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On the nature of the active centres in the anionic polymerization of methacrylonitrile—II. Initiation with organomagnesium compounds
t~m'ol}t,un Potvmcr Journal. Vol. 15. pp. 503 to 507 '~ Pergamon Press Lid 1979. Printed in Great Britain
0014-3057'79 0501-0503502.(Y,) 0
O N THE N A T U R E OF THE ACTIVE C E N T R E S IN THE A N I O N I C P O L Y M E R I Z A T I O N O F METHACRYLONITRILE--II INITIATION
WITH
ORGANOMAGNESIUM
COMPOUNDS*
CH. B. TSVETANOV Central Laboratory for Polymers, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria (Received 18 July 1978) Abstract--It is shown by i.r. spectroscopy that the anionic polymerization of methacrylonitrile using
organomagnesium compounds as initiators leads to two types of "'living" ends i.e. 1-)
+
~-)
~ ( C H 3 ) C N , MgX(X = Br, R) and
2+
(-)
~ ( C H 3 J C N , Mg, NC(CH3)C--.
The active centres form complexes with the monomer, as well as with the nitrile groups of the monomer units in the polymer chain, in a hydrocarbon medium. The complexes are characterized by bands in the 2250-2280cm-1 region. Addition of donors destroys the complexes and in some cases (hexamethylphosphorous triamide) converts the complex of organomagnesium compound with monomer into a "living" end. In ethers as solvent, the dominant side-reaction is addition of the initiator to the nitrile group of the monomer.
Study of the active centres (AC) in the anionic polymerization of acrylonitrile (AN) and methacrylonitrile (MAN) gives an insight into the mechanisms of initiation and growth of the polymer chain as well as of the deactivation of the initiator or of the AC. We earlier reported physico-chemical data on the "living" ends of A N and M A N with alkali metals as counterions [1-3]. i.r. Spectroscopy is especially convenient for studying the character and changes of the AC during polymerization. These spectra of solutions containing AC of "living" A N and M A N chains exhibit characteristic absorption bands in the 2020-2060 c m - 1 region [2, 3]. Two types of AC interaction have been found. The first is the so-called " p e n u l t i m a t e " effect characterized by a shift towards higher wave numbers of the bands at 2020-2060 c m with increase of the average n u m b e r of included m o n o m e r i c units, the interaction of the AC being limited to the third unit from the "living" end. The other interaction ig the observed formation in hydroc a r b o n media of a complex between the lithium counterion and pivalonitrile [3] similar to tha.t between Lil and a nitrile [4], characterized by the appearance of a band at 2255-2260 c m - 1. The same band is also observed when more than 3 units of A N or M A N add to the AC. The complexes formed are probably intermediates in the Z i e g l e r - T h o r p e side reaction [5]. The study of AC with bi-valent metal counterions was considered worthwhile. In these cases the following "living"' ends can be observed:
(-)
2+
(-)
- - C ( C H 3 ) C N , Mt, N C ( C H 3 ) C - AC II In the present study we paid attention to the range of organomagnesium c o m p o u n d s such as organomagnesium halides free from ether, typical Grignard reagents, dialkylmagnesium compounds in ethereal and h y d r o c a r b o n media as well as alcoxyalkylmagnesium •reagents [6]. • The aim of the present work was to investigate the nature of AC with magnesium counterions during anionic polymerization of MAN, and also to examine the character of the interaction between the AC and the m o n o m e r or m o n o m e r units. EXPERIMENTAL
The experiments were conducted using a vacuum apparatus. The initiators were prepared thus: cyclohexylmagnesium bromide (RMgBr) in.ether was prepared in the usual manner; dodecylmagnesium bromide (RaMg2Br) in toluene was obtained after reacting dodecyl bromide with magnesium in ether, filtering the reaction mixture, distilling off the solvent, dissolving the residue in toluene, again distilling off the solvent and heating the residue at 70¢ for 8 hr at a pressure of 10-5 Tort; the ether-free" organomagnesium compound was dissolved in toluene; d ialkylmagnesium compounds (R = diethyl dipropyl-, di-tert, butyl-) were prepared by removing the halogen from Schlenk's equilibrium[7 ] 2RMgX ~ R2Mg + MgX2 by adding dioxane; ether-free dialkylmagnesium compound was synthesized by the procedure described above; alkoxyalkylmagnesium compound (ethylmagnesium methoxyethanolate) was made by the procedure of Narita et al. [8]. The solvents and the monomer (MAN, ether, dioxane, toluene, THF) were purified as already described [2] and [3]. The MAN active centres were obtained by allowing its vapours to reach the vigorously stirred solutions
( - ) ~
~ O ( C H 3 ) C N , MtX AC ! * The contents of this paper have been reported at Microsymposium "Ionic polymerization" held in Varna, Bulgaria 1977. 503
504
CH. B. TSVETANOV
of the organomagnesium compounds at -70, - 4 0 and 10~. The infrared spectra were recorded on a UR-20 Carl Zeiss Jena soectrophotometer using KBr cells of 0.26, 1.0
~:H3
~H3 CH3"-~;H
RMgBr
+
~
~;H3
CHa'--~;H
CN
"
CHg--C;H
CN.RMgBr
"~CN 2245 cm-1
RESULTS AND DISCUSSION
I. Organomagnesium ha/ides as initiators of polymerization The polymerization mixtures of cyclohexylmagneslum bromide in ether with monomer/initiator (M/I) ratios between 2 and 4 exhibit absorption bands at 2020 cm- 10), 2234 cm- l(II) and 2270 cm- l(III) as well as at 1650cm -~ (Fig. 1). Bands I, III as well as one at 1650 cm-~ disappear on addition of ethanol. Band I indicates the presence of +
(-)
R--C=NMgBr
~CN 2285 cm-1
and 2.0mm optical paths. The spectra taken in the -30-30 ° temperature interval were recorded with the aid of a special system of temperature-controlled cells and a freon-eontaining refrigerator [9].
