Conformations of the Polymers

Conformations of the polymers were studied by CD and optical rotation measurements. Poly-L-lysine is known to exist in disordered, helical and -conformation, depending on the temperature, pH of the system and the solvent used. The side chain of the polymer has a significant effect on the backbone conformation. At neutral pH, poly-L-lysine exists in a random coil structure while at pH above 10, the e-amino group becomes a neutral form and the polymer undergoes transition to a helical structure. In order to elucidate the effect of base substituents on the conformation of poly-L-lysine, CD spectra of the copolymer were measured. 

From the CD spectra of PLL-A-67 (poly-L-lysine having 67% adenine units) in acidic aqueous solution, the residual elliptidty at 222 nm ([ J222)is plotted against the pH of the system (Fig. 21). The value [0)222 is known to be related to the helix content of poly-(a-amino acid)73.  

The helical structure observed here may be stabilized by interactions which is revealed by the formation of a double helix of poly A in acidic aqueous solution74. With rising pH of the system, helicity of the polymer increases due to release of the electrostatic repulsion between positively charged side chains. Above pH 2.5, the spectra cannot be measured, as the polymer begins to precipitate in aqueous solution. By adding EG, helicity tends to increase (Fig. 22). In EG, however, poly-L-lysine - HBr still exists in a random coil structure. Therefore, it can be assumed that EG rather depresses the electrostatic repulsion between piotonated adenine units. 

For PLL-T-93 and PLL-T-79, the values of [0]222 in alkaline pH region are plotted against the pH (Fig. 23). These polymers tend to exist in a helical conformation at neutral pH while poly-L-lysine exists in a random coil structure. In contrast to the latter, helicity of PLL-Ts decreases with increasing pH of the system. The decrease in helicity may be caused by the electrostatic repulsion between negatively charged thymine basses which are formed by deprotonation at N-3 in the base. The helicity of PLL-T-79 is lower at neutral pH and higher at alkaline pH than that of PLL-T-93. This can be explained by the fact that the unreacted free amino units in poly-L-lysine at neutral pH assume a random coil structure, whereas at alkaline pH they exist in a helical conformation. A similar tendency was observed in the case of PLL-U-93 and PLL-U-76. 

Another important factor for the polymer conformation is the solvent effect. As the usual solvents for the copolymers in question are DMSO and DMF, which have absorption in the UV region, CD spectral measurements are impossible. However, the optical rotation measurements and analyses using the Moffitt-Yang equation give the Moffitt parameter bo for the copolymers (Table 21). Hie parameter is known to be related to the helix content of poly-(a-amino acids). The b0 value of polycarboben- 

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The rheological behaviour of polymeric solutions is strongly influenced by the conformation of the polymer. In principle one has to deal with three different conformations, namely (1) random coil polymers (2) semi-flexible rod-like macromolecules and (2) rigid rods. It is easily understood that the hydrody-namically effective volume increases in the sequence mentioned, i.e. molecules with an equal degree of polymerisation exhibit drastically larger viscosities in a rod-like conformation than as statistical coil molecules. An experimental parameter, easily determined, for the conformation of a polymer is the exponent a of the Mark-Houwink relationship [25,26]. In the case of coiled polymers a is between 0.5 and 0.9,semi-flexible rods exhibit values between 1 and 1.3, whereas for an ideal rod the intrinsic viscosity is found to be proportional to M2. 

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

Experimental and theoretical results are presented for four nonlinear electrooptic and dielectric effects, as they pertain to flexible polymers. They are the Kerr effect, electric field induced light scattering, dielectric saturation and electric field induced second harmonic generation. We show the relationship between the dipole moment, polarizability, hyperpolarizability, the conformation of the polymer and these electrooptic and dielectric effects. We find that these effects are very sensitive to the details of polymer structure such as the rotational isomeric states, tacticity, and in the case of a copolymer, the comonomer composition. 

As discussed in Section 1, general requirements for the crystallizability of polymers are the regularity of the chemical constitution and of the configuration of long sequences of monomeric units. In these conditions the conformation of the chains is also regular (equivalence principle). However, some three-dimensional long-range (crystalline) order may be maintained even when disorder is present in the conformation of the polymer chains. 

The secondary structure describes the molecular shape or conformation of the polymer chain. For most linear polymers this shape approaches a helical or pleated skirt (or sheet) arrangement depending on the nature of the polymer, treatment, and function. Examples of secondary structures appear in Figure 2.13. 

