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.

Evaluating Trans-Benzocyclobutene-Fused Cyclooctene as a Monomer for Chemically Recyclable Polymer.

Chemically recyclable polymers offer a promising solution to address the issues associated with the unsustainable use of plastics by converting the traditional linear plastic economy into a circular one. Central to developing chemically recyclable polymers is to identify the appropriate monomers that enable practical conditions for polymerization and depolymerization and ensure useful stability and material properties. Our group has recently demonstrated that trans-cyclobutane-fused cyclooctene (tCBCO) meets the abovementioned requirements and is a promising candidate for developing chemically recyclable polymers. Herein, encouraged by the success with tCBCO, we investigate the thermodynamics of polymerization of a relevant system, trans-benzocyclobutene-fused-cyclooctene, which can be viewed as tCBCO with an additional benzene ring. The study shows that introducing an additional benzene ring favors polymerization and disfavors depolymerization, and the effect is predominantly entropic. The benzo-effect can be leveraged to fine-tune the thermodynamics of polymerization and depolymerization to facilitate the chemical recycling of polymers.

Mechanochemically accessing a challenging-to-synthesize depolymerizable polymer.

Polymers with low ceiling temperatures (Tc) are highly desirable as they can depolymerize under mild conditions, but they typically suffer from demanding synthetic conditions and poor stability. We envision that this challenge can be addressed by developing high-Tc polymers that can be converted into low-Tc polymers on demand. Here, we demonstrate the mechanochemical generation of a low-Tc polymer, poly(2,5-dihydrofuran) (PDHF), from an unsaturated polyether that contains cyclobutane-fused THF in each repeat unit. Upon mechanically induced cycloreversion of cyclobutane, each repeat unit generates three repeat units of PDHF. The resulting PDHF completely depolymerizes into 2,5-dihydrofuran in the presence of a ruthenium catalyst. The mechanochemical generation of the otherwise difficult-to-synthesize PDHF highlights the power of polymer mechanochemistry in accessing elusive structures. The concept of mechanochemically regulating the Tc of polymers can be applied to develop next-generation sustainable plastics.