Polymers polysilazane

The susceptibility of cyclodisilazanes to nucleophilic attack by aromatic amines has also been used to prepare silazane containing polymers. Polysilazane cyclo-linear chains with aromatic spacing groups, synthesized by polycondensations of difunctional cyclodisilazanes with bis-phenols and N.N -diorganosilane diamines, have been reported (13). 

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. 

Organometallic polymer precursors offer the potential to manufacture shaped forms of advanced ceramic materials using low temperature processing. Polysilazanes, compounds containing Si-N bonds in the polymer backbone, can be used as precursors to silicon nitride containing ceramic materials. This chapter provides an overview of the general synthetic approaches to polysilazanes with particular emphasis on the synthesis of preceramic polysilazanes. 

With the exception of reaction (3), the utility of all of the above reactions for the synthesis of oligo- and polysilazanes has been examined in varying detail. In the following paragraphs, we attempt to examine the pertinent studies in each area especially as it relates to the synthesis of preceramic polymers. 

Polymers of this type were found to have the highest molecular weight, Mjj 10K D, of any polysilazanes produced to date. Unfortunately, no one has examined the utility of these compounds as preceramic polymers. 

None of the above described ring opening polymerization methods has, as yet, proved useful for the formation of polysilazane preceramic polymers. However, Si-N bond cleavage and reformation, as it occurs in reaction (13), is probably responsible in part for the curing or thermoset step in transition metal catalyzed dehydrocoupling polymerization of hydridosilazanes (31), as described below. 

Surprisingly, pyrolysis of these polysilazane polymers gave mostly SiC rather than the expected Si3N. It was later found that the use of HSiCl-5 in place of MeSiCl-i provides useful precursors to Si-iN/. (42) J J ... 

Introductory Remarks. In contrast with the popularity and usefulness of the polysiloxane chains, which constitute the structural backbone of silicones, the knowledge of polymers based on the silazane unit is still limited. In the sixties there was still some hope of the possibility of producing long chain polysilazane molecules and a number of laboratories were active in seeking convenient methods for their synthesis (e.g. see review by Aylett (12)). 

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. 

Summary A brief review of the preparation of silicon containing preceramic polymers to prepare silicon carbide and silicon carbonitride fibers is given. Methylchlorodisilanes are converted to polysilanes and polysilazanes which yield ceramic fibers after meltspinning, curing, and pyrolysis. 

Gaul and co-workers [15] prepared a variety of polysilazanes by using chlorosilanes and hexa-methyldisilazane to control polymer molecular weight, rheology, and spinnability (Eq. 9). 

Polysilazane fibers are rendered infusible by humidity or in the absence of oxygen by ammonia. The final step of producing ceramic fibers is the pyrolysis. The cured fibers are heated at 1200 -1300°C in argon, nitrogen, or in vacuo, and SiC- or SiC/SijN fibers with a diameter of around 15 /xm are obtained. Heating up silicon-polymers, whether polysilanes or polysilazanes, results in the evolution of CH4 and H2. 

Polymer or polymer network with a skeletal strueture that does not inelude carbon atoms. Note Examples include polyphosphazenes, polysilieates, polysiloxanes, polysilanes, polysilazanes, polygermanes, polysulfides, ete. 

Development has been conducted to fabricate SiC-based matrix/Si-C-based fiber composites by polymer infiltration. Nicalon fibers are used, and various polymers such as polysilazanes, polysiloxanes, and polycarbosilanes are used to yield matrices of SiCO, SiNC, and SiC. As with CVI of SiC/SiC composites, carbon acts as an interface layer but does not result in stability at high temperatures. 

More recently, we have seen other new plastics arrive, including such exotic, high performance materials as the polybenzimidazoles, polyoxadiazoles, polyperfluorotriazines, polyphenylenes, and such inorganic materials as the boron polymers, the metalloxanes, and the polysilazanes, to name only a few. These exotic materials are mostly development products today, they are very expensive and will undoubtedly find their first uses, if any, in the aerospace industry which requires the high performance offered by these materials and can afford to pay for them. Some of these new materials will become commercial successes, others will not. 

The coupling of a trialkylsilane and an amine with loss of H2, catalyzed by palladium on carbon, was first reported by Sommer and Citron in 1967.178 More recent work by Laine and Blum has involved the application of catalytic dehydrocoupling of compounds containing Si-H and N-H bonds to form aligo- and polysilazanes. These polymers, with silicon-nitrogen bonds in the backbone, are useful precursors to silicon nitride. In the presence of Ru3(CO)i2, silicon-nitrogen bonds are cleaved and reformed... 

One other Si—N—B—X precursor type, where X = Ti, has been made by several groups85,90,91. The approach used has been to react Ti(NMe2)4 with a polysilazane as illustrated in equation 32. Note that the initial reaction with Ti(NMe2)4 can be exothermic and even violent if Si—H bonds are present. However, processable polymers are... 

The most important class of silicon-containing polymers that have not yet been covered are the polysilazanes, shown in 7.1. These polymers, or precursors to them, are generally prepared by the reaction of organic-substituted chlorosilanes with ammonia or amines as is shown in reaction (l).32... 

The initial polysilane is rearranged to a polycarbosilane by thermolysis at 743 K. The resulting polymer has no well-defined structure. The use of polysilazanes, having a -[RjSi-NH]- structural unit, allowed the production of mixed SiC/Si3N4 ceramics.12 By a variation of the processing conditions, both compounds can be selectively... 

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