Backbone Degradation of Polymethacrylates via Metal-Free Ambient-Temperature Photoinduced Single-Electron Transfer

Abstract

Polymeric materials comprised of all-carbon backbones are ubiquitous to modern society due to their low cost, impressive robustness, and unparalleled physical properties. It is well-known that these materials often persist long beyond their intended usage lifetime, resulting in environmental accumulation of plastic waste. A substantial barrier to the breakdown of these polymers is the relative chemical inertness of carbon–carbon bonds within their backbone. Herein, we describe a photocatalytic strategy for cleaving carbon-based polymer backbones. Inclusion of a low mole percent of a redox-active comonomer allows for a dramatic reduction in polymer molecular weight upon exposure to light. The N-(acyloxy)­phthalimide comonomer, upon reception of an electron from a single-electron transfer (SET) donor, undergoes decarboxylation to yield a backbone-centered radical. Depending on the nature of this backbone radical, as well as the substitution on neighboring monomer repeat units, a β-scission pathway is thermodynamically favored, resulting in backbone cleavage. In this way, polymers with an all-carbon backbone may be degraded at ambient temperature under metal-free conditions.

Photocatalytic Direct Decarboxylation of Carboxylic Acids to Derivatize or Degrade Polymers

“…[28,29] Additionally, comonomers that can generate carbon-centered radicals and subsequently lead to cleavage of adjacent CÀ C bonds may be integrated in poly(meth)acrylate backbones to achieve degradation. [12,30] We demonstrate that this approach is especially facile and effective when MAA is used to form backbone radicals.…”

“… [6, 7] This method has been employed to accomplish a number of transformations, including amination, conjugate addition, dehydrodecarboxylation, sulfonylation, and thiolation [7, 8] . Previous approaches for post‐polymerization decarboxylation relied on redox‐active esters subjected to single‐electron reduction by a photoredox catalyst to prompt cleavage of the N−O bond and decarboxylation [9–12] . However, these methods required stoichiometric amounts of reducing agent and pre‐activation of the carboxylic acid, which is synthetically inconvenient in many cases and suffers from poor atom economy [13] .…”

“…An alternative method that does not rely on chain‐end triggers is the incorporation of hydrolysable groups into the polymer chain through radical ring‐opening copolymerization with thionolactones, macrocyclic allylic sulfones, or cyclic ketene acetals [28, 29] . Additionally, comonomers that can generate carbon‐centered radicals and subsequently lead to cleavage of adjacent C−C bonds may be integrated in poly(meth)acrylate backbones to achieve degradation [12, 30] . We demonstrate that this approach is especially facile and effective when MAA is used to form backbone radicals.…”

“…[7,8] Previous approaches for post-polymerization decarboxylation relied on redox-active esters subjected to single-electron reduction by a photoredox catalyst to prompt cleavage of the NÀ O bond and decarboxylation. [9][10][11][12] However, these methods required stoichiometric amounts of reducing agent and pre-activation of the carboxylic acid, which is synthetically inconvenient in many cases and suffers from poor atom economy. [13] In addition, polymeric precursors with pendent carboxylic acids have extended shelf-lives due to their hydrolytic stability.…”

Photoassisted Radical Depolymerization

“…Unfortunately, these same properties pose a challenge from a sustainability perspective (i.e., lack of degradability), as polymeric materials can persist for long periods after end-of-life use. , Most of the degradation methods explored to date have relied on backbone chain scission to produce lower molecular weight polymers . For example, we recently reported a strategy for backbone degradation of polymethacrylates by photoinduced single-electron transfer. − While these findings and those of other recent reports focused on degradation strategies for polymers with all-carbon backbones, there are clear opportunities to enhance polymer life-cycle circularity by focusing on reversion from polymer to monomer …”