A theoretical study of ethylene–styrene copolymerization by using half-sandwich Cp-based titanium catalysts

In this work we present the first attempt to study the copolymerization process of ethylene with styrene by computer simulation. Density functional theory calculations have been carried out on the detailed mechanism of ethylene–styrene copolymerization using non-bridge half-sandwich cationic species, generated by the system CpTiCl3 activated with the methylaluminoxane cocatalyst. The goal of the study is to examine how the different active oxidation states of the transition metal can influence their abilities to produce a variety of polymers. This is of particular interest when considering that under the polymerization conditions Ti(IV) can be reduced to Ti(III) catalytic species. The theoretical calculations are in good agreement with the experimentally published results. It could be shown that the cationic species [CpTiMe2]+ produces a mixture of ethylene homopolymer and ethylene–styrene copolymer whereas the active species [CpTiMe]+ is responsible for promoting only styrene homopolymerization. For the Ti(IV) catalytic system, the activation energy for the first ethylene monomer insertion is 2.9 kcal/mol, while for the styrene monomer is notably higher, being 10.4 and 9.4 kcal/mol for the 2,1 secondary and 1,2 primary insertion, respectively. The position of the phenyl ring of the styrene monomer plays a very important role in the polymerization reaction. Thus, in the 1,2 primary styrene insertion, the phenyl ring strongly interacts with the active site, blocking the active centers and avoiding the polymerization, whereas in the 2,1 secondary styrene insertion the interaction leaves one position free so that the polymerization can carry on. The monomer complexation energies after the 2,1 secondary styrene insertion is −0.9 kcal/mol for the ethylene and 0.8 and −3.3 kcal/mol for the 1,2- or 2,1-styrene monomer. On the other hand the energy barriers for monomer insertions are 3,3 kcal/mol for the ethylene and 20.6 kcal/mol for the 2,1 secondary styrene insertion. When the two monomers are competing for the Ti(III) catalytic species the energy barrier for the first monomer insertion is very high for the ethylene (18.2 kcal/mol) and 9.7 kcal/mol for the styrene monomer. Once the initial styrene insertion has taken place further ethylene insertion has also a very high energy barrier of 15.5 kcal/mol as compared to 5.8 kcal/mol for the styrene. Therefore, styrene polymerization is far more likely to occur than ethylene polymerization.