Resonance promoted ring-opening metathesis polymerization of twisted amides

The twisting of an amide bond provides a new driving force for living ring-opening metathesis polymerization through resonance destabilization.


General Ring-Opening Metathesis Polymerization (ROMP) Procedure/Examples
Twisted amide 1 was dissolved in DCM in a 4 mL vial, followed by rapid addition of a G3 stock solution in DCM to form reaction solution (0.2 M for the monomer). The reaction was stirred at room temperature for indicated time, and then quenched by ethyl vinyl ether (EVE) and the solvent was removed under vacuum. A small amount of the crude product was dissolved in CDCl3 to determine the monomer conversion by 1 H NMR. The product was purified by precipitating into Et2O/hexane (1/1, v/v), and then characterized using SEC, NMR and MALDI-TOF-MS analyses.  the reaction was quenched by EVE (100 µL) and the solvent was removed under vacuum. A small amount of the crude product was dissolved in CDCl3 to determine the monomer conversion by 1 H NMR (91%). The crude polymer P1ROMP-100 was precipitated three times by slowly adding a concentrated DCM solution (2 mL) into a Et2O/hexane mixture (9 mL/9 mL) with vigorous stirring, and then dried under vacuum. The purified polymer was then characterized using 1 H-NMR and SEC analyses (Mn: 15.2 kg/mol, Ð: 1.20).

Attempted Polymerization of Ketone 2
Ketone 2 (18.4 mg, 0.1 mmol, 50 equiv) was dissolved in DCM (450 µL) in a 4 mL vial. A solution of G3 (1.45 mg, 0.002 mmol, 1 equiv) in DCM (50 µL) was rapidly added to the ketone solution with stirring. After stirred at room temperature for 3 h, the reaction was quenched by EVE (100 µL) and the solvent was removed under vacuum. The crude mixture was analyzed by 1 H NMR, and 2 remained intact with no polymerization occurring.

Synthesis of Polymer P2 through halide-rebound polymerization (HaRP)
According to our reported method, twisted amide monomer S4 was synthesized and corresponding HaRP was performed to synthesize polymer H2-P1HaRP (targeting DP 100). 5 The NMR data of S4 and H2-P1HaRP matched those from literature.
The reaction was then cooled down to room temperature, and diluted with DCM, and washed with 0.1M HCl, aqueous sat. NaHCO3, water and brine. The organic phase was dried (Na2SO4), filtered and concentrated. The crude mixture was precipitated for three times     [2b]). The assignments of aromatic proton b and proton d were determined by COSY experiments of H2-P1HaRP and H2-P1ROMP ( Figure S38 and S43), respectively. In addition, based on HMBC spectra of H2-P1HaRP and H2-P1ROMP, ( Figure S17B and C), carbon a (37.1 ppm) correlating with aromatic proton b is assigned as the benzylic carbon in H2-P1HaRP, and carbon c (37.2, 37.9 ppm) correlating with aromatic proton d is assigned as the benzylic carbon in H2-P1ROMP. Therefore, HSQC spectra of H2-P1HaRP and H2-P1ROMP ( Figure S17D and E) confirm the assignments of proton a (2.91 ppm) and proton c (2.91, 2.73 ppm) as the benzylic protons in H2-P1HaRP and H2-P1ROMP, respectively. The existence of two types of benzylic protons c in H2-P1ROMP is consistent with the two possible types of connection (tail-to-tail and tail-to-head) on the benzylic position ( Figure S17F).

Polymer Thermal Properties
TGA curves of purified P1ROMP and H2-P1ROMP samples were obtained in a nitrogen atmosphere at a heating rate of 10 °C/min.

Computational Studies
Computational Methods. All the calculations were carried out using Psi4 with the standard grid size (75,302). 7 All of the geometry optimizations and frequency analysis were performed at the B3LYP-D3MBJ/6-311++G(d,p) level of theory in the gas phase. The absence of imaginary frequencies was used to characterize the structures as minima on the potential energy surface. All of the optimized geometries were verified as minima (no imaginary frequencies). Electronic and thermal energies were calculated for all structures. Energetic parameters were calculated under standard conditions (298.15 K and 1 atm). 8 Strain energies and resonance energies were calculated using isodesmic reactions based on total energies (with zero-point energy and thermal corrections) for optimized structures. Gibbs free energies were determined using vibrational frequency calculations.