Polymerization of methylsilylenes into polymethylsilanes or polycarbosilanes after dechlorination of dichloromethylsilanes?

Abstract

Competition between two different polymerization pathways to polymethylsilanes (PMS) and polycarbosilanes (PCS) was studied. This occurs in a Wurtz reduction-coupling reaction system during dechlorination of several dichloromethylsilanes via sodium in toluene. These two reactions do not carry on pari-passu, i.e., PMS is exclusively formed. However, PCS can be formed dominantly when some zirconocenes are introduced. This competition was introduced as a result of a tautomeric transformation from methylsilylene (MeRSi:) into 1-silene (CH₂=SiRH). These are formed simultaneously as two intermediates but are influenced by the substitution of methyl, ethyl, phenyl or vinyl groups on silicon in the dichloromethylsilanes. Polymerization of methylsilylenes into PMS can be inhibited by deactivation of the sodium surface via chemical adsorption of zirconocene dichloride. Hence, the catalytic insertion polymerization of 1-silene intermediates into PCS occurs at the active sites of zirconocenes—this leads to the formation of PCS conversion ratios of 82% to 93% from dichloromethylsilanes with methyl, ethyl and phenyl as substituents. However, catalytic polymerization does not happen from dichloromethylvinylsilane. The mechanisms of insertion polymerization as well as the thermal dynamic barrier from polymethylvinylsilane (PMVS) to polyvinylcarbosilane (PVCS) are discussed in detail.

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Introduction

Polycarbosilanes (PCSs) may be cataloged as a large group of polymers in which silicon and carbon atoms are connected in many manners and sequences to form the backbone of the macromolecule.¹ However, PCSs with alternating silicon to carbon sequences in their main chains are still the most important organic precursor products for silicon carbide ceramic, which can be a fiber and matrix for fiber-reinforced composites and monoliths.^2,3 Since the 1950s, G. Fritz, M. Kumada, and especially S. Yajima have pioneered the synthesis of cross-linked PCS via thermal rearrangement of polydimethylsilane (PDMS) which is synthesized via a Wurtz reduction-coupling reaction of dichlorodimethylsilane (SiCl₂Me₂:DCDMS) by sodium—both are cheap raw chemicals.^2,4 In this process, PCS is thus derived not directly from DCDMS but with PDMS as a medium which is thermally cracked at high temperatures and undergoes further complicated radical rearrangements (this process is now known as the Kumada rearrangement⁵):

During this energy-intensive process, PCS was produced as a condensed polymer, accompanied by lots of volatile and flammable cyclic silanes as byproducts. The total yield from DCDMS to PCS is usually below 50%. PCS-470 is known as a commercial product and generally exhibits a mean molecular weight of around 1000. Meanwhile it has significant oxygen contamination, because water or alcohols are used to remove the residual sodium in a washing procedure.²

On the other hand, single-site catalysts (SSCs) of metallocene containing transition metals of titanium, zirconium and hafnium combining with MAO (methylaluminoxide) have been widely used for some 1-olefin polymerizations.^6–9 Such soluble metallocenes are very efficient for 1-olefin polymerization and deliver polymers with much narrower molecular distributions than those with traditional insoluble Ziegler–Natta multi-site catalysts.⁸

In organosilicon chemistry, zirconocenes have also found many applications. Some organosilane monomers containing Si–H bonds can be linked through the formation of Si–Si bonds in the coordination sphere—this offers access to polysilanes but with low molecular weights.^10,11 The mechanism of this process was assumed to be a catalytic disproportionation of Si–H to Si–Si and H–H (hydrogen elimination) via a pathway of metal–silicon (Zr–Si) bonds. Meanwhile, some zirconocenes of ZrCp₂H₂, ZrCp₂HCl and ZrCp₂(CH=CH₂)₂ were used to modify and cross-link polyvinylsilane and polymethylsilanes to improve their ceramic yields or form hybrid pre-ceramic precursors containing metal carbides,^12–14 in which zirconocenes were found to functionalize branch groups through a hydrogen elimination reaction between Si–H (in polymethylsilane) and C–H (in the substituted vinyl group) of polysilanes.¹² In these chemical reactions, the mechanism clearly differs from the insertion of 1-olefin coordinative into the metal–carbon (Zr–C) bond (Patat–Sinn mechanism).⁸

