Cross-linking pendant groups

The presence of pendant reactive vinyl groups through 1,2 and 3,4 addition provides a site for branching and cross-linking since these may be involved in other chain reactions. Because of this a 1,4 polymer is generally to be desired. 

Their main point of difference is that the phenoxies are of much higher molecular weight ( 25 000). The phenoxies are also said to be slightly branched. Like the epoxide resins they are capable of cross-linking via the pendant hydroxyl groups, in this instance by di-isocyanates and other agents. 

The solubility parameter is about 19.2MPa and being amorphous they dissolve in such solvents as tetrahydrofuran, mesityl oxide, diacetone alcohol and dioxane. Since the main chain is composed of stable C—C and C—O—C linkages the polymer is relatively stable to chemical attack, particularly from acids and alkalis. As already mentioned, the pendant hydroxyl groups are reactive and provide a site for cross-linking. 

The dry peroxide, added to cause cross-linking in liquid polysulfide polymers with pendant thiol groups, caused sparking or ignition, depending on the scale of the... 

Interfacial behavior of different silicones was extensively studied, as indicated in Section 3.12.4.6. To add a few more examples, solution behavior of water-soluble polysiloxanes carrying different pendant hydrophilic groups, thus differing in hydrophobicity, was reported.584 A study of the aggregation phenomena of POSS in the presence of amphiphilic PDMS at the air/water interface was conducted in an attempt to elucidate nanofiller-aggregation mechanisms.585 An interesting phenomenon of the spontaneous formation of stable microtopographical surface domains, composed primarily of PDMS surrounded by polyurethane matrix, was observed in the synthesis of a cross-linked PDMS-polyurethane films.586... 

Figure 8.2 Polyacrylamide gel formation and hydrolysis of acrylamide to acrylate. (A) Acrylamide and A,A-methylenebisacrylamide (bis) are copolymerized in a reaction catalyzed by ammonium persulfate [(NH4)2S208] and TEMED. (B) A very short stretch of cross-linked polyacrylamide is represented. Cross-linking between similar structures leads to the formation of ropelike bundles of polyacrylamide that are themselves cross-linked together forming the gel matrix. In the lower portion of (B) is shown how pendant, neutral carboxamide groups can become hydrolyzed to charged carboxyls.

The pendant hydroxyl groups of cellobiose have been confirmed to be useful for crosslinking of its polymers in the presence of additional amount of diisocyanate. Cross-linked insoluble films have been obtained by casting a polymer solution in dimethylacetamide containing 7% of additional amount of MDI. 

Glass Transition Temperature Increases with the presence of bulky pendant groups stiffening groups as 1,4-phenylene chain symmetry polar groups cross-linking... 

Make sketches or diagrams showing (a) a linear polymer, (b) a polymer with pendant groups, (c) a polymer with short branches, (d) a polymer with long branches, and cross-linked polymers with (e) low and (f) high cross-linked density. 

High molecular weight thermoplastics called phenoxy resins are formed by the hydrolysis of the epoxy resins so that no epoxy groups are present. These transparent resins can be further reacted, forming cross-linked material through reaction of the hydroxyl pendant groups with diisocyanates or cyclic anhydrides. 

The absence of strong intermolecular forces, the presence of pendant methyl groups discouraging close association, and crankshaft action associated with the cis isomer all contribute to the flexibility of NR. The introduction of a few cross-links by vulcanization with sulfur reduces slippage of chains but still permits good flexibility. 

Increased length of flexible side, pendant groups Inclusion of chain-stiffening units Addition of a and a,a substituents Addition of polar groups Increase in cross-link density... 

The thermal- and flame-retardant properties of the copolymers obtained could be varied by changing the ratio of both monomers. Furthermore, in a related work, unsaturated polyesters containing pendant hydroxyl groups were prepared via ADMET and subsequently acrylated [130]. Further radical cross-linking afforded flame-retardant thermosets. 

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