Microgels formation

Storey [32] observed some anomalies in the dependence of the gel point at higher concentrations of DVB which suggested some inhomogeneity and a tendency to microgel formation which explained the shift of the gel point towards higher conversions. 

In the following a possible mechanism of microgel formation in crosslinking EP, using water soluble initiators, is given [79,84,90,91 ]. 

Microgel Formation in Solution by Free-Radical Crosslinking Copolymerization... 

Among the divinyl monomers, 1,4-DVB and EDM A are the most extensively studied monomers for microgel formation by anionic polymerization. Com-... 

Fig. 51. Schematic illustration of the mechanism of microgel formation in the anionic dispersion polymerization of 1,4-DVB initiated by living PBS chains in heptane. [Reprinted with permission from Ref. 247, Copyright 1995, American Chemical Society].

The free-radical crosslinking polymerization can be regarded as a special example of specific diffusion control, in which the tendency to microgel formation and decrease of apparent reactivity of Internal double bonds depends on the size of the mlcrogel which in turn depends on the molecular weight of the primary chain. Polymerization of diallyl monomers exhibits much less of these features (W) because the degree of polymerization of their primary chains is extremely low due to degradative chain transfer. 

High dichloroethane ratios reduce microgel formation by dilution and/or chain transfer action. 

As in most chainwise polymerizations, q -> 1, the first observation arising from Eq. (3.180) is the very low value of the gel conversion. In actual systems, intramolecular cyclization and microgel formation produce an increase in the gel conversion. But reported values of xgei for the free-radical polymerization of systems containing multifunctional monomers are usually below 0.10. This is the case for the crosslinking of unsaturated polyesters (Af) with styrene (A2 ). 

Hybrid colloids can be prepared by encapsulation of nanoparticles (NPs) during microgel formation. 

Radical polymerization techniques that are for example used for the crosslinking of methacrylate-modified polysaccharides [9] are not included in this chapter. Prepolymer crosslinking through radical polymerization techniques as well as non-covalent microgel formation has recently been reviewed in a number of excellent review articles [2, 4, 10-14] and is thus not discussed within this chapter. 

As mentioned above, the preparation of nanogels by addition reactions of functional macromolecular precursors is mainly used for biomedical applications. Thus, the choice of synthetic precursors for microgel formation is restricted to biocompatible materials. Moreover, as most applications are in drug delivery, the molecular weight of the gel precursors should be below the threshold for renal clearance, a value that depends on the molecular architecture and chemical nature of the polymer but that is usually smaller than 30kDa, which is set as the limit for linear PEG [97], Polymers that are mostly used and thus presented in more detail here are PEG, poly(glycidol) (PG), and polyethylene imine) (PEI). 

Sol-gel analysis is an important tool for distinguishing between polymers that can be cross-linked by irradiation and polymers that are not cross-linkable by radiation techniques. These experiments are typically done for wall-to-wall gels and offer the possibility to determine the irradiation conditions for microgel formation. Charlesby and Pinner first obtained a simple expression relating the soluble part of a polymer sample, the sol fraction (5) or sol content (in contrast to the insoluble part, the gel fraction (g) or gel content) to the absorbed dose D of radiation [10, 12],... 

Fig. 4 Microgel formation during y-ray irradiation. (Reprinted with permission from [53], copyright 2009 American Chemical Society)...

Schmitz KS, Wang BL, Kokufuta E (2001) Mechanism of microgel formation via cross-linking of polymers in their dilute solutions mathematical explanation with computer simulations. Macromolecules 34(23) 8370-8377... 

The fact that a viscosity increase after phase segregation (for t > tp) is connected with such mechanism is evidenced by the results of gel chromatographic (GPC) analysis of solfi action in the network formation process of low-molecular siloxane rubbers (Fig. 15). As the reaction proceeds the molecular mass of the sol fraction decreases and so does its viscosity. However, network formation of a number of epoxy resins cured with amines or other curing agents conform the homogeneous model without microgel formation [88 a]. 

The move towards catalytic reactions was initiated in 1983 by Hopkins and William. Phthalimide attached to a methacrylate residue acted as template. Hydrolysis after microgel formation with methyl methacrylate, 2-ethoxyethyl-methacrylate and ethylene glycol dimethacrylate left behind cavities possessing primary amine groups. Sha selectivity was confirmed by determination of hydrolysis rates between different 4-nitrophenyl esters. An acetate was hydrolysed considerably faster than a caproate [147]. Four years later Leonhardt and... 

---