c i b l e , a n d the m a x i m u m d e v i a t i o n s o b s e r v e d in t h e m e a s u r e d c o n c e n t r a t i o n s were o f the o r d e r o f 5 p e r cent.
Effect of type of alcohol I n t h e p r e v i o u s w o r k o n the b u l k o l i g o m e r i z a t i o n , (2) r e a c t i o n c o n d i t i o n s were used so t h a t h i g h c o n v e r s i o n s (80-90 p e r cent) a n d relatively high degrees o f o l i g o m e r i z a t i o n ( ] ~ ) were o b t a i n e d . I n t h e p r e s e n t study, the r e a c t i o n c o n d i t i o n s were such t h a t the DlSs o b t a i n e d were n o t high, a n d u n d e r such c o n d i t i o n s c o n v e r s i o n s o f a b o u t 4 0 - 5 0 p e r cent were o b t a i n e d after 2 - 3 hr.
286
GIORA TEICHMAN and ALBERT ZILKHA
The effect of the type of alcohol was investigated using methanol, ethanol and isopropanol which differ appreciably in the pKa values; thus methanol is the strongest acid, while the methoxide initiator is the weakest derived base. Figure 1 shows that at a ratio [MAN]/[ROH], (R)= 1.5, the rate of conversion was highest with/so-propanol, as expected, although the catalyst concentration used was lower. 3
4(:
~
o,'~°
3G .,
"i 20 c~ ~o
0
40
Time,
rain
FIG. 1. Dependence of conversion on type of alcohol ([MAN]/[ROH]-- 1"5) [MAN]0 [RONa]0 (mole/L) (mole/L) I. Methanol; 9.04 0.120 2. Ethanol; 8.17 0.100 3. Iso-propanol; 7.43 0-095
The number-average degree of oligomerization, D-P., was dependent on the type of alcohol. Under comparable conditions, D-'Pnwas highest in the presence of methanol, and lowest in the presence of iso-propanol, both at (R)-- 1-5 (Fig. 2) and at (R) = 2 "0 (Fig. 3). At (R)= 1.0, the D-P, observed was approximately the same with the three alcohols, and after 2 hr it was not more than 1.1.
1.5-1.4--
/
,3-
/:/
/
+1.2- / . / . I II
I/
,.+-"
2f* 3 e ~ e a,,~..~0 ~
,io
+
Time,
rain
o
,+
FIG. 2. Dependence of ]~Pn on type of a]cohol ([MAN]/[ROHI-- 1.5).
1. Methanol; 2. Ethanol; 3. !so-propanol;
[MANI0
[RONal0
(mole/L) 9-04 8.]7 7.43
(mole/L)
0-100 0.100 0.095
Studies in the Anionic Oligomerization of Methacrylonitrile by Alkali Metal Alkoxides
287
1,5
,.,k// ....---" Time,
rain
FIG. 3. Dependence of ~iin on type of alcohol ([MAN]/[ROH]=2-0) [MAN]o [RONa]0 (mole/L) (mole/L) I. Methanol; 9.68 0.100 2. Ethanol; 8.84 0"100 3. Iso-propanol; 8.18 0.095
Effect of the ratio, [MAN]/[ROH], (R) Increasing (R) increased b o t h the rate o f conversion a n d the D-'P~ as f o u n d before Ct, 21 (Fig. 4). T h e increase in rate shows that the m o n o m e r is m o r e effective in increasing
8(
,oh
i/ ] '/
/
/
"t"
~ ~ . ~~' ' - ~ -~--~'---
o
0
40
80I Time,
,~o
,70
min
FIG. 4. D e p e n d e n c e o f per ce~ntag¢ c o n v e r s i o n o n initiator c o n c e n t r a t i o n .
1. 2. 3. 4.
[MAN]o (mole/L) 8.84 8-84 8"17 8"17
[ethanol]o (mole/L) 4.40 4'40 5"40 5-40
[Na-ethoxide]o (mole/L) 0.216 0.087 0"087 0"017
288
G I O R A T E I C H M A N and A L B E R T z I L K H A
'I 1.7--
1.6
1.5
c
I~
,4
1.2
15.
/2
i.i
I.c ~'/~'~1"-~' 40
"I'
80 Time,
I
17'0 rain
I
160
200
FIG. 5. Dependence of~-Pn on [MAN]/[lacohol] ratio (methanol) [Methanol]o [MAN]0 (mole/L) (mole/L) I. 4.82 9-60 2. 5.98 9.04 3. 8.05 8.06 [K-methoxide]0 = 0.120 mole/l, in all cases
1.5--
1.4~
/J
I /.1"
I.)l-
"~0
I
I
o'-° 2
40
3
80 Time,
I20 min
160
FIG. 6. Dependence of ]~Ein on [MAN]/[~Icohol] ratio (ethanol). [MAN]0 [ethanol]o (mole/L) (mole/L) 1. 8.84 4.40 2. 8.17 5.40 3. 7-16 6.86 [Na-ethoxide]0 = 0-087 mole/i, in all cases
Studies in the Anionic Oligomerization of Methacrylonitrile by Alkali Metal Alkoxides
289
the rate of the oligomerization than the alcohol. At (R)= 1.0, under the conditions investigated the main product obtained was ~-alkoxy iso-butyronitrile (n--1, where n is the degree of polymerization). Figures 5, 6, and 7 show the variation of D-P, with time in the case of methanol, ethanol, and/so-propanol respectively. In all cases, DP, increased continuously with the time of reaction. The effect of increase of (R) on 13-P, was greater in the case of methanol than in the case of/so-propanol. I.I~.
I
f
/"
2/"
~a
Time, rain FIO. 7. Dependence of ]51s. on [MAN]/[lacohol] ratio (isopropanol). [MAN]0 [iso-propanol]0 1. 8.18 4.10 2. 7.43 4.93 3. 6.24 6.23 [Na-iso-propoxide]o=O.095 mole/1, in all cases.
