Isospecific Centers Research Articles

The role of alkoxysilanes in propylene polymerization catalysis. A study of model Ti(Oi‐C3H7)4‐Al(C2H5)2Cl/Mg(C4H9)2 catalyst system

AbstractThe article describes the modifying effect of methylcyclohexyldimethoxysilane (CH3)(cyclo‐C6H11)Si(OCH3)2 in propylene polymerization reactions with a pseudo‐homogeneous catalyst system: Ti(Oi‐C3H7)4‐Al(C2H5)2Cl/Mg(C4H9)2. This catalyst system is a convenient model for the analysis of modifier effects (effects of ‘external donors’) in supported Ziegler–Natta polymerization catalysts of the fourth generation. A combination of chromatographic, spectroscopic and calorimetric data for crystalline and amorphous fractions of polypropylene prepared in the presence of the silane shows that it has a dual modification effect: (i) the silane is a potent poison of stereo‐aspecific active centers in the catalyst (the centers producing amorphous polypropylene fraction) and (ii) silane molecules coordinate at isospecific centers in the catalyst and changes their nature: the centers produce isotactic polypropylene of higher molecular weight and a higher isotacticity level. © 2023 Society of Industrial Chemistry.

Effects of titanium dispersion state on distribution and reactivity of active centers in propylene polymerization with MgCl2‐supported Ziegler‐Natta catalysts: A kinetic study based on active center counting

AbstractKinetics of propylene polymerization with two TiCl4/MgCl2 model catalysts of different Ti load (Cat‐1: Ti 0.1 wt%; Cat‐2: Ti 1.0 wt%) was investigated in this work. Time‐dependent variations of active centers fraction ([C*]/[Ti]) and active center distribution of the two catalysts were compared. In propylene polymerization with Cat‐1, [C*]/[Ti] was only 7.8 % in the initial stage, but increased for more than three times in reaction period of 600 s. Propylene polymerization with Cat‐2 had much lower [C*]/[Ti]. Active center distributions of the two catalysts were markedly different. Cat‐1 has evidently higher proportion of aspecific active centers and less isospecific ones than that of Cat‐2. These catalysts also showed different time‐dependent variations of active center distribution. The chain propagation rate constant of isospecific centers in Cat‐2 was evidently higher than that in Cat‐1, but propagation rate constants of either aspecific and medium‐isospecific centers were only slightly changed by enhancing Ti content of the catalyst. The results suggest that isospecific centers are mainly originated from clustered Ti species, but non‐clustered Ti species are the main source of active centers with lower stereoselectivity. By considering factors like dispersion states of Ti species and release of shielded active species through disintegration of catalyst particles, active center models have been proposed to explain the different kinetic behaviors of the two model catalysts.

Active centers in propylene polymerization catalysts of the fourth generation

Isospecific centers in propylene polymerization catalysts of the 4th generation [combinations of solid catalysts of the TiCl4/aromatic diester/MgCl2 composition and binary cocatalysts containing Al(C2H5)3 and methoxysilanes RnSi(OCH3)4−n] contain several populations of active centers, which differ in kinetic parameters and in the level of stereospecificity control. The differences in the kinetic parameters between the active centers translate into large differences in the average polymerization degree of the polymer material the centers produce, from 500–800 to 8000–17,000. Differences in the stereospecificity level between the active centers result in large differences in melting points of the polymer components. Methoxysilanes RnSi(OCH3)4−n directly affect each type of the polymerization center; both the kinetic parameters of the centers and their level of stereocontrol are influenced by the nature of the alkyl groups R.

