Reactivity of Solvated Lithium n-Butyldimethylsilanolate with Organosiloxane Substrates William A. Fessler' and Peter C. Juliano Corporate Research and Development, General Electric Co., Schenectady, N.Y. ib301
The reaction of lithium n-butyldimethylsilanolate, I, in the presence of solvating agents with organosiloxane substrates was examined. In the reaction of solvated I with hexamethylcyclotrisiloxane, D3, the following order of rates was observed: hexamethylphosphortriamide (HMPT) dimethoxyethane (DME) tetrahydrofuran (THF). Products isolated (after derivatization with TMCS) in the DME solvation experiment were organosiloxane homologs of the type BuMezSiOSiMea and BuMe2SiOMe2SiOSiMe3.To account for these products and the rates at which they were formed, a mechanism was proposed that involved siloxanolate-silanolate redistribution reactions.
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P r e v i o u s studies (Juliano et al., 1971a,b) have demonstrated that proton magnetic resonance (pmr) spectroscopy could be used to study specific solvation effects with lithium nbutyldimethylsilanolate, I, and various solvating agents. These experiments rvere conducted in the presence of hexamethylcyclotrisiloxane, D3, and either hexamethylphosphortriamide (HXIPT), tetrahydrofuran (THF), or dimethoxyethane (DME). We observed (Juliano et al., 1971a,b) that all of these solvating agents a t the 1: 1 level (based on Li) promoted the oligomerization of D 3 by I. Preliminary examination indicated that the products of oligomerization were organosiloxanes and gave a qualitative measure of the relative rates of reaction in the presence of each of the solvating agents. However, the products of these reactions m-ere not identified. This report examines in greater detail the products and the relative rates of oligomerization of D3 in the presence of solvated I. I n addition, the nature of the reaction of solvated I with other organosiloxane substrates was examined. Experimental
When present in equimolar amounts, n-butyllithium and D3 react in hexane solution to give I and unreacted DS as s h o w (Frye et al., 1970). I n the absence of solvating agents, no BuLi
+ D3 D
---f
=
BuDILi
+ 2//3D3
(CH3)2SiOz/z
further reaction is observed a t 25°C over a period of one week. The addition of solvating agents t o this solution proPresent address, Group Technical Resources Operation, General Electric Co., 100 Woodlawn Avenue, Pittsfield, Mass. 01201. To whom correspondence should be addressed.
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motes the reaction of D3 and I. By increasing the amount of n-butyllithium in the reaction with D3, it is possible to react all of the D3 to I in hexane solution. 3BuLi
+ D3 +3BuDLi
The addition of organosiloxane compounds t o this solution of I along with solvating agents allows one to study the reactions of compounds other than D3 with solvated I. A11 manipulations, including monomer, solvent, and solvating agent purification, were carried out as before (Juliano et al., 1971a,b). Reactions were performed in a nitrogen-filled dry box a t 25 + 1°C by use of standard syringe techniques (Frye et al., 1970). Reaction of I and D3 in Presence of Solvating Agent. mol) of D 3 in 70.0 ml of n-hexane To 5.55 grams (2.5 X were added 10.25 ml of 2 . 4 4 s n-butyllithium (2.5 X 10-2 mol). After the reaction was complete, 25-ml aliquots (7.18 X mol of I in each) of the solution were delivered to three clean, dry 2-02 screw-cap bottles. Into each of the bottles was placed 7.18 X mol of one of the solvating agents, T H F , DME, or HMPT. The reaction time of I and D3 in the presence of the solvating agents was marked from the time of addition of the solvating agents. Periodically, 5.0 ml (1.44 X mol I) were withdrawn and derivatized nith 0.02 ml (1.57 x mol) of trimethylchlorosilane, TLICS. Immediately prior to derivatization, an additional 1.0 ml of T H F was added only to those reaction mixtures that contained THF originally (Juliano et al., 1971a,b). The derivatized reaction mixtures were analyzed by glpc. Reaction of I and Organosiloxane Substrates in Presmol) of D3 in ence of DME. To 1.85 grams (8.33 X 30.0 ml of n-hexane and 10.00 ml of benzene were added 10.25 ml of 2.44N n-butyllithium (2.5 X mol). The concentraInd. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 4, 1972
407
Table 1. Characterization of Bu(D),SiMe3 from Reaction of I and D3 Elemental analvses
c, 70 n
Found
H, Colcd
Found
%
Si, 70 Calcd
Found
Mol wt Colcd
1 52.9 52.9 12.3 11.8 27.8 27.5 2 47.0 47.4 10.9 10.8 29.9 30.2 3 44.6 44.3 10.2 10.3 31.8 31.8 4 42.2 42.2 10.1 9.9 32.5 32.9 5 41.2 40.8 9.8 9.7 32.3 33.6 a Cryoscopic mol wt determination. * Vapor pressure lowering (Mechrolab 301A).
