Polymer precursor

The chemistry of these polymers is based on the dianhydride of 4,4 -[2,2,2-trifluoro( 1 -trifluoromethyl)ethylidene]bis(l,2-benzenedicarboxylic acid) 5 (6FDA) that is reacted with ODA 2 to produce polyimide 8 (Du Pont NR 150 A2 ). The other polymers in this series are copolyimide NR 150 B2 composed of repeating units 8, 9 and 10 obtained by reacting dianhydride 5, with ODA 2, 1,3-benzenediamine 6 (mefa-phenylenediamine, MPDA) and 1,4-benzenediamine 7 

The macromolecule is formed of randomly distributed repeating units 8, 9 and 10. 

One point that deserves to be mentioned is that the synthesis of polyamic acids is a solid—liquid interfacial polymerization because of the low solubility of aromatic dianhydrides in organic solvents. This is also the predominant mechanism when 

The chemistry of these polymers is based on the dianhydride of 4,4 -[2,2,2-trifluoro(l-triliuoro methyl)ethylidene]bis( 1,2-benzenedicarboxylic acid) 

Polymers discussed in the previous section consist of linear poiyimide precursors (polyamic acids) that are transformed into polyimides during the processing stage. This means that water molecules evolve within the adhesive layer and must be removed to avoid the formation of large voided areas. To circumvent this drawback, research was focused on the synthesis of fully cyclized heterocyclic polymers, which are intrinsically thermoplastic, even though the Tg can be extremely high. Polyphenylquinoxalines, poly 

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A hquid-phase reaction in which TiCl is reacted with hquid ammonia at —35 C to form an adduct that is subsequendy calcined at 1000°C has also been proposed (35). Preparation of titanium nitride and titanium carbonitride by the pyrolysis of titanium-containing polymer precursors has also been reported (36). 

Preceramic polymer precursors (45,68) can be used to make ceramic composites from polymer ceramic mixtures that transform to the desired material when heated. Preceramic polymers have been used to produce oxide ceramics and are of considerable interest in nonoxide ceramic powder processing. Low ceramic yields and incomplete burnout currently limit the use of preceramic polymers in ceramics processing. 

Deformation of low molecular weight polymer or polymer precursor such as in the casting of acrylic sheet and preparation of glass-reinforced laminates. 

Turning now to other types of ceramic fibre, the most important material made by pyrolysis of organic polymer precursors is silicon carbide fibre. This is commonly made from a poly(diorgano)silane precursor, as described in detail by Riedel (1996) and more concisely by Chawla (1998). Silicon nitride fibres are also made by this sort of approach. Much of this work originates in Japan, where Yajima (1976) was a notable pioneer. 

Rosin and its derivates have shown wide compatibility with a broad range of acrylics and other PSA polymer precursors. This property has made them one of the most common tackifiers in the industry. 

Diphenol/thiophenol is one of the most important polymer precursors for synthesis of poly(aryl ethers) or poly-(aryl sulfides) in displacement polymerizations. Commonly used bisphenols are 4,4 -isopropylidene diphenol or bisphenol-A (BPA) due to their low price and easy availability. Other commercial bisphenols have also been reported [7,24,25]. Recently, synthesis of poly(aryl ethers) by the reaction of new bisphenol monomers with activated aromatic dihalides has been reported. The structures of the polymer precursors are described in Table 2. Poly(aryl ether phenylquinoxalines) have been synthesized by Connell et al. [26], by the reaction of bisphenols containing a preformed quinoxaline ring with... 

Mohanty et al. were the first to introduce pendent r-butyl groups in die polymer backbones. The resulting material was quite soluble in aprotic dipolar solvents.83 The PEEK precursors were prepared under a mild reaction condition at 170°C. The polymer precursor can be converted to PEEK in die presence of Lewis acid catalyst A1C13 via a retro Friedel-Crafts alkylation. Approximately 50% of die rerr-butyl substitutes were removed due to die insolubility of the product in die solvent used. Later, Risse et al. showed diat complete cleavage of f< rf-butyl substitutes could be achieved using a strong Lewis acid CF3SO3H as both die catalyst and the reaction medium (Scheme 6.15).84... 

Another commercially important crosslinking process that involves unsaturated polymer precursors is the so-called drying of alkyd resins in paints. This process is not drying at all, at least not in the sense of mere loss of solvent to leave behind a solid residue. Instead, the main process is the conversion of high relative molar mass molecules to a crosslinked structure via... 

Chan, VZH Hoffman, J Lee, VY latrou, H Avgeropoulos, A Hadjichristidis, N Miller, RD Thomas, EL, Ordered Bicontinuous Nanoporous and Nanorelief Ceramic Pihns from Self-Assembling Polymer Precursors, Science 286, 1716, 1999. 

FIG. 5 Model of alkylammonium-exchanged clay swollen by monomer or polymer precursors such as styrene, e-caprolactam, and epoxide. 

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. 

A ceramic fiber with Si-C-N-0 composition can be prepared by melt-spinning, cure and pyrolysis of a polymethyldisilylazane polymer precursor (14, 15), which is the reaction product of a mixture of 50 mol % 1,1,2,2- tetrachloro-1,2-dimethyldisilane (la), 40 mol % 1,1,2-trichloro- 1,2,2-trimethyldisilane (lb) and 10 mol %... 

Significant recent advances have occured with phosphazenes (J ) and to a lesser extent silizanes (6) in contrast, polymers based on phosphorus(III) and nitrogen are virtually unknown. Because such systems offer opportunities for metal coordination, and in their oxidized forms could be valuable polymer precursors to PON ceramics (2), we have begun systematic studies of the structural and reactivity factors necessary for their formation. 

The gel stiffness, S, was also found to depend on the molecular weight of the polymer precursor. For end-linking PDMS, S decreases with increasing... 

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. 

Byrom D (1994) In Mobley DP (ed) Plastics from microbes microbial synthesis of polymers and polymer precursors. Hanser Munich, p 5... 

Byrom D (1994) Plastics From Microbes Microbial Synthesis of Polymers and Polymer Precursors. Hansen, New York... 

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