Ru-mediated ring-opening metathesis polymerization for the synthesis of complex polymer architectures

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Virginia Tech

Ring-opening metathesis polymerization (ROMP) has gained attention over the last few decades for its versatility and robustness. Through the use of highly active metal catalysts, such as Grubbs' first-generation catalyst [G1, (PCy3)2(Cl)2RuCHPh] and Grubbs' third-generation catalyst [G3, (H2IMes)(Cl)2(pyr)2RuCHPh], ROMP can exhibit living characteristics for some monomer classes, most commonly substituted norbornenes. The high livingness of ROMP makes it well-suited for the synthesis of complex polymer architectures (e.g., bottlebrush polymers, star polymers, and (multi)block copolymers, among others). However, compared to other living polymerizations, quantitative studies on the kinetic factors affecting living characteristics in ROMP are lacking.
This work describes the effects of several reaction factors on the livingness in Ru-mediated ROMP of norbornene-based monomers. We performed thorough studies on the effects of the anchor group, the series of atoms directly attached to the norbornene, in the synthesis of both linear and bottlebrush polymers. Using small molecule norbornene monomers, we studied monomer HOMO energy, rate of propagation (kp), catalyst decomposition [a proxy for the termination rate constant (kt)], and livingness in ROMP as measured by the kp/kt ratio. HOMO energies were used to predict the reactivity of various monomers based on the hypothesis that high HOMO energy would lead to high monomer reactivity and high kp values. We observed a positive correlation between the HOMO energy and kp with both G1 and G3 catalysts, but we observed a plateau in kp for monomers with the highest HOMO energy when polymerized with G3 catalyst. These results suggested that above a certain level, HOMO energy no longer influenced the rate-determining step. Additionally, the anchor group had no apparent effect on catalyst decomposition with either catalyst. Therefore, when examining the livingness in ROMP of linear polymers, differences in the kp/kt ratios were primarily controlled by the kp value. When studying the synthesis of bottlebrush polymers, we found a similar positive correlation between HOMO energy and kp for five macromonomer (MM) species. To evaluate livingness of MMs, we targeted various backbone degrees of polymerization (Nbb), 100–2000, and found that MMs with high kp,obs values reached higher conversion with lower dispersities (Đ) at high target Nbb values than MMs with low kp,obs values. Finally, we investigated the synthesis of bottlebrush pseudo-pentablock copolymers using MMs with the highest and lowest kp anchor groups. This study revealed higher MM conversion and lower Đ values for each block for the MM with the highest kp anchor group, compared to the lowest kp MM. Furthering the anchor group study, we synthesized a small molecule monomer and two macromonomers containing a norbornene–benzoladderene structure. We found this anchor group had a higher HOMO energy than all other (macro)monomers previously studied. However, kp was not higher for the small molecule monomer compared to the other monomers studied, supporting the plateau in rate observed for monomers above a certain HOMO energy. The higher HOMO energy for MMs did increase kp for one of the MM compared to other MMs previously studied, however the MM with side-chains in the ortho position on the anchor group had a lower kp than expected. This low kp was attributed to the side-chains being in closer proximity to the reactive chain end, compared to the MM with side-chains in the meta position, hindering addition of new MM units. Altogether, these experiments revealed how the anchor group impacts kp and livingness in ROMP, two factors that are essential for the synthesis of precise bottlebrush (co)polymers. The final chapter of this dissertation evaluates the effect of reaction atmosphere on livingness in Ru-mediated ROMP. Here we envisioned that changing the atmosphere, from under air to on a Schlenk line under N2 to in an N2-filled glovebox, would reduce the rate of chain termination and improve livingness in ROMP of small molecule and macromonomers. We synthesized linear or bottlebrush pseudo-pentablock copolymers to evaluate the livingness of foud (macro)monomer structures in three different atmospheres. We found better agreement between Mn,expected and Mn,obs as well as lower overall and apparent Đ values for the bottlebrush polymers polymerized in N2 atmospheres. Interestingly, the effect was more prominent for low kp (macro)monomers; in other words, ROMP reactions of (macro)monomers with high kp values were successful under air, but for those with low kp values, a glovebox was required to observe good control.

Livingness, Catalyst, Propagation, Termination, Rate, Monomer, Macromonomer