Stereostructure of Polysilanes by Ring-Opening Polymerization

Chapter 4

Stereostructure of Polysilanes by Ring-Opening Polymerization 1

Eric Fossum, Jerzy Chrusciel, and Krzysztof Matyjaszewski

Downloaded by UNIV LAVAL on May 16, 2016 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch004

Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213

The stereoisomers of 1,2,3,4-tetramethyl-1,2,3,4-tetraphenyl-cyclotetrasilane were identified using a combination of spin labeling and chemical derivatization. The dominating isomer obtained in the synthesis possesses an all-trans configuration of the methyl and phenyl substituents. Polymerization of this isomer using silyl cuprates proceeds with two inversions of configuration and results in poly(methylphenylsilylene) with 25% isotactic, no syndiotactic, and 75% heterotactic triads as determined by Si N M R studies. 29

Polysilanes (polysilylenes) consist of a linear chain of silicon atoms carrying two substituents, generally, either alkyl or aryl.(l,2,3) Interest in these materials stems from their unique properties, such as sigma-catenation and thermochromic behavior. They have potential applications as photoresists, electro-optical devices, non-linear optical materials, and also as precursors to silicon carbide. Polysilanes have been prepared by several methods including: a) the reductive coupling of dichlorosilanes,(l,2) b) dehydrogenative coupling of hydridosilanes,(4) c) anionic polymerization of masked disilenes,(5) and d) ring opening polymerization of cyclotetrasilanes.(6) Each of these methods possesses its own advantages and disadvantages. The ring opening route allows for the preparation of well-defined polysilanes with controlled molecular weights, relatively low polydispersities, and defect free structures. In addition, it allows for potential control over the resulting microstructure if the monomers, with known configurations of substituents, can be opened in a controlled manner. Because the electronic properties of polysilanes are dependent on the conformation of the backbone,which may depend on the configuration of the substituents, it is necessary to prepare polymers with well-defined microstructures. In ring opening polymerization, the resulting polymer microstructure depends on two factors. The first one is the configuration of the substituents in the cyclic monomers. The second factor is the mechanism of the ring opening polymerization. This paper is mainly focused on the discussion of stereochemistry in the cyclotetrasilanes. 1

Corresponding author 0097-6156/94/0572-0032$08.00/0 © 1994 American Chemical Society Wisian-Neilson et al.; Inorganic and Organometallic Polymers II ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

4. FOSSUM ET AL.

33

Stereostructure of Polysilanes

Results and Discussion


Monomer Synthesis. The synthesis of l,2,3,4-tetramethyl-l,2,3,4tetraphenylcyclotetrasilane is shown in Scheme 1.(6) A slurry of octaphenylcyclotetrasilane in methylene chloride is treated with four equivalents of trifluoromethanesulfonic acid resulting in l,2,3,4-tetrakis(trifluoromethanesulfonoxy)1,2,3,4-tetraphenylcyclotetrasilane. The solvent is then removed under vacuum and the triflate derivative dissolved in benzene/toluene. This mixture is then reacted, at -30 °C, with four equivalents of methyl magnesium bromide resulting in a mixture of stereoisomers of l,2,3,4-tetramemyl-l,2,3,4-tetraphenylcyclotetrasilane. Three of the four possible isomers shown in Scheme I are obtained. The fourth isomer, having all four methyl groups on one side of the ring plane, has not been detected. The isomer possessing a l,2,3-up-4-down, lc, configuration of the methyl groups can be identified easily by the 1:2:1 pattern of peaks it givesriseto in Scheme I.

la

lb

lc

Id

2

and 29si NMR spectra. The ^ S i NMR spectrum is shown in Figure 1(*H and show similar patterns). In this spectrum isomer lc is identified, along with tentative assignments for isomers la and lb, both of which would giveriseto only one signal in all NMR spectra. If this mixture is allowed to stand in hexane at sub-ambient temperatures one of the isomers, either la or lb, precipitates out of the solution in « 95% purity. This compound givesriseto the single peak shown in Figure lb. Preliminary Assignments The compound, which can be purified to 95%, has previously been assigned to isomer la, based on intuitive arguments. Because isomer la has an all-trans configuration of substituents it can adopt a preferred conformation where all four phenylringsoccupy pseudo-equatorial positions and the four methyl groups are in pseudo-axial positions. Isomer lb cannot adopt such a conformation because it would always have two phenyl groups in pseudo-axial positions and two in pseudo-equatorial postions. Therefore, la should be sterically less crowded, and present in a higher proportion. This argumentation is based on thermodynamics and not kinetics because once the cyclotetrasilanes are formed the configurations are permanent.

