Investigating the effects of bulky allylic substituents on the regioregularity and thermodynamics of ROMP on cyclopentene

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.

Regulating the Thermodynamics and Thermal Properties of Depolymerizable Polycyclooctenes through Substituent Effects

Chemical recycling to monomer (CRM) is a promising route for transitioning to a circular polymer economy. To develop new CRM systems with useful properties, it is important to understand the effects of monomer structure on polymerization/depolymerization behavior. In earlier work, this group demonstrated chemically recyclable polymers prepared by ring-opening metathesis polymerization of trans-cyclobutane fused cyclooctenes (tCBCO). Here, it is investigated how different substituents on cyclobutane impact the thermodynamics and thermal properties of tCBCO polymers. Introducing additional substituents to a cis-diester functionalized tCBCO is found to favor the conversion of polymerization; increased polymerization conversion is also observed when the cis-diester is isomerized into its trans counterpart. The effects of these structural features on the thermal properties are also studied. These findings can provide important insights into designing next-generation CRM polymers.

A Single Functionalization Agent for Heterotelechelic ROMP Polymers

Heterotelechelic polymers are an important class of materials finding applications in bioconjugation, imaging, sensing, and synthesis of organic/inorganic hybrid systems with interesting features. However, the synthesis of such polymers is challenging. Here, we report a mechanistically unique and most efficient method based on a single functionalization agent to prepare heterotelechelic polymers by a ring-opening metathesis polymerization. Different functionalization agents can be synthesized in one simple step from inexpensive commercial starting materials. The functionalization agents initially generate a functional initiator from commercial Grubbs' first-generation ruthenium benzylidene catalyst. During this process, a functional dihydrofuran derivative is produced. After functional initiation and propagation of a suitable monomer, the dihydrofuran derivative functionally terminates the polymerization yielding a primary alcohol-terminated heterotelechelic polymer. Molecular weight control is achieved by varying the ratio between monomer and Grubbs' first-generation catalyst. This method may emerge as a popular choice to prepare heterotelechelic polymers due to its simplicity and efficiency.

Ring-Opening Metathesis Polymerization of δ-Pinene: Well-Defined Polyolefins from Pine Sap

Well-controlled ring-opening metathesis polymerization (ROMP) of δ-pinene is reported. The monomer is produced through a facile, metal-free, three-step synthesis from highly abundant and sustainable α-pinene. Using Grubbs third-generation catalyst, δ-pinene undergoes ROMP to high conversion (>95%) with molar mass up to 70 kg mol^-1 and narrow dispersity (<1.2). A highly regioregular propagation mechanism was concluded by NMR spectroscopic analysis that revealed a head-to-tail (HT, >95%) microstructure and high trans content (>98%). Successful ROMP is corroborated with density functional theory calculations on δ-pinene's ring strain energy (∼35 kJ mol^-1). Poly(δ-pinene) has a high glass transition temperature (∼104 °C) and a unique chiral microstructure bearing gem-dimethylcyclobutane rings. Controlled ROMP also allowed the synthesis of block copolymers containing segments of poly(δ-pinene) and polynorbornene which are discussed. Finally, bulk polymerization of δ-pinene is possible, indicating a greener approach to these materials, albeit with some loss of control.

Selectively Depolymerizable Polyurethanes from Unsaturated Polyols Cleavable by Olefin Metathesis

This communication describes a novel series of linear and crosslinked polyurethanes (PUs) and their selective depolymerization under mild conditions. Two unique polyols are synthesized bearing unsaturated units in a configuration designed to favor ring-closing metathesis (RCM) to five- and six-membered cycloalkenes. These polyols are co-polymerized with toluene diisocyanate to generate linear PUs and trifunctional hexamethylene- and diphenylmethane-based isocyanates to generate crosslinked PUs. The polyol design is such that the RCM reaction cleaves the backbone of the polymer chain. Upon exposure to dilute solutions of Grubbs' catalyst under ambient conditions, the PUs are rapidly depolymerized to low molecular weight, soluble products bearing vinyl and cycloalkene functionalities. These functionalities enable further re-polymerization by traditional strategies for polymerization of double bonds. It is anticipated that this general approach can be expanded to develop a range of chemically recyclable condensation polymers that are readily depolymerized by orthogonal metathesis chemistry.