Plastic waste is pervasive throughout the world, from massive garbage patches in the ocean to microplastics found in human blood, and it continues to accumulate at an alarming rate. Not only is this directly harmful to human and environmental health, but it also wastes the valuable resources required for manufacturing plastics and thus drives the continued extraction of fossil fuels. Recycling is one strategy to both reduce this waste and minimize the extraction of fossil fuels for new plastic products. However, conventional thermo-mechanical recycling methods degrade critical properties of the polymer and are therefore not a viable solution for the long-term recycling of waste plastic. Monomer-to-monomer recycling, where polymers are depolymerized to regain pure monomers that can then be synthesized into a new polymer like new, can truly lead to infinite recycling. Currently, monomer-to-monomer recycling is not feasible for most conventional plastics due to the difficulty of selectively cleaving a carbon-carbon backbone. One solution is to design polymers to incorporate bonds that can be selectively broken in specific chemical processes. This work demonstrates how computational chemistry can be used to discover mechanistic insights into how bond chemistry enables monomer-to-monomer recycling and develop design rules to accelerate the development of sustainable polymers. Specifically, this work investigates the effect of polymer chemistry on the recycling of a new polymer platform, polydikeotenamines (PDKs), which can be depolymerized with high purity in strong acid but are inert in mildly acidic to strongly basic conditions. We build an atomistic understanding of how designing the PDK chemistry with specific heteroatom chemistry and placement dictates the kinetics of the diketoenamine acidolysis reaction that enables its depolymerization. Through the study of these effects, we also develop insight into how computational chemistry methods, from quantum chemistry to quasi-classical molecular dynamics, can best be used to study acid-catalyzed hydrolysis kinetics.

We find that the depolymerization rate of PDKs can be greatly varied through heteroatom and functional group substitutions on the monomer and crosslinker. By calculating the energetic pathway for the acidolysis of a small molecule representative of the PDK, referred to as a diketoenamine (DKE), a mechanistic understanding of these phenomena can be developed. For example, heteroatoms incorporated into the cyclic region of the monomer can vary the DKE acidolysis rate by an order of magnitude. Analyzing the energetic pathway for DKEs with different cyclic heteroatom substitutions reveals that heteroatoms dictate the reaction rate by changing the energy required for the iminium at the DKE reaction center to rotate into the conformation of a low-energy transition state. Not only does this finding build understanding of the acidolysis required for PDK depolymerization, but the discovery of variable depolymerization rates also unlocks the ability to synthesize multi-material PDK composites that can be selectively deconstructed back to pure monomers. In addition, the geometry of the low-energy transition state required for room temperature acidolysis suggests that remote heteroatoms may play an equally important role to those that alter the electronic structure of the reaction center. By systematically studying this feature, it is found that a remote heteroatom capable of hydrogen-bonding with the attacking water dramatically accelerates PDK depolymerization. Moreover, the spacing between this heteroatom and the diketoenamine bond offers a previously unexplored opportunity to tune PDK depolymerization rates. Because novel PDKs that target diverse applications require further engineering of the crosslinker and monomer far from the reaction center, these findings suggest new targets to diversify PDK properties while ensuring circularity in chemical recycling. In addition, these studies emphasized the role of the conformation of the flexible moiety containing the remote heteroatoms in calculations of the acidolysis rate. To account for this in simulations, a multi-path transition state theory becomes critical, and an efficient method for accounting for all low-energy reactive pathways is necessary to accurately calculate DKE acidolysis kinetics.

The studies of heteroatom and functional group substitutions on the monomer and crosslinker focus on the bimolecular reaction kinetics of one hydronium ion reacting with one DKE and make the simplifying assumption that the effect of the environment can be modeled as a dielectric continuum, which allows the use of highly accurate quantum chemistry methods. However, explicit interactions between the PDK and solvent can also play a significant role in the depolymerization kinetics of certain chemistries. Reactive molecular dynamics simulations, treating the solvent explicitly, can elucidate the role of the environment in the acidolysis of DKEs. We highlight a case study in which transition path sampling methods for calculating the rate in explicit solvent correct an error where the energetic pathway that utilizes an implicit solvent yields an incorrect prediction of the relative kinetics of two different DKEs. Analyzing the reactive trajectories, and comparing these to the energetic pathway, it is evident that the interaction of specific PDK chemistries with the solvent can fundamentally alter the acidolysis reaction. As the design of polymers for monomer-to-monomer recycling continues to develop, these studies demonstrate how simulations will play an integral role in elucidating structure-property relationships, from systematically assessing the impact of polymer chemistry on reaction rate to delving into detailed analyses of reaction mechanisms.