Silicones and Silicone-Modified Materials - American Chemical Society

Chapter 29

Downloaded by UCSF LIB CKM RSCS MGMT on November 28, 2014 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch029

A Review of the Ruthenium-Catalyzed Copolymerization of Aromatic Ketones and 1,3-Divinyltetramethyldisiloxane: Preparation of alt-Poly(carbosilane-siloxanes) William P. Weber, Hongjie Guo, Cindy L. Kepler, Timothy M . Londergan, Ping L u , Jyri Paulsaari, Jonathan R. Sargent, Mark A . Tapsak, and Guohong Wang D. P. and K . B. Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, Los Angeles, C A 90089-1661

Step-growth copolymerization of aromatic ketones and 1,3-divinyltetramethyldisiloxane is found to be catalyzed by (Ph3P)3RuH2CO. Polymerization occurs by the anti-Markovnikov addition of the C-H bonds, which are ortho to the carbonyl group of the aromatic ketone, across the C - C double bonds of 1,3-divinyltetramethyldisiloxane. Prior activation of the catalyst by treatment with a stoichiometric amount of styrene results in formation of ethylbenzene and higher molecular weight copolymers. The scope and current mechanistic understanding of this copolymerization reaction are considered. Preparation of aft-Poly(carbosilane/siloxane) - Previous Methodologies

a/?-Poly(carbosilane/siloxanes) have a backbone which contain C-C, Si-C and Si-O-Si bonds. Monomers required for such polymers have been prepared by several approaches. At least two of these involve the formation of Si-C bonds as an essential step. The first involves the formation of Si-C bonds by nucleophilic displacement of chloride from silicon by Grignard or organolithium reagents. For example, 1,4bis(hydroxydimethylsilyl)benzene, the key monomer for the synthesis of the well studied silphenylene/siloxanes, has been prepared by an in-situ Grignard reaction between 1,4-dibromobenzene and dimethylchlorosilane [1,2], as shown in Figure 1. H I

Figure 1 © 2000 American Chemical Society

In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

433

434


Alternatively, monomers required for the synthesis of a/f-poly(carbosilane/ siloxanes) have been prepared by formation of Si-C bonds by Pt catalyzed hydrosilation reactions. For example, Pt catalyzed hydrosilation of the C - C double bond of the Diels-Alder adduct of cyclopentadiene and maleic anhydride with symtetramethyldisiloxane yields a difunctional monomer 5,5'-bis( 1,1,3,3-tetramethyl-1,3disiloxanediyl)norborane-2,3-dicarboxylic anhydride, see Figure 2. Reaction of this with diamines such as 4,4-diaminobiphenyl yields a thermally stable poly[imidea/r(carbosilane/siloxane)] [3].

Figure 2 Unsaturated a/r(carbosilane/siloxane) polymers have been prepared by Wagener's group by use of alicyclic diene metathesis (ADMET) reactions [4]. As shown in Figure 3, the polymer results from the formation of new C-C double bonds. Both the molybdenum and tungsten versions of the Schrock metathesis catalyst are effective [5],

I

I Figure 3

Ru catalyzed synthesis of aft-poly(carbosilane/siloxanes) The reaction, whose scope, limitation and mechanism we are going to review, directly yields a/*-poly(carbosilane/siloxanes) by the (Ph P) RuH CO (Ru) catalyzed copolymerization of aromatic ketones and 1,3-divinyltetramethyldisiloxane, as shown in Figure 4. The key step in this process involves the ruthenium catalyzed activation of an aromatic C - H bond which is ortho to a carbonyl group for anti-Markovnikov addition across the C-C double bond of 1,3-divinyltetramethyldisiloxane. Each time 3

3

2


435

this occurs a C - C bond is formed and the polymer grows. For example, the Ru catalyzed copolymerization of acetophenone and 1,3-divinyltetramethyldisiloxane yields copoly(2-acetyl-1,3-phenylene/3,3,5,5-tetramethyl-4-oxa-3,5-disilaheptanylene), as shown in Figure 4 [6].


