3 Anionic Polymerization of Ethylene Oxide with Lithium Catalysts Solution Properties of Styrene-Ethylene Oxide Block Polymers 1
RODERIC P. QUIRK and NORMAN S. SEUNG
2
1
Institute of Polymer Science, The University of Akron, Akron, OH 44325 Michigan Molecular Institute, Midland, MI 48640
2
The anionic polymerization of ethylene oxide has been investigated using p o l y ( s t y r y l ) l i t h i u m , α-lithium poly(methyl methacrylate), cumylpotassium, and sodium t r i b u t y l magnesate as i n i t i a t o r s . No ethylene oxide polymerization was detected i n tetrahydrofuran using i n i t i a t o r s with the l i t h i u m counterion. In a mixture of benzene and dimethylsulfoxide (2/1), ethylene oxide polymerization was observed using the ethylene oxide adduct of p o l y ( s t y r y l ) l i t h i u m . Size exclusion chroma tography and v i s c o s i t y measurements indicate that the hydrodynamic volume of the t r i b l o c k polymer, poly(styrene-b-ethylene oxide-b-styrene) i s very similar to that of the corresponding diblock polymer, poly(stryene -b-ethylene oxide) with half the molecular weight. Ethylene oxide i s an inherently reactive monomer from a thermodynamic point of view. Because of the ring s t r a i n i n the three-membered r i n g the enthalpy of polymerization of ethylene oxide i s comparable to that of cyclopropane, -27 kcal/mole(l). A variety of simple and com plex catalysts and i n i t i a t o r s can be used to e f f e c t the polymeri zation of ethylene oxide and homologous compounds (_2-4)· Therefore, i t i s somewhat surprising that lithium hydroxide and l i t h i u m alkox ide s have been reported to be i n e f f e c t i v e i n i t i a t o r s for the polym e r i z a t i o n of ethylene oxide and propylene oxide(_5-9). This apparent lack of r e a c t i v i t y of these lithium s a l t s stands i n sharp contrast with k i n e t i c studies of the reactions of ethylene oxide with a l k a l i metal derivatives of fluoradenyl(10) and polystyryl(11) carbanions where the l i t h i u m derivatives are the most reactive species by sev e r a l powers of ten. This lack of polymerization a c t i v i t y has been used to advantage for the hydroxyethylation of simple(12) and poly meric (2,9,13) organolithium compounds i n high y i e l d s (Equation 1).
PLi
2) Η ^
7
ΐ
θ
η
β
°
X
i
d
e
>
PCH
2CH 0H 2
0097-6156/ 85/ 0286-0037S06.00/ 0 © 1985 American Chemical Society
(1)
RING-OPENING POLYMERIZATION
38
However, there are various, fragmentary reports that l i t h i u m based i n i t i a t o r s may, under certain conditions, e f f e c t the polymeri zation of ethylene oxide (11,14,15). Since anionic polymerizations of non-polar monomers involving l i t h i u m o f f e r considerable advantages in terms of control of the major variables a f f e c t i n g polymer properties(13), i t i s important to determine i f conditions can be found to use well-characterized, polymeric organolithium compounds to polymer ize ethylene oxide. Herein are reported results of investigations of reactions of polymeric organolithium compounds with ethylene oxide. Also, preliminary r e s u l t s of the use of a new, hydrocarbon-soluble i n i t i a t o r (sodium t r i b u t y l magnesate, NaMgBua) has been examined. In addition, evidence for unusual hydrodynamic e f f e c t s of poly(styrene_b-ethylene oxide) vs. poly(styrene-b-ethylene oxide-b-styrene) w i l l be described. Experimental Styrene (99%, Aldrich) was p u r i f i e d by i n i t i a l s t i r r i n g and degassing over freshly-crushed CaH on a high vacuum l i n e followed by d i s t i l l a t i o n onto dibutylmagnesium (Lithium Corporation). F i n a l p u r i f i cation involved d i s t i l l a t i o n from t h i s solution d i r e c t l y into c a l i brated ampoules. Ethylene oxide (99.7% min. p u r i t y , Matheson) was condensed d i r e c t l y from the storage cylinder into a flask with freshly-crushed CaH on the vacuum l i n e . After s t i r r i n g and degas sing, further p u r i f i c a t i o n involved d i s t i l l a t i o n onto sodium disper sion (Alpha), s t i r r i n g and degassing, d i s t i l l a t i o n onto dibutylmag nesium and f i n a l d i s t i l l a t i o n into a calibrated ampoule. Tetrahydro furan (Fisher S c i e n t i f i c , c e r t i f i e d spectranalyzed, no preservatives) was s t i r r e d and degassed over LiAlHi* followed by d i s t i l l a t i o n and storage over sodium benzophenone k e t y l . Dimethylsulfoxide (Fisher S c i e n t i f i c ) was stored over and d i s t i l l e d under vacuum from 4A molecu l a r sieves. Cumylpotassium i n tetrahydrofuran was prepared from methyl cumyl ether and sodium-potassium alloy(16). Sodium t r i b u t y l nagnesate [1.47N(0.49 molar)] was used as received from Lithium Cor poration. sec-Butyllithium (Lithium Corporation of America, 12.0 wt% in cyclohexane) was used as received. Polymerizations were carried out i n a l l - g l a s s , sealed reactors using breakseals and standard high vacuum techniques(17). Polymeri zations using sodium t r i b u t y l magnesate were carried out i n flasks equipped with teflon Rotoflo stopcocks. Number-average molecular weights were determined i n toluene solutions using a membrane osmometer (Mechrolab, Hewlett-Packard 503 with S & S-08 membranes). Size exclusion chromatographic analyses i n chloroform were performed by HPLC (Perkin-Elmer 601 HPLC) using two μ-Styragel columns (10 *, 10 A) a f t e r c a l i b r a t i o n with standard poly styrene samples. Size exclusion chromatographic analyses i n tetrahydrofuran were performed with a Waters 150C GPC with s i x μ-Styragel columns having a continuous porosity range of Ι Ο - 10 À and also with the PerkinElmer Model 601 HPLC with three μ-Styragel columns (ΙΟ , 1 θ \ 10 A) after c a l i b r a t i o n with standard polystyrene samples. I n t r i n s i c v i s c o s i t i e s were measured i n chloroform at 30.0°C and i n tetrahydrofuran at 40°C using an Ubbelohde type viscometer. 2
2
1
3
6
2
o
5
3
3.
QUIRK A N D SEUNG
Anionic Polymerization of Ethylene Oxide
39
The u l tr ac en tr i f ligation experiment was performed using a Beckman Model Ε instrument with a c a p i l l a r y synthetic boundary c e l l at 30°C. Toluene was used as the solvent and solution concentrations were 1% (w/v). Results and Discussion Ethylene Oxide Polymerization. The apparent i n a b i l i t y of l i t h i u m bases to e f f e c t the anionic polymerization of ethylene oxide and i t s homologues i s unique among the a l k a l i metals. At l e a s t part of the unreactivity of lithium alkoxides can be ascribed to their strong association i n solution as shown i n Table 1(18). However, i t can be seen that the corresponding sodium and potassium alkoxides are also highly associated i n solution and yet they are active i n i t i a t o r s for the polymerization of ethylene oxide. Indeed, i t has been reported that polymeric sodium and potassium alkoxides are associated into dimers even i n hexamethylphosporictriamide at 40°C(19). It can be concluded that the l i t h i u m alkoxide unreactivity i s not due to the phenomenon of association per se, but Table I.
Alkoxide
Degree of Association of A l k a l i Metal Alkoxides i n Various Solvents(18).
