Ring‐Opening Metathesis Polymerization: Mechanisms

Abstract

This article covers the basic principles of modern, well‐defined group 6 and group 8 metal alkylidene catalysts and their use in ring‐opening metathesis polymerization (ROMP). In this context, the expression “well‐defined” refers to catalytic systems that are characterized by a uniform and stoichiometric composition and for which the actual propagating species is well known and characterized. Such catalysts allow for the controlled, sometimes living, ROMP, which is of advantage for a variety of purposes. Generally, one particularity of ROMP is the formation of either cis ‐ or trans ‐double bonds along the polymer chain. Their formation during ROMP is strongly related to the catalyst used. In addition, tactic ROMP‐derived polymers are accessible. The influence of the alkylidene configuration and chirality of the catalysts on the microstructure of the resulting polymers as well as the different mechanisms that lead to tactic structures are outlined.

Complex Polymer Architectures Using Ring-Opening Metathesis Polymerization: Synthesis, Applications, and Practical Considerations

“…Many of these catalysts are based on Mo or W transition metals, although some Ru-based catalysts with control over polymer microstructure have been developed. These topics were recently reviewed by Buchmeiser . In figures here we do not intend to imply control over backbone olefin stereochemistry.…”

cis-Selective Acyclic Diene Metathesis Polymerization Using Bulky Cyclometalated Ruthenium Carbene Catalysts

“…Evidenced by the extensive studies on polyisoprenes with all- cis , all- trans , or mixed cis – trans structures, − the regulation of cis – trans configuration of main-chain double bonds offers an important stereochemical handle for adjusting and enhancing properties of polymers with repeating alkene units . Among various approaches toward olefin-containing polymers, olefin metathesis-based polymerization methods, including both ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis (ADMET) polymerization, are ubiquitously practiced due to their broad structural and functional group compatibility. − Often, the formation of a sterically less congested olefin configuration is preferred. Yet, the nature of double-bond formation at a metal center renders metathesis polymerization with valuable opportunities to kinetically tune the stereochemistry of the resulting olefins. − The selective formation of thermodynamically less favored olefin configurations, such as cis -1,2-disubstituted olefins, could be realized by a series of elegantly developed Mo, W, and Ru catalysts. − Therefore, while a classic metathesis polymerization often results in mixed olefin configurations favoring the trans form, cis -enriched polymer backbones are accessible (Scheme a). , For example, significant progress has been made in cis -selective ROMP, leading to nearly perfect stereochemical control − and tailored polymer properties including light-responsiveness and improved shear stability. , Further, an ADMET-based approach to cis -polymer backbones has been elegantly demonstrated recently by using a stereoretentive Ru-dithiolate catalyst, which largely broadens the scope of attainable cis -polyalkenamers beyond cis -selective ROMP .…”