+

/\ /\ Figure 4

It should be noted that the Ru catalyst is easily prepared by reaction of RuCb hydrate with triphenylphosphine, sodium borohydride and formaldehyde [7]. While ruthenium is a rare element, whose natural abundance is reported to be only one tenth that of platinum, its catalytic utility in several different types of polymerization reactions has recently been reported. Among these are ring opening metathesis polymerizations (ROMP) reactions [8], as shown in Figure 5, as well as in isomerization of allyl ethers to reactive propenyl ethers which undergo facile acid catalyzed polymerization [9], as seen in Figure 6. A D M E T polymerization can also be achieved using Grubbs Ru-carbene complexes. However, it should be noted that 1,3divinyltetramethyldisiloxane has been reported not to undergo A D M E T type Ru catalyzed polymerization. Me. +Me CI"

0

Cvx^Cy

Cy

Cy

N(CH ) Cr 3

Figure 5


3

+

436

-o

o

o

Ru(PPh3) CI 3

Ar l"X H0


2

o-

2

+

2

hv

Polymer Figure 6 The first examples of this Ru catalyzed reaction in monomer systems were reported by S. Murai, who found that acetophenone and vinyltrimethylsilane undergo Ru catalyzed reaction to yield 2-(2'-trimethylsilylethyl)acetophenone [10,11]. A catalytic cycle, seen in Figure 7, involving coordination of the unsaturated Ru center by the oxygen of the acetyl group followed by insertion of Ru into an adjacent ortho C - H bond to form Ru-C and Ru-H bonds has been proposed. Coordination of the C-C double bond of the vinyl silane to the Ru center followed by anti-Markovnikov addition across the C - C double bond regenerates the coordinately unsaturated Ru center and completes the catalytic cycle.

Figure 7 Unfortunately, the reaction fails with most a,o>dienes. One reason for this is that the Ru catalyst isomerizes terminal oe,o>dienes such as 1,7-heptadiene to internal dienes which are not reactive. Conjugated dienes such as 1,3-butadiene or 2,3-dimethyl-l,3-butadiene are also unreactive. Further C-C double bonds substituted with electron withdrawing groups such those of methyl acrylate, acrylonitrile, or methyl vinyl ketones do not react. So why are the C-C double bonds of vinylsilanes, vinylsiloxanes and styrenes reactive? Perhaps the simplest explanation is that silicon and In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

437


phenyl groups are not strongly electron withdrawing and do not permit isomerization of adjacent C - C double bonds. On the other hand, a variety of aromatic ketones have proved to be reactive. Polycyclic aromatic ketones such as an throne, fluorenone, and xanthone all undergo successful Ru catalyzed copolymerization with 1,3-divinyltetramethyl-disiloxane [12], see Figure 8. It has also been possible to incorporate phenanthrene units, which strongly absorb in ultraviolet and fluoresce, into an a/f(carbosilane/siloxane) by copolymerization of 2-acetylphenanthrene and 1,3-divinyltetramethyldisiloxane [13]. Initially, the molecular weight of these materials was quite low [12].

Figure 8 End group analysis by N M R spectroscopy suggested the presence of ethyl groups attached to silicon. This could be explained if the Ru catalyst is capable of catalyzing the hydrogenation of the vinyl groups of 1,3-divinyltetramethyldisiloxane as well as the copolymerization reaction. In fact, we have suggested that loss of hydrogen from the ruthenium center, which creates a site of coordinate unsaturation, may be essential for activating the catalyst for copolymerization [12]. Exact stoichiometric balance is essential in a step-growth copolymerization if one is to achieve high molecular weights. Loss of hydrogen from Ru by hydrogenation of a C-C double bond of 1,3-divinyltetramethyldisiloxane not only activates the catalyst - but also converts a difunctional oo,G>-diene into a mono-functional alkene which ultimately becomes an end group. This limits the molecular weight of the copolymer. To overcome this problem, we have treated Ru with a stoichiometric amount of styrene for a few minutes at 135°C. In this way, styrene is converted to ethylbenzene and the activated catalyst is formed. Subsequent addition of an equal molar mixture of an aromatic ketone and 1,3-divinyltetramethyldisiloxane to the activated catalyst yields significantly higher molecular weight copolymers. Solutions of the activated complex are catalytically active for at least twenty-four hours [14]. The activated complex has been characterized by N M R and X-ray crystallography. The signals in the H N M R at -8.18 ppm and -6.35 ppm due to the two nonequivalent ruthenium hydrogen bonds of Ru are no longer observed. Further, examination by P N M R of the activated catalyst reveals the presence of two nonequivalent triphenylphosphines at 46.21 ppm and 46.78 ppm, as well as one free triphenylphosphine ligand. This suggests that the activated complex is "[PhaP^RuCO" or a dimer or trimer species which has this stoichiometry. In this regard, polynuclear ruthenium carbonyl species are well known. Addition of ortho acetylstyrene, which has a perfect stoichiometric balance of terminal vinyl groups to ortho C - H bonds, does not yield polymer but rather a 1:1 complex of [Pfi3P]2RuCO", see Figure 9. The l

