Linear architecture

It is clear that the combination of different architectures and the precise localization of functionalities within a single macromolecule provide unique opportunities for the control of molecular shape as well as molecular, optical, and electronic properties. A significant hurdle that still remains today is the relatively demanding multistep process used to prepare dendrons and hybrids. This, in turn, translates into limited availability but, as high added-value applications emerge, it is clear that current, as well as yet-to-be-developed, syntheses will be used to prepare specialty materials that benefit from the unique properties derived from the combination of dendritic and linear architectures. 

The actual knowledge of the basic molecular structure property relationships relies mainly on the availability of well defined linear architectures the di, tri- and multiblock copolymers. New and well controlled molecular structures could undoubtedly provide a deep understanding of the behavior of block copolymers and a more efficient mastering of their applications. ... 

The observation of a bent Cr-H-Cr bond in the tetraethylammonium salt without an accompanying substantial deformation of the linear architecture of the nonhydrogen atoms in the [Cr2(CO)io(M2-H)] monoanion reflects the inherent flexibility of the bond. The deformability of the[M2(CO)io(M2-H)] monoanion species to adopt an appreciably bent, staggered carbonyl structure was first reported by Bau and co-workers (23) from neutron diffraction studies of two crystalline modifications of the electronically equivalent, neutral W2(CO)9(NO)(m2-H) molecule. Subsequent x-ray diffraction studies (15) of the analogous [W2(CO)io(m2-H)] monoanion found that the nonhydrogen backbone can have either an appreciably bent structure for the bis(triphenylphosphine)-iminium salt or a linear structure for the tetraethylammonium salt, with the W-W separation 0.11 A less in the bent form. Crystal packing forces probably were responsible (15) for the different molecular configurations of the monoanion in the two lattices. In solution, however, all known salts of the [W2(CO)io(m2-H)] monoanion exhibit the same three-band carbonyl ir absorption spectrum char-... 

Cationic synthesis of block copolymers with non-linear architectures has been reviewed recently [72]. These block copolymers have served as model materials for systematic studies on architecture/property relationships of macromolecules. (AB)n type star-block copolymers, where n represents the number of arms, have been prepared by the living cationic polymerization using three different methods (i) via multifunctional initiators, (ii) via multifunctional coupling agents, and (iii) via linking agents. 

Aside from the linear architecture, BCs can be prepared with advanced architectures such as miktoarm star structures, i.e., BCs where arms of different chemical nature are linked to the same branch point [20]. Unique segregation properties are expected of these polymers [21, 22]. 

Figure 7 Schematic representation of non-linear architectures, (a) and (c) are star and branched structures, respectively, that may he available by statisticallpragmatic modification of free-radical polymerisation, (b) and (d) are controlled analogues which could be made by living free-radical polymerisation...

Template-associated synthetic proteins (TASP), proteins that have been designed and synthesized de novo. Artificial tertiary structures can be constructed by covalent attachment of secondary structure building blocks, e.g., /3-sheets, a-helices, and turns, to a topological template. This results in a non-linear architecture of protein modules that is nevertheless able to mimic native proteins. The combination of different secondary structure motifs in TASP provides, e.g., /Sa/3-structures, helix-bundles, or /3-barrel tertiary structures. The design of TASP makes extensive use of molecular modeling techniques. Solid-phase synthesis and spot synthesis have been used to obtain TASP [G. Tuchscherer, M. Mutter,... 

Frechet [43,106] was the first to compare viscosity parameters for (A) linear topologies, as well as (B) random hyperbranched polymers and (C) dendrimers. More recently, we reported such parameters for (D) dendrigraft polymers [105] as shown in Fig. 42.15. It is clear that all three dendritic topologies behave differently than the linear architecture. There is, however, a continuum of behavior wherein, random hyperbranched polymers behave most nearly like the linear systems. Dendrigrafts exhibit intermediary behavior whereas, dendrimers show a completely different relationship as a function of molecular weight. 

This long-chain molecular geometry has interesting consequences in terms of properties and is invariably the reason behind the success of plastics as a material. Some important practical consequences of linear architecture of molecules will be considered in this section. 

Sugar-based fatty acid ester diols have also been prepared by transesterification of epoxidised oleates with methyl a-D-glucopyranoside and sucrose, followed by hydrolysis of the oxirane ring [57]. These fully bio-based monomers were polymerised with an aliphatic diisocyanate to produce PU whose structure could be oriented toward a linear architecture (when the sugar OH groups were not involved) or a network (if at least some of them participated in the polycondensation) by changing the solvent medium. 

Materials with multiaiyl cores e.g., biphenyls, terphenyls and phenylpyrimidines) and esters that include biphenol or bipheitylcarboxylic acid moieties are somewhat more difficult to prepare because of the direct bond between the aiyl sections. Traditionally, the biphenyl liquid crystals are prepared by the modification of bipheityl in a linear fashion as distinct from a convergent approach. For example, a synthesis of 4-cyano-4 -pentylbiphenyl (28) from biphei rl (24) is shown in Scheme 6. Liquid crystals require a long, linear architecture and hence most simple structures are 4,4 -disubstituted. Fortunately, such linear syntheses work fairly well but overall yields are considerably reduced as the number of steps increases. 

The underlying factors behind the selectivity differences between anion-exchange materials based on styrenic and methacrylate monomers are not fully understood. It is likely that these differences can be attributed to the n electrons in the styrenic polymer. Thus, Pohl and Saini [38] were searching for a methodology to synthesize an anion-exchange material free of any k electron character in the polymer backbone. A particularly interesting system for synthesizing a cationic condensation polymer involves the reaction of diepoxides with primary amines. When diepoxides and primary amines are allowed to react in a 1 1 molar ratio, the product is a water-soluble polymer with a predominantly linear architecture. 

The rheology of blends of linear and branched PLA architectures has also been comprehensively investigated [42, 44]. For linear architectures, the Cox-Merz rule relating complex viscosity to shear viscosity is valid for a large range of shear rates and frequencies. The branched architecture deviates from the Cox-Merz equality and blends show intermediate behavior. Both the zero shear viscosity and the elasticity (as measured by the recoverable shear compliance) increase with increasing branched content. For the linear chain, the compliance is independent of temperature, but this behavior is apparently lost for the branched and blended materials. These authors use the Carreau-Ya-suda model. Equation 10.29, to describe the viscosity shear rate dependence of both linear and branched PLAs and their blends ... 

Figure 26 Frechet-type monodendrons (generations 1, 2, and 3) possessing various reactive groups at the focal point. Rod-shaped, dendritic-linear architectural copolymers obtained by coupling respective dendrons to linear poly(para-phenylene) (PPP) backbone. (Courtesy J. Am. Chem. Soc. 119 3297, 1997. Copyright 1997 American Chemical Society.)...

The first traditional polymerization strategies generally produced linear architectures however, it was soon found that branched topologies may be formed either by chain transfer processes or intentionally introduced by grafting techniques. In any case, the linear and branched architectural classes have traditionally defined the broad area of thermoplastics. Of equal importance is the major architectural class... 

In the following, typical examples of these recent studies will be discussed, with the focus on the micellization of water-soluble A-B copolymers with non-linear architectures as well as of A-B-C copolymers having at least one hydrophilic block. 

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