Polymer Conformational Control

Although conformational control of synthetic macromolecules is a daunting proposition, given the enormous entropy cost that has to be borne, approaches to achieve this are being actively researched. Typically, synthetic polymer chains adopt a random coil conformation in solution, unlike biological macromolecules which 

A straightforward consequence of such strong interactions (in the absence of inter-chain interactions, such as in very dilute solutions) would be aggregation (or clustering) of these segments, a situation that can be termed self-association. Such a 

Bond angle constraint aided by solvophobic effect Fig. 15.1. Examples of polymers with conformational restriction due to steric and bond angle constraints. 

Bond angle constraint aided by electrostatic interactions 

Directional interactions could lead to self-assembly 

---

In order to relax 1 mol of compacted polymeric segments, the material has to be subjected to an anodic potential (E) higher than the oxidation potential (E0) of the conducting polymer (the starting oxidation potential of the nonstoichiometric compound in the absence of any conformational control). Since the relaxation-nucleation processes (Fig. 37) are faster the higher the anodic limit of a potential step from the same cathodic potential limit, we assume that the energy involved in this relaxation is proportional to the anodic overpotential (rj)... 

Equations (37) and (38), along with Eqs. (29) and (30), define the electrochemical oxidation process of a conducting polymer film controlled by conformational relaxation and diffusion processes in the polymeric structure. It must be remarked that if the initial potential is more anodic than Es, then the term depending on the cathodic overpotential vanishes and the oxidation process becomes only diffusion controlled. So the most usual oxidation processes studied in conducting polymers, which are controlled by diffusion of counter-ions in the polymer, can be considered as a particular case of a more general model of oxidation under conformational relaxation control. The addition of relaxation and diffusion components provides a complete description of the shapes of chronocoulograms and chronoamperograms in any experimental condition ... 

From these experimental results it was assumed that conformational transitions within single polymer chains — controlled via intramolecular hydrogen bonds — are causing the viscosity decrease. Chain scission could be excluded. 

The azo-group capabilities to change their configuration under UV-VIS irradiation induce the possibility of the polymer conformational photo-control, with potential application in optoelectronics or biology fields. One of the consequences of the... 

Somasundaran P, Xiang Y, Krishnakumar S. Role of conformation and orientation of surfactants and polymers in controlling flocculation and dispersion of aqueous and nonaqueous suspensions. Colloids Surf A 1998, 133, 125-133. 

Al-Anazi, H. A., Sharma, M.M., 2002. Use of pH sensitive polymer for conformance control. Paper SPE 73782 presented at SPE International Symposium Formation Damage Control, Lafayette, Louisiana, 20-21 February. 

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. 

Most properties of linear polymers are controlled by two different factors. The chemical constitution of the monomers determines the interaction strength between the chains, the interactions of the polymer with host molecules or with interfaces. The monomer structure also determines the possible local conformations of the polymer chain. This relationship between the molecular structure and any interaction with surrounding molecules is similar to that found for low-molecular-weight compounds. The second important parameter that controls polymer properties is the molecular weight. Contraiy to the situation for low-molecular-weight compounds, it plays a fundamental role in polymer behaviour. It determines the slow-mode dynamics and the viscosity of polymers in solutions and in the melt. These properties are of utmost importance in polymer rheology and condition their processability. The mechanical properties, solubility and miscibility of different polymers also depend on their molecular weights. 

Introduction of relatively weak functional groups, such as carbonyl, hydroxyl, nitro, amide, etc., in the nanochannels of PCPs would affect the monomer alignment, which may lead to precision control of stereoselectivity and regioselectivity of the resulting polymers. In particular, PCPs with either helical or chiral structures on the pore surface are of intense interest in chemistry and such porous solids are potentially useful to find applications in enantioselective sorption/separation and catalysis [34, 38 0, 42, 45]. Of considerable interest is the use of the chiral channels to affect asymmetric polymerizations such as asymmetric selective polymerization of racemic monomers as well as asymmetric polymerization of prochiral monomers, which may give helical polymer conformations. 

---