Zirconocene-catalyzed propene polymerization: a quenched-flow kinetic study

Area of Science:

• Polymer Chemistry
• Organometallic Chemistry
• Catalysis

Background:

• Ansa-metallocenes are crucial catalysts for alkene polymerization, but their mechanistic details, especially regarding active versus dormant species, remain incompletely understood.
• The distribution of catalyst species between active chain growth and dormant states significantly impacts polymerization kinetics and efficiency.

Purpose of the Study:

• To investigate the kinetics of propene polymerization using two distinct ansa-metallocene catalyst systems.
• To elucidate fundamental mechanistic aspects, including catalyst initiation, chain propagation rates, and the nature of dormant species.
• To establish a quantitative correlation between catalyst structure and polymerization rates.

Main Methods:

• Utilized quenched-flow techniques to study propene polymerization kinetics under varying monomer and catalyst concentrations.
• Employed two catalyst systems: (SBI)ZrMe2/Al(i)Bu3/[Ph3C][CN[B(C6F5)3]2] and (SBI)ZrCl(2)/methylalumoxane (MAO).
• Analyzed polymer yield (Y) versus reaction time (t) and number-average polymer molecular weight evolution.

Main Results:

• Both catalyst systems exhibited first-order dependence on propene and zirconocene concentrations.
• Apparent chain growth rate constants (k(p)(app)) were determined, revealing that only approximately 8% of the total catalyst was actively involved in chain growth for both systems.
• The borate system showed significantly faster propagation relative to initiation (k(p)/k(i) ≈ 6000) compared to the MAO system (k(p)/k(i) ≈ 800).

Conclusions:

• 2,1-regioerrors in propene insertion are identified as a primary cause for chain termination and the formation of dormant species, even at short reaction times.
• The energetics of chain growth are strongly influenced by the counteranion's nature, suggesting associated ion pairs play a role.
• Despite differences in activity, the fraction of active catalyst species remained consistent, highlighting the importance of dormant species in metallocene catalysis.