New Quantum Mechanical/Stochastic Description of Copolymerization Mechanism

Polyolefin elastomers based on ethene and propene are important players in the field of synthetic rubbers and the molecular features that control bulk properties are strictly related to the monomer distributions along the chains (1). Usually, the chemical reactivity of a propagating chain is considered to depend only on the identity of monomer units at the growing end. Two reactivity ratio, r1 and r2, were defined starting from the kinetic expressions of the uncontrolled radical mechanism in order to describe the radical copolymer microstructure; mathematical models (e.g. Mayo-Lewis, Fineman-Ross and Kelen-Tudos), derived from such approach, have also been exploited to characterize homogeneous catalyzed copolymers despite the latters could present a more complicated mechanism, involving coordination preequilibria (2,3). In this study, we describe a theoretical study of E/P copolymerization by a C2-symmetric metallocene catalyst, rac-Me2C-Ind2ZrCl2 via a synergic DFT/kMC approach offering a robust alternative interpretation of homogeneously catalyzed copolymer process: the mechanism consists of at least two steps, equally important, namely coordination and insertion. If this holds, the application of Markovian models to copolymerization induced by homogeneous catalyst might request extreme care, as they hinge on the mechanism proposed for uncontrolled radical polymerization, which neglects coordination and pre-equilibria.