The relationship between catalyst precursors and chain end groups in homogeneous propene polymerization catalysis

The chain transfer to monomer reactions promoted by primary and secondary growing chains in the propene polymerization promoted by ansa‐zirconocenes and postmetallocene precursors are studied by using DFT methods. From the theoretical results it comes out that the prevalence of propene insertion over β‐hydrogen transfer to the monomer decreases drastically in the presence of a secondary chain. Furthermore, we explained the reason why C₂‐symmetric metallocene catalysts promote the selective formation of cis but‐2‐enyls end group after a 2,1 inserted unit whereas for octahedral bis(phenoxy‐imine)titanium‐based catalysts, chain release promotes exclusively the formation of allyl terminated chain end. These results might be useful to design ligand precursors able to obtain not only high M_n PP polymers but also tuned chain end groups to build new polymer architectures. Overall, a more general picture of the enantioselectivity of the chain transfer to monomer processes is reported. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 699–708, 2010     

Asymmetric Markovian analysis of the polymerization mechanism of propene by ziegler-natta catalysis and external alkoxysilane donors

The formulae for conditional probabilities are derived with the term of NMR triad tacticities for the polymers whose polymerization process is described by an asymmetric Markovian process. The formulae obtained are applied to the polypropylene polymerized by Ziegler-Natta catalysis with external alkoxysilane donors. For both, unfractionated samples and heptane-insoluble fractions, four evidences are clarified as follows: Don the transformation from symmetric to (fluctuating) asymmetric sites the transformation of one (hereafter, L) site is much more enhanced than that of the other (D) site, 2) monotonous increase of isotacticity with an increase of the concentration of the donor is due to the increase of p_LL, 3) aromatic donors are more effective than aliphatic ones for the improvement of isotacticity, and 4) the improvement of isotacticity after an addition of alkoxysilane is not due to the contribution from both of L and D sites, but the competition between the larger positive contribution from the former and the smaller negative one from the latter.     

Syndiotactic-specific polymerization of propene with a Ni-based catalyst

Polymerization of propene at −78°C in the presence of a homogeneous catalytic system based on a Ni(II) diimine derivative and methylaluminoxane affords prevailingly syndiotactic crystalline poly(propylene) (rr triad content ≈80%). ¹³C NMR analysis of the polymer microstructure indicates that the stereochemistry of the monomer insertion is controlled by the configuration of the methine carbon of the growing chain end (unlike-1,3 asymmetric induction). This is the first example of stereospecific homopolymerization of propene promoted by a late transition metal catalyst.     

Zirconocene-catalyzed propene polymerization: a quenched-flow kinetic study

The kinetics of propene polymerization catalyzed by ansa-metallocenes were studied using quenched-flow techniques. Two catalyst systems were investigated, (SBI)ZrMe2/Al(i)Bu3/[Ph3C][CN[B(C6F5)3]2] (1:100:1) at 25.0 degrees C and (SBI)ZrCl(2)/methylalumoxane at 40.0 degrees C (Al:Zr = 2400:1) (SBI = rac-Me(2)Si(1-Indenyl)2). The aims of the study were to address fundamental mechanistic aspects of metallocene-catalyzed alkene polymerizations, catalyst initiation, the quantitative correlation between catalyst structure and the rate of chain propagation, and the nature of dormant states. One of the most important but largely unknown factors in metallocene catalysis is the distribution of the catalyst between dormant states and species actively involved in polymer chain growth. Measurements of polymer yield Y versus reaction time t for propene concentrations [M] = 0.15-0.59 mol L(-1) and zirconocene concentrations in the range [Zr] = (2.38-9.52) x 10(-5) mol L(-1) for the borate system showed first-order dependence on [M] and [Zr]. Up to t approximately 1 s, the half-life of catalyst initiation is comparable to the half-life of chain growth; that is, this phase is governed by non-steady-state kinetics. We propose a rate law which takes account of this and accurately describes the initial rates. Curve fitting of Y(t) data provides an apparent chain growth rate constant k(p)(app) on the order of 10(3) L mol(-1) s(-1). By contrast, the evolution with time of the number-average polymer molecular weight, which is independent of the concentration of catalyst involved, leads to a k(p) which is an order of magnitude larger, (17.2 +/- 1.4) x 10(3) L mol(-1) s(-1). The ratio k(p)(app)/k(p) = 0.08 indicates that under the given conditions only about 8% of the total catalyst is actively engaged in chain growth at any one time. The system (SBI)ZrCl(2)/methylalumoxane is significantly less active, k(p)(app) = 48.4 +/- 2.7 and k(p) = (6 +/- 2) x 10(2) L mol(-1) s(-1), while, surprisingly, the mole fraction of active species is essentially identical, 8%. Evidently, the energetics of the chain growth sequence are strongly modulated by the nature of the counteranion. Increasing the counteranion/zirconium ratio from 1:1 to 20:1 has no influence on catalyst activity. These findings are consistent with a model of closely associated ion pairs throughout the chain growth sequence. For the borate system, propagation is approximately 6000 times faster than initiation, while for the MAO catalyst, k(p)/k(i) approximately 800. Polymers obtained at 25 degrees C show 0.1-0.2 mol % 2,1-regioerrors, and end-group analysis identifies 2,1-misinsertions as the main cause for chain termination (66%), as compared to 34% for the vinylidene end groups. The results suggest that 2,1-regioerrors are a major contributor to the formation of dormant species, even at short reaction times.     

