1,4-Polybutadienes with Pendant Hydroxyl Functionalities by ROMP: Synthetic and Mechanistic Insights

The reactivity of <i>cis</i>-3,4-bis­(hydroxymethyl)­cyclobutene derivatives bearing free and protected hydroxyl groups during ring-opening metathesis polymerization (ROMP) was investigated using ruthenium-based initiators. It was found that the ROMP of <i>cis</i>-4-benzyloxymethyl-3-hydroxymethylcyclobutene (<b>1</b>) using highly reactive initiators containing <i>N</i>-heterocyclic carbenes as nonlabile ligands leads to well-defined polymers while <i>cis</i>-3,4-bis­(hydroxymethyl)­cyclobutene (<b>2</b>) was reluctant to polymerize under the same conditions. Kinetic studies were performed to assess a number of critical reaction parameters: initiator structure, solvent, and temperature. The results demonstrate that Grubbs’ second- and third-generation catalysts are the best initiators to prepare well-defined 1,4-polybutadienes containing simultaneously free and protected hydroxyl side groups with predictable molecular weights (up to 40 000 g mol^–1) and narrow molecular weight distributions. Besides, low values of <i>k</i>_p/<i>k</i>_i (the ratio of the rate constant of propagation to the rate constant of initiation) were found for the ROMP of monomer <b>1</b> with Grubbs’ second-generation catalyst in chloroform or THF as the solvent, demonstrating a living process. The so-obtained polymers having hydroxyl side groups are an ideal platform to prepare original well-defined graft copolymers through the grafting-from strategy