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

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 (kp) 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 kp values, consistent with the high HOMO energies of the anchor groups, a predictor of kp. Conversely, when the side-chain was in the ortho position, closer to the reactive olefin, kp 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 kp 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. We report high kp (macro)monomer structures for use in grafting-through ring-opening metathesis polymerization to make linear and bottlebrush polymers.

Automated summary

This study introduces a new anchor group, NBL, for ROMP and evaluates the polymerization rates of different positions of side-chain macromonomers. The results show that macromonomers with side-chains in the meta position have higher polymerization rates. Conversely, macromonomers with side-chains in the ortho position have lower polymerization rates, possibly due to steric interference.