Polymer backbone, cross-linking

In summary, we described a system in which a backbone containing a highly polar group contains a degree of cross-linking. Put another way, the system is a three-dimensional network of water-soluble polymer and cross-linking that serves as the basis for all hydrogels, natural or synthetic. 

Stationary phases in capillary SFC [30] are usually based on a polysiloxane backbone, cross-linked by means of a free-radical initiator, and methyl, phenyl, octyl, biphenyl, and cyanopropyl substituents are incorporated into them. These polysiloxane phases exhibit wide ranges between their glass transition and decomposition temperatures, and this feature sets them apart from most high molecular-mass polymers, as they allow useful operating conditions for chromatography. Polysiloxanes can be tailor-made for specific... 

Reactions that modify either the backbone or side chains of polymers without cross-linking or chain-breaking side reactions can have more utility in that the resulting polymers can... 

This process is shown in Box 2.2 to occur predominantly for polyethylene. Elimination in polymer degradation occurs whenever the chemical bonds on substituents are weaker than the C—C backbone bonds. As shown in Box 2.2, for PVC (or for polyvinyl acetate), the chloride bond (or acetate) breaks first and HCl (or acetic acid) is liberated. Normally, the elimination of HCl (or acetic acid) does not lead to a considerable decrease in molecular weight. However, because of the formation of double bonds on the backbone, cross- linking occurs as shown. Intramolecular cyclization in a polymer is known to occiu at high temperatiues... 

The presence of the unsaturated substituent along this polyester backbone gives this polymer crosslinking possibilities through a secondary reaction of the double bond. These polymers are used in paints, varnishes, and lacquers, where the ultimate cross-linked product results from the oxidation of the double bond as the coating cures. A cross-linked polyester could also result from reaction (5.J) without the unsaturated carboxylic acid, but the latter would produce a gel in which the entire reaction mass solidified and is not as well suited to coatings applications as the polymer that crosslinks upon drying. ... 

If a modest number of cross-links between the polymer backbone are introduced, the polymer Hquid crystal takes on elastomeric properties. The useflilness of these materials probably Hes in the coupling of mechanical and optical effects. 

Similar types of cross-linking reactions are observed for polymers to which photosensitive molecules ate chemically attached to the backbone of the polymer stmcture (Fig. 7). Radiation curing of polymers using uv and visible light energies is used widely in photoimaging and photoresist technology (Table 8) (58,59). 

Fig. 3. Mechanisms for polymer degradation. The illustration is a schematic representation of three degradation mechanisms I, cleavage of cross-links II, hydrolysis, ionisa tion, or protonation of pendent groups III, backbone cleavage. Actual biodegradation may be a combination of these mechanisms.

There are no known practical peroxide cure systems for the PO—AGE polymers. Apparentiy the peroxide attacks the polymer backbone at a rate that is unfavorably competitive with the cross-linking rate. A typical sulfur cure system consists of zinc oxide [1314-13-2] tetramethylthiuram mono sulfide (TMTM), 2-2-mercaptobenzothiazole [149-30-4] (MBT), and sulfur. A sulfur donor cure system is zinc oxide, di-o-tolylguanidine [97-39-2] (DOTG) and tetramethylthiuram hexasulftde. 

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