IiHMPT
00'
, 10
I
20 TIME (HRS)
1
1
30
40
Figure 1 . Disappearance of DB by reaction with I solvated with HMPT, DME, and THF
tion of I was 4.75 X lo-' mol, and no D3 was present after complete reaction. Into two clean, dry 2-0s screw-cap bottles were delivered 20-ml aliquots of the above reaction mixture. DME (4.75 X mol) was added to each of the bottles. To mol of octamethylcyclotetraone of the bottles, 9.5 X siloxane, D4 (bp = 175OC) (Patnode and Wilcock, 1946), was added. To the other bottle was added 9.5 X mol of 1,1,1,3,3,5,5,7,7,9,9,11,11,I 1-tetradecamethylhexasiloxane, MDIM (bp = 141°C/20 mm) (Kantor et al., 1954). The reaction time was marked from the time of addition of the organosiloxane substrate. Periodically, 4.0-ml aliquots were withdrawn and derivatized with 0.23 ml of TMCS. The derivatized reaction mixtures were analyzed by glpc. Synthesis of Bu(D).SiMe, Oligomers for Preparative Glpc. BuDISiMed. A 3:l (mol) BuLi:Dg reaction was carried out in hexane as described above. After complete reaction, a 3: 1 molar ratio of T H F : T M C S was added (1:l TMCS:Li), and LiCl was removed by filtration. The solution was vacuum distilled, and the fraction boiling at 70-80"/45 mm was collected and further purified by preparative glpc. Bu(D),SiMe3 (2 5 n l 5 ) . A 1 : l (mol) BuLi:D3 (11.35 grams) reaction was carried out in hexane. After 4 hr the reaction was complete and 2.62 ml of D M E (1 :1 based on Li) was added. The reaction of D M E solvated I and excess D3 was allowed to continue for 48 hr. TMCS (11.6 ml) was then added to derivatize the reaction mixture. The LiCl was removed by filtration. The mixture of siloxane oligomers from this reaction was separated by preparative glpc. Analytical and Preparative Glpc. After derivatization, the product mixtures were analyzed by glpc on 6-ft siliconegum columns in a n HP-5750 instrument equipped with thermal conductivity detectors. The injection of sample (3 ~ l ) was done with a springdriven syringe (Hamilton Co., Whittier, Calif., #700-20), and the peak areas were reproducible within + 1%. After 2 min a t 100°C, the oven temperature was increased to 300°C a t a rate of 15"C/min. Retention times (fR) were reproducible within *O.l min. The weight percent of 408
Ind. Cng. Chern. Prod. Res. Develop., Vol. 1 1 ,
No. 4, 1972
Glpc, t ~rnin ,
3.4 6.1 8.2 10.0 11.5
Calcd
204 278 352 426 500
Found
...
272. ... ...