---C(CH3)CN, MgX as an AC, to be designated as AC I. In structure and CH RMgX
since this process would give rise to an AC absorbing in the 2020-2040 cm-1 region. The following reaction can be envisaged:
"~C_~1658 cm"1
The bands at 1658 cm- ~ and 2285 cm- ~ disappear on addition of ethanol. Absorption bands I, II and one at 1610cm -~ appear if the interaction between RMgBr and MAN takes place in THF instead of ether. No complex is formed in THF as indicated by the absence of band III, to be ascribed to the better electron donating properties of THF compared with ether which in turn causes the magnesium coordination sphere to consist of THF molecules only. An entirely different picture is observed when polymerization is conducted in a hydrocarbon medium. Bands I, II and III, but not, however, that at 1650cm -1 can be observed for initiation with RMgBr-etherate in toluene. This is an indication that under these conditions the processess of initiation and addition of several monomer units proceed without addition of initiator to the monomer nitrile function which is characterized by absorption bands in the 1600-1660em-a region. The following reactions are believed to take place:
complex
RMgX'NC--C=CHz
+ CH2=C ~ , ~
c'.
~)2270 cm -1
ct., -
R~CH2-~ ~ , MgX
+
AC I , ~ 2 0 2 O c m -1
CN
geometry, they resemble the AC with alkali metal counterions [2, 3]. Band II is due to MAN units included in the polymer chain; band III arises from the complex formed between the AC and the nitrile groups of the polymer units or the monomer [3]. The band at 1650 cm- 1 is probably due to the
I
------C---~-NM gX function obtained by addition of the initiator to the nitrile group of the monomer. These assumptions are supported by the infrared spectra of the reaction products formed by mixing RMgBr with isobutyr0nitrile at room temperature. Three characteristic bands, one at 2245 cm-~ due to unreacted nitrile, one at 2285 cm- ~ due to the nitrile-magnesium salt complex and one at 1658cm -~ are obselved. Obviously, no proton transfer takes place under these conditions, R,
R
X
"" / "', /
R/
""R/
(_),~
(+,_~.~)
~C(CH3)CN. MgX
I R~N,
X
"'- /
"'R/
"""
+MAN
*HM P T . ~
2020 cm -1
MgX.
No polymer formation takes place on mixing the ether-free organomagnesium halide [(C~2H3s)3Mg2Br]n in toluene with MAN. The infrared spectrum of the mixture exhibits an absorption band at 2263cm-1: Adding equivalent amounts of THF causes band I[I to disappear, while the intensity of the band due to the free monomer (2225cm-~1 increases. Addition of a strong donor (hexamethylphosphorous triamide, HMPT) not only results in the disappearance of band III, but also brings into being band I, an indication of the existence of the characteristic centre of growth AC I (Fig. 2). Higher M/I ratios lead to polymer formation. The observed interactions can be represented schematically thus:
R
"', /
"'x /
i.e. the complex (band IIl) is not converted into the corresponding ketimine
MAN
complex
M """ f
~
MAN ~CN 2263
+THF
RMgX.THF + MAN 2225 cm -I
Active centers in the anionic polymerization of methacrylonitrile
lI
505
I00
I00
5.
E~
-7-Q.
.o
a.
.E) <
ot.,-,_--....-.::.;.:.:. 2000
..........
2100
.
0
[
.
.
~. ~-_
-..._.-.... 1-.:.~
2000
I 2100
-.. -.._..
2200
.
.
; . -,... ',Y~...-~,~...
. . <~~ _ / l 2200
cm-I
cm-I
Fig. 1. Infrared spectra of the reaction mixture MANRMgBr at various MAN/Mg ratios: - 1.2, Ether: . . . . . . 0.7. THF: . . . . . 1.7. THF.
Fig. 2. Infrared spectra of the reaction mixture MAN[R3Mg2Br], in toluene: ; . . . . . after adding THF: . . . . . after adding HMPT.