Effect of initiator concentration On increasing the alkoxide concentration the rate of conversion increased considerably (Fig. 4), as well as the ~Pn; the effect being similar to that of increasing (R). This effect was general and was found in all the systems investigated (Figs. 8-11). This important effect on ]~Pn was not noticed before. <1'2)
Effect of the alkali metal counterion~ In the previously studied oligomerizations, no difference in behaviour was noticed for sodium, potassium, and quaternary ammonium methoxides c2) in solution C1) or in bulk. <2) In the present work the same behaviour was found. Oligomerizations, performed using sodium or potassium methoxides in the range (R)-- 0.5-2 and initiator concentrations up to 0.2 mole/L, gave similar results for the rate of conversion and the formation of the different oligomers. 19
290
GIORA TEICHMAN and ALBERT ZILKI-IA
I~ 1.2-
LO0
o
o
• .~"-T'" 40
~
80
120
"
160
Time, rain
Fin. 8. Effect of initiator concentration on ]5~.([MAN]/[ethanol]-- 1.5). [Na-ethoxid¢]0, mole/l.: (1) 0.087; (2) 0-017
1.2
0
40
~ Time,
I
I
rain
FIG. 9. Effect of initiator concentration on ]3"~.([MAN]/[ethanol]=2.0). [Na-ethoxide]0, mol©/l.: (l) 0.216; (2) 0.100; (3) 0.071 The behaviour of lithium methoxide was, however, completely different. Thus a reaction carried out at ( R ) = 2 , ( M A N ) = 9 . 6 0 mole/1, and LiOCH3=0-175 mole/l. gave 2"5 per cent conversion to (n = l) after 4-5 hr, and no higher oligomers were formed. Increasing the initiator concentration to 0.206 mole/l, and (R) to 4, gave only 3 per cent conversion to (n = 1). Similar results were obtained with lithium ethoxide.
Studies in the Anionic Oligomerization of Methacrylonitril¢ by Alkali Metal Alkoxides
291
2.0 I.E
1.4 1.2 4to
t Time,
min
Flo. 10. Effect of initiator concentration on ZYPn([MAN]/[methanol]--2-0). [Na-methoxide]o,mole/l.: (1) 0.18; (2) 0-12
//
+
1.2 /
Time,
mln
FI~. 11. Effect of initiator concentration on ~Pn([MAN]l[methanol]-- 1-5 (1) [K-methoxide]0ffi0-18;(2) [Na-methoxide]0--0-10mole/l. T o find a way of increasing the yield with lithium, the alkoxide was f o r m e d / n situ. Lithium metal was added to a mixture of monomer and alcohol. Different experiments were carried out using ethanol,/so-propanol and tert-butanol at (R) = 2. Conversions of up to 10 per cent only were obtained, and the products were high oligomers (n >~3).
Relation between com,ersion and ~ , In most of the oligomerizations, ]3~P, increased slowly at first with conversion up to 20-30 per cent, and then the increase was faster (Figs. 12-14).
292
G I O R A T E I C H M A N and ALBERT Z I L K H A
I
I.I
o
I.C )
20 Conversion,
1.2
40 %
1.0
of
I.I
20 Conversion,
40 %
1.0
4[0 Conversion,
o/.
FIG. 12 (left). Dependence o f ] ~ n on percentage conversion. [Iso-propanol]o = 4.93; [MAN]o= 7-43; [Na-iso-propoxide]0 = 0.095 mole/l. Fie. 13 (centre). Dependence of 1S]5. on percentage conversion. [Ethanol]0=4.40; [MAN]o=8.84; [Na--ethoxide]o=0.10 mole/l. FIG. 14 (right). Dependence of ISI~. on percentage conversion. [Ethanol]0=5.40; [MAN]0=8.17; [Na-ethoxide]0=0.087 mole/l. TABLE 2. OUGOM~ZATION OF M A N BY C H 3 0 - / C H 3 O H Run* No. IA3
IA2
IC1
IA4
Time (rain) 0 30 60 90 120 150 180 0 2 15 45 75 105 135 165 0 15 40 75 0 2 8"5 20 37 58 81 105 134 174
[MAN] (mole/L)
[CH3OH] (mole/L)
[ n = 1] (mole/L)
[n--2] (mole/L)
[ n = 3] (mole/l.)
9"06 7-30 6"65 5"85 5"50 5"30 5"10 9"60 9"30 7"95 6"20 5.00 4-12 3"50 3"03 9-60 8"65 7"10 5"80 9"04 8"77 8"25 8.23 8"22 8.12 8"07 7.90 7.63 6.56
5.93 4.37 4"03 3"64 3"63 3"63 3"62 4"82 4"52 3"46 2"58 2" 12 1"78 1"59 1"46 4"82 4"01 3"08 2"36 5"98 5"71 5"21 5.22 5-25 5.25 5-25 5-20 5.10 4.59
0 1-35 1"41 1"49 1"27 1.06 0"91 0 0"30 1"08 1"16 1"03 0"99 0"93 0"88 0 0"66 1"00 1"17 0 0"26 0"74 0.70 0-61 0.56 0.46 0"41 0.36 0.30
0 0"21 0"46 0"69 0"83 1"00 1"14 0 0.0008 0"272 1"020 1'440 1-650 1.730 1"730 0 0"15 0"72 1"23 0 0"006 0"024 0.055 0-105 0.175 0.255 0-366 0"527 1.090
0 0 0"03 0" 11 0"21 0"24 0"26 0 0 0-007 0"065 0.234 0"397 0"572 0-751 0 0 0"017 0.057
* Experimental conditions: IA3 : [MAN]][CH3OH] = 1.5; IA2: [MAN]/[CH3OH]=2"0; ICI : [MAN]/[CH3OH]---2"0; IA4: [MAN]/[CH3OH]-- 1.5;
CH~ONa = 0.180 mole/l. CH3ONa--0"I80 mole/l. CH3OK=0-120 mole/l. CH3OK--0-120 mole/l.
