Molecular architectures

Bates [3] discussed the role of molecular architecture in polymer-polymer phase behavior. There are a number of molecular configurations available to a pair of chenucally distinct polymer species. Star polymers with a specific arm number with predeternuned molecular weights and with narrow molecular weight distribution can be synthesized using anionic polymerization. Diblock and multiblock arrangements are possible. Different polymers can be combined into a single material in several different ways that can lead to a variety of phase behaviors. Four factors control the phase behavior of polymer-polymer systems [3]. These are 

The Flory-Huggins segment-segment interaction parameter can be written as 

The contact energy between i and j segments for nonpolar polymer such as polyethylene, polystyrene, and polyisoprene that are characterized by dispersive van der Waal interactions can be represented by 

In general, a and (5 represent experimentally determined enthalpy and excess entropy coefficients for a particular composition a and p may be a function of 

Specific interactions, preferential segment orientation, and EOS effects play an important role, although not well understood, in dictating phase behavior. Phase state is governed by a balance between enthalpic and entopic contributions. Thus, 

The porphyrinic complexes are all essentially planar. However, the simple porphyrin and tbp macrocycles are somewhat flexible, and can adopt a ruffled, or saddle-shaped distortion through twisting at the methine carbon atoms. These deformations cause almost negligible changes in electronic structure, but the conformational mobility offers an added element of subunit variability. 

This conformational mobility is exemplified in the structures of Ni(oep) (Fig. 26). In the triclinic form of Ni(oep)194-1 the molecules are planar and centrosymmetric (Fig. 26 a) and are stacked inclined relative to the stacking axis in a herringbone pattern similar to that adopted by most of the metallophthalocyanines. By change of solvent, Ni(oep) can be made to crystallize in a tetragonal form195) in which the molecule has crystallographi- 

3 The Ni(pc) molecule is not included in any comparisons since the structure of Ni(pc), completed in 1937I96), is not as accurate as the structures of the other M(pc) systems. 

Pt(pc) are stable, and both have been structurally characterized201,2021. In these systems, the angle between the perpendicular to the PtN4 plane and the stacking axis is 25.3 and 30.8° for the a- and y-polymorphs, respectively, as compared with the 47° angle found in the -polymorphs described above. As a result, the intermolecular distance between the Pt atom and the closest azamethine nitrogen atom in the a- and y-polymorphs is increased (relative to the /9-polymorphs) to 3.8 A. 

Of the metallophthalocyanines that crystallize in forms other than the a-, j3-, and y-polymorphs, the most notable are Pb(pc)2031 and Ga(pc)F1301. The Pb(pc) molecules are stacked metal-over-metal, as shown in Fig. 28, and the Pb-Pb spacing is 3.73 A. The phthalocyanine ring deviates markedly from planarity, although the separate isoindole moieties retain their planarity. In Ga(pc)F the Ga atoms are symmetrically bridged by F, with a Ga-F distance of 3.92 A the pc rings are eclipsed, rather than staggered1301. 

Polymers can be classified, based on structural shape of polymer molecules, as linear, branched, or network (cross-linked). Schematic representations are given in Fig. 1.7. 

Branched polymers may be formed either because at least one of the monomers has functionality greater than 2 or because the polymerization process itself creates branching points on the polymer chain. An example of the first type is the polymer made, for instance, from styrene and a very small amount of divinyl benzene (VIII). A segment of such a macromolecule might look like (X)  

A good example of the second type of branched polymer is the polyethylene that is made by free radical polymerization at high temperatures (100-300°C) and pressures (1,000-3,000 atm). The extent of branching varies considerably depending on reaction conditions and may reach as high as 30 branches per 500 

The term branched commonly implies that the polymer molecules are discrete, which means that they can generally be dissolved in a solvent and their sizes can be measured by some of the methods described in Chapter 4. 

A network polymer [Fig. 1.7(d)], on the other hand, can be described as an interconnected branched polymer. For example, a three-dimensional or space network structure will develop, instead of the branched structure (X), if styrene is copolymerized with higher concentrations of divinyl benzene. In a network structure, all polymer chains are linked to form one giant molecule and the molecular weight is infinite in the sense that it is too high to be measured by standard techniques (see Problem 1.5). Because of their network structure such polymers cannot be dissolved in solvents and cannot be melted by heat strong heating only causes decomposition. 

Polyesters based on HBAor hydroquinone (HQ)/TPA do not exhibit a hquid crystaUine state due to their high melting temperature. To lower the melting temperature of the polymers based on HBA, three approaches have been employed [10]  

Incorporation of unsubstituted rigid, rodlike segments such as 4,4 -biphenylene and 2,6-disubstituted naphthalene ring into the main chain 

Incorporation of flexible aliphatic units such as 1,2-dioxyethylene into the main chain 

Incorporation of rigid kinks such as 1,3-phenylene ring into the main chain 

On the other hand, to lower the melting temperature of polymers based on HQ/TPA, two approaches had been employed [11]  

Problem 1.5 For a network polymer sample in the form of a sphere of 1 cm diameter with a density of 1.0 g/cm, estimate the molecular weight assuming that the sample constitutes a single molecule. (Avogadro number = 6.02x10 molecules/mol). 

Short-chain branched Long-chain branched Ladder 

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Chain models capture the basic elements of the amphiphilic behaviour by retaining details of the molecular architecture. Ben-Shaul et aJ [ ] and others [ ] explored the organization of tlie hydrophobic portion in lipid micelles and bilayers by retaining the confonuational statistics of the hydrocarbon tail withm the RIS (rotational isomeric state) model [4, 5] while representing the hydrophilic/liydrophobic mterface merely by an... 

