Cyclo-depolymerization of olefin-containing polymers to give macrocyclic oligomers by metathesis and the entropically-driven ROMP of the olefin-containing macrocyclic esters

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.

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.

Mechanical gating of a mechanochemical reaction cascade

Covalent polymer mechanochemistry offers promising opportunities for the control and engineering of reactivity. To date, covalent mechanochemistry has largely been limited to individual reactions, but it also presents potential for intricate reaction systems and feedback loops. Here we report a molecular architecture, in which a cyclobutane mechanophore functions as a gate to regulate the activation of a second mechanophore, dichlorocyclopropane, resulting in a mechanochemical cascade reaction. Single-molecule force spectroscopy, pulsed ultrasonication experiments and DFT-level calculations support gating and indicate that extra force of >0.5 nN needs to be applied to a polymer of gated gDCC than of free gDCC for the mechanochemical isomerization gDCC to proceed at equal rate. The gating concept provides a mechanism by which to regulate stress-responsive behaviours, such as load-strengthening and mechanochromism, in future materials designs.

Sequence-Controlled Copolymers Prepared via Entropy-Driven Ring-Opening Metathesis Polymerization

A new general synthetic approach to sequenced macromolecules was developed and applied to the synthesis of polymers comprising lactic acid (L), glycolic acid (G), and ε-caprolactone (C)-derived monomer units. The new method employs entropy-driven ring-opening metathesis polymerization (ED-ROMP) to prepare copolymers with embedded sequences and controlled molecular weights. Cyclic macromonomer precursors were prepared by ring-closing metathesis of ethylene glycol (Eg)-linked sequenced oligomers bearing terminal olefins. ED-ROMP of the resulting macrocycles using Grubbs' second generation catalyst yielded poly(CL-Eg-LC-Oed), poly(CLL-Eg-LLC-Oed), poly(LGL-Eg-LGL-Oed), and poly(LGL-Eg-LGL-Hed) (Oed = octenedioc acid; Hed = hexenedioc acid). Hydrogenation produced the saturated sequenced copolymers. Molecular weight was well-controlled and could be adjusted by varying the monomer-to-catalyst ratio. M_ns of 26-60 kDa were obtained (dispersities = 1.1-1.3). The methodology proved general for three different sequences and two olefinic metathesis groups.

Relative Mechanical Strengths of Weak Bonds in Sonochemical Polymer Mechanochemistry

The mechanical strength of scissile chemical bonds plays a role in material failure and in the mechanical activation of latent reactivity, but quantitative measures of mechanical strength are rare. Here, we report the relative mechanical strength of polymers bearing three putatively "weak" scissile bonds: the carbon-nitrogen bond of an azobisdialkylnitrile (<30 kcal mol(-1)), the carbon-sulfur bond of a thioether (71-74 kcal mol(-1)), and the carbon-oxygen bond of a benzylphenyl ether (52-54 kcal mol(-1)). The mechanical strengths are assessed in the context of chain scission triggered by pulsed sonication of polymer solutions, by using two complementary techniques: (i) the competition within a single polymer chain between the bond scission of interest and the nonscissile mechanochemical ring opening of gem-dichlorocyclopropane mechanophores and (ii) the molecular weights at long (4 h) sonication times of multimechanophore polymers. The two methods produce a consistent story: in contrast to their thermodynamic strengths, the relative mechanical strengths of the three weak bonds are azobisdialkylnitrile (weakest) < thioether < benzylphenyl ether. The greater mechanical strength of the benzylphenyl ether relative to the thermodynamically stronger carbon-sulfur bond is ascribed to poor mechanochemical coupling, at least in part as a result of the rehybridization that accompanies carbon-oxygen bond scission.

Entropically-driven ring-opening metathesis polymerization (ED-ROMP) of macrocyclic olefins prepared from deoxycholic acid to give functionalized polymers

The macrocycles shown were prepared, then polymerized and copolymerized by ROMP. This gave polymers with free OH or free CO₂R groups. Treatment of the polymers having R =t-Bu with trifluoroacetic acid gave polymers with free CO₂H groups.

