Synthesis of ABA triblock copolymers of norbornenes and 7-oxanorbornenes via living ring-opening metathesis polymerization using well-defined, bimetallic ruthenium catalysts

Norbornene and 7-oxanorbornene derivatives were polymerized by ring-opening metathesis polymerization (ROMP) in a living manner by employing three new well-defined, bimetallic ruthenium catalysts (PR₃)₂Cl₂Ru(=CH-p-C₆H ₄C(H)=)RuCl₂(PR₃)₂ [R = Ph, 1; R = Cy (cyclohexyl), 2; R = Cyp (cyclopentyl), 3] as initiators to obtain ABA triblock copolymers with low polydispersities. 1-3 were synthesized via alkylidene transfer from 1,4-bis(α-diazomethyl)benzene to RuCl₂(PPh₃)₃ and subsequent phosphine exchange. Reactions of 7-oxanorbornenes 5 and 6 or a silicon-containing norbornene 7 with 3 resulted in polymers with low polydispersities ranging from 1.10 to 1.22, while polymerizations initiated by 2 displayed higher polydispersities ranging from 1.20 to 1.35. Polymerizations of 5-7 catalyzed by 2 and 3 fulfill the requirements for a living polymerization. However, catalyst 1 is not reactive enough to polymerize functionalized norbornenes or 7-oxanorbornenes. The living polymerizations were successfully used to prepare three ABA triblock copolymers of monomers 5-7. In all cases, these ABA triblock copolymers have higher molecular weights than the homopolymers, and all polydispersities remained low.