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Polymeric systems fundamental feature

The predominant approach toward the synthesis of olefin-based BCPs has focused on development of living coordination polymerization systems. Unfortunately, one feature that makes coordination polymerization catalysts so efficient for production of RCPs also limits their use for synthesis of conventional BCPs. These catalysts are susceptible to several chain termination and transfer mechanisms and typically produce many chains during polymerization. Therefore, a sequential monomer addition scheme produces a physical polymer blend with a conventional catalyst (Scheme 1). However, by designing systems that suppress these termination processes, advanced catalysts have been used to make BCPs via sequential monomer addition techniques (Scheme 1). These systems have produced many new BCPs with interesting structures. Unfortunately, the fundamental features that enable precision synthesis also make the processes very inefficient and thus of limited commercial appeal. Conventional catalysts produce hundreds to thousands of chains per metal center, but these living systems produce only one. For these materials to be competitive with other large-volume TPEs, more efficient protocols for BCP synthesis must be developed. [Pg.701]

The diffusion of small molecules in polymeric solids has been a subject in which relatively little interest has been shown by the polymer chemist, in contrast to its counterpart, i.e., the diffusion of macromolecules in dilute solutions. However, during the past ten years there has been a great accumulation of important data on this subject, both experimental and theoretical, and it has become apparent that in many cases diffusion in polymers exhibits features which cannot be expected from classical theories and that such departures are related to the molecular structure characteristic of polymeric solids and gels. Also there have been a number of important contributions to the procedures by which diffusion coefficients of given systems can be determined accurately from experiment. It is impossible, and apparently beyond the author s ability, to treat all these recent investigations in the limited space allowed. So, in this article, the author wishes to discuss some selected topics with which he has a relatively greater acquaintance but which he feels are of fundamental importance for understanding the current situation in this field of polymer research. Thus the present paper is a kind of personal note, rather than a balanced review of diverse aspects of recent diffusion studies. [Pg.1]

The preceding, brief discussion about the complex structure of the cell walls and the fine features of the cellulosic microfibrils clearly indicates that the formation and functioning of this system must involve not only the chemical synthesis of the raw materials, but also other fundamental processes, such as the packing of the cellulose molecules in the microfibrils and the orientation of the microfibrils within the cell walls. These complex processes are so closely related in biological processes that a discussion of one aspect without consideration of others would be rather meaningless. However, for the sake of convenience and simplicity, the biosynthesis of cellulose, or the process of polymerization of the D-glucose residues, will be considered first, irrespective of the physical structure of the final product. [Pg.319]

For a specified solvent system, water or aqueous solutions for example, there are two variables that must be considered in the solubilization process (1) the molecular nature, purity, and homogeneity of the surfactant and (2) the chemical nature of the additive. From a technological viewpoint, it is important to understand exactly what surfactant structural features serve to maximize the desired solubilizing effect, and the best way to achieve that understanding is through a fundamental knowledge of the molecular and thermodynamic processes involved. In addition, since most technological applications of solubilization involve complex multicomponent systems, such factors as temperature, electrolyte content, and the presence of polymeric species and other solutes must be examined. [Pg.398]

It is well-known that the crystallinity of EP copolymers mainly depends on comonomer content (composition) and comonomer distribution (constitution). It is the objective of this section to demonstrate that the tacticity of propylene, that is, the regio- and stereoregularity of propylene insertion, also plays a fundamental role in controlling crystallinity. As discussed earlier in detail, all of these microstruc-tural features in turn essentially depend on the catalytic system and on polymerization conditions. [Pg.332]


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