Step-ladder" structures

Structurally Defined Poly(para-phenylene)s with "Step-Ladder" Structure... 

Thus, it was logical to combine the principle for preparation of "step-ladder" structures with new, efficient and selective methods for aryl-aryl coupling. A first attempt at this was from Yamamoto et al. [17]. They coupled 2,7-dibromo-9,10-dihy-drophenanthrene to give an ethano-bridged poly( arflf-phenylene) derivative (poly(9,10-dihydrophenanthrene-2,7-diyl)) (fi) by way of low-valent nickel complexes, which were used either stoichiometrically as reagent (Ni(COD)2) or were genera-... 

This ajqnxjadi, using 1,4,5,8-naphthakne tetracarbox ic acid and 3,3 diamino benzidine, has yielded a step-ladder structure, which can be spun to a fiber from sulfuric add solution and which undergoes no weight loss upon heating to 600 C in Ng 102—105). 

Treatment of 1,4-dimethylpiperazine (dmp) with lithium anilide LiNHPh gave [(LiNHPh)5(dmp)2]. " It has a distorted ladder structure with five Li—N rings. However, it is U-shaped rather than stepped as shown by 4 [N = NHPh N(dmp) = 1,4-dimethylpi-peridine] from which it can be seen that the bridgehead lithium is bound to... 

Perhaps one of the best known syntheses of a heterocyclic polymer via the modification method is the generation of nitrogen-containing ladder polymers by pyrolysis of polyacrylonitrile) (77MI11109). The thermolysis is known to take place in discrete steps. The first step in the sequence, which can take place with explosive violence if the heating rate is not sufficiently slow, occurs at about 150 °C and can be detected by the onset of intense color formation. The product of this reaction (Scheme 101) is the cyclic tetrahydropyridine ladder structure (209). The next step, which is conducted in the presence of air at ca. 250 °C, involves the thermooxidation of polymer (209) to form what is best described as terpolymer (210) containing dihydropyridine, pyridone and pyridine units. 

Figure 4.27a shows the equivalent circuit of a simple example of the ladder structure, for electrochemical systems that are known as Faradaic reactions involving one adsorbed species. This heterogeneous reaction occurs in two steps in the absence of diffusion limitation ... 

The simulated complex-plane impedance diagram is shown in Figure 4.27b. As can be seen in the figure, this ladder structure is characterized by two semicircles with two time constants, r, = RclCd] and r2 = R3C2, accounting for the two-step reaction. The element C2 symbolizes the adsorption capacitance, and r2 represents the relaxation of the adsorbing process. 

An electrophilic substitution reaction has been used for the key ladderforming step in the synthesis of soluble ladder-type poly(phenylene)s [51-53]. These aromatic polymers have a ribbon-like rigid, planar structure. They are of interest because of their optical and electronic properties [51,54,55]. The preparation of these polymers was accomplished by two basic steps. The first step was the construction of a substituted poly(p-phenylene) backbone. The ladder structure was obtained by a subsequent intramolecular electrophilic ring closure reaction. For example, the syn-... 

Polyacrylonitrile (PAN) is the most common precursor used to make carbon fibers. A flow diagram showing the steps involved in making PAN-based carbon fiber is shown in Fig. 8.3. The PAN precursor has a flexible polymer chain structure like any other polymer, but it has an all carbon backbone chain that contains polar nitrile groups, as shown in Fig. 8.4. During the stabilization treatment, the PAN precursor fiber is heated to 200-220 C, under tension. When this is done oxygen is absorbed, and it serves to cross-link the chains the fibers turn black, and a stable ladder structure is formed. A ladder polymer is a rigid... 

A LiBr complex with another, rather weaker, monodentate pyridine donor in the same ligand salt ratio results in a more aggregated structure, [(2,6-Me2CsH3N)6Li4Br4], and is a product of dissolution in excess donor. A 4-runged step ladder arrangement results because the internal lithiums are solvated giving a pseudotetrahedral 4-coordination at each... 

The authors feel, that the classification of the synthetic principles applied here is somewhat arbitrary. Multifunctional polycondensations which are conducted in a two-step manner (generation of single-strand intermediates, followed by cyclization), could be classified with the same justification as stepwise processes. On the other hand some of the stepwise syntheses of ladder structures constitute condensations of multifunctional monomers (e.g. the use of butadiynes as starting compounds, see Sect. 4.1.). 

One of the most significant steps in the preparation of carbon fibers from acrylic precursor fibers is the oligomerization of the nitrile groups. This reaction has originally been studied in context with the problem of thermal discoloration of PAN (e.g. McCarthney Grassie and McNeill Grassie and Hay It was supposed to lead to a so-called ladder structure ... 

The first commercially feasible process for converting acrylic fibers to carbon fibers was developed by Walt, Phillips, and Johnson of the Royal Aircraft Establishment (RAE) in collaboration with the acrylic fiber producer, Courtaulds [621]. In the RAE process, the acrylic precursor is converted to carbon fiber in a two-step process [622]. Preoxidation or filament stabilization is carried out in the first stage. The precursor is heated in an oxygen atmosphere under tension at a temperature of approximately 200 250°C, well below its carbonizing temperature (approximately 800°C). At this temperature, the nitrile groups react with each other via a free radical addition process leading to the so-called ladder structure shown in reaction 12.34 [609,621 625]. 

In a similar manner, polymers with pendant unsaturation undeigo cyclization reactions in benzene in the presence of BF3 or POCI3 yielding ladder structures. The exact nature of the initiation process is not clear. Water may be need for the initiation step ... 

Ladder polymers. A type of high-temperature polymer. Double linear chains of the macromolecules are periodically linked together (Fig. 1.1). They are insoluble and infusible, being unsuitable for thermoplastic processing and thus are limited in applications. In the macromolecules of step-ladder polymers shorter units of cross-linked double linear chains (ladder structures) are joined by single bonds (Fig. 1.2). An example of a step-ladder polymer is polyimide. 

Polyimides, Common name of polymers characterized by repeated imide groups in the macromolecule (Fig. 1.4). Their structure corresponds to that of the step-ladder polymers. Thermally stable infusible insoluble thermosets. Their intermediate prepolymers can be shaped. Modified polyimides have repeated, mainly aliphatic, ester or amide groups in the main chain besides the imide units, in order to facilitate processing. Examples of applications high-temperature electric insulating materials... 

Two structural types can be distinguished for tetrameric alkali metal amides. [ Li(pyrrolidyl) 2(tmeda)]2, 119, associates in the form of the familiar ladder or step-like structure containing three fused four-membered rings. The same central core is present in the related hexanuclear adduct [ Li(pyrrolidyl) 3(pmdta)]2, 125... 

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