Ladder structure advantages

Summary Comparative analysis is carried out of some physical-chemical and physical-mechanical properties of polyorganosiloxane block copolymers of linear and linear-ladder structure. Advantages of polyorganosiloxane block copolymers are demonstrated in terms of thermal stability and physical-mechanical performance. There is an opportunity for the development of new heat-resistant composites for various branches of industry. 

Another advantage of the ladder structure is its resistance to degradative chain scission. The structure will not be degraded into two shorter ladder structures except in the very unlikely event that two single-chain scissions occur directly across from one another (25). 

Polyorganosiloxane block copolymers of linear-ladder structure are the most promising compounds for the development of new composites because of their unusual stmcture. The advantages of block copolymers in comparison with a,to-dihydroxypolydimethylsiloxane become obvious on comparative analysis of their physical-chemical and physical-mechanical properties. 

Although elimination reactions are clearly degradation, leading to color and char formation, they can have advantages. Conjugated sequences and ladder structures confer thermal stability on the polymers, forming stable chars. This is the basis of the preparation of carbon fibers by pyrolysis of polyacrylonitrile. It is also one reason for the use of plasticized PVC in electrical insulation in a... 

The multistep routes always appear to be particularly advantageous when conjugated ladder structures are the synthetic aim, as the subsequent examples demonstrate. 

Conjugated polymers with ladder structures have become more significant in the last few years with respect to preparative and theoretical aspects [198,199]. They combine the advantage of high stabilities with interesting electrical and optical properties. The most suitable method to synthesize these polymers and their precursor molecules (oligomers) is the repetitive Diels-Alder reac-... 

It is supposed that any unit of the chain may form a contact with any unit of another chain. Variations of parameters allow the kinetics of dissociation and association in the system to be controlled. Rate constants of formation and disappearance of one contact were estimated122. It is seen on the computer display screen that a strongly defect structures are formed first. In time, these structures go over to completely stretched ones. A completely stretched ladder form of the complex was found to be the most advantageous from the energetic point of view. 

Several structural types are based on the combination of two units of (13). The edge-to-edge combination of (13) yields a ladder -type structure (19). Of course there are various combinations of solvent donor ligands and/or chelate donors possible in (20) and (21). The face-to-face combination of (13) can produce a relatively cubic infrastructure (22) as previously seen in the enolates (5) and (0). Distorted variations of the cube (22) are observed, such as (23), or alternatively as another variation (24), with opposite square faces offset from one another (by nearly 90 in 24). Such variations may be described as a tetrahedron within a tetrahedron . It is noteworthy that the cube (22) can be derived from the ladder (21) simply by decreasing the appropriate internal bond angles to about 90 as indicated by the sequence (tf formulae (13) — (19) — (21) — (22). An advantage of the closed cubic structure (22) over the ladder (19) is the additional coordination of the terminal metal cations (M) to a third anion. The cube (22) is most frequendy observed with four-coordinate metal cations, as in (25) and not in its unsolvated form (22) (Scheme 3). 

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