Noncovalent polymers

This chapter will outline the synthesis of polymeric materials pursuing structural diversity and prepared by equilibrium reactions through DCLs. In particular, the dynamic covalent polymers will be focused upon because of their high stability and processability. In addition, advanced approaches to polymeric materials in DCC will be outlined. In this chapter, the authors will only discuss covalent polymers, excluding noncovalent polymers (supramo-lecular polymers) that can be found in References 7 and 8. 

In the case of ditopic molecules with two hydrogen bonding groups, linear noncovalent polymers can form in either solution or the solid state (Fig. 4.1). In this case, the degree of noncovalent polymerization (DP) depends directly on the association constant (ATJ in the medium and the concentration of the molecule (c) as shown in Eq. (4.3) (Xu et al. 2004) ... 

Noncovalent assemblies of covalent or noncovalent polymers, e.g., the gel of the vitreous humor (Figure 1.5.1), may remain unchanged for several decades under favorable conditions but may also decompose upon simple heating to relatively low temperatures (e.g. 60°C) or application of modest mechanical pressure. 

Spherical, Fibrous Rock- and Sheet-like Noncovalent Polymers... 

Noncovalent polymers are defined here as molecular assemblies with a high degree of polymerization (PN > 100) and well-defined molecular arrangements in solution as well as in the dry state. We also include membrane-coated colloidal metal or silicate particles with rigid nanometer gaps. The spherical or planar carrier is needed to fixate the membrane structures, which are not distinguishable by substances in the bulk water or solvent phase from clefts in the surfaces or polymers with an organic core. 

Noncovalent spherical assemblies of amphiphilic lipids, on the other hand, are either short-lived already in aqueous solution (micelles) or collapse immediately upon drying (vesicles).This behavior is due to the character of the forces which form them. Curvature is retained by repulsive hydration forces. If the hydration sphere is removed from the head groups, the amphiphiles will pack together and form crystalline sheets and 3-D crystals. Neither crystallites nor micelles and vesicles can be considered as noncovalent polymers, because they change their molecular arrangement drastically when going from the dissolved to the dry state. They do not have material properties. 

Fig. 3.1 Transmission electron micrograh (TEM) and model of a multilayered micelle made of the depicted amphiphilic ruthenium complex with PF6 -counterions. The whole bilayer assembly does not measureably dissociate in aqueous solution and can be isolated in the dry state. It is therefore a noncovalent polymer. ...

Sheets of noncovalent polymers, which would be analogous to the p-pleated sheets of silk, are commonplace on water and solid surfaces. The molecular weight and extension of these surface mono- and bilayers is unlimited. Of current interest are well-defined rigid gaps within such monolayers. They provide small reactive domains which can be manipulated on a molecular scale. It was found possible, for example, to fixate porphyrin heterodimers with a plane-to-plane distance of 10 or 20 A within such gaps and to position tyrosine monomers as electron relays between them. These sheets can then be fixated on spherical colloids (see Fig. 3.4) and we are back to spherical polymers. ... 

Porphyrin bolaamphiphiles have been shown to form platelets with a thickness of 8 A in bulk water. Well-defined monolayers have been obtained with bolaamphiphilic porphyrins with two positive charges on each edge. Charge repulsion obviously first prevents crystallization and second unlimited growth of the isolated sheets. Their surface area is a few [ixn. In aqueous solution lecithin bilayers of unlimited extension (myelin figures) are known to have erne s below 10" M and can thus be considered as noncovalent polymers. ... 

Both biopolymers and noncovalent polymers are produced at the borderline between hydrophobic membrane structures and bulk water. The hydrophobic effect enforces curvature in both, hydrogen bonds lead to linear chains, chirality produces helices, sheets are often insoluble and are applied only occasionally. All kinds of planar thin layers occur, however, on solid surfaces, e.g., bones and minerals in nature or gold, graphite and mica in model systems. 

In our context of a comparison between covalent and noncovalent polymers important differences appear in general and should be considered with respect to any application. 

The analysis of heterologous proteins in recombinant hosts, such as E. coll, presents many challenges to the analytical biochemist. The cells must be lysed and the inclusion bodies solubilized prior to quantification. Cell lysis and protein solubilization can be accomplished chemically through the use of SDS. As the proteins in inclusion bodies can exist as a distribution of forms, such as covalent and noncovalent polymers, it is crucial to convert the target protein into a single molecular entity prior to analysis. This can be achieved by unfolding the proteins and disrupting the inter- and intramolecular disulfide bonds via reduction or sulfitolysis. The complexity of the matrix adds to the difficulty in the determination of the recombinant protein, as both the whole cell and the inclusion bodies can also contain nucleic acids, salts, lipids, and other host molecules, in addition to proteinaceous material. 

Fig. 13. (a) Bisthymine cross-linkers 13-15. (b,c) LCSM micrographs of microspheres generated from combination of polymer 6 and bisthymine (BT) 16. (d) Schematic depiction of noncovalent polymer cross-linking. 

Supramolecular chemistry as we understand it today has evolved to encompass not just host and guest chemistry but also all aspects of self-assembly. It includes the design and function of molecular devices and molecular assemblies, noncovalent polymers, and soft materials such as... 

Palanikumar L, Choi ES, Cheon lY, loo SH, Ryu J-H. Noncovalent Polymer-Gatekeeper in Mesoporous Silica Nanoparticles as a Targeted Drug Delivery Platform. Adv Funct Mat... 

Caro et al. [155] s)mthesized three polymers using 4-chlorophenol (4-CP) as the template, following different protocols (noncovalent and semicovalent), and used different functional comonomers, 4-vinylpyridine (4-VP) and methacrylic acid (MAA). They have evaluated the selectivity of the polymers as MIPs sorbent in SPE coupled online to LC. They found out that the 4-VP noncovalent polymer was the only polymer that showed a clear imprint effect. This MIP also showed cross-reactivity for the 4-chloro-substituted phenols and for 4-NP from a mixture containing the 11 priority EPA phenolic compounds and 4-CP. The MIP was applied to selectively extract the 4-chloro-substituted compounds and 4-NP from river water samples. Figure 16.4 shows the chromatograms obtained by online MISPE with the 4-VP noncovalent 4-CP imprinted polymer of 10 mL standard solution (pH 2.5) spiked at 10 mg/L with each phenolic compoimd. 

There are also noncovalent polymers, such as those assembled through hydrogen bonds. 

The term polymer reactions is used in a very broad sense. It refers not only to the usual step growth and chain growth reactions that produce polymers, but also to postpolymerization modifications, conjugation, degradation processes, noncovalent polymers, physical reactions producing aggregates and gels, and supramacromolecular assemblies. 

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