Dielectric susceptibility and electric conductivity measurements carried out o n the organomagnesium c o m p o u n d during " t i t r a t i o n " with M A N show that the M A N / M g ratio in the complex is 2:1 (Fig. 3).
iminonitrile anion for, if it was due to a ZieglerThorpe reaction product, one would expect with increase in M/I the intensity of the 2120cm 1 band to increase as well: the reverse is actually observed. One can assume that in T H F an equilibrium exists between the various ionic forms and that the band at 2120cm - I is characteristic for one of them. lonic equilibria in solutions are influenced by the type of medium and the temperature. No change in the intensity ratios of the bands at 2020cm i and 2120cm occurs on changing the solvent (THF with dioxane or toluene) or by recording the infrared spectra within the temperature range - 3 5 ° to + 30 c. These experiments do not confirm the assumption of the existence of various ionic forms and associates in equilibrium. The formation of a metallation product is most likely :
II. Dialkylmagnesium compounds as initiators of poly,m, rizat ion Bands I, II and III as well as those at 1 6 0 5 c m - 1 and 1665 c m - ' are registered on initiation of polymerization in dioxane (M/I between 1 and 4 at 10:). Addition of equivalent amounts of ethanol results in the disappearance of bands I and III as well as of the bands in the 1600-1660 c m - 1 interval; new bands appear at 2048 and 1630cm -x. Polymerization in dioxane obviously leads to the formation of AC I of structure 1-)
1+)
---C(CH 3)CN, MgR. The addition of the initiator to the nitrile group (bands at 1607 c m - 1 and 1665 c m - x) proceeds simultaneously, the b a n d at 1630 c m - 1 being characteristic of the ketimine function
\ C~NH
/ on protonation. The appearance of a new band at 2048 c m - I on addition of an equivalent a m o u n t of ethanol is worth noting, the b a n d vanishing totally when the ethanol is in considerable excess (5:1). It can be assumed that the m a x i m u m at 2048 c m - 1 is characteristic of the group (-) (+) ,----.-.-.-.,.----~ ~
--C(CH3)CN, MgOR. Indeed, on initiation of M A N polymerization with alkoxyalkylmagnesium and particularly with ethylmagnesium methoxyethanolate, only one band at 2060 c m - 1 is observed. Polymerization in T H F (M/I = 1) leads to the appearance of another band at 2120cm -1, but no bands in the region 2 2 6 0 - 2 2 8 0 c m - 1 are seen. The maximum at 2120cm - I cannot be ascribed to the
t~H3 CH2~-C
+
+
R2Mg
-
RH
+
C / ' ,"{''CH2
~M--'~"
CN
since on acidification of the reaction mixture the presence of M A N was established by gas chromatographic analysis. Bands at 2034, 2075, 2120 and 2235 cm t appear when the polymerization is conducted in toluene in the presence of catalytic amounts of T H F . Even insignificant quantities of T H F lead to the appearance of the m a x i m u m at 2 1 2 0 c m - 1 . A new band at 2075 c m - 1 (IV), becoming more intense with increase of the initial M/! ratio, is also found. The use of ether-free dialkylmagnesium initiators brings about formation of bands at 2034, 2075, 2235 and 2270 c m - l : no bands are, however seen between 1600 and 1660 c m - l, (Fig. 4). Only bands I and llI are observed at ratios of M/I = 1. With the increase of M/I, band I decreases at the expense of band IV: the intensity of band III also decreases. The band at 1650cm -1 and bands I and 1V arise on mixing isobutyronitrile and dialkylmagnesium compounds in THF. The metallation reaction, as well as the addition to the nitrile group, are comparatively slow at room temperature. The bands at 2034 and 1650cm -~ appear first while that at 2075cm 1 is observed after 24hr, its intensity growing at the
,o_
2,oo
506
CH. B. TSVETANOV
two types of AC i,e. AC I and 2+
[(CH3)C- CN]2 Mg (AC II)
I
0
I
I
I
2
3
MAN/Mg Fig. 3. Dielectric susceptibility E and equivalent conductivity measurements of O.05n [RaMg2Br]. solutions in toluene at different M A N / M g ratios.
expense of that of Band I. All three bands vanish on protonation with ethanol, while the maximum due to a free nitrile (2245 cm- ~) increases in intensitv and there is a band at 1630cm -1 ascribed to ) C = NH. The interaction between isobutyronitrile and R 2 Mg can be depicted thus :
(CH~)zCH ÷ RzMq ~ CN ~
a
- R H ~
in contrast to the polymerization in ether media where only AC I was observed. Initiating the polymerization with R2Mg in toluene results in a decrease of intensity of band III at lower values of the ratio of AC II/AC I. A possible explanation for this is the assumption that the AC-nitrile complex characterized by band III arises solely from AC I, since the relative decrease in the concentration of AC 1 at higher initial M/I ratios should lower also the concentration of the complex. Indeed, when a magnesium atom is attached to two (-)
--C(CHa)CN groups, its free coordination sites will be occupied by the nitrogen atoms of the terminal units thus hindering the approach of another nitrile group. A similar coordination of magnesium can also be
I
L
CN
~
CNJ2
(CH3)zCH R~C~---N,MqR
The consecutive formation of the bands at 2035 and 2075 c m - 1 as well as their conversion into the band at 2245 cm-1 corresponding to the initial nitrile on protonation, give grounds for their assignment to the mono- and double derivatives of organomagnesium compounds of model AC in the polymerization of MAN. The identity of the bands obtained with reaction mixtures from MAN polymerization in toluene and with those from the interaction between R2Mg and isobutyronitrile (bands I and IV) shows that in a hydrocarbon medium the growth of a polymer chain during anionic polymerization of MAN proceeds via
I00
I
o
r,
.".
