Why do C1-symmetric ansa-zirconocene catalysts produce lower molecular weight polymer's for ethylene/propylene copolymerization than for ethylene/propylene homopolymerization?

We have carried out a combined QM/MM study to rationalize the factors that can affect the performance of C-1-symmetric ansa-zirconocene catalysts that contain bridged cyclopentadienyl (Cp) and fluorenyl (Flu) ligands in olefin homo- and copolymerization. Two growing chains with different beta-C (tertiary or secondary) and two olefins (propene and ethylene) have been used for this purpose. Our calculations indicate that chain transfer has a higher barrier than chain propagation in EE (ethylene homopolymerization), PP (propylene homopolymerization), and PE (propylene complexation to a metal with a propyl chain) systems. However, the two processes are competitive in EP (ethylene complexation to a metal with a 2-methylpropyl chain) system. Substituents on the carbon in a C-H link weaken the C-H bond. This in turn determines the order EP < EE < PP < PE for the heat of reaction of the beta-hydrogen transfer process, giving rise to the chain termination, where the process with the most negative reaction heat is the more thermodynamically favorable. It is further argued that the barriers for the termination process must follow the same order of EP < EE < PP < PE. For the insertion process the barrier increases with the number of substituents on the olefin and the C-beta atom of the growing chain as PP > PE similar to EP > EE. The different propensity of the four systems for termination and propagation results in the higher barrier of termination for EE, PP, and PE, whereas the barriers are similar for EP. Our analysis explains why ethylene/propylene homopolymerization affords high molecular weight polymers, whereas ethylene/propylene copolymerization affords low molecular weight polymers.