Garret M. Miyake

Organocatalyzed atom transfer radical polymerization driven by visible light

Precise control from a metal-free catalyst Polymerization can be a rather dangerous free for all, with molecules joining randomly in chains at a chaotic pace. One of modern chemistrys great accomplishments has been the development of methods to assemble polymers in steady, orderly steps. However, order comes at a price, and often its the need for metal catalysts that are hard to remove from the plastic product. Theriot et al. used theory to guide the design of a metal-free light-activated catalyst that offers precise control in atom transfer radical polymerization, alleviating concerns about residual metal contamination (see the Perspective by Shanmugam and Boyer). Science, this issue p. 1082; see also p. 1053 A metal-free catalyst offers comparable control to more commonly used metals without the drawback of product contamination. Atom transfer radical polymerization (ATRP) has become one of the most implemented methods for polymer synthesis, owing to impressive control over polymer composition and associated properties. However, contamination of the polymer by the metal catalyst remains a major limitation. Organic ATRP photoredox catalysts have been sought to address this difficult challenge but have not achieved the precision performance of metal catalysts. Here, we introduce diaryl dihydrophenazines, identified through computationally directed discovery, as a class of strongly reducing photoredox catalysts. These catalysts achieve high initiator efficiencies through activation by visible light to synthesize polymers with tunable molecular weights and low dispersities.

Alane‐Based Classical and Frustrated Lewis Pairs in Polymer Synthesis: Rapid Polymerization of MMA and Naturally Renewable Methylene Butyrolactones into High‐Molecular‐Weight Polymers

The seminal works by the research groups of Stephan and Erker uncovered the concept of “frustrated lewis pairs” (FLPs) to describe pairs formed by a sterically encumbered borane Lewis acid (most commonly B(C6F5)3) and a base (e.g. (tBu)3P), in which these two components are sterically precluded from forming classical donor/acceptor adducts Instead, the unquenched, opposite reactivity of FLPs can carry out unusual reactions or reactions that were previously known to be possible only by transition-metal complexes, such as activation of H2, CO2, N2O, B H bonds, alkenes and alkynes, as well as catalytic hydrogenation. The direct use of FLPs in polymer synthesis and/or in polymerization catalysis is currently still missing from this list. Our research group reported in 2002 that strongly acidic, sterically encumbered alane Al(C6F5)3 [5] and bulky 2,6-di-tertbutyl pyridine, which do not form a classical acid/base adduct when the unsolvated alane is used, work cooperatively to activate and break arene C H bonds when the toluene/alane adduct, C7H8·Al(C6F5)3, [6] is used, or when toluene is added to the above-mentioned mixture (Scheme 1). Preceding the current term FLP, this result was an early example of the unusual reactivity of FLPs relative to what was generally recognized. Since then, our interest has continued to focus on utilizing the high Lewis acidity of Al(C6F5)3 and, more importantly in many cases, the unique catalytic feature of the active species derived from this alane, for the polymerization of functionalized alkenes. Reported herein is a significant development in this continuing effort: Al(C6F5)3based Lewis pairs rapidly polymerize polar vinyl monomers at room temperature (Scheme 2), including methyl methacrylate (MMA) and the naturally renewable methylene butyr-

Rapid self-assembly of block copolymers to photonic crystals

The reduced chain entanglement of brush polymers over their linear analogs drastically lowers the energetic barriers to reorganization. In this report, we demonstrate the rapid self-assembly of brush block copolymers to nanostructures with photonic bandgaps spanning the entire visible spectrum, from ultraviolet (UV) to near infrared (NIR). Linear relationships were observed between the peak wavelengths of reflection and polymer molecular weights. This work enables “bottom-up” fabrication of photonic crystals with application-tailored bandgaps, through synthetic control of the polymer molecular weight and the method of self-assembly. These polymers could be developed into NIR-reflective paints, to combat the “urban heat island effect” due to NIR photon thermalization.

Synthesis of Isocyanate-Based Brush Block Copolymers and Their Rapid Self-Assembly to Infrared-Reflecting Photonic Crystals

The synthesis of rigid-rod, helical isocyanate-based macromonomers was achieved through the polymerization of hexyl isocyanate and 4-phenylbutyl isocyanate, initiated by an exo-norbornene functionalized half-titanocene complex. Sequential ruthenium-mediated ring-opening metathesis polymerization of these macromonomers readily afforded well-defined brush block copolymers, with precisely tunable molecular weights ranging from high (1512 kDa) to ultrahigh (7119 kDa), while maintaining narrow molecular weight distributions (PDI = 1.08-1.39). The self-assembly of these brush block copolymers to solid thin-films and their photonic properties were investigated. Due to the rigid architecture of these novel polymeric materials, they rapidly self-assemble through simple controlled evaporation to photonic crystal materials that reflect light from the ultra-violet, through the visible, to the near-infrared. The wavelength of reflectance is linearly related to the brush block copolymer molecular weight, allowing for predictable tuning of the band gap through synthetic control of the polymer molecular weight. A combination of scanning electron microscopy and optical modeling was employed to explain the origin of reflectivity.

Precisely Tunable Photonic Crystals From Rapidly Self‐Assembling Brush Block Copolymer Blends

Colorful: enabled by their reduced capacity for chain entanglement, high-molecular-weight brush block copolymers can rapidly self-assemble to photonic crystals. The blending of two polymers of different molecular weight can predictably modulate the sizes of the polymer domains, giving rise to a facile means of precision tuning of these photonic-band-gap materials.