/ ~ ' , , ........ ~, ..
0
I 2000
.... .. ../ \ .;;,',~
I 2100
1 2200
I
cm-I
Fig. 4. Infrared spectra of the reaction mixture MAN-tBu:Mg in toluene at various MAN/Mg ratios: - - 0.9, . . . . . 2.4: . . . . . 3.2.
expected to occur with AC during polymerization of MAN with RMgOCH2CH2OCH a, since in this case in toluene no band at 2260-2280 cm- 1 is observed. CONCLUSION
The investigations have shown that two types of AC are formed during polymerization of MAN initiated by organomagnesium compounds. Growth of the polymer chain in ether solvents proceeds through AC I on initiation with organomagnesium halides or dialkylmagnesium compounds. The "living" end is AC II for polymerization in toluene initiated with R2Mg. It seems reasonable to expect that the different mechanisms of growth should lead to significant alterations in the molecular masses and in the degree of ordering of the polymer. It is interesting that Joch et aL [10] suggest, on the basis of molecular mass distribution, the existence of two types of AC in the polymerization of MAN in toluene initiated with diethylmagnesium. The Ziegler-Thorpe cyclization occurs to a lesser extent with organomagnesium compounds as initiators. The main side reaction is addition of the initiator to the nitrile group giving rise to ketimines. This side reaction becomes less marked when the polymerization is conducted in toluene since there are no intensive bands due to the
I
--C~-N, MgX
and
I
~ N H
groups. The question regarding the origin of the band at 2120cm -1 which appears during polymerization in THF or in other media containing THF remains
Active centers in the anionic polymerization of methacrylonitrile--ll open. Our proposed mechanism of metallation is refuted by V a n k e r c k h o v e n and Van Beylen [11] while Tsuruta and Erussalimsky [12, 13] assume that it operates. C o o r d i n a t i o n c o m p o u n d s of magnesium with molecules of the m o n o m e r or with nitrile functions of the monomeric units (bands at 2250-2280 c m - ~ ) are formed in n o n p o l a r solvents. This observation is especially worth noting, since it permits a more detailed investigation of the initial act of complex formation between the initiator and the m o n o m e r or between the AC and the monomer. The organomagnesium c o m p o u n d s are suitable for such studies because of their lower reactivity: thus in h y d r o c a r b o n media, they do not initiate polymerization at temperatures below - 6 0 : . Addition of very small quantities of H M P T converts the complexes into AC. This fact confirms the assumption of Erussalimsky and Krasnoselskaya [14] that, in the presence of H M P T , the rate of AN polymerization in toluene initiated with organomagnesium halides increases because of the higher concentration of the AC.
Acknowledgement The author thanks Professor I. M. Panayotov for helpful discussions.
507
REFERENCES
1. Ch. l-svetanov, V. Zgonnik, B. Erussalimsky and 1. Panayotov, Justus Liebigs Annln Chem. 763, 53 (1972). 2. Ch. Tsvetanov, I. Panayotov and B. Erussalimsky. Europ. Polym. J. 10, 557 (1974). 3. Ch. Tsvetanov and I. Panayotov, Europ. Polym. J. ! l, 209 (1975). 4. I. S. Pominov and A. Z. Gadjiev, lzr. Vuzov SSSR, Ph)'.~. 19, (1965). 5. A. Ottolenghi and A. Zilkha, J. Poiym. Sci. AI, 687 (1963). 6. B. J. Wakefield, Organomet. Chem. Rer. 1, 13l (t966). 7. W. Schlenk and W. Schtenk, Ber. 62, 920 (1929). 8. T. Narita, T. Yasumura, T. Tsuruta, Polym. 3". 4, 421 (1973). 9. V. N. Zgonnik, K. K. Kalnin'sh, Ch. B. Tsvetanov and N. I. Nikolaev, Vysokomolek. Soedin. 15A, 900 (1973). 10. Y. Joch, S. Kurihara, T. Sakurai, T. Tomita, J. Polym. Sci. A-I 8, 2383 (1970): Y. Joch, S. Kurihara. T. Tomita, J. Polym. Sci. B 9, 1463 (1971). 11. H. Vankerckhoven and M. Van Beylen, Europ. Polym. 9'. 14, 189 (1978). 12. N. Kawabata and T. Tsuruta, Makromolek. Chem. 98, 262 (1966). 13. B. L. Erussalimsky and A. V. Novoselova, Fa~erJbr~ch. Text Tech. 26, 293 (1975). 14. B. L. Erussalimsky and I. G. Krasnoselskaya, Makromolek. Chem. 123, 70 (1969).
