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.
Highlights
In the case of the anionic polymerization of styrene in non-polar solvents, it is generally accepted that polystyryllitium is mainly associated into dimeric species (PStLi)2 in equilibrium with a small amount of non-associated PStLi chains [1,2,3,4,5]
In the case of the anionic polymerization of styrene in non-polar solvents, it is generally accepted that polystyryllitium is mainly associated into dimeric species and only a small amount of non-associated polystyryllithium species can propagate
It was shown that polystyryllithium mainly associated into dimeric species and a small amount of non-associated species reacted with styrene; its relative enthalpy of transition state in cyclohexane agreed with the apparent activation energies experimentally observed by Worsfold et al and Ohlinger et al Further, the most stable transition state was found to be the one with a new structure and the reason for the penultimate unit effect was described
Summary
In the case of the anionic polymerization of styrene in non-polar solvents, it is generally accepted that polystyryllitium is mainly associated into dimeric species (PStLi) in equilibrium with a small amount of non-associated PStLi chains [1,2,3,4,5]. The possibility of a reaction of styrene with dimeric (PStLi) or higher aggregates was proposed based on experimental data, such as the addition of butadiene to freeze-dried polystyryllithium and the Polymers 2019, 11, 1022; doi:10.3390/polym11061022 www.mdpi.com/journal/polymers. In our previous study [20], we described the optimization of the anionic polymerization of styrene with non-associated polystyryllithium using the density functional theory (DFT) calculation method, M062x/6-31+G(d) [21]; the following aspects were observed in that case. The most stable transition state was the one in which Li was located near the phenyl rings of both styrene and polystyryllithium chain end, and the other transition state in which Li was located near the side chains of both styrene and polystyryllithium chain end, which was previously supposed to be the only transition state, was less stable than the former
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