Polymer routes

The soluble nonconjugated precursor polymer route to Durham polyacetylene via thermal elimination. 

One of the main advantages of the polymer route to ceramics is the preparation of ceramic fibers, a shape difficult to achieve by other methods. Ceramic fiber-based composites are becoming an increasingly important group of structural materials (12, 13). 

Boron-containing nonoxide amorphous or crystalline advanced ceramics, including boron nitride (BN), boron carbide (B4C), boron carbonitride (B/C/N), and boron silicon carbonitride Si/B/C/N, can be prepared via the preceramic polymers route called the polymer-derived ceramics (PDCs) route, using convenient thermal and chemical processes. Because the preparation of BN has been the most in demand and widespread boron-based material during the past two decades, this chapter provides an overview of the conversion of boron- and nitrogen-containing polymers into advanced BN materials. 

The problems associated with route B also have something to do with steric hindrance. Here the critical point is the steric demand of both monomer and chain end. Incoming monomer will only be connected to the chain end, if steric hindrance is not too high. Otherwise this process will be slowed down or even rendered impossible. Depending on the kind of polyreaction applied, this may lead to termination of the reactive chain end and/or to side reactions of the monomer, like loss of coupling functionality as in some polycondensations or auto-initiation specifically in radical polymerizations. From this discussion it can be extracted that the basic problems for both routes are incomplete coverage (route A) and low molecular weight dendronized polymer (route B). 

Initial reports on chemoenzymatic block copolymer synthesis focus on the enzymatic macroinitiation from chemically obtained hydroxy-functional polymers (route A in Fig. 4). The first report on enzymatic macroinitiation was published by Kumar et ah, who used anionically synthesized hydroxy-functional polybutadiene of various molecular weights ranging from 2600 to 19,000Da (Fig. 5) [16]. In a systematic study, the authors investigated the efficiency of the macroinitiation of CL and PDF by Novozym 435 as a function of the polybutadiene macroinitiator. The reaction profile showed that polybutadiene consumption steadily increased with the reaction... 

Extension of DKR to polymer chemistry would readily result in chiral polyesters, polycarbonates, or polyamides from an optically inactive monomer mixture. Scheme 10 describes three variants of chemoenzymatic catalysis applied in polymer chemistry that recently appeared in the literature. Route A uses AA and BB monomers to prepare chiral polymers from racemic/diasteromeric diols. Route B converts an enantiomer mixture of AB monomers to homochiral polymers. Route C is the enzymatic ring-opening polymerization of co-methylated lactones to homochiral polyesters. Details will be given in Sect. 3.4.2. 

To further test the functional purity of hydroxyl polymers prepared by the protected initiator and polymer route, two stage chain extensions of two polystyrene diols were carried out. The first stage involved conversion of the diols to acids with succinic anhydride the second stage involved chain extension with a diepoxide (Table III). If one assumes an overall conversion of 97-98%, a UP of 19 requires a functionality 1.93-1.95 based upon step-growth polymeri-... 

A preceramic, carrier polymer route to boron carbide has been reported via the pyrolysis of a polynorbomene that bears decaborane side groups.69 An important feature of this development is the ability to produce nanofibers of boron carbide in the following way. A solution of the poly(norbomenyldecaborane) in THF is subjected to the process of... 

Various preceramic oligomer and polymer routes to aluminum nitride have been investigated.70 For example, the reaction of LiAlH4 or A1H3 with ammonia initially yields A1(NH2)3, which loses ammonia and hydrogen during pyrolysis and leaves AIN contaminated by carbon from the initial reaction solvent.71... 

Figure 6.20 Steps involved in the fabrication of ceramic fibers via the polymer route.

P (Polymer route) are obtained in quantitative yield using this reaction pathway. 

This chapter gives an introduction to the preceramic polymer route to ceramic materials and focuses on the reasons why this new approach was needed and on the chemical considerations important in its implementation, with examples from research on organosilicon polymers. Novel polysilazanes have been prepared by the dehydro-cyclodimerization reaction, a new method for polymerizing suitably substituted cyclooligosilazanes. The living polymer intermediate in this reaction has been used to convert Si-H-containing organosilicon polymers that are not suitable for pyrolytic conversion to ceramics into useful preceramic polymers. 

SCHEME 18.28 Synthesis of boron-modified polysilazanes by ammonolysis of tris(chlorosilylethylene)boranes (M, monomer route) and by hydroboration of vinyl-substituted polysilazanes (P, polymer route). 

Light-emitting diodes (LEDs) based on PPP52 (see Fig. 8.12), obtained by a precursor polymer route after Ballard et al.,53 give blue-light emission (kmax =... 

The deactivation by formation of polymers, route 2, is enhanced at lower temperatures where the rate of hydrocarbon adsorption becomes higher than the hydrocracking rate of the adsorbed species (1,2,8). The deactivation rate by gum formation is usually defined in terms of a resistance number defined as kg hydrocarbon feed required to... 

If we use the term alternative routes to SiC here, we want to exclude this Acheson process as well as the main topic of this paper, the polymer route ... 

By the way, although a dominant majority of papers concerning the formation of amorphous SiC layers describes appUcations of CVD or PVD techniques, there have also been some attempts to use the polymer route for preparing SiC films. Starting from solutions of various polysilanes or polycarbosilanes, frequently films are formed by spin-coating and pyrolyzed under inert atmosphere [215-218]. Of course, such a procedure does not form a part of this section SiC layers via gas phase reactions . However, in this connection it should be mentioned that polysilanes are also applied to form films via evaporation, not only with the aim to build amorphous and/or crystalline SiC films, but also to use special properties of the polysilane films themselves, i.e. without a subsequent pyrolysis of these films. Such amorphous films are characterized by non-linear optical effects [219, 220] and their properties may be controlled by the uniformity of the orientation of polysilane chains which is susceptible to epitaxial influences [221-223]. 

In the following section (Sect. 5) an overview will be given over thin SiC fibers characterized by small diameters of one to few tens of a micrometer and produced via the polymer route (in most cases). This kind of fiber is applied as rovings consisting of some hundred up to more than a thousand single filaments, so-called multifilament fibers, which may be woven and is predominantly used for reinforcing brittle matrices (ceramics and glasses). 

A further technological advantage of applying the polymer route for composite matrices is that no mechanical damage of the reinforcing fibers is caused. The scheme in Fig. 13 demonstrates the steps for making composites by the liquid polymer infiltration process [250, 251]. 

This so-called polymer route was introduced by Chantrell and Popper [10], who proposed the use of inorganic polymers as starting materials for the preparation of ceramics, opening up the rather ambitious perspective of easily shaping monolithic green bodies via this route. At the end of the 1960s, Winter et al. [11, 12] pioneered such a process for the production of Si/C/N fibers, and developed it to technical feasibility. Following the route developed by Yajima et al. [13, 14], ceramic fibers (SiC, Nicalon) have been available on... 

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