Accurate Prediction of Copolymerization Statistics in Molecular Olefin Polymerization Catalysis: The Role of Entropic, Electronic, and Steric Effects in Catalyst Comonomer Affinity

Accurate in silico prediction of
copolymerization performance of olefin polymerization catalysts is
demonstrated. It is shown by the example of 19 metallocene and post-metallocene
group IV metal (Ti, Zr, Hf) systems that DFT (M06-2X­(PCM)/TZ//TPSSTPSS/DZ)
can accurately describe the copolymerization factor re: i.e., the competition of ethene and propene for insertion
in metal n-alkyl bonds. Experimental re values were computationally reproduced with a mean average
deviation (MAD) and maximum deviation of only 0.2 and 0.5 kcal/mol,
respectively. Both dispersion and solvent corrections play a crucial
role in achieving this accuracy. Ethene insertion is found to be entropically
favored for all catalysts due to a combination of symmetry factors
and less congested insertion geometries. The enthalpic preference
for either ethene or propene is catalyst dependent. The predictions
are based on straightforward calculation of relevant insertion transition
state energies; there are no indications for a shift in rate-limiting
step from insertion to e.g. olefin capture or chain rotation.