Kinetics of propylene and ethylene polymerization reactions with heterogeneous ziegler-natta catalysts: Recent results

The article discusses recent results of kinetic analysis of propylene and ethylene polymerization reactions with several types of Ti-based catalysts. All these catalysts, after activation with organoaluminum cocatalysts, contain from two to four types of highly isospecific centers (which produce the bulk of the crystalline fraction of polypropylene) as well as several centers of reduced isospecificity. The following subjects are discussed: the distribution of active centers with respect to isospecificity, the effect of hydrogen on polymerization rates of propylene and ethylene, and similarities and differences between active centers in propylene and ethylene polymerization reactions over the same catalysts. Ti-based catalysts contain two families of active centers. The centers of the first family are capable of polymerizing and copolymerizing all α-olefins and ethylene. The centers of the second family efficiently polymerize only ethylene. Differences in the kinetic effects of hydrogen and α-olefins on polymerization reactions of ethylene and propylene can be rationalized using a single assumption that active centers with alkyl groups containing methyl groups in the β-position with respect to the Ti atom, Ti-CH(CH3)R, are unusually unreactive in olefin insertion reactions. In the case of ethylene polymerization reactions, such an alkyl group is the ethyl group (in the Ti-C2H5 moiety) and, in the case of propylene polymerization reactions, it is predominantly the isopropyl group in the Ti-CH(CH3)2 moiety.

Isoselectivity Distribution of Isospecific Centers in Supported Titanium‐Based Ziegler‐Natta Catalysts

AbstractSummary: This article describes the quantitative aspects of the isoselectivity distribution of isospecific centers in five supported titanium‐based Ziegler–Natta catalyst systems containing different internal and external organic donors. Highly crystalline poly(propylene) fractions of the polymers (insoluble in o‐xylene at 95 °C) were analyzed by analytical Tref and gel permeation chromatography (GPC) methods. Separation of the Tref curves into the peaks of sterically uniform components (each represented by the Lorentz function) showed that all these “isotactic” poly(propylene) fractions are nonuniform materials. Each highly crystalline fraction contains several distinct components that differ in stereoregularity. The [mmmm] values for different components (which have approximately the same isotacticity in different polymers) range from as high as 0.986–0.993 to much lower values, in the 0.935–0.955 range. The number of Tref components and their relative contents are substantially different for the crystalline fractions of the polymers produced with different catalyst systems; these parameters depend on the nature of the internal and the external electron donors in the catalysts. GPC analysis of the crystalline poly(propylene) fractions showed that each consists of four or five Flory components (the components produced by kinetically uniform active centers) with widely different average molecular weights, from ≈1 × 104 to ≈1.3 × 106. In general, the probabilities of chain transfer reactions for different active centers decrease with the increase in their isospecificity.Resolution of the Tref curve of the highly crystalline fraction of poly(propylene) prepared with catalyst system 3.magnified imageResolution of the Tref curve of the highly crystalline fraction of poly(propylene) prepared with catalyst system 3.

Polymerization Stereochemistry with Ziegler−Natta Catalysts Containing Dialkylpropane Diethers: A Tool for Understanding Internal/External Donor Relationships

A study on a new generation of MgCl2-supported Ziegler−Natta catalysts, which contain dialkylpropane diethers as internal donors and have the property of producing highly active and stereospecific catalysts without any external donor, has been performed. The behavior of three different diethers (2,2-diisobutyl-1,3-dimethoxypropane, 2,2-dicyclopentyl-1,3-dimethoxypropane, and 2-ethyl-2-butyl-1,3-dimethoxypropane) as internal donors is compared with the behavior of the same diethers when they are used as external donors with a catalyst containing diisobutyl phthalate as internal donor. The results obtained show that the same isospecific centers are produced when the same diether donor is present as an internal donor or replaces diisobutyl phthalate during polymerization when it is added as an external donor. The sum of the present results and of the previously obtained findings on traditional supported Ziegler−Natta catalysts has enabled us to reach some conclusions on the role and reciprocal behavior of internal and external donors.

Silica‐supported metallocenes: stereochemical comparison between homogeneous and heterogeneous catalysis

AbstractPropylene was polymerized in the presence of the isospecific Et(Ind)2ZrCl2 (Et: ethylene, Ind: indenyl) and the aspecific (Ind)2ZrCl2 complexes in solution and anchored to SiO2 and SiO2/MAO (MAO: methylaluminoxane) supports. From the stereochemical analysis of the polypropene samples obtained it can be deduced that (i) the same active species is formed when a metallocene is in solution and when it is anchored to the SiO2/MAO support and (ii) a completely different active species is formed when the metallocene is anchored to the silica. The fact that both systems Et(Ind)2ZrCl2SiO2 and (Ind)2ZrCl2SiO2 produce the same prevailingly isospecific polymer suggests that only isospecific centers are formed in this case, independently of the metallocene stereochemical structure.