0014-3057'79 0501-0503502.(Y,) 0
O N THE N A T U R E OF THE ACTIVE C E N T R E S IN THE A N I O N I C P O L Y M E R I Z A T I O N O F METHACRYLONITRILE--II INITIATION
WITH
ORGANOMAGNESIUM
COMPOUNDS*
CH. B. TSVETANOV Central Laboratory for Polymers, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria (Received 18 July 1978) Abstract--It is shown by i.r. spectroscopy that the anionic polymerization of methacrylonitrile using
organomagnesium compounds as initiators leads to two types of "'living" ends i.e. 1-)
+
~-)
~ ( C H 3 ) C N , MgX(X = Br, R) and
2+
(-)
~ ( C H 3 J C N , Mg, NC(CH3)C--.
The active centres form complexes with the monomer, as well as with the nitrile groups of the monomer units in the polymer chain, in a hydrocarbon medium. The complexes are characterized by bands in the 2250-2280cm-1 region. Addition of donors destroys the complexes and in some cases (hexamethylphosphorous triamide) converts the complex of organomagnesium compound with monomer into a "living" end. In ethers as solvent, the dominant side-reaction is addition of the initiator to the nitrile group of the monomer.
Study of the active centres (AC) in the anionic polymerization of acrylonitrile (AN) and methacrylonitrile (MAN) gives an insight into the mechanisms of initiation and growth of the polymer chain as well as of the deactivation of the initiator or of the AC. We earlier reported physico-chemical data on the "living" ends of A N and M A N with alkali metals as counterions [1-3]. i.r. Spectroscopy is especially convenient for studying the character and changes of the AC during polymerization. These spectra of solutions containing AC of "living" A N and M A N chains exhibit characteristic absorption bands in the 2020-2060 c m - 1 region [2, 3]. Two types of AC interaction have been found. The first is the so-called " p e n u l t i m a t e " effect characterized by a shift towards higher wave numbers of the bands at 2020-2060 c m with increase of the average n u m b e r of included m o n o m e r i c units, the interaction of the AC being limited to the third unit from the "living" end. The other interaction ig the observed formation in hydroc a r b o n media of a complex between the lithium counterion and pivalonitrile [3] similar to tha.t between Lil and a nitrile [4], characterized by the appearance of a band at 2255-2260 c m - 1. The same band is also observed when more than 3 units of A N or M A N add to the AC. The complexes formed are probably intermediates in the Z i e g l e r - T h o r p e side reaction [5]. The study of AC with bi-valent metal counterions was considered worthwhile. In these cases the following "living"' ends can be observed:
(-)
2+
(-)
- - C ( C H 3 ) C N , Mt, N C ( C H 3 ) C - AC II In the present study we paid attention to the range of organomagnesium c o m p o u n d s such as organomagnesium halides free from ether, typical Grignard reagents, dialkylmagnesium compounds in ethereal and h y d r o c a r b o n media as well as alcoxyalkylmagnesium •reagents [6]. • The aim of the present work was to investigate the nature of AC with magnesium counterions during anionic polymerization of MAN, and also to examine the character of the interaction between the AC and the m o n o m e r or m o n o m e r units. EXPERIMENTAL
The experiments were conducted using a vacuum apparatus. The initiators were prepared thus: cyclohexylmagnesium bromide (RMgBr) in.ether was prepared in the usual manner; dodecylmagnesium bromide (RaMg2Br) in toluene was obtained after reacting dodecyl bromide with magnesium in ether, filtering the reaction mixture, distilling off the solvent, dissolving the residue in toluene, again distilling off the solvent and heating the residue at 70¢ for 8 hr at a pressure of 10-5 Tort; the ether-free" organomagnesium compound was dissolved in toluene; d ialkylmagnesium compounds (R = diethyl dipropyl-, di-tert, butyl-) were prepared by removing the halogen from Schlenk's equilibrium[7 ] 2RMgX ~ R2Mg + MgX2 by adding dioxane; ether-free dialkylmagnesium compound was synthesized by the procedure described above; alkoxyalkylmagnesium compound (ethylmagnesium methoxyethanolate) was made by the procedure of Narita et al. [8]. The solvents and the monomer (MAN, ether, dioxane, toluene, THF) were purified as already described [2] and [3]. The MAN active centres were obtained by allowing its vapours to reach the vigorously stirred solutions
( - ) ~
~ O ( C H 3 ) C N , MtX AC ! * The contents of this paper have been reported at Microsymposium "Ionic polymerization" held in Varna, Bulgaria 1977. 503
504
CH. B. TSVETANOV
of the organomagnesium compounds at -70, - 4 0 and 10~. The infrared spectra were recorded on a UR-20 Carl Zeiss Jena soectrophotometer using KBr cells of 0.26, 1.0
~:H3
~H3 CH3"-~;H
RMgBr
+
~
~;H3
CHa'--~;H
CN
"
CHg--C;H
CN.RMgBr
"~CN 2245 cm-1
RESULTS AND DISCUSSION
I. Organomagnesium ha/ides as initiators of polymerization The polymerization mixtures of cyclohexylmagneslum bromide in ether with monomer/initiator (M/I) ratios between 2 and 4 exhibit absorption bands at 2020 cm- 10), 2234 cm- l(II) and 2270 cm- l(III) as well as at 1650cm -~ (Fig. 1). Bands I, III as well as one at 1650 cm-~ disappear on addition of ethanol. Band I indicates the presence of +
(-)
R--C=NMgBr
~CN 2285 cm-1
and 2.0mm optical paths. The spectra taken in the -30-30 ° temperature interval were recorded with the aid of a special system of temperature-controlled cells and a freon-eontaining refrigerator [9].