Organocatalyzed Atom Transfer Radical Polymerization Using N-Aryl Phenoxazines as Photoredox Catalysts

N-Aryl phenoxazines have been synthesized and introduced as strongly reducing metal-free photoredox catalysts in organocatalyzed atom transfer radical polymerization for the synthesis of well-defined polymers. Experiments confirmed quantum chemical predictions that, like their dihydrophenazine analogs, the photoexcited states of phenoxazine photoredox catalysts are strongly reducing and achieve superior performance when they possess charge transfer character. We compare phenoxazines to previously reported dihydrophenazines and phenothiazines as photoredox catalysts to gain insight into the performance of these catalysts and establish principles for catalyst design. A key finding reveals that maintenance of a planar conformation of the phenoxazine catalyst during the catalytic cycle encourages the synthesis of well-defined macromolecules. Using these principles, we realized a core substituted phenoxazine as a visible light photoredox catalyst that performed superior to UV-absorbing phenoxazines as well as previously reported organic photocatalysts in organocatalyzed atom transfer radical polymerization. Using this catalyst and irradiating with white LEDs resulted in the production of polymers with targeted molecular weights through achieving quantitative initiator efficiencies, which possess dispersities ranging from 1.13 to 1.31.

Intramolecular Charge Transfer and Ion Pairing in N,N-Diaryl Dihydrophenazine Photoredox Catalysts for Efficient Organocatalyzed Atom Transfer Radical Polymerization

Photoexcited intramolecular charge transfer (CT) states in N,N-diaryl dihydrophenazine photoredox catalysts are accessed through catalyst design and investigated through combined experimental studies and density functional theory (DFT) calculations. These CT states are reminiscent of the metal to ligand charge transfer (MLCT) states of ruthenium and iridium polypyridyl complexes. For cases where the polar CT state is the lowest energy excited state, we observe its population through significant solvatochromic shifts in emission wavelength across the visible spectrum by varying solvent polarity. We propose the importance of accessing CT states for photoredox catalysis of atom transfer radical polymerization lies in their ability to minimize fluorescence while enhancing electron transfer rates between the photoexcited photoredox catalyst and the substrate. Additionally, solvent polarity influences the deactivation pathway, greatly affecting the strength of ion pairing between the oxidized photocatalyst and the bromide anion and thus the ability to realize a controlled radical polymerization. Greater understanding of these photoredox catalysts with respect to CT and ion pairing enables their application toward the polymerization of methyl methacrylate for the synthesis of polymers with precisely tunable molecular weights and dispersities typically lower than 1.10.

Highly Ordered Dielectric Mirrors via the Self-Assembly of Dendronized Block Copolymers

Dendronized block copolymers were synthesized by ruthenium-mediated ring-opening methathesis polymerization of exo-norbornene functionalized dendrimer monomers, and their self-assembly to dielectric mirrors was investigated. The rigid-rod main-chain conformation of these polymers drastically lowers the energetic barrier for reorganization, enabling their rapid self-assembly to long-range, highly ordered nanostructures. The high fidelity of these dielectric mirrors is attributed to the uniform polymer architecture achieved from the construction of discrete dendritic repeat units. These materials exhibit light-reflecting properties due to the multilayer architecture, presenting an attractive bottom-up approach to efficient dielectric mirrors with narrow band gaps. The wavelength of reflectance scales linearly with block-copolymer molecular weight, ranging from the ultraviolet, through the visible, to the near-infrared. This allows for the modulation of photonic properties through synthetic control of the polymer molecular weight. This work represents a significant advancement in closing the gap between the precision obtained from top-down and bottom-up approaches.

Photoinduced Organocatalyzed Atom Transfer Radical Polymerization Using Continuous Flow

Organocatalyzed atom transfer radical polymerization (O-ATRP) has emerged as a metal-free variant of historically transition-metal reliant atom transfer radical polymerization. Strongly reducing organic photoredox catalysts have proven capable of mediating O-ATRP. To date, operation of photoinduced O-ATRP has been demonstrated in batch reactions. However, continuous flow approaches can provide efficient irradiation reaction conditions and thus enable increased polymerization performance. Herein, the adaptation of O-ATRP to a continuous flow approach has been performed with multiple visible-light absorbing photoredox catalysts. Using continuous flow conditions, improved polymerization results were achieved, consisting of narrow molecular weight distributions as low as 1.05 and quantitative initiator efficiencies. This system demonstrated success with 0.01% photocatalyst loadings and a diverse methacrylate monomer scope. Additionally, successful chain-extension polymerizations using 0.01 mol % photocatalyst loadings reveal continuous flow O-ATRP to be a robust and versatile method of polymerization.

Organocatalyzed Atom Transfer Radical Polymerization: Perspectives on Catalyst Design and Performance

The recent development of organocatalyzed atom transfer radical polymerization (O-ATRP) represents a significant advancement in the field of controlled radical polymerizations. A number of classes of photoredox catalysts have been employed thus far in O-ATRP. Analysis of the proposed mechanism gives insight into the relevant photophysical and chemical properties that determine catalyst performance. Discussion of each of the classes of O-ATRP catalysts highlights their previous uses, their roles in the development of O-ATRP, and the distinctive properties that govern their polymerization behavior, leading to a set of design principles for O-ATRP catalysts. Remaining challenges for O-ATRP are presented, as well as prospects for further improvement in the application scope of O-ATRP.