Microgels bonds

Reactive microgels may be incorporated into plastics by covalent bonds. It could be demonstrated that substantial amounts of polymer chains from bifunctional monomers can be attached at microgel surfaces and thus become insoluble [313, 377-380]. 

An interesting way to prepare shock-resistant coatings [381] follows the synthesis of the ABS-terpolymers, e.g. shock-resistant polystyrene, where a soft, elastomeric phase is incorporated in a hard polymer matrix via covalent bonds. Because organic coatings solidify in situ, elastomeric microgels have been synthesized and mixed to a binder which forms the hard matrix phase before the application of this mixture as a coating material. 

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

Attainment of a maximum double bond conversion is typical in multifunctional monomer polymerizations and results from the severe restriction on bulk mobility of reacting species in highly crosslinked networks [26]. In particular, radicals become trapped or shielded within densely crosslinked regions known as microgels, and the rate of polymerization becomes diffusion limited. Further double bond conversion is almost impossible at this point, and the polymerization stops prior to 100% functional group conversion. In polymeric dental composites, which use multifunctional methacrylate monomers, final double bond conversions have been reported ranging anywhere from 55-75% [22,27-29]. 

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. 

As the reaction proceeds to higher conversions, the unreacted pendant double bonds eventually become shielded in the microgel regions, and the reactivity of the monomeric double bond approaches, and in some cases sur-... 

The following two sections will describe how these materials have been exploited so far to improve the activity of systems, previously developed as a bulk or newly designed, creating molecularly imprinted microgels with hydrolytic activity or C-C bond formation capability. 

---