Designing Degradable Polymers from Tricycloalkenes via Complete Cascade Metathesis Polymerization.

Cascade metathesis polymerization has been developed as a promising method to synthesize complex but well-defined polymers from monomers containing multiple reactive functional groups. However, this approach has been limited to the monomers involving simple alkene/alkyne moieties or produced mainly non-degradable polymers. In this study, we demonstrate a complete cascade ring-opening/ring-closing metathesis polymerization (RORCMP) using various tricycloalkenes and two strategies for the efficient degradation. Through rational design of tricycloalkene monomers, the structure and reactivity relationship was explored. For example, tricycloalkenes with trans configuration in the central ring enabled faster and better selective cascade RORCMP than the corresponding cis isomers. Also, a 4-substituted cyclopentene moiety in the monomers significantly enhanced the overall cascade RORCMP performance, with the maximum turnover number (TON) reaching almost 10,000 and molecular weight up to 170 kg/mol using an amide-containing monomer. Furthermore, we achieved one-shot cascade multiple olefin metathesis polymerization using tricycloalkenes and a diacrylate, to produce new highly A,B-alternating copolymers with full degradability. Lastly, we successfully designed xylose-based tricycloalkenes to give well-defined polymers that underwent ultra-fast and complete degradation under mild conditions.

Living Alternating Ring-Opening Metathesis Copolymerization of 2,3-Dihydrofuran to Provide Completely Degradable Polymers.

2,3-Dihydrofuran (DHF) has recently been gaining significant attention as a comonomer in metathesis polymerization, thanks to its ability to provide the resultant polymer backbones with stimuli-responsive degradability. In this report, we present living alternating copolymerization of DHF with less reactive endo-tricyclo[4.2.2.02,5 ]deca-3,9-dienes (TDs) and endo-oxonorbornenes (oxoNBs). By carefully controlling the reactivity of both the Ru initiators and the monomers, we have achieved outstanding A, B-alternation (up to 98 %) under near stoichiometric DHF loading conditions. Notably, we have also found that the use of a more sterically hindered Ru initiator helps to attain polymer backbones with higher DHF incorporation and superior A, B-alternation. While preserving the living characteristics of DHF copolymerization, as evidenced by controlled molecular weights (up to 73.9 kDa), narrow dispersities (down to 1.05), and block copolymer formation, our DHF copolymers could be broken down to a single repeat unit level under acidic conditions. 1 H NMR analysis of the model copolymer revealed that after 24 hours of degradation, up to 80 % of the initial polymer was transformed into a single small molecule product, and after purification, up to 66 % of the degradation product was retrieved. This study provides a versatile approach to improve the alternation and degradability of DHF copolymers.

Monodisperse Cyclic Polymer Mechanochemistry: Scission Kinetics and the Dynamic Memory Effect with Ultrasonication and Ball-Mill Grinding.

A series of monodisperse cyclic and linear poly(d,l-lactide)s (c-PLA and l-PLA, respectively) were prepared with various degrees of polymerization (DP) using an iterative convergent synthesis approach. The absence of a molecular weight distribution provided us a chance to study their mechanochemical reactivity without obstructions arising from the size distribution. Additionally, we prepared l- and c-PLAs with identical DPs, which enabled us to attribute differences in scission rates to the cyclic polymer architecture alone. The polymers were subjected to ultrasonication (US) and ball-mill grinding (BMG), and their degradation kinetics were explored. Up to 9.0 times larger scission rates were observed for l-PLA (compared to c-PLA) with US, but the difference was less than 1.9 times with BMG. Fragmentation requires two backbone scission events for c-PLA, and we were able to observe linear intermediates (formed after a single scission) for the first time. We also developed a new method of studying the dynamic memory effect in US by characterizing and comparing the daughter fragment molecular weight distributions of l- and c-PLAs. These results provide new insights into the influence of the cyclic polymer architecture on mechanochemical reactions as well as differences in reactivity observed with US and BMG.

Living Suzuki-Miyaura Catalyst-Transfer Polymerization for Precision Synthesis of Length-Controlled Armchair Graphene Nanoribbons and Their Block Copolymers.

