Synthesis and Mechanochemical Activation of Ladderene–Norbornene Block Copolymers

Ring-Opening Metathesis Polymerization of the Dewar Isomer of 1,2-Azaborinine, a B-N Isostere of Benzene

The successful polymerization of the Dewar isomer of an azaborinine heterocycle is reported. Controlled ring-opening metathesis polymerization was accomplished with Grubbs and Hoveya-Grubbs second generation catalysts (G2, HG2), as well as a Z-selective Ru catalyst (HGM2001). The structure of the polymers containing 4-membered B-N heterocycles was verified by GPC and multinuclear and 2D NMR. Differences in stereochemistry of polymers derived from G2/HG2 versus the Z-selective catalyst HGM2001 were substantiated by 2D NOESY, FT-IR, and Raman analyses. The incorporation of B-N heterocycles into these polymer structures is promising as a route to functional polymers that contain polar side groups.

A Metal-Free Cyclobutadiene Reagent for Intermolecular [4 + 2] Cycloadditions

Cyclobutadiene is a highly reactive antiaromatic hydrocarbon that has fascinated chemists for over 60 years. However, its preparation and uses in chemical synthesis are sparing, in part due to its lengthy synthesis that generates hazardous byproducts including excess heavy metals. Herein, we report a scalable, metal-free cyclobutadiene reagent, diethyldiazabicyclohexene dicarboxylate, and explore its intermolecular [4 + 2] cycloaddition with various electron-deficient alkenes. We also demonstrate its utility in a three-step synthesis of dipiperamide G and a diverse array of product derivatizations including bromocyclobutadiene.

Ring-opening metathesis polymerization of ester-functionalized endo-tricyclo[4.2.2.02,5]deca-3,9-dienes and thermal properties of the resulting polymers

We investigated the ROMP of 1 and 2, and revealed the glass transition temperature and pyrolysis mechanisms of the resulting (co)polymers (with norbornene).

Computational Study of Mechanochemical Activation in Nanostructured Triblock Copolymers

Force-driven chemical reactions have emerged as an attractive platform for diverse applications in polymeric materials. However, the microscopic chain conformations and topologies necessary for efficiently transducing macroscopic forces to the molecular scale are not well-understood. In this work, we use a coarse-grained model to investigate the impact of network-like topologies on mechanochemical activation in self-assembled triblock copolymers. We find that mechanochemical activation during tensile deformation depends strongly on both the polymer composition and chain conformation in these materials. Activation primarily occurs in the tie chains connecting different glassy domains and in loop chains that are hooked onto each other by physical entanglements. Activation also requires a higher stress in materials having a higher glassy block content. Overall, the lamellar samples show the highest percent activation at high stress. In contrast, at low stress, the spherical morphology, which has the lowest glassy fraction, shows the highest activation. Additionally, we observe a spatial pattern of activation, which appears to be tied to distortion of the self-assembled morphology. Higher activation is observed in the tips of the chevrons formed during deformation of lamellar samples as well as in the centers between the cylinders in the cylindrical morphology. Our work shows that changes in the network-like topology in different morphologies significantly impact mechanochemical activation efficiencies in these materials, suggesting that this area will be a fruitful avenue for further experimental research.

Ultrasound triggered organic mechanoluminescence materials

Ultrasound induced organic mechanoluminescence materials have become one of the focal topics in wireless light sources since they exhibit high spatiotemporal resolution, biocompatibility and excellent tissue penetration depth. These properties promote great potential in ultrahigh sensitive bioimaging with no background noise and noninvasive nanodevices. Recent advances in chemistry, nanotechnology and biomedical research are revolutionizing ultrasound induced organic mechanoluminescence. Herein, we try to summarize some recent researches in ultrasound induced mechanoluminescence that use various materials design strategies based on the molecular conformational changes and cycloreversion reaction. Practical applications, like noninvasive bioimaging and noninvasive optogenetics, are also presented and prospected.

The Mechanochemical Synthesis and Activation of Carbon-Rich π-Conjugated Materials

Mechanochemistry uses mechanical force to break, form, and manipulate chemical bonds to achieve functional transformations and syntheses. Over the last years, many innovative applications of mechanochemistry have been developed. Specifically for the synthesis and activation of carbon-rich π-conjugated materials, mechanochemistry offers reaction pathways that either are inaccessible with other stimuli, such as light and heat, or improve reaction yields, energy consumption, and substrate scope. Therefore, this review summarizes the recent advances in this research field combining the viewpoints of polymer and trituration mechanochemistry. The highlighted mechanochemical transformations include π-conjugated materials as optical force probes, the force-induced release of small dye molecules, and the mechanochemical synthesis of polyacetylene, carbon allotropes, and other π-conjugated materials.