3 1

M


438


"[PhiPhRuCO", see Figure 9. The structure of this crystalline complex has been determined by X-ray crystallography, as shown in Figure 10. This complex also catalyzes the copolymerization of a 1:1 molar mixture of acetophenone and 1,3divinyltetramethyldisiloxane. These results suggest that "[PhsPhRuCO" may be the catalytically active species [15].

Figure 10 Hiraki and coworkers have recently reported the X-ray structure of a 1:1 complex of "[Ph P]2RuCO" with N-benzylideneaniline. This complex has both Ru-C and Ru-H bonds formed by insertion of the nitrogen coordinated Ru center into one of the adjacent ortho C - H bonds [16]. The relationship of this complex to the catalytic cycle previously proposed should be evident. 3


439

M

We have also reacted activated catalyst [Ph P]2RuCO" with dimethyldivinylsilane and obtained a 1:1 complex which demonstrates dynamic NMR spectra depending on temperature. This fluxional complex is also capable of catalyzing the copolymerization of acetophenone and 1,3-divinyltetramethyldisiloxane [17].


3

Acetophenones substituted with methoxy or phenoxy groups in the para position had been shown to undergo successful activated Ru catalyzed copolymerization with 1,3-divinyltetramethyldisiloxane [18], Activated Ru catalyzed copolymerization of 4-acetylbenzo crown ethers with 1,3-divinyltetramethyldisiloxane provides a synthetic route to a/f(carbosilane/siloxane) copolymers which incorporate crown ethers, as shown in Figure 11. Polymeric crown ether/lithium perchlorate complexes have been prepared. Of particular note, the sterically congested ortho C-H bond, which is between the acetyl group and the crown ether ring, is still reactive [19,20]

Figure 11 Likewise, activated Ru catalyzed copolymerization of acetophenones which are substituted with dialkylamino groups in the para position proceed with facility [21]. We have previously noted that while terminal C - C double bonds of vinylsilanes, vinylsiloxanes and styrenes are reactive, internal C-C double bonds are not. This selectivity has been exploited in the activated Ru catalyzed copolymerization of 4-acetylstilbenes with 1,3-divinyltetramethyldisiloxane. As expected, the internal C-C double bond of the stilbene does not react [22], see Figure 12.

OCH

0CH3

3

Figure 12 Chemical modification of unsaturated polysiloxanes with pendant or terminal Si-vinyl groups has been achieved by Ru catalyzed reaction with the single reactive ortho C - H bond of 2-methylacetophenone. In this way, Si-vinyl groups are converted to 2-(2'-acetyl-3'-methyl-phenethyl) groups [23], see Figure 13. It should be noted that In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

440


there is considerable current interest in chemical modification of polymers [24,25]. Similar Ru catalyzed reaction of unsaturated polysiloxanes with pendant Si-vinyl groups with the two reactive ortho C - H bonds of acetophenone leads to crosslinking.

Figure 13 On the basis of the above results, it is surprising that Ru catalyzed reaction between benzophenone which has four potentially reactive ortho C - H bonds and 1,3divinyltetramethyldisiloxane does not yield a crosslinked material - but rather a low molecular weight polymer as well as a cyclic monomer, as shown in Figure 14. Similar results have been obtained in the ruthenium catalyzed reaction with 4-benzoylpyridine and 1,3-divinyltetramethyldisiloxane. On the other hand, 4-acetylpyridine is unreactive. Apparently once one of the ortho C - H bond in benzophenone or 4benzoylpyridine has reacted, the second ortho C - H bond in the substituted aromatic ring suffers a significant decrease in reactivity. For this reason, the second ortho C-H bond which reacts is usually in the unsubstituted aromatic ring. The formation of cyclic compounds often occurs competitively in polymerization reactions [26].

