End-Capping Reaction of Living Anionic Poly(benzyl methacrylate) with a Pentafluorophenyl Ester for a Norbornenyl-ω-End Macromonomer with a Long Flexible Spacer: Advantage in the Well-Controlled Synthesis of Ultrahigh-Molecular-Weight Bottlebrush Polymers

12-(cis-5-Norbornene-exo-2,3-dicarboximido)dodecanoate pentafluorophenyl ester (exo-NBC12-PFP) was used as a norbornene-substituted end-capping terminator of living anionic polymers. Polystyrene with a terminal 1,1-diphenylethyllithium (PSt(DPE)−Li+), poly(2-vinylpyridine) with a terminal 2-pyridinylethyllithium (P2VP–Li+), and poly(benzyl methacrylate) with a terminal lithium ester enolate (PBzMA–Li+) reacted with exo-NBC12-PFP under appropriate reaction conditions to generate norbornenyl-ω-end macromonomers, NBC12-PSt, NBC12-P2VP, and NBC12-PBzMA, respectively, each with a 12-carbon spacer. To estimate the efficiency of end-capping, these macromonomers were polymerized by grafting-through ring-opening metathesis polymerization (ROMP). The end-capping reaction of PSt(DPE)−Li+ and P2VP–Li+ suffered from side reactions resulting in low end-capping efficiencies and the generation of by-products. On the other hand, the side reactions were minimal in the end-capping reaction of PBzMA–Li+, resulting in a high end-capping efficiency of 95%. The ROMP of NBC12-PBzMA allowed the synthesis of poly[12-(5-norbornene-exo-2,3-dicarboximido)dodecanoylate]-graft-poly(benzyl methacrylate)s (PNBC12-g-PBzMAs) with a wide range of controlled molecular weights (Mn = 436–4048 kDa, Đ = 1.07–1.23).