Polymer conformational isomerism

Dendritic polymers are most often reported to be amorphous, which can be anticipated from their highly branched architecture. However, some exceptions are presented in the literature. Percec et al. [34,35] reported on liquid crystalline (LC) hyperbranched polymers where the LC-phase was achieved by conformational isomerism. Various repeat units of A2B type have been used where a flex-... 

Experimental data collected so far show that most secondary relaxations (transitions, dispersions) in glassy polymers are a consequence of the conformational isomerization of short sections of main or side chains and that their kinetics may be satisfactorily described by means of the site model in which stable conformations are separated by a potential... 

The compact conformation returns to the initial extended conformation by the thermal or photochemical isomerization of the chromophores from the cis to trans form. Polyamides with azobenzene groups in the polymer backbone are among the earliest in which trans-cis isomerizable chromophores are used to regulate the conformation of polymer chains(4.5). 

The initial four chapters of the book concern several important aspects of polymer science which are relevant to a course in polymer chemistry. Following Chapter 1, which is a general introduction aimed at giving the reader an appreciation for the language, applications, and versatility of synthetic polymers. Chapter 2 is devoted to polymer characterization dealing with the size and shape of a polymer chain, polymer isomerism, polymer conformation, and thermal transitions in polymers. 

Values for tte internal variabtes in thetmodynamic, internal equilibriwn are generally uniquely defined by the values for the external variables. For instance, in a simple, thermomechanical system (i.e. one that reacts mechanically solely volume-elastically) the equilibrium concentrations of the conformational isomers are uniquely described by temperature and pressure. In this case the conformational isomerism is not explicitly percqitible, but causes only overall effects, for example in the system s enthalpy or entropy. Elastic macroscopic effects may, however, occur when the relationship between internal and external variables is not single-valued. Then the response-functions of the system diverge or show discontinuities. The Systran undergoes a thermodynamic transformation. The best-known example of sudi a transformation based on conformational isomerism is the helix-coil transition displayed by sonte polymers in solution. An example in the scdid state is the crystal-to-condis crystal transition discussed in this paper. The conditions under which such transformations occur are dealt with in more detail in Sect 2.2. 

The Eqs. (2.1a) and (2.1b) apply thus actually to a rate scale and, in the frequait case of cyclic exposure, to a frequency scale co. If a thermorheologically simple system is considei ed the fr juency scale can be replaced by a temperature sale 1/T. Steps A that satisfy Eqs. (2.1a) and (2.1b) appear then in the response-functions for systmis of this nature that are measured as a function of temperatiue at pven, fixed paturbation rate. The temperature at whidi the steps occur depends, however, on the rate of external i rturbution. The temperature-dependent thawing of conformational isomers in thermorheologically complicated systems can be similariy observed in the response-functions, but the steps no longer satisfy Eqs. (2.1 a) and (2.1 b). These two equations lose, in addition, their validity with rrapect to the rate scale if, as is the case of polymers, several mutually independent, internal variables are required in order to uniquely define the conformational isomerism. In this case. Eqs. (2.1a) and (2.1b) become inequalities... 

Flory (1969 1971 1974) has developed the rotational isomeric state theory for predicting the conformation of polymer molecules. This incorporates the interdependence of the bond rotational potentials. This theory is fully explained in Flory s comprehensive monograph (Flory, 1969) and only the barest details will be mentioned here. 

Percec and coworkers have shown that the oriented fibers of dendronized polyacetylenes can undergo thermally induced mechanical motion [38]. In this system (Figure 37.12), the dendrons around the polymer backbone become self-organized and thus facilitate a reversible ciscoid-to-transoid conformational isomerism of the polymer backbone. This isomerism is then converted into unidirectional macroscopic motion along the fiber axis. Remarkably, this mechanical motion could be used to displace objects with a mass of up to 250-fold that of the polymer fiber. 

In conclusion, form II of l,4-tra s-poly(l,3-butadiene) represents a case of conformational isomerism of polymers in the crystalline state where different conformations occur at random within the same chain and long-range order is maintained among parallel chains [58]. 

