Mechanochemically Promoted Functionalization of Postconsumer Poly(Methyl Methacrylate) and Poly(α-Methylstyrene) for Bulk Depolymerization

We describe a methodology of post-polymerization functionalization to enable subsequent bulk depolymerization to monomer by utilizing mechanochemical macro-radical generation. By harnessing ultrasonic chain-scission in the presence of N-hydroxyphthalimide methacrylate (PhthMA), we successfully chain-end functionalize polymers to promote subsequent depolymerization in bulk, achieving up to 82 % depolymerization of poly(methyl methacrylate) (PMMA) and poly(α-methylstyrene) (PAMS) within 30 min. This method of depolymerization yields a high-purity monomer that can be repolymerized. Moreover, as compared to the most common methods of depolymerization, this work is most efficient with ultra-high molecular weight (UHMW) polymers, establishing a method with the potential to address highly persistent, non-degradable all-carbon backbone plastic materials. Lastly, we demonstrate the expansion of this depolymerization method to commercial cell cast PMMA, achieving high degrees of depolymerization from post-consumer waste. This work is the first demonstration of applying PhthMA-promoted depolymerization strategies in homopolymer PMMA and PAMS prepared by conventional polymerization methods.

Electrochemical Postpolymerization Modification and Deconstruction of Macromolecules

Electrolysis is an emerging approach to polymer postpolymerization modification, deconstruction, and depolymerization. Electrochemical reactions are particularly appealing for macromolecular transformations because of their high selectivity, ability to be externally monitored, and intrinsic scalability. Despite these desirable features and the recent resurgent use of small-molecule electrochemical reactions, the development of macromolecular electrolysis has been limited. Herein, we highlight recent examples of polymer transformations driven by heterogeneous redox chemistry. Throughout our exploration of macromolecular electrolysis, we provide our perspective on opportunities for continued investigation in this nascent field. Specifically, we highlight how targeted reaction development through deeper mechanistic insight will expand the scope of materials that can be (de)constructed with electrochemical methods. As this insight is developed, we expect macromolecular electrolysis to emerge as a high-functioning and complementary tool for macromolecular functionalization and deconstruction.

β-Triketones as Reactive Handles for Polymer Diversification via Dynamic Catalyst-Free Diketoenamine Click Chemistry

The spontaneous condensation of amines with β-triketones (TK), forming β,β'-diketoenamines (DKE) and releasing water as the sole byproduct, exhibits many of the hallmarks of "click" reactions. Such characteristics render TKs as a highly advantageous platform for efficient polymer diversification, even in biological contexts. Leveraging reversible addition-fragmentation chain transfer (RAFT) and photoiniferter polymerization of novel TK-containing vinylic monomers, we synthesized polymers containing pendent TKs with excellent control of molecular weights, even in excess of 106 g mol-1. Under mild, catalyst-free conditions, poly(β-triketone methacrylate) could be modified with a diverse scope of amines containing a plethora of functional groups. The high efficiency of this functionalization approach was further emphasized when grafting-to with poly(ethylene glycol)-amine resulting in bottlebrushes with molecular weights reaching 2.0 × 107 g mol-1. Critically, while the formed DKE linkages are stable under ambient conditions, they undergo catalyst-free, dynamic transamination at elevated temperatures, paving the way for associative covalent adaptable networks. Overall, we introduce pendent triketone moieties into methacrylate and acrylamide polymers, establishing a novel postpolymerization modification technique that facilitates catalyst-free ligation of amines under highly permissible conditions.

Selective Electrochemical Modification and Degradation of Polymers

We demonstrate that electrochemical-induced decarboxylation enables reliable post-polymerization modification and degradation of polymers. Polymers containing N-(acryloxy)phthalimides were subjected to electrochemical decarboxylation under mild conditions, which led to the formation of transient alkyl radicals. By installing these redox-active units, we systematically modified the pendent groups and chain ends of polyacrylates. This approach enabled the production of poly(ethylene-co-methyl acrylate) and poly(propylene-co-methyl acrylate) copolymers, which are difficult to synthesize by direct polymerization. Spectroscopic and chromatographic techniques reveal these transformations are near-quantitative on several polymer systems. Electrochemical decarboxylation also enables the degradation of all-methacrylate poly(N-(methacryloxy)phthalimide-co-methyl methacrylate) copolymers with a degradation efficiency of >95 %. Chain cleavage is achieved through the decarboxylation of the N-hydroxyphthalimide ester and subsequent β-scission of the backbone radical. Electrochemistry is thus shown to be a powerful tool in selective polymer transformations and controlled macromolecular degradation.

