Experimental Characterization of Structural Features during Radical Chain Homopolymerization of Multifunctional Monomers Prior to Macroscopic Gelation

Abstract

An experimental methodology is presented to characterize the evolution of structural heterogeneities in cross-linked polymer networks formed by homopolymerizations of multifunctional monomers. In particular, intramolecularly cross-linked macromolecules (ICMs) were synthesized by individual initiation events and characterized. A set of complementary experimental tools, including atomic force microscopy (AFM), size exclusion chromatography, light scattering, and NMR, is used to provide information about size, intramolecular cross-link density, and kinetic chain length for poly(methacrylic anhydride) (PMA) ICMs. Images of PMA ICMs, which contain ca. 1000−2000 monomers, on average, are captured with AFM. Varying polymerization temperature from −13 to 70 °C led to an increase in the apparent size of the ICMs and an increase in the fraction of doubly reacted monomers. Furthermore, ICMs synthesized in the presence of tetraethylthiuram disulfide (TED), a photoiniferter precursor, were characterized. Iniferter-mediated polymerization of methacrylic anhydride created smaller ICMs with higher intramolecular cross-link density.

Enhancing molecularly imprinted polymer binding properties via controlled/living radical polymerization and reaction analysis

“…The limitations and structural heterogeneity of radical polymerizations caused by fast termination reactions can be reduced since iniferters provide a reversible termination reaction. Recently, the use of iniferter has been shown to decrease the kinetic chain length for the homopolymerization of methacrylic anhydride [40].…”

Toward an enhanced understanding and implementation of photopolymerization reactions

“…34,53,137 Unfortunately, the strong spatial correlations that lead to the enhanced reactivity also lead to the consumption of these potential crosslinks by the relatively ineffective reaction that forms primary cycles, ultimately leading to the formation of what has classically been referred to as ''microgels'' or internally crosslinked polymers. 34,77,[137][138][139][140] At the earliest stages of the polymerization, before macrogelation, microgels exist as independent regions of high crosslink density in a monomer pool. As the polymerization proceeds, microgels continue to form and their growth ultimately leads to macrogelation of the polymer, though at higher conversions than predicted classically due to cyclization.…”

“…56 In the earliest stages of the reaction, radicals are formed from initiator cleavage with initiation sites well dispersed within a nearly infinite pool of unreacted monomer, which yields a locally high degree of polymerization, promoting intramolecular crosslinking (or primary cyclization) of these radicals and leading to microgel formation. 34,76,77 The relatively high conversion of pendant functional groups in microgels ultimately does not lead to significant network development. These ''wasted'' crosslinks delay the gel-point significantly, often by as much as 5-10% conversion, from the classical predictions, many of which would predict ideal gel-points in these systems to be below 1% double bond conversion.…”

Enhancing therapeutic loading and delaying transport via molecular imprinting and living/controlled polymerization

“…“Living/controlled” polymerization (LCP) reactions have been used to create linear polymers of uniform chain length,15–18 specific block copolymers,19–24 and grafted polymers on silicon surfaces 25–30. Shorter kinetic chain lengths have been shown with “living/controlled” homopolymerization of methacrylic anhydride,31 and structural evolution of networks has been studied with living polymerization 32. Surprisingly, there is not much work in the literature on the use of living polymerization to produce and control the formation of polymer networks.…”