Analogously to the catalytic polymerization of 1-olefin, metallocene catalytic insertion polymerization of a 1-methylsilene monomer directly into PCS has been found in our group recently,¹⁵ in which the 1-methylsilene monomer was formed from DCDMS during dechlorination by sodium at temperatures around 100 °C when metallocenes were introduced into the Wurtz reaction system. PCS synthesized via this molecular insertion process exhibits very small polymolecularities (M_w/M_n < 1.5) with a linear chain structure. Hence, it is better for spinning into long ceramic fibers. Transition metal complexes of Ti, Zr and Hf do not need to be removed from PCS because they are still converted during pyrolysis into metal carbides which can then act as reinforcements to improve the thermal and oxidation resistance of the SiC ceramic.^16–18

Theoretically, 1-methylsilene monomers can be formed from any dichloromethylsilane with various substituents to silicon. This is because 1-silene is a tautomeric intermediate co-existing with its twin moiety—methylsilylene—a bi-radical after dechlorination of DCMS. Conversion between these two tautomeric states is reversible depending on their exposed UV-visible irradiation energy (wavelength-dependent).^19,20 Progressively, the polymerizability from methylsilylene to polymethylsilane was confirmed to be a kinetic fast process simply via a merging and coupling reaction of this bi-radical.²¹ However, competitive polymerization from the 1-methylsilene tautomer to PCS has more constraints including the catalytic effectiveness of metallocenes, the competition from the fast dimerization of silylenes into polysilane that can exhaust the silicon source, and the stability of the resulting 1-silene intermediates.

PCSs with different substituents play very important roles in not only increasing ceramic yields, but also in improving the composition of the pyrolysized ceramics,^4,22 which is important for its applications in illumination and semiconductors.²³ Therefore, four kinds of DCMSs with different substituents of methyl, ethyl, phenyl and vinyl groups were used to investigate the possibility of synthesizing various polymeric precursors as well as their general mechanisms of catalytic insertion polymerization.

Experimental section

Synthesis

The synthesis procedures were as follows: 100 ml of toluene and a certain amount of zirconocene dichloride and sodium were put in a 500 ml four-necked flask with a dropping funnel and reflux condenser. Four kinds of dichloromethylsilane were used with different substituents including dichlorodimethylsilane (DCDMS), dichloromethylphenylsilane (DCMPS), dichloromethylethylsilane (DCMES) and dichloromethylvinylsilane (DCMVS). The mixture was heated to 100 °C under a nitrogen atmosphere. When the sodium melted, the corresponding amount of dichloromethylsilane was introduced to the system with a dropping funnel and fast stirring. The color of the solution quickly changed to wine red, and the reaction was terminated when the system atmosphere became neutral after ∼5 hours. During the process of reaction, water and air, including those dissolved in the solvent, needed to be isolated from the reaction system because of the sodium. The precipitate was removed via suction filtration from the wine red solution. Residual sodium was eliminated with methanol. The PCS products were separated from the solvent by rotary evaporator including polymethylcarbosilane (PMCS), polyphenylcarbosilane (PPCS) and polyethylcarbosilane (PECS). The so-called “polyvinylcarbosilane (PVCS)” cannot be synthesized via this catalytic process. The ratios of zirconocene dichloride and dichloromethylsilane used vary in terms of achieving different Zr/Si mole ratios ranging from 1/3 to 1/50.