Polymerization-Depolymerization System Based on Reversible Addition-Dissociation Reaction of 1,3-Benzoxazine with Thiol

The reversible nature of the addition reaction of 1,3-benzoxazine and thiol at ambient temperature was discovered by investigating the reaction with using p-cresol-derived N-phenyl benzoxazine 1 and 1-octadecanethiol 2. The reaction was performed in several deuteriated media involving CDCl₃ and CDCl₃ + CD₃OD, for monitoring their reaction by ¹H NMR spectrometry. CDCl₃ was a favorable solvent for the efficient progress of the reaction, and its combination with CD₃OD allowed further acceleration of the reaction. In both cases, the reaction proceeded until conversion of 1 reached a certain ceiling value, to suggest that the reaction was reversible. This reversible nature was concretely confirmed by finding a dissociation reaction of isolated 3 into 1 and 2 in CDCl₃. Analogously, a bisphenol A-derived bifunctional benzoxazine 4 and 1,6-hexanedithiol 5 underwent the polyaddition in CDCl₃ + CD₃OD at ambient temperature to afford the corresponding polymer 6. Successful depolymerization of 6 into small fragments was achieved by dissolving 6 in CDCl₃.

Ring-chain equilibria of R-but-3-enoate esters — A quantum mechanical study of direct and indirect ring-closing reactions

Macrocyclic structures can be synthesized through two simultaneous olefin metathesis reactions: either directly through ring-closing metathesis (RCM) or indirectly through an intermediate formed by acyclic diene metathesis (ADMET). The proclivity of a homologous series of 16 R-but-3-enoate esters to form lactones through one of these two processes is studied at the HF/6–31G(d), B3LYP/6–31G(d), and MP2(full)/6–31G(d) levels of theory. Computed Gibbs free energies are used to determine ΔGRCM, ΔGADMET, and ΔΔG (ΔGRCM – ΔGADMET). ΔΔG is evaluated to compare the relative favourability of the RCM and ADMET reactions for the various R-but-3-enoate esters, where each system is differentiated by the number of methylene groups (n) added to the ester chain. When n = 0, 1, 10, or 13, cyclic lactone formation by direct RCM is predicted to be thermodynamically favoured, and the indirect synthesis is preferred for all other heterocyclic structures. The same trend holds between 298.15 and 333.15 K, therefore, the gas-phase model is a reasonable approximation of the experimental reaction conditions. The theoretical model is sufficient for smaller systems, but molecules larger than the n = 6 case do not follow experimental results for similar saturated structures. Hence, the assumptions pertaining to straight-chain and cis-ring conformations need to be re-evaluated. In particular, chain flexibility should be further examined.

Synthesis of copolymers by alternating ROMP (AROMP)

The alternating polymerization of cyclobutene 1-carboxylic esters and cyclohexene derivatives with the precatalyst [(H(2)IMes)(3-Br-pyr)(2)Cl(2)Ru=CHPh] is described. This reaction is synthetically accessible and provides (AB)(n) heteropolymers with an alternating backbone and alternating functionality. The regiocontrol of heteropolymer formation derives from the inability of the cyclobutene ester and cyclohexene monomers to undergo homopolymerization in combination with the favorable kinetics of cross polymerization.

Ring-expansion metathesis polymerization: catalyst-dependent polymerization profiles

Ring-expansion metathesis polymerization (REMP) mediated by recently developed cyclic Ru catalysts has been studied in detail with a focus on the polymer products obtained under varied reaction conditions and catalyst architectures. Depending upon the nature of the catalyst structure, two distinct molecular weight evolutions were observed. Polymerization conducted with catalysts bearing six-carbon tethers displayed rapid polymer molecular weight growth which reached a maximum value at ca. 70% monomer conversion, resembling a chain-growth polymerization mechanism. In contrast, five-carbon-tethered catalysts led to molecular weight growth that resembled a step-growth mechanism with a steep increase occurring only after 95% monomer conversion. The underlying reason for these mechanistic differences appeared to be ready release of five-carbon-tethered catalysts from growing polymer rings, which competed significantly with propagation. Owing to reversible chain transfer and the lack of end groups in REMP, the final molecular weights of cyclic polymers was controlled by thermodynamic equilibria. Large ring sizes in the range of 60-120 kDa were observed at equilibrium for polycyclooctene and polycyclododecatriene, which were found to be independent of catalyst structure and initial monomer/catalyst ratio. While six-carbon-tethered catalysts were slowly incorporated into the formed cyclic polymer, the incorporation of five-carbon-tethered catalysts was minimal, as revealed by ICP-MS. Further polymer analysis was conducted using melt-state magic-angle spinning (13)C NMR spectroscopy of both linear and cyclic polymers, which revealed little or no chain ends for the latter topology.