Studies in the Anionic Oligomefization of Methacrylonitrile by Alkali Metal Alkoxides
293
TAm.E 3. Ouoo~a~IzT,'no~ oF M A N nY C2HsO-IC2HsOH Run* No. A
D
E
C
Time (rain)
[MAN] (mole/L)
[C2HsOH] (mole/L)
[n = 1] (mole/L)
[n = 2] (mole/L)
[n = 3] (mole/L)
0 2 10 20 28 38 56 73 81 107 149 0 5 20 35 55 71 90 106 130
8.17 8.00 7.17 6.70 6.36 6.16 5.85 5-62 5.57 5"46 5"26 8.84 7-64 6.54 6.20 5.84 5"58 5.27 4-95 4.66 8-84 6.50 5.60 5.00 4.71 4"27 4-03 8.17 8-12 8-05 7"91 7-82 7"67 7"43 7-33 7-26 7"24 7"07 6-95 6-74 6.74 6"72
5-40 5.23 4.43 4.00 3-66 3-53 3-29 3.13 3"12 3.08 3"05 4.40 3-22 2"27 2"12 1"96 1.87 1"76 1"65 1"54 4-40 2"29 1.74 1"47 1"43 1.29 1.25 5.40 5.35 5'29 5,14 5.05 4,90 4.66 4.56 4-49 4.47 4.30 4-18 3.97 3.97 3.95
0 0-17 0.95 1.37 1.67 1.77 1.93 2.04 2.02 1"98 1.96 0 1"17 2.00 2-00 2.00 1-97 1"95 1.92 1.90
0 0 0.013 0.023 0.046 0-072 0.135 0.183 0-212 0.260 0.380 0 0.01 0"10 0.20 0.31 0"38 0.46 0.52 0-60
0 0 0-008 0.012 0-015 0.032 0.042 0-046 0.050 0.071 0-097 0 0 0.03 0"08 0.13 0"18 0"23 0"31 t" 0"36~
0
30 60 90 120 150 180 0 1.5 9 19 27.5 37.5 55.5 72-5 80 93.5 106.5 148.5 244 267 287
0
0
0
1.91 2.17 2-13 2.04 1"97 1.90 0 0.05 0.11 0-25 0"35 0.50 0.73 0.84 0-91 0.93 1.10 1"21 1.41 1.41 1"41
0.17 0"40 0.55 0"65 0.80 0.84 0 0.00015 0.0011 0.0023 0.0033 0.0044 0.0061 0.0075 0.0081 0.0089 0.0097 0.0112 0.0129 0.0131 0"0133
0.031 0.090 0-170 0.235 0-328 0-410 0 0 0.0001 0.0005 0.0008 0.0012 0"0018 0-0023 0"0026 0"0029 0"0032 0.0041 0.0053 0"0055 0.0057
* Experimental conditions: A: [MAN]/[C,2HsOH]--- 1"5; C2HsONa-- 0.087 mole/l. D : [MAN]/[C2H~OH]=2.0; C2HsONa--0.100 mole/l. E: [MAN]I[C2HsOH]=2"0; C2HsONa=0"087 mole/l. C: [MAN]/[C2HsOH]= 1.5; C2HsONa=0"0172 mole/l. t [n~4]=0-047 mole/L; no tetramer was found at shorter reaction time. :l: [n~4]~0-117 mole/1.
Kinetics o f the oligomerization system Experiments were performed to discover, on the one hand, the rates of disappearance of monomer and alcohol, and, on the other, the rate of formation of the different
294
GIORA TEICHMAN and ALBERT ZILKI-IA
oligomers. The results using methanol and ethanol are given in Tables 2 and 3. An important observation was that in most of the reactions the amount of (n = 1) increased to a m a x i m u m during the oligomerization and then decreased. The lowering in concentration of (n = 1) during the reaction seemed to be due to the reversibility of the reaction: (I)
R O - + CH2 = C - - C N ~-ROCH2--C----CN
I
I
CH3
CH3
(I) + ROH ~-RO- + ROCH2CH---CN
(II)
I
CH3 To confirm this point, experiments were carried out in which ethanolic sodium ethoxide was added to (II) ( R = C 2 H s ) under oligomerization conditions (Table 4). I t is seen that there is a rapid elimination of ethanol and formation of M A N . Even after 1 rain, there was about 15 per cent elimination. An equilibrium was established after about 2 hr and remained stable up to about 4 hr. When the reaction was continued up to 11.5 hr, a small amount of dimer (0.5 mole/L) was formed. It must be noted that the ratio of M A N to ethanol was less than unity, which is not in favour of the formation of oligomers higher than n = 1. A duplicate experiment gave similar results. TABLE 4. RI~VERSIBILITYOF THE CYANOETHYLATIONREACTION*
Time (rain)
[n ffi1] (mole/L)
[MAN] (mole/L)
0 1 11 20 35 55 82 100 128
7-09 5.99 5"19 4.54 4.19 3.82 3"49 3.27 3.03
0 1"I0 1.90 2-45 2.90 3-27 3.60 3.72 4.04
[C2HsOH] (mole/L) 1"46 2"56 3"36 3"91 4.36 4-73 5.06 5-18 5"50
* Sodium ethoxide (1-1 ml of 1.32 N solution) was added to fl-ethoxy iso-butyronitrile (7-09 mole/L) under the usual oligomerization conditions. To investigate further the equilibrium reaction, we carried out experiments in which (R) ~0.1, i.e. using a large excess o f alcohol. Various concentrations of initiator (0.05-0.15 mole/L) were used. In all experiments, the system reached equilibrium within 15 rain and no changes were observed even after several hours. The equilibrium concentrations were the same in all cases, and independent of the initiator concentration. The equilibrium concentrations with ethanol were: [ M A N ] = 0 - 5 2 mole/L, [C2HsOH]= 14-7 mole/L, and [ n = 1]=0.98 mole/1. With methanol using methoxide concentrations between 0.12-0.18 mole/L, the average values o f the equilibrium concentrations were [MAN] = 5.07 mole/L, [CH3OH] = 11.07 mole/l., and In = 1] = 1.02 mole/1.