The bottom-up synthesis of graphene nanoribbons (GNRs) offers a promising approach for designing atomically precise GNRs with tuneable photophysical properties, but controlling their length remains a challenge. Herein, we report an efficient synthetic protocol for producing length-controlled armchair GNRs (AGNRs) through living Suzuki-Miyaura catalyst-transfer polymerization (SCTP) using RuPhos-Pd catalyst and mild graphitization methods. Initially, SCTP of a dialkynylphenylene monomer was optimized by modifying boronates and halide moieties on the monomers, affording poly(2,5-dialkynyl-p-phenylene) (PDAPP) with controlled molecular weight (Mn up to 29.8k) and narrow dispersity (D = 1.14-1.39) in excellent yield (>85%). Subsequently, we successfully obtained N = 5 AGNRs by employing a mild alkyne benzannulation reaction on the PDAPP precursor and confirmed their length retention by size-exclusion chromatography. In addition, photophysical characterization revealed that a molar absorptivity was directly proportional to the length of the AGNR, while its highest occupied molecular orbital (HOMO) energy level remained constant within the given AGNR length. Furthermore, we prepared, for the very first time, N = 5 AGNR block copolymers with widely used donor or acceptor-conjugated polymers by taking advantage of the living SCTP. Finally, we achieved the lateral extension of AGNRs from N = 5 to 11 by oxidative cyclodehydrogenation in solution and confirmed their chemical structure and low band gap by various spectroscopic analyses.

Size-Tunable Semiconducting 2D Nanorectangles from Conjugated Polyenyne Homopolymer Synthesized via Cascade Metathesis and Metallotropy Polymerization.

Size-tunable semiconducting two-dimensional (2D) nanosheets from conjugated homopolymers are promising materials for easy access to optoelectronic applications, but it has been challenging due to the low solubility of conjugated homopolymers. Herein, we report size-tunable and uniform semiconducting 2D nanorectangles via living crystallization-driven self-assembly (CDSA) of a fully conjugated polyenyne homopolymer prepared by cascade metathesis and metallotropy (M&M) polymerization. The resulting polyenyne with enhanced solubility successfully underwent living CDSA via biaxial growth mechanism, thereby producing 2D nanorectangles with sizes precisely tuned from 0.1 to 3.0 µm2 with narrow dispersity mostly less than 1.1 and low aspect ratios less than 3.1. Furthermore, living CDSA produced complex 2D block comicelles with different heights from various degrees of polymerization (DPs) of unimers. Based on diffraction analyses and DFT calculations, we proposed an interdigitating packing model with an orthorhombic crystal lattice of semiconducting 2D nanorectangles.

Scalable and continuous access to pure cyclic polymers enabled by 'quarantined' heterogeneous catalysts.

Cyclic polymers are topologically interesting and envisioned as a lubricant material. However, scalable synthesis of pure cyclic polymers remains elusive. The most straightforward way is to recover a used catalyst after the synthesis of cyclic polymers and reuse it. Unfortunately, this is demanding because of the catalyst's vulnerability and inseparability from polymers, which reduce the practicality of the process. Here we develop a continuous circular process, where polymerization, polymer separation and catalyst recovery happen in situ, to dispense a pure cyclic polymer after bulk ring-expansion metathesis polymerization of cyclopentene. It is enabled by introducing silica-supported ruthenium catalysts and newly designed glassware. Different depolymerization kinetics of the cyclic polymer from its linear analogue are also discussed. This process minimizes manual labour, maximizes the security of vulnerable catalysts and guarantees the purity of cyclic polymers, thereby showcasing a prototype of a scalable access to cyclic polymers with increased turnovers (≥415,000) of precious catalysts.

Cascade Cyclopolymerization of 5-Ethynyl-1,8-Nonadiyne Derivatives to Synthesize Low Band Gap Conjugated Polyacetylenes Containing a Fused Bicyclic Structure.

Cyclopolymerization is a powerful method for synthesizing polyacetylenes containing four- to seven-membered rings. However, the structure of the repeat unit only consists of mono-cycloalkene due to the single cyclization of diyne monomers. Herein, we demonstrate a novel cascade cyclopolymerization to synthesize polyacetylenes containing fused bicyclic rings from triyne monomers containing bulky dendrons via sequential cascade ring-closing metathesis. These dendrons provided solubility and stability to the rigid bicyclic polyacetylene backbone. In addition, we controlled the regioselectivity of the catalyst approach by altering its structure and synthesized polymers containing fused bicyclo[4,3,0] or [4,4,0] rings with high molecular weights of up to 120 kg mol-1 . Interestingly, the resulting polymers showed narrower band gaps (down to 1.6 eV) than polymers with mono-cycloalkene repeat units due to the planarization of the conjugated segment resulting from the fused bicyclic structure.