Photocatalytic Upcycling and Depolymerization of Vinyl Polymers

Photocatalytic upcycling and depolymerization of vinyl polymers have emerged as promising strategies to combat plastic pollution and promote a circular economy. This mini review critically summarizes current developments in the upcycling and degradation of vinyl polymers including polystyrene and poly(meth)acrylates. Of these material classes, polymethacrylates possess the unique possibility to undergo a photocatalytic depolymerization back to monomer under thermodynamically favourable conditions, thus presenting significant advantages over traditional thermal strategies. Our perspective on current formidable challenges and potential future directions are also discussed.

Bulk Depolymerization of Methacrylate Polymers via Pendent Group Activation

In this study, we present an efficient approach for the depolymerization of poly(methyl methacrylate) (PMMA) copolymers synthesized via conventional radical polymerization. By incorporating low mol % phthalimide ester-containing monomers during the polymerization process, colorless and transparent polymers closely resembling unfunctionalized PMMA are obtained, which can achieve >95% reversion to methyl methacrylate (MMA). Notably, our catalyst-free bulk depolymerization method exhibits exceptional efficiency, even for high-molecular-weight polymers, including ultrahigh-molecular-weight (106-107 g/mol) PMMA, where near-quantitative depolymerization is achieved. Moreover, this approach yields polymer byproducts of significantly lower molecular weight, distinguishing it from bulk depolymerization methods initiated from chain ends. Furthermore, we extend our investigation to polymethacrylate networks, demonstrating high extents of depolymerization. This innovative depolymerization strategy offers promising opportunities for the development of sustainable polymethacrylate materials, holding great potential for various applications in polymer science.

RAFT Polymerisation by the Radical Decarboxylation of Carboxylic Acids

The controlled grafting of polymers from small- and macro-molecular substrates is an essential process for many advanced polymer applications. This usually requires the pre-functionalisation of substrates with an appropriate functional group, such as a RAFT agent or ATRP initiator, which requires additional synthetic steps. In this paper, we describe the direct grafting of RAFT polymers from carboxylate containing small molecules and polymers via photochemical radical decarboxylation. This method utilises the innate functional groups present in the substrates, and achieves efficient polymer initiation in a single step with excellent control of molecular weight and dispersity.

Diversification of Acrylamide Polymers via Direct Transamidation of Unactivated Tertiary Amides

Postpolymerization modification offers a versatile strategy for synthesizing complex macromolecules, yet modifying acrylamide polymers like poly(N,N-dimethylacrylamide) (PDMA) is notoriously challenging due to the inherent stability and low reactivity of amide bonds. In this study, we unveil a novel approach for the direct transamidation of PDMA, leveraging recent advances in the transamidation of unactivated tertiary amide substrates. By exploiting photoiniferter polymerization, we extended this direct transamidation approach to ultrahigh-molecular-weight (UHMW) PDMA, showcasing the unprecedented postpolymerization modification of synthetic polymers exceeding 106 g/mol. We also designed acrylamide copolymers comprising both the moderately reactive N-methyl-N-phenyl tertiary amides, along with the less reactive, fully alkyl-substituted N,N-dimethyl amides inherent to PDMA. This disparate reactivity enabled a sequential, chemoselective transamidation by initially targeting the more reactive pendant aryl amides with less nucleophilic aromatic amines, and second, transamidating the untouched N,N-dimethyl amide moieties with more nucleophilic aliphatic amines, yielding a uniquely diversified acrylamide copolymer. This work not only broadens the scope of postpolymerization modification strategies by pioneering direct transamidation of unactivated amides but also provides a robust platform for the design of intricate macromolecules, particularly in the realm of UHMW polymers.

Alternating styrene-propylene and styrene-ethylene copolymers prepared by photocatalytic decarboxylation

Synthesis of olefin-styrene copolymers with defined architecture is challenging due to the limitations associated with the inherent reactivity ratios for these monomers in radical or metal-catalyzed polymerizations. Herein, we developed a straightforward approach to alternating styrene-propylene and styrene-ethylene copolymers by combining radical polymerizations and powerful post-polymerization modification reactions. We employed reversible addition-fragmentation chain transfer (RAFT) copolymerization between styrene derivatives and saccharin (meth)acrylamide to generate alternating copolymers. Once polymerized, the amide bond of the saccharin monomers was highly reactive toward hydrolysis, an observation exploited to obtain alternating styrene-acrylic acid/methacrylic acid copolymers. Subsequent mild decarboxylation of the (meth)acrylic acid groups in the presence of a photocatalyst and a hydrogen source under visible light resulted in the styrene-alt-ethylene/propylene copolymers. Alternating copolymers comprised of either propylene or ethylene units alternating with functional styrene derivatives were also prepared, illustrating the compatibility of this approach for functional polymer synthesis. Finally, the thermal properties of the alternating copolymers were compared to those from statistical copolymer analogs to elucidate the effect of microarchitecture and styrene substituents on the glass transition temperature.