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295
To find out whether the formation of oligomers higher than (n = 1) was also reversible under the general conditions of the ofigomerization, experiments similar t o those carried out with (n= 1) were conducted. The dimer (RO-C2HsO) was heated with sodium ethoxide in ethanol, and even after 8 hr no MAN, no (n= 1) or a higher oligomer was found in the reaction mixture, indicating that in the ollgomerization system studied the reversibility of the formation of (n = 1) alone was important but not of the higher oligomers. DISCUSSION It was shown that the only significantly reversible step of the oligomerization was that of the decomposition of (n-- 1) to MAN and ROH. Therefore the oligomerization reaction may be represented by the scheme: R O - + CH2-, C(CH3)CN~.
kl "~RO--CH z---C(CH3)C"N (Mc) kl"
(i)
k2 MI-+ROH~.
>MI+RO-
(ii)
>M2-
(iii)
k2"
MI- + M A N " M2-+ROH M2- + MAN'
k3
k4 > M z + R O ks
(iv)
~Ms-
(v)
k6 Ms- + R O H - - : - + M s + RO-, etc.
(vi)
M~ = RO--I-CHr--C(C~,-13XC-~]o_i CH~-~(CI-13)CN Mn I RO--[-CHr--C(CH3XCN)]~--H
The oligomerization system as shown consists of a series of consecutive and competitive reactions, and their exact kinetic treatment is very complex. However we tried by using approximations to estimate some of the rate constants of the reactions.
Equilibrium constant of the addition reaction (steps i and ii) When reactions (i) and (ii) reach equilibrium there exist the following relationships:
kl/k1' = [MI-]/[MAN][RO-]
(I)
k2'/k2 = [MI-] [ROH]/[MI] [RO-]
(2)
Dividing (2) by (I) gives K~. = k1'k2'/klk2 = [ M A N ] [ROH]
[MI]
(3)
Substitution of the values of [MAN], [ROH] and [MI] in Eqn. 3 with the equilibrium concentrations determined before, gives the following equilibrium constants: K~. (methanol) = 55 _ 5 mole/l. K,q. (ethanol)= 7.8 mole/l.
296
GIORA TEICHMAN and ALBERT ZILKHA
Rate constants of initiation, propagations and terminations
In order to estimate these rate constants, two plausible assumptions were made. (a) The oligomerizations were carried out in bulk, and it was assumed that the changes in the composition of the reaction medium did not affect appreciably the rate constants. (b) A stationary state was assumed for the concentrations of the different anions present. This assumption is only true for the middle of the reaction (between about 15-150 mill). According to the kinetic scheme, d[M2] d----~ = k4[M2--] [ROH] and
d[M3] dt = k6[Ma-] [ROH]
In the assumption that [Mz-] and [M3-] are constant, it follows that for a certain reaction time, t, [M2] = k4[M2--] fo [ROH]dt
(4)
[M31 = k6[M3-] fo [ROH]dt.
(5)
The integrals, ~ [ROH]dt, were evaluated graphically for any time, t, at which measurements of the different species present in the reaction mixture were made. Using Eqns. 4 and 5, the values of k4[M2-] and k6[M3- ] were thus calculated for the different reaction times (in the range of about 15-150 rain) and were found to be constant for every run within 10-20 per cent error. Since the oligomerizations were carded out under conditions such that no significant amount of tetramer was formed, its concentration may be neglected, and therefore the stationary state concentration of [M3-] is given by ks[M2-] [MAN] = kdM3-] [ROH]
(6)
The stationary state concentration of [M2-] is given by k3[M1- ] [MAN] = k4[M2-] [ROH]+ks[M2-] [MAN] and
k3[M1-] = [ROH]/[MAN].k4[M2-]+ks[M2-]
(7)
The rate of disappearance of the monomer, based on the oligomerization steps, is given by -d[MAN] -- (kl [RO-] +k3[M1- l +ks[M2-]) [MAN]t-kI'[MI-] dt The solution of this differential equation is [MAN], -- [MAN]*+{[MAN]0- [MAN]*}e-pt
(8)
p = kl[RO-]+k3[Mt-]+ks[M2-].
(9)
where
It is based on the assumption that the different anions are present in stationary concentrations. Now [MAN]* should have been the value of [MAN] at infinite reaction
Studies in the Anionic Oligomerization o f Methacrylonitrile by Alkali Metal Alkoxides
297
time, but according to the oligomerization scheme, this value should be zero. Since at infinite reaction time the assumption of stationary state conditions is not legitimate, the value of [MAN]* cannot be the real value of [MAN]® but that obtained by extrapolation to t ® of the stationary state prevailing at the middle of the reaction (for which the equation was derived). Accordinglyp for each run was determined graphically from the slope of the graph ofln ([MAN]t- [MAN] *) vs, t. The value of [MAN]* was chosen to fit the equation so that the linear slope has an intercept at t = 0 equal to In {[MAN] 0 - [MAN]*}. From Eqns 1-9, kl[RO-], k3[M1-], ks[M2-], k4[MZ] and k6[M3-] were evaluated. The values were constant within 15-20 per cent. TABI.~ 5. PS~-DO-RAT~ CONSTANTSFOR THB SYSTEMM E r H A C R Y L O N I T R I I . ~ O L * Run No.
kl[RO-]xl0 s
ka[M1-]xl(P
ks[M2-]xl0 3
k4[M2-]×10 3
k6[Ms-]xl03
IA3 IA2 IC1
14"2 I 1.2 12"5
1"27 2.87 1.84
0"24 0.41 0-65
1-60 5-72 3.50
0"36 0.94 0.17
IA4
5.6
0"36
0
0-56
0
* The run numbers are as in Table 2. The rate constants are in 1. mole -1 min -1.