A Single Functionalization Agent for Heterotelechelic ROMP Polymers

Heterotelechelic polymers are an important class of materials finding applications in bioconjugation, imaging, sensing, and synthesis of organic/inorganic hybrid systems with interesting features. However, the synthesis of such polymers is challenging. Here, we report a mechanistically unique and most efficient method based on a single functionalization agent to prepare heterotelechelic polymers by a ring-opening metathesis polymerization. Different functionalization agents can be synthesized in one simple step from inexpensive commercial starting materials. The functionalization agents initially generate a functional initiator from commercial Grubbs' first-generation ruthenium benzylidene catalyst. During this process, a functional dihydrofuran derivative is produced. After functional initiation and propagation of a suitable monomer, the dihydrofuran derivative functionally terminates the polymerization yielding a primary alcohol-terminated heterotelechelic polymer. Molecular weight control is achieved by varying the ratio between monomer and Grubbs' first-generation catalyst. This method may emerge as a popular choice to prepare heterotelechelic polymers due to its simplicity and efficiency.

Mechanochromic polymers with a multimodal chromic transition: mechanophore design and transduction mechanism

This review presents the recent progress in multi-chromic polymers embedded with mechanophores concentrating on transduction mechanisms and design concepts.

Heterocyclic Mechanophores in Polymer Mechanochemistry

This Account covers the recent progress made on heterocyclic mechanophores in the field of polymer mechanochemistry. In particular, the types of such mechanophores as well as the mechanisms and applications of their force-induced structural transformations are discussed and related perspectives and future challenges proposed.

1 Introduction

2 Types of Mechanophores

3 Methods to Incorporate Heterocycle Mechanophores into Polymer Systems

4 Mechanochemical Reactions of Heterocyclic Mechanophores

4.1 Three-Membered-Ring Mechanophores

4.2 Four-Membered-Ring Mechanophores

4.3 Six-Membered-Ring Mechanophores

4.4 Bicyclic Mechanophores

5 Applications

5.1 Cross-Linking of Polymer

5.2 Degradable Polymer

5.3 Mechanochromic Polymer

6 Concluding Remarks and Outlook

Understanding the Mechanochemistry of Ladder-Type Cyclobutane Mechanophores by Single Molecule Force Spectroscopy

We have recently reported a series of ladder-type cyclobutane mechanophores, polymers of which can transform from nonconjugated structures to conjugated structures and change many properties at once. These multicyclic mechanophores, namely, exo-ladderane/ene, endo-benzoladderene, and exo-bicyclohexene-peri-naphthalene, have different ring structures fused to the first cyclobutane, significantly different free energy changes for ring-opening, and different stereochemistry. To better understand their mechanochemistry, we used single molecule force spectroscopy (SMFS) to characterize their force-extension behavior and measure the threshold forces. The threshold forces correlate with the activation energy of the first bond, but not with the strain of the fused rings distal to the polymer main chain, suggesting that the activation of these ladder-type mechanophores occurs with similar early transition states, which is supported by force-modified potential energy surface calculations. We further determined the stereochemistry of the mechanically generated dienes and observed significant and variable contour length elongation for these mechanophores both experimentally and computationally. The fundamental understanding of ladder-type mechanophores will facilitate future design of multicyclic mechanophores with amplified force-response and their applications as mechanically responsive materials.

Mechanochemical generation of acid-degradable poly(enol ether)s

In an effort to develop polymers that can undergo extensive backbone degradation in response to mechanical stress, we report a polymer system that is hydrolytically stable but unmasks easily hydrolysable enol ether backbone linkages when force is applied. These polymers were synthesized by ring-opening metathesis polymerization (ROMP) of a novel mechanophore monomer consisting of cyclic ether fused bicyclohexene. Hydrogenation of the resulting polymers led to significantly enhanced thermal stability (T_d > 400 °C) and excellent resistance toward acidic or basic conditions. Solution ultrasonication of the polymers resulted in up to 65% activation of the mechanophore units and conversion to backbone enol ether linkages, which then allowed facile degradation of the polymers to generate small molecule or oligomeric species under mildly acidic conditions. We also achieved solid-state mechano-activation and polymer degradation via grinding the solid polymer. Force-induced hydrolytic polymer degradability can enable materials that are stable under force-free conditions but readily degrade under stress. Facile degradation of mechanically activated polymechanophores also facilitates the analysis of mechanochemical products.

A mechanically responsive polymer system that is hydrolytically stable without stress, but unmasks enol ether backbone linkages under force to allow facile hydrolytic degradation.

