John R. Severn

Immobilisation of homogeneous olefin polymerisation catalysts. Factors influencing activity and stability

The activity and stability of homogeneous olefin polymerisation catalysts, when immobilised on a support, are dependent on both chemical and physical effects. Chemical factors affecting catalyst activity include the ease of formation of the active species, which is strongly dependent on the transition metal. Catalyst productivity is dependent on the balance between activity and stability. Immobilisation can lead to a lower proportion of active species and therefore lower initial polymerisation activity, but nevertheless give higher polymer yields in cases where increased catalyst stability is obtained. Important physical factors are support porosity and the ability of a support to undergo progressive fragmentation during polymerisation, facilitating monomer diffusion through the growing catalyst/polymer particle. This article illustrates the importance of these factors in olefin polymerisation with both early- and late-transition metal catalysts, with particular reference to the use of silica and magnesium chloride supports as well as to effects of immobilisation on polymer structure and properties.

Formation of Octameric Methylaluminoxanes by Hydrolysis of Trimethylaluminum and the Mechanisms of Catalyst Activation in Single‐Site α‐Olefin Polymerization Catalysis

Hydrolysis of trimethylaluminum (TMA) leads to the formation of methylaluminoxanes (MAO) of general formula (MeAlO)n (AlMe3)m. The thermodynamically favored pathway of MAO formation is followed up to n=8, showing the major impact of associated TMA on the structural characteristics of the MAOs. The MAOs bind up to five TMA molecules, thereby inducing transition from cages into rings and sheets. Zirconocene catalyst activation studies using model MAO co-catalysts show the decisive role of the associated TMA in forming the catalytically active sites. Catalyst activation can take place either by Lewis-acidic abstraction of an alkyl or halide ligand from the precatalyst or by reaction of the precatalyst with an MAO-derived AlMe2(+) cation. Thermodynamics suggest that activation through AlMe2(+) transfer is the dominant mechanism because sites that are able to release AlMe2(+) are more abundant than Lewis-acidic sites. The model catalyst system is demonstrated to polymerize ethene.

Catalyst behaviour for 1-pentene and 4-methyl-1-pentene polymerisation for C2-, Cs- and C1-symmetric zirconocenes

1-Pentene and 4-methyl-1-pentene (4M1P) have been polymerised using several C2-symmetric ansa-zirconocene catalysts rac-X(2-R1,4-R2-Ind)2ZrCl2 [X = C2H4, R1 = R2 = H (1); X = SiMe2, R1 = R2 = H (2); X = SiMe2, R1 = Me, R2 = H (3); X = SiMe2, R1 = H, R2 = Ph (4); X = SiMe2, R1 = Me, R2 = Ph (5)] with MAO as cocatalyst. The effects of polymerisation conditions as well as substituents on the indenyl ligand were studied. Except for the poly-1-pentenes synthesized with 3 and 5 at low