[Ru]-Catalyzed Olefin Metathesis and Ethenolysis for the Synthesis and Recycling of Bio-Based Polycarbonates and Polycyanurates

Abstract

Eugenol, an abundant, naturally occurring phenolic compound, was converted into a thermoplastic polycarbonate by olefin metathesis followed by interfacial polymerization with triphosghene. This resulted in polymers with Mn ranging from 5300 – 12700 g mol-1 and an average glass transition temperature (Tg) of 82 °C. The polycarbonates were depolymerized via ethenolysis reactions under modest ethylene pressures (150 – 240 psi) in the presence of [Ru]-metathesis catalysts to yield a discrete monomer [bis(4-allyl-2-methoxyphenol) carbonate, compound 2]. 2 was then polymerized with a second generation Grubbs catalyst (M204) to produce a recycled polymer with Mn = 7500 g mol-1 and a Tg of 114 °C. The 32 °C increase in Tg was due to the isomerization of the allyl group to internal positions, which then allowed for the formation of stilbene and 3-carbon unsaturated linkages between aromatic groups. To expand the ethenolysis recycling approach to cross-linked networks, eugenol was converted into a cyanate ester (3), which was then thermally cyclotrimerized to generate 2,4,6-tris(4-allyl-2-methoxyphenoxy)-1,3,5-triazine (4), a monomer with a triazine core and three pendent aromatic rings with methoxy and allyl substituents. 4 was cross-linked via olefin metathesis (M204 catalyst) to generate a network with Mn = 8600 g mol-1 and a Tg of 180 °C. Similar to the polycarbonate, the polycyanurate was efficiently depolymerized in the presence of ethylene to regenerate 4. Compound 4 was then polymerized and depolymerized two additional times, demonstrating full circularity for the triazine monomer/network. The recycled networks exhibited similar Tgs (167–184 °C) and thermal stability compared to the virgin polymer. Overall, this work demonstrates that both thermoplastic and cross-linked networks can be readily prepared from eugenol and catalytically recycled under standard ethenolysis conditions. Unlike many conventional approaches, the recycled polymers described in this work exhibited no significant degradation in thermomechanical properties. This type of approach supports a circular bioeconomy and may help to reduce plastic waste and the accumulation of micro/nanoplastic particles in the environment.