Molecular structure is used to describe those attributes which are characteristic of individual polymer chains. Here we are concerned with the way in which the monomers are joined together. Within this heading are included tacticity, conformational isomerism, monomer sequence distribution, branching and the presence of minor structural defects. It is in studies of this kind that the spectroscopic techniques particularly excel. In recent years, the advent of the microcomputer for data collection and manipulation, in conjunction with modern Fourier-transform instruments, has allowed enormous advances in the quality of information that is now routinely available. 

The key factor responsible for the photoinduced variations of conformation is the affinity of the azo-polymer for the micelles. Such an affinity, in fact, is likely to be different when the azo side chains are in trans or in cis configuration. When azo-units are in the planar, apolar, trans form, they dissolve within the hydrophobic core of the micelles, forcing the polypeptide chains to assume a coil conformation. Isomerization of the azo units to the skewed, polar, cis form inhibits hydrophobic interactions and causes the azo-units to leave the micelles, thus allowing the polypeptide chains to adopt the a-helix structure (which is favored in the absence of micelles). In other words, the primary photochemical event is the trans cis isomerization of the azobenzene units, but the driving force of the photoresponse should be the different location of the macromolecules relative to the micelles. 

For the above, detailed knowledge of chain architecture is not necessary. It is well known, however, that the modulus or of each polymer are strongly dependent on the detailed structure, actual and virtual bond lengths, rotational and valence bond angles, and the conformational isomeric states of the individual chains or segments. In a crystalline polymer, the highest elastic modulus is one parallel to the chain axis. The lowest modulus is in the plane transverse to the chain axis. In this plane the interactions are exclusively intermolecular in character and contain no intramolecular, covalent bonds. The intermolecular interactions may be common van der Waals dispersive forces, the somewhat... 

A final twist to the interpretation of the IR spectra of polymers is that apparently conformational isomerism is also present in these materials. Thus,... 

This concludes an introduction to the IR spectra of polymers. Clearly, the interpretation of polymer spectra can be reasonably straightforward based on the group frequency logic that has been developed to deal with relatively small organic molecules. There are, however, complexities which result from the fact that these solid systems may be associated with configurational isomerism, stereoisomerism, and conformational isomerism. These added complexities affect the spectra and must be remembered and taken into account in the interpretive process. Finally, there are aids such as the flow... 

Chapters 10-12 are three chapters that address special areas of interpretation. Chapter 10 is focused on the interpretation of polymer spectra. Exercise Sections 1 and II have three exercises that involve the identification of relatively simple polymer spectra. These spectra were introduced to demonstrate to the reader that the extension of the group frequencies approach to the interpretation of polymer spectra is, in general, straightforward. However because of the importance of polymer spectra, we now consider this area in some detail in Chapter 10. Section I of the chapter builds on the interpretation of the spectra of hydrocarbon polymers started in exercise sections I and II. In section II the problem of the presence of plasticizers is examined and in addition the polymerization of hetero-atom monomers is explored. The sampling of polymers to acquire infrared and Raman spectra often requires specialized techniques. A short introduction to a few of these techniques is given in Section III. The chemistry involved in the formation of polymers is reviewed in part IV with examples of condensation (nylon) and addition (polyethylene) polymerization presented. Copolymers are examined next (V) with methylmethacrylate-stryene used as an example. The effects on the spectra of block and random copolymerization are also noted. Next crosslinked polymerization is studied (VI) with phenol-formaldehyde. Tacticity (VII) is then explored with evidence for its presence in the spectra of polypropylene. This discussion leads to a concise examination of conformational isomerism (VIII) and the impact of this... 

The rotational isomeric state (RIS) model assumes that conformational angles can take only certain values. It can be used to generate trial conformations, for which energies can be computed using molecular mechanics. This assumption is physically reasonable while allowing statistical averages to be computed easily. This model is used to derive simple analytic equations that predict polymer properties based on a few values, such as the preferred angle... 

The temperature of esterification has a significant influence on isomerization rate, which does not proceed above 50% at reaction temperatures below 150°C. In resins produced rapidly by using propylene oxide and mixed phthaUc and maleic anhydrides at 150°C, the polyester polymers, which can be formed almost exclusively in the maleate conformation, show low cross-linking reaction rates with styrene. 

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