Regulation of Stereospecificity of Titanium–Magnesium Catalysts in Propylene Polymerization Reactions

This article discusses the similarities and differences between active centers in propylene and ethylene polymerization reactions over the same Ti‐based catalysts. These correlations were examined by comparing the polymerization kinetics of both monomers over two different Ti‐based catalyst systems, δ‐TiCl3‐AlEt3 and TiCl4/DBP/MgCl2‐AlEt3/PhSi(OEt)3, by comparing the molecular weight distributions of respective polymers, in consecutive ethylene/propylene and propylene/ethylene homopolymerization reactions, and by examining the IR spectra of “impact‐resistant” polypropylene (a mixture of isotactic polypropylene and an ethylene/propylene copolymer). The results of these experiments indicated that Ti‐based catalysts contain two families of active centers. The centers of the first family, which are relatively unstable kinetically, are capable of polymerizing and copolymerizing all olefins. This family includes from four to six populations of centers that differ in their stereospecificity, average molecular weights of polymer molecules they produce, and in the values of reactivity ratios in olefin copolymerization reactions. The centers of the second family (two populations of centers) efficiently polymerize only ethylene. They do not homopolymerize α‐olefins and, if used in ethylene/α‐olefin copolymerization reactions, incorporate α‐olefin molecules very poorly. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1745–1758, 2003

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

The hydrogenation of rac-[1-(9-η5-fluorenyl)-2-(5,6-cylopenta-2-methyl-1-η5-indenyl)ethane] metallocene dichlorides (1a: Zr, 1b: Hf) with PtO2/H2O/H2 is reported. The diastereoselective formation of exclusively rac-[1-(2,3,4,5,6,7,8,9-octahydro-η5-fluorenyl)-2-(2-methyl- 1,4,4a(R;S),5,6,7,7a(S;R),8-octahydro-s-η5-indacenyl)ethane]metallocene dichlorides (2a: Zr, 2b: Hf) was shown by 1H-NMR and by X-ray analysis of 2a. Both compounds were activated in situ with triisobutylaluminum/PhC+ 3 [B(C6F5)4]D and tested as catalysts in propylene polymerization reactions. Comparison to the non-hydrogenated complexes revealed a decrease in molecular weight of the polymer and in catalyst activity. Experiments at elevated temperatures resulted in a lower stereospecificity and reduced isotacticity values indicating a polymerization mechanism analogous to C2-symmetric catalysts.

Cocatalyst effect in propylene polymerization reactions with post-metallocene catalysts

The hydrogen activation effect in propylene polymerization reactions with Ti‐based Ziegler–Natta catalysts is usually explained by hydrogenolysis of dormant active centers formed after secondary insertion of a propylene molecule into the growing polymer chain. This article proposes a different mechanism for the hydrogen activation effect due to hydrogenolysis of the Tiiso‐C3H7 group. This group can be formed in two reactions: (1) after secondary propylene insertion into the TiH bond (which is generated after β‐hydrogen elimination in the growing polymer chain or after chain transfer with hydrogen), and (2) in the chain transfer with propylene if a propylene molecule is coordinated to the Ti atom in the secondary orientation. The TiCH(CH3)2 species is relatively stable, possibly because of the β‐agostic interaction between the H atom of one of its CH3 groups and the Ti atom. The validity of this mechanism was demonstrated in a gas chromatography study of oligomers formed in ethylene/α‐olefin copolymerization reactions with δ‐TiCl3/AlEt3 and TiCl4/dibutyl phthalate/MgCl2–AlEt3 catalysts. A quantitative analysis of gas chromatography data for ethylene/propylene co‐oligomers showed that the probability of secondary propylene insertion into the TiH bond was only 3–4 times lower than the probability of primary insertion. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1353–1365, 2002

Hydrogen is a very effective chain‐transfer agent in propylene polymerization reactions with Ti‐based Ziegler–Natta catalysts. However, measurements of the hydrogen concentration effect on the molecular weight of polypropylene prepared with a supported TiCl4/dibutyl phthalate/MgCl2 catalyst show a peculiar effect: hydrogen efficiency in the chain transfer significantly decreases with concentration, and at very high concentrations, hydrogen no longer affects the molecular weight of polypropylene. A detailed analysis of kinetic features of chain‐transfer reactions for different types of active centers in the catalyst suggests that chain transfer with hydrogen is not merely the hydrogenolysis reaction of the TiC bond in an active center but proceeds with the participation of a coordinated propylene molecule. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1899–1911, 2002

