Solution-Phase Conformational/Vibrational Anharmonicity in Comonomer Incorporation Polyolefin Catalysis

The prediction of comonomer incorporation statistics
in polyolefin
catalysis necessitates an accurate calculation of free energies corresponding
to monomer binding and insertion, often requiring sub-kcal/mol resolution
to resolve experimental free energies. Batch reactor experiments are
used to probe incorporation statistics of ethene and larger α-olefins
for three constrained geometry complexes which are employed as model
systems. Herein, over 6 ns of quantum mechanics/molecular mechanics
(QM/MM) molecular dynamics is performed in combination with the zero-temperature
string method to characterize the solution-phase insertion barrier
and to analyze the contributions from conformational and vibrational
anharmonicity arising both in vacuum and in solution. Conformational
sampling in the solution-phase results in 0–2 kcal/mol corrections
to the insertion barrier which are on the same scale necessary to
resolve experimental free energies. Anharmonic contributions from
conformational sampling in the solution phase are crucial energy contributions
missing from static density functional theory calculations and implicit
solvation models, and the accurate calculation of these contributions
is a key step toward the quantitative prediction of comonomer incorporation
statistics.