517b
each organosiloxane component in the derivatixed reaction mixture was calculated from the normalized area percent (DalNogare and Juver, 1962). Samples of the various components were obtained for characterization by preparative glpc on 3 or 12-ft silicone-gum cnlumns in an Aerograph 1520 instrument. The purity and the retention times of the samples obtained from preparative chromatography were checked by analytical glpc following the standard procedure. I n all cases, the samples were greater than 99% pure, and the retention times agreed within experimental error with the retention times found by analysis of the reaction mixtures. Characterization of Products of D3 Oligomerization. The derivatized products obtained from preparative glpc were characterized by elemental analysis, molecular weight, mass spectra, and pmr spectra. The results of the elemental analyses and molecular-weight determinations are summarized along with the glpc retention times in Table I. The pmr spectra of all the compounds listed in Table I showed resonance peaks for both the butyl protons and the silicon methyl protons. As the siloxane chain length increased, the intensity of the butyl proton resonance decreased relative to the intensity of the silicon methyl proton resonance. The complexity of the silicon methyl proton resonance also increased as the siloxane chain became longer. This latter phenomenon has been reported (Liu, 1969) previously and is due to the presence of terminal as well as various types of interior silicon methyl groups. The compounds listed in Table I, as well as several model compounds, were examined on a CEC21-104 medium resolution mass spectrometer. None of the siloxanes examined gave a parent ion peak (Dibeler e t al., 1953). For the lower values of n (Table I), the base peak was m/e = P-57 corresponding to loss of the butyl group. Fragmentation also occurred t o produce a peak at m/e = 73 (loss of trimethylsilyl). The P-15 peak owing to loss of methyl from silicon was always less intense than the P-57 peak As the siloxane chain length increased, the fragmentation became more complex, and the peak a t m/e = P-57, though still observed, was no longer the base peak. The peak a t m/e = 73 (trimethylsilyl) increased in relative intensity and became the base peak as n increased. Results
Displayed in Figure 1 is the disappearance of Da during the reaction of D3 and solvated I a t 25OC. With H M P T solvated I, no D3 was detected after 4 min of reaction time. D3 disappeared faster with DME solvated I than with T H F solvated I. Shown in Figure 2 is the disappearance of solvated I in these same experiments. The products isolated from the reaction of DME solvated I and DI are shown in Table I. The formation and disappearance of these various organosiloxanes are shown in Figure 3.
The reaction of D M E solvated I was examined with D4 and hID4XI under the same conditions of temperature (25 l'C) and reactant concentrations as with D3. After 48 hr there was no detectable reaction of D M E solvated I with MD4M. With D4, only one product was formed, BuD2SilLle3, which was identified by its glpc retention time. After 40 hr the amount of BuDnSiMes was 6.4 wt % of the siloxane mixture as compared with 69 wt 70in the D3 reaction (Figure 3). Also, there was no buildup of higher BuD,SiMe3 oligomers in the D4 reaction.
*
Discussion
The reaction of HMPT, DME, or T H F solvated I and D S vias reported previously in a proton magnetic resonance study (Juliano et al., 1971). These observations were interpreted in a qualitative manner, that is, in terms of the relative rates in which the pmr spectra became more complex owing to oligomer formation. To obtain more quantitative data concerning the reactions of solvated I and D3, the formation of oligomers was followed by glpc. I n Figures 1 and 2 these observations are displayed quantitatively as the disappearance of D3 and solvated I as a function of reaction time. It is quite obvious that HhIPT solvated I disappears more rapidly thar. D M E or T H F solvated I. The observed order parallels the known basicities of the solvating agents: H M P T > D M E > T H F (Xormant, 1967; Garst, 1969). I n the remainder of the discussion, it is to be understood that all lithium atoms are solvated by D M E unless other solvents are indicated. Furthermore, the derivatized products in Table I are understood to represent the active silanolate or siloxanolate species present in the reaction mixture at" the time of addition of TMCS. Thus, the derivatized and underivatized products are a t times used interchangeably in the text. Derivatization occurs via the reaction BuD,Li
+ Me3SiCl +BuD,SiMe3 + LiCl
The oligomers (derivatized with TMCS) isolated from a 48hr reaction of D M E solvated I and D3 gave a homologous series of BuDzSiMe,, BuD3SiMer, etc. (Table I). Thus, homologation of the type shown below did not occur.
