Seminar Abstract:

To address the plastics waste crisis, significant efforts have been directed towards the design and/or use of inherently recyclable polymers with competitive properties. However, industrial adoption of these materials remains challenging due to their expense and practicality. For this reason, in addition to the necessary mitigation of current plastic pollution, it is important to direct efforts towards the deconstruction of commodity materials. To this end, we propose a method for the transformation of waste polypropylene ((i)PP) into poly(alpha-methylstyrene) (PAMS), the latter of which is depolymerizable to monomer, 𝛼-methylstyrene (AMS), under mild conditions. This proposal thereby establishes a closed-loop lifecycle for (i)PP and a synthetic route to (P)AMS, simultaneously providing an economic incentive for plastic waste recycling and making the expansion of inherently circular (P)AMS applications feasible by circumventing its synthetic cost.

The substituted backbone structure of (i)PP allows for relatively facile tertiary radical formation via hydrogen abstraction. This phenomenon has been exploited previously for radical grafting of (i)PP. Based on this reactivity, we propose our intended transformation through concurrent generation of cationic benzene radicals via photocatalytic oxidation and tertiary radical formation via hydrogen abstraction of the (i)PP backbone, which couple, and upon deprotonation of the benzene moiety, form a single (P)AMS unit. In Aim I, we seek to establish proof-of-concept for this strategy on small-molecule (i)PP mimics, which can be easily analyzed with NMR. Initial experiments will be carried out in solution using commercially available 1-methyl quinolinum ion (PC+) for the previously reported photocatalytic oxidation of benzene. Different radical initiators (RO-OR) will be tested under these conditions for their reactivity towards hydrogen abstraction of the (i)PP backbone. To prevent side reactions such as (i)PP crosslinking, 𝛽-scission or chain transfer, coupling of radical initiators and benzene radicals, and benzene radical self-coupling, we will focus on limiting individual radical concentrations through balancing the rates of hydrogen abstraction and photocatalyst activation. In this vein, model reactions will be carried out to probe the possibility for sacrificial oxidants (S.O.) to act as hydrogen abstractors, a strategy that would circumvent the need for stoichiometric radical initiators and link the photocatalyst oxidative regeneration with the generation of radicals on the (i)PP backbone. Similarly, changes in light intensity and duration will be used to tailor the photocatalyst activation and reactivity to generate benzene radicals before (i)PP radicals can undergo significant chain transfer or 𝛽-scission.

Aim II will apply the reactions to polymeric (i)PP, which will be monitored with SEC tracking and NMR to gauge the efficacy of the desired reaction relative to chain scission and the stability of the newly formed PAMS chains. Studies will be performed with different solvent combinations and reaction conditions to achieve high conversions as well as prevent premature PAMS depolymerization or interference with the (i)PP transformation. These experiments will include probes into the method efficacy on post-consumer (i)PP and multi-component mixtures to mimic a more realistic industrial environment and determine the sensitivity to additives and impurities commonly present in plastic waste. Lastly, Aim III will explore the applicability of this strategy to other starting commodity polymers and alternative transformations.