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. 2019 Aug 30;10(42):9729-9734.
doi: 10.1039/c9sc03602d. eCollection 2019 Nov 14.

Resonance promoted ring-opening metathesis polymerization of twisted amides

Mizhi Xu  1 , Krista K Bullard  1 , Aja M Nicely  1 , Will R Gutekunst  1
Affiliations

Resonance promoted ring-opening metathesis polymerization of twisted amides

Mizhi Xu et al. Chem Sci. .

Abstract

The living ring-opening metathesis polymerization (ROMP) of an unsaturated twisted amide using the third-generation Grubbs initiator is described. Unlike prior examples of ROMP monomers that rely on angular or steric strain for propagation, this system is driven by resonance destabilization of the amide that arises from geometric constraints of the bicyclic framework. Upon ring-opening, the amide can rotate and rehybridize to give a stabilized and planar conjugated system that promotes living propagation. The absence of other strain elements in the twisted amide is supported by the inability of a carbon analogue of the monomer to polymerize and computational studies that find resonance destabilization accounts for 11.3 kcal mol-1 of the overall 12.0 kcal mol-1 ring strain. The twisted amide polymerization is capable of preparing high molecular weight polymers rapidly at room temperature, and post-polymerization modification combined with 2D NMR spectroscopy confirms a regioirregular polymer microstructure.

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Figures

Fig. 1
Fig. 1. (A) Typical ROMP and cycloolefin monomers. (B) Halide-rebound polymerization of twisted amide 1. (C) Resonance promoted ROMP of 1.
Fig. 2
Fig. 2. (A) Size-exclusion chromatograms of P1ROMP targeting different DPs. (B) MALDI-TOF-MS spectrum of P1ROMP. (C) First-order kinetic plot for 1 targeting DP 100. (D) Mn-conversion correlation (blue) and Ð-conversion correlation (red).
Fig. 3
Fig. 3. (A) Attempted ROMP of ketone 2. (B) Calculated ring strain energies of twisted amide 1 and ketone 2 from isodesmic ring-opening reaction with ethylene. (C) Resonance energies of amide 1 and 3 determined by COSNAR method (B3LYP-D3MBJ/6-311++G(d,p)).
Fig. 4
Fig. 4. (A). Reduction of P1ROMP to generate saturated polymer H2-P1ROMP. (B) Stacked 1H NMR spectra and microstructures of H2-P1HaRP and H2-P1ROMP.
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