Effect of the external donors on the polymerization of 4‐methyl‐1‐pentene with high activity MgCl2/TiCl4 catalytic system

AbstractIn order to find out correlations between the structure of an external donor and the obtained polymer, the effects of different external donors on the activity and stereospecificity of the MgCl2/TiCl4 catalytic system in the bulk polymerization of 4‐methyl‐1‐pentene (4MP) were carefully studied. Different silane compounds of the structure RnSi(OR')4‐n (where: n = 1–3, R = alkyl/phenyl, R = alkyl) and Al(i‐Bu)3 (TIBA) were used as external donors and cocatalyst, respectively. The effect of the donor/TIBA mole ratio on the activity and stereospecificity of the catalytic system was studied. Some major effects were observed for the three different external donors, namely, Me3Si(OMe), Me2Si(OMe)2, and Me3Si(OBu)3, in the 4MP polymerization process. It was observed that the effect of the external silane donor on the polymerization strongly depends upon the size of the alkoxy and hydrocarbon (alkyl/phenyl) groups which are attached to the silicon atom. A selective deactivation of the non‐stereospecific centers, as well as a transformation of the non‐stereospecific into isospecific centers, is assumed to occur. On the basis of the obtained results and literature data available for the propene polymerization, the concept of structural conformity between the ligand‐surrounded active center and the monomer molecule was carried forward.

The trigger mechanism for polymerization of α-olefins with Ziegler-Natta catalysts: A new model based on interaction of two monomers at the transition state and monomer activation of the catalytic centers

The trigger mechanism involves a two-monomer transition state, where the entering of a new monomer unit triggers the insertion of the already complexed monomer. The catalytic center is activated through the insertion of the first monomer, and a reaction scheme (the dynamic steady-state approach) for a repeated activation/deactivation of the active center is formulated. The model explains kinetic phenomena as a reaction rate order vs monomer concentration between 1.0 and 2.0, the complex kinetics of ethene/α-olefin copolymerization, and phenomena observed when reaction parameters are changed during the polymerization. If the catalytic center is an octahedral titanium complex with three facial ligands bridging a support surface, an isospecific insertion is predicted as a result of the interaction between the two monomers and between the monomers and the surface. The model predicts a very high and self-correcting microisotacticity, self-correcting stereoselectivity, and a lower regularity for the first insertion. A reaction rate higher for the isospecific centers compared to that of nonspecific centers is rationalized. Syndiospecific polymerization is predicted for nonchiral homogeneous catalysts.

Stereochemical investigation of the initiation step in MgCl2-supported Ziegler-Natta catalysts: the Lewis base activation effect

The 13 C NMR analysis of the initiation step is applied to the study of the isotactic propene polymerization. Catalytic systems containing internal bases in the absence and in the presence of external bases are studied. The fact that the increase of isotactic productivity here observed, due to the presence of either internal or the external base, is accompanied by a change of the first step stereoregulatory gives evidence of the direct participation of the Lewis base on the formation of the isospecific centers

Evaluation of olefin reactivity ratios over highly isospecific Ziegler‐Natta catalyst

AbstractThe recently found catalyst system, composed of Solvay‐type TiCl3 and (RCp)2TiMe2, producing extremely high isospecific polyolefins, was used for copolymerizations of ethylene with various α‐olefins (propene, 1‐butene,1‐hexene and 4‐methyl‐1‐pentene) to obtain monomer reactivity ratios for isospecific centers. It was found that no homopolymer was formed actually. Using the copolymer composition obtained by 13C NMR, the monomer reactivity ratios were calculated according to the Fineman‐Ross and Kelen‐Tüdős method. For the ethylene/propene pair the same values of reactivity ratio were also obtained from the diad distribution of comonomers. The relative reactivity of these olefins was found to decrease in the following order; ethylene > propene > 1‐butene > 1‐hexene > 4‐methyl‐1‐pentene.