TABLE 6. STATIONARY C O N ~ T I O N
OF THE ANIONS IN THE SYSTEM METHACRYLONITRILE-METHANOL*
Run No.
[RO-]o x I0 a
[RO-] x I0 a
[ M c ] x 10 3
[M2- ] x 10 3
[Ma-] x 10 3
IA3 IA2 IC1 IA4
180 180 120 120
142 112 85 113
26 40 22 5
10 23 12 2
2 5 1 0
* The run numbers are as in Table 2. The concentrations are in mole/1.
TAeI~ 7. RATZ CONS'rANTSVOlt THE svsrmd METHACRYLONITRILE--METHANOL* Run
No.
kl
k3
k5
k4
k6
IA3 IA2 IC1 IA4
0.100 0.100 0"147 0"050
0-049 0-072 0-084 0"072
0"024 0.018 0-054 --
0.160 0.249 0"292 0"280
0"180 0.190 0" 170 --
Average Standard
0.120
(~060
0.036
0.250
0.180
deviation
(0-031)
(0-016)
(0.016)
(0-042)
(0.008)
* The Run Nos. are as in Table 2. The rate constants are in 1. mole -1 rain -1.
G I O R A T E I C H M A N and ALBERT Z I L K H A
298
TAm.E 8. PSEUDO-RATECONSTANTSFOR THE SYSTEMMETHACRYLONITRILE--ETHANOL* Run NO.
kl[RO-]xl03
k3[M1-]x103
ks[M2-]xl03
k4[M2-]x103
k6[M3-]x103
A D
14"5 6"5
0.493 1.220
0.103 0.450
0.684 2-300
0"181 1-300
E C
11-8 9"1
0.387 0-017
0.102 0.004
0-930 0.021
0.330 0.006
* The run numbers are as in Table 3. The rate constants are in 1. mole -1 min-L TABLE 9. STATIONARY CONCENTRATION OF THE ANIONS IN THE SYSTEM METHACRYLONITRII.EETHANOL* Run
No.
[gO-J0 × 103
A D E C
87 100 87 17"2
[RO-] x 103 30 12 25 15"7
[Ml-] x 103 28 38 24 0"8
[ M 2 - I x 103
[Ms-] x 103
19 31 22 0"5
10 19 16 0.2
* The run numbers are as in Table 3. The concentrations are in mole/litre.
TABLE 10. R A n CONSTANTS FOR THE SYSTEM MErHACRYLON1TRILE-LrrHANOL * Run
No.
kl
ks
ks
k4
ke
A D E C
0-485 0.530 0.472 0-576
0.0176 0.0320 0.0161 0.0225
0-0054 0.0145 0-0064 0-0076
0-036 0-074 0.042 0.032
0.018 0.068 0.021 0.025
0.520
0.023
0.0093
0-049
0.039
(0.031)
(0-006)
(0.0036)
(0-017)
(0.021)
Average Standard deviation
* The run numbers are as in Table 3. The rate constants are in L mole -x min-l.
To estimate the rate constants alone, we assumed the f o l l o w i n g plausible assumption, [M~-] I> [M2-] i> [M3-']
(I0)
[RO=]0 = [RO-] + [MI-] + [M2-] + M3-].
(II)
Stoichiometrically,
From these last two relationshipsusing least square analyses, we sought numerical values for the stationarystateconcentrationsof the differentanions which would give approximately constant values for the rate constants of the differentreactions. The resultsof these calculationsare given in Tables 5-7 for C H 3 0 - / C H 3 O H and in Tables 8-I0 for CzHsO-/CzHsOH oligomerization.
Studies in the Anionic Oligomerization of MethacrylonRrileby Alkali Metal Alkoxides
299
Comparison of the rate constants of the oligomerizations From Tables 7 and 10 it is seen that for oligomerizations in both methanol and ethanol, the initiation rate constant kl is greater than both the rate constants of propagation. In the first case kl/k3=2, and in the second case kl/k3--23. This is because k3 (methanol)> k3 (ethanol) while kx (ethanol)> kl (methanol). The former may be due, at least in part, to the greater basicity of the anion M1- with methoxy end group, as compared to that with an ethoxy end group (compare the pKo of methoxy acetic acid=3.53 with that of ethoxy acetic acid=3.60). °) The larger value of kx (ethanol) may be due to the greater nucleophilic activity of the initiator, C2H50-. Such differences between the rate constants of initiation and propagation were observed in a similar system, namely that of the polymerization of acrylonitrile in dimethylformamide by CH30-/CH3OH, where the initiation rate constant (as measured from cyanoethylation) was about seventeen times greater than the propagation rate constant. (4) These differences between the rates of initiation and propagation may explain the observed changes in ] ~ , with conversion (Figs. 12-14). Thus up to about 20-30 per cent conversion there was only a slow increase of D-P~, which was followed by a faster increase at higher conversions. Due to the higher values of kt, most of the monomer is initially consumed in the formation of (n--1) in a relatively fast reaction. The equilibrium of the addition reaction is achieved rapidly, and after this stage, the monomer consumed is used in the formation of the higher oligomers at a slower rate. The observed reversibility of the addition reaction, as compared to the further oligomerization reactions, also predicts that, on lengthening the reaction time, it would be possible to increase the ]SIsn by converting the (n = 1) to higher oligomers. In the previous oligomerization of MAN in dimethylformamide by CH30-/CH3OH, an induction period was observed before the formation of high oligomers; it was dependent on the variables of the polymerization system. (1) It seems that the cause for this is the fast addition reaction, and the induction period is the time required to achieve the equilibrium of the addition reaction. Differences between the rates of the propagation reactions were also observed. Thus k3/ks=l'7 (methanol) and k3/ks--2"5 (ethanol). This means that the anion CH30,-CHz--C-(CH3)(CN) is more active than the dimer anion C H 3 0 - - C H 2 C(CH3)(CN)---CH2--C-(CH3)(CN ). The former anion seems to be actually the more basic (compare the pKa of methoxy acetic acid---3-53 with that of/3-cyano propionic acid = 3.99). (3) An interesting observation is that the rate constants for termination, k4 and k 6 (protonation by alcohol of the growing anions) are only 2-5 times greater than the propagation rate constants. In the case of ethanol, they are even much smaller than the initiation rate constant, kx. A partial explanation for this behaviour may be found in the fact that the oligomerization was carried out in bulk, not in the presence of excess alcohol or alcohol as solvent. The effective concentration of the alcohol may be reduced by such factors as solvation of the growing anions, of RO-, and of the polar nitrile groups by alcohol, in addition to association and hydrogen bonding between the alcohol molecules. This reduction of the effective concentration of the alcohol may be, at least in part, the cause for the relatively low values of the observed termination rate constants.