Wisian-Neilson et al.; Inorganic and Organometallic Polymers II ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

34

INORGANIC AND ORGANOMETALLIC POLYMERS II


The line widths of the peaks for la and lb also give some insight into the system. In the *H and ^Si NMR spectra, the peak tentatively assigned to isomer la has line narrower widths than those assigned to lb (1.5:1). This may be a result of the preferred conformation that can be adopted by isomer la, whereas isomer lb can be rapidly equilibrating between two states of equal energy resulting in somewhat broader lines. However, these intuitive arguments are not sufficient to unambiguously assign the configuration of the dominating isomer. Because the stereochemistry of theringsis highly important for a detailed study of the microstructure of the resulting polymer, it was necessary to take a closer look at the problem. Several attempts at obtaining crystals pure enough for an X-ray study were unsuccessful and thus, the methods of spin labeling and chemical derivatization were employed.

-24X0

-26X0 PPM

-26X0 PPM

•28X0

29

Figure 1. S i NMR specta of a) the mixture of stereoisomers and b) purified to either isomer la or lb. Spin-Labeling Study Because the triflate derivatives are so versatile, it is quite simple to introduce many different functionalities onto the ring. Thus, introducing methyl groups which have 99% labels was achieved simply by substituting labeled methyl magnesium iodide in the second step of monomer preparation. This afforded compound 1*, now with !3c labeled methyl groups.(7) By examining the coupling patterns of the resulting NMR spectra some information about the configuration of the substituents around the ring of the starting compound could be obtained. The *H NMR spectrum of 1* in C6D6 is shown in Figure 2. The methyl region is a duplicate of the spectrum obtainedfromthe non-labeled compound, 1, but now split into doublets with a *JC-H of 123 Hz. This spectrum served as confirmation that the desired product had indeed been prepared. However, no useful information about the configuration of substituents was obtained. The spectrum was an exact copy of that observed for 1; no coupling from adjacent methyl groups was detected. This implies one of two possibilities: 1) the coupling constant JC-C is below the line width 3


4. FOSSUM ET AL.

35



resolution («1 Hz), or 2) the structures are rapidly interconverting between conformations and therefore, averaging each methyl group into equivalent positions wherein no C-C coupling would be present

1

ι

1 .961

•

1 .80

«

1 .70

1

——ι .60

1

PPM

Figure 2. 300 MHz *H NMR spectrum of the methyl region of compound 1*. 2

The ^ S i NMR spectrum of 1* is shown in Figure 3, in contrast to the spectrum of 1, the spectrum for 1* contains fine structure which provides deeper insight into the stereochemistry of each isomer. A simple splitting scheme with Lorentzian line shapes and spectral superposition was employed to derive the coupling constants shown in Table I. In order to simplify the discussion of the coupling constants in the stereoisomers, it should be emphasized that the each of the four silicon atoms in isomer lc is in an environment resembling that of the silicon atoms in la, lb, or Id. The two equivalent silicon atoms (lc*-Sib) in isomer lc resemble the environment of the silicon atoms in lb, and therefore, should be present with twice the intensity of those resembling la and Id. Analysis of the observed coupling constants from Table I indicates large values of J for all of the systems. Apparently, all of the values of *J for lc are higher than those for la and lb, regardless of the stereochemical environment. These values can be strongly dominated by the hybridization in the corresponding Si-C bonds. The values of 2j are quite similar in all cases, but smaller than 3j values; the latter provide the most useful information. The J value for lc*-Sib is the smallest («1.8 Hz). Values of 3j for the lc*S i and lc*-Sid are 4.6 and 6.8 Hz. The 3j values for the two synthetically available isomers la and lb are 1.9 and 4.6 Hz, respectively. Similar values of the coupling constants for lc*-Sib, present with twice the intensity of both lc*-Sia and lc*Sid, and isomer lb enable the assignment to lb. The equivalent coupling constants for lc*-Sia and la confirm the assignment to la. It is anticipated that 3j value for the unavailable isomer Id could be similar to that for lc*-Sid and equal to 3j « 6.8 Hz. 1

