Are polymer gels synthesized by free radical polymerization with cleavable crosslinkers really degradable?

Using cleavable crosslinkers is a straightforward way to impart degradability to gels/networks from vinyl polymers. However, if synthesized by conventional free-radical polymerization (FRP), such networks are often resistant to degradation, despite containing cleavable bonds. Indeed, the literature contains conflicting reports, suggesting a more complex relationship between the polymer type, preparation conditions and the ability of a network to degrade. To address this, we present a systematic study on the degradation of a series of polymer networks synthesized via FRP and containing disulfide crosslinkers. Poly(methyl methacrylate) (PMMA), polystyrene (PS), poly(methyl acrylate) (PMA), and poly(N,N-dimethylacrylamide) (PDMAm) networks were synthesized under standardized polymerization conditions and subjected to degradation by thiol-disulfide exchange. Interestingly, PMMA and PS networks fully degraded and dissolved, however only at relatively low crosslinker loadings (< 2 mol% vs monomer). In contrast, PMA and PDMAm networks were found not to degrade at any crosslinking densities. By analysis of the polymerization kinetics, equilibrium swelling ratios pre- and post- attempted degradation and theoretical studies, we show that the inability of the FRP networks to fully degrade results from the presence of microclusters that form due to the rapid polymerization and extensive intramolecular cyclization. These heterogeneous structures do not swell, which prevents a small fraction of the disulfide bonds from being cleaved. Furthermore, degradability can be afforded to these networks by significantly reducing the initial monomer concentration, however at the expense of effective crosslinking density, thus explaining the literature discrepancies. Alternatively, strand-cleaving comonomers can be employed instead of cleavable crosslinkers to make the FRP networks fully degradable.