Biomass-Derived Degradable Polymers via Alternating Ring-Opening Metathesis Polymerization of Exo-Oxanorbornenes and Cyclic Enol Ethers

Ring Opening Metathesis Polymerization of Cyclooctadiene and Cyclooctene with Dihydrofuran: Influence of Ru Fischer Carbene

Vinyl ethers are commonly used to deactivate Grubbs catalysts and terminate ring opening metathesis polymerization (ROMP) by forming Fischer carbene species with attenuated metathesis reactivity. However, we recently demonstrated that a cyclic enol ether, 2,3-dihydrofuran (DHF), can in fact be homopolymerized or copolymerized with norbornene derivatives. 1,5-Cyclooctadiene (COD) and cyclooctene (COE) consist of an important class of ROMP monomers, and we describe here a study of their copolymerization with DHF. Addition of DHF greatly suppressed the ROMP activity of COD and COE and resulted in significant alkene isomerization of COD. Chloranil was found to be an effective additive to prevent undesired isomerization and promote copolymerization. As a result, high molecular weight COD/COE and DHF copolymers were synthesized. Hydrolysis of the enol ether main chain linkages yields polyalkenamers with alcohol and aldehyde end groups. This study encourages further exploration of the in situ formed Ru Fischer carbene species in ROMP to access degradable polymers.

Quantitative, Regiospecific, and Stereoselective Radical Ring-Opening Polymerization of Monosaccharide Cyclic Ketene Acetals

Cyclic ketene acetals (CKAs) are among the most well-studied monomers for radical ring-opening polymerization (rROP). However, ring-retaining side reactions and low reactivities in homopolymerization and copolymerization remain significant challenges for the existing CKAs. Here, we report that a class of monosaccharide CKAs can be facilely prepared from a short and scalable synthetic route and can undergo quantitative, regiospecific, and stereoselective rROP. NMR analyses and degradation experiments revealed a reaction mechanism involving a propagating radical at the C2 position of pyranose with different monosaccharides exhibiting distinct stereoselectivity in the radical addition of the monomer. Furthermore, the addition of maleimide was found to improve the incorporation efficiency of monosaccharide CKA in the copolymerization with vinyl monomers and produced unique degradable terpolymers with carbohydrate motifs in the polymer backbone.

Chemical recycling of polyolefins via ring-closing metathesis depolymerization

The current insufficient recycling of commodity polymer waste has resulted in pressing environmental and human health issues in our modern society. In the quest for next-generation polymer materials, chemists have recently shifted their attention to the design of chemically recyclable polymers that can undergo depolymerization to regenerate monomers under mild conditions. During the past decade, ring-closing metathesis reactions have been demonstrated to be a robust approach for the depolymerization of polyolefins, producing low-strain cyclic alkene products which can be repolymerized back to new batches of polymers. In this review, we aim to highlight the recent advances in chemical recycling of polyolefins enabled by ring-closing metathesis depolymerization (RCMD). A library of depolymerizable polyolefins will be covered based on the ring size of their monomers or depolymerization products, including five-membered, six-membered, eight-membered, and macrocyclic rings. Moreover, current limitations, potential applications, and future opportunities of the RCMD approach will be discussed. It is clear from recent research in this field that RCMD represents a powerful strategy towards closed-loop chemical recycling of novel polyolefin materials.