---C(CH3)CN, MgX as an AC, to be designated as AC I. In structure and CH RMgX
since this process would give rise to an AC absorbing in the 2020-2040 cm-1 region. The following reaction can be envisaged:
"~C_~1658 cm"1
The bands at 1658 cm- ~ and 2285 cm- ~ disappear on addition of ethanol. Absorption bands I, II and one at 1610cm -~ appear if the interaction between RMgBr and MAN takes place in THF instead of ether. No complex is formed in THF as indicated by the absence of band III, to be ascribed to the better electron donating properties of THF compared with ether which in turn causes the magnesium coordination sphere to consist of THF molecules only. An entirely different picture is observed when polymerization is conducted in a hydrocarbon medium. Bands I, II and III, but not, however, that at 1650cm -1 can be observed for initiation with RMgBr-etherate in toluene. This is an indication that under these conditions the processess of initiation and addition of several monomer units proceed without addition of initiator to the monomer nitrile function which is characterized by absorption bands in the 1600-1660em-a region. The following reactions are believed to take place:
complex
RMgX'NC--C=CHz
+ CH2=C ~ , ~
c'.
~)2270 cm -1
ct., -
R~CH2-~ ~ , MgX
+
AC I , ~ 2 0 2 O c m -1
CN
geometry, they resemble the AC with alkali metal counterions [2, 3]. Band II is due to MAN units included in the polymer chain; band III arises from the complex formed between the AC and the nitrile groups of the polymer units or the monomer [3]. The band at 1650 cm- 1 is probably due to the
I
------C---~-NM gX function obtained by addition of the initiator to the nitrile group of the monomer. These assumptions are supported by the infrared spectra of the reaction products formed by mixing RMgBr with isobutyr0nitrile at room temperature. Three characteristic bands, one at 2245 cm-~ due to unreacted nitrile, one at 2285 cm- ~ due to the nitrile-magnesium salt complex and one at 1658cm -~ are obselved. Obviously, no proton transfer takes place under these conditions, R,
R
X
"" / "', /
R/
""R/
(_),~
(+,_~.~)
~C(CH3)CN. MgX
I R~N,
X
"'- /
"'R/
"""
+MAN
*HM P T . ~
2020 cm -1
MgX.
No polymer formation takes place on mixing the ether-free organomagnesium halide [(C~2H3s)3Mg2Br]n in toluene with MAN. The infrared spectrum of the mixture exhibits an absorption band at 2263cm-1: Adding equivalent amounts of THF causes band I[I to disappear, while the intensity of the band due to the free monomer (2225cm-~1 increases. Addition of a strong donor (hexamethylphosphorous triamide, HMPT) not only results in the disappearance of band III, but also brings into being band I, an indication of the existence of the characteristic centre of growth AC I (Fig. 2). Higher M/I ratios lead to polymer formation. The observed interactions can be represented schematically thus:
R
"', /
"'x /
i.e. the complex (band IIl) is not converted into the corresponding ketimine
MAN
complex
M """ f
~
MAN ~CN 2263
+THF
RMgX.THF + MAN 2225 cm -I
Active centers in the anionic polymerization of methacrylonitrile
lI
505
I00
I00
5.
E~
-7-Q.
.o
a.
.E) <
ot.,-,_--....-.::.;.:.:. 2000
..........
2100
.
0
[
.
.
~. ~-_
-..._.-.... 1-.:.~
2000
I 2100
-.. -.._..
2200
.
.
; . -,... ',Y~...-~,~...
. . <~~ _ / l 2200
cm-I
cm-I
Fig. 1. Infrared spectra of the reaction mixture MANRMgBr at various MAN/Mg ratios: - 1.2, Ether: . . . . . . 0.7. THF: . . . . . 1.7. THF.
Fig. 2. Infrared spectra of the reaction mixture MAN[R3Mg2Br], in toluene: ; . . . . . after adding THF: . . . . . after adding HMPT.
