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

Accurate in silica 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 r(e): i.e., the competition of ethene and propene for insertion in metal n-alkyl bonds. Experimental r(e) 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.