Cyclohexane
Lithium ^-butoxide Sodium t-butoxide Potassium t-butoxide
5.8 8.2 -
Degree of Association Diethyl TetraBenzene ether hydrofuran 6.2 8.3 -
5.9 4.3 3.9
4.1 3.9 4.0
Pyridine 4.0 3.9
to the strength of the association, i . e . , the lack of d i s s o c i a t i o n . Two approaches have been taken to promote d i s s o c i a t i o n (and presum ably r e a c t i v i t y ) of l i t h i u m alkoxides: (a) addition of s p e c i f i c lithium-complexing agents (12-Crown-4 ether and Ν,Ν,Ν ,N'-tetramethylethylenediamine); and (b) addition of dipolar aprotic solvents. P o l y ( s t y r y l ) l i t h i u m (^=15,000) i n benzene solution was reacted with excess ethylene oxide i n the presence of Ν,Ν,Ν',N'-tetramethylethylenediamine (TMEDA, [TMEDA]/[Li]=3.2). After 12 days at 25-30°C, size exclusion chromatographic analyses indicated no s i g n i f i c a n t ethylene oxide polymerization. Hydroxyethylated polystyrene was recovered i n e s s e n t i a l l y quantitative y i e l d . α-Lithium poly(methyl methacrylate) (M =28,000) was prepared by polymerization of methyl methacrylate with l,l-diphenyl(hexyl)lithium as i n i t i a t o r ( 2 0 ) at -78°C i n tetrahydrofuran i n the presence of 12Crown-4 ether (Aldrich, [Crown]/[Li]=2). Once again after reaction with excess ethylene oxide for 12 days at 25°C, the homopolymer, poly(methyl methacrylate), presumably hydroxyethylated(20), was i s o lated e s s e n t i a l l y quantitatively and no s i g n i f i c a n t ethylene oxide polymerization was evident by size exclusion chromatographic analyses. In view of the r e s u l t s described herein with dimethylsulfoxide, how ever, i t would be imprudent to conclude that ethylene oxide polymeri zation would not occur with the l i t h i u m counterion at elevated tem peratures in the presence of 12-Crown-4 ether or TMEDA. 1
n
RING-OPENING POLYMERIZATION
40
P o l y ( s t y r y l ) l i t h i u m (Mn=2400) was terminated with ethylene oxide in benzene solution at 25°C as shown i n Equation 2. The GPC retenp
s
L
i
ethylene o x i d e
>
p
A
s
C
H
2
C
H
2
0
L
i
( 2 )
Β
t i o n of t h i s derivative (B) a f t e r hydrolysis had e s s e n t i a l l y the same retention volume as an aliquot of the o r i g i n a l p o l y ( s t y r y l ) l i t h i u m (A) which had been quenched with _t-butanol (see Figure 1). The molecular weight d i s t r i b u t i o n of A as calculated from the GPC data was 1.04 (Mtf/tyO. An aliquot of the polymeric l i t h i u m alkoxide (B) (8 mmoles) was dissolved i n ca. 300 mL of a 2/1 (v/v) mixture of ben zene and dimethylsulfoxide using high vacuum techniques(17) a f t e r removal of the o r i g i n a l benzene and excess ethylene oxide from the i n i t i a l alkoxyethylation reaction (Equation 2). Ethylene oxide (0.52 moles) was then condensed into the high-vacuum reactor which was then placed i n a 40°C bath for 4 days followed by 4 days at 60°C. The reaction was terminated by addition of a few mL of degassed acetic acid. The polymer product isolated a f t e r p r e c i p i t a t i o n and drying cor responded to an o v e r a l l y i e l d of 83%, which indicates a 70% conver sion of ethylene oxide. The size exclusion chromatographic retention volume_of__this product (see Figure 2) corresponds to an Μ η ^ ^ 3 4 0 0 with (M /M )Qpc=1.04. Several s a l i e n t features of these r e s u l t s deserve s p e c i f i c comment. Both the narrow molecular weight d i s t r i bution of the product, PS-PEO, diblock polymer and the absence of any observable peak corresponding to the o r i g i n a l polystyrene block (observable by GPC i n synthetic mixtures of hydrolyzed A and PS-PEO), indicate that (a) the hydroxyethylation reaction (Equation 2) occurs e s s e n t i a l l y quantitatively i n benzene solution; and (b) no evidence for chain termination or chain transfer i s apparent i n the polymeri zation of ethylene oxide with l i t h i u m as counterion i n a mixture of benzene/dimethylsulfoxide. The 60 MHZ H-NMR spectrum of the PS-PEO diblock (Figure 3) c l e a r l y indicates the presence of the p o l y ( e t h y l ene oxide) segment. The calculated r a t i o of the integrated i n t e n s i t i e s for the aromatic to -CH 0- protons corresponds to 1.1, while the i n t e n s i t y r a t i o calculated from the GPC molecular weights corresponds to 1.0. Thus, a dipolar aprotic solvent such as dimethylsulfoxide pro vides the necessary solvation and p o l a r i t y to render l i t h i u m alkoxides as e f f e c t i v e i n i t i a t o r s for ethylene oxide polymerization. Work i s underway to further explore the scope and k i n e t i c s of t h i s important polymerization system. Several polymerizations of ethylene oxide with sodium t r i b u t y l magnesate have been performed. Reactions at 60°C for 3 days produced polymer i n 22% y i e l d . It was necessary to heat the reactions mix tures for 12 days at 60°C to achieve a conversion of 56%. Molecular weights were l e s s than stoichiometric and the molecular weight d i s t r i b u t i o n s as determined by GPC were somewhat broad but symmetrical. β
w
n
X
2
Solution Properties of Styrene-Ethylene Oxide Block Polymers. During the course of our studies of synthetic routes to poly(styrene-bethylene oxide), we have undertaken an investigation of the solution
QUIRK AND SEUNG
Anionic Polymerization of Ethylene Oxide
ELUTION'VOLUME(ML) Figure 1.