Dielectric susceptibility and electric conductivity measurements carried out o n the organomagnesium c o m p o u n d during " t i t r a t i o n " with M A N show that the M A N / M g ratio in the complex is 2:1 (Fig. 3).
iminonitrile anion for, if it was due to a ZieglerThorpe reaction product, one would expect with increase in M/I the intensity of the 2120cm 1 band to increase as well: the reverse is actually observed. One can assume that in T H F an equilibrium exists between the various ionic forms and that the band at 2120cm - I is characteristic for one of them. lonic equilibria in solutions are influenced by the type of medium and the temperature. No change in the intensity ratios of the bands at 2020cm i and 2120cm occurs on changing the solvent (THF with dioxane or toluene) or by recording the infrared spectra within the temperature range - 3 5 ° to + 30 c. These experiments do not confirm the assumption of the existence of various ionic forms and associates in equilibrium. The formation of a metallation product is most likely :
II. Dialkylmagnesium compounds as initiators of poly,m, rizat ion Bands I, II and III as well as those at 1 6 0 5 c m - 1 and 1665 c m - ' are registered on initiation of polymerization in dioxane (M/I between 1 and 4 at 10:). Addition of equivalent amounts of ethanol results in the disappearance of bands I and III as well as of the bands in the 1600-1660 c m - 1 interval; new bands appear at 2048 and 1630cm -x. Polymerization in dioxane obviously leads to the formation of AC I of structure 1-)
1+)
---C(CH 3)CN, MgR. The addition of the initiator to the nitrile group (bands at 1607 c m - 1 and 1665 c m - x) proceeds simultaneously, the b a n d at 1630 c m - 1 being characteristic of the ketimine function
\ C~NH
/ on protonation. The appearance of a new band at 2048 c m - I on addition of an equivalent a m o u n t of ethanol is worth noting, the b a n d vanishing totally when the ethanol is in considerable excess (5:1). It can be assumed that the m a x i m u m at 2048 c m - 1 is characteristic of the group (-) (+) ,----.-.-.-.,.----~ ~
--C(CH3)CN, MgOR. Indeed, on initiation of M A N polymerization with alkoxyalkylmagnesium and particularly with ethylmagnesium methoxyethanolate, only one band at 2060 c m - 1 is observed. Polymerization in T H F (M/I = 1) leads to the appearance of another band at 2120cm -1, but no bands in the region 2 2 6 0 - 2 2 8 0 c m - 1 are seen. The maximum at 2120cm - I cannot be ascribed to the
t~H3 CH2~-C
+
+
R2Mg
-
RH
+
C / ' ,"{''CH2
~M--'~"
CN
since on acidification of the reaction mixture the presence of M A N was established by gas chromatographic analysis. Bands at 2034, 2075, 2120 and 2235 cm t appear when the polymerization is conducted in toluene in the presence of catalytic amounts of T H F . Even insignificant quantities of T H F lead to the appearance of the m a x i m u m at 2 1 2 0 c m - 1 . A new band at 2075 c m - 1 (IV), becoming more intense with increase of the initial M/! ratio, is also found. The use of ether-free dialkylmagnesium initiators brings about formation of bands at 2034, 2075, 2235 and 2270 c m - l : no bands are, however seen between 1600 and 1660 c m - l, (Fig. 4). Only bands I and llI are observed at ratios of M/I = 1. With the increase of M/I, band I decreases at the expense of band IV: the intensity of band III also decreases. The band at 1650cm -1 and bands I and 1V arise on mixing isobutyronitrile and dialkylmagnesium compounds in THF. The metallation reaction, as well as the addition to the nitrile group, are comparatively slow at room temperature. The bands at 2034 and 1650cm -~ appear first while that at 2075cm 1 is observed after 24hr, its intensity growing at the
,o_
2,oo
506
CH. B. TSVETANOV
two types of AC i,e. AC I and 2+
[(CH3)C- CN]2 Mg (AC II)
I
0
I
I
I
2
3
MAN/Mg Fig. 3. Dielectric susceptibility E and equivalent conductivity measurements of O.05n [RaMg2Br]. solutions in toluene at different M A N / M g ratios.
expense of that of Band I. All three bands vanish on protonation with ethanol, while the maximum due to a free nitrile (2245 cm- ~) increases in intensitv and there is a band at 1630cm -1 ascribed to ) C = NH. The interaction between isobutyronitrile and R 2 Mg can be depicted thus :
(CH~)zCH ÷ RzMq ~ CN ~
a
- R H ~
in contrast to the polymerization in ether media where only AC I was observed. Initiating the polymerization with R2Mg in toluene results in a decrease of intensity of band III at lower values of the ratio of AC II/AC I. A possible explanation for this is the assumption that the AC-nitrile complex characterized by band III arises solely from AC I, since the relative decrease in the concentration of AC 1 at higher initial M/I ratios should lower also the concentration of the complex. Indeed, when a magnesium atom is attached to two (-)
--C(CHa)CN groups, its free coordination sites will be occupied by the nitrogen atoms of the terminal units thus hindering the approach of another nitrile group. A similar coordination of magnesium can also be
I
L
CN
~
CNJ2
(CH3)zCH R~C~---N,MqR
The consecutive formation of the bands at 2035 and 2075 c m - 1 as well as their conversion into the band at 2245 cm-1 corresponding to the initial nitrile on protonation, give grounds for their assignment to the mono- and double derivatives of organomagnesium compounds of model AC in the polymerization of MAN. The identity of the bands obtained with reaction mixtures from MAN polymerization in toluene and with those from the interaction between R2Mg and isobutyronitrile (bands I and IV) shows that in a hydrocarbon medium the growth of a polymer chain during anionic polymerization of MAN proceeds via
I00
I
o
r,
.".