3

a



36


124

45

46

47

3Γ

ppm Figure 3. S i NMR spectrum of the mixture of stereoisomers of compound lc*. 2 9

Table L Carbon-Silicon Coupling Constants for the Stereoisomers of 1* Stereoisomer

ij

2j

3j

lb*

37.8 + 0.2 Hz

2.1 ± 0.2 Hz

1.9 ±0.2 Hz

la*

37.6 ± 0.2 Hz

2.1+0.2 Hz

4.6 + 0.2 Hz

38.5 ± 0.2 Hz

1.8 ± 0.2 Hz

4.5 ±0.2 Hz

lc*-Sib (x 2)

38.6 ± 0.2 Hz

2.1 ±0.2 Hz

1.8 ±0.2 Hz

lc*-Si

39.8 + 0.2 Hz

1.7 ± 0.2 Hz

6.8 ±0.2 Hz

lc*-Si

a

d

Si is the coupling constant between the observed silicon atom,and the methyl carbon directly attached. J C - S i is the coupling to the methyl carbons on two neighboring a silicon atoms, and ^JC-Si is the coupling constant to the methyl carbon on the b silicon atom. 2

Chemical Derivatization Because the above study left some ambiguity as to the configuration of the dominating isomer, further evidence was needed. The relatively high symmetry of both l a and l b makes differentiating between them quite difficult. Therefore, if the symmetry of the molecules could be lowered or destroyed completely, some insight into the configurations could be found. One possibility to change the symmetry was to remove


4.

FOSSUMETAL.

37


one more aromatic ring and replace it with a methyl group, thereby reducing the symmetry. This procedure is outlined in Scheme Π. If the dominating isomer is indeed la, treating this compound with one equivalent of triflic acid, followed by methylation, should yield compound 2 a (Me5Ph3Si4). This new compound should give rise to NMR spectra which could be used to determine the structure of the starting material. A NMR spectrum with four peaks in the ratio 2:1:1:1 is expected, and also a ^ s i NMR spectrum with three peaks in the ratio 2:1:1 should be observed. If the dominating isomer is lb, treatment with one equivalent of triflic acid, followed by methylation should lead to compound 2b. This compound should give rise to a completely different pattern of peaks in the NMR spectra. The *H NMR 2


Scheme Π.

spectrum should appear asfiveresonances in the ratio 1:1:1:1:1 for the methyl region and the ^ S i NMR spectrum should appear as four individual peaks in the ratio 1:1:1:1. 2

. 2 la

1 I—

—ι— —ι— -26.5 -27. β PPM Figure 4. S i NMR spectrum of the products of the reaction of la with one equivalent of triflic acid, followed be methylation. -25. 0

-25.5

-26. e

29


38


The resulting ^Si NMR spectrum is shown in Figure 4. As is apparent, the dominating isomer possesses the expected all-trans configuration of substituents. In addition to unreacted Me4Ph4Si4, mere are present, as much lower intensity peaks, which can be attributed to Me6Ph2Si4. The triflation reaction is known to occur with limited chemoselectively and this leads to the mixture of products.(8) However, the major peaks in both spectra give sufficient evidence for assignment of the dominating isomer to la.


Microstructure of PMPS With the configuration of the monomer known, it was then possible to obtain a clearer picture of the microstructures obtained from ring opening polymerization. The microstructure of PMPS presents an interesting problem. The simplest description of the microstructure of PMPS consists of triad data where groups of three silicon atoms in the chain are considered. There are three different possibilities for the stereochemistry at each group. Schematic representations of the three possibilities are shown in Scheme ΙΠ. Looking at the central silicon atom of the group of three, the configurations of the neighboring atoms are considered. If both adjacent atoms have the same configuration as the central atom, two meso junctions are present. This results in a mm triad, which corresponds to isotactic polymer. Scheme ΙΠ.

mm, isotactic

rr, syndiotactic

mr or rm, heterotactic If the neighboring silicon atoms have different configurations from that of the central atom, two racemic junctions are then present, and a rr triad results, which


4. F O S S U M E T A L .

Stereostructure ofPolysilanes

39

corresponds to syndiotactic polymer. If the configurations of the two adjacent silicon atoms are not the same, then bom meso and racemic junctures are present and this gives rise to either mr or rm triads result, corresponding to heterotactic polymer. 2 9

Assignments of Configuration. In the S i NMR spectrum of PMPS prepared by the reductive coupling of dichloromethylphenysilane there are three broad resonances present at -38.5, -39.0, and -41.0 ppm. West et al. have assigned these resonances to isotactic, syndiotactic, and heterotactic PMPS respectively.(9) The assignments were based on the corresponding six membered rings where the molecules are forced into configurations which may be quite different from those in the linear chain; cis and gauche for the cyclics vs. trans and gauche for the linear chain. Therefore, the shielding effects of the aromatic rings may be quite differentfromthose present in the polymer.


