Intermediate Polymerization

The strategy of Kaeriyama represents a so-called precursor route and was developed to overcome the characteristic shortcomings (insolubility, lack of process-ability) of previous PPP syntheses. The condensation reaction is carried out with solubilized monomers, leading to a soluble polymeric intermediate. In the final reaction step this intermediate is then converted, preferentially in the solid state allowing the formation of homogeneous PPP films or layers, into PPP (or other poly(arylene)s). 

Preparation of poly(dichlorophosphazene), (NPCl2)n> a polymeric intermediate from which the great majority of POPs have been prepared by nucleophilic substitution of the highly reactive chlorine atoms with carefully selected organic substituents... 

The silyloxy-substituted precursor polymers (88) can be photocyclized in a polymer-analogous fashion to yield polymeric intermediates (90) containing the 3,6-phenanthrylene unit. These intermediates can be converted thermally to yield conjugated polyarylenevinylenes, in this case poly(3,6-phenanthrylene-vinylene) (91), a polymer that displays a long-wavelength absorption maximum at about 360 nm 1114]. 

Tetravalent silicon reagents are less suitable for transient protection of any hydroxyl groups present, because the resulting activated oligomeric or polymeric intermediates cannot be defined. It can, furthermore, be expected that any derivatives utilizing tetravalent silicon are much more polar and less lipophilic than... 

The polymeric intermediate 2209a, derived, e.g., from substituted N,0-bissilyl-ated 4-aminobenzoic acids 2206 b or 2206 c and free or silylated terephthalic acid, affords, on heating to 180-200°C, the fire resistant polybenzoxazole (PBO) 2210 and H2O [20, 21] (Scheme 14.7). O-Silylated 2209b should, likewise, cyclize on heating to give the polymer PBO 2210 with formation of the volatile trimethylsila-... 

The synthesis of poly(organophosphazenes) represents probably the best example of a central theme of Inorganic macromolecules Preparation of a reactive polymeric intermediate, poly(dichlorophos-phazene), and subsequent use in a wide variety of side group replacement reactions (Figure 1). This concept has been demonstrated in a number of laboratories (3) and has provided a wide variety of polymers with different properties. 

Because the opportunities for controlled chain growth are more restricted in inorganic than in organic systems, an alternative approach to polymer synthesis becomes appealing. This involves the use of substitution processes carried out on a preformed reactive polymeric intermediate. In this way molecular diversity can be introduced by different substitution reactions rather than by a diversification of the polymerization process. 

Two key principles have played a pivotal role in our exploration and development in this field. First, unlike most macromolecules, nearly all poly(organophosphazenes) are prepared by a substitutive technique, in which a broad range of different substituent groups are introduced via a reactive polymeric intermediate (II). Second, because substitution reactions play such an important role in the chemistry of these... 

Water treatment of the intermediate. In order to recover the maximum quantity of ethyleneurea, it was necessary to treat the resinous polymeric intermediate with water. If the resin was reasonably fluid at 234—270° it was possible to recover ethyleneurea in high yield simply by passing superheated steam through it at these temperatures. 

Fortunately, aromatic polyimides could be used as materials because they can be prepared through a multistep process, being applicable in the state of soluble polymeric intermediate. Nevertheless, the transformation into polyimides at the moment of application is an approach far from being optimal in most cases, and it can be said that, for many years, aromatic homopolyimides could be successfully applied only in the form of films or coatings [2,3]. 

In general, the synthesis of polyphosphazene polymers is unique in that, in theory, an infinite number of polymers with a variety of properties can be derived from the common polymeric intermediate, poly(dichlorophosphazene) (PNCI2), by replacing the chlorines with different nucleophiles. If the polydichlorophosphazene precursor is reacted with the sodium salts of trifluoroethanol and a mixed fluorotelomer alcohol, a poly(fluoroaIkoxyphosphazene) elastomer (FZ elastomer) is obtained. It contains a small amount of an unsaturated substituent as a curing site. The polymer is a soft gum, which can be compounded with carbon blacks and fillers and cured with sulfur or peroxides or by radiation. 

The electronic structures of silicate minerals of polymerization intermediate between nesosilicates and tektosilicates have been studied to a lesser extent than have SiOj or the olivines. The complexity of their crystal structures makes calculation difficult, and their diversity in terms of local chemical environment makes phenomenological assignment of their spectra difficult. Nonetheless, some recent comparative studies have given valuable electronic structure information on such materials. 

For studies related to the characterization of Ziegle-r—Natta polymerization intermediates with Cp2ZrCl2 as catalyst, see P. Gassman, M. R. 

The rationale for this approach further involved the hypothesis that swelling in water occurs by virtue of water uptake in the amorphous regions of the polymers with network structure provided by relatively water-insensitive crystallites in the polymers. In this approach, vinyl trifluoroacetate was polymerized or copolymerized to provide highly syndiotactic thermoplastic polymeric intermediates that were converted to the corresponding syndiotactic PVA homopolymers or copolymers by solvolysis with mild nucleophilic reagents in media that were not solvents for both starting and end-product polymers. 

This is in agreement with experiment, according to which no solid state syntheses of multinary compounds within the system Si/B/N/C from the binary border phases have been successful so far (for the corresponding thermodynamic considerations cf. [9]). In the course of the past decades, however, considerable progress has been achieved in the synthesis of multinary compounds by applying an alternative synthetic approach via molecular and polymeric intermediates (cf. Fig. 1). 

Since all polymeric intermediates, and in many instances also the final ceramics, are amorphous, only thermal and spectroscopic methods can be utilized to characterize the thermal conversion. The most extensive studies have been performed on the polymer N-methylpolyborosileizane (PBS-Me), made from the single source precursor TADB. The pyrolysis has been monitored in situ by differential thermal analysis combined with thermo-gravimetric analysis and mass spectrometry (DTA/TG/MS). For ex situ investigations, batches of the polymer were treated at different temperatures, cooled to room temperature, and characterized by infrared spectroscopy and nuclear magnetic resonance spectroscopy. 

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