Effect of macromonomer chemical structure on the rate of grafting-through ring-opening metathesis polymerization

Abstract

Ring-opening metathesis polymerization (ROMP) of norbornenyl macromonomers (MM) has become a preferred method to synthesize bottlebrush (BB) polymers with perfect grafting density. Prior investigations had established that fast ROMP rates were necessary to synthesize clean BB polymers and that the chemical structure of the anchor group linking the MM polymeric chain and norbornene unit had the most influence on the rate of ROMP. While the order of fastest anchor groups was predicted to be a general trend across all MM chemistry, we observed that the fastest anchor group for a poly(propylene oxide) (PPO) MM is not the fastest for a poly(lactic acid) (PLA) MM. From our kinetic analysis, we showed that PLA MMs are faster with a methylene linker and that PPO MMs are faster with a dicarboximide linker. Interestingly, however, the fast rate of the dicarboximide PPO MM did not lead to the synthesis of a PPO BB polymer with lower dispersity. While the root cause for this MM chemistry sensitivity is still unclear, this study shows that despite being far from the reactive group of the norbornene unit, the chemical structure of MM polymeric chain influences the rate of ROMP beyond the anchor group chemistry.