300
GIORA TEICHMAN and ALBERT ZILKHA
Another interesting observation is that the estimated stationary concentrations of the different anions, MT, M~- and M~ vary with the reaction conditions (Tables 6 and 9). Under conditions where the addition reaction alone is of importance and almost only (n = 1) is formed, the stationary state concentrations of all these anions are low (Run C, Table 9). On the other hand, under reaction conditions leading to the formation of a high concentration of oligomers with D-P, > 1"5-2, the sum of the concentrations of these anions may reach almost 90 per cent of the total initial concentration of initiator, and are much higher than the stationary concentration of the RO- present in the oligomerization system (Run D, Table 9). This seems to result from the small differences found between the propagation and termination rate constants. Effect of the type of alcohol. When considering the differences in behaviour observed between the various alcohols, it is important to remember that the reactions were carried out under bulk conditions, and there are differences between the dielectric constants of the reaction media. Further, there may be differences in the extent of solvation of the anions by alcohol molecules and in the degree of association of the different alcohols. These factors may themselves lead to quantitative differences in the rate constants, and the differences found must be considered with reservation. However, it can be seen that the initiation rate constant, kl, for C2HsO-/C2HsOH is 4-5 times greater than that for CH30-/CH3OH, and yet the termination rate constants, k4 and k6, differ in the opposite sense. This observation is compatible with the order of acidity of the alcohols, and the order of basicity of the alkoxides. It follows that if the formation of (n = 1) was not an equilibrium reaction, then under the same conditions higher D-'Ps should have been attained in the case of C2HsO-/C2HsOH than in the case of CH30-/CH3OH, even though the propagation rate constants in the former case are about one-third those of the latter. However, the results show that under comparable conditions, lower D--Ps were obtained with ethanol. This may be understood as follows. If the equilibrium constants of the addition reaction, Kin. =
[ROH] [MAN] [MI] '
are compared (55 mole/l, for methanol and 7.8 mole/l, for ethanol) it can be seen that in the oligomerization with methanol, the equilibrium is shifted more to the reactants than in the case of ethanol. This means that after the fast step of the oligomerization in which the addition product, M1, is formed and dynamic equilibrium is established, more monomer will be available for propagation in the case of methanol, due to the high value of K~. This leads to the higher D-Ps found in methanol as compared with ethanol. From Tables 2 and 3, it was seen that the maximum concentration attained of (n = 1) was lower in the case of methanol and was only about half that for ethanol. This also shows that the elimination of a proton from (n = 1) in the reversible reaction, is easier in the case of methanol than in that of ethanol, leading to more availability of M1- (or monomer) for further propagation. These considerations apply equally well to the case of isopropanol in comparison with the more acidic alcohols. It was found that the rate of conversion was in the order isopropanol > ethanol >
Studies in the AnionicOligomerizationof Methacryionitrileby Alkali MetalAlkoxides
301
methanol. The conversion rate is expected to be higher firstlywhen the basic strength of the initiatoris greater leading to higher initiationrates, and secondly when the K~q. value of the addition reaction,which is relativelyfast,is smaller. Thus it can be seen that the experimental results are in accord with the higher initiationrates and lower equilibrium constants of the addition reaction observed with the less acidic alcohols. In the earlier preliminary study of the bulk oligomcrization of M A N , (e) higher D--Pswere obtained in the case of ethanol than methanol, contrary to the findingsof the present study. However, closerexamination of the previous reaction conditions shows first,that the reactions were carried out to high conversions, contrary to the present study. Thus higher I~-Psmay be obtained in converting more of the (n = I) oligomcr, via the reversiblereaction, to higher oligomers. Secondly, the previous oligomcrizations with the two alcohols were not carried out under the same conditions. A higher initiatorconcentration was used in the case of ethanol as compared with methanol (0.272 mole/l, instead of 0-187 mole/L) and as seen from the results of the present study, the concentration of the initiatorisunexpectedly an important factorcontrolling the ]5"i5, higher initiatorconcentrations leading to higher molecular weights. Previously(e)the ~ of the oligomcrization was connected with the concentrations of reactants thus: ..[MAN] Derivation of this equation was based on the simple relationship existing in polymer systems where transfer is dominant, namely [MAN] = Rp/Rt = Kp/Kt[ ROH ]" From the results of the present work, it is seen that this equation is oversimplified. It does not take into consideration the reversibility of the formation of (n = 1), and the differences, in the early stages, of the propagation and termination rate constants. Also it does not show the effect of the initiator concentration on ]515. Therefore the previously derived equation may be regarded as only a rough approximation and the K-values observed ce) are a function of initiator concentration.
