Cascade Polymerization via Controlled Tandem Olefin Metathesis/Metallotropic 1,3-Shift Reactions for the Synthesis of Fully Conjugated Polyenynes

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.

Conjugated [5]Cumulene Polymers Enabled by Condensation Polymerization of Propargylic Electrophiles

Cumulenes, sp-hybridized carbon motifs featuring consecutive double bonds, have rarely been explored as π-elements for conjugated polymers. Long cumulenic conjugated polymers can serve as models for approaching carbyne, an intriguing yet elusive carbon allotrope. However, their synthesis is notoriously difficult due to intrinsic instability. To date, only few [3]cumulene-based polymers have been synthesized, mostly relying on surface chemistry. Higher cumulene-based polymers remain unknown. Here, we present a "meet in the middle" strategy to overcome this challenge and synthesize high-molecular-weight, stable, and solution-processable conjugated [5]cumulene polymers (Mw up to 67.9 kg/mol). Our approach involves a new polymerization method called step-growth condensation polymerization of propargylic electrophiles (step-growth CPPE). The structures and molecular weights of the cumulenic polymers are established by various spectroscopic methods, including a comparative analysis of a discrete oligomer series. By introducing ortho-substituents on the aryl side groups, we successfully address the stability-conjugation dilemma. Electronic communication between cumulene units is found to be contingent upon the aromaticity of the π-spacers, enabling flexible energy-level adjustment and new narrow band gap polymers. The synthetic methodology and structure-property relationship established in this work serve as the starting points for the exploration of this fascinating family of sp-carbon-rich materials.

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.

Alkyne Polymers from Stable Butatriene Homologues: Controlled Radical Polymerization of Vinylidenecyclopropanes

Controlled polymerization of cumulenic monomers represents a promising yet underdeveloped strategy toward well-defined alkyne polymers. Here we report a stereoelectronic effect-inspired approach using simple vinylidenecyclopropanes (VDCPs) as butatriene homologues in controlled radical ring-opening polymerizations. While being thermally stable, VDCPs mimic butatrienes via conjugation of the cyclopropane ring. This leads to exclusive terminal-selective propagation that affords a highly structurally regular alkyne-based backbone, featuring complete ring-opening and no backbiting regardless of polymerization conditions.

Practical Route for Catalytic Ring-Opening Metathesis Polymerization

Norbornene derivatives are typical monomers for ring-opening metathesis polymerization (ROMP) for synthesizing highly functional polymers. However, the lack of catalytic methods, that is, the lack of readily available chain transfer agents (CTAs) for these monomers has been a significant cost limitation when large-scale syntheses are required. Here, we report commercially available styrene and its derivatives as efficient regioselective CTAs for the catalytic synthesis of metathesis polymers requiring up to 1000 times less ruthenium than in classical ROMP experiments. The molecular weight of the synthesized polymers was controlled by the monomer-to-CTA ratio. Low molecular weight ROMP polymers known for their antimicrobial properties were also synthesized on a gram scale in this report. Polymers were characterized by SEC, ¹H NMR spectroscopy, and isotopically resolved MALDI-TOF MS. This approach describes a greener, more cost-effective, and eco-friendly methodology for the preparation of metathesis-based materials on the multigram scale.

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.

Controlled Alternating Metathesis Copolymerization of Terminal Alkynes

Terminal alkynes display high reactivity toward Ru-carbene metathesis catalysts. However, the formation of a less reactive bulky carbene hinders their homopolymerization. Simultaneously, the higher reactivity of alkynes does not allow efficient cross propagation with sterically less-hindered cycloalkene monomers, resulting in inefficient copolymerization. Nonetheless, terminal alkynes undergo rapid cross-metathesis with vinyl ethers. Therefore, an efficient cross propagation can be achieved with terminal alkynes and cyclic enol ether monomers. Here, we show that terminal alkyne derivatives can be copolymerized in an alternating fashion with 2,3-dihydrofuran using Grubbs' third generation catalyst (G3). A linear relationship of the number-average molecular weight versus monomer to initiator ratio and block copolymer synthesis confirmed a controlled copolymerization. The SEC and NMR analyses of the synthesized copolymers confirmed the excellent control over molecular weight and exclusive alternating nature of the copolymer. The regioselective chain transfer of G3 to vinyl ether and the high reactivity of the Fischer-type Ru carbene toward terminal alkynes was also exploited for polymer conjugation. Finally, the presence of an acid labile backbone functionality in the synthesized alternating copolymers allowed complete degradation of the copolymer within a short time interval which was confirmed by SEC analyses.

