Influence of the Norbornene Anchor Group in Ru-Mediated Ring-Opening Metathesis Polymerization: Synthesis of Bottlebrush Polymers

Ring-opening metathesis polymerization (ROMP) mediatedby Grubbs'third-generation catalyst [G3, (H(2)IMes)-(Cl)(2)(pyr)(2)RuCHPh] is widely used to make bottlebrush polymersby polymerization of a macromonomer (MM), typically a low molecularweight polymer functionalized with a norbornene. Termed the grafting-throughmethod, this strategy requires a high degree of living character (livingness)to form well-defined bottlebrush polymers. Here, we studied how variousanchor groups, the series of atoms connecting the polymerizable norborneneunit to the polymer side-chain, affect livingness in ROMP in a seriesof exo-norbornene polystyrene MMs. First, we calculatedthe HOMO and HOMO/LUMO gap energies of MM structures containing fivedifferent anchor groups using density functional theory methods, findingthat these energies spanned a range of 10 kcal/mol. We then performedkinetics experiments on each MM with target backbone degrees of polymerization(N (bb)) of 100 to measure the propagationrate constant (k (p,obs)) under identicalconditions. A positive correlation between the HOMO energy and measured k (p,obs) values emerged, revealing a 7-fold variationin k (p,obs) values across the five MMs,suggesting different degrees of livingness among the anchor groups.A series of studies targeting N (bb) valuesranging from 100 to 2000 further highlighted these differences: TheMMs with high k (p,obs) values reached higherconversions at high target N (bb) valueswith lower dispersities (D) than the MMs withlower k (p,obs) values. Finally, we evaluatedthe synthesis of bottlebrush pentablock copolymers using the MMs atthe two extremes by injecting an MM aliquot into a catalyst solutionfive consecutive times, allowing for polymerization of each blockbefore the next injection. MM conversion at each step was higher,and the D values for each block were lowerfor the MM with the highest k (p) anchorgroup compared to the lowest k (p) anchorgroup. Taken together, these studies highlight how the anchor groupdramatically affects both k (p) and livingnessin ROMP, which is crucial for the synthesis of precise bottlebrush(co)-polymers.

Automated summary

Ring-opening metathesis polymerization (ROMP) mediated by Grubbs' third-generation catalyst (G3) is widely used to make bottlebrush polymers. This study investigated the effect of anchor groups on the livingness of ROMP, finding a positive correlation between anchor group energy levels and reaction rate constants, suggesting different degrees of livingness among the anchor groups.