Ring Opening Metathesis Polymerization of Cyclic Allenes

Synthesis of Linear and Cyclic Poly(allenamer)s by Powerful Cyclic-Alkyl-Amino-Carbene (CAAC) Ruthenium Catalysts and Facile Post-modification

Cyclic polymers are very attractive due to their unique properties; however, so far, they have simple and less reactive backbone structures due to synthetic limitations, restricting their further post-modification. Notably, allenes present a potentially useful platform in polymer chemistry due to their well-established toolbox in organic chemistry. Nevertheless, the biggest challenge remains in synthesizing poly(allenamer)s with high allene contents or polymerization efficiency, as well as synthesizing different types of cyclic poly(allenamer)s. Herein, we synthesized linear and cyclic poly(allenamer)s via ring-opening metathesis polymerization (ROMP) and ring-expansion metathesis polymerization (REMP), employing highly efficient cyclic-alkyl-amino-carbene (CAAC) ruthenium catalysts. Mechanistic studies suggested CAAC ligands enhanced stability of propagating Ru vinylidene, enabling various linear and cyclic poly(allenamer)s with turnover number up to 1360 and molecular weight reaching 549 kDa. Their cyclic architecture was thoroughly characterized by multiangle light scattering size-exclusion chromatography (MALS SEC) with viscometer. Moreover, controlled ROMP of a highly reactive α-substituted cyclic allene was achieved using third-generation Grubbs' catalyst. Finally, we demonstrated highly efficient and selective post-modifications on poly(allenamer)s with primary and secondary alcohols. This broadens the scope of cyclic polymers with improved efficiency and structural control, affording a practical platform for diverse macromolecules.

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.

Carbodiimide Ring-Opening Metathesis Polymerization

Controlled incorporation of nitrogen into macromolecular skeletons is a long-standing challenge whose resolution would enable the preparation of soft materials with the scalability of man-made plastics and functionality of Nature's proteins. Nylons and polyurethanes notwithstanding, nitrogen-rich polymer backbones remain scarce, and their synthesis typically lacks precision. Here we report a strategy that begins to address this limitation founded on a mechanistic discovery: ring-opening metathesis polymerization (ROMP) of carbodiimides followed by carbodiimide derivatization. An iridium guanidinate complex was found to initiate and catalyze ROMP of N-aryl and N-alkyl cyclic carbodiimides. Nucleophilic addition to the resulting polycarbodiimides enabled the preparation of polyureas, polythioureas, and polyguanidinates with varied architectures. This work advances the foundations of metathesis chemistry and opens the door to systematic investigations of structure-folding-property relationships in nitrogen-rich macromolecules.

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.

Synthesis of Polyallenoates through Copper-Mediated Cross-Coupling of Dialkynes and Bis-α-Diazoesters

The copper-catalyzed cross-coupling of alkynes and α-diazoesters have been applied in the synthesis of polyallenoates for the first time. The polymerization tolerated various functional groups and afforded the polyallenoates with...

Recent Advances in Polyallenes: Preparation, Self-Assembly, and Stimuli-Responsiveness

Polyallenes, as a typical type of reactive polymers, are of great significance and have aroused widespread interest because they contain double bonds that can be post-modified into other functionalities to afford varieties of functional materials. This Minireview firstly highlights the recent advances in the preparation of polyallenes, including preparation of helical polyallenes through directly polymerization of chiral allene monomers or helix-sense-selective polymerization (HSSP) of achiral allene monomers, synthesis of 1,2-regulated polyallenes and 2,3-regulated polyallenes via selective polymerization of allene monomers, polymerization of allene monomers catalyzed by Ni(II)-terminated poly(3-hexylthiophene) (P3HT), and so on. Then, latest progress on the self-assembly and stimuli-responses of polyallene-based diblock, ABA and ABC triblock copolymers is summarized. We hope this Minireview will inspire more interest in developing polyallenes and encourage further advances in functional materials.