Hafnocene catalysts for selective propylene oligomerization: efficient synthesis of 4-methyl-1-pentene by beta-methyl transfer

A series of hafnocene complexes (eta5-C5Me4R1)(eta5-C5Me4R2)HfCl2 with [R1, R2] = [H, H] (1), [Me, H] (2), [Me, Me] (3), [Et, Me] (4), [(i)Pr, Me] (5), [SiMe(3), Me] (6), [(t)Bu, Me] (7), [(n)Bu, Me] (8), [(i)Bu, Me] (9), [Et, Et] (10), [(n)Bu, (n)Bu] (11), [(i)Bu, (i)Bu] (12) was tested as catalyst precursors for propylene oligomerization. Upon activation with methylaluminoxane or [Ph(3)C][B(C(6)F(5))(4)]/Al(i)Bu(3), complexes 2-4 and 8-12 catalyzed the dimerization of propylene to produce 4-methyl-1-pentene with selectivities ranging from 23.9 to 61.6 wt % in the product mixture. The selectivity was dependent on the nature of the substituents R(1) and R(2), with the highest value found for (eta5-C5Me4(i)Bu)2HfCl2 (12). Rapid deactivation was observed for 5-7, whereas (eta5-C5Me4H)2HfCl2 (1) polymerized propylene. 4-Methyl-1-pentene is proposed to form by repeated 1,2-insertion of propylene into the hafnocene methyl cation, followed by selective beta-methyl elimination. Detailed analysis of the byproduct distribution (isobutene, 1-pentene, 2-methyl-1-pentene, 2,4-dimethyl-1-pentene, 4-methyl-1-heptene, 4,6-dimethyl-1-heptene), determined by gas chromatography, was performed with the aid of a stochastic simulation involving rate constants for the propagation by insertion, beta-hydride elimination, and beta-methyl elimination. The rate of termination is dependent on the structure of the growing chain of the active species as well as on the bulkiness of the cyclopentadienyl ligands. The selectivity highly depends on the reaction conditions (pressure, temperature, concentration of methylaluminoxane). The rates of beta-methyl elimination leading to 4-methyl-1-pentene were proportional to propylene pressure for 2-4 and 8-10 but practically independent from propylene pressure for the sterically bulkier derivatives 11-12.     

Crystal structure of isotactic poly(4-methyl-1-pentene)

The crystal structure of isotactic poly(4-methyl-1-pentene) was determined by x-ray analysis. The unit cell is tetragonal, P4 b2, with α = 18.70 Å and c (fiber axis) = 13.68 Å; it contains four molecular chains each consisting of seven monomeric units in the fiber period. The molecular conformation is essentially a (7/2) helix, but deviates slightly from the uniform (7/2) helix. The unusual low density is discussed from the structural point of view.     

Gas sorption-induced dilation of poly(4-methyl-1-pentene)

Sorption and volume dilation isotherms of semicrystalline poly(4-methyl-1-pentene) (PMP) were measured using CO₂ and C₃H₈ as penetrants, which have sieving diameters of 3.3 and 4.3 Å, respectively. On the other hand, the PMP crystal has a void width of approximately 4 Å as estimated by X-ray diffraction, so it was anticipated that CO₂ would be able to sorb into the PMP crystal while C₃H₈ would not. The data show that C₃H₈ has a constant partial molar volume of approximately 87 cc/mol, just above the value reported in other rubbery polymers, and are consistent with the hypothesis that the C₃H₈ molecules are too large to sorb into the PMP crystals. The partial molar volume of CO₂ was found to be 39 cc/mol for CO₂ weight fractions of up to 0.03. Since the typical partial molar volume of CO₂ in rubbery materials is 46 cc/mol, the lower values in this study were attributed to CO₂ sorption into crystalline regions of the polymer, which provided no dilation. Application of a two-phase model using the assumption of Henry's law sorption showed that apparently all C₃H₈ sorption was occurring in the amorphous region but approximately 16% of CO₂ sorption occurred in the crystalline regions. © 1996 John Wiley & Sons, Inc.     

Radiation-induced copolymerization of hexafluoroacetone with 2-methyl-1-pentene

Copolymerization of hexafluoroacetone (HFA) with 2-methyl-1-pentene (2MP) in trichlorotrifluoroethane (R-113) was carried out by γ-ray irradiation in a low-temperature region of ?100 to 0°C. Though HFA does not homopolymerize and 2MP scarcely does, the copolymerization took place at various monomer compositions. The copolymerization rate and the molecular weight in the low-temperature region were much higher than those at 0°C. Above room temperature the copolymerization did not take place and only the adduct of monomers was formed. The copolymerization was inhibited to some extent by cation scavengers, but not by radical or electron scavengers. Elemental analysis and nuclear magnetic resonance (NMR) spectra show that the copolymer consists of almost equimolar monomer units and has two types of structure, head to tail and head to head or tail to tail. It has been concluded that copolymerization probably proceeds via a cationic mechanism to form an alternating copolymer.     