Inverse Miniemulsion Enables the Continuous-Flow Synthesis of Controlled Ultra-High Molecular Weight Polymers

We report the controlled synthesis of ultra-high molecular weight (UHMW) polymers (Mn ≥ 106 g/mol) via continuous flow in a tubular reactor. At high monomer conversion, UHMW polymers in homogeneous batch polymerization exhibit high viscosities that pose challenges for employing continuous flow reactors. However, under heterogeneous inverse miniemulsion (IME) conditions, UHMW polymers can be produced within the dispersed phase, while the viscosity of the heterogeneous mixture remains approximately the same as the viscosity of the continuous phase. Conducting such IME polymerizations in flow results in a faster rate of polymerization compared to batch IME polymerizations while still providing excellent control over molecular weight up to 106 g/mol. Crucial emulsion parameters, such as particle size and stability under continuous flow conditions, were examined using dynamic light scattering. A range of poly(N,N-dimethylacrylamide) and poly(4-acryloylmorpholine) polymers with molecular weights of 104-106 g/mol (Đ ≤ 1.31) were produced by this method using water-soluble trithiocarbonates as photoiniferters.

Degradation of Polyacrylates by One-Pot Sequential Dehydrodecarboxylation and Ozonolysis

We establish a synthetically convenient method to degrade polyacrylate homopolymers. Carboxylic acids are installed along the polymer backbone by partial hydrolysis of the ester side chains, and then, in a one-pot sequential procedure, the carboxylic acids are converted into alkenes and oxidatively cleaved. This process enables the robustness and properties of polyacrylates to be maintained during their usable lifetime. The ability to tune the degree of degradation was demonstrated by varying the carboxylic acid content of the polymers. This method is compatible with a wide range of polymers prepared from vinyl monomers through copolymerization of acrylic acid with different monomers including acrylates, acrylamides, and styrenics.

Excitation Dependence in Photoiniferter Polymerization

We report on a fundamental feature of photoiniferter polymerizations mediated with trithiocarbonates and xanthates. The polymerizations were found to be highly dependent on the activated electronic excitation of the iniferter. Enhanced rates of polymerization and greater control over molecular weights were observed for trithiocarbonate- and xanthate-mediated photoiniferter polymerizations when the n → π* transition of the iniferter was targeted compared to the polymerizations activating the π → π* transition. The disparities in rates of polymerization were attributed to the increased rate of C-S photolysis which was confirmed using model trapping studies. This study provides valuable insight into the role of electronic excitations in photoiniferter polymerization and provides guidance when selecting irradiation conditions for applications where light sensitivity is important.

Photoassisted Radical Depolymerization

Controlled radical polymerization techniques enable the synthesis of polymers with predetermined molecular weights, narrow molecular weight distributions, and controlled architectures. Moreover, these polymerization approaches have been routinely shown to result in retained end-group functionality that can be reactivated to continue polymerization. However, reactivation of these end groups under conditions that instead promote depropagation is a viable route to initiate depolymerization and potentially enable closed-loop recycling from polymer to monomer. In this report, we investigate light as a trigger for thermal depolymerization of polymers prepared by reversible-addition-fragmentation chain-transfer (RAFT) polymerization. We study the role of irradiation wavelength by targeting the n → π* and π → π* electronic transitions of the thiocarbonylthio end-groups of RAFT-generated polymers to enhance depolymerization via terminal bond homolysis. Specifically, we explore depolymerization of polymers with trithiocarbonate, dithiocarbamate, and p-substituted dithiobenzoate end groups with the purpose of increasing depolymerization efficiency with light. As the wavelength decreases from the visible range to the UV range, the rate of depolymerization is dramatically increased. This method of photoassisted depolymerization allows up to 87% depolymerization efficiency within 1 h, results that may further the advancement of recyclable materials and life-cycle circularity.

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

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.

Decarboxylation of Poly[N-(acryloyloxy)phthalimide] as a Versatile Tool for Post-Polymerization Modification

Herein we report on the decarboxylation of poly[N-(acryloyloxy)phthalimide] (PAP) for the synthesis of functionalized polymers. PAP homopolymer and block copolymers are used as precursor polymers for the straightforward functionalization via decarboxylation and subsequent Michael-type addition or nitroxide radical coupling (NRC). This article is protected by copyright. All rights reserved.

Anthranilamide-protected vinylboronic acid: rational monomer design for improved polymerization/transformation ability providing access to conventionally inaccessible copolymers

We designed a vinyl-boronic-acid protected by anthranilamide as a “transformable” monomer in radical polymerization to synthesize conventionally inaccessible copolymers, such as poly(vinyl alcohol-co-styrene) and poly(ethylene-co-acrylate).