Iron-Catalyzed Vinylsilane Dimerization and Cross-Cycloadditions with 1,3-Dienes: Probing the Origins of Chemo- and Regioselectivity

The selective, intermolecular, homodimerization and cross-cycloaddition of vinylsilanes with unbiased 1,3-dienes, catalyzed by a pyridine-2,6-diimine (PDI) iron complex is described. In the absence of a diene coupling partner, vinylsilane hydroalkenylation products were obtained chemoselectively with unusual head-to-head regioselectivity (up to >98% purity, 98:2 E/Z). In the presence of a 4- or 2-substituted diene coupling partner, under otherwise identical reaction conditions, formation of value-added [2+2]- and [4+2]-cycloadducts, respectively, was observed. The chemoselectivity profile was distinct from that observed for analogous α-olefin dimerization and cross-reactions with 1,3-dienes. Mechanistic studies conducted with well-defined, single-component precatalysts (MePDI)Fe(L2) (where MePDI = 2,6-(2,6-Me2-C6H3N═CMe)2C5H3N; L2 = butadiene or 2(N2)) provided insights into the kinetic and thermodynamic factors contributing to the substrate-controlled regioselectivity for both the homodimerization and cross cycloadditions. Diamagnetic iron diene and paramagnetic iron olefin complexes were identified as catalyst resting states, were characterized by in situ NMR and Mössbauer spectroscopic studies, and were corroborated with DFT calculations. Stoichiometric reactions and computational models provided evidence for a common mechanistic regime where competing steric and orbital-symmetry requirements dictate the regioselectivity of oxidative cyclization. Although distinct chemoselectivity profiles were observed in cross-cycloadditions with the vinylsilane congeners of α-olefins, these products arose from metallacycles with the same connectivity. The silyl substituents ultimately governed the relative rates of β-H elimination and C-C reductive elimination to dictate final product formation.

Perspectives from nearly five decades of total synthesis of natural products and their analogues for biology and medicine

Covering: 1970 to 2020By definition total synthesis is the art and science of making the molecules of living Nature in the laboratory, and by extension, their analogues. Although obvious, its application to the synthesis of molecules for biology and medicine was not always the purpose of total synthesis. In recent years, however, the field has acquired momentum as its power to reach higher molecular complexity and diversity is increasing, and as the demand for rare bioactive natural products and their analogues is expanding due to their recognised potential to facilitate biology and drug discovery and development. Today this component of total synthesis endeavors is considered highly desirable, and could be part of interdisciplinary academic and/or industrial partnerships, providing further inspiration and momentum to the field. In this review we provide a brief historical background of the emergence of the field of total synthesis as it relates to making molecules for biology and medicine. We then discuss specific examples of this practice from our laboratories as they developed over the years. The review ends with a conclusion and future perspectives for natural products chemistry and its applications to biology and medicine and other added-value contributions to science and society.

Enhanced polymer mechanical degradation through mechanochemically unveiled lactonization

The mechanical degradation of polymers is typically limited to a single chain scission per triggering chain stretching event, and the loss of stress transfer that results from the scission limits the extent of degradation that can be achieved. Here, we report that the mechanically triggered ring-opening of a [4.2.0]bicyclooctene (BCOE) mechanophore sets up a delayed, force-free cascade lactonization that results in chain scission. Delayed chain scission allows many eventual scission events to be initiated within a single polymer chain. Ultrasonication of a 120 kDa BCOE copolymer mechanically remodels the polymer backbone, and subsequent lactonization slowly (~days) degrades the molecular weight to 4.4 kDa, > 10× smaller than control polymers in which lactonization is blocked. The force-coupled kinetics of ring-opening are probed by single molecule force spectroscopy, and mechanical degradation in the bulk is demonstrated. Delayed scission offers a strategy to enhanced mechanical degradation and programmed obsolescence in structural polymeric materials.

Controlled Cyclopolymerization of 1,5-Hexadiynes to Give Narrow Band Gap Conjugated Polyacetylenes Containing Highly Strained Cyclobutenes

For decades, cyclopolymerization of α,ω-diyne derivatives has been an effective method to synthesize various soluble polyacetylenes containing five- to seven-membered rings in the backbone. However, cyclopolymerization to form four-membered carbocycles was considered impossible due to their exceptionally high ring strain (∼30 kcal/mol). Herein, we demonstrate the successful cyclopolymerization of rationally designed 1,5-hexadiyne derivatives to afford various polyacetylenes containing highly strained cyclobutenes in each repeat unit. After screening, Ru catalysts containing bulky diisopropylphenyl groups promoted challenging four-membered ring cyclization efficiently from various monomers, enabling the synthesis of high molecular weight (up to 40 kDa) polyacetylenes in a controlled manner. Furthermore, living polymerization allowed for block copolymer synthesis by combining with ring-opening metathesis polymerization as well as block copolymerization of two different 1,5-hexadiyne monomers to give a fully conjugated polyacetylene. These new polymers unexpectedly showed much narrower band gaps than conventional substituted polyacetylenes by >0.2 eV. Interestingly, computational studies showed much smaller bond length alternation in the conjugated backbone containing cyclobutenes, resulting in highly delocalized π electrons along the polymer chain and lower band gaps.