Associative charge transfer reactions. Temperature effects and mechanism of the gas-phase polymerization of propene initiated by a benzene radical cation

In associative charge transfer (ACT) reactions, a core ion activates ligand molecules by partial charge transfer. The activated ligand polymerizes, and the product oligomer takes up the full charge from the core ion. In the present system, benzene(+*) (Bz(+*)) reacts with two propene (Pr) molecules to form a covalently bonded ion, C(6)H(6)(+*) + 2 C(3)H(6) --> C(6)H(12)(+*) + C(6)H(6). The ACT reaction is activated by a partial charge transfer from Bz(+*) to Pr in the complex, and driven to completion by the formation of a covalent bond in the polymerized product. An alternative channel forms a stable association product (Bz.Pr)(+*), with an ACT/association product ratio of 60:40% that is independent of pressure and temperature. In contrast to the Bz(+*)/propene system, ACT polymerization is not observed in the Bz(+*)/ethylene (Et) system since charge transfer in the Bz(+*)(Et) complex is inefficient to activate the reaction. The roles of charge transfer in these complexes are verified by ab initio calculations. The overall reaction of Bz(+*) with Pr follows second-order kinetics with a rate constant of k (304 K) = 2.1 x 10(-12) cm(3) s(-1) and a negative temperature coefficient of k = aT(-5.9) (or an activation energy of -3 kcal/mol). The kinetic behavior is similar to sterically hindered reactions and suggests a [Bz(+*) (Pr)]* activated complex that proceeds to products through a low-entropy transition state. The temperature dependence shows that ACT reactions can reach a unit collision efficiency below 100 K, suggesting that ACT can initiate polymerization in cold astrochemical environments.     

Stereoregularity and regioregularity of active centers in propene polymerization

The relation between stereoregularity of active centers on a MgCl₂/TiCl₄ catalyst and functions of inside donor (ID) and outside donor (OD) was investigated in the case of ethyl benzoate (EB)/methyl p-toluate (MPT) as an ID/OD pair. The results indicate that stereregularity depends merely on the amount of MPT supported on the catalyst. On the other hand, regioregularity of active centers was investigated with a MgCl₂/TiCl₄/dioctyl phthalate(DOP)-Et₃Al/diphenyldimethoxysilane(DPDMS) catalyst system. Regio-irregular inserted units were detected only in end groups of PP. It indicates that regio-irregular insertion leads to dormant centers with respect to propene insertion, though such centers are active for hydrogen transfer.     

Some peculiar aspects of a new catalyst for propene polymerization

An investigation was carried out on the preparation of highly disordered samples of MgCl₂ (in delta -form), starting from different Mg-alkyls and chlorinating agents. Successively we studied which kind of silica carrier had to be preferred and which treatment was strictly' necessary to achieve a qualified catalyst precursor showing outstanding performance. Different routes were investigated to support the active, disordered MgCl₂ on the silica carrier and then some following ways to complete the catalyst. The final result of this investigation was the synthesis of a catalyst showing the best performance with reference to the parameters of use in every polymerization process. It has to be e.g. mentioned: — high yield, referred to the residual content of Mg, Ti and Cl; — no activity decrease when operating at a very high hydrogen concentration; — possibility to polymerize also at elevated temperatures; — very high isotactic index and stereoregular structure, as measured by the NMR technique; — possibility to reach high MFR (Melt Flow Rate) (up to 400 g IPP/10 min at 230°C, 2,16 kg, ASTM D 1238L); — outstanding morphology and flowability.     

Ethylene polymerization catalysis over chromium oxide

A study of the nature of polymerization catalysis over CrO₃–silica and of the nature of the polyethylene obtained is presented. A fixed surface chromate (or possibly dichromate) species was shown to be activated for polymerization by an oxidation–reduction reaction with the monomer or with other reactive compounds such as CO. A change in catalyst color from orange to blue occurred simultaneously, and an indigoblue color was present only during ethylene addition, indicating involvement of Cr d orbitals. A spectacular chemiluminescence, due to excitation of oxygen, occurred when CO-treated catalyst was exposed to air. Active site population and the rate at which each site produced polymer molecules were calculated. A reaction mechanism compatible with the experimental data is depicted.     

