Mechanism-Guided Discovery of Cleavable Comonomers for Backbone Deconstructable Poly(methyl methacrylate)

Abstract

The development of cleavable comonomers (CCs) with suitable copolymerization reactivity paves the way for the introduction of backbone deconstructability into polymers. Recent advancements in thionolactone-based CCs, exemplified by dibenzo[c,e]-oxepine-5(7H)-thione (DOT), have opened promising avenues for the selective deconstruction of multiple classes of vinyl polymers, including polyacrylates, polyacrylamides, and polystyrenics. To date, however, no thionolactone CC has been shown to copolymerize with methacrylates to an appreciable extent to enable polymer deconstruction. Here, we overcome this challenge through the design of a new class of benzyl-functionalized thionolactones (bDOTs). Guided by detailed mechanistic analyses, we find that the introduction of radical-stabilizing substituents to bDOTs enables markedly increased and tunable copolymerization reactivity with methyl methacrylate (MMA). Through iterative optimizations of the molecular structure, a specific bDOT, F-p-CF 3 PhDOT, is discovered to copolymerize efficiently with MMA. High molar mass deconstructable PMMA-based copolymers (dPMMA, M n > 120 kDa) with low percentages of F-p-CF 3 PhDOT (1.8 and 3.8 mol%) are prepared using industrially relevant bulk free radical copolymerization conditions. The thermomechanical properties of dPMMA are similar to PMMA; however, the former is shown to degrade into low molar mass fragments (<6.5 kDa) under mild aminolysis conditions. This work presents the first example of a radical ring-opening CC capable of nearly random copolymerization with MMA without the possibility of cross-linking and provides a workflow for the mechanism-guided design of deconstructable copolymers in the future.

Plenty of Space in the Backbone: Radical Ring‐Opening Polymerization

“…A different DFT based workflow was recently reported by the team of Johnson to develop benzyl-functionalized thionolactones (bDOTs) that overcame a previous key limitation of thionolactones: copolymerization with methacrylates. [83] In contrast to defining a composite rate constant to cover the multistep radical addition process, the entire radical addition potential energy landscape was evaluated via DFT to identify the reactivity ratio defining key transition states (Figure 4A). These calculations indicated that the introduction of radicalstabilizing substituents onto the benzylic carbon of DOT may lower the energy of the rate-determining transition state of the DOT ring-opening, and thereby enable copolymerization with MMA (Figure 4B).…”

Accurate Determination of Reactivity Ratios for Copolymerization Reactions with Reversible Propagation Mechanisms

“…In the context of ring-opening metathesis polymerization (ROMP), low-strain cyclic silyl ethers and enol ethers were recently reported by our groups to copolymerize with traditional norbornene-based monomers to yield deconstructable linear, graft, and thermoset copolymers. − Similarly, cyclic disulfides, , isocyanides, and thioamides have recently emerged as competent cleavable comonomers with broad classes of vinyl monomers via radical polymerization. In this context, the distribution of these comonomers within a polymer backbone (i.e., the comonomer sequence) dictates degradation properties; thus, accurate understanding of comonomer reactivity is vital for predicting and engineering the end-of-life properties of these materials. , Finally, the emerging use of data-driven research practices in combination with machine learning methods relies on large bodies of accurate and high-quality data to develop broadly useful tools for property and reactivity predictions. − …”