The resulting chromatogram is then a reflection of the molecular size distribution. The relationship between molecular size and molecular weight depends on the conformation of the polymer in solution. As long as the polymer conformation remains constant, which is... 

Organic polymers are responsible for the very life—both plant and animal—that exists. Their complexity allows for the variety that is necessary for life to occur, reproduce, and adapt. Structures of largely linear natural and synthetic polymers can be divided into primary structures, which are used to describe the particular sequence of (approximate) repeat units secondary structures, which are used to describe the molecular shape or conformation of the polymer tertiary structures, which describe the shaping or folding of macromolecules and quaternary structures, which give the overall shape to groups of tertiary-structured macromolecules. The two basic secondary structures are the helix and the sheet. 

The distinct properties of liquid-crystalline polymer solutions arise mainly from extended conformations of the polymers. Thus it is reasonable to start theoretical considerations of liquid-crystalline polymers from those of straight rods. Long ago, Onsager [2] and Flory [3] worked out statistical thermodynamic theories for rodlike polymer solutions, which aimed at explaining the isotropic-liquid crystal phase behavior of liquid-crystalline polymer solutions. Dynamical properties of these systems have often been discussed by using the tube model theory for rodlike polymer solutions due originally to Doi and Edwards [4], This theory, the counterpart of Doi and Edward s tube model theory for flexible polymers, can intuitively explain the dynamic difference between rodlike and flexible polymers in concentrated systems [4]. 

In view of the marked kinetic effects described so far, it has seemed wise to carry out some examination of the three-dimensional disposition of the groups on the polyethylenimines. It has been our hope that even a rudimentary insight into the conformation of the polymer would provide a guide to the search for other types of chemical reaction that might be facilitated in the presence of these polymers. The results of investigations with two conformational probes, 19F nuclear magnetic resonance and excimer fluorescence, are described. 

Professor Ubbelohde s suggestions are certainly worthwhile. But there are many directions that we are trying to pursue more detailed studies of the conformation of the polymers, introduction of new functional groups, macro molecular characterization by hydrodynamic methods, and so on. If I were a professor at Imperial College, I could assign all these problems, and those you suggest, to members of the staff but I am constrained by a smaller research group. 

First a complex is prepared between a polymer ligand, eg. PVP,and a metal ion(M,) and a crosslinking agent is added to the solution of the polymer complex. The metal ion M( is (hen removed by treating the resin with an acid. If the conformation of the polymer-ligand chains in this resin remains the best for the metal ion Mj, used as the template, then the resin should preferentially form complexes with the metal ion M j when dipped into a solution containing various metal ions. 

AS = 13 e.u. for the Cu-template resin, and AH = -0.8, AS = 9,8 (K - 540) for the resin synthesized without any template ion. The larger change in entropy observed in the complexation of the Cu-template resin indicated that die Cu-template resin selectively adsorbed Cu ions by entropic effect. Furthermore, the absorption spectrum of the Cu complex of the Cu template resin was located at a wavelength 10—20 nm shorter than those of the other resins70 and the ESR parameters of the Cu complex of the Cu-template resin were similar to those of the non-distorted planar Cu complex71. From these results, it was suggested that the conformation of the polymer-ligand chain in the Cu template resin remained the best one for the Cu ion. 

It is thus expected that the conformation of the polymer-ligand chain will influence the reactivity of a metal complex. The influence of the conformational change of a poly(N-vinyl-2-methylimidazole)(PVMI 3) ligand has been studied in the electron-transfer reaction of a Co(III) complex in aqueous-alcoholic solvents87. ... 

The higher reactivity of the PVMI-Co(III) complex is attributed to the electrostatic domain of the polymer complex, as in the above PVP system. When the PVMI chain contracts, the charge density in the polymer domain increases and the reaction rate also increases. On the other hand, when the polymer chain expands, the electrostatic domain is weakened, which produces a fall in reactivity. These results confirm that the conformation of the polymer complex is closely related to the strength of its electrostatic domain and to the reaction rate. The effects of the polymer chain on reactivity are to be understood not only in terms of static chemical environment but also as dynamic effects which vary with the solution conditions, e.g. pH, ionic strength, solvent composition, temperature, and so on. 

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