A Quantum Mechanical Approach for Accurate Rate Parameters of Free-Radical Polymerization Reactions

Abstract This chapter provides a robust and computationally inexpensive DFT protocol to accurately estimate reaction rate parameters for free-radical polymerization (FRP). The protocol is applied to a series of small molecules and excellent agreement is achieved with experimental data in gas and condensed phases. It is also shown that including the contributions from multiple reaction pathways is extremely important. In gas phase, the raw (CP)M06-2X/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) calculations are sufficient to provide the desired accuracy. In condensed phase with a low dielectric constant, the best overall rate parameters are obtained using gas-phase calculations. However, the agreement is fortuitous only due to under-prediction of the Arrhenius parameters. A direct implicit solvation model and thermodynamic cycle calculations deliver an accurate activation energy but suffer from a low pre-exponential factor. We recommend that a two-parameter entropy scaling model be utilized to reach the targeted accuracy of the overall rate coefficient and the individual Arrhenius parameters.