Controlled Living Cascade Polymerization of Polycyclic Enyne Monomers: Leveraging Complete Degradability for a Stereochemical and Structural Investigation.

Cascade polymerizations recently gained significant attention due to their use of unique transformations, involving multiple bond making and/or breaking steps, when converting monomers to repeat units. However, designing complex cascade polymerizations which proceed in a controlled manner is very challenging. Various side reactions can hamper polymerization performance and the efficiency of the cascade. In this work, we explore a metathesis-based cascade polymerization of unique polycyclic enyne monomers, which contain a terminal alkyne and two cyclic alkenes. By modifying the monomer's stereochemistry, linkers, and ring types, we were able to modulate the polymerization performance and the extent to which a complete cascade reaction occurs. Upon subjecting the resulting polymers to mild acidic conditions and analyzing the degradation products, we were able to calculate the percentage of repeat units derived from a complete cascade reaction (termed the cascade efficiency). In addition to identifying how various structural parameters in the monomer influence the success of a cascade polymerization, we were able to achieve controlled living cascade polymerizations of multiple monomers with >99% cascade efficiency and produce various block copolymers.

Precision Synthesis of Various Low-Bandgap Donor-Acceptor Alternating Conjugated Polymers via Living Suzuki-Miyaura Catalyst-Transfer Polymerization.

Herein, we demonstrate that living Suzuki-Miyaura catalyst-transfer polymerization (SCTP) using a RuPhos Pd G3 precatalyst is a versatile method for the precision synthesis of various donor-acceptor alternating conjugated polymers (DA ACPs). First, the living SCTP of biaryl monomers with combinations of both medium to strong A and D were optimized to produce DA ACPs with controlled number average molecular weight (Mn ), narrow dispersity (Ð, 1.05-1.29), and high yield (>87 %). Moreover, its expansion to controlled polymerization (Mn =9.2-40.0 kg mol-1 ) of an A1 -D-A2 -D quateraryl monomer containing diketopyrrolopyrrole (DPP; strong A) was successful. The living SCTP also enabled the efficient one-pot synthesis of various diblock and triblock copolymers. Lastly, the DA ACPs showed tunable optical band gap (Eg opt , from 1.29 to 1.77 eV) and highest occupied molecular orbital (HOMO) level (from -5.57 to -4.75 eV), while their block copolymers exhibited broad absorption ranges and promising visible light-harvesting properties.

Conformation Dynamics of Single Polymer Strands in Solution.

Conformational changes in macromolecules significantly affect their functions and assembly into high-level structures. Despite advances in theoretical and experimental studies, investigations into the intrinsic conformational variations and dynamic motions of single macromolecules remain challenging. Here, liquid-phase transmission electron microscopy enables the real-time tracking of single-chain polymers. Imaging linear polymers, synthetically dendronized with conjugated aromatic groups, in organic solvent confined within graphene liquid cells, directly exhibits chain-resolved conformational dynamics of individual semiflexible polymers. These experimental and theoretical analyses reveal that the dynamic conformational transitions of the single-chain polymer originate from the degree of intrachain interactions. In situ observations also show that such dynamics of the single-chain polymer are significantly affected by environmental factors, including surfaces and interfaces.

Synchronous Preparation of Length-Controllable 1D Nanoparticles via Crystallization-Driven In Situ Nanoparticlization of Conjugated Polymers.

Precise size control of semiconducting nanomaterials from polymers is crucial for optoelectronic applications, but the low solubility of conjugated polymers makes this challenging. Herein, we prepared length-controlled semiconducting one-dimensional (1D) nanoparticles by synchronous self-assembly during polymerization. First, we succeeded in unprecedented living polymerization of highly soluble conjugated poly(3,4-dihexylthiophene). Then, block copolymerization of poly(3,4-dihexylthiophene)-block-polythiophene spontaneously produced narrow-dispersed 1D nanoparticles with lengths from 15 to 282 nm according to the size of a crystalline polythiophene core. The key factors for high efficiency and length control are a highly solubilizing shell and slow polymerization of the core, thereby favoring nucleation elongation over isodesmic growth. Combining kinetics and high-resolution imaging analyses, we propose a unique mechanism called crystallization-driven in situ nanoparticlization of conjugated polymers (CD-INCP) where spontaneous nucleation creates seeds, followed by seeded growth in units of micelles. Also, we achieved "living" CD-INCP through a chain-extension experiment. We further simplified CD-INCP by adding both monomers together in one-shot copolymerization but still producing length-controlled nanoparticles.