Size exclusion chromatogram of polystyrene.
ELUTION VOLUME(ML) Figure 2.
Size exclusion chromatogram of poly(styrene-b-ethy 1 oxide).
RING-OPENING
42
POLYMERIZATION
properties of these polymers. We have compared a diblock polymer, po l y ( s tyrene-b-ethylene oxide) (PS-PEO), with the corresponding t r i b l o c k polymer, poly(styrene-b-ethylene oxide-b-styrene)(PS-PEO-PS). The t r i b l o c k polymer has the same styrene end segment lengths as the diblock polymer, and the poly(ethylene oxide) center block i n the t r i b l o c k i s twice the poly(ethylene oxide) segment length i n the diblock as shown i n Table I I . Both PS-PEO and PS-PEO-PS exhibited the same retention volume i n tetrahydrofuran using a three-column set Table I I .
Molecular Weight Characterization of PS-PEO and PS-PEO-PS
Polymer PS-PEO (Diblock) PS-PEO-PS (Triblock)
Stoichiometric Molecular Weight (g/mol) a
83,000 (35 - 48) 166,000 (35-96-35)
M
n (g/mol)
b
82,800 163,900
The numbers i n parentheses correspond to the stoichiometric molecular weights f o r the individual block segments ( x l 0 ~ ) based on the r a t i o of gm of monomer charged to the moles of initiator. 3
Determined by membrane osmometry. of μ-styragel columns. This surprising r e s u l t was compounded by the fact that the measured i n t r i n s i c v i s c o s i t i e s i n tetrahydrofuran were 75 ml/g and 71 ml/g for the diblock and the t r i b l o c k , respectively. Thus, the t r i b l o c k polymer apparently exhibits the same hydrodynamic volume and v i s c o s i t y as the diblock polymer which has one-half of the molecular weight of the t r i b l o c k . These unusual observations probably r e f l e c t the fact that tetrahydrofuran i s l i s t e d as non-solvent for poly(ethylene oxide)(21) and p r e c i p i t a t e s from a 1% solution at 18°C (22). This phenomenon was explored further by examining the behavior of these block polymers i n chloroform, a good solvent for polystyrene and poly(ethylene oxide)(21). The size exclusion chromatograms of these polymers i n chloroform are shown i n Figure 4. The retention volumes were 13.5 ml and 14.0 ml for the t r i b l o c k and diblock, respectively. For the polystyrene standards, a retention volume d i f ference of 1.2 ml (versus 0.5 ml observed for the block polymers) would be expected for a doubling of the molecular weight from 83,000 to 166,000. Further evidence for the unusually small increase i n hydrodynamic volume for the t r i b l o c k polymer r e l a t i v e to the diblock i n chloroform has been obtained from their i n t r i n s i c v i s c o s i t i e s and second v i r i a l c o e f f i c i e n t s as shown i n Table I I I . It i t noteworthy that the diblock and t r i b l o c k polymers i n toluene solution could not be separated by u l t r a c e n t r i f u g a t i o n . A 50/50 mixture of the two polymers i n toluene exhibited a single peak throughout the sedimentation process with the ultracentrifuge oper ating at a speed of 28,000 rpm. In conclusion, a l l of the evidence from solution properties of a PS-PEO-PS block polymer indicates that
3.
