Polycarbosilane polymers

The discovery by Yajima that polysilanes could be pyrolyzed to silicon carbide was mentioned in the introduction.7 In this process, either (Me2Si) or the cyclic oligomer (Me2Si)6 are synthesized from Me2SiCl2 and are then heated to near 450 °C (Scheme 5.10). This discovery has been commercialized by the Nippon Carbon Co. for the production of NICALON silicon carbide fibers. In this process, methylene groups become inserted into many of the Si-Si bonds to give a polycarbosilane polymer with the idealized 5.14. 

However, interest in this technology was ignited by the extensive and pioneering work of Yajima and co-workers (6, 7). In an early work, Yajima and Hayashi (6) applied the known Kumada rearrangement to Burkhard s poly(dimethylsilane) polymer and obtained melt-spinnable polycarbosilane polymers (equation 2). 

Fig. 28.21 Reactions involved in the radiation and thermal curing processes in the production of cross-linked polycarbosilane polymers.

The purpose of this part of the work is to obtain new polycarbosilane polymers and systematically study their gas separation properties. 

Polymetallocarbosilanes. PolymetaHocarbosilanes having a number-average molecular weight of 700—100,000 can be prepared by reaction of polycarbosilane, /2 2 fx where R is H, or lower alkyl, with a tetraalkyl titanate, to give a mono-, di-, tri-, or tetrafunctional polymer... 

Polycarbosilanes are an interesting class of polymers from a materials standpoint however, they do not occur naturally. A variety of synthetic methods can be employed to produce such polymers, but this part of the chapter will focus on using ADMET as a viable route to synthesize polycarbosilanes, siloxanes, and other silicon-containing polymers. 

The first type of polycarbosilane synthesized by using ADMET methodology was a poly[carbo(dimethyl)silane].14c Linear poly(carbosilanes) are an important class of silicon-containing polymers due to their thermal, electronic, and optical properties.41 They are also ceramic precursors to silicon carbide after pyrolysis. ADMET opens up a new route to synthesize poly(carbosilanes), one that avoids many of the limitations found in earlier synthetic methods.41... 

There are, however, other classes of inorganic and organometallic polymers that deserve consideration due to their considerable scientific and applicative relevance, such as polysilanes [28-31], polycarbosilanes [32,33],polysilazanes [33],polyborazines [34,35],polythiazenes [36], and, as an example,polymetal-locenylsilanes [37]. 

West (p. 6), Miller (p. 43), Zeigler (10), and Sawan (p. 112) outline the synthesis of a wide variety of soluble, processable polydiorganosilanes, a class of polymers which not long ago was thought to be intractable. Matyjaszewski (p. 78) has found significant improvements in the synthetic method for polydiorganosilane synthesis as well as new synthetic routes to unusual substituted polydiorganosilanes. Seyferth (p. 21, 143) reports synthetic routes to a number of new polycarbosilanes and polysilazanes which may be used as precursors to ceramic materials. 

Polycarbosilanes, in their broadest definition, are organosilicon polymers whose backbone is composed of silicon atoms, appropriately substituted, and difunctional organic groups which bridge the silicon atoms, as shown in formula 1. The polycarbosilanes may be linear, or they can be cyclic or polycyclic -... 

In conclusion, the lesson learned from the research carried out to date on the subject of polycarbosilanes is that the general rule that linear, noncrosslinked polymers are not suitable preceramic polymers applies here as well. Crosslinked network-type polymers are needed. Such structures can be generated in more than one way, but in the case of the polycarbosilanes they have, to date, been obtained mainly by thermolytic routes thermal treatment (with or without other chemical additives) in the case of the Yajima polycarbosilanes and the thermolysis of tetramethylsilane in the case of the Bayer process-derived polycarbosilane. 

The cyclotrisilazane (R = Me) produced in reaction (14) is recycled at 650°C [by reaction with MeNHo) the reverse of reaction (14)] to increase the yield of processible polymer. Physicochemical characterization of this material shows it to have a softening point at 190°C and a C Si ratio of 1 1.18. Filaments 5-18 pm in diameter can be spun at 315°C. The precursor fiber is then rendered infusible by exposure to air and transformed into a ceramic fiber by heating to 1200°C under N2- The ceramic yield is on the order of 54% although, the composition of the resulting amorphous product is not reported. The approach used by Verbeek is quite similar to that employed by Yajima et al. (13) in the pyrolytic preparation of polycarbosilane and its transformation into SiC fibers. 

The first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by S. Yajima and his coworkers at Tohoku University in Japan [5]. As might be expected on the basis of the 2 C/l Si ratio of the (CH3)2SiCl2 starting material used in this process, the ceramic fibers contain free carbon as well as silicon carbide. A typical analysis [5] showed a composition 1 SiC/0.78 C/0.22 Si02- (The latter is introduced in the oxidative cure step of the polycarbosilane fiber). 

The chemistry which is involved in the "graft" and "in situ" procedures and the structures of the hybrid polymers which are formed remain to be elucidated. However, there is no doubt that these procedures are useful ones. We have used them also to form new and useful hybrid preceramic polymers from the Yajima polycarbosilane which contains a plurality of [CH3Si(H)CH2l units [14]. 

The incorporation of decaborane into polycarbosilane-based polymers has been used in the densification of the polymer-derived SiC fibers. Improved densifi-cation resulted in the formation of fibers with density as high as 2840 kg/m3 corresponding to an increase of 89%.154... 

The spectrum of silicon based polymers is enriched by high tech ceramics like silicon nitride and carbide, respectively. These materials are produced by pyrolysis of appropriate polymeric precursors such as polysilanes, polycarbosilanes and polysilazanes (preceramics). These synthetic ceramics display a certain analogy to silicates, having SiC, SiN, or Si(C,N) as structural subunits instead ofSiO. 

Other organosilicon polymer precursors for ceramics have either been prepared or improved by means of transition metal complex-catalyzed chemistry. For instance, the Nicalon silicon carbide-based ceramic fibers are fabricated from a polycarbosilane that is produced by thermal rearrangement of poly(dimethylsilylene) [18]. The CH3(H)SiCH2 group is the major constituent of this polycarbosilane. 

Polysilanes containing phenyl groups are thermally stable up to 350°C while methylmethoxypoly-silanes without phenyl groups decompose at 250 - 300°C. When methylmethoxyphenylpolysilanes are heated to 350°C methylmethoxysilanes distil off and a polymer is formed which is nearly free of methoxy groups. In contrast to the thermal reaction of polydimethylsilanes no polycarbosilane structures can be observed even if polyborodiphenylsiloxane is added as catalyst (Figure 3.). 

Linear polycarbosilanes and polycarbosiloxanes-especially those containing arylene units in the chain-have specific physico-chemical properties which can be applicable in heat-resistant materials [29-31]. Phenylene-silylene-ethylene-polymers, which may serve as potential substrates for applications as membrane materials are usually obtained in the presence of platinum catalysts [32], although other transihon-metal complexes have also been tested in this process. 

The other process is the transformation of an organic precursor into a continuous thin ceramic fiber. In the spinning process, polycarbosilane, a high molecular weight polymer containing Si and C, is obtained by thermal decomposition and polymerization of polydimethylsilane. The fiber thus produced consists of a mixture of P-SiC, carbon crystallite and SiO. The presence of carbon crystallite suppresses the growth of SiC crystals. Yajima and coworkers (Yajima et al., 1976, 1978, 1979) were the first to produce fine (10-30 pm in diameter), continuous and flexible fibers, which are commercialized with the trade name of Nicalon (Nippon Carbon Co.). 

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