Characterization

The synthesized polymers were subjected to the following measurements: Fourier transform infrared spectra (FTIR), detection was performed using a Thermo Nicolet IS50 FTIR from 4000 to 400 cm⁻¹. Ultraviolet-visible spectroscopy (UV) was measured on an ultraviolet-visible light detector (HitachiU-4100, Hitachi Limited Japan) from 200 nm to 800 nm. The molecular weight of the polymer was determined on a MALDI-TOF (time of flight mass spectrometer, Bruker Autoflex III). Gel permeation chromatography (GPC) analysis of the polymer was done on a Waters1515 GPC Instrument using toluene as the eluent and polystyrene as the standard. DFT calculation was conducted on a Gaussian09 platform using ‘wb97xd’ with a basis set of 6-311++g**.

Results

Firstly, we experimentally confirmed that DCDMS, DCMPS, DCMES and DCMVS could be reduction-coupled into linear and cyclic polysilanes via sodium in toluene. This was done either independently or via copolymerization.^21,24 Their polymeric products are PDMS, PPMS, PEMS and PVMS.

Secondly, we found that wine-red polycarbosilanes preferentially formed in the toluene when the zirconocene dichloride was introduced into the Wurtz-type system. Dichlormethylsilanes of DCDMS, DCMPS and DCMES were used as described in the Experimental section. Based on the weight of the various dichlorosilanes, the resulting toluene-soluble polymers had yields of 90.1, 93.8 and 82.6%, respectively. Preferential formation of these PCSs (PMCS, PPCS and PECS) indicates a very strong inhibition of polymethylsilane formation via the introduced zirconocene. However, “polyvinylcarbosilane” was not obtained when DCMVS was used in the reaction system, i.e., PMVS was exclusively formed as an insoluble precipitate in toluene—it showed no interaction with the zirconocene.

In contrast to the polymethylsilanes, PCS with a repeating unit of (–CH₂–SiHR–) can be easily recognized by its infrared spectra adsorption. The FT-IR spectra of polymers synthesized with various dichloromethylsilanes are shown in Fig. 1. The spectra top to bottom are of the polymers made from SiCl₂(CH₃)₂, SiCl₂CH₃C₆H₅, and SiCl₂CH₃C₂H₅. The Si–H bonds and Si–CH₂–Si bridge bonds are generated from PCS but absent in PDMS. Bands were observed at 2100 cm⁻¹ (Si–H stretching), 880 cm⁻¹ (Si–H deformation), 1020 cm⁻¹ (Si–CH₂–Si stretching) and 1355 cm⁻¹ (Si–CH₂–Si bending) for PMCS. Absorption at 600 to 920 cm⁻¹ (Si–CH₃ rocking and stretching), 1250 cm⁻¹ (Si–CH₃ symmetric deformation), 1400 cm⁻¹ (Si–CH₃ asymmetric deformation), and 2900 and 2950 cm⁻¹ (C–H stretching of Si–CH₃) was observed in all samples. This is evidence of Si–CH₃ in the polymers. Therefore, polycarbosilane was formed preferentially when zirconocenes were added into the reaction system. In addition, the intensity of the Si–H bond is different when the Si–CH₃ bonds are at the same strength, which indicates that there are different amounts of Si–H bonds in the three polycarbosilanes.

Fig. 1 FT-IR spectra of PDMS and the catalytic synthesized PMCS, PECS and PPCS.

The existing Si–Si segments in the backbone of PMCS were investigated with UV spectroscopy (Fig. 2). There is a broad absorption with a peak at 265 nm in the spectrum. Polysilane has no π-electrons in the linear system, but the absorption shifts to longer wavelengths with increased length of the Si–Si chain and accompanying electron conjugation. When there are 5 monomers connected with Si–Si bonds, the UV maximum absorbance is ∼272 nm.^24,25 This implies that the resulting polymer has a chain structure in which there are fewer than 5 monomers connected with the Si–Si bond. The reasons underlying this will be discussed with the catalytic polymerization mechanism in the Discussion.

Fig. 2 UV spectra of the synthesized PMCS.

