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

Organophotoredox-Catalyzed Postfunctionalization of Poly(methacrylate) Derivatives via Radical-Polar Crossover Phosphonylation

Photoredox-catalyzed postfunctionalization of poly(meth)acrylate-based copolymers bearing phthalimide moieties can produce novel polymer architectures that are inaccessible using polymerization approaches; however, this method is restricted to radical reactions. To expand the scope of reactions applicable to photoredox-catalyzed postfunctionalization, this study focused on the radical-polar crossover (RPC) process, in which a carbocation equivalent is generated to react with nucleophiles. The organophotoredox-catalyzed reaction of a random copolymer of N-(methacryloxy)phthalimide and methyl acrylate using trialkyl phosphite nucleophiles afforded the corresponding phosphonate-containing polymers. Although simultaneous hydrogenation was inevitable, a novel polymer comprising dialkyl isopropenylphosphonate, propylene, and methyl acrylate units was obtained, which is of interest because phosphonate-containing polymers are potentially applicable as thermoresponsive materials, flame-retardant materials, and additives in Li-ion batteries. A series of copolymers with different components were successfully applied to the postfunctionalization method. The organophotoredox catalysis-based postfunctionalization strategy developed in this study provides versatile functional polymers in a simple, green, and sustainable manner.

Redox-Selective Macromolecular Electrolysis for Sequential Functionalization and Deconstruction

This study demonstrates that selective macromolecular electrolysis can be achieved on copolymers containing redox-orthogonal targets by controlling the externally applied voltage. We designed macromolecules containing phthalimide (E1/2 = -1.8 V vs Ag/AgNO3) and tetrachlorophthalimide (E1/2 = -1.3 V vs Ag/AgNO3) (meth)acrylates that have significantly different reduction potentials such that they are separately redox-addressable. The polymer-centered radicals generated by decarboxylation can either undergo (1) hydrogen atom transfer to form olefinic repeat units or (2) β-scission to deconstruct the polymer backbone. Our results reveal selective electrochemical control over postpolymerization modifications, which enables sequential transformations that tune the glass transition temperature of electrochemically generated copolymers over a range of -54 to 125 °C. This method was also shown to maintain its selectivity in a polymer blend and provided access to copolymers (poly(styrene-co-propylene-co-ethylene)) that would be challenging to prepare in other ways. These results demonstrate the potential of macromolecular electrolysis for selective material functionalization and degradation. This approach expands the toolbox for postpolymerization modification and targeted polymer degradation with applications in macromolecular information processing, spatiotemporal patterning, and producing materials with complex architectures that are driven by external stimuli.

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.

Acridine/Lewis Acid Complexes as Powerful Photocatalysts: A Combined Experimental and Mechanistic Study

A class of in-situ generated Lewis acid (LA) activated acridine complexes is reported, which act as potent photochemical catalysts for the oxidation of a variety of protected secondary amines. Acridine/LA complexes exhibit tunable excited state reduction potentials ranging from +2.07 to 2.38 V vs. SCE. The ytterbium triflate complex of 3,6-di-t-butyl-9-mesitylacridine catalyzes a photochemical Giese-type reaction of Boc-protected secondary amines with challenging conjugate acceptors such as acrylates, that are inaccessible to the analogous acridinium (t-Bu-Mes-Acr⊕) catalyzed reaction. The mechanism of this reaction was investigated using a suite of physical organic probes including intramolecular 13C kinetic isotope effects (KIEs), variable time normalization analysis (VTNA) kinetics, determination of redox potentials, and computational studies. In the reaction catalyzed by t-Bu-Mes-Acr⊕, mechanistic studies are consistent with single-electron transfer (SET) from the ground-state reduced t-Bu-Mes-Acr• to the α-keto radical intermediate as the first irreversible step in the catalytic cycle. Intriguingly, we find that the reduced acridine/LA complexes are better ground state reductants (-0.72 to -0.74 V vs SCE) relative to t-Bu-Mes-Acr• (-0.59 V vs SCE) and predict that the increased substrate reactivity stems from a lower energy barrier for this key SET event.

Photocatalytic Decarboxylative Alkylation of Cyclic Imine-BF3 Complexes: A Modular Route to Functionalized Azacycles

Alkyl radicals generated via an acridine photocatalyzed decarboxylation reaction of feedstock carboxylic acids engage with a range of cyclic imine-BF3 complexes to provide α-functionalized azacycles in an operationally simple process. A three-component variant of this transformation incorporating [1.1.1]propellane as an additional reaction partner enables the synthesis of valuable bicyclopentane (BCP)-containing azacycles. Reactions exhibit good functional group compatibility, enabling late-stage modification of complex bioactive molecules.

Alternating Graft Copolymer Carrying PLA Graft Chains at Every Other Unit: Sequence Impacts on Crystallization Behaviors

Alternating graft copolymers were precisely synthesized via selective cyclopolymerization of pendant-transformable divinyl monomer (1), post-polymerization modification via aminolysis with alkylamine, and ring-opening polymerization of l-lactide (LLA) from the hydroxy pendant group in alternating sequence. The poly(LLA) (PLLA) graft chain on the alternating copolymer gave a higher crystallization degree on the isothermal treatment than that on the random counterpart likely because of the periodic sequence. The comonomer pendant group from alkylamine in the aminolysis reaction in the alternating sequence affected the crystallization behaviors, and the oligoethylene glycol (OEG) group promoted the crystallization thanks to the larger free volume effect. As for the stereocomplex formation of the racemic mixture of enantiomeric PLLA and poly(d-lactide) (PDLA) chains, the alternating graft copolymer gave a higher degree of stereocomplex crystallization in the mixture with the enantiomer homopolymer than the random analogue.

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.

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.

Access to Functionalized Materials by Metal-Free Ring-Opening Metathesis Polymerization of Active Esters and Divergent Postpolymerization Modification

Metal-free ring-opening metathesis polymerization (MF-ROMP) is an emerging polymerization strategy that provides access to ROMP materials by using organic initiators and photoredox catalysts. Unlike metal-mediated ROMP, MF-ROMP is not highly tolerant toward functionalized monomers. Herein, we report that pentafluorophenyl esters are polymerizable under MF-ROMP conditions to produce homopolymers, statistical copolymers, and block copolymers. Amine coupling agents were then used to install a range of functional groups via acyl substitution including alkynes, amino acid derivatives, fluorophores, and redox active moieties. Overall, these findings provide a framework to prepare functionalized ROMP polymers without the risk of metal contamination.