Synthesis of Polydiynes via an Unexpected Dimerization/Polymerization Sequence of C3 Propargylic Electrophiles

Here, we describe the unexpected discovery of a Cu-catalyzed condensation polymerization reaction of propargylic electrophiles (CPPE) that transforms simple C3 building blocks into polydiynes of C6 repeating units. This reaction was achieved by a simple system composed of a copper acetylide initiator and an electron-rich phosphine ligand. Alkyne polymers (up to 33.8 kg/mol) were produced in good yields and exclusive regioselectivity with high functional group compatibility. Hydrogenation of the product afforded a new polyolefin-type backbone, while base-mediated isomerization led to a new type of dienyne-based electron-deficient conjugated polymer. Mechanistic studies revealed a new α-α selective Cu-catalyzed dimerization pathway of the C3 unit, followed by in situ organocopper-mediated chain-growth propagation. These insights not only provide an important understanding of the Cu-catalyzed CPPE of C3, C4, and C6 monomers in general but also lead to a significantly improved synthesis of polydiynes from simpler starting materials with handles for the incorporation of an α-end functional group.

Selective Ring-Opening Allene Metathesis: Polymerization or Ruthenium Vinylidene Formation

Selective ring-opening allene metathesis polymerization (ROAlMP) and ruthenium vinylidene formation from 1,2-cyclononadiene (1) by simple catalyst selection are discussed. Grubbs second-generation catalyst (G2) favors the formation of an alkylidene leading to the ROAlMP of (1). Grubbs first-generation catalyst (G1) favors vinylidene formation and prevents the homopolymerization of (1) even at elevated temperatures. Isolation and characterization of poly(1) by NMR analysis and MALDI-TOF confirms the generation of a well-defined polyallene that exhibits good thermal stability (T_D ca. 390 °C) and fluorescent properties. Ring-opening metathesis polymerization (ROMP) of a highly strained norbornene derivative (NBE-ⁱPr) at 80 °C using the ruthenium vinylidene generated from (1) is also investigated. The discovery of ROAlMP allows for the simple access of well-defined polyallenes from commercially available catalysts and will ultimately guide structure-property determinations of polyallenes.

Ruthenabenzene: A Robust Precatalyst

Metallaaromatics constitute a unique class of aromatic compounds where one or more transition metal elements are incorporated into the aromatic system, the parent of which is metallabenzene. One of the main concerns about metallabenzenes generally deals with the structural characterization related to their relative aromaticity compared to the carbon archetype. Transition metal-containing metallabenzenes are also implicated in certain catalytic processes such as alkyne metathesis polymerization; however, these transition metal-based metallaaromatic compounds have not been developed as a catalyst. Herein, we describe an effective strategy to generate diverse arrays of ruthenabenzenes and demonstrated them as an aromatic equivalent of the Grubbs-type ruthenium alkylidene catalysts. These ruthenabenzenes can be prepared via an enyne metathesis and metallotropic [1,3]-shift cascade process to form alkyne-chelated ruthenium alkylidene intermediates followed by spontaneous cycloaromatization. The aromatic nature of these complexes was confirmed by spectroscopic and X-ray crystallographic data, and the mechanistic pathways for the cycloaromatization process were studied by DFT calculations. These ruthenabenzenes display robust catalytic activity for metathesis and other transformations, which illustrates that metallabenzenes are not only compounds of structural and theoretical interests but also are a novel platform for new catalyst development.