Recent Advances in Diversity-Oriented Polymerization Using Cu-Catalyzed Multicomponent Reactions.

Diversification of polymer structures is important for imparting various properties and functions to polymers, so as to realize novel applications of these polymers. In this regard, diversity-oriented polymerization (DOP) is a powerful synthetic strategy for producing diverse and complex polymer structures. Multicomponent polymerization (MCP) is a key method for realizing DOP owing to its combinatorial features and high efficiency. Among the MCP methods, Cu-catalyzed MCP (Cu-MCP) has recently paved the way for DOP by overcoming the synthetic challenges of the previous MCP methods. Here the emergence and progress of Cu-MCP, its current challenges, and future perspectives are discussed.

Powerful Direct C-H Amidation Polymerization Affords Single-Fluorophore-Based White-Light-Emitting Polysulfonamides by Fine-Tuning Hydrogen Bonds.

The development of white-light-emitting polymers has been actively pursued because of the importance of such polymers in various applications, such as lighting sources and displays. To generate white-light, numerous research efforts have focused on synthesizing multifluorophore-based random copolymers to effectively cover the entire visible region. However, due to their intrinsic synthetic and structural features, this strategy has limitations in securing color reproducibility and stability. Herein, we report the development of single-fluorophore-based white-light-emitting homopolymers with excellent color reproducibility. A powerful direct C-H amidation polymerization (DCAP) strategy enabled the synthesis of defect-free polysulfonamides that emit white-light via excited-state intramolecular proton-transfer (ESIPT). To gain structural insights for designing such polymers, we conducted detailed model studies by varying the electronic nature of substituents that allow facile tuning of the emission colors. Further analysis revealed precise control of the thermodynamics of the ESIPT process by fine-tuning the strength of the intramolecular hydrogen bond. By applying this design principle to polymerization, we successfully produced a series of well-defined polysulfonamides with single-fluorophore emitting white-light. The resulting polymers emitted consistent fluorescence, regardless of their molecular weights or phases (i.e., solution, powder, or thin film), guaranteeing excellent color reproducibility. With these advantages in hand, we also demonstrated practical use of our DCAP system by fabricating a white-light-emitting coated LED.

Universal Suzuki-Miyaura Catalyst-Transfer Polymerization for Precision Synthesis of Strong Donor/Acceptor-Based Conjugated Polymers and Their Sequence Engineering.

Catalyst-transfer polymerization has revolutionized the field of polymer synthesis due to its living character, but for a given catalyst system, the polymer scope is rather narrow. Herein we report a highly efficient Suzuki-Miyaura catalyst-transfer polymerization (SCTP) that covers a wide range of monomers from electron-rich (donor, D) to electron-deficient (acceptor, A) (hetero)arenes by rationally designing boronate monomers and using commercially available Buchwald RuPhos and SPhos Pd G3 precatalysts. Initially, we optimized the controlled polymerization of 3,4-propylenedioxythiophene (ProDOT), benzotriazole (BTz), quinoxaline (QX), and 2,3-diphenylquinoxaline (QXPh) by introducing new boronates, such as 4,4,8,8-tetramethyl-1,3,6,2-dioxazaborocane and its N-benzylated derivative, to modulate the reactivity and stability of the monomers. As a result, PProDOT, PBTz, PQX, and PQXPh were prepared with controlled molecular weight and narrow dispersity (D < 1.29) in excellent yield (>85%). A detailed investigation of the polymer structures using 1H NMR and MALDI-TOF spectrometry supported the chain-growth mechanism and the high initiation efficiency of the SCTP method. In addition, the use of RuPhos-Pd showing excellent catalyst-transfer ability on both D/A monomers led to unprecedented controlled D-A statistical copolymerization, thereby modulating the HOMO energy level (from -5.11 to -4.80 eV) and band gap energy (from 1.68 to 1.91 eV) of the resulting copolymers. Moreover, to demonstrate the living nature of SCTP, various combinations of D-A and A-A block copolymers (PBTz-b-PProDOT, PQX-b-PProDOT, and PQX-b-PBTz) were successfully prepared by the sequential addition method. Finally, simple but powerful one-shot D-A block copolymerization was achieved by maximizing the rate difference between a fast-propagating pinacol boronate donor and a slow-propagating acceptor to afford well-defined poly(3-hexylthiophene)-b-poly(benzotriazole).