Novel synthesis of electroconducting polymers from simple monomers with transition metal complex catalysts

Electroconductive polyaromatics or polyarylenes have been successfully prepared by the oxidative polymerization of the corresponding simple monomers over the transition metal complex catalyst. For example, poly(1,4-phenylene) was prepared from benzene by using oxygen as an oxidant and a copper(I) chloride-aluminum chloride double complex as the catalyst. The same catalytic system was applied to the preparation of poly(2,5-pyrrolylene) and poly(2,5-thienylene) from pyrrole and 2,2′-bithiophen, respectively. The electrochemical oxidative polymerization was also carried out for the preparation of the poly(1,4-phenylene) and poly(1,4-naphthylene) films in the presence of copper(I) chloride and aluminum chloride.     

Small-angle X-ray scattering of isotactic poly(4-methyl-1-pentene)

Although the crystalline phase of poly(4-methyl-1-pentene) (P4MP) is less dense than the amorphous phase, the density difference is very small at room temperature. This makes it difficult to study the fine structure of P4MP by small-angle x-ray scattering (SAXS). It has therefore not been clear whether a so-called superlattice, or long-spacing structure, is formed in P4MP as in other synthetic polymers. However, it is found that raising the temperature increases the density difference sufficiently to make SAXS study feasible, and the existence of the long-spacing structure of P4MP is indicated. Annealing studies reveal two characteristic temperatures of 170 and 200°C: above 170°C the long-spacing structure is formed with an abrupt increase of crystallinity, and at 200°C the structure is further developed.     

Polymerization of 1-methoxy-1-ethynylcyclohexane by transition metal catalysts

The polymerization of 1-methoxy-1-ethynylcyclohexane (MEC) was carried out by various transition metal catalysts. The catalysts MoCl₅, MoCl₄, and WCl₆ gave a relatively low yield of polymer (≤ 16%). The catalytic activity of Mo-based chloride catalyst was greater than that of W-based chloride catalyst. However, catalyst tungsten carbene complex (I) gave a larger molar mass and higher yield in the presence of a Lewis acid such as AlCl₃ than in the absence of a Lewis acid. The activity of the tungsten carbene complex was obviously affected by Lewis acidity. The catalyst PdCl₂ was a very effective catalyst for the present polymerization and gave polymers in a high yield. The structure of the resulting poly(MEC) was identified by various instrumental methods as a conjugated polyene structure having an α-methoxycyclohexyl substituent. The poly(MEC)s were mostly light-brown powders and completely soluble in various organic solvents such as tetrahydrofuran (THF), chloroform (CHCl₃), ethylacetate, n-butylacetate, dimethylformamide, benzene, xylene, dimethylacetamide, 1,4-dioxane, pyridine, and 1-methyl-2-pyrrolidinone. Thermogravimetric analysis showed that the polymer started to lose mass at 125°C and that maximum decomposition occurred at 418°C. The x-ray diffraction diagram shows that poly(MEC) has an amorphous structure. © 1997 John Wiley & Sons, Inc.     

Functionalized polyolefins prepared by the combination of borane monomers and transition metal catalysts

This paper summarizes the new developments in the functionalization of polyolefins by borane monomers and transition metal catalysts. It is becoming clear that the borane groups are stable to transition metal catalysts and borane groups in polyolefins are valuable intermediates which not only can be quantitatively converted to a wide range of functional groups but also can be used as initiator for free radical graft-from copolymerizations. Some interesting functionalized polymers, such as polyethylene (PE) and polypropene (PP) with -OH, -NH₂ and halide groups, and graft copolymers of polypropene-graft-poly(methyl methacrylate) (PP-g-PMMA), polypropene-graft-polycapolactone (PP-g-PCL) and PE-g-PMMA, have been synthesized with controllable compositions and molecular microstructures. Most of them would be otherwise very difficult to prepare by the other existing methods.     

Equibiaxial deformation of poly(4-methyl-1-pentene) by a forging process

Poly(4-methyl-1-pentene) (PMP) has been uniaxially compressed by a forging (equibiaxial) process. The rheology of the process has been examined for this semicrystalline polyolefin, melting point about 235°C. The yield energy, area under the compressive stress-strain curve up to the yield point, as a function of temperature was found to consist of two linear components of different slope. These two linear relations arise from the glassy and crystalline phases of PMP. The intercept temperature (T_i) at zero yield energy for the glassy phase has been evaluated. The attainable maximum compression ratio without sample rupture (CR_max) increased steadily on increasing forging temperature above T_i, and below T_m. In this range, the crystalline relaxation temperature (T_c), evaluated from an Arrhenius plot of yield stress was 160°C. Above T_c, a CR_max of 240 was reached. This value is five times higher than that attained for isotactic polypropylene (i-PP). However, the draw efficiency evaluated by elastic recovery in the plane direction of PMP (0.76) is lower than for i-PP (0.97). Differential scanning calorimetry analyses showed that the melting peak became a complex doublet on increasing compression ratio ( > 100). The drawing and stress-strain behavior of PMP are compared with i-PP. © 1994 John Wiley & Sons, Inc.     