Stereoselective Ring Opening of la Knowing the configuration of the monomer allows a detailed look at the microstructure of PMPSfromring opening polymerization. The all-trans monomer, la, can be opened via three different pathways. Scheme IV depicts the possibilities. If ring opening occurs with two retentions of configuration at the attacked silicon atom and the newly formed reactive center, then a rrr sequence is built up with the possibility for defects to occur at the connections between monomer units because the monomer unit can be attacked on either of two faces of each of the prochiral silicon atoms in the ring. This results in a sequence of [(rrr)m/r], where the ratio m/r is equal to 1, leading to triad data which would consist of 75% syndiotactic, 25% heterotactic, and 0% isotactic triads, respectively.

2XRet.

[(rrr)r/m]

n

If ring opening occurs via two inversions of configuration the sequence of [(mrm)m/r] would result. This corresponds to 75% heterotactic, 25% isotactic, and 0% syndiotactic triads, respectively. The third potential pathway involves both a retention and an inversion of configuration. Regardless of whether the retention or inversion occurs first, the configuration sequence of [(mrr)m/r] is observed. This would result in a polymer with 37.5% syndiotactic, 12.5% isotactic, and 50% heterotactic triads, respectively.


40


Ring Opening Polymerization The 29si NMR spectrum of the polymer prepared by ring opening polymerization of the mixture of stereoisomers, 1, using BuLi/cryptand in benzene, was very similar to that observed for the polymer prepared via the reductive coupling route. One possible explanation for this observation may involve scrambling (transsilylation) reactions. If this process occurs via an intramolecular pathway, the formation of macrocycles should be observed. However, the polymerizations were terminated before the scrambling could occur. Apparently, with L i as the counterion, very little, if any, control over the pathway of ring opening can be achieved. The result is then a random distribution of inversions and retentions leading to stereorandom polymer. Therefore, to achieve some degree of control over whether the polymerization occurs with retentions or inversion of configuration at the attacked silicon atom and the newly formed reactive center, a modified initiator must be employed.


+

Silyl Cuprates. The use of organocuprates in organic synthesis has been studied intensively.(lO) More recently, silyl cuprates have received some attention for use in organic chemistry and also for polymer synthesis. Silyl cuprates are prepared by the reaction of varying numbers of equivalents of R3SiLi with CuX. Oehlschlager and Sharma have studied the reaction of PhMe2SiLi with CuCN in tetrahydrofuran.(ll) They have found that there are many equilibria present in solutions of cuprates prepared with various ratios of PhMe2SiIi to CuCN. The reaction of one equivalent of silyl lithium results in the monosilyl cuprate and no free silyl lithium present. At two equivalents, the majority of the cuprates exists as the disilyl compounds with only a small amount of both the mono- and trisilyl compounds present in solution. A third equivalent displaces the cyano group as LiCN giving the trisilyl cuprate. These equilibria are shown in Scheme V. Scheme V. PhMe SiLi+ CuCN

PhMe SiCu(CN)Li

2

2

PhMe SiLi [ 2

PhMe SiLi 2

L i C N + (PhMe Si) CuLi 2

3

2

*

(PhMe Si) Cu(CN)Li 2

2

2

Ring Opening Using Silyl Cuprates Ring opening polymerization of l a was attempted with both the monosilyl and disilyl cuprates. The monosilyl cuprate was used unsuccessfully to initiate polymerization, however, use of the disilyl cuprate resulted in nearly quantitative conversion to monomodal polymer with polydispersities in the range of M / M = 1.5. The polymerizations were carried out in THF at room temperature, which is interesting in itself, because if the polymerizations using BuLi are carried out in THF there is rapid decomposition of the polymer to five and six-membered cyclic polysilanes, however, with the silyl cuprates, the formation of cyclics is not observed, even after 24 hours. The most interesting result of the polymerizations employing silyl cuprates is the remarkably different microstructures of the formed polysilanes. The *H and l^C NMR w