Effect of initiator concentration This effect was found to be considerable, and it should be considered when planning oligomerization conditions for attaining a desirable 13"P. N o simple relationship between ~ and initiator concentration could be derived, but ]3-P increased always with increase of the initiator concentration. Increase of the initiator concentration also leads to higher conversion rates, as expected. Although Keq. of the addition reaction is not influenced by the catalyst concentration, the rate of its attainment is expected to rise with increasing initiator concentration. This means that more monomer (or M1-) may be available for oligomerization, thus leading to increase in ]3-15 with increasing initiator concentration. However, many other factors also may be the cause for the dependence of ]515 on initiator concentration. In an ordinary anionic polymerization, where transfer to solvent is dominant, 15P does not depend on initiator concentration. But in the present case, some of the
302
GIORA TEICHMAN and ALBERT ZILKHA
assumptions on which the derivation of the dependence of D-P on the reaction variables are based, such as constancy of the propagation or termination rate constants from the first stages o f the polymerization, are not justified. Further, an important factor in the system is the reversibility of the addition reaction. Thus the observed effect of initiator on ~ in oligomerization reactions m a y be expected.
Effect of the counter-ion While no difference was observed between sodium and potassium alkoxides in the oligomerization, with lithium alkoxides practically no oligomerization occurred. This is interesting since in the previously studied (2) cyanoethylation of methanol in methano[ as solvent, no difference in the rate constants was found between potassium, sodium and lithium methoxides. In that case it is obvious that the various alkoxides are dissociated completely to ions, and thus there was no effect o f the electropositivity of the alkali metal counter-ion. Under the oligomerization conditions investigated, the results obtained indicate that the alkoxide is dissociated to ions in the case of potassium and sodium but is in the form of ion-pairs, or more highly associated, in the case o f lithium, so lowering considerablythe catalyticactivity. Comparison of the behaviour of methacrylonitrile and acrylonitrile under oligomerization conditions Unlike methacrylonitrile, acrylonitrile was not found to yield oligomers under comparable conditions using R O - / R O H solutions. ~) fl-Alkoxy propionitrile was solely formed with all the alcohol present, and excess m o n o m e r was polymerized to relatively high molecular weight polymer. This may be due to a rapid cyanoethylation reaction, which is much faster than propagation, thus leading to complete consumption of the alcohol before the start of the polymerization. Subsequent polymerization will occur in the absence of a chain terminator (alcohol). Further, it seems that the reversibility of the formation o f fl-alkoxy propionitrile is negligible under these conditions, so that no alcohol is released later in the reaction to cause termination o f very short polymer chains and formation of oligomers. REFERENCES (1) B. A. Feit, J. Wallach and A. Zilkha, J. Polym. ScL A2, 4743 (1964). (2) B. A. Fcit, E. Heller and A. Zilkha, J. Polym. Sci..4,4, 1499 (1966). (3) E. A. Braude and F. C. Nachod, editors. Determination of Organic Structures by Phy$ica Methods, p. 578, Academic Press, New York (1955). (4) B. A. Feit, Ph.D. Thesis, Th© Hebrew University, Jerusalem (1962). (5) B. A. Feit and A. Zilkha, J. org. Chem.28, 406 (1963). (6) Y. Barzilai, B. A. Feit and A. Zilkha, unpublished results. ROann6---L'oligom(~risation anionique en vrac de m6thacrylonitrile (MAN) par des solutions R O - / R O H a ~t6 ~tudi~e h temp6rature constante. La disparition des monom/~reset des alcools, e~ la formation des diff~rents oligom6res jusqu'/L n ffi3 (trim/~res) furent suivies quantitativement par chromatographie des gazet le changement de ]~YPdes oligom~res avec conversion fut d6terrnin(~. Les effets du type d'alcool, du rapport [MAN]/[ROH], de la concentration de rinitiateur, et la nature de l'ion d'opposition de m6tal alcalin furent examine. Z[3J~ diminua dans l'ordre m(~thanol> 6thanol > isopropanol, et le taux de conversion vice versa. L,augmentation du rapport [MAN]/[ROH] et de la concentration de l'initiateur augmenta ]3~. Tandis que le meme comportement fut observ~ clans le cas des oxydes alcooliques de sodium et de potassium, pour les oxydes alcooliques dc lithium pratiquementaucune oligorn~'risation n'a lieu meme darts des conditions plus rigoureuses.
Studies in the Anionic Oligomerization of Methacrylonitrile by Alkali Metal Alkoxides
303
Dans les conditions ~tudi~es, la formation du produit d'addition RO---MAN (n ffi 1) ~tait r6versible, ~t l'oppos6 de la formation d'oligom~res plus 6iev~. Le/~q. de la r~action r6versible [MAN] [ROH]/ [RO--MAN] fur ~tabli ~t partir de r6~ctions utilisant ROH en exc~; pour du rn~qhanol il ~tait de 55 mole/L, et pour l'~thanol de 7.8 mole/l. Quelques unes des constantes du rapport des diff~rents stades des r~actions d'oligon~isation pour le m~thanol et l'6thanol furent ~ t l u / ~ s en tenant compte de la r~versibilit6 de la r~tction d'initiation, et en supposant des conditions d'~tat stable dam la concentration des diff&ents anions presents dans le syst~me. La constante du taux de la r ~ ' t i o n d'initiation ~tait plus ~lev~e que les constantes du taux de propagation; la constante du taux de propagation de M l - (formation d'anion dim~re) ~tait plus grande clue celle