Conformational bias in density functional theory ring strain energy calculations of cyclopentene derivatives: Towards predictive design of chemically recyclable elastomers

Abstract

Equilibrium ring opening metathesis polymerization of low strained cycloolefins is opportunistic for the development of novel materials capable of chemical recycling to monomer (CRM). However, many of the potential materials for CRM contain complex side chains complicating predictions of their ring strain energies (RSE). The effects of different conformational considerations on RSE predictions using density functional theory (DFT) are explored. New homodesmotic equations are investigated to capture changes in olefin conformation upon polymerization. The employment of cis‐2‐butene as a corrective factor with a 2,7‐nonadiene linear analog bearing one cis and one trans olefin (H2cis) resulted in RSEs similar to previously reported ΔHp values. Different consideration of possible conformers aside from their lowest energy counterparts leads to a range of predicted RSE values. Similarly, the application of a Boltzmann distribution resulted in negligible differences in RSE. Therefore, RSE predictions using the lowest energy structures with H2cis calculated at B3LYP/6‐31+G* in toluene is a sufficient approach for predicting RSE of monomers with multiple conformers. This method can be used to screen a monomer's potential for CRM to reduce the time, cost, waste, and effort necessary to research new materials towards a more circular polymer economy.

Substituent Effects on Torsional Strain in Cyclopentene Derivatives: A Computational Study

“…We recently found that RSE calculations for CPs using homodesmotic equations can be highly sensitive to conformational considerations, but RSEs calculated using a Boltzmann ensemble of conformers yielded negligible differences to RSEs calculated using only the low-energy conformers. RSEs also agreed well with previously reported Δ H p values with a mean average deviation (MAD) and root-mean-square deviation (RMSD) of 0.97 and 1.22 kcal mol –1 , respectively, instilling confidence that more complex substituents can begin to be investigated using this method . CPs are unique since they are the only monomer amenable to ROMP with RSEs dominated by torsional strain enabling a means to study the effects of complex substituents on the most understudied type of RSE.…”

“…54,57,65−68 We also recently found that olefin geometry, presented in the H2 equations, can systematically shift RSEs by ∼2 kcal mol −1 depending on the relative numbers of cis/trans olefins on both sides of the equation. 7 Therefore, a similar phenomenon may be occurring within these calculations. B3LYP is known to ineffectively capture mid-to-long-range electron correlation energies, while the D3 correction can be used to address this shortcoming.…”

“…These calculations employed B3LYP/6-31G** and provided good agreement. 3,6,7 However, B3LYP is known to have limitations in calculating electron correlation energies, which depreciates the accuracy of larger molecules where midrange correlations are important. 48,54−57 Thermal corrections are also not reported in the previous calculations, suggesting room for improvement in the computational methods used to calculate these enthalpies.…”

“…Density functional theory (DFT) calculations have been useful in determining and studying ring strain energies (RSEs). Calculations for RSEs using homodesmotic reactions modeled after ring closing metathesis (RCM) have shown good agreement with the enthalpy of polymerization (Δ H p ) for the ring-opening metathesis polymerization (ROMP) of both cyclopentenes (CPs) and cyclooctenes. − For small-to-medium-sized rings, Δ H p is usually the driving force for polymerization where RSE is commonly perceived as the dominant energetic term in Δ H p . − Therefore, a better understanding of the complex relationships between substituents, structure, and RSE can facilitate and expedite the experimental design and synthesis of novel polymers. ,,,− …”

“…RSEs also agreed well with previously reported ΔH p values with a mean average deviation (MAD) and root-mean-square deviation (RMSD) of 0.97 and 1.22 kcal mol −1 , respectively, instilling confidence that more complex substituents can begin to be investigated using this method. 7 CPs are unique since they are the only monomer amenable to ROMP with RSEs dominated by torsional strain enabling a means to study the effects of complex substituents on the most understudied type of RSE. In this study, we expand the library of RSEs to 73 CP derivatives while simultaneously studying various potential substituent trends.…”