DFT Study of Ethylene and Propylene Copolymerization over a Heterogeneous Catalyst with a Coordinating Lewis Base

Abstract

The copolymerization of ethylene and propylene over a heterogeneous Ti(III) catalyst containing tetrahydrofuran (THF) as a Lewis base and MgCl2 as a support has been studied by means of DFT. Two feasible models of active sites have been examined thoroughly, and one of them turned out to be favorable in terms of both catalytic activity and the microstructure of the resulting polymer. The external barriers of olefin insertion for this model range from 3.1 to 16.0 kcal/mol and are influenced by a variety of factors, such as the structure of the growing polymer chain and the nature of the incoming olefin as well as the orientation of the ligands around the titanium atom. Stochastic simulations performed on the basis of insertion and termination barriers provided us with the insight into the composition and microstructure of the copolymer as well as its molecular weight as a function of comonomer partial pressures. It is demonstrated that the reactivity of ethylene in the copolymerization process is significantly higher than that of propylene, which is consistent with known experimental data. Our results also indicate moderate regiospecificity and stereoselectivity toward propylene, depending on the partial pressures of the comonomers.

Coadsorption model for first-principle description of roles of donors in heterogeneous Ziegler–Natta propylene polymerization

“…However, in separate calculations using MgCl 2 clusters with tractable sizes, these contributions were mostly constant among different pathways for the insertion and chain transfer: about 16-17 kcal/mol at 350 K for the entropy and about À1 kcal/mol for the zero-point vibration. Similar insensitivities were previously reported by Ziegler and his coworkers [70,71]. Consequently, the energies of arbitrary two pathways were believed to be nicely compared even without those contributions, for example, in estimating the stereo-and regioselectivities of propylene insertion, the most probable molecular weight, and so on.…”

“…The DE ad values calculated for the propylene p complexation with Ti-iBu were always lower than the entropic losses around 14 kcal/mol, estimated using a cluster model at 350 K. This fact assures that active sites are far from fully covered by propylene at typical polymerization temperatures, thus validating the usage of the apparent activation energies in subsequent insertion and chain-transfer reactions [14,70]. Summing up the equilibrium populations calculated for different structures of the p complexes at 350 K (Tables 3 and 4), the equilibrium populations of the p complexes with specific enantiofaces of propylene or with specific conformations of the growing chain were given (Table 5).…”

“…Apparent activation energies are normally equal to those experimentally obtained from the Arrhenius plots for heterogeneous catalysis, as long as the adsorption of reagents onto active sites is far from being saturated, which is the case for ZN olefin polymerization under usual conditions [14,70]. We have neglected the contributions of entropy and zero-point vibration in the reactions, in cost of the relatively large unit cells.…”

“…The other is an electronic origin that the electron-donating methyl group destabilizes the transition state of the 2,1 insertion by further donating electron density to negatively charged C 2 . Judging from the fact that only quantum chemical calculations have succeeded to reproduce clear preference to the 1,2 insertion in heterogeneous systems [44,49,70] (while molecular mechanics calculations failed [30]), the electronic origin must be very important. In our additional calculations using chloroethylene having electron-withdrawing Cl (whose size is similar to the methyl group for propylene), the 1,2 insertion was still advantageous but only by 0.6 kcal/mol over the 2,1 insertion.…”

“Dormant” secondary metal-alkyl complexes are not omnipresent

“…We decided to compare two catalysts that launched the debate on dormancy, i.e., the homogeneous OOONONOOO postmetallocene system studied by Busico et al (5) and the indenyl system discussed by Landis et al (4). The third case, a heterogeneous catalyst with a Lewis base, has been described previously (11); therefore, only a brief reference to this work will be given in the present paper.…”