de M2-. Les constantes de taux de terminaison des diffh~*nts anions oligom~res par ROH n'~taient que de 2-4 fois plus grandes qu¢ ies constantes des taux de propagation correspondants. Les concentrations ~ ~tat stable M~-, M2- et M3furent ~valu~s, et darts certains cas ~taient relativement ~ de telle sorte que leur total ~tait d'autant que 90 ~ de la concentration initiale de l'initiateur d'oxyde d'alcool. La constante du taux d'initiation avec C2H~O-/C2H~OH ~tait d'environ 4-5 fois plus grande que celie avec CH30-/CH3OH, et les constantes de taux de terminaison correspondames ~qaient plus petites clans ie rapport d'un facteur de 4-5, c¢ qui est conforme ~ l'acidit6 des alcools. Sommsrlo---l~ stata studiata la oligomer~r,'~=ione anionica del metacrilonitrile (MAN) in presenza di soluzioni RO-IROH a temperatura costant¢. La scomparsa del monomero • dell'alcol • la formazione dei vari oligomeri fino a nm 3 (trimeri) 6 stata seguita quantitativamente per via gas-cromatografica ed ~ stata determinata la variazione del ]3'P degli oligomeri con la conversione. Si ~ studiata l'influenza del tipo di aicol, del rapporto (MAN)/(ROH), delia concentrazione dell'iniziatore e della natura del metallo alcalino che fa da controione. ]3]s diminuisce in questo ordine: metanolo > etanolo > isopropanolo, mentre l'inverso si verifica per la velocitb di conversione. Un aumento del rapporto (MAN)/(ROH) e della concentrazione dell'iniziatore porta ad un aumento di ]515. Con gli alcolati di sodio • potassio si osserva 1o sUmo comportamento, mentre col corrispondente alcoiato di litio non awiene, anche in condizioni molto pi~ drastiche, alcuna ofigomerJ-~ione. Nelle condizioni di esperienza, la formazione del prodotto di addizione R O - - M A N (n--l) reversibile, contrariamente alia formazione di oligomeri con n maggiore. La /~q. della reazione reversibile, (MAN) (ROI-I)/(RO--MAN), t stata determinata daUe reazioni nsando un ~ di ROH; nel caso del metanolo risulta 55 moli/l., • per l'etanolo 7.8 moli/l. Alcune delle costanti di veloci~ dei vari stadi delle reazioni di oligomeri per il metanolo • per l'etanolo sono state determinate tenendo conto della reversibiliUt della reazione di iniziazione • assumendo condizioni di stato stazionario per le concentrazioni dei vari anioni prescnti nel sistema. La costante di velociflt della reazione di iniziazione ~ maggiore delie costanti di velocit~ di propa~aTione; la costante di velocit~ della propagazione di M l - (formazione dell'anione dimero) ~ maggiore di quella di M2-. Le constanti di velocit~ della terminazione dei vari anioni oligomeri da parte di ROH sono solo 2-4 V pi/~ grandi delle corrispondenti costanti di propagazione. Le concantrazioni stazionarie di M~-, M2- e M3- sono state valutate e in alcuni casi risultavano relativamente elevate tanto chela loro somma risultava pari al 90 por cente della concentrazione iniziale dell'iniziatore alcolato. Le costanti di velociUt di iniziazione con C2HsO-/C2HsOH risultavano 4-5 V pi~ grandi di quelie con CH30-/CHsOH • le corrispondenti costanti di velocit~ di terminazione erano pit~ piccole di un fattore 4-5: cib ~ in accordo con l'acidit~ degli alcoli. Zussameafassag--Die anionische Oligomerisation in Substanz yon Methacrylnitril (MAN) durch RO-/ROH LOsungen wurde bei konstanter Temperatur untersucht. Die Abnahme von Monomerund Alkohol, sowie die Bildong der verschiedenen Oligomeren bis zu n = 3 (Trirnere) wurde durch Gaschromatographie quantitativ verfolgt, und die Verinderung des [YP der Oligomeren mit dem Urnmtz bestimmt. Der EinfluB tier Art des Aikohols, des (MAN)/(ROH) VerhiUtnisses, der Initiatorkonzentration und der Natur des Alkalimetall Gegenions wurde untersucht. Der D'P nahm ab in der Reihenfolge Methanol > ~thanol > Isopropanol, und im umgekehrten Sinne mit dem Umsatz. Eine Zunahme im Verhaltnis (MAN)/(ROH) und in der Initiatorkonzentration ergab eine Zunahme des DI s. Wfihrend/'fir Natrium- und Kaliumalkoxyde gieiches Verhalten beobachtet wurde, fand mit den entsprechenden Lithium-alkoxyden praktisch keine Oligomerisation start, auch unter energischeren Reaktionsbedingungen. Unter den untersuchten Bedingungen war die Bildung des Additionsproduktes, RO-MAN (nffi 1) reversibel, im Gegensatz zur Bildung tier hOheren Oligomeren. Das Kc¢. der reversiblen Reaktion, (MAN)(ROH)/(RO-MAN) wurde dutch Reaktionen mit iiberschiissigem ROH bestimmt. Fiir Methanol ergab sich ein Wert yon 55 Mol/l. und fth",~thanol von 7.8 Mol/L Einige der Geschwindigkeitskonstanten der verschiedenen Stufen der Oligomerisationsreaktionen wurden for Methanol und ,~thanol abgeschatzt unter Beriicksichtigung der Reversibili~t der Startreaktion und unter der Annahme von steady-state Bedingongen fOr die Konzentration tier verschiedenen im System vor-
304
GIORA TEICHMAN ann ALBERT ZILKHA
handenen Anionen. Die Geschwindigkeitskomtante der Startreaktion war gr6Ber als die Geschwindigkeitskonstanten des Wachstumsschrittes. Die Geschwindigkeitskonstame des Wachstums von M l - (Bildung des Dimer-Anions) war gr6Ber als die f'tir M2-. Die Geschwindigkeitskonstanten f'ur den Abbruch der verschiedenen Oligomer-Anionen durch ROH waren nur 2-4 mal gr6Ber also die entsprechenden Wachstumskonstanten. Die steady state Konzentrationen yon MI-, M2- und M3wurden bestimmt und waren in einigen Fallen relativ hoch, so dab ihre Summe bis zu 9 0 ~ der anf~mglichen Konzentration an Alkoxyd-Initiator ausmachte. Die Geschwindigkeitskonstante der Startreaktion mit C2HsO-/C2HsOH war etwa 4-5 mal gr6Ber als die mit CH30-/CH3OH, und die entsprechenden Abbruchskonstanten waren um den Faktor 4-5 kleiner, was mit der Azidit~t der Alkohole in Einldang steht.