Ordered polymer conformations

Major questions concerning the differences between small and big molecules and between natural and synthetic polymers oriented a part of the effort to verify the existence or nonexistence of ordered polymer conformations and of molecular or ionic interactions. 

Mesomorphic forms characterized by conformationally ordered polymer chains packed in lattices with different kinds of lateral disorder have been described for various isotactic and syndiotactic polymers. For instance, for iPP,706 sPP,201 sPS,202 syndiotactic poly(p-methylstyrene) (sPPMS),203 and syndiotactic poly(m -methylstyrene),204 mesomorphic forms have been found. In all of these cases the X-ray fiber diffraction patterns show diffraction confined in well-defined layer lines, indicating order in the conformation of the chains, but broad reflections and diffuse haloes on the equator and on the other layer lines, indicating the presence of disorder in the arrangement of the chain axes as well as the absence of long-range lateral correlations between the chains. 

From this point of view, polypeptides containing photochromic units in the side chains are quite special polymers. They can exist in ordered or disordered conformations, and photoisomerization of their photochromic side chains can produce order = disorder conformational changes. These photostimulated structural variations, such as random coil a-helix, take place as highly cooperative transitions therefore photochromic polypeptides actually work as amplifiers and transducers of the primary photochemical events occurring in the photosensitive side chains. 

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 fundamental questions are What is the microscopic mechanism or driving force for the transition, and what physical factors are important Two distinct possibilities have been advanced side-chain crystallization (5, 6, 17-19), which is postulated to induce polymer backbone ordering, and conformation-dependent polymer-solvent interactions that arise explicitly from electron delocalization and that stabilize an ordered rodlike conformation (20-24). Side-chain crystallization remains a qualitative suggestion that has not been developed to the point where it has predictive power and can be critically tested. However, in the solid state, the enhanced importance of packing effects makes such a mechanism more plausible (18, 19). 

In the preceding section, the remarkable salt concentration effect on the acid dissociation equilibria of weak polyelectrolytes has been interpreted in a unified manner. In this treatment, the p/( ,pp values determined experimentally are believed to reflect directly the electrostatic and/or hydrophobic nature of polyelectrolyte solutions at a particular condition. It has been proposed that the nonideality term (Ap/Q corresponds to the activity ratio of H+ between the poly electrolyte phase and the bulk solution phase, and that the ion distribution equilibria between the two phases follow Donnan s law. In this section, the Gibbs-Donnan approach is extended to the equilibrium analysis of metal complexation of both weak acidic and weak basic polyelectrolytes, i.e., the ratio of the free metal ion activity or concentration in the vicinity of polyion molecules to that of bulk solution phase is expressed by the ApAT term. In Section III.A, a generalized analytical treatment of the equilibria based on the phase separation model is presented, which gives information on the intrinsic complexation equilibria at a molecular level. In Secs. B and C, which follow, two representative examples of the equilibrium analyses with weak acidic (PAA) and weak basic (PVIm) functionalities have been presented separately, in order to validate the present approach. The effect of polymer conformation on the apparent complexation equilibria has been described in Sec. III.D by exemplifying PMA. 

The molecular conformation of a macromolecule is one of the fundamental physical properties of polymers, since it controls macroscopic properties, such as viscosity or solubility. There have been many attempts to stimulate reversible changes in polymer conformation under controlled and reproducible conditions in order to create responsive polymers. One approach is to induce a structural change in photosensitive groups incorporated into the polymer chains, such as a trans-cis isomerization. Another method is to generate ionic charges along the polymer chains. The repulsive interactions thus created force the chain to adopt a different conformation. 

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