Polymers reversible covalent bonds

Similarly to the phospholipid polymers, the MPC polymers show excellent biocompatibility and blood compatibility [43—48]. These properties are based on the bioinert character of the MPC polymers, i.e., inhibition of specific interaction with biomolecules [49, 50]. Recently, the MPC polymers have been applied to various medical and pharmaceutical applications [44-47, 51-55]. The crosslinked MPC polymers provide good hydrogels and they have been used in the manufacture of soft contact lenses. We have applied the MPC polymer hydrogel as a cell-encapsulation matrix due to its excellent cytocompatibility. At the same time, to prepare a spontaneously forming reversible hydrogel, we focused on the reversible covalent bonding formed between phenylboronic acid and polyol in an aqueous system. 

Two main approaches are used to produce MIPs the noncovalent [48] and the covalent [49] approach. In the covalent approach (Figure 5.12a), the functional monomer is covalently bonded to the template molecule before polymerization. When polymerization is complete, the covalent bonds between the template molecule and the polymer are cleaved and the template molecule is extracted. The resulting imprint is then able to recognize and rebind the imprinted analyte via reversible covalent bonds. However, this technique suffers from lack of generality owing to the difficulties of finding suitable monomers. 

The covalent approach or the pre-organized approach implies the formation of a template-functional monomer complex through reversible covalent bonds prior to polymerization. After synthesis and removal of the template, in the subsequent rebinding step, the initial covalent linkage is reconstituted between the polymer and template. Therefore, only a low number of non-selective binding sites are expected to be formed because of the well-defined stoichiometry taking place between the functional monomer and template. Unfortunately, this approach is only applicable to a limited number of template molecules. 

The semi-covalent approach combines the advantages of the previous two methods, employing reversible covalent bonds in the imprinting step and noncovalent interactions in the recognition process, after the cleavage of the template from the polymer. 

Fig. 2.12 Schematic representation of the molecularly imprinting process the formation of reversible interactions between the template and polymerizable functionality may involve one or more of the following interactions (A) reversible covalent bond, (B) covalently attached polymerizable binding groups that are activated for nrai-covalent interaction by template cleavage, (C) electrostatic interactions, (D) hydrophobic or Van der Waals interactirais, (E) co-ordination with a metal center each kind of interaction occurs with complementary functional groups or structural elements of the template, (a-e), respectively. A subsequent polymerization in the presence of crosslinker(s) results in the formation of an insoluble matrix in which the template sites reside. Template is then removed from the polymCT through disruptirai of polymer-template interactions, and extraction from the matrix. (Reproduced fiom Ref. [97] with the permission of Wiley)...

Table 6.1 Reversible covalent bonds used in DCC of polymers.

Over recent years, the group of Lehn has studied constitutionally dynamic polymers ( dynamers ), which result from the connection of monomers via reversible covalent bonds [3], In this context, they have investigated the condensation reaction of a 1 1 1 mixture of 2,7-diaminofluorene (1), tra s-l,4-diaminocyclohexane (2), and 2,7-fluorene-bis-carboxaldehyde (3) in the presence of variable amounts of [Zn(BF4)2(H20)8] (Scheme 7.1) (4, 5]. 

The strategies presented so far in this chapter rely on a common concept polymers with a network structure based on reversible covalent bonds. Healing is permitted by a temporary transition from a thermoset state to a thermoplastic behavior with enough chains mobility at ambient temperature. Considering this, alternative routes based on the same concept could be... 

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... 

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