Deconstruction of Polymers through Olefin Metathesis

The consumption of synthetic polymers has ballooned; so has the amount of post-consumer waste generated. The current polymer economy, however, is largely linear with most of the post-consumer waste being either landfilled or incinerated. The lack of recycling, together with the sizable carbon footprint of the polymer industry, has led to major negative environmental impacts. Over the past few years, chemical recycling technologies have gained significant traction as a possible technological route to tackle these challenges. In this regard, olefin metathesis, with its versatility and ease of operation, has emerged as an attractive tool. Here, we discuss the developments in olefin-metathesis-based chemical recycling technologies, including the development of new materials and the application of olefin metathesis to the recycling of commercial materials. We delve into structure-reactivity relationships in the context of polymerization-depolymerization behavior, how experimental conditions influence deconstruction outcomes, and the reaction pathways underlying these approaches. We also look at the current hurdles in adopting these technologies and relevant future directions for the field.

Reactivity and Selectivity in Ruthenium Sulfur-Chelated Diiodo Catalysts

A trifluoromethyl sulfur-chelated ruthenium benzylidene, Ru-S-CF₃ -I, was synthesized and characterized. This latent precatalyst provides a distinct activity and selectivity profiles for olefin metathesis reactions depending on the substrate. For example, 1,3-divinyl-hexahydropentalene derivatives were efficiently obtained by ring-opening metathesis (ROM) of dicyclopentadiene (DCPD). Ru-S-CF₃ -I also presented a much more effective photoisomerization process from the inactive cis-diiodo to the active trans-diiodo configuration after exposure to 510 nm (green light), allowing for a wide scope of photoinduced olefin metathesis reactions. DFT calculations suggest a faster formation and enhanced stability of the active trans-diiodo species of Ru-S-CF₃ -I compared with Ru-S-Ph-I, explaining its higher reactivity. In addition, the photochemical release of chloride anions by irradiation of Cl-BODIPY in the presence of DCPD derivatives with diiodo Ru benzylidenes, led to in situ generation of chloride complexes, which quickly produced the corresponding cross-linked polymers. Thus, novel selective pathways that use visible light to guide olefin metathesis based synthetic sequences is presented.

Unprecedented Selectivity of Ruthenium Iodide Benzylidenes in Olefin Metathesis Reactions

The development of selective olefin metathesis catalysts is crucial to achieving new synthetic pathways. Herein, we show that cis-diiodo/sulfur-chelated ruthenium benzylidenes do not react with strained cycloalkenes and internal olefins, but can effectively catalyze metathesis reactions of terminal dienes. Surprisingly, internal olefins may partake in olefin metathesis reactions once the ruthenium methylidene intermediate has been generated. This unexpected behavior allows the facile formation of strained cis-cyclooctene by the RCM reaction of 1,9-undecadiene. Moreover, cis-1,4-polybutadiene may be transformed into small cyclic molecules, including its smallest precursor, 1,5-cyclooctadiene, by the use of this novel sequence. Norbornenes, including the reactive dicyclopentadiene (DCPD), remain unscathed even in the presence of terminal olefin substrates as they are too bulky to approach the diiodo ruthenium methylidene. The experimental results are accompanied by thorough DFT calculations.

Preparation and property investigation of chain end functionalized cis-1,4 polybutadienes via de-polymerization and cross metathesis of cis-1,4 polybutadienes

The end-functional cis-1,4 polybutadiene displayed improved thermal stability and mechanistic properties.

Ethenolysis: A Green Catalytic Tool to Cleave Carbon-Carbon Double Bonds

Remarkable innovations have been made in the field of olefin metathesis due to the design and preparation of new catalysts. Ethenolysis, which is cross-metathesis with ethylene, represents one catalytic transformation that has been used with the purpose of cleaving internal carbon-carbon double bonds. The objectives were either the ring opening of cyclic olefins to produce dienes or the shortening of unsaturated hydrocarbon chains to degrade polymers or generate valuable shorter terminal olefins in a controlled manner. This Review summarizes several aspects of this reaction: the catalysts, their degradation in the presence of ethylene, some parameters driving their productivity, the side reactions, and the applications of ethenolysis in organic synthesis and in potential industrial applications.

End-of-life treatment of poly(vinyl chloride) and chlorinated polyethylene by dehydrochlorination in ionic liquids

There is an urgent need for green technologies to remove halogens from halogenated polymers at the end of their lifetime. Ionic liquids (ILs) were used to dehydrochlorinate and/or dissolve the chlorinated polymers poly(vinyl chloride) (PVC) and chlorinated polyethylene (CPE). The dehydrochlorination activity of an IL depends mainly on its anion and is related to the high hydrogen-bond-accepting ability (β value) of the anion. Different phosphonium ILs successfully dissolve and dehydrochlorinate PVC and CPE at temperatures from 80 °C. PVC is dehydrochlorinated up to 98 % after 60 min in tetrabutylphosphonium chloride ([P4444 ][Cl]) at 180 °C. PVC pieces stabilized by calcium stearate (4 mm(3) ) are dehydrochlorinated more slowly; conversions of 85 and 96 % are reached after 1 and 8 h, respectively. Smaller pieces are dehydrochlorinated faster. High loadings, for example, 0.3 g stabilized PVC in 0.5 g IL, can be applied with only a minor loss of conversion. [P4444 ][Cl] proved to be stable during several consecutive reactions; after each run more than 99 % of the IL can be recovered. The structure of the dehydrochlorinated PVC was studied by (13) C cross-polarization magic-angle spinning NMR and FTIR spectroscopy; the removal of Cl and the formation of double bonds were confirmed. Carefully dehydrochlorinated CPE was processed further by acyclic diene metathesis depolymerization with ethylene and the Hoveyda-Grubbs second-generation catalyst to yield α,ω-dienes such as 1,5-hexadiene and 1,6-heptadiene.

Metathesis depolymerization for removable surfactant templates

Current methodologies for the production of meso- and nanoporous materials include the use of a surfactant to produce a self-assembled template around which the material is formed. However, post-production surfactant removal often requires centrifugation, calcination, and/or solvent washing which can damage the initially formed material architecture(s). Surfactants that can be disassembled into easily removable fragments following material preparation would minimize processing damage to the material structure, facilitating formation of templated hybrid architectures. Herein, we describe the design and synthesis of novel cationic and anionic surfactants with regularly spaced unsaturation in their hydrophobic hydrocarbon tails and the first application of ring closing metathesis depolymerization to surfactant degradation resulting in the mild, facile decomposition of these new compounds to produce relatively volatile nonsurface active remnants.