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

Abstract

Ring-opening metathesis polymerization (ROMP) mediated by Grubbs' first-generation catalyst [G1, (PCy 3 ) 2 (Cl) 2 RuCHPh] and Grubbs' third-generation catalyst [G3, (H 2 IMes)(Cl) 2 (pyr) 2 RuCHPh] can exhibit living characteristics for some monomer classes, most commonly substituted norbornenes. Here, we studied how various anchor groups, the series of atoms connecting the polymerizable norbornene unit to a functional group, affect livingness in ROMP in a series of small-molecule exo-norbornene monomers. We first designed and calculated the HOMO energy of 61 monomers using density functional theory methods, finding that these energies spanned a range of 25 kcal/mol. We then performed kinetics experiments using 1 H NMR spectroscopy to measure the propagation rate constant (k p,obs ) under identical conditions for eight selected monomers with different anchor groups across the range of HOMO energies. We observed a positive correlation between the HOMO energy or the HOMO/LUMO energy gap and measured k p,obs values for both catalysts, revealing a 30-fold and a 10-fold variation in k p,obs values across the series for G1 and G3, respectively. Interestingly, we observed a plateau for the three monomers with the highest HOMO energies for G3 catalyst, suggesting that above a certain level, the HOMO energy no longer influenced the rate-determining step under the conditions studied here. Chelation studies revealed that only one of the eight monomers showed measurable binding of electron-rich groups on the monomer to the catalyst, but with no apparent effect on k p . Finally, we utilized 1 H NMR spectroscopy to measure the rate of catalyst decomposition in the presence of each monomer, a key termination pathway in ROMP. Ultimately, we determined that the anchor group did not substantially affect catalyst decomposition, a proxy for the termination rate constant (k t ). In sum, these combined computational and experimental studies collectively demonstrate that livingness in ROMP of exo-norbornene monomers using G1 and G3 catalysts, as measured by relative k p /k t ratios, is primarily controlled by the k p of the anchor group, which is correlated with the HOMO energy.

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

“…Polymerizations of all MMs were mediated by G3 catalyst with a targeted N bb of 100 at a concentration of 20 mM in CDCl 3 , under air, and at room temperature. Aliquots were removed As expected, each MM polymerized slower, by approximately an order of magnitude, compared with analogue small molecule monomer structures, 49 where the only difference lies in the presence of the PS side-chain. MM x-ME'-PS (2) had the highest k p,obs out of all the MMs tested and polymerized 7fold faster than the MM with the lowest k p,obs [xx-IM 2 E'-PS (5)].…”

“…In our companion paper to this one, we calculated the HOMO and HOMO/LUMO gap energies of 61 monomers with different anchor groups for the synthesis of linear polymers via ROMP. 49 All HOMOs were centered on the norbornene alkene, which interacts with the Ru center in the ratedetermining metallacyclobutane formation step in ROMP. 51 Of these 61 monomers, we selected eight anchor groups to synthesize and study experimentally with the goal of identifying the effects of the anchor group on k p and livingness in ROMP.…”

“…The HOMOs localized on the reactive olefins span energies in the −197 to −187 kcal/mol range, similar to analogous small molecule structures in our companion paper. 49 In this related paper, 49 we also calculated the HOMO/LUMO energy gap for each monomer based on the concept that multiple orbital interactions occur during the formation of the metallacyclobutane ring, as suggested by Suresh and Koga. 54 Here, we hypothesized that MMs with higher norbornene HOMO energies and lower LUMO olefin energies would exhibit faster polymerization rates (k p values) and higher livingness in general, as measured in maximum obtainable N bb studies and bottlebrush pentablock copolymer chain extension studies.…”

“…48 In a companion paper to this one, we presented a thorough investigation of this phenomenon through a combined computational and experimental approach, measuring the propagation and termination rates of small molecule monomers in the synthesis of linear polymers. 49 In this work, we monitored the polymerization of eight different monomers mediated by G3 catalyst as well as the less active Grubbs' 1st generation catalyst [G1, (PCy 3 ) 2 (Cl) 2 RuCHPh]. The monomers had varying anchor groups, which are the series of atoms directly connected to the polymerizable unit used in ROMP (norbornene here).…”

“…We set out to study five anchor groups of interest, specifically those suitable for attaching a polymer chain to the norbornene as well as easily comparable to equivalent structures in our companion paper. 49 Through the use of computational methods to determine HOMO energy values and experimental methods to measure k p values, we investigated the effects of the anchor group in the ROMP of MMs. Additionally, we aimed to study how k p affects livingness in ROMP by monitoring MM conversion and the maximum obtainable bottlebrush polymer N bb at high [MM]/[G3] ratios (up to 2000).…”