Sacrificial Synthesis of Hydroxy-Functionalized ROMP Polymers: An Efficiency Study

Practical Route for Catalytic Ring-Opening Metathesis Polymerization

Norbornene derivatives are typical monomers for ring-opening metathesis polymerization (ROMP) for synthesizing highly functional polymers. However, the lack of catalytic methods, that is, the lack of readily available chain transfer agents (CTAs) for these monomers has been a significant cost limitation when large-scale syntheses are required. Here, we report commercially available styrene and its derivatives as efficient regioselective CTAs for the catalytic synthesis of metathesis polymers requiring up to 1000 times less ruthenium than in classical ROMP experiments. The molecular weight of the synthesized polymers was controlled by the monomer-to-CTA ratio. Low molecular weight ROMP polymers known for their antimicrobial properties were also synthesized on a gram scale in this report. Polymers were characterized by SEC, ¹H NMR spectroscopy, and isotopically resolved MALDI-TOF MS. This approach describes a greener, more cost-effective, and eco-friendly methodology for the preparation of metathesis-based materials on the multigram scale.

Degradable polymers via olefin metathesis polymerization

The development of degradable polymers has commanded significant attention over the past half century. Approaches have predominantly relied on ring-opening polymerization of cyclic esters (e.g., lactones, lactides) and N-carboxyanhydrides, as well as radical ring-opening polymerizations of cyclic ketene acetals. In recent years, there has been a significant effort applied to expand the family of degradable polymers accessible via olefin metathesis polymerization. Given the excellent functional group tolerance of olefin metathesis polymerization reactions generally, a broad range of conceivable degradable moieties can be incorporated into appropriate monomers and thus into polymer backbones. This approach has proven particularly versatile in synthesizing a broad spectrum of degradable polymers including poly(ester), poly(amino acid), poly(acetal), poly(carbonate), poly(phosphoester), poly(phosphoramidate), poly(enol ether), poly(azobenzene), poly(disulfide), poly(sulfonate ester), poly(silyl ether), and poly(oxazinone) among others. In this review, we will highlight the main olefin metathesis polymerization strategies that have been used to access degradable polymers, including (i) acyclic diene metathesis polymerization, (ii) entropy-driven and (iii) enthalpy-driven ring-opening metathesis polymerization, as well as (iv) cascade enyne metathesis polymerization. In addition, the livingness or control of polymerization reactions via different strategies are highlighted and compared. Potential applications, challenges and future perspectives of this new library of degradable polyolefins are discussed. It is clear from recent and accelerating developments in this field that olefin metathesis polymerization represents a powerful synthetic tool towards degradable polymers with novel structures and properties inaccessible by other polymerization approaches.

Clip Chemistry: Diverse (Bio)(macro)molecular and Material Function through Breaking Covalent Bonds

In the two decades since the introduction of the "click chemistry" concept, the toolbox of "click reactions" has continually expanded, enabling chemists, materials scientists, and biologists to rapidly and selectively build complexity for their applications of interest. Similarly, selective and efficient covalent bond breaking reactions have provided and will continue to provide transformative advances. Here, we review key examples and applications of efficient, selective covalent bond cleavage reactions, which we refer to herein as "clip reactions." The strategic application of clip reactions offers opportunities to tailor the compositions and structures of complex (bio)(macro)molecular systems with exquisite control. Working in concert, click chemistry and clip chemistry offer scientists and engineers powerful methods to address next-generation challenges across the chemical sciences.

Alternating Ring-Opening Metathesis Polymerization Provides Easy Access to Functional and Fully Degradable Polymers

Polymers with hydrolyzable groups in their backbones have numerous potential applications in biomedicine, lithography, energy storage and electronics. In this study, acetal and ester functionalities were incorporated into the backbones of copolymers by means of alternating ring-opening metathesis polymerization catalyzed by third-generation Grubbs ruthenium catalyst. Specifically, combining large-ring (7-10 atoms) cyclic acetal or lactone monomers with bicyclo[4.2.0]oct-1(8)-ene-8-carboxamide monomers provided perfectly alternating copolymers with acetal or ester functionality in the backbones and low to moderate molecular weight distribution (Đ M = 1.2-1.6). Copolymers containing ester and acetal backbones hydrolyzed to significant extent under basic condition (pH 13) and acidic conditions (pH ≤ 5) respectively to yield the expected by-products within 30 hours at moderate temperature. Unlike the copolymer with all-carbon backbone, copolymers with heteroatom-containing backbone exhibited viscoelastic behavior with crossover frequency which decreases as the size of the R group on the acetal increases. In contrast, the glass transition temperature (T g) decreases as the size of the R group decreases. The rate of hydrolysis of the acetal copolymers was also dependent on the R group. Thus, ruthenium-catalyzed alternating ring-opening metathesis copolymerization provides heterofunctional copolymers whose degradation rates, glass transition temperatures, and viscoelastic moduli can be controlled.

Advances in the synthesis of silyl-modified polymers (SMPs)

Silyl-modified polymers (SMPs) are being synthesized from chemical modification and olefin metathesis strategies.

Branched Polymers via ROMP of Termimers

Today's olefin metathesis catalysts show high reactivity, selectivity, and functional group tolerance and allow the design of new syntheses of precisely functionalized polymers. Here the synthesis of a new end-capping reagent is investigated allowing the introduction of a highly reactive activated ester end-group at the polymer chain end as well as its prefunctionalization to directly introduce functional moieties. The versatility of this new end-capping reagent is demonstrated by utilizing it to synthesize a so-called termimer (a monomer with termination capabilities). Copolymerization of a norbornene derivative with the termimer leads to hyperbranched ring-opening metathesis polymerization polymers as proven by gel permeation chromatography and MALDI-ToF-(matrix-assisted laser desorption/ionization time of flight) mass spectrometry.

Sequential ROMP of cyclooctenes as a route to linear polyethylene block copolymers

AB diblock copolymers were prepared by sequential ring-opening metathesis polymerization of cyclooctenes catalyzed by a Ru-based Grubbs catalyst. The relatively slow polymerization of cis-3-phenylcyclooct-1-ene (3PC) or cis-cyclooct-2-en-1-yl acetate (3AC) was first carried out and then followed by the faster polymerization of unsubstituted cis-cyclooctene (COE) from the active Ru-alkylidene chain ends. In contrast, simultaneous polymerization of the two monomers provides copolymers with a statistical monomer distribution owing to extensive chain transfer. The resulting poly(3PC-b-COE) and poly(3AC-b-COE) diblock copolymers were subjected to hydrogenation to selectively saturate the backbone alkenes. The consequences of architectural variance between the materials from simultaneous vs. sequential polymerizations are reflected by the contrasting thermal characteristics.