Polymeric ladder structure

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... 

By using a combination of gas-phase synthesis and millimeter/submillimeter-wave spectroscopy, LiNH2 was found to be a monomeric unsolvated planar molecule. The lithium amide [H2NCH2CH2N(H)Li]co has a polymeric ladder structure with two types of (NLi)2 ring which alternate throughout its infinite length. ... 

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. 

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)... 

Depending on the template activity , the ratio Pa/pb is different. If the template is selected properly, distances between groups are such that tensions in a ladder structure are as small as possible (for instance for 6 member ring), and bonds are flexible (e.g, ether bonds). In this case, the ratio Pa/pb is high. In the opposite case, if the substrate molecules are stiff and distances between reacting groups are too large (or too small), we can expect that template makes polymerization more difficult (pa/pb < D or eventually can make it impossible pa = 0. A competition between these two reactions leads to the dif-... 

Dimeric and higher aggregate lithium amides can generally be classified into the coordination motifs illustrated in Scheme 2.2. The four-membered (LiN)2 ring is ubiquitous in lithium amide chemistry and is observed both in discrete dimeric structures in either planar (Scheme 2.2, A) or non-planar (Scheme 2.2, B) geometries as well as in oligomeric and polymeric (ladder) frameworks (Scheme 2.2, C). Trimeric six-membered (LiN), ring... 

The ladder structures formed by lithium amides and their heavier group 15 analogues stand in contrast to those formed by the related lithium alkyls which generally prefer aggregates with three-dimensional or one-dimensional polymeric structures. 

Antimony forms polymeric oxyhalides, and not metallic as in BiOCl. The fluoride, SbOF, has been prepared in two forms V, with a ladder structure, and iM which has a layered structure. Both forms have a trigonal bipyramidal structure about antimony with three oxygens, one fluorine and one lone pair. Structural parameters are given in Table 15, from which it can be seen that L-SbOF heads the table as the nearest to an ideal fit for trigonal bipyramidal geometry. 

In the center ofthecubane complex [LiCl HMPAH (HMPAis (Me2N)3P=0), each Li+ is bonded to three Cl- and one oxygen of HMPA, as shown in Fig. 12.2.6(b). The complex (LiCl)6(TMEDA)2 has a complicated polymeric structure based on a (LiCl)6 core, as shown in Fig. 12.2.6(c). Tetrameric [LiBr]4-6[2,6-Me2Py] has a staggered ladder structure, as shown... 

That these compounds are polymeric there can be little doubt, and spectroscopic evidence favors a cross-linked, reticular lattice rather than a ladder structure (151, and references therein). However, if bulky hydrocarbon groups are present on tin then soluble oligomers are formed, which to date have all been trimers [Eq. (42)] (152-154). The (SnO)3 ring (51) is planar... 

Polymeric addition compounds (16) of furan and ethylene, where n is a whole number from 1 to 50, have been prepared (58) by addition of >2 mole furan to 1 mole ethylene at 120 °C to 250 °C and high pressures followed by fractional extraction of the solid product formed. These polymers have interesting physical and chemical properties. They appear to be highly crystalline on analysis by X-ray. As a consequence, the polymers show good thermal stability, which increases directly with molecular weight. The presence of ether bridges makes possible conversion (Scheme 4) to other compounds that retain the double hydrocarbon chain (ladder) structure and may bear various func-... 

The scope and limitation of a two-fold, stepwise, polymerization process toward ladder structures is discussed for three representative examples. 

A single deviation from the perfect ladder structure, and thus the formation of single standed subunits, produces the point of attack for chemical decomposition as, induced by thermolysis or hydrolysis. An additional complication arises because one often compares macroscopic properties, deduced from TGA or DSC measurements, for example, with the spectroscopic description of the molecular structure. Even if the spwtroscopic characterization of the ladder polymere, say by C-NMR spectroscopy, is supported by the inclusion of well-defined low molecular weight model components, the limit of detection for structural defects will not exceed about 1 %. Such a degree of inhomogeneity, however, may be disastrous for many material properties. 

Amidolithium complexes of type RR NLi (e.g. R and R = alkyl, aryl, silyl) exhibit a fascinating structural diversity as above, bulky amido ligands are essential for complex stabilization. Planar Li2N2-rings are common structural units, and these appear in a variety of laddered structures which may be polymeric or discrete molecular as in [ BuHNLijgj (Figure 10.10). 

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