Ladder-like structures

Potyimides obtained by reacting pyromellitic dianhydride with aromatic amines can have ladder-like structures, and commercial materials are available which may be used to temperatures in excess of 300°C. They are, however, somewhat difficult to process and modified polymers such as the polyamide-imides are slightly more processable, but with some loss of heat resistance. One disadvantage of polyimides is their limited resistance to hydrolysis, and they may crack in aqueous environments above 100°C. 

Evaluation of the peak pattern gives evidence of silsesquioxanes and their homo-derivatives as well as high molecular reaction products with ladder-like structures. The presence of cage-like polysiloxanes is remarkable since their formation generally requires completely different preparative conditions [10],... 

The ladder-like structure, where complex formation takes place on a molecular level via conformational adaptation (zip mechanism),... 

Remember here (Section II) that, depending on the counterion, some [M(dithiolene)2] systems have a tendency to form stacks with side-by-side S S interactions between stacks. Therefore, such systems with a spin S = seemed to be good candidates for the construction of ladder-like structures. 

Figure 15. Ladder-like structure of [Cp2W(dmit)]+ in [Cp2W(dmit)](TCNQF4). [Adapted from the data available in (313).]...

Scheme 6 Ladder-like structure of polysilsesquioxane (See also Plate 20 in the Colour Plate Section)...

Opening the cube leads to ladder-like structures such as Li4Cl4(azetidine)2[N-(3-aminopropyl)-azetidine]2, Li4l4(2,4,6-trimethylpyridine)6, and Li4Br4(2,6-dimethylpyridine)6. The latter, prepared by recrystallizing LiBr in pyridine, has a structure typical for the class i.e., a stepped tetramer (see Pigure 97). A more complex species, LiLLiClLiL(THP) 2, is formed as a side product of the reaction of [LiL(THP)J (L = A,A-dimethyl-V -trimethylsilylethane-1,2-diamide)... 

Pr(C103)3 2H20 has praseodymium coordinated by seven different chlorates and two water molecules." Lanthanum is nine-coordinate in Rb[La(OAc)4] in which each La is coordinated by six different acetate groups in a chain structure. " Thulium nitrilotriacetate, [Tm(NTA)-(H20)2] 2H20, has a ladder-like structure." ... 

Polyfluorenes are sometimes referred to as stepladder poly(przra-phenylene) (PPP) polymers as they have a backbone with a planar, ladder-like structure. The first poly-fluorene LED was made by Yoshino and coworkers using poly(9,9-dihexylfluorene) 16 [50]. 

Delnoye, D.A.P. et al., Pi-conjugated oligomers and polymers with a self-assembled ladder-like structure, J. Am. Chem. Soc. 118, 8717-8718, 1996. 

The aromatic rings of the base pairs are parallel to each other, forming a twisted ladder-like structure. The diameter of a DNA helix molecule is about 2 nm. With millions of nucleotides in the strand, the length of a DNA molecule when laid out straight can measure several centimetres. 

Figure 3.6 Schematic of (a) a network-like structure and (b) a ladder-like structure of self-assembled diacid and tetrapyridyl components (redrawn from [10]).

Deoxyribonucleic acid (DNA) (Sections 25.1 and 25.4A) One of the two molecules (the other is RNA) that carry genetic information in cells. Two molecular strands held together by hydrogen bonds give DNA a twisted ladder -like structure, with four typ>es of heterocyclic bases (adenine, cytosine, thymine, and guanine) making up the rungs of the ladder. 

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