Activation crosslinking

The first paints were based upon linseed oil (obtained from flax). This is an unsaturated long-chain triglyceride, which, with metal activators, crosslinks via radical pathways to form a continuous film on the substrate. Modern paints use synthetic polymers together with either a solvent or suspending medium (e.g., water), which evaporates leaving the deposited film. Exceptions are powder coatings, which require heat for completion. 

Figure 5.4 Schematic representation of sol and a part of the gel DC dangling chains, EANC elastically active network chains, EAC elastically active crosslinks...

Photochemically reactive molecules have often been used as labels for specific sites in proteins and nucleic acids. Psoralen derivatives serve as relatively nonspecific photochemically activated crosslinking agents for DNA and double-stranded RNA.195 Aryl azides are converted by light to aryl nitrenes, which react in a variety of ways including insertion into C-H bonds (Eq. 23-27).200 201 In some cases UV irradiation can be used to join natural substrates to enzymes or hormones to receptors. For example, progesterone, testosterone, and other steroids have been used for direct photoaffinity labeling of their receptors.202 Synthetic benzophenones have also been used widely as photoactivated probes.203... 

A new high-performance elastomer, prepared from tetrafluoroethylene (TFE) and perfluoro(methyl vinyl ether) (PMVE), and characterized by outstanding resistance to chemical attack and excellent thermal stability, was reported recently by Barney et al. (I) of our laboratory. That paper described a rubbery perfluorinated dipolymer that could not be crosslinked using ordinary techniques because of its chemical inertness, and also a terpolymer in which an unspecified third monomer was used to introduce active crosslinking sites. 

All these immobilization techniques run the risk of altering activity compared to the native enzyme. Improved activity is occasionally reported, but this is the exception. The immobilization techniques fisted above are in approximate order of loss in activity. Physical entrapment normally causes no change. Adsorption will distort the shape of the molecule compared to the native state. The effect of covalent bonding depends on the location of the bond relative to an active site. If remote from the site, it may have no effect. The chemical nature of the support can affect activity. Crosslinking requires two covalent attachments per enzyme molecule and is thus likely to distort the shape of the enzyme to the point that catalytic activity is lost. Such distortions are even more likely, but not inevitable, for coagulated or flocculated enzymes. On the positive side, immobilization tends to stabilize enzymes against deactivation. 

Amphiphilic peptides lonically crosslinked micro- and nano-spheres Biotinylated microspheres Photopolymerizable hydrogels Thermally activated crosslinked gels Two-component polyurethanes Alginates... 

Tris (dimethylaminomethyl) phenol activator, crosslinking peroxide vulcanization... 

Assumptions made in the derivation of these relationships are as follows, a) The fraction of the total number of junctions in the gel that contribute to the elasticity, i.e. the fraction of the number of elastically active crosslinks, is a function of concentration and not of temperature. This assumption is questionable, since the degree of intramolecular crosslinking will depend on the concentration, becoming smaller with increasing concentration. However, the variation in T with concentration is relatively small, so that the question of the strict validity of this assumption is rather irrelevant. 

The process of gelling water soluble polymers with metal ions can be broken down Into 3 basic steps 1) production of active crosslinker, 2) attachment of active crosslinker to the polymer, and 3) crosslinking of two polymer segments. ... 

Discriminating between the various effects of heat and moisture on the strength of cyanoacrylate metal-to-metal bonds is not easy. The environment can affect the adhesive, the metal surface, or the interface between them. The reduction in strength may be due to heat alone, or to water, or to both. The most durable adhesive possible today would probably contain a room temperature active crosslinking agent and an adhesion promoter resistant to both heat and moisture. Table XII lists the water durability modifiers discussed in this section. 

Another example (which up to now seems very difficult to achieve) is based on chains dissolved in a cholesteric phase. This is a liquid where the molecules locally have one direction of alignment but where this direction has a helical twist in space. If we start with chains which are not optically active, crosslink them by an optically inactive agent, and then wash out the cholesteric solvent (replacing it by an achiral solvent), we should obtain a gel which has an optical rotatory power (a memory of its preparative state) although all its components do not distinguish right from left. ... 

Certain difunctional compoimds are used to crosslink elastomers by reaeting to bridge polymer chains. For example, diamines (e.g., hexamethylenedia-mine carbamate) are used as crosslinks for fluoroelastomers p-quinone dioxime is oxidized to p-dinitrosobenzene as the active crosslink for bridging at the polymer double bonds of butyl rubber and methylol terminated phenol-formaldehyde resins will likewise bridge butyl rubber chains (with SnCl2 activation) as well as other unsaturated elastomers. 

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