The molecular weight and distribution of the synthesized PCS were determined with a TOF-MS. The absolute weight values and molecular structure information can be analyzed directly from the TOF-MS determination, which is shown for PCS in Fig. 3. The molecular weight of PMCS is near 1100 Da with a maximum value of up to 1800 Da. The molecular weight distributions of all PCSs are very narrow. The polymolecularity (ratio of M_w to M_n) is 1.31—this is quite small for most polymers especially those including traditional PCS-470 with very broad polymolecularity around 20. This is because PCS formed via the decomposition of PDMS exhibits a complex cross-linked molecular structure in contrast to the linear chains. These narrow molecular weight distributions of PCS agree with the characteristic features of the metallocene catalytic insertion polymerization of 1-olefins.⁸ Pyrolysis of the synthesized PCSs was also investigated by heating the polymers up to 1500 °C in argon. This offers ceramic yields from 60 to 73% with ZrC, SiC and free carbon in the residues.

Fig. 3 Molecular weight and distribution of the PMCS determined by MALDI-TOF-MS.

Discussion

Thermodynamic considerations

The thermodynamics of two polymerization pathways leading to PMS and PCS should be considered. It is somewhat obvious that PCS is more stable than the PMS isomer because it provides the base for a thermal Kumada-rearrangement reaction.^4,5 However, the two possible polymerization pathways start simultaneously from a pair of tautomers including methylsilylene (I) and 1-silene (II) in a metallocene catalytic Wurtz-type reaction system. Previously,¹⁹ the molecular ratios of these two tautomeric moieties have been shown to depend on the exposure to UV-visible irradiation. Photoselection of the 450 nm band converts I into II while photoselection at 260 nm converts II back into I. This wavelength dependence indicates the presence of an energy difference between the two tautomers. We calculated the energy barriers of conversion from methylsilylene to 1-silene with various substituents including methyl, ethyl, phenyl and vinyl using a traditional DFT program.²⁶ The results are summarized in Table 1, and the energy data of conversion from dimethylsilylene to 1-methylsilene is illustrated in Fig. 4.

Table 1 DFT calculated energies of various tautomeric intermediates of methylsilylene (CH₃RSi:) with various substituents (states corresponding to Fig. 4)

P	R
	–CH3	–C6H5	–C2H5	–C2H3 (cis)	–C2H3 (trans)
	E kJ mol^−1
a Excitation energy of reactant.b Energy barrier of positive reaction.c Energy barrier of reverse reaction.d Excitation energy of products.
A–A*^a	107.99	104.39	114.34	102.88	108.49
A*–B^b	191.77	199.40	185.43	199.62	198.25
B–C^c	157.82	159.85	157.98	158.09	157.81
C–D^d	145.77	135.14	141.63	134.66	140.82
(D–A)	−3.89	−8.80	−0.16	−9.75	−8.11

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Fig. 4 DFT calculation of the energies of various tautomeric intermediates and conversion pathway.

As shown in Fig. 4, singlet methylsilylene is set in the calculation as the ground state (A). This is excited into its triplet state (A*) with about 103–114 kJ mol⁻¹ energy adsorption. This subsequently forms its transient state (B) containing a Si–C–H triangular bond with a total energy adsorption ranging from 299–306.6 kJ mol⁻¹ depending on different substituents. The 1-silene (D) is formed via a triplet transition state of C with an energy difference of 135–145 kJ mol⁻¹ (see Table 1). However, the energy differences between intermediates with different substituents are quite small. Therefore, the energy barrier existing in the conversion from methylsilylenes to 1-silenes is as high as ∼300 kJ mol⁻¹ but shows very small differences influenced by the substituents. Energy differences also exist between the two tautomers of methylsilylenes (A) and 1-silenes (D) varying from −0.16 to −9.75 kJ mol⁻¹. This indicates that 1-silenes are slightly more thermally stable than methylsilylenes.