Cascade Ring-Opening/Ring-Closing Metathesis Polymerization of a Monomer Containing a Norbornene and a Cyclohexene Ring

Norbornene is polymerized extremely fast when reacted with Grubbs' first (G1) or third generation catalyst (G3) because of its very high ring strain energy. Cyclohexene, on the other hand, cannot be polymerized using G1 or G3 due to its very low ring strain energy. Subsequently, the sequence-selective polymerization of these two monomers is extremely challenging. A sequence-selective cascade ring-opening/ring-closing metathesis polymerization of the monomer M containing both the norbornene and the cyclohexene ring using G1 or G3 is reported. The polymer structure was analyzed by ¹H NMR, ¹H-¹H COSY, and ¹H-¹H ROESY spectroscopy and MALDI-ToF mass spectrometry. Polymers with moderate molecular weight dispersities and good molecular weight control were achieved by varying the ratio between monomer M and G1.

Recent progress in the construction of polymers with advanced chain structures via hybrid, switchable, and cascade chain-growth polymerizations

Recent progress of hybrid, switchable, and cascade chain-growth polymerizations for the preparation of polymers with advanced chain structures with diverse compositions has been summarized.

Cascade polymerizations: recent developments in the formation of polymer repeat units by cascade reactions

Traditionally, most polymerizations rely on simple reactions such as alkene addition, ring-opening, and condensation because they are robust, highly efficient, and selective. These reactions, however, generally only yield a single new C-C or C-O bond during each propagation step. In recent years, novel macromolecules have been prepared with propagation steps that involve cascade reactions, enabling various combinations of bond making and breaking steps to form more complex repeat units. These polymerizations are often challenging, given the requirements for high conversion and selectivity in controlled polymerizations, yet they provide polymers with unique chemical structures and significantly broaden the scope of how polymers can be made. In this perspective, we summarize the recent developments in cascade polymerizations, primarily focusing on single-component cascades (rather than multi-component polymerizations). Polymerization performance, monomer scope, and mechanisms are discussed for polymerizations utilizing radical, ionic, and metathesis-based mechanisms.

The Introduction of the Radical Cascade Reaction into Polymer Chemistry: A One-Step Strategy for Synchronized Polymerization and Modification

Polymerization and modification play central roles in polymer chemistry and are generally implemented in two steps, which suffer from the time-consuming two-step strategy and present considerable challenge for complete modification. By introducing the radical cascade reaction (RCR) into polymer chemistry, a one-step strategy is demonstrated to achieve synchronized polymerization and complete modification in situ. Attributed to the cascade feature of iron-catalyzed three-component alkene carboazidation RCR exhibiting carbon-carbon bond formation and carbon-azide bond formation with extremely high efficiency and selectivity in one step, radical cascade polymerization therefore enables the in situ synchronized polymerization through continuous carbon-carbon bond formation and complete modification through carbon-azide bond formation simultaneously. This results in a series of α, β, and γ poly(amino acid) precursors. This result not only expands the methodology library of polymerization, but also the possibility for polymer science to achieve functional polymers with tailored chemical functionality from in situ polymerization.

Synthesis of Conjugated Polyenynes with Alternating Six- and Five-Membered Rings via β-Selective Cascade Metathesis and Metallotropy Polymerization

Cascade metathesis and metallotropy (M&M) polymerization, which involves sequential olefin metathesis and metallotropic 1,3-shift reactions specifically from multiyne monomers, is the only method reported so far to prepare conjugated polyenynes via the chain-growth mechanism. Using this method, various conjugated polyenynes containing cyclopentene units in the backbone could be synthesized via exclusive α-addition by using the third-generation Grubbs catalyst. Herein, we demonstrate the complete switch of regioselectivity toward β-addition using a Ru carbene containing a dithiolate ligand, and thus, synthesized unique conjugated polyenynes having alternating cyclohexene and cyclopentene units in the backbone. Furthermore, detailed in situ NMR studies revealed an interesting phenomenon that the adjacent triple bond strongly chelates to the propagating Ru carbene during the polymerization.

