Double Tethered Metallacyclobutane Catalyst for Cyclic Polymer Synthesis

Concentration-Driven Ring Expansion Metathesis Polymerization via Tunable Ring Transfer Processes

Ring expansion metathesis polymerization (REMP) is a robust and versatile method used to access polymeric cyclic architectures for applications in biomedicine, electronics, and performance engineering. Cyclic Ru-benzylidene REMP catalyst CB6 demonstrates higher stability and polymerization rates compared to other Ru-based systems. However, CB6 also exhibits an unusual molar mass evolution profile where high-molar-mass cyclic polymers are produced at early time points followed by a gradual decrease in molar mass. For broad cyclic polymer applications to be fully realized, a mechanistic understanding of REMP is crucial. In this work, we investigate the polymerization profiles of CB6 using a series of mechanistic studies to probe the requisite ring transfer steps envisaged for such a polymerization profile. Furthermore, our studies reveal an intricate relationship between reaction concentration and experimental molar mass. These collective studies demonstrate CB6's role not only as an initiator but also as a catalytic ring transfer agent. Overall, we showcase a new toolkit by which to control REMP that will allow further optimization of catalyst design and the creation of novel cyclic materials.

Ring-Expansion Metathesis Polymerization under Confinement

The cationic molybdenum alkylidyne N-heterocyclic carbene (NHC) complex [Mo(C-p-OMeC6H4)(OCMe(CF3)2)2 (IMes)][B(ArF4] (IMes = 1,3-dimesitylimidazol-2-ylidene) was selectively immobilized inside the pores of ordered mesoporous silica (OMS) with pore diameters of 66, 56, and 28 Å and used in the ring-expansion metathesis polymerization (REMP) of cyclic olefins to yield cyclic polymers. A strong confinement effect was observed for cis-cyclooctene (cCOE), 1,5-cyclooctadiene (COD), (+)-2,3-endo,exo-dicarbomethoxynorborn-5-ene ((+)-DCMNBE), and 2-methyl-2-phenylcycloprop-1-ene (MPCP), allowing for the synthesis of low-molecular-weight cyclic polymers even at a high monomer concentration. The exclusive formation of cyclic polymers was demonstrated by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry. Confinement also influences stereoselectivity, resulting in a pronounced increase in Z-selectivity and in an increased cis-syndiospecificity.

A Pseudo-Block Copolymerization Access to Cyclic Alternating Copolymers through Segment-Selective Transesterification

Efficient synthesis of cyclic polymers remains a frontier challenge. We report here that macromolecular transesterification during a pseudoblock copolymerization process can be utilized for such a purpose. Organobase-catalyzed ring-opening alternating copolymerization of 3,4-dihydrocoumarin and epoxide is conducted with four-armed poly(ethylene oxide) (PEO) as a macroinitiator. Intramolecular transesterification (backbiting) occurs selectively on the newly formed polyester segments. The disconnected cyclic alternating copolymers can be easily isolated by precipitation owing to their substantial solubility difference from the PEO-containing acyclic parts. The obtained cyclic alternating copolymers exhibit low dispersity (<1.2) and a molar mass of around 3 kg mol-1, irrespective of the monomer-to-initiator feed ratio, indicating thermodynamic control over the ring size. The macrocyclic structure is confirmed by both mass spectroscopy and microscopic visualization and then utilized to prepare cyclic-brush terpolymer by thiol-ene modification, followed by graft polymerization of propylene oxide.

Air and Thermally Stable Cyclic (Alkyl)(amino)carbene Ruthenium Complexes for Efficient Ring Expansion Metathesis Polymerization

Ring expansion metathesis polymerization (REMP) has emerged as a potent strategy for obtaining cyclic polymers over the past two decades. The scope of monomers, however, remains limited due to the poor functional group tolerance and stability of the catalyst, necessitating a rational catalyst design to address this constraint. Here, we present ruthenium complexes featuring tethered cyclic (alkyl)(amino)carbene ligands for REMP, aiming to deepen our understanding of the structure-property relationship in newly designed catalysts. Notably, these ruthenium catalysts exhibit remarkable thermal stability even in air, as confirmed through monitoring the air-exposed solution at 80 °C. In addition, the initiation rate of the catalysts was effectively modulated by tuning the steric hindrance of the N-aryl substituent, adjusting tethered chain lengths, or introducing a Blechert-type ligand. Based on systematic studies of catalysts, we successfully demonstrate challenging REMP of a cyclic allene (2,8-dimethylnona-4,5-diene) for the first time, as well as methyl-5-norbornene-2-carboxylate, resulting in cyclic polymers. We also note that the exceptional stability of the catalyst enables REMP under air. This study provides valuable insights into the rational design of catalysts and introduces a novel, user-friendly platform for the synthesis of cyclic polymers.

