Pendant-group cleavage

As pointed out by Heller (2), polymer erosion can be controlled by the following three types of mechanisms (1) water-soluble polymers insolubilized by hydrolytically unstable cross-links (2) water-insoluble polymers solubilized by hydrolysis, ionization, or protonation of pendant groups (3) hydrophobic polymers solubilized by backbone cleavage to small water soluble molecules. These mechanisms represent extreme cases the actual erosion may occur by a combination of mechanisms. In addition to poly (lactic acid), poly (glycolic acid), and lactic/glycolic acid copolymers, other commonly used bioerodible/biodegradable polymers include polyorthoesters, polycaprolactone, polyaminoacids, polyanhydrides, and half esters of methyl vinyl ether-maleic anhydride copolymers (3). 

Arenes of the type ArX-Y, where X may be O, S, or NR, frequently undergo facile photoinduced homolytic cleavage of the X-Y bond with the Y radical subsequently attacking the aromatic ring. The photo-Fries reaction is the most common process of this type and has been reported within the year for aromatic esters which form part of a ptolymer chain or are pendant groups on a polymer chain. In the former case, the rearrangement of fluorene-based polyacrylates [for example, (292)] was studied. Formation of the o-hydroxybenzophenone moiety in the product (293) was monitored by u.v. and... 

In general terms. Type I erosion encompasses water-soluble polymers that have been insolubilized by hydrolytically unstable crosslinks. Type II erosion includes polymers that are initially water-insoluble and are solubilized by hydrolysis, ionization, or protonation of a pendant group. Type III erosion includes hydrophobic polymers that are converted to small water-soluble molecules by backbone cleavage. 

Physically cross-linked hydrogels can degrade by processes that reverse the gelation mechanism or disturb the noncovalent interactions of the cross-links. Chemically cross-linked hydrogels can be degraded via several mechanisms (Fig. 10.17) and include cleavage of the backbone chain, cross-linker, or pendant groups. 

Figure 10.17 Degradation strategies for chemically cross-linked hydrogels. Degradation (a) via cleavage of the polymer backbone, (b) cross-linker, and (c) pendant-group (Kharkar et al., 2013).

Amino Acid and Carboxylato Complexes. Rate data for the formation kf) and dissociation (fcj of [Cr(X)Y] " ions from the reactions of [Cr(Y)(OH2)] "" " ions from the reactions of [Cr(Y)(OH2)] " ions H4Y = edta, H3Y = A-(hydroxyethyl)ethylenediamine-A, A, 7V -triacetic acid (hed-tra), ethylenediamine-N,A, A -triacetic acid (edtra), and A -methylethyl-enediamine-A,A, N -triacetic acid (medtra) with anions X" (NOi",Nf, NCS") are collected in Table 5.11. The accelerated rates of anation and aquation are ascribed to transient coordination of the pendant groups of ligands Y"". Nitrite ion probably reacts without Cr-0 bond fission in the case of [Cr(hedtra)(H20)]. However, for [Cr(medtra)(H20)] nitrosation proceeds via parallel reactions, one with and one without Cr-0 bond cleavage. 

---