Polymers central structural feature

According to Ref. [12], template for synthesis of nanomaterials is defined as a central structure within which a network forms in such a way that removal of this template creates a filled cavity with morphological or stereochemical features related to those of the template. The template synthesis was applied for preparation of various nanostructures inside different three-dimensional nanoporous structures. Chemically, these materials are presented by polymers, metals, oxides, carbides and other substances. Synthetic methods include electrochemical deposition, electroless deposition, chemical polymerization, sol-gel deposition and chemical vapor deposition. These works were reviewed in Refs. [12,20]. An essential feature of this... 

Chapter 1 is devoted to introductory concepts and de nitions, while Chapter 2 deals with physical and molecular aspects of polymers, that is, those relating to molecular shape and size, distinctive characteristics, conformational and con gurational behavior, structural features, morphology, thermal transitional phenomena, and relaxation properties. Chapter 3 discusses polymer solution behavior, the emphasis being on thermodynamics, phase equilibria, solubility, swelling, frictional properties, and viscosity. Molecnlar weight determination, which is one of the rst steps of polymer characterization and a centrally important topic of polymer science, mostly involves... 

Recently the development of dendritic and hyperbranched polymers (HBPs) has attracted much attention (Tomalia, 1985, Newkome et al, 1985, Webster, 1991, Chu and Hawker, 1993, Wooley et al, 1994, Feast and Stanton, 1995, Malmstrom et al, 1995, Kim, 1998). The key features of the macromolecular architecture of dendrimers and HBPs are given in Section 1.2, and their synthesis by stepwise polymerization is discussed in Section 1.2.1. Dendrimers and HBPs are globular macromolecules that have a highly branched structure with multiple reactive chain ends (shell), which converge to a central focal point (core) see Figure 5.1, where I is the core, 11 is the structure and 111 is the shell. 

Dendrimer-like polymers are dendrimers that not only feature tmly macromolecular generations, but can also be viewed as an extension of star polymers with the presence of a central core (Figure 27.1) (7, 10, 11]. Like dendrimers, these polymers possess a precise number of branching points that are distributed regularly throughout the dendritic structure (in contrast, for instance, to dendtigrafts), as well as a more dense surface functionahty than do stars, due to their numerous terminal functions. Although more... 

When the fundamentals of the structure of semi-crystaUine polymers with stacks of layer-lrke crystalUtes with thicknesses in the nm-range being embedded in an amorphous matrix were revealed in the Fifties, considerations about the mechanism of the formation of these structures started immediately. In the Sixties and Seventies, they became a major field of research and a focus of interest, discussed as a central topic in all structure oriented polymer conferences (see, for example, [1] with the lectures at the Faraday Discussion in Cambridge 1979). In the years which followed, one approach gained superiority - the one put forward by Hoffman, Lauritzen and their co-workers [2], It was accepted and used in data evaluations by more and more workers, due to some appealing features ... 

Metallocene catalysts are organometallic coordination compoimds in which one or two cyclopentadienyl rings or substituted cyclopentadienyl rings are ir-bonded to a central transition metal atom (Figure 1). The most remarkable feature of these catalysts is that their molecular structure can be designed to create active centre types to produce polymers with entirely novel properties. 

Broadening the scope, we may briefly consider a nonexhaustive panorama of various types and features of supramolecular polymers depending on their constitution, characterized by three main parameters the nature of the core/framework of the monomers, the type of noncovalent interaction(s), and the eventual incorporation of functional subunits. The interactions may involve complementary arrays of hydrogen-bonding sites, electrostatic forces, electronic donor-acceptor interactions, metalion coordination, etc. The polyassociated structure itself may be of main-chain, side-chain, or branched, dendritic type, depending on the number and disposition of the interaction subunits. The central question is that of the size and the polydispersity of the polymeric supramolecular species formed. Of course their size is expected to increase with concentration and the polydispersity depends on the stability constants for successive associations. The dependence of the molecular weight distribution on these parameters may be simulated by a mathematical model [19]. These features are detailed in Chapters 2, 3, and 6 for various growth mechanisms. 

Although the structure of a polyethylene chain is drawn in Rgure 25.1 as a straight line, the bond angles around a central carbon atom with four bonds are approximately 109.5 each [IH Section 9.1], Thus, the straight chain really zigzags, instead. The polymer chain bends, too, because there is free rotation about carbon-carbon single bonds. These molecular-level features manifest themselves in such macroscopic properties as flexibility. 

Branched macromolecules fall into three main classes star-branched polymers, characterized by multiple chains linked at one central point (Roovers, 1985), comb-branched polymers, having one linear backbone and side chains randomly distributed along it (Rempp et al., 1988), and dendritic polymers, with a multilevel branched architecture (Tomalia and Frechet, 2001). The cascade-branched structure of dendritic polymers is typically derived from polyfunctional monomers under more or less strictly controlled polymerization conditions. This class of macromolecules has a unique combination of features and, as a result, a broad spectrum of applications is being developed for these materials in areas including microencapsulation, drag delivery, nanotechnology, polymer processing additives, and catalysis. 

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