Iron Olefin Metathesis: Unlocking Reactivity and Mechanistic Insights

Olefin metathesis catalyzed by iron complexes has garnered substantial interest due to iron's abundance and nontoxicity relative to ruthenium, yet its full potential remains untapped, largely because of the propensity of iron carbenes to undergo cyclopropanation instead of cycloreversion from a metallacycle intermediate. In this report, we elucidate the reactions of [{PC(sp2)P}Fe(L)(N2)], ([PC(sp2)P] = bis[2-(diisopropylphosphino)phenyl]methylene) with strained olefins, unveiling their capability to yield metathesis-related products. Our investigations led to the isolation of a structurally characterized metallacyclobutane during the reaction with norbornadiene derivatives, ultimately leading to a ring-opened iron alkylidene. These findings provide compelling evidence that iron complexes adhere to the Chauvin olefin metathesis mechanism.

Stereoselective Ring Expansion Metathesis Polymerization with Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Complexes

Molybdenum alkylidyne N-heterocyclic carbene (NHC) complexes of the type [Mo(C-p-C6H4Y)(OC(R)(CF3)2)2 (L)(NHC)][B(ArF)4] (Y = OMe, NO2; R = CH3, CF3; L = none, pivalonitrile, tetrahydrofuran; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-3,4-dihydroimidazol-2-ylidene (IMesH2), 1,3-dimesityl-3,4-dichloroimidazol-2-ylidene (IMesCl2), 1,3-diisopropylimidazol-2-ylidene (IiPr); B(ArF)4- = tetrakis(3,5-bis(trifluoromethyl)phen-1-yl)borate) were used in the ring expansion metathesis polymerization (REMP) of cyclic olefins. With cis-cyclooctene (cCOE) cyclic, low molecular weight oligomers were obtained at low monomer concentrations and the cyclic nature of the polymer was confirmed by MALDI-TOF measurements. High-molecular weight cyclic poly(cCOE) became available at high monomer concentrations. Also, post-REMP allowed for converting low-molecular-weight cyclic poly(cCOE) into high-molecular-weight cyclic poly(cCOE). Tailored catalysts together with suitable additives offered access to the stereoselective REMP of functional norbornenes providing functional cis-isotactic (cis-it), cis-syndiotactic (cis-st) and trans-it poly(norbornene)s with up to 99% stereoselectivity. Mechanistic details supported by density functional theory (DFT) calculations are outlined.

Polyacetylene-Based Asymmetric Bicyclic Polymer by Blocking-Cyclization Technique

A rare asymmetric bicyclic polymer containing different length of conjugated polyacetylene segments is synthesized by metathesis cyclopolymerization-mediated blocking-cyclization technique. The size of each single ring differs from each other, and the unique cyclic polymer topology is controlled by adjusting the feed ratio of monofunctional monomer to catalyst. The topological difference between linear and bicyclic polymers is confirmed by several techniques, and the visualized morphology of asymmetric bicyclic polymer is directly observed without tedious post-modification process. The photoelectric and thermal properties of polymers are investigated. This work expands the pathway for the derivation of cyclic polymers, and such unique topological structure enriches the diversity of cyclic polymer classes.

Ring Expansion Alkyne Metathesis Polymerization

The synthesis, characterization, and preliminary activity of an unprecedented tethered alkylidyne tungsten complex for ring expansion alkyne metathesis polymerization (REAMP) are reported. The tethered alkylidyne 7 is generated rapidly by combining alkylidyne W(CtBu)(CH2tBu)(O-2,6-i-Pr2C6H3)2 (6) with 1 equiv of an yne-ol proligand (5). Characterized by NMR studies and nuclear Overhauser effect spectroscopy, complex 7 is a dimer. Each metal center contains a tungsten-carbon triple bond tethered to the metal center via an alkoxide ligand. The polymerization of the strained cycloalkyne 3,8-didodecyloxy-5,6-dihydro-11,12-didehydrodibenzo[a,e]-[8]annulene, 8, to generate cyclic polymers was demonstrated. Size exclusion chromatography (SEC) and intrinsic viscosity (η) measurements confirm the polymer's cyclic topology.

Cyclic polymers: synthesis, characteristics, and emerging applications

Cyclic polymers with a ring-like topology and no chain ends are a unique class of macromolecules. In the past several decades, significant advances have been made to prepare these fascinating polymers, which allow for the exploration of their topological effects and potential applications in various fields. In this Review, we first describe representative synthetic strategies for making cyclic polymers and their derivative topological polymers with more complex structures. Second, the unique physical properties and self-assembly behavior of cyclic polymers are discussed by comparing them with their linear analogues. Special attention is paid to highlight how polymeric rings can assemble into hierarchical macromolecular architectures. Subsequently, representative applications of cyclic polymers in different fields such as drug and gene delivery and surface functionalization are presented. Last, we envision the following key challenges and opportunities for cyclic polymers that may attract future attention: large-scale synthesis, efficient purification, programmable folding and assembly, and expansion of applications.

Synthesis of conjugated segments-based cyclic polymers for direct imaging of cyclic molecular topology

Conjugated polyacetylene-based monocyclic and bicyclic polymers were synthesized by blocking-cyclization metathesis polymerization using the short ladderphanes as the intial motif and multi-cyclizing unit, and fully characterized to elucidate the cyclic...