In summary, the tautomers of 1-silenes and methylsilylenes are both thermally dynamically favored. Polydimethylsilane is not formed via a polymerization of dimethylsilylenes in a Wurtz reduction system but rather proceeds via a step-by-step radical merging of dimethylchlorosilane (CH₃)₂ClSi˙. This leads to a chain propagation or cyclization.²¹ Here, the energy changes from the two dimethylsilylenes to their dimer and further to their isomer of two kinds of carbosilanes—A and B—were roughly calculated and are shown in Fig. 5. These are all energy decreasing pathways because of the formation of new Si–Si and Si–C bonds. However, energy differences between the polysilane dimer and methylcarbosilane-A isomers are still big (23 kJ mol⁻¹). This means that the methylcarbosilane-A is more stable than the dimer of dimethylsilane simply because one Si–C bond plus one Si–H bond is stronger than the sum of one Si–Si and one C–H bond. In other words, conversion from PDMS to PCS is a thermo-dynamically preferred pathway as illustrated by the experimental results of Kumada and Fritz many years ago. This must proceed via insertions of –CH₂– groups into the Si–Si chain one-by-one as indicated by this calculation.^2,3

Fig. 5 Thermodynamic consideration of two pathways of polymerization.

However, the conversion of a dimer of dimethylsilylene to methylcarbosilane-B has an energy barrier of 86.5 kJ mol⁻¹ (see to Fig. 5). This high energy barrier may inhibit this conversion during the dechlorination of DCDMS by sodium because the dimerization of methylsilylenes is a much faster kinetic process that leads directly to PDMS which can only be thermally converted into polycarbosilanes or post-processed at high temperatures.^2–5 On the other hand, the transition from methylsilylene to methylcarbosilane-B is also a thermally dynamically favored pathway that results in an energy decrease to about 100 kJ mol⁻¹. Hence, PCS can be formed through two competitive pathways, one is a thermal crack and conversion of PDMS at high temperatures, and the other is a catalytic polymerization of silenes that need strong inhibition of the dimerization of methylsilylenes.

Calculations of the energy between a methylsilylene-dimer and carbosilane with other substituents were not performed because of a shortage of chemical data. However, the above thermal dynamic considerations should also be suitable to explain the failure of catalytic syntheses of PVCS. According to the DFT data (Table 1), we inferred that if PVCS cannot be formed through a metallocene catalytic process, then it should also not be formed via thermal cracking and rearrangement of PMVS. This is supported by the experimental data. PMVS heated to 400 °C under 4 MPa pressure of argon does not undergo a Kumada rearrangement into a carbosilane. The FT-IR spectra of PMVS before and after the heat-treatments are given in Fig. 6b. In these, the characteristics of PCS with a repeating unit of (–CH₂–SiH–) and Si–H adsorption are not observed. As a comparison, these adsorption bands do appear via heating PMPS under the same experimental conditions (Fig. 6a). In other words, PVCS cannot be formed through either a metallocene catalytic process nor by thermal cracking and rearrangement. So, the only thing we are sure of is that the failure of the synthesis of PVCS is not controlled by thermodynamics but some other factors. Further investigation must be done to confirm the factors.

Fig. 6 FT-IR spectra of PMPS and PMVS before and after high pressure thermal treatment at 400 °C.

The mechanism of polymerization

According to the results in Fig. 1 and 2, insertion polymerization of 1-silene (R–SiH=CH₂, R = CH₃, C₂H₅ and phenyl) into PCS was achieved during dechlorination of dichloromethylsilanes while zirconocenes were used as catalysts. During the dechlorination reactions, the 1-silene was assumed to be trapped in the catalytic centers of activated metallocenes from its tautomeric hybrids of methylsilylenes (CH₃RSi:). Polymerization or cyclization of these highly reactive intermediates is simply inhibited by deactivation of the sodium surface by chemisorption of zirconocene dichloride (schematically illustrated in Fig. 7). Therefore, diffusion-limited dechlorination reactions combined with the highly effective metallocene catalytic polymerization results in a one-by-one insertion of 1-silene into PCS with yields as high as 90%. The resulting macromolecules exhibit an average molecular weight around 1800 and a very narrow distribution of 1.31. The mechanism underlying this catalytic insertion polymerization of 1-silenes into polycarbosilanes should be similar to that of 1-olefin.^6–9 However, methylalumoxanes (MAOs) were not used in this catalytic polymerization system although they are essential to the catalytic polymerization of the 1-olefin.