/ ~ ' , , ........ ~, ..
0
I 2000
.... .. ../ \ .;;,',~
I 2100
1 2200
I
cm-I
Fig. 4. Infrared spectra of the reaction mixture MAN-tBu:Mg in toluene at various MAN/Mg ratios: - - 0.9, . . . . . 2.4: . . . . . 3.2.
expected to occur with AC during polymerization of MAN with RMgOCH2CH2OCH a, since in this case in toluene no band at 2260-2280 cm- 1 is observed. CONCLUSION
The investigations have shown that two types of AC are formed during polymerization of MAN initiated by organomagnesium compounds. Growth of the polymer chain in ether solvents proceeds through AC I on initiation with organomagnesium halides or dialkylmagnesium compounds. The "living" end is AC II for polymerization in toluene initiated with R2Mg. It seems reasonable to expect that the different mechanisms of growth should lead to significant alterations in the molecular masses and in the degree of ordering of the polymer. It is interesting that Joch et aL [10] suggest, on the basis of molecular mass distribution, the existence of two types of AC in the polymerization of MAN in toluene initiated with diethylmagnesium. The Ziegler-Thorpe cyclization occurs to a lesser extent with organomagnesium compounds as initiators. The main side reaction is addition of the initiator to the nitrile group giving rise to ketimines. This side reaction becomes less marked when the polymerization is conducted in toluene since there are no intensive bands due to the
I
--C~-N, MgX
and
I
~ N H
groups. The question regarding the origin of the band at 2120cm -1 which appears during polymerization in THF or in other media containing THF remains
Active centers in the anionic polymerization of methacrylonitrile--ll open. Our proposed mechanism of metallation is refuted by V a n k e r c k h o v e n and Van Beylen [11] while Tsuruta and Erussalimsky [12, 13] assume that it operates. C o o r d i n a t i o n c o m p o u n d s of magnesium with molecules of the m o n o m e r or with nitrile functions of the monomeric units (bands at 2250-2280 c m - ~ ) are formed in n o n p o l a r solvents. This observation is especially worth noting, since it permits a more detailed investigation of the initial act of complex formation between the initiator and the m o n o m e r or between the AC and the monomer. The organomagnesium c o m p o u n d s are suitable for such studies because of their lower reactivity: thus in h y d r o c a r b o n media, they do not initiate polymerization at temperatures below - 6 0 : . Addition of very small quantities of H M P T converts the complexes into AC. This fact confirms the assumption of Erussalimsky and Krasnoselskaya [14] that, in the presence of H M P T , the rate of AN polymerization in toluene initiated with organomagnesium halides increases because of the higher concentration of the AC.
Acknowledgement The author thanks Professor I. M. Panayotov for helpful discussions.
507
REFERENCES
1. Ch. l-svetanov, V. Zgonnik, B. Erussalimsky and 1. Panayotov, Justus Liebigs Annln Chem. 763, 53 (1972). 2. Ch. Tsvetanov, I. Panayotov and B. Erussalimsky. Europ. Polym. J. 10, 557 (1974). 3. Ch. Tsvetanov and I. Panayotov, Europ. Polym. J. ! l, 209 (1975). 4. I. S. Pominov and A. Z. Gadjiev, lzr. Vuzov SSSR, Ph)'.~. 19, (1965). 5. A. Ottolenghi and A. Zilkha, J. Poiym. Sci. AI, 687 (1963). 6. B. J. Wakefield, Organomet. Chem. Rer. 1, 13l (t966). 7. W. Schlenk and W. Schtenk, Ber. 62, 920 (1929). 8. T. Narita, T. Yasumura, T. Tsuruta, Polym. 3". 4, 421 (1973). 9. V. N. Zgonnik, K. K. Kalnin'sh, Ch. B. Tsvetanov and N. I. Nikolaev, Vysokomolek. Soedin. 15A, 900 (1973). 10. Y. Joch, S. Kurihara, T. Sakurai, T. Tomita, J. Polym. Sci. A-I 8, 2383 (1970): Y. Joch, S. Kurihara. T. Tomita, J. Polym. Sci. B 9, 1463 (1971). 11. H. Vankerckhoven and M. Van Beylen, Europ. Polym. 9'. 14, 189 (1978). 12. N. Kawabata and T. Tsuruta, Makromolek. Chem. 98, 262 (1966). 13. B. L. Erussalimsky and A. V. Novoselova, Fa~erJbr~ch. Text Tech. 26, 293 (1975). 14. B. L. Erussalimsky and I. G. Krasnoselskaya, Makromolek. Chem. 123, 70 (1969).