Figure 14 A similar pattern of reactivity is observed with ferrocenyl ketones. Thus acetyl ferrocene is unreactive, while only one of the ortho C-H bonds of benzoyl ferrocene undergoes Ru catalyzed anti-Markovnikov addition reaction across the C - C double bonds of 1,3 inyltetramethyldisiloxane to yield a monomeric product. On the other hand, 1,1 -dibenzoylferrocene undergoes Ru catalyzed copolymerization with 1,3In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

441

divinyltetramethyldisiloxane, as shown in Figure 15. The reversible electron chemical oxidation of this material has been studied by cyclic voltametry [27].


o=

Figure 15 Macromolecules which are highly branched are of substantial interest due to their unusual properties [28-30], Hyperbranched materials do not have a well-defined architectural structure, by comparison to dendrimers, but rather an irregular pattern of branching. Hyperbranched materials can be prepared in a single step from a monomer which has two mutually reactive functional groups A and B present in a ratio such as AB2 [31,32]. l-P-(4 -Acetylphenyl)vinyl-3-vinyl-l,l,3,3-tetramethyldisiloxane is an AB2 type monomer in that it has two reactive C - H bonds which are ortho to the acetyl and a single reactive terminal Si-vinyl group. Reaction of this with activated Ru leads to a hyperbranched material [33], as shown in Figure 16. Similarly, Ru catalyzed reaction of 4-acetylstyrene gives a hyperbranched material [34]. ,

Figure 16 While there is considerable interest in conjugated copoly(arylene/l,2vinylene)s, there has been much less work on unsaturated cross-conjugated copoly(arylene/l,l-vinylene)s [35]. The Ru catalyzed reaction between a-tetralone and internal acetylenes such as phenyethynyltrimethylsilane has been reported by Murai to In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

442

yield E/Z-a-(8-a-tetralonyl)-p-trimethylstyrene [36]. We have applied this Ru catalyzed Markovnikov addition of the C-H bonds of aromatic ketones across the C - C triple bond of internal acetylene to directly prepare cross-conjugated copoly(arylene/l,l-vinylenes). Thus activated Ru catalyzed reaction of acetophenone and l,4-bis(trimethylsilylethynyl)benzene directly yields copoly[2-acetyl-1,3phenylene/aja'-bisCtrimethylsilylmethyleneVl^-xylenylene], as shown in Figure 17. Similar Ru catalyzed reactions between acetophenone and l,4-bis(phenylethynyl)benzene are also successful [37]. Downloaded by UCSF LIB CKM RSCS MGMT on November 28, 2014 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch029

ortho

Figure 17 The fluorescence spectra of these cross conjugated materials were performed on a PTI instrument equipped with a model A1010 Xe/Hg lamp and a model 710 photomultiplier detraction detector. Spectra were obtained on methylene chloride, toluene and D M S O solutions which had been degassed by bubbling argon through them for 10 min. Fluorescence quantum yields were determined relative to that of 7diethylamino-4-methyl coumarin [38]. The fluorescence emission of these materials occurs in the blue, as shown in Figure 18. However, the quantum yield for fluorescence is very low. It is probable that the acetyl group contributes to the low fluorescence. uv

Xnm

Figure 18 The Ru catalyzed reaction of aromatic ketones with vinyl and acetylenic ketones to yield a/f-poly(siloxane/carbosilanes) is a reaction which was first reported in 1994. While experimental results to date have helped to elucidate the mechanism, In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

443

scope and limitations of this reaction, much still remains to be done. It is our hope that further study will demonstrate that the Murai reaction will prove to be a highly versatile and valuable type of step growth polymerization reaction.