+ D3+ BuD4Li BuD4Li + D3 +BuD7Li I
(Ib)
ki
+ D3 +BuD4Li
@a)
k2
+ BuDILi ++3BuDzLi ka BuDzLi + D 3 +BuD5Li BuDzLi + BuDsLi +BuD3Li + BuD4Li 2I
I
I
TIME (HRSI
Figure 3. Oligomerization of Ds by DME solvated I
in hexane) allows us to extend the order of apparent reactivity of various solvated and unsolvated lithium bases with D3:
> n-BuLi (unsolvated in hexane) >> Bu;LlezSiOLi.DiCIE > BuMesSiOLi.THF >
BuMezSiOLi.H M P T
BuMesSiOMe2SiOLi~ DME
(la)
The results shown in Figure 3 are best explained by the reactions shown below.
I
Figure 2. Disappearance of solvated I by reaction with Da
(2b) (2c)
kr
(2d)
Since BuDzLi is formed as rapidly as BuDILi (I) disappears, kz must be greater than kl. If this were not so, there would be more of the BuD3Li and BuDILi than observed in the initial stages of the reaction (Figure 3). The buildup of BuD3Li and BuDdLi is relatively slow and results from Reactions 2c and 2d. From the shapes of the formation and disappearance curves for BuDzLi and BuDILi, kl is greater than k3. Thus, a D M E solvated lithium triorganosilanolate is more reactive toward D3 than a DATE solvated lithium siloxanolate. This result, combined with the findings of Frye et al. (1970) (n-BuLi
These comparisons are based on the disappearance of lithium base when reacted with D3 in hexane solution. Where solvating agents are indicated, they are present in stoichiometric amounts based on lithium. The reaction of BuDILi and Dh was also examined. This reaction was much slower than the reaction of BuDlLi and D3. After 40 hr of reaction with D3, there was less than 470 BuD1Li remaining in the siloxane mixture, whereas with D4,41% of BuDILi remained after the same reaction time. The enhanced reactivity of D3 is presumably due to the decreased Si-0Si bond angle, 125O, vs. 142' in D4and an increased polar contribution to the resonance bond (Noll, 1968). The same order of reactivity of other bases with D3 and Dq is well-documented (Frye et al., 1970; Kantor et al., 1954). That onlyBuDpLiis formed from the reaction of BuDILi and D4indicates that as long as BuDILi (41y0 after 40 hr) remains in the reaction mixture, any higher oligomer from the reaction of D4 will be redistributed back to BuD2Li by the excess BuDILi: k6
BuDILi
+ Dd +BuD6Li
(3%)
kz
3BuD1Li
+ BuD5Li ++4BuD2Li
Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 4, 1972
(3b) 409
BuD2Li 4BuD1Li
+ D4
k7 ---f
BUD&
(3c)
+5BuD2Li
(3d)
ka
+ BUD&
----f
However, from the amount of BuDnSiMeafound after 40 hr (6.470 of the reaction mixture), both kg and ki are smaller than kl (Reaction 2a). It is obvious from the product analysis, Table I, that a redistribution reaction of higher mol wt siloxanes t o lower mol wt siloxanes is necessary, as depicted in Reactions 2a-2d. The mechanism of the redistribution reaction will be dealt with in this section. With Reaction 2b as a model, DME solvated I could react a t the following numbered silicon sites of the transient BuDaLi. Reactions a t the various sites in the siloxanolate molecule are distinguished in the following way as siloxane-silanolate or siloxanolate-silanolate reactions. 1
2
3
4
Bu-Si-O-Si-O-Si-O-Si-OeLi@.
