Pulsed-Addition ROMP: Catalytic Syntheses of Heterotelechelic Polymers via Regioselective Chain Transfer Agents

A Versatile Reversible, Degenerative Chain Transfer Mechanism for the Catalytic Living Ring-Opening Metathesis Polymerization

Most metathesis polymers based on norbornene derivatives carry a vinyl end group. Here we show that these vinyl end groups readily undergo a degenerative exchange of the terminal methylene unit in the presence of sub-stoichiometric amounts of a propagating metathesis polymer carrying a Grubbs ruthenium complex. We show that this degenerative exchange can be exploited in synthesizing ROMP polymers in a catalytic living fashion. Chain transfer agents based on styrene, or monosubstituted conjugated 1,3 diene derivatives are used as initiating sites for the catalytic living polymerization. Suitable derivatives of these chain transfer agents not only allow the linear living growth of polymers but also the synthesis of block copolymers from macro-initiators or star polymers from multi-functional chain transfer agents. This reversible exchange mechanism offers a cheaper, greener, and more sustainable alternative for the synthesis of living ROMP polymers compared to the classical synthetic route.

Efficient Dienyl End-Capping of Ruthenium Catalyzed Ring Opening Metathesis Polymerization with Allyl Compounds through Base-Promoted Metallacyclobutane Decomposition

Herein we demonstrate an effective and facile end-capping technique for ring-opening metathesis polymerization (ROMP) using readily available allyl compounds as a new type of terminating agents. This new type of end-capping reactions, which are based on the base-promoted decomposition of ruthenocyclobutane intermediates, introduce diene moiety onto the chain end of ROMP polymers while simultaneously deactivating the ruthenium complex. These termination reactions are highly efficient, typically completing within 1 minute at 0 °C with >95 % end-capping fidelity.

Catalytic Living Ring-Opening Metathesis Polymerization Using Vinyl Ethers as Effective Chain-Transfer Agents

A catalytic living ring-opening metathesis copolymerization (ROMP) method is described that relies on a degenerative, reversible and regioselective exchange of propagating Fischer-carbenes. All characteristics of a living polymerization such as narrow dispersity, excellent molar mass control and the ability to form block copolymers are achieved by this method. The method allows the use of up to 200 times less ruthenium complex than traditional living ROMP. We demonstrate the synthesis of ROMP-ROMP diblock copolymers, ATRP from a ROMP macro-initiator and living ROMP from a PEG-based macro chain transfer agent. The cost-effective, sustainable and environmentally friendly synthesis of degradable polymers and block copolymers enabled by this strategy will find various applications in biomedicine, materials science, and technology.

Ring-Opening Metathesis Polymerization and Related Olefin Metathesis Reactions in Benzotrifluoride as an Environmentally Advantageous Medium

A tremendous number of solvents, either as liquids or vapors, contaminate the environment on a daily basis worldwide. Olefin metathesis, which has been widely used as high-yielding protocols for ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM), and isomerization reactions, is typically performed in toxic and volatile solvents such as dichloromethane. In this study, the results of our systematic experiments with the Grubbs G1, G2, and Hoveyda-Grubbs HG2 catalysts proved that benzotrifluoride (BTF) can replace dichloromethane (DCM) in these reactions, providing high yields and similar or even higher reaction rates in certain cases. The ROMP of norbornene resulted not only in high yields but also in polynorbornenes with a high molecular weight at low catalyst loadings. Ring-closing metathesis (RCM) experiments proved that, with the exception of the G1 catalyst, RCM occurs with similar high efficiencies in BTF as in DCM. It was found that isomerization of (Z)-but-2-ene-1,4-diyl diacetate with the G2 and HG2 catalysts proceeds at significantly higher initial rates in BTF than in DCM, leading to rapid isomerization with high yields in a short time. Overall, BTF is a suitable solvent for olefin metathesis, such as polymer syntheses by ROMP and the ring-closing and isomerization reactions.

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.

Chain transfer agents for the catalytic ring opening metathesis polymerization of norbornenes

Here, we present a detailed study of the metathesis activity of conjugated 1,3 diene derivatives in ring opening metathesis polymerization (ROMP) using Grubbs' 3rd generation catalyst (G3). A comprehensive screening of those derivatives revealed that monosubstituted 1,3 dienes show similar reactivities towards G3-alkylidenes as norbornene derivatives. Therefore, they represent perfect candidates for chain transfer agents in a kinetically controlled catalytic ROMP. This unprecedented reactivity allowed us to catalytically synthesize mono-end-functional poly(norborneneimide)s on the gram scale. Much more complex architectures such as star-shaped polymers could also be synthesized catalytically for the very first time via ROMP. This inexpensive and greener route to produce telechelic ROMP polymers was further utilized to synthesize ROMP block copolymers using bifunctional ROMP and ATRP/NCL initiators. Finally, the regioselective reaction of G3 with 1,3 diene derivatives was also exploited in the synthesis of a ROMP-PEG diblock copolymer initiated from a PEG macroinitiator.