A multitude of different variants of this model has been investigated using Monte Carlo simulations (see, for example [M])- The studies aim at correlating the phase behaviour with the molecular architecture and revealing the local structure of the aggregates. This type of model has also proven useful for studying rather complex structures (e.g., vesicles or pores in bilayers). 

These chain models are well suited to investigate the dependence of tire phase behaviour on the molecular architecture and to explore the local properties (e.g., enriclnnent of amphiphiles at interfaces, molecular confonnations at interfaces). In order to investigate the effect of fluctuations on large length scales or the shapes of vesicles, more coarse-grained descriptions have to be explored. 

Dietel E, Hirsch A, Zhou J and Rieker A 1998 Synthesis and electrochemical investigations of molecular architectures involving Cgg and tetraphenylporphyrin as building blocks J. Chem. See., Perkin Trans. 2 1357-64... 

Next let us consider the differences in molecular architecture between polymers which exclusively display viscous flow and those which display a purely elastic response. To attribute the entire effect to molecular structure we assume the polymers are compared at the same temperature. Crosslinking between different chains is the structural feature responsible for elastic response in polymer samples. If the crosslinking is totally effective, we can regard the entire sample as one giant molecule, since the entire volume is permeated by a continuous network of chains. This result was anticipated in the discussion of the Bueche theory for chain entanglements in the last chapter, when we observed that viscosity would be infinite with entanglements if there were no slippage between chains. 

Once the potential associated with this aspect of molecular architecture is recognized, the principles of the last section coupled with the richness of organic (and inorganic) chemistry suggest numerous synthetic possibilities. We shall not attempt to be comprehensive in discussing this facet of polymer chemistry instead we cite only a few examples of step-growth polymers which incorporate... 

R. D. Preston, The Molecular Architecture of Plant Cell flY/A, John Wiley Sons, Inc., New York, 1952. 

There is a whole science called molecular architecture devoted to making all sorts of chains and trying to arrange them in all sorts of ways to make the final material. There are currently thousands of different polymeric materials, all having different properties - and new ones are under development. This sounds like bad news, but we need only a few six basic polymers account for almost 95% of all current production. We will meet them later. 

Sygusch, J., Beaudry, D., Allaire, M. Molecular architecture of rabbit skeletal muscle aldolase at 2.7 A resolution. Proe. Natl. Aead. Sei. USA 84 ... 

Weiss, M.S., et al. Molecular architecture and electrostatic properties of a bacterial porin. Science 254 1627-1630, 1991. 

The discovery and development of polypropylene, the one genuinely new large tonnage thermoplastics material developed since World War II, forms part of what is arguably the most important episode in the history of polymer science. For many years it had been recognised that natural polymers were far more regular in their structure than synthetic polymers. Whilst there had been some improvement in controlling molecular architecture, the man-made materials, relative to the natural materials, were structurally crude. 

In addition to plastics materials, many fibres, surface coatings and rubbers are also basically high polymers, whilst in nature itself there is an abundance of polymeric material. Proteins, cellulose, starch, lignin and natural rubber are high polymers. The detailed structures of these materials are complex and highly sophisticated in comparison the synthetic polymers produced by man are crude in the quality of their molecular architecture. 

A chemical property of silicones is the possibility of building reactivity on the polymer [1,32,33]. This allows the building of cured silicone networks of controlled molecular architectures with specific adhesion properties while maintaining the inherent physical properties of the PDMS chains. The combination of the unique bulk characteristics of the silicone networks, the surface properties of the PDMS segments, and the specificity and controllability of the reactive groups, produces unique materials useful as adhesives, protective encapsulants, coatings and sealants. 

A somewhat different approach to providing tailored cavities for metal cations was taken by the groups of Cram and Lehn °. Graf and Lehn prepared the spheroidal molecule 21 which has an interesting molecular architecture. The molecule has ten coordination sites within it, six which form an octahedral array and four which are in a tetrahedral arrangement. This remarkable compound is soluble in all solvents from petroleum... 

The Molecular Architecture of Photo.synthedc Reaction Centers... 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

Eukaryotic Reaction Centers The Molecular Architecture of PSII... 

FIGURE 22.19 The molecular architecture of PSII. The core of the PSII complex consists of the two polypeptides (D1 and D2) that bind P680, pheophytin (Pheo), and the quinones, Qb- Additional components of this complex include cytochrome -6559,... 

FIGURE 22.20 The molecular architecture of PSI. PsaA and PsaB constitute the reaction center dimer, an integral membrane complex P700 is located at the lumenal side of this dimer. PsaC, which bears Fe-S centers and Fb, and PsaD, the interaction site for ferre-doxin, are on the stromal side of the thylakoid membrane. PsaF, which provides the plasto-cyaiiin interaction site, is on the lumenal side. (Adapted from Golbeck, J. H., 1992. Amiual Review of Plant Physiology and. Plant Molecular Biology 43 293-324.)... 

The properties of a molecule are primarily deter- and by the molecular architecture. By archi-mined by the bond types which hold it together tecture we mean the structure of the molecule—... 

From the data presented here, the orbitals involved in bonding correlate with the molecular architecture. The relationships are summarized in Table 16-IV. 

Consider the fluorides of the second-row elements. There is a continuous change in ionic character of the bonds fluorine forms with the elements F, O, N, C, B, Be, and Li. The ionic character increases as the difference in ionization energies increases (see Table 16-11). This ionic character results in an electric dipole in each bond. The molecular dipole will be determined by the sum of the dipoles of all of the bonds, taking into account the geometry of the molecule. Since the properties of the molecule are strongly influenced by the molecular dipole, we shall investigate how it is determined by the molecular architecture and the ionic character of the individual bonds. For this study we shall begin at the left side of the periodic table. 

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