About the Seminar:

For as long as we have understood the negative impacts of plastic waste on human health and the environment, polymer chemists have been motivated to develop cost effective and industrially viable strategies to mitigate the historically unfortunate externalities inherent to these essential materials. These efforts have resulted in an array of potential solutions, one of the most compelling being the complete depolymerization of used plastic back into its virgin monomer, which establishes a closed-loop material life cycle. Traditionally, selective depolymerization relies on catalysis to lower the activation energy of active species generation and/or the ensuing depropagation. An alternative, yet less explored, strategy is to instead promote depolymerization via thermodynamic manipulation of the polymer mainchain. To better investigate the potential of this unorthodox approach, we propose the development of a model system centering on the incorporation of comonomers containing photoswitch moieties to strategically induce mainchain scission on demand through photoisomerization induced bond strain. Owing to the highly efficient reversible cis-trans photoisomerization of azobenzene compounds and their chemical similarity to styrene, we propose polystyrene doped with styrenic azobenzene dimers to be an ideal candidate for initial studies. Moreover, the easily modified benzene scaffold of the azobenzene containing comonomer may be judiciously derivatized to improve the polymer’s working state orthogonality via manipulating the quantum yield of photoisomerization, raising the cis-trans vs. trans-cis interconversion excitation energy gap, and suppressing thermally induced isomerization. We hypothesize that incorporation of the cis form of the azobenzene comonomer will form a strained ring during polymerization, which upon photoexcitation and the ensuing availability of interconversion to its trans isomer will promote regioselective cleavage of one of the enthalpically weakened mainchain [sp3-sp3] C-C single bonds between the dimerized styrenic repeat units. This cleavage will produce radicals capable of subsequently regenerating the original photoswitch comonomer and monomeric styrene under depolymerization conditions. The propensity of this strategy to promote depolymerization and its ultimate efficiency will be evaluated systematically through a series polystyrene depolymerization studies designed to isolate the effects of the degree of photoswitch comonomer doping on the ultimate temperature required for depolymerization, selectivity for monomer regeneration, and the apparent rate of depolymerization. Furthermore, these runs will be contrasted to identical metal oxide catalyzed depolymerizations to both assess the core postulate of thermodynamically promoted depolymerization and to explore a potential synergy in combining both approaches – ultimately seeking to lower the energy requirement of depolymerization and improve monomer recovery. Overall, through the development and analysis of the proposed model system, this work aims to deepen our understanding of the chemical dynamics of depolymerization and introduce a new branch of tunable depolymerization chemistry with the potential to enable a scalable circular materials economy.