ABSTRACTMedium‐ and high‐resolution SEM analysis of several Ti‐based MgCl2‐supported Ziegler–Natta catalysts and isotactic polypropylene produced with them is carried out. Each catalyst particle, 35–55 μ in size, produces one polymer particle with an average size of 1.5–2 mm, which replicates the shape of the catalyst particle. Polymer particles contain two distinct morphological features. The larger of them are globules with Dav ∼400 nm; from 1 to 2 × 1011 globules per particle. Each globule represents the combined polymer output of a single active center. The globules consist of ∼2500 microglobules with an average size of ∼20 nm. The microglobules contain several folded polymer molecules; they are the smallest thermodynamically stable macromolecular ensembles in propylene polymerization reactions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3832–3841

The hydrogen activation effect in propylene polymerization reactions with Ti—based catalysts is usually explained in the literature as a consequence of the secondary insertion of a propylene molecule into the Ti—CH2CH(CH3)—Polymer bond resulting in the formation of a sleeping active center. This article proposes a different mechanism of the hydrogen activation effect. It is based on the assumption that propylene insertion into the Ti—H bond has poor regioselectivity, in contrast to its insertion into the Ti—C bond, which is highly regioselective. Ti—CH(CH3)2 species formed after the secondary insertion of propylene into the Ti—H bond is stable due to the s—agostic interaction between the H atom of its CH3 group and the Ti atom. However, the Ti—H bond is restored in a reaction with hydrogen. Validity of this mechanism is demonstrated in the study of ethylene/α—olefin copolymerization reactions with two Ti—based catalysts. GC analysis of ethylene—propylene co—oligomers prepared in the presence of hydrogen showed that the probability of the secondary insertion of a propylene molecule into the Ti—H bond is only 3-4 times lower than the probability of its primary insertion.

Stereospecificity of catalytic systems MCl 3AlEt 3 in propylene polymerization reactions

The 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.

Olefin polymerizations with zirconocene supported on SiO 2 modified by MgO, NaOH and LiOH

For Abstract see ChemInform Abstract in Full Text.

This report describes propylene polymerization reactions with titanium complexes bearing carbamato ligands, Ti(O2CNMe2)Cl2 (I) and Ti(O2CR2)4 [R2 = NMe2 (II), NEt2 (III) and (IV)]. Combinations of these complexes and MAO form catalysts for the synthesis of atactic polypropylene, as confirmed by FT-IR, DSC and 13C NMR analysis. Effects of main reaction parameters on the catalyst activity were studied including the type of complex, solvent, temperature, and the [Al]/[Ti] molar ratio. The highest activity was observed when chlorobenzene was used as a solvent and AlMe3-depleted MAO was employed as a cocatalyst. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4095–4102

Highly Efficient AuPd Catalyst for Synthesizing Polybenzoxazole with Controlled Polymerization

Organotin compounds have been used to catalyze the condensation of various α,ω-dihydroxyl terminated organosiloxane diols. This tin catalyzed reaction extends the length of siloxane chains and it also produces narrow molar mass distribution polyorganodisilanols (Mw/Mn = 1.4). The reaction occurs between a variety of siloxanes that are terminated with hydroxyl groups and it does not depend on the organic side groups connected to silicon for the systems studied here. These systems include silicon atoms bearing dimethyl, methylphenyl or methyl 1,1,1 trifluoropropyl substituents. The organotin catalyst in the reaction facilitates the organosilanol condensation releasing water as a byproduct. However it does not appear to facilitate the opening of siloxane bonds nor the redistribution of siloxane bonds under the conditions employed here. Copolymerization of linear oligomeric dimethylsiloxane diol and linear oligomeric methylphenylsiloxanediol was found to give a relatively equal reactivity of homo polymerization and hetro polymerization in the condensation and randomly alternating segmented block copolymers that were formed. The reaction kinetics of the polymerization was used to experimentally verify the fact that there is a chain length dependence of the reacting silanol end-groups. The molar mass values during the polymerizations were determined using gel permeation chromatography. The chromatography, viscosity and FTIR results demonstrate that the reactivity of the hydroxyl end-groups in the polycondensation reaction decreases upon increasing the chain length of the siloxane. It therefore appears that these tin catalyzed siloxane systems deviate from the widely demonstrated hypothesis of Paul Flory on the “equal reactivity of functional groups” for step-growth polymerizations.