Three-dimensional polymer backbones

Two- or three-carbon backboned bi- and tridentates can also bridge adjacent metal centres, e.g. with Cu(I), Ag(I), Sb(III) and Bi(III), giving rise to one-, two- or three-dimensional polymers.130-133... 

It should be mentioned that these silicon-backbone polymers are highly absorptive, and must therefore be used in very thin films ( 30 nm), which can be prone to defects. Also, the principal electronic transition in these polysilane (with two-dimensional silicon backbone) and polysilane (three-dimensional silicon backbone) polymers when irradiated at 193 nm is most likely a (t-ct transition involving the all-silicon backbone. Due to this excitation, reaction pathways leading to scission of the Si-Si bonds lead to photodegradation of the polymer. 

Silicones are essentially quartz-like structures in which the three-dimensional SiCh backbone has been modified by incorporation of methyl groups. This progressive saturation leads ultimately to low-molecular weight polymers [4],... 

A functionalized oligomer, which by polymerizing will constitute the backbone of the three dimensional polymer network formed ... 

Ion exchangers are polymer electrolytes prepared a priori as insoluble solids (salts, acids, bases hydrated, possibly gel-Uke). Their polymer backbone is three-dimensional. Many are polyvinyl compounds (substituted polyethylenes) having the general formula [-CH2-CXH-] , where different substituents X lead to rather different products ... 

If a large number of branches exist that connect all of the backbone molecules into a three-dimensional network, the material will not flow when heated, and it is considered a thermoset resin. Vulcanized rubber is an example where the sulfur linkages create a three-dimensional network, converting the precursor rubber into a solid thermoset material. Crosslinked backbone chains are shown in Fig. 2.8(e). When extruding many thermoplastics, the polymer can undergo chemical reactions to form small amounts of crosslinked material. Partial crosslinking is a problem with some PE resins that contain residual double bonds that are made using... 

The complex three-dimensional structure of these materials is determined by their carbon-based polymers (such as cellulose and lignin), and it is this backbone that gives the final carbon structure after thermal degradation. These materials, therefore, produce a very porous high-surface-area carbon solid. In addition, the carbon has to be activated so that it will interact with and physisorb (i.e., adsorb physically, without forming a chemical bond) a wide range of compounds. This activation process involves controlled oxidation of the surface to produce polar sites. 

Structurally, the Fido polymers consist of a conjugated backbone with rigid, three-dimensional pentiptycene groups (Fig. 9.2). The rigidity and three-dimensional... 

In summary, we described a system in which a backbone containing a highly polar group contains a degree of cross-linking. Put another way, the system is a three-dimensional network of water-soluble polymer and cross-linking that serves as the basis for all hydrogels, natural or synthetic. 

All the polyrotaxanes discussed thus far are based on linear backbones. Viewing new properties and applications of branched and cross-linked polymers, three-dimensional polyrotaxanes will surely be interesting in terms of both topology and potential applications. Gong and Gibson extended the hydrogen-bonding theo-... 

While for nematic polymers the statistical distribution of the centers of gravity of the mesogenic side chains is compatible with a more or less statistical main chain conformation, for smectic polymers a three dimensional coil conformation is no longer consistent with the layered structure of the mesogenic side chains. The backbone has to be restricted in its conformation, which will cause a more pronounced interaction between the main chain and the anisotropically ordered mesogenic side chains, compared to nematic and cholesteric polymers. 

In any case, both models have in common that owing to the positional ordering of the mesogenic side chains, the polymer backbone no longer exhibits a statistical three dimensional coil conformation. Therefore at the phase transformation isotropic to smectic or nematic (cholesteric) to smectic, in addition to the change of the anisotropic packing of the side chains, the main chain has to change its conformation, which must be consistent with the layered smectic structure. A direct interaction... 

The conformational transitions observed in the simulations [97] resemble in some aspects the so-called zipping transitions [211], the process in which two strongly attracting strands composing the polymer come in contact in such a way as to form a bound double structure, which remains swollen and does not assume compact configurations. The cylindrical-shaped conformations in which the hydrophobic backbone is in a locally collapsed state (Figs. 41e and f) look a lot like three-dimensional zipped structures. 

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