Cross-linking heterogeneity

Most of these polymers have multi-functional character, which results in cross-linked heterogeneous products. In contrast, monomethoxy polyethylene glycol (PEG) presents only one reactive terminal group per polymer chain. Once PEGy-lated with these compounds, the protein acquires a brush-like shape, with the hydrophilic PEG chains extended from the protein to the solvent. 

It appears that during the drying of paint or varnish films the presence of solvent molecules interferes with the process of cross-linking consequently the films have a heterogeneous structure and films of improved protective quality arise when solvents are eliminated. 

The immobilization of metal catalysts onto sohd supports has become an important research area, as catalyst recovery, recycling as well as product separation is easier under heterogeneous conditions. In this respect, the iron complex of the Schiff base HPPn 15 (HPPn = iVA -bis(o-hydroxyacetophenone) propylene diamine) was supported onto cross-linked chloromethylated polystyrene beads. Interestingly, the supported catalyst showed higher catalytic activity than the free metal complex (Scheme 8) [50, 51]. In terms of chemical stability, particularly with... 

Figure 17 Schematic representation of heterogeneous portions of curdlan hydrogel (left) (A) liquid-like portion, (B) portion of intermediate mobility, and (C) triple-helical cross-links in the solid-like portion and crystallites as additional cross-links, and branched glucans (triple helical chains) (right). From Ref. 117 with permission.

Structural as well as dynamical studies of membrane proteins, constituting an estimated 30% of all proteins, are very important in view of their crucial biological importance. These membrane proteins are obviously dynamically heterogeneous systems, even though they are structurally homogeneous without any cross-links or aggregation so far discussed. 

Fig. 16.3 Heterogenization of RhCI(PPh3)2 by grafting to a cross-linked phosphinated styrene/divinylbenzene resin.

---