Mechanochemical Reactivity of Bottlebrush and Dendronized Polymers: Solid vs. Solution States.

We explored the mechanochemical degradation of bottlebrush and dendronized polymers in solution (with ultrasonication, US) and solid states (with ball-mill grinding, BMG). Over 50 polymers were prepared with varying backbone length and arm architecture, composition, and size. With US, we found that bottlebrush and dendronized polymers exhibited consistent backbone scission behavior, which was related to their elongated conformations in solution. Considerably different behavior was observed with BMG, as arm architecture and composition had a significant impact on backbone scission rates. Arm scission was also observed for bottlebrush polymers in both solution and solid states, but only in the solid state for dendronized polymers. Motivated by these results, multi-mechanophore polymers with bottlebrush and dendronized polymer architectures were prepared and their reactivity was compared. Although dendronized polymers showed slower arm-scission, the selectivity for mechanophore activation was much higher. Overall, these results have important implications to the development of new mechanoresponsive materials.

The influence of polymer architecture in polymer mechanochemistry.

Polymer architecture is an important factor in polymer mechanochemistry. In this Feature Article, we summarize recent developments in utilizing polymer architecture to modulate mechanochemical reactions within polymers, or more specifically, the location and rates of bond scission events that lead to polymer fragmentation or mechanophore activation. Various well-defined architectures have been explored, including those of cyclic, intramolecularly cross-linked, dendritic, star, bottlebrush, and dendronized polymers. We primarily focus on describing the enhancement or suppression of mechanochemical reactivity, with respect to analogous linear polymers, as well as differences in solution- and solid-state behavior.

Semi-conducting 2D rectangles with tunable length via uniaxial living crystallization-driven self-assembly of homopolymer.

Semi-conducting two-dimensional (2D) nanoobjects, prepared by self-assembly of conjugated polymers, are promising materials for optoelectronic applications. However, no examples of self-assembled semi-conducting 2D nanosheets whose lengths and aspect ratios are controlled at the same time have been reported. Herein, we successfully prepared uniform semi-conducting 2D sheets using a conjugated poly(cyclopentenylene vinylene) homopolymer and its block copolymer by blending and heating. Using these as 2D seeds, living crystallization-driven self-assembly (CDSA) was achieved by adding the homopolymer as a unimer. Interestingly, unlike typical 2D CDSA examples showing radial growth, this homopolymer assembled only in one direction. Owing to this uniaxial growth, the lengths of the 2D nanosheets could be precisely tuned from 1.5 to 8.8 µm with narrow dispersity according to the unimer-to-seed ratio. We also studied the growth kinetics of the living 2D CDSA and confirmed first-order kinetics. Subsequently, we prepared several 2D block comicelles (BCMs), including penta-BCMs in a one-shot method.

Sugar-Based Polymers from d-Xylose: Living Cascade Polymerization, Tunable Degradation, and Small Molecule Release.

Enyne monomers derived from D-xylose underwent living cascade polymerizations to prepare new polymers with a ring-opened sugar and degradable linkage incorporated into every repeat unit of the backbone. Polymerizations were well-controlled and had living character, which enabled the preparation of high molecular weight polymers with narrow molecular weight dispersity values and a block copolymer. By tuning the type of acid-sensitive linkage (hemi-aminal ether, acetal, or ether functional groups), we could change the degradation profile of the polymer and the identity of the resulting degradation products. For instance, the large difference in degradation rates between hemi-aminal ether and ether-based polymers enabled the sequential degradation of a block copolymer. Furthermore, we exploited the generation of furan-based degradation products, from an acetal-based polymer, to achieve the release of covalently bound reporter molecules upon degradation.