Polysiloxanes conformation

An important source of experimental and theoretical studies of equilibria in ring formation is represented by the field of so-called macrocyclisation equilibria (Flory, 1969). Interest in this field appears to have been restricted so far to chemists conventionally labelled as polymer chemists. Experimental evidence of cyclic oligomer populations of ring-chain equilibrates such as those obtained in polysiloxanes (Brown and Slusarczuk, 1965) may be delated to the statistical conformation of the corresponding open-chain molecules (Jacobson and Stockmayer, 1950 Flory, 1969). In these studies experimental results are expressed in terms of molar cyclisation equilibrium constants Kx (14) related to the x-meric cyclic species Mx in equilibrium with the... 

Chain flexibility also effects the ability of a polymer to crystallize. Excessive flexibility in a polymer chain as in polysiloxanes and natural rubber leads to an inability of the chains to pack. The chain conformations required for packing cannot be maintained because of the high flexibility of the chains. The flexibility in the cases of the polysiloxanes and natural rubber is due to the bulky Si—O and rxv-olelin groups, respectively. Such polymers remain as almost completely amorphous materials, which, however, show the important property of elastic behavior. 

Other crystalline inorganic polymers such as poly(dichlorophosphazene), poly(aryloxyphosphazenes), liquid crystalline polysiloxanes and poly(dichloro-silane) have also been studied by X-ray diffraction methods, enabling the conformations in the crystallites in the solid state to be established. 

Conformations and Spatial Configurations Symmetrically Substituted Polysiloxanes... 

The crystallization of 3D-ordered crystalline phases from thermotropic mesophases, envisaged as stable pre-crystalline partially ordered intermediates, is an additional interesting issue which should be considered with care experimentally, theoretically, and with appropriate simulation approaches. Depending upon the nature of the mesophase it can be seen as a crystal-crystal transition or, for conformationally disordered, columnar mesophases, it approaches a true crystallization process. It is quite clear that the preexisting order will play a major role for example if the mesophase is chain-extended, bundle equilibria and chain-folding should not play any role. Indeed available experimental evidence supports this idea. Mechanistic and kinetic features should in general differ widely from the standard chain-folded crystallization processes yielding thin lamellar structures. In a number of cases (polyphosphazenes, polysiloxanes, see below) the crystalline polymorphs obtained from the chain-extended precursor differ from those obtained from solution. 

Several research groups are currently investigating the inherent thermochromism of various polymers. Certain polysiloxanes exhibit reversible thermochromic activity [59, 60]. The thermochromic behavior of these macromolecules is due to order-disorder conformational changes that accompany a particular temperature change. This transition perturbs the electron delocalization of the silicone backbone and results in a shift in the absorption maxima in the UV-visible range. 

The ordering chain tendency induced by the presence of the azo-phenolic groups in the polysiloxanic chain is evidenced by the minimum energy conformation of a homo-polysiloxane containing only p-structural units (completely modified with azobenzene). A very nice helix structure is obtained (Fig. 7) if compared with the unmodified polysiloxane presented in Fig. 3. 

Fig. 6 Minimum energy conformation corresponding to a polysiloxane modified with azobenzene (DP=25) (See also Plate 12 in the Colour Plate Section)...

The Si-O-Si linkage has very specific properties. For example, in polysiloxanes [13-16], the structural parameters that determine their general behavior are (a) the relatively high length of the Si-0 bond (1.64 A), (b) an important free volume by segment (c) the conformational flexibihty of the polysiloxane bridge (Si-O-Si can vary... 

This principle also governs the separation on the commercially available Chirasil-Val [37,38]. In Chirasil-Val , the chiral entity was incorporated in a polysiloxane backbone for higher thermal stability. Some of the compounds separated on Chirasil-Val contained only groups, such as V-TFA-proline esters, that are able to accept hydrogen bonding. To undergo such an interaction, the diamide phase has to have a conformation where both NH groups point toward the selectand in a conformation similar to the a-helix structure of proteins [36]. 

The basic strategy in studying the conformation of polymers is to identify the principal factors that are responsible for their physical properties. Polysiloxanes constitute a suitable class of polymers, because they show two levels of flexibility in terms of molecular structure torsional flexibility, which... 

Three aspects of chain flexibility in polysiloxanes will be discussed in this section (1) the nature of the bending flexibility of the Si-O-Si angle, (2) the effects of this flexibility on the conformational analysis performed with simple scanning and with scanning that allows for torsional relaxation, and (3) the conformational analysis of various pendant groups attached to the polysiloxane bond. 

High static and dynamic flexibility of the polysiloxane chain, associated with a very low energy barrier to rotation around their skeletal bonds and a low energy of deformation of the SiOSi bond angle, make the polymer soluble in many solvents. The catalyst attached to such a mobile polymer chain, which can adopt many conformations, is available for the interaction with reactants in a... 

Polysiloxane 134 having chiral phthalocyanine moieties as repeating constituents takes a helical conformation in a chloroform solution.232 The helical structure was indicated to be stable at up to 120 °C in a dodecane solution. On the basis of the CD spectra, the helix was found to be left-handed. 

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