Ruthenium ring-opening polymerization

The earliest reported ring-opening polymerizations of functionalized norbornenes were carried out in protic solvents (alcohol, water) using iridium, ruthenium, or osmium salts. Thus, norbornenes substituted with ester (93-95), hydroxy (95), chlorine (96), alkoxy (97), and imide (93) groups have been polymerized via metathesis using noble metal catalysts. 

Well-defined bimetallic ruthenium catalysts (R3P)2Cl2Ru(=CHp-C6H4CH=)RuCl2(PR3)2 [R = Ph, Cy (cyclohexyl), Cp (cyclopentyl)) (12) have been employed successfully by Grubbs and coworkers for ring-opening polymerization of norbornene and nor-bornene derivatives [35]. 

Optimal yields were obtained by slow addition of the alkene substrates to a solution of the ruthenium vinylalkylidene and this allowed just two equivalents of the acyclic alkene to be used without significant formation of polymeric products. Unlike the acyclic cross-metathesis reactions, which generally favour the formation of tram products, the above ring-opening metathesis reactions yielded products in which the cis stereoisomer is predominant. Particularly noteworthy was the absence of significant amounts of products of type 31, formed from metathesis of one cyclic and two acyclic alkenes. In fact, considering the number of possible ring-opened products that could have been formed, these reactions showed remarkable selectivity (GC yields > 80%). 

Fig.4A,B. Ring-opening metathesis polymerization (ROMP) A Structures of organometal-lic initiators that have been used in ROMP to generate neobiopolymers. B General pathway for polymer synthesis using ROMP. Molybdenum-initiated reactions are typically capped with aldehydes and ruthenium-initiated with end ethers.

The stable ruthenium alkyhdenes, used for catalysis of ring opening metathesis polymerizations, were found to exchange the alkylidene proton for a deuteron in D2O or in CD3OD (Scheme 9.4) [13],... 

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