Macrogel, formation

Thus, in order to interpret the above correlations between mechanical properties and cocrosslinkers, the function of glycol bis(allyl phthalate)s as cocrosslinkers on the polymerization process of DAP beyond the gel-point conversion should be considered in connection with the microgel and, moreover, macrogel formation. Here it should be recalled that PEGBAP showed a rather drastic effect ... 

Intermolecular crosslinking between pendant vinyl groups and radical centers located on different macromolecules produce crosslinks that are responsible for the aggregation of macromolecules, which leads to the formation of a macrogel. It must be remembered that both normal and multiple crosslinks may contribute to the rubber elasticity of a network, whereas small cycles are wasted links. 

The divinyl monomers can thus be found in macromolecules as units which bear pendant vinyl groups or which are involved in cycles, crosslinks or multiple crosslinks. Since the number of crosslinks necessary for the onset of macrogelation is very low [64], pendant vinyl groups in RCC are mainly consumed in cycles and multiple crosslinks. Therefore, the reaction rate of pendant vinyl groups is a very sensitive indicator for the formation of cycles and multiple crosslinks in finite species [100,147,157-160]. 

In Fig. 48, the regions of the formation of linear or branched polymers, microgels and macrogels are shown as a function of the concentration of 1,4-DVB and of n-BuLi. Reactive microgels can be obtained at a monomer concentration below 50 g/1 and between 3 and 16 mol % of n-BuLi. The polymer structure approaches that of a macrogel when the concentration of 1,4-DVB or n-BuLi is increased. 

Fig. 49. Calculated dependence of the polymer structure on the initial 1,4-DVB and n-BuLi concentrations in the anionic 1,4-DVB polymerization. The numbers I to IV represent the region for the formation of linear, branched, microgel and macrogel structures, respectively. The solid and dashed curves represent the transition regions between these structures. [Reproduced from Ref. 239 with permission, Hiithig Wepf Publ., Zug, Switzerland]. 

Okamoto and Mita studied the anionic polymerization of 1,4-DIPB in THF [261]. They found the reactivity of the pendant vinyl groups by about three to four orders of magnitude lower than that of the vinyl groups of the monomers. Popov et al. compared the reactivities of 1,4-DVB and 1,4-DIPB in the reaction with polystyryl dianions in THF/benzene mixtures [262]. While addition of 1,4-DVB to the dianion solution caused an immediate macrogelation, no gel formation was observed on the addition of 1,4-DIPB. Anionic polymerization of 1,3-DIPB was also studied by several research groups [263-265]. They reported formation of low molar mass species. 

Both the refractory and labile fractions of HMW DOM can be lost from seawater through formation of macrogels that aggregate into marine snow. The labile fraction that is known to participate in marine snow formation are the TEPs, such as mucopolysaccharides found in the mucus sheaths surrounding fecal pellets and plankton colonies. HMW DOM is also lost from seawater via (1) adsorption onto sinking POM and minerals, (2) conversion into POM at the sea surfece by turbulence associated with bursting bubbles, and (3) photochemical degradation. 

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