I
Men
I
Me2
I
Mez
DME
I
Me2
Reaction a t site 1 (or a t any site) to produce dilithiumsiloxane-diolate and BuD,Bu compounds is ruled out since no products derived from such reactions are observed. Alternate reaction a t site 1 to generate BuLi would be energetically unfavorable. Reaction a t site 2 or a n internal siloxane silicon would be a siloxane-silanolate redistribution. The penultimate silicon, site 3, would be another of the possible siloxanesilanolate reactions resulting in redistribution. Reaction in site 4 or the terminal, active, siloxanolate silicon, would require a siloxanolate-silanolate reaction to effect redistribution. Sincewe were unable to detect any reaction of D M E solvated I and MDJI, reaction a t site 2 is probably unimportant in the mechanism of the redistribution reaction, thus : BuDiLi
+ lIDJI
UuD1-JI
+ l1D4-*Li
(4a)
I n no case did we observe the formation of higher cyclic siloxanes. This is in agreement with no silanolate-siloxanetype reaction (Reaction 4a). Attack of D M E solvated I a t site 3 is also unimportant because there is a rapid formation of BuDQLi in the virtual absence of BuD3Li (Figure 3). Attack a t the penultimate silicon (site 3) would require equal amounts of BuDzLi and BuD3Li. Further, to account for the absence of BuD3Li, it would also require the reaction of D N E solvated I with BuD3Li to be much greater than with BuD4Li. There is no reason to believe that the reaction with BuDaLi should be favored. Thus, penultimate siloxane silicon-silanolate reaction cannot explain the redistribution of higher molecular-rveight siloxanolates (Reactions 2b or 2d). We must conclude, then, that silanolate-siloxanolate (Reaction 2b) and, to a lesser extent, siloxanolate-siloxanolate-type redistribution reactions are involved in the mechanism of oligomer formation. Thus,
410 Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 4, 1972
siloxanolate redistribution requires the reaction of two active, terminal sites (site 4). Since the redistribution reaction requires the attack of one active terminal site on another active terminal site, Me2
Me2
I I ,Si-O--Si-O”Li@~
‘! f
D l l E Lie ?O-Sillc2
DlIE
0- Siw
+Redistribution products
Mei
reaction of this type would be bimolecular with respect to the active sites. Since the ring opening of cyclosiloxanes would be unimolecular with respect to the active ends, one would expect that as the concentration of active ends decreased, the redistribution reaction would become unimportant relative to the opening of cyclosiloxane rings. As a consequence, formation of high-molecular-weight siloxanes from D3 initiated by lithium bases in ether solvents would proceed with no detectable redistribution. Such polymerizations have indeed been report’ed (Bostick, 1969; Lee e t al., 1969). On t’he other hand, formation of low-molecularweight oligomers from D3, IVhere the concentration of active ends would be much higher, would produce redistributed products under the same conditions of solvent and temperature. The reaction of cis-2,4,6-trimethyl-2-4,6-triphen)-lcyclotrisilosane and solvated lithium bases has been examined to determine the stereochemical effects iii the redistribut’ion reactions. The results of these experiments will be reported a t a later date. Literature Cited
Bostick, E. E., Polym. Prepr., 10, 877 (1969). DalXogare, S., Juvet, R. S., “Gas Liquid Chromatography,” p 2.57, Interscience, ?Jew York, X.Y., 1962. Dibeler, V. H., Rlohler, F. L., Reese, R . AI., J . Chem. Phys., 21, 180 (1953). Frye, C. L., Salinger, R. M., Fearon, F. W. G., Klosowski, J. lI., DeYoung, T., J . Org. Chem., 35, 1308 (1970). Garst, J. F., “Solute-Solvent Interactions,” Chap. 8, J. F. Coetzee, C. D. Ritchie, Eds., Marcel Dekker, New York, X.Y., 1969. Juliano, P. C., Fessler, W. A., Cargioli, J. D., XXIII Int. Conof Pure and Applied Chem., Mucromol. Prepr., 2, 1212 __ -gress (1971). Juliano, P. C., Fessler, W. A , , Cargioli, J. D., Polym. Prepr., 12, 158 (1971). Kantor, S. W., Grubb, W. T., Osthoff, R . C., J . Amer. Chem. Soc., 76,5190 (1954). Lee, C. L., Frye, C. L., Johannson, 0. K., Polym. Prepr., 10, 1361 (1969). Liu, K.-J., Makromol. Chem., 126, 187 (1969). Noll, W., “Chemistry and Technology of Silicones,” p 313, Academic Press, New York, X.Y., 1968. Xormant, H., Angm. Chem., Int. Ed., 6 , 1046 (1967). Patnode, W. J., Wilcock, D. F., J . Amer. Chem. SOC.,68, 358 (1946). RECEIVED for review October 15, 1971 ACCEPTEDAugust 5, 1972 Presented at the Division of Polymer Chemistry, 162nd Meeting, ACS, Washington, D.C., September 1971.
