The Mechanism of the Propagation in the Anionic Polymerization of Polystyryllithium in Non-Polar Solvents Elucidated by Density Functional Theory Calculations. A Study of the Negligible Part Played by Dimeric Ion-Pairs under Usual Polymerization Conditions

Abstract

The elementary processes occurring in the anionic polymerization of styrene with dimerically associated polystyryllithium (propagation during the anionic polymerization of dimeric polystyryllithium) in the gas phase and cyclohexane were studied using MX062X/6-31+G(d), a recently developed density functional theory (DFT) method and compared with the polymerization of styrene with non-associated polystyryllithium, which was described in a previous study. The most stable transition state in the reaction of styrene with dimeric polystyryllithium has a structure in which the side chains of styrene and the two chain end units of polystyryllithium are located in the same direction around the Li atom near the reactive site. The relative enthalpy for this transition state in cyclohexane is 28 kJ·mol−1, which is much lower than that for the reaction of non-associated polystyryllithium (51 kJ·mol−1). However, the relative free energy (which determines the rate constant) for the former is 93 kJ·mol−1, which is greater than that for the latter by 7 kJ·mol−1, indicating that the latter reaction (reaction with non-associated polystyryllithium) is advantageous over the former (reaction with dimeric polystyrylllithium). Their rates of reaction are also affected by initiator concentrations; in the case of reactions with low initiator concentrations, from which high molecular weight polymers are usually obtained, the rate of reaction corresponding to non-associated polystyryllithium is much larger than that corresponding to dimeric polystyryllithium.

Keywords

• anionic polymerization
• dimeric ion-pairs
• dimeric polystyryllithium
• density functional theory
• non-polar solvent
• polymerization mechanism
• styrene