Site chirality as a messenger in chain-end stereocontrolled propene polymerization

The origin of stereoselectivity in the chain-end controlled syndiospecific polymerization of propene with octahedral Ti-catalysts is unclear. We present a possible mechanism which is based on the site chirality as a messenger of information between the chirality of the chain-end and the chirality of monomer insertion which can operate for secondary propagation. This mechanism could be operative also for the industrially relevant V-based homogeneous catalysts.     

Kinetic investigation of propene polymerization with stopped‐flow method

The stopped‐flow technique, by which a quasi‐living polymerization process can be performed within an extremely short period (ca. 0.2s), is a powerful method for kinetic investigation in Ziegler catalysis. Significant understanding of many controversial problems was achieved, such as arguments concerning the nonuniformity of the active sites, the accurate kinetic parameters, the active sites formed in the different stages, the role of cocatalyst, the effects of hydrogen and electron donors, etc..     

Catalytic activity of some organolanthanoid derivatives in styrene and propene polymerization

Organolanthanoids of several classes were examined as potential styrene and propene polymerization catalysts. They are: molecular hydrides of divalent lanthanoids (samarium, europium, ytterbium); naphthalene and stilbene complexes of neodymium(III), samarium(II), europium(II), ytterbium(II), lutetium(III); amides and alkoxides (including heterobifunctional derivatives) of praseodymium(III), neodymium(III), samarium(II), europium(II), thulium(III), ytterbium(II, III); thiolate of samarium(III); phenyl and phenylethinyl derivatives of europium(II), thulium(III), ytterbium(II); methylytterbium cluster Yb₈ (μ‐CH₃)₁₄(μ‐CH₂)(THF)₆; heterobimetallic samarium(II), ytterbium(II, III) complexes; diazabutadiene ytterbium(III) derivatives; metallic praseodymium and ytterbium, activated by iodine. The highest activity in styrene polymerization revealed hydrides, naphthalene and stilbene complexes of samarium(II), europium(II) and ytterbium(II). In the propene polymerization only [(η⁵‐C₅H₄)CH₂CH­(CH₂OBu)(η¹‐O)]YbMe(THF) displayed noticeable activity.Copyright © 2001 John Wiley & Sons, Ltd.     

Polyolefin catalysis of propene, 1-butene and isobutene monitored using hyperpolarized NMR

Polymerization reactions of the dissolved gases propene, 1-butene, and isobutene catalyzed by [Zr(Cp)₂Me][B(C₆F₅)₄] were characterized using in situ NMR. Hyperpolarization of ¹³C spins by the dissolution dynamic nuclear polarization (DNP) technique provided a signal enhancement of up to 5000-fold for these monomers. For DNP hyperpolarization, liquid aliquots containing monomers were prepared at a temperature between the freezing point of the solvent toluene and the boiling point of the monomer, mixed with the polarizing agent α,γ-bis-diphenylene-β-phenylallyl free radical, and subsequently frozen. The hyperpolarized signals after dissolution enabled the observation of reaction kinetics, as well as polymer products and side products within a time of 30 s from the start of the reaction. The observed kinetic rate constants for polymerization followed a decreasing trend for propene, 1-butene, and isobutene, with the lowest rate constant for the latter explained by steric bulk. For all reactions, partial deactivation was further observed during the measurement time. The line shape and the chemical shift of the monomer signals with respect to a toluene signal were both dependent on catalyst concentration and reaction time, with the strongest dependence observed for isobutene. These changes are consistent with the characteristics of a rapid binding and unbinding process of the monomer to the catalyst occurring during the reaction.

Hyperpolarization by dissolution dynamic nuclear polarization (DNP) enhances ¹³C NMR signals of normally gaseous olefins. The polymerization reactions of these dissolved gases catalyzed by a metallocene catalyst are characterized in real time.     

Unusual temperature effects in propene polymerization using stereorigid zirconocene catalysts.

Free Car-Parrinello molecular dynamics (CPMD) simulations of four diastereomers of the zirconium-propene complexes [{iPr(3-iPr-CpFlu)}ZriBu(C3H6)]+ (Cp=cyclopentadienyl; Flu=fluorenyl) provide valuable insight into the mechanism and stereocontrol of propene polymerization with stereorigid metallocenes. Spontaneous insertion of propene into the zirconium-isobutyl bond is not observed, and propene is found to be weakly bound and to rotate relatively freely around the C--C bond to be formed. Large-amplitude rotation of the isopropyl substituent around the Cp--iPr bond may play a role in triggering dissociation of propene. Three of the four diastereomers eliminate propene during the course of the simulations, which makes dissociation the dominating event on a 20-ps timescale. The CPMD simulations thus support the validity of the assumption, fundamental to statistical propagation models, that each insertion is independent of the preceding insertions. Using insertion barriers from static density functional calculations, the statistical model predicts the polypropene microstructure in good agreement with experiment at low polymerization temperatures for the catalysts {iPr(3-R-CpFlu)}ZrCl2 (R=H, iPr, tBu). The predictions become less accurate at higher temperatures, probably due to the onset of the competing back-skip reaction, which is not included in the model.