Resonance promoted ring-opening metathesis polymerization of twisted amides

The living ring-opening metathesis polymerization (ROMP) of an unsaturated twisted amide using the third-generation Grubbs initiator is described. Unlike prior examples of ROMP monomers that rely on angular or steric strain for propagation, this system is driven by resonance destabilization of the amide that arises from geometric constraints of the bicyclic framework. Upon ring-opening, the amide can rotate and rehybridize to give a stabilized and planar conjugated system that promotes living propagation. The absence of other strain elements in the twisted amide is supported by the inability of a carbon analogue of the monomer to polymerize and computational studies that find resonance destabilization accounts for 11.3 kcal mol-1 of the overall 12.0 kcal mol-1 ring strain. The twisted amide polymerization is capable of preparing high molecular weight polymers rapidly at room temperature, and post-polymerization modification combined with 2D NMR spectroscopy confirms a regioirregular polymer microstructure.

Synthesis of Functional Polyacetylenes via Cyclopolymerization of Diyne Monomers with Grubbs-type Catalysts

Metathesis cyclopolymerization (CP) of α,ω-diynes is a powerful method to prepare functional polyacetylenes (PAs). PAs have long been studied due to their interesting electrical, optical, photonic, and magnetic properties which make them candidates for use in various advanced applications. Grubbs catalysts are widely used throughout synthetic chemistry, largely due to their accessibility, high reactivity, and tolerance to air, moisture, and many functional groups. Prior to our entrance into this field, only a few examples of CP using modified Grubbs catalysts existed. Inspired by these works, we saw an opportunity to expand the accessibility and utility of Grubbs-catalyzed CPs. We began by exploring CP with popular and commercially available Grubbs catalysts. We found Grubbs third-generation catalyst (G3) to be an excellent catalyst when we used strategies to stabilize the propagating Ru carbene, such as decreasing the polymerization temperature or using weakly coordinating solvent or ligands. Controlled living polymerizations were demonstrated using various 1,6-heptadiyne monomers and yielded polymers with exclusively 5-membered rings (via α-addition) in the polymer backbone. The strategy of stabilizing the Ru carbene was also critical to successful CP with Hoveyda-Grubbs second-generation (HG2) and Grubbs first-generation (G1) catalysts. We found that decomposed Ru species were catalyzing side reactions which could be completely shut down by decreasing the reaction temperature or using weakly coordinating ligands. While HG2 generally led to uncontrolled polymerizations, we found it to be an effective catalyst for monomers with very large side chains. G1 displayed broader functional group tolerance and thus broader monomer scope than G3. We next looked at our ability to change the regioselectivity of the polymerization by using Z-selective catalysts which favor β-addition and the formation of 6-membered rings in the polymer backbone. While modest β-selectivity could be obtained using Grubbs Z-selective catalyst at low temperatures, we found that by using one of Hoveyda and co-workers' catalysts with decreased carbene electrophilicity, we could achieve exclusive formation of 6-membered rings. We also pursued alternative routes to achieve 6+-membered rings in the polymer backbone by using diyne monomers with increased distance between alkynes. We found that optimizing the monomer structure for CP was an effective strategy to achieve controlled polymerizations. By using bulky substituents (maximizing the Thorpe-Ingold effect) and/or using heteroatoms (shorter bonds) to bring the alkynes closer together, controlled living CP could be achieved with various 1,7-octadiyne and 1,8-nonadiyne monomers. Finally, we took advantage of several inherent properties of controlled CP techniques to prepare polymers with advanced architectures and nanostructures. For instance, the living nature of the polymerization enabled production of block copolymers, the tolerance of very large substituents enabled production of dendronized and brush polymers, and the insolubility or crystallinity of some monomers was utilized for the spontaneous self-assembly of polymers into various one- and two-dimensional nanostructures. Overall, the strategies of stabilizing the propagating Ru carbene, modulating the selectivity and reactivity of the Ru carbene, and enhancing the inherent reactivity of monomers were key to improving the utility and performance of CP with Grubbs-type catalysts. The insight provided by these studies will be important for future developments of CP and other metathesis polymerizations utilizing ring-closing steps.

