Macromolecular Structural Control

It is appropriate to compare the well-known concepts of covalent bond formation in traditional organic chemistry with those that apply to classical polymer chemistry and to dendritic macromolecular chemistry. This allows one to fully appreciate the differences between the three areas in the context of structure control, in concert with issues related to terminal group and mass amplification. 

Since that time, synthetic chemists have explored numerous routes to these statistically hyperbranched macromolecular structures. They are recognized to constitute the least controlled subset of structures in the major class of dendritic polymer architecture. In theory, all polymer-forming reactions can be utilized for the synthesis of hyperbranched polymers however, in practice some reactions are more suitable than others. 

In general, three major macromolecular classes, namely linear, cross-linked, and branched architectures, are produced by statistical polymerization processes. Therefore their architectures are not structurally-controlled as in many biological systems [37]. However, the discovery of dendrimers has changed this paradigm. 

As in linear polymers, the relative influence of the molecular structure (scale of nanometers and monomers), and the macromolecular structure (crosslink density), on network properties, depends on temperature, as shown in Fig. 10.9. In the glassy state, the physical behavior is essentially controlled by cohesion and local molecular mobility, both properties being mainly under the dependence of the molecular scale structure. As expected, there are only second-order differences between linear and network polymers. Here, most of the results of polymer physics, established on linear polymers, can be used to predict the properties of thermosets. Open questions in this domain concern the local mobility (location and amplitude of the (3 transition). 

Dieckmann et al. in 2003 described an amphiphilic a-helical peptide specifically designed to coat and solubilize CNTs and to control the assembly of the peptide-coated nanotubes into macromolecular structures through peptide-peptide interactions between adjacent peptide-wrapped nanotubes [227]. They claimed that the peptide folds into an amphiphilic a-helix in the presence of carbon nanotubes and disperses them in aqueous solution by noncovalent interactions with the nanotube surface. EM and polarized Raman studies revealed that the peptide-coated nanotubes assemble into fibers with the nanotubes aligned along the fiber axis. The size and morphology of the fibers could be controlled by manipulating the solution conditions that affect peptide-peptide interactions [227]. 

In this chapter, we intend to revise the most recent contributions to the aforementioned aspects of Pc research. We will describe how the versatile chemistry of Pcs makes possible the preparation of monofunctionalized macrocycles, mainly aimed at preparing multicomponent systems through reaction with other electroactive moieties. The controlled organization of Pcs in solution and the incorporation of these chromophores into macromolecular structures, as well as the preparation of mono-, bi-, and three-dimensional nanostructures, will be the object of study. Finally, some examples of Pc-based devices (solar cells, sensors, transistors, etc.) will also be given as an example of the real applicability of these molecules. 

These seven italicized criteria are integrated into a variety of (GDS) schemes thus allowing construction of hyperbranched macromolecular structures referred to as dendrons or dendrimers . A direct consequence of this strategy is a systematic molecular morphogenesis [1] with an opportunity to control "critical molecular design parameters (CMDP s) (i.e., size, shape, surface chemistry, topology and flexibility) as one advances with covalent connectivity from molecular reference points (seeds) of picoscopic/sub-nanoscopic size (i.e.. 0.01-1.0 nm) to precise macromolecular structures of nanoscopic dimensions (i.e., 1.0-100 nm) [2]. Genealogically directed synthesis offers a broad and versatile approach to the construction of precise, abiotic nanostructures with predictable sizes, shapes and surface chemistries. 

---