References 1. Hani, R.; Lenz, R. W., in "Silicon Based Polymer Science: A Comprehensive Resource", Ed. by Zeigler J. M . ; Gordon Fearon, F. W. ACS, Adv. in Chem. Series 224, American Chemical Society, Washington, D. C. 1990, p 741-752. 2. Merker, R. L.; Scott, M . J., J. Polymer Science A, 1964,2, 15. 3. Keohan F. L.; Hallgren, J. E., in Silicon Based Polymer Science: A Comprehensive Resource, Ed. by J. M . Zeigler and F. W. Gordon Fearon, ACS, Adv. in Chem. Series, 224, American Chemical Society, Washington, D. C. 1990, p 165-179. 4. Cummings, S.; Ginsburg, E.; Miller, R.; Portmess, J.; Smith, D. W. Jr.; Wagener, K., in "Step-Growth Polymers for High Performance Materials, New Synthetic Methods", Ed. by Hedrick, J. L.; Labadie, J. W., ACS Symposium Series 624, American Chemical Society, Washington, D.C. 1996. 5. Schrock, R. R.; DePue, R. T.; Feldman, J.; Schaverien, C. J.; Dewan, S. C.; Liu, A. H., J. Am. Chem. Soc., 1988, 110, 1423. 6. Guo, H.; Weber, W. P., Polymer Bull., 1994, 32, 525. 7. Levison, J. J.; Robinson, S. D., J. Chem. Soc., 1970, A , 2947. 8. Lynn, D.M.; Kanaoka, S.; Grubbs, R.H. J. Am. Chem. Soc., 1996, 118, 784. 9. Crivello, J. V.; Kim, W. G., Pure and Applied Chem., 1994, A31, 1005. 10. Murai, S.; Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Sonoda, M . ; Chatani, N., Nature, 1993, 366, 529. 11. Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M . ; Chatani, M . ; Murai, S., Bull. Chem. Soc. Jpn., 1995, 68, 62. 12. Tapsak, M . A.; Guo, H.; Weber, W. P., Polymer Bull., 1995, 34,49. 13. Guo, H.; Wang, G.; Weber, W. P., Polymer Bull., 1996,37,4231. 14. Guo, H.; Wang, G.; Tapsak, M . A.; Weber, W. P., Macromolecules, 1995,28, 5686. 15. Lu, P.; Paulasaari, J.; Jin, K.; Bau, R.; Weber, W. P., Organometallics, 1998, 17, 584. 16. Hiraki, K.; Koizumi, M . ; Kira, S. I.; Kawano, H., Chem. Letters, 1998, 47. 17. Paulasaari, J.; Weber, W. P., unpublished results. 18. Guo, H.; Weber, W. P., Polymer Bull., 1995, 35, 259. 19. Wang, G.; Guo, H.; Weber, W. P. Polymer Bull., 1996, 37, 169. 20. Wang, G.; Guo, H.; Weber, W. P., J. Organometal. Chem., 1996, 521, 351. 21. Guo, H.; Tapsak, M . A.; Weber, W. P., Macromolecules, 1995, 28, 4714. 22. Londergan, T. M . ; Weber, W. P., Makromol. Rapid Commun., 1997, 18, 207. 23. Guo, H.; Tapsak, M . A.; Weber, W. P., Polymer Bull., 1994, 33, 417. 24. Carraher, C. E . Jr.; Moore, J. A.,"Modification of Polymers", Plenum Press, New York 1983. 25. Benham, J. L.; Kinstle, J. F., "Chemical Reactions on Polymers", A C S Symposium Series 363, American Chemical Society, Washington, D. C. 1988.


444


26. Kepler, C. L.; Londergan, T. M . ; Lu, J.; Paulasaari, J.; Weber, W. P., Polymer in press. 27. Sargent J. R.; Weber, W. P., Polymer Preprints, 1998, 39-I, 274. 28. Tomalia, D.; Adv. Mater., 1994, 6, 529. 29. Frechet, J. M . , Science, 1994, 263, 1710. 30. Mekelburger, H . B.; Jaworek, W.; Vogtle, F., Angew. Chem., Int. Ed. Engl., 1992, 231, 1571. 31. Kim, Y. H. Webster, O. W., J. Am. Chem. Soc., 1990,112,4592. 32. Kim, Y. H. Webster, O. W., Macromolecules, 1992, 25, 5561. 33. Londergan, T. M . ; Weber, W. P., Polymer Bull., 1998, 40, 15. 34. Lu, P.; Paulasaari, J.; Weber, W. P., Macromolecules, 1996, 29, 8583. 35. Mao, S. S. H.; Tilley, T. D., J. Organometal Chem., 1996,521,425. 36. Kakiuchi, F.; Yamamoto, Y.; Chatani, N.; Murai, S., Chemistry Letters, 1995,681. 37. Londergan, T. M . ; You, Y.; Thompson, M . E.; Weber, W. P., Macromolecules, in press 1998. 38. Murov, S. L.; Carmichael, I.; Hug, G. L., Handbook of Photochemistry 2nd Ed., Marcel Dekker, Inc., New York, New York, 1993, p 24.


Silicones and Silicone-Modified Materials - American Chemical Society

Recommend Documents