Search

A density functional theory (DFT) study has been carried out to investigate the effect of various cocatalysts/chain transfer agents (such as Al(R)3, Mg(R)2, (R)Mg(Cl), Zn(R)2, (R)Zn(Cl), B(C6F5)3, and Al(C6F5)3, here, R=alkyl group) on catalyst activation and ethylene polymerization including chain transfer steps using (CpR, R')2M(Cl)2 (M=Zr, Hf and R, R'=alkyl group) metallocenes. Initially, the catalyst alkylation step (where one of the chlorine atoms of the catalyst is replaced by an alkyl group of the cocatalyst) has been studied using the Al(Me)3, Mg(Me)2, (Me)Mg(Cl), Zn(Me)2, and (Me)Zn(Cl) moieties, and it was observed that the AlMe3 species is more effective for the alkylation step. Furthermore, a modification in the metallocene, such as (CpR, R')2Zr(Cl)2, where R and R'=H, Me, Et, n‐propyl, iso‐propyl, iso‐butyl, tert‐butyl, and different combinations of those, has been performed to evaluate the effect of different alkyl substituents on the alkylation process, whereby it was found that the mono‐substituted metallocene favors the alkylation in comparison to bi‐substituted metallocene. Moreover, a detailed fundamental understanding of the ion pair formation step was gained by incorporating various steric and electronic factors of different alkyl groups, including fluorinated alkyl group bound on the Cp ligand. These results suggest that only B(C6F5)3 and Al(C6F5)3 activators could form the ion pairs by abstracting a methyl group from the metallocene. Additionally, the ethylene polymerization has also been investigated using B(C6F5)3 or Al(C6F5)3 cocatalyst, where B(C6F5)3 was found to be more effective towards polymerization. Furthermore, the Al(Me)3, Mg(R)2, (Me)Mg(Cl), Zn(R)2, and (Me)Zn(Cl) species have been employed as chain transfer agent (CTA). It has been observed that the magnesium‐ and zinc‐based species are more likely to behave as CTAs, and this process can be improved by changing the alkyl substituents on them. These chemical calculations shed light on catalyst alkylation and ethylene polymerization, including the chain transfer process, in metallocenes. [ABSTRACT FROM AUTHOR]

Developing donors for Ziegler‐Natta (ZN) catalysis to control the polymerization reaction and produce polymers with desirable properties has always been challenging due to the multi‐component nature of the catalytic systems. Here, we have developed a new synthetic protocol for making two external donors, D1 (2,2,2‐trifluoroethyl myristate) and D2 (2,2,2‐trifluoroethyl palmitate) that show self‐extinguishing properties, followed by a systematic DFT study to understand this peculiar property of these donors. D1 and D2 can undergo parallel reactions with aluminum and titanium species present in the system to produce ketones and aldehydes, which are poisons for ZN catalytic systems, thus explaining their self‐extinguishing nature. The non‐covalent interaction between the long alkyl chain of the donors with the surface plays a vital role in determining the donors′ self‐extinguishing nature. There is a significant thermodynamic preference for the binding of the donor with the longer alkyl chain at the titanium center. The current work, therefore, provides interesting insights into how self‐extinguishing donors function in ZN catalytic systems. [ABSTRACT FROM AUTHOR]

Full quantum chemical calculations, using density functional theory (DFT), have been conducted to explain the effect of donors on the 'activation mechanism' in the Ziegler-Natta (Z-N) catalyst system. In the activation mechanism, the inactive TiIVCl4 catalyst converts into the active TiIIICl2Et catalyst with the help of the AlEt3 present in the system. The donors that have been considered in this study are: ethyl benzoate ( eb), two representative diether cases, a phthalate donor, and a silyl ester donor. The results indicate that eb and the diether donor cases donor have a negative effect on the barriers for the activation mechanism. However, the eb donor can be displaced from the MgCl2 surface by AlEt3, which matches experimental observations. For the phthalate, silyl ester and TiCl3-OC4H8Cl cases, the results indicate that a significant induction period would be present in Z-N systems employing such donors or having such a catalytic center, before catalysis could commence. [ABSTRACT FROM AUTHOR]

Full quantum chemical calculations with density functional theory (DFT) show that a principal role of donors in Ziegler-Natta (ZN) olefin polymerization catalysts is to coordinate to the metal center at the active sites on the MgCl2 surface. Thereby, the behavior of the catalyst is modulated to favor insertion over termination and, thus, polymerization occurs. This is shown to be true for a range of different donors. The calculations indicate that active sites that feature anionic chloride ligands at the titanium center (the conventional model for the active site) would lead to lower-molecular-weight polymers. If an -OC2H5 group were present instead of a chloride ligand, the active site would much more effectively produce long chain polymers. Therefore, the current work provides important new insights into the nature of the ZN polymerization process. [ABSTRACT FROM AUTHOR]

The development of new donors (Lewisbases, usually containingoxygen atoms) is one of the chief areas of research in Ziegler–Natta(ZN) olefin polymerization systems. The addition of such donors hasled to improvement in the activity and selectivity of ZN systems.However, in order for the donor to be effective, it has to be chemicallystable and resistant to decomposition by Lewis acidic species suchas AlEt3. Discussed in the current work is the chemicalstability of different ester donors, including aromatic benzoate donorsand the silyl estera promising new donor class in ZN systems.Full quantum chemical calculations with density functional theory(DFT) indicate that esters can undergo decomposition through differentpathways upon interaction with species such as the AlEt3dimer: Al2Et6. Moreover, the studies showthat the active, supported titanium catalyst species can cause donordecomposition and, in fact, is likely to be the greater threat todonor decomposition than Al2Et6. This explainswhy the addition of excess donors can lead to the poisoning of theactive site in ZN systems. We have also computationally investigatedmeans of improving the silyl ester donors in order to make them morerobust and resilient to donor decomposition by Al2Et6and the supported active titanium species. [ABSTRACT FROM AUTHOR]