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

Abstract

Ring-opening metathesis polymerization (ROMP) mediated by Grubbs’ third-generation catalyst [G3, (H2IMes)(Cl)2(pyr)2RuCHPh] is widely used to make bottlebrush polymers by polymerization of a macromonomer (MM), typically a low molecular weight polymer functionalized with a norbornene. Termed the grafting-through method, this strategy requires a high degree of living character (“livingness”) to form well-defined bottlebrush polymers. Here, we studied how various anchor groups, the series of atoms connecting the polymerizable norbornene unit to the polymer side-chain, affect livingness in ROMP in a series of exo-norbornene polystyrene MMs. First, we calculated the HOMO and HOMO/LUMO gap energies of MM structures containing five different anchor groups using density functional theory methods, finding that these energies spanned a range of 10 kcal/mol. We then performed kinetics experiments on each MM with target backbone degrees of polymerization (N bb) of 100 to measure the propagation rate constant (k p,obs) under identical conditions. A positive correlation between the HOMO energy and measured k p,obs values emerged, revealing a 7-fold variation in k p,obs values across the five MMs, suggesting different degrees of livingness among the anchor groups. A series of studies targeting N bb values ranging from 100 to 2000 further highlighted these differences: The MMs with high k p,obs values reached higher conversions at high target N bb values with lower dispersities (D̵) than the MMs with lower k p,obs values. Finally, we evaluated the synthesis of bottlebrush pentablock copolymers using the MMs at the two extremes by injecting an MM aliquot into a catalyst solution five consecutive times, allowing for polymerization of each block before the next injection. MM conversion at each step was higher, and the D̵ values for each block were lower for the MM with the highest k p anchor group compared to the lowest k p anchor group. Taken together, these studies highlight how the anchor group dramatically affects both k p and livingness in ROMP, which is crucial for the synthesis of precise bottlebrush (co)polymers.

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

“…We attribute this to an apparent influence of the degree of polymerization on k p , where k p decreases at high conversion due to the steric demands of the growing bottlebrush macromolecule, a phenomenon we have observed previously. 49 Additionally, instead of reaching complete conversion, both MMs leveled off at 95% conversion based on SEC, even though 1 H NMR spectroscopy confirmed complete (>99%) consumption of the norbornene in all three cases. This phenomenon was previously observed in other norbornene MMs made by ATRP and is attributed to the presence of ∼5% MM species that lack a norbornene end group.…”

“…25,46,47 Our group has studied the effects of the anchor group in ROMP, and we have found that the energy of the HOMO centered on the norbornene olefin of various (macro)monomer structures is a reasonable predictor for olefin reactivity, showing a positive correlation with k p values. 48,49 Therefore, development and characterization of new anchor groups broadens the scope of bottlebrush polymer materials by increasing (macro)monomer structural diversity and allowing for tunability of monomer olefin reactivity.…”

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

“…When synthesizing more complex topologies by ROMP, i.e., bottlebrush polymers, choice of anchor group becomes critical to reach high degrees of polymerization and achieve low dispersity polymers. A companion paper to this one focuses on this topic …”

Solution Structure and Scaling Laws of Cylindrical and Tapered Bottlebrush Polymers

“…Chemists have established precise polymerization techniques to create polymers with complex topologies that resemble macroscopic architectures found in nature . Such complex polymer topologies are not merely academic curiosities; polymers with sophisticated and distinct structures allow for creating materials with specialized properties and functionalities. , In particular, the bottlebrush polymer architecture, which contains a linear backbone with densely grafted polymeric side chains (Figure , left), provides exceptional potential for controlling polymer structure and properties by modifying the composition and degree of polymerization of the backbone and side chains ( N bb and N sc , respectively). − Recently, our group and others have added a new dimension to bottlebrush polymer synthesis by systematically varying the molecular weight of the side chains along the backbone to create nanoscale cones termed tapered bottlebrush polymers (Figure , right). − Cone-shaped structures appear in nature, for example, in the HIV viral envelope and in the protein assemblies that form the tobacco mosaic virus. , Cone-shaped materials and other noncentrosymmetric structures have also been the center of experimental − and computational − investigations of self-assembled nanostructures that mimic naturally occurring molecular architectures or that readily integrate into biological materials.…”