Ladder structures

Spiro polymers are also sometimes classified as ladder polymers, and molecules in which the ladder structure is interrupted by periodic single bonds are called semiladders. Consisting entirely of fused ring structures, ladder polymers possess very rigid chains with excellent thermal stability. 

The product from acrylonitrile will withstand a bunsen flame in the open air and is the basis of one type of carbon fibre. None of the polymers produced by this route have a high degree of perfection in their ladder structure. 

Figure 4.14 Schemauc represemaiion of (a) 4-membered, (b) 6-membered and (c) 8-membered (LiN) heterocydes showing pendant groups on N lying both above and below the plane of the ring, (d) the laddered structure formed by lateral bonding of iwo l.iiN uniK.

Coordination complexes with a ladder structure have been developed by Anderson (36,37). These ladders are prepared from covalently linked... 

As outlined elsewhere (A), we have employed a variation of the original reaction hy Lambourne (3) in exploring condensation products leading to the drum composition as well as to a mixed oxycarboxylate-tricarboxylate formulation, [ (R Sn(0)02CR) 2R Sn CR) 3 ] 2. The latter was identified as having an unfolded drum or ladder structure (A). 

Our investigation using 119Sn NMR establishes the retention of the drum and ladder structures in solution and shows their interconversion according to Equation 3. 

Figure 6. A suggested initial intermediate resulting in the hydrolysis of a ladder structure as represented by Equation 3.

A possible explanation for the formation of this planar ladder structure can be found in the close geometrical similarity of 7 to (3-gallium. The relative positions of the Ga atoms in 7 are comparable to those found in (3-gallium (see Fig. 18), but as expected, the Ga-Ga distances in 7 are all shorter than in (3-gallium due to a more molecular kind of bonding. This means that in 7 the formation of metalloid structures is preferred over the formation of polyhedral structures, which results in the unusual arrangement of the Ga atoms. 

The structures of the radical anions were confirmed by the following experiment (Scheme 9). The reduction of the ladder polysilanes was monitored by UY-visible-NIR spectroscopy. When the absorption of the ladder polysilanes was completely replaced by the absorption of the radical anions, the sealed tube was opened. The radical anions were immediately oxidized, and the starting ladder polysilanes were recovered in high isolated yields. It is reasonable to conclude that the radical anions of the ladder polysilanes retain the ladder structure, and the Si-Si bond cleavage or skeletal rearrangement does not occur. 

A similar investigation of the base adducts of K(PBu Ph) shows that [ KfPBuTh fTHF)], (34), [ K(PButPh) 2(AT-MeIm)]I (35), and I K(PBu Ph ) 2(py) lx (36) also adopt extended polymeric ladder structures in the solid state (74). These adducts resemble the Rb and Cs complexes 28—33 however, the base coligands in 34—36 do not bridge the potassium atoms but are bound in a terminal fashion. In each case there are two types of potassium atom in alternate positions... 

Fig. 16. Ladder structure of [Li(CH2PPh2)(THF)L. Reproduced with permission from D. Steinborn et al., Polyhedron 1998, 17, 351. Copyright 1997, Elsevier Science Ltd. 

Two nitrogen-containing polymeric materials with extended aromatic ladder structures have been chosen for direct fluorination studies (Figure 14.9).57 Pyrolyzed polyacrylonitrile (3) and paracyanogen (4) [poly(pyrazinopryazine)] have been subjected to direct fluorination to produce perfluorinated analogues. 

Polyquinoxalines, ladder structured conducting, 7 522 Polyrotaxane structures, 23 733 Polysaccharide extraction, 10 307... 

The structural constraints imposed by the fused cyclic structure of the catenates and the ability to cleanly separate product mixtures greatly facilitates crystallization, and single crystal X-ray structures have been obtained for a number of ladder polysilanes.3 8 For the all-anti molecules, the non-planarity of the SI rings induces a unidirectional helical twist to the ladder structure, as is evident in Figure 63. 

Fig. 22 Hydrogen bonded ladder structure of ((NH2)3C)(CH3CN n CTV)[Co(C2B9Hn)2] 41 with N-H...OMe hydrogen bonding between guanidinium cations and CTV [90]... 

For a-Ga (coordination number 1 + 2 + 2 + 2) the short Ga-Ga bond distance of 2.45 A of every Ga atom with one of its seven neighbors is characteristic, so that a-Ga is also described as a molecular metal with Ga2 dumbbells. For the low-temperature phases / -, y-, and r)-Ca the following characteristic units are observed the ladder structure (coordination number 2 + 2 + 2 + 2) for /i-gallium, Gayrings that stack to form tubes and a centered Ga wire , observed for y-Ga, and interpenetrating Gan icosahedra for b-Ga. 

Scheme 3.6-3. Homo-aggregation of lithium ketimide rings showing the general preference for stack, as opposed to ladder, structures.

Considering the importance of alkali metal phosphanides it is not surprising that numerous review articles have dealt with this subject [34-36]. The solid state and solution structures vary from dimers with central M2 P2 cycles to larger rings and from chain to ladder structures as described for the lithium amides (see Sections 3.6.1 and 3.6.2). Cage compounds in the field of lithium phosphanides are unusual... 

Composite crystals, in which the structure can be described as resulting from two or more substructures (related to two or more sets of three dimensional lattices) having different periodicity along at least one direction (chimney-ladder structures, vernier structures, misfit-layer structures, etc.). See the scheme presented in Fig. 3.42. 

A group of crystals show diffraction patterns in which two or more 3D lattices having periods commensurate or incommensurate to each other may be recognized. In other words, the crystal consists of two or more interpenetrating substructures (two or more different atom sets) with different periods at least along one direction (see Fig. 3.42). Names such as composite crystals, vernier structures, misfit-layer structures, and chimney-ladder structures have been used for this group of structures. 

Nowotny phases, chimney-ladder structures. The Nowotny chimney-ladder phases are an example in alloy field chemistry of composite structures. They are a series of intermetallic T X , compounds formed by transition metals T from the 4th to 9th groups with p-block elements X from the 13th to the 15th groups. 

Figure 3.43. Examples of chimney-ladder structures (b)-(d) and the reference oF24-TiSi2 type structure (a) presented in terms of a tetragonal pseudo-cell (12 atoms in the pseudo-cell). Notice that the metal atoms (black circles) form sequences of (3Sn like cells.

The ratio of the oxide formed to the metal consumed is called the Piling and Bedworth number. When the number is over 1, the metal rusts. Aluminum and magnesium are the best examples of metals that do not rust because a protective oxide coat forms that is, they have a Piling to Bedworth number of 1. Scratch an aluminum ladder and notice a bright fissure forms and quickly self-coats. The heat release in the sealing aluminum oxide is dissipated to the ladder structure. 

Cyclization is a key reaction in the production of carbon fibers from polyacrylonitrile (PAN) (acrylic fiber see Sec. 3-14d-2). The acrylic fiber used for this purpose usually contains no more than 0.5-5% comonomer (usually methyl acrylate or methacrylate or methacrylic acid). Highly drawn (oriented) fibers are subjected to successive thermal treatments—initially 200-300°C in air followed by 1200-2000°C in nitrogen [Riggs, 1985]. PAN undergoes cyclization via polymerization through the nitrile groups to form a ladder structure (XXVII). Further reaction results in aromatization to the polyquinizarine structure (XXVIII)... 

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