Microgel-like network polymer precursor formation in free-radical cross-linking multiallyl polymerization

Abstract

The present study is an extension of our preceding work on gelation behavior. We observed no substantial difference in the actual gel points among three isomeric diallyl phthalates: diallyl phthalate, diallyl isophthalate and diallyl terephthalate. The resulting network polymer precursors (NPPs) were characterized by size-exclusion chromatography with both multiangle laser light scattering and viscometry. It is of note that the structure of NPP, consisting of oligomeric primary polymer chains, becomes core-shell type dendritic or nanogel-like with the progress of polymerization. The nanogel-like NPPs can then collide with one another to form cross-links, eventually leading to gelation. Although the concentration of NPP should be high at the conversion close to the gel point, the dilution of NPP by adding monomer could prevent the cross-link formation among NPPs, and consequently lead to the successive growth of high-molecular-weight NPP from a nanogel to a microgel. The further growth of the microgel as an inhomogeneous NPP with high cross-link density could eventually reach an extremely inhomogeneous network polymer. These processes were pursued as typical examples using the bulk polymerization of DAT. Polymer Journal ( Keywords: cross-linking; gelation; microgel; multiallyl polymerization; nanogel; network polymer precursorIn our previous work, 1 the gel points in the free-radical polymerizations of diallyl aromatic dicarboxylates-including three isomeric diallyl phthalates: diallyl phthalate (DAP), diallyl isophthalate (DAI) and diallyl terephthalate (DAT)-were experimentally reexamined in detail and discussed according to Gordon's theory. 2 Although the discrepancy between actual and theoretical gel-point conversions was quite large and, moreover, it was enhanced in the order DAP o DAI o DAT, no substantial difference in the actual gel points was observed between the three isomeric DAPs. This interesting gelation behavior was discussed in detail 3 in terms of the correlation between gelation and the difference in cyclization modes, 4 as well as the difference in reactivity between the uncyclized and cyclized radicals for cross-linking. Conclusively, the nonconsecutive addition in DAT polymerization led to a delayed gelation, and the cyclized radical in DAP polymerization showed an enhanced reactivity for cross-linking.Then, we tried to extend our previous discussion 3 to the polymerization of triallyl trimellitate (TAT) because the chemical structure of TAT essentially exhibits the characteristics of three isomeric DAPs (Figure 1). Therefore, the enhanced gelation was expected in TAT polymerization, involving both abundant unreacted pendant double bonds and cyclized radicals as two significant factors responsible for an enhanced intermolecular cross-linking. However, no promoted gelation was observed in TAT polymerization when compared with DAP, DAI and DAT polymerizations.For a full understanding of the gelation behavior in cross-linking multiallyl polymerization, our preceding work 5 was focused on the cha...

Improved Dental Adhesive Formulations Based on Reactive Nanogel Additives

“…Since this approach allows for retention of the basic adhesive monomer formulation, the water-chasing capacity, which depends on both resin and solvent composition, is expected to be maintained. Upon solvent removal and polymerization shrinkage of the matrix resin, discrete nanogel particles partially or fully coalesce to potentially reinforce the adhesive resin network with some similarity to interpenetrating polymer networks (IPN) (Sahiner et al, 2006) or as network precursors (Hamamoto et al, 2010), with the aim to increase bond strength and limit water percolation as well as network swelling. The goal is to develop adhesive materials that allow for simple dentin-bonding procedures that provide long-term stability of bonded interfaces.…”