Fig. 7 Schematic illustrations of two polymerization pathways to PDMS (a) and PCS via zirconocene catalytic insertion (b).

Sodium combined with the formed silane radicals plays an alternative role in the active site initiation and regeneration on these metallocenes. Some possible steps of active center generation by sodium and silanes are listed in Fig. 8. The mechanism includes several steps: (1) initiation by dechlorination and methylsilylene formation; (2) tautomeric conversation between methylsilylene and 1-silene; (3) formation of catalytic sites via dechlorination and zirconium carbonization or siliconization; and (4) insertion of Si=C into the Zr–C active site. Clearly, this mechanism is an analog to the polymerization of 1-olefins. According to this suggested mechanism, small silane dimer or trimer segments should exist in the polycarbosilanes that were also detected with UV spectra (Fig. 2).

Fig. 8 A suggested mechanism of the metallocene catalytic insertion polymerization of 1-silene.

Molecular weight of PCS

A detailed mechanism for the termination of the growing chain of polycarbosilane is still lacking. This assumes that β-hydrogen elimination takes place in the metallocene catalytic insertion polymerization of 1-olefins.⁷ However, metallocenes do not have to be removed from PCS after termination of their chain growth. This allows conversion to carbide ceramics together with SiC to form a homogeneously dispersed composite ceramics on the nanometer scale during pyrolysis at elevated temperatures.

According to the molecular weight determination using TOF-MS and GPC, the average molecular weights of all PCSs synthesized using various zirconocenes as catalysts are small; however, there are differences in terms of the maximum value for each. Polyolefins synthesized via metallocene catalytic processes exhibit both high molecular weights and extremely narrow polymolecularities. The former is caused by high catalytic effectiveness of the activated metallocenes, and the latter is from their unique characteristics including a single site catalytic (SSC) structure. Contrary to the homogeneous catalytic polymerization of olefins, insertion polymerization of 1-methylsilene can only occur on the surface of sodium in which two tautomeric intermediates of dimethylsilylene and methylsilene are formed and subsequently and preferentially converted into polycarbosilane via the SSC insertion polymerization. Therefore, termination of this living chain can be caused by: (1) lack of sodium particles which are continuously consumed during the dechlorination of dichlorometallocene and dichlorosilanes, or (2) cleavage of metallocenes from the surface of sodium or leaving from the diffusion zone around a sodium particle. Continuous growth of carbosilane segments following a metallocene catalyst adsorbed on the sodium surface results in growth in the toluene solvent or other aromatic hydrocarbons where PCS exhibits very high solubility. Therefore, it is logical to conclude that longer polymer chains can be obtained either via sodium particles with bigger diameters and adequate excess stoichiometric ratios for the dechlorination reactions, or through the use of alkanes as solvents, in which polycarbosilane exhibits lower solubility and there are weaker forces abstracting these growing chains from a sodium surface. Both effects have been considered and confirmed. Polycarbosilanes with an average molecular weight above 5000 can be obtained using octane instead of toluene.

Conclusions

In a Wurtz dechlorination coupling reaction system, polymethylsilanes can be formed from transient intermediates of methylsilylenes with various substituents to silicon such as methyl, ethyl, phenyl, vinyl and hydrogen. However, polycarbosilanes can be directly formed from some of these dichloromethylsilanes as long as zirconocene dichloride is used as a catalyst. It was assumed that the catalytic insertion polymerization of methylsilenes, as well as the tautomeric intermediates of methylsilylenes, occurs on a sodium surface, and that the formation of polymethylsilanes was inhibited simply because of a depletion of methylsilylene by transforming into the tautomer. PCSs cannot be formed from DMVS—this is not caused by stereospecific blockade effect because insertion polymerization happens with dichloro-methylphenylsilane and -methylethylsilane. The DFT energy calculations as well as thermal cracking experiments indicate a thermally dynamic limitation.

Acknowledgements

This work was supported by the Chinese Academy of Sciences, National Natural Science Foundation of China (51472243) and National High-tech R&D Program (MOST).

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