Macrogels

The chromatographic procedure [11] is carried out using a wide-bore fused-silica column 30 m long and 0.53 mm in internal diameter coated with macrogel 20 000 2-nitrotere-phthalate with film thickness of 0.5 pm. In addition, for chromatography, helium should be used as the carrier gas at a flow rate of 8.0 mL/min using a flame ionization detector. 

The experimental data obtained with macrogels formed in the presence of solvents, agreed well with Eq. (5) [99,105,108]. In order to check the applicability of this equation to microgels, the experimental data reported by Hoffmann [70] are used. He prepared a series of microgels with different crosslink densities, using toluene as a solvent, at Q°° = 5. Qvwas calculated from the reported data... 

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 order to check these results, Lutz et al. degraded polymer samples which had been isolated shortly before macrogelation, by ultrasonic waves [213]. Figure 38A shows the decrease of Mw and of the hydrodynamic diameter dz, measured by static and dynamic light scattering respectively, on ultrasonic treatment of a polymer of Mw = 2.2 X10. Both Mw and dz decrease first abruptly but then... 

Fig. 38. A Degradation experiments with pregel polymers isolated prior to the onset of macrogelation in 1,4-DVB polymerization [209] Variation of Mw ( ) and dz (O) with the time of ultrasonic degradation. The polymer sample was prepared at 5 g/100 mL monomer concentration and its initial Mw was 2.2 X106 g/mol. The dotted horizontal line shows Mw of zero conversion polymers ( individual microgels ). B Variation of Mw with the polymerization time t and monomer conversion x in 1,4-DVB polymerization at 5 g/100 mL monomer concentration. The region 1 in the box represents the limiting Mw reached by degradation experiments. [Reprinted with permission from Ref. 209,Copyright 1995, American Chemical Society].

In t-DVB/S copolymerization, Antonietti and Rosenauer isolated microgels slightly below the macrogelation point [221]. Using small angle neutron scattering measurements they demonstrated that these microgels exhibit fractal behavior, i.e. they are self-similar like the critically branched structures formed close to the sol-gel transition. 

Hiller and Funke extensively investigated the change of the polymer structure as a function of the monomer and the initiator concentration in various solvents [231]. The content of pendant vinyl groups in the polymer was about 100% for n-BuLi concentrations below 2 mol % and for the whole range of the monomer concentration studied (20-100 g/1). The content of pendant groups decreased when the n-BuLi concentration increased and approached 80% in the transition region of a soluble polymer to a macrogel. As seen in Fig. 45, the decrease of pen-... 

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. 

Anionic polymerization of 1,4-DVB by n-BuLi leading to the microgels was also reported by Eschwey et al. [236,237]. In their experiments, n-BuLi was used at very high concentrations of 17 and 200 mol % of the monomer. Contrary to the results of Hiller and Funke [231], they observed a transition from microgel to macrogel with decreasing n-BuLi concentration. Similar results were also reported by Lutz and Rempp [238]. They used potassium naphthalene as the initiator of the 1,4-DVB polymerization and THF as the solvent. Soluble polymers could only be obtained above 33 mol % initiator, whereas below this value macrogels were obtained as by-products. 

Fig. 48. Dependence of the polymer structure on the initial concentrations of n-BuLi and 1,4-DVB in the anionic 1,4-DVB polymerization in THF. The symbols represent linear ( ) branched (T) microgel ( ) and macrogel (M) structures. [Reproduced from Ref. 231 with permission, Hiithig Wepf Publ., Zug, Switzerland]. 

The solvent used in the anionic polymerization of 1,4-DVB by n-BuLi also has an important effect on the polymer structure. If polymerization reactions are carried out in benzene/THF mixtures, the onset of macrogelation can be retarded by increasing the THF fraction in the solvent mixture [230]. Hexane, that is a solvent for the monomer but a precipitant for the resulting polymer, was not suitable because an insoluble aggregate was formed within a few minutes [230]. For hexane /THF mixtures with equal volumes, the conditions for the synthesis of a soluble polymer depends on the concentrations of 1,4-DVB and n-BuLi (Fig. 50). The course of the curve in the transition region from a soluble polymer to a macrogel is similar to that shown in Fig. 48 for n-BuLi/THF. 

Under the same reaction conditions macrogelation occurs later in the polymerization of 1,3-DVB. Moreover, the [r ] of the microgels from 1,3-DVB is much smaller than that from 1,4-DVB. The exponent a of Mark-Houwink equation for the 1,3-DVB polymers in toluene was found to be only 0.25 [250] and 0.29 [251 ] compared with 0.48 for 1,4-DVB polymers obtained under similar reaction conditions [230]. The delay of the gel point and the small hydro-dynamic volumes of 1,3-DVB microgels, compared with 1,4-DVB microgels also illustrate that the extent of cyclization is much higher in 1,3-DVB polymerization. 

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. 

Macrogel Semi-rigid polystyrene Synthetic polymers... 

---