Force-Induced Cycloaddition of Aziridine: Can We Force a New Route?

Cycloaddition reactions of aziridines with dipolarophiles under traditional thermal or photochemical conditions entail destructive routes to form reactive intermediates such as an azomethine ylide. This article highlights a recent study that demonstrates a cycloaddition reaction of aziridine induced by mechanical force. Experimental results suggest that the force-induced cycloaddition of aziridine with dimethyl acetylenedicarboxylate as a dipolarophile does not seem to involve an ylide, with implications for a possible new reaction route.

1 Rivalry between Aziridine and Epoxide

2 Mechanochemically Responsive Polymers

3 Aziridine Mechanophore

4 Concluding Remarks and Outlook

Bicyclohexene-peri-naphthalenes: Scalable Synthesis, Diverse Functionalization, Efficient Polymerization, and Facile Mechanoactivation of Their Polymers

Pursuing polymers that can transform from a nonconjugated to a conjugated state under mechanical stress to significantly change their properties, we developed a new generation of ladder-type mechanophore monomers, bicyclo[2.2.0]hex-5-ene-peri-naphthalene (BCH-Naph), that can be directly and efficiently polymerized by ring-opening metathesis polymerization (ROMP). BCH-Naphs can be synthesized in multigram quantities and functionalized with a wide range of electron-rich and electron-poor substituents, allowing tuning of the optoelectronic and physical properties of mechanically generated conjugated polymers. Efficient ROMP of BCH-Naphs yielded ultrahigh molecular weight polymechanophores with controlled MWs and low dispersity. The resulting poly(BCH-Naph)s can be mechanically activated into conjugated polymers using ultrasonication, grinding, and even simple stirring of the dilute solutions, leading to changes in absorption and fluorescence. Poly(BCH-Naph)s represent an attractive polymechanophore system to explore multifaceted mechanical response in solution and solid states, owing to the synthetic scalability, functional diversity, efficient polymerization, and facile mechanoactivation.

A Polymer with "Locked" Degradability: Superior Backbone Stability and Accessible Degradability Enabled by Mechanophore Installation

Though numerous applications require degradable polymers, there are surprisingly few polymer systems that combine superior stability and controllable degradability. Particularly, the degradability of a conventional degradable polymer is typically enabled by cleavable groups on the backbone, which can be attacked by stimuli in ambient conditions, causing undesirable material deterioration. Here we report a general strategy to overcome this issue: "locking" the degradability during handling and use of the polymers and "unlocking" it when degradation is needed. This strategy is demonstrated with a cyclobutane-fused lactone (CBL) polymer. The cyclobutane keeps polymer backbone intact under conditions that hydrolyze the lactone and allows the ester group to be recovered when undesirable hydrolysis occurs. When backbone degradation is needed, the degradability can be unlocked by mechanochemical activation that converts the polyCBL into a linear polyester. The rare combination of two intrinsically conflicting properties, i.e., backbone stability and accessible degradability, can make this polymer a potential option for new sustainable materials.

Polymer mechanochemistry-enabled pericyclic reactions

Polymer mechanochemical pericyclic reactions are reviewed with regard to their structural features and substitution prerequisites to the polymer framework.

ROMPI-CDSA: ring-opening metathesis polymerization-induced crystallization-driven self-assembly of metallo-block copolymers

Polymerization-induced self-assembly (PISA) and crystallization-driven self-assembly (CDSA) are among the most prevailing methods for block copolymer self-assembly. Taking the merits of scalability of PISA and dimension control of CDSA, we report one-pot synchronous PISA and CDSA via ring-opening metathesis polymerization (ROMP) to prepare nano-objects based on a crystalline poly(ruthenocene) motif. We denote this self-assembly methodology as ROMPI-CDSA to enable a simple, yet robust approach for the preparation of functional nanomaterials.

Flow-facilitated ring opening metathesis polymerization (ROMP) and post-polymerization modification reactions

Continuous flow facilitates ROMP reactions to prepare homopolymers and block copolymers and allows for in-line post-polymerization click modifications.