Polymer covalent bonds

Table 9.1 Covalently polymer bonding surface modifications...

Preparation of TRCSs by covalent polymer bonding surface modification... 

Chemically, proteins are unbranched polymers of amino acids linked head to tail, from carboxyl group to amino group, through formation of covalent peptide bonds, a type of amide linkage (Figure 5.1). 

Molecularly imprinted polymers (MIPs) can be prepared according to a number of approaches that are different in the way the template is linked to the functional monomer and subsequently to the polymeric binding sites (Fig. 6-1). Thus, the template can be linked and subsequently recognized by virtually any combination of cleavable covalent bonds, metal ion co-ordination or noncovalent bonds. The first example of molecular imprinting of organic network polymers introduced by Wulff was based on a covalent attachment strategy i.e. covalent monomer-template, covalent polymer-template [12]. 

There are a number of methods of classifying polymers. One is to adopt the approach of using their response to thermal treatment and to divide them into thermoplastics and thermosets. Thermoplastics are polymers which melt when heated and resolidify when cooled, while thermosets are those which do not melt when heated but, at sufficiently high temperatures, decompose irreversibly. This system has the benefit that there is a useful chemical distinction between the two groups. Thermoplastics comprise essentially linear or lightly branched polymer molecules, while thermosets are substantially crosslinked materials, consisting of an extensive three-dimensional network of covalent chemical bonding. 

There is some need for new pH indicators with improved characteristics which allow also covalent binding. P. Makedonski report about new kind of reactive azo dyes and their application as reversible pH sensors35. They prepare a new pH indicating sensors based on thin films prepared from azo dyes that are covalently bonded by an acetal linkage to a vinylalcohol ethylene copolymer (Figure 7). The absorption spectra of the polymer bond... 

Covalent polymers with reversible properties arising from dynamic covalent bonds such as disulfide exchange reaction [47 9], transesterification [50,51], transetherification [52], and boronate ester formation [53] were reported without respect to DCC. These studies should involve DCLs in... 

Further oxidation of the nine-atom clusters to formal [Ge9] leads to linear polymers oi[-Ge9-] with two covalent intercluster bonds (Fig. 2i). Trimers [Ge9=Ge9=Ge9] (Fig. 6a) and tetramers [Ge9=Ge9=Ge9=Ge9] (Fig. 6b) occur via nonclassical bond formation between two neighboring atoms of the triangular prism basis planes of the c/oio-shaped clusters, which results in Ge-Ge-Ge bond angles of 90° and in considerably longer Ge-Ge contacts between the cluster units. Quantum-chemical calculations have shown that the exo-bonds participate in a delocalized electronic system that comprises the whole anion [204]. 

Wilson AJ. Non-covalent polymer assembly using arrays of hydrogen-bonds. Soft Mater 2007 3 409-425. 

FIGURE 27-41 Three-step cascade pathway by which ubiquitin is attached to a protein. Two different enzyme-ubiquitin intermediates are involved. The free carboxyl group of ubiquitin s carboxyl-terminal Gly residue is ultimately linked through an amide (isopeptide) bond to an e-amino group of a Lys residue of the target protein. Additional cycles produce polyubiquitin, a covalent polymer of ubiquitin subunits that targets the attached protein for destruction in eukaryotes. 

There are cases where it is not easy to draw a clear distinction between molecular Lewis acid/Lewis base complexes and ionic substances of the kind discussed immediately above. For example, in the compound formulated as [IC12][AsF6], each I atom has as near neighbours two F atoms from the anion at a distance of only 265 pm, compared with 350 pm for the sum of the Van der Waals radii. The compound might be alternatively formulated as a covalent polymer with (admittedly rather long) I-F covalent bonds. 

As the science of adhesion has developed, various theories of adhesion have been advocated for one material or another. With wood as a substrate, mechanical interlocking, interdiffusion of polymers, intermolecular attractive forces, and covalent chemical bonding all have been proposed, either individually or collectively, to explain adhesion. In reality, no experiments reported to date have been able to disprove the existence of any one of these mechanisms, or to quantify their relative importance. A most exasperating feature of research on adhesion to wood is that factors presumed to be independent in experiments are never totally independent. 

Polymerisation carried out in the presence of a coordination catalyst is referred to as coordination polymerisation , when each polymerisation step involves the complexation of the monomer before its enchainment at the active site of the catalyst. The active site in each coordination catalyst comprises the metal atom (Mt), surrounded with ligands, one of which (X) forms a covalent active bond (Mt X) with this metal atom. This implies that the growing polymer chain is covalently bound to the metal atom. A characteristic feature of coordination polymerisation is the mutual activation of the reacting bonds of both the monomer (M) and the active site (Mt-X) through the complexation of the monomer with the metal atom at this site, which results in the cleavage of these bonds in the concerted reaction. 

---