QUIRK A N D SEUNG
8.0 Figure 3.
7.0
Anionic Polymerization of Ethylene Oxide
6.0
5.0 """' 4.0
3.0
2.0
1.0
43
0
60 MHz *H-NMR spectrum of poly(styrene-b-ethylene oxide),
...Triblock Diblock
10
15
20
v («i) E
ELUTION VOLUME(ML)
Figure 4.
Size exclusion chromatograms of poly(styrene-b-ethylene
oxide-b-styrene) and poly ( s tyrene-b-ethylene
oxide).
RING-OPENING POLYMERIZATION
44
Table I I I .
Polymer
[η] (ml/g)
PS-PEO PS-PEO-PS
124 132
Solution Characterization of PS-PEO and PS-PEO-PS i n Chloroform. Huggins Constant, k i 0.40 0.34
Kraemer Constant, k -0.13 -0.14
AaxlO** (ml/mol g ) 2
2
6.63 5.01
this polymer exhibits unique hydrodynamic properties when compared to the corresponding diblock polymer with one-half of the molecular weight of the t r i b l o c k . Further work i s i n progress to characterize the solution properties of these polymers. Acknowledgments The authors would l i k e to acknowledge the able assistance of Mr. Dennis McFay who carried out the i n i t i a l synthetic and solution property experiments at Michigan Molecular I n s t i t u t e .
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
Sawada, H. J. Macromol. Sci.-Rev. Macromol. Chem., 1970, (C5(1), 151. Morton, M. "Anionic Polymerization: Principles and Practice"; Academic Press: New York, 1983; p. 52. Boileau, S. in "Anionic Polymerization: Kinetics, Mechanisms, and Synthesis"; McGrath, J.E., Ed.; ACS Symposium Series No. 166, American Chemical Society: Washington, D.C., 1981; p. 283. Bailey, F.E.; Koleske, J.V. "Poly(ethylene Oxide)"; Academic Press: New York, 1976. St. Pierre, L.E.; Price, C.C. J. Am. Chem. Soc., 1956, 78, 3432. Lebedev, N.N.; Baranov, Yu.I. Polym. Sci. USSR, 1966, 8, 211. Doroshenko, N.P.; Spirin, Yu.L. Polym. Sci. USSR, 1970, 12, 2812. Cabasso, F.; Zilkha, A. J. Macromol. Sci.-Chem., 1974, A8(8), 1313. Guilbert, Y.; Brossas, J. Polym. Bull., 1979, 1, 293. Chang, C.J.; Kiesel, R.F.; Hogen-Esch, T.E. J. Am. Chem. Soc., 1973, 95, 8446. Solov'yanov, A.A.; Kazanskii, K.S. Polym. Sci. USSR, 1970, 12, 2812. Wakefield, B.J. "The Chemistry of Organolithium Compounds"; Pergamon Press: Elmsford, N.Y., 1974; p. 199. Young, R.N.; Quirk, R.P.; Fetters, L.J. Adv. Polym. Sci., 1984, 56, 1. Dudek, T.J. Ph.D. Thesis, University of Akron, 1961, p. 74. Kobayashi, S.; Kaku, M.; Mizutani, T.; Saegusa, T. Polym. Bull., 1983, 9, 169. Ziegler, K.; Dislich, H. Chem. Ber., 1957, 90, 1107. Morton, M.M.; Fetters, L.J. Rubber Chem. Tech., 48, 359 (1975). Halaska, V.; Lochmann, L.; Lim, D.; Coll. Czech. Chem. Commun., 1968, 33, 3245. Figueruelo, J.E.; Worsfold, D.J. Eur. Polym. J., 1968. 4, 439.
3.
QUIRK A N D SEUNG
Anionic Polymerization of Ethylene Oxide
20.
45
Anderson, B.C.; Andrews, G.D.; Arthur, P., Jr.; Jacobson, H.W.; Melby, L.R.; Playtis, A.J.; Sharkey, W.H. Macromolecules, 1981, 14, 1599. 21. Fuchs, O.; Suhr, H.-H. in "Polymer Handbook," Second ed.; Brandrup, J.; Immergut, E.H., Eds.; Wiley-Interscience: New York, 1975; p. IV-241-265. 22. Stone, F.W.; Stratta, J.J. in "Encyclopedia of Polymer Science and Technology"; N. Bikales, Ed.; John Wiley and Sons, Inc.: New York, 1967; Vol. 6; p. 114. RECEIVED September 14, 1984
