Ring-opening metathesis polymerization of norbornene–benzoladderene (macro)monomers

Abstract

Norbornene-based monomers are common starting points in ring-opening metathesis polymerization (ROMP) due to their high ring strain, and recent reports highlight how the structure of norbornene derivatives influences polymerization rates. In particular, the anchor group, the series of atoms directly connected to the norbornene, critically affects polymerization. In this work, we introduce a new anchor group, the exo-norbornene–benzoladderene (NBL), for use in ROMP. We synthesized one small molecule NBL monomer and two polystyrene (PS) macromonomers (MMs), each containing a polystyrene side-chain, varying the position where the side-chain was attached to the anchor group arene ring. We then evaluated propagation rate constants (k_p) and examined their correlation with calculated HOMO energies using density functional theory. The small molecule monomer and the MM with the PS side-chain in the meta position had high k_p values, consistent with the high HOMO energies of the anchor groups, a predictor of k_p. Conversely, when the side-chain was in the ortho position, closer to the reactive olefin, k_p was 3.3-fold lower than the meta-MM. We hypothesized that the unexpectedly slow polymerization of the ortho-MM was due to steric interference between the growing bottlebrush side-chains and the coordinating MM, resulting in a lower k_p than predicted. In copolymerization studies of these MMs with a small molecule diluent monomer, we found a negligible difference in the MM propagation rate between the two MMs, supporting our hypothesis. Ultimately, the introduction of the NBL structure as an anchor group broadens the scope of (macro)monomer structures available for ROMP.