Living Metathesis and Metallotropy Polymerization Gives Conjugated Polyenynes from Multialkynes: How to Design Sequence-Specific Cascades for Polymers

On the basis of a combined experimental and computational study, a novel method for preparing fully conjugated polyenynes via cascade metathesis and metallotropy (M&M) polymerization of various multialkynes is developed. DFT calculations elucidate the detailed mechanism of the metallotropic 1,3-shift, which is a key process of M&M polymerization. An α,β-(C,C,C)-agostic interaction stabilizing the metallacyclobutadiene transition state is found to be critically important for the successful polymerization with excellent specificity. The polymerization efficiency displayed by the tetrayne monomer is controlled by the steric demands of its substituents, and more complex hexayne monomers can be successfully polymerized to give access to highly conjugated polyenynes via a series of intramolecular metathesis and metallotropic shift cascade reactions. Furthermore, living polymerization led to the synthesis of block copolymers consisting of fully conjugated polyenyne backbones. The implementation of pentayne monomers provides polyenynes with successive C-C triple bonds via consecutive metallotropic 1,3-shift. In short, the design of multialkynes enables the preparation of diverse conjugated polyenyne motifs via selective M&M cascade reactions.

Cyclopolymerizations: Synthetic Tools for the Precision Synthesis of Macromolecular Architectures

Monomers possessing two functionalities suitable for polymerization are often designed and utilized in syntheses directed to the formation of cross-linked macromolecules. In this review, we give an account of recent developments related to the use of such monomers in cyclopolymerization processes, in order to form linear, soluble macromolecules. These processes can be activated by means of radical, ionic, or transition-metal mediated chain-growth polymerization mechanisms, to achieve cyclic moieties of variable ring size which are embedded within the polymer backbone, driving and tuning peculiar physical properties of the resulting macromolecules. The two functionalities are covalently linked by a "tether", which can be appropriately designed in order to "imprint" elements of chemical information into the polymer backbone during the synthesis and, in some cases, be removed by postpolymerization reactions. The two functionalities can possess identical or even very different reactivities toward the polymerization mechanism involved; in the latter case, consequences and outcomes related to the sequence-controlled, precision synthesis of macromolecules have been demonstrated. Recent advances in new initiating systems and polymerization catalysts enabled the precision syntheses of polymers with regulated cyclic structures by highly regio- and/or stereoselective cyclopolymerization. Cyclopolymerizations involving double cyclization, ring-opening, or isomerization have been also developed, generating unique repeating structures, which can hardly be obtained by conventional polymerization methods.

Unusual Superior Activity of the First Generation Grubbs Catalyst in Cascade Olefin Metathesis Polymerization

Recently, we reported a new cascade ring-opening/closing metathesis polymerization of monomers containing two cyclopentene moieties. Several Ru catalysts were tested, but the best polymerization results were unexpectedly obtained using the first-generation Grubbs catalyst (G1). This was puzzling since the second- and third-generation Grubbs catalysts are well-known for their higher activities compared to G1. In order to explain the unique and superior activity of G1, we conducted a series of kinetics experiments for the polymerization of 3,3'-oxydicyclopent-1-ene, a representative monomer of this cascade polymerization, as well as the competition polymerization with cycloheptene using the various Grubbs catalysts. Based on our results, we propose a model in which the differences in the steric hindrance between the different ligands and the monomer determine the selectivity of the catalyst approach to the monomer and, therefore, the extent to which the productive pathway leads to successful cascade polymerization. In short, G1 with the smaller ligand showed a high preference for the productive pathway.