Comparing Long-Chain Branching Mechanisms for Ethylene Polymerization with Metallocenes and Other Single-Site Catalysts: What Simulated Microstructures Can Teach Us

Three different long-chain branch (LCB) formation mechanisms for ethylene polymerization with metallocenes in solution polymerization semi-batch and continuous stirred-tank reactors are modeled to predict the microstructure of the resulting polymer. The three mechanisms are terminal branching, C-H bond activation, and intramolecular random incorporation. Selected polymerization parameters are varied to observe how each mechanism affects polymer microstructure. Increasing the ethylene concentration during semi-batch polymerization reduces the LCB frequency of polymers made with the terminal branching and intramolecular mechanisms, but has no effect on those made with the C-H bond activation mechanism, which disagrees with most previous data published in the literature. The intramolecular mechanism predicts that LCB frequencies hardly depend on polymerization time or ethylene conversion, which also disagrees with the published experimental data for these systems. For continuous polymerization reactors, experimental data relating polydispersity to LCB frequency can be well described with the terminal branching mechanism, but both C-H bond activation and intramolecular models fail to describe this experimental relationship. Therefore, detailed simulations confirm that the terminal branching mechanism is indeed the most likely mechanism for LCB formation when ethylene is polymerized with single-site coordination catalysts such as metallocenes in solution polymerization reactors.