Ring-opening metathesis polymerization with the second generation Hoveyda-Grubbs catalyst: an efficient approach...

Herein, we present a facile and general strategy to prepare functionalized macrocyclic oligo(cyclooctene)s (cOCOEs) in high purity and high yield by exploiting the ring-opening metathesis polymerization (ROMP) intramolecular backbiting process with the commercially available second generation Hoveyda-Grubbs (HG2) catalyst. In the first instance, ROMP of 5-acetyloxycyclooct-1-ene (ACOE) followed by efficient quenching and removal of the catalyst using an isocyanide derivative afforded macrocyclic oligo(5-acetyloxycyclooct-1-ene) (cOACOE) in high yield (95%), with a weight-average molecular weight (Mw) of 1.6 kDa and polydispersity index (PDI) of 1.6, as determined by gel permeation chromatography (GPC). The structure and purity of the macrocycles were confirmed by NMR spectroscopy and elemental analysis, which indicated the complete absence of end-groups. This was further supported by GPC-matrix assisted laser desorption ionization time-of-flight mass spectroscopy (GPC-MALDI ToF MS), which revealed the exclusive formation of macrocyclic derivatives composed of up to 45 repeat units. Complete removal of residual ruthenium from the macrocycles was confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The same methodology was subsequently extended to the ROMP of 5-bromocyclooct-1-ene and 1,5-cyclooctadiene to prepare their macrocyclic derivatives, which were further derivatized to produce a library of functionalized macrocyclic oligo(cyclooctene)s. A comparative study using the second and third generation Grubbs catalysts in place of the HG2 catalyst for the polymerization of ACOE provided macrocycles contaminated with linear species, thus indicating that the bidendate benzylidene ligand of the Hoveyda-Grubbs catalyst plays an important role in the observed product distributions.