ESR spectra of spin-adducts of hydroxyalkyl radicals with perfluoro-4-methyl-2-pentene

A study was carried out on the ESR spectra of the spin-adducts of hydroxyalkyl radicals with perfluoro-4-methyl-2-pentene (I) formed upon the photolysis of alcoholic solutions of this perfluoroolefin. The stable conformational positions of the substituents in the spin-adducts depend on the structure of the hydroxyalkyl radicals. The spin-adducts are annihilated as a consequence of hydrogen atom abstraction as indicated in a preparative experiment.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 730–732, March, 1991.     

Polymerization of mixtures of alkyl methacrylates or alkyl acrylates with 4-methyl-1-pentene by Ziegler-Natta catalyst

Mixtures of methyl methacrylate (MMA) and 4-methyl-1-pentene (4MP)(molar ratio MMA/4MP = 3–0.1) have been subjected to polymerization by VOCl₃/Al(C₂H₅)₃. The amorphous polymeric products, extractable with boiling methanol up to 75%, consist mainly of MMA monomeric units (～80%). The composition of the product was almost independent of the starting MMA/4MP ratio. Comparison of these results with thoseof homopolymerization experiments shows that the presence of MMA drastically reduces the polymerization rate of 4MP. Moreover, 4MP is polymerized with rather low stereospecificity in the presence of MMA. Fractionation by solvent extraction of the unchanged polymeric products as well as of hydrolyzed samples seems to exclude the formation of random copolymers, suggesting to us that the polymerization of the two monomers takes place by different mechanisms. On taking into account these data and analogous data obtained with 4MP and alkyl acrylates or tert-butyl methacrylate, is it suggested that, contrary to what has previously been proposed, the MMA polymerization by Ziegler-Natta catalysts does not take place at the same centers which polymerize 4MP; moreover a coordinated anionic mechanism for MMA polymerization does not seem to be very probable.     

Structural analysis of five-coordinate transition metal boryl complexes with different d-electron configurations

The site preference of boryl ligands in five-coordinate transition metal boryl complexes has been investigated with the aid of density functional theory calculations. The preferred site for a boryl ligand depends on the electron count of the complex under consideration. Our studies show that the very strong sigma-donating boryl ligands choose to occupy coordination sites such that those orbitals accommodating metal d electrons have minimal metal-boryl sigma-antibonding character.     

Stability of transition metal complexes with sulfonated polymers

Stability constants of macromolecular metallocomplexes of transition metal ions (Ag⁺, Cu²⁺, Ni²⁺, Fe³⁺) with sulfonated polymers in water and aqueous HCl and NaCl solutions were determined from quenching by transition metal ions of the luminescence of macromolecules labeled with luminescent groups.     

Isomeric active ends of growing chains in the cationic polymerization of 4-methyl-1-pentene

Quantum chemical investigation of the relative stabilities of carbenium ions modelling the end units of growing chains in the cationic polymerization of 4-methyl-1-pentene has been carried out. The investigation was performed by the CNDO/2 method and by partial optimization of geometry. It is shown that the polymer chain can be adequately modelled by a methyl group. The calculated values of relative energies ΔE, geometric parameters and charge distribution are given for free cations, ion-pairs and for the cation-electron donor and cation-electron acceptor systems (H₂O, HCN and NO₂). All the additives show donor behaviour with respect to cations. The differences in the energies of secondary and tertiary cations decrease on going from free ions to ion-pairs or cation-additive molecule system.     

On thermal properties of poly(4-methyl-1-pentene) membranes cast from solution

Thermoanalytical investigations of poly(4-methyl-1-pentene) membranes of two substantially different types (opaque and transparent), cast from solution, have been carried out using the derivatograph (Q-1500 D). On the basis of simultaneous DTA, TG and DTG measurements, enthalpy changes, weight losses and rates of weight losses have been determined, respectively. DSC has been done using the Perkin-Elmer 7-Series Thermal Analysis System. The results obtained have been used to get some structural information like decompositions and glass as well as crystallization properties of the membrane material.     

聚4-甲基-1-戊烯膜的制备与应用研究进展

聚4-甲基-1-戊烯是一种具有低密度、高透气性等优良性质的膜材料,具有广泛的应用前景.本文从制备方法和具体应用两个方面综述了近年来国内外PMP膜的研究进展,重点介绍了热致相分离法、熔融拉伸法、溶剂挥发法及静电纺丝法、非溶剂致相分离法、热致相分离-熔融拉伸耦合法等其它方法进行PMP膜制备的原理及影响因素,并对PMP膜在体...