A latent s‐chelated ruthenium benzylidene initiator for ring‐opening metathesis polymerization

Abstract

Olefin metathesis has greatly impacted polymer, organic, and organometallic chemistry during the last decades. Thus, the development of well-defined metathesis catalysts and the search to expand the substrate range under various conditions is a highly active area of research. Ruthenium olefin metathesis catalysts (Fig. 1) are of special interest as a result of their higher stability and leniency towards various functional groups. Olefin metathesis reactions may be classified into three main categories. Ring-closing metathesis (RCM) is characterized by an intermolecular reaction between catalyst and substrate followed by an intramolecular reaction, resulting in product formation and regeneration of the active catalytic species. Cross metathesis (CM) or acyclic diene metathesis polymerization (ADMET) proceeds by two subsequent bimolecular steps, also releasing the active catalyst to the solution. On the other hand, in ring-opening metathesis polymerization (ROMP), the initiator reacts with a monomer and remains attached to the product until termination (decomposition). ROMP and ADMET have shown many important industrial applications, consequently, the study and development of new methods in metatheses polymerizations is a valued objective. Although stability and activity are fundamental factors when thinking about any catalysis, precise mechanistic control is certainly a central concern in polymer production, since the physical properties of the product are strongly dependent on polymer size, polydispersity, and tacticity. In typical ROMP reactions, following the initial exchange of the benzylidene with a monomer, the produced propagating species is virtually equivalent for catalysts 2, 3, and 4 (Scheme 1). The disparities between these kind of catalysts, therefore, should only be interpreted as a consequence of initiation rate differences. An important distinction in the case of bidentate chelated catalysts is that the substituted styrene may return to the 14-electron active catalytic form via CM, regenerating the precatalytic 16-electron species with a longer half-life; this is known as the ‘‘boomerang effect.’’ However, in ROMP, chelation has a lesspronounced effect on stability (the propagating species is more stable due to the absence of methylidene complexes), and furthermore, it has been shown to impose a negative effect on polydispersity. Recently, we developed highly stable sulfur-chelated ruthenium complexes that display latency at room temperature, and determined that their activities at higher temperatures could be modulated as a function of steric acceleration factors. As dormancy in polymerization reactions is a sought asset, we decided to study whether this appealing characteristic is maintained in Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 4209–4213 (2009)