n


4. F O S S U M E T A K

Stereostructure ofPolysilanes

41


+

spectra have much sharper peaks than those observed using L i as the counterion ion and also from polymers prepared by the reductive coupling route. However, the most dramatic differences can be seen in the 29si NMR spectrum shown in Figure 5. The spectrum shows only two signals present at -38.5 and -41.0 ppm, in the ratio 3:1, no signal at -39.0 ppm is detected From the previously discussed possibilities of ring opening, the two microstructures which can be obtained are 3:1 syndiotactic:heterotactic and 3:1 heterotactictisotactic, corresponding to two retentions and two inversions of configuration, respectively. On the other hand, polymerization of the mixture of all three steroisomers in the ratio 28% (la): 14% (lb):58% (lc) provides three signals in the ratio 58% (-38.5 ppm): 15% (-39ppm): 27% (-41 ppm). Combination of the results obtained by the polymerization of die mixture and all trans isomer suggests the following assignment: -38.5 ppm (heterotactic), -39 ppm (syndiotactic), and -41 ppm (isotactic). This also indicates that ring-opening polymerization with silyl cuprates occurs with two inversions of configuration at the attacked silicon atom and at the newly developed growing center.

-40.00 PPM

-40.00 PPM

29

Figure 5. S i NMR spectra of a) PMPS prepared by reductive coupling and b) PMPS prepared by theringopening polymerization of la with silyl cuprates. Polymer Properties. The absorption and emission spectra of the polymer prepared by the silyl cuprate polymerization of la are very similar to those of PMPS prepared by reductive coupling. The room temperature absorption maximum, = 336 nm, is 2 nm lower than for the polymer prepared by the reductive coupling process. No thermal transitions were observed during differential scanning calorimetry measurements, indicating a low degree of crystallinity in the polymer, which is consistent with a highly heterotactic polymer.


42


Conclusions Ring opening polymerization of cyclotetrasilanes provides a unique and quite promising route to polysilanes. l,2,3,4-Tetramethyl-l,2,3,4-tetraphenylcyclotetrasilane can be prepared and the dominating isomer possessing an all-trans configuration of substituents can be isolated in up to 95% purity. Ring opening polymerization of the monomer using silyl cuprates results in poly (methylphenylsilylene) with 75% heterotactic triads and 25% isotactic triads. The polymerization occurs via two inversions of configuration at the attacked silicon atom and also at the newly formed reactive center. Acknowledgements. Partial support from the National Science Foundation and the Office of Naval Research is kindly acknowledged. Downloaded by UNIV LAVAL on May 16, 2016 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch004

Literature Cited. 1. Miller, R.D.; Michl, J. Chem. Rev. 1989, 89, 1359. 2. West, R. J. Organomet. Chem. 1986, 300, 327. 3. Matyjaszewski, K . ; Cypryk, M.; Frey, H . ; Hrkach, J.; K i m , H.K.; Moeller, M.; Ruehl, K . ; White, M. J. Macromol. Sci.-Chem. 1991 A28, 1151. 4. Tilley, T.D. Acc. Chem. Res. 1993, 26, 22. 5. Sakamoto, K.; Obata, K . ; Hirata, H . ; Nakajima, M.; Sakurai, H. J. Am. Chem. Soc. 1989, 111, 7641 . 6. Matyjaszewski, K.; Gupta, Y . ; Cypryk, M. J. Am. Chem. Soc. 1991, 113, 1046 . 7. Fossum, E.; Gordon-Wylie, S. W.; Matyjaszewski, K . Organometallics, in press. 8. Chrusciel, J.; Cypryk, M.; Fossum, E.; Matyjaszewski, K.; Organometallics 1992, 11, 3257. 9. Maxka, J.; Mitter, F.K.; Powell, D.R.; West, R. Organometallics 1991, 10, 660. 10. Lipshutz, Β. H. Synthesis 1987, 4, 325 . 11. Sharma, S.; Oehlschlager, A . Tetrahedron 1989,45, 557. R E C E I V E D April 8, 1994


Stereostructure of Polysilanes by Ring-Opening Polymerization

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