Abstract
The elementary processes of anionic styrene polymerization in the gas phase and in cyclohexane were studied using M062X (a recently developed density functional theory (DFT) method) combined with the 6-31+G(d) basis sets, in order to clarify the complicated phenomena caused by the association of the active chain-ends and elucidate the details of the polymerization mechanism. Three types of HSt2Li (a model structure of polystyryllithium chain-ends) were obtained; the well-known first structure in which Li is coordinated to the side chain, the second structure in which Li is coordinated to the phenyl ring, (both without the penultimate unit coordination), and the third structure in which Li is coordinated to both the chain-end unit and the penultimate styrene unit. Although the third HSt2Li is the most stable as expected, the free energy for the transition state of its reaction with styrene is higher than those for the other two transition states due to its steric hindrance. The free energy for the transition state of the reaction of the second HSt2Li with styrene is the lowest, suggesting that the route through it is the predominant reaction path. The penultimate unit effect, slower addition of styrene to HSt2Li than to HStLi, is attributed to coordination of the penultimate styrene units of the polystyryllithium dimer (one of the starting materials) to its Li atoms. The calculated enthalpy for the reaction barrier of the second HSt2Li with styrene in cyclohexane was found to agree with the observed apparent activation energy in benzene.
Highlights
Anionic polymerization of vinyl monomers such as dienes and styrene is currently the best known and most controllable of all types of polymerization
The purpose of this study is to elucidate in detail the mechanism of styrene anionic polymerization with organolithium initiators in non-polar solvents by means of a recently developed theoretical method
Polystyryllithium species that are obtained by the addition of styrene to alkyllithium can be expressed as R(St)m Li, where R denotes the alkyl group of the initiator
Summary
Anionic polymerization of vinyl monomers such as dienes and styrene is currently the best known and most controllable of all types of polymerization. Its course and mechanism, studied by M. For the anionic polymerization of styrene in non-polar solvents, it is generally accepted that polystyryllithium (PStLi) is mainly associated into dimers (PStLi) in equilibrium with a small amount of non-associated PStLi chains. A possibility of the reaction of dimeric (PStLi) and higher aggregates with styrene was suggested on the basis of the experimental data such as addition of butadiene to freeze-dried polystyryllithium and existence of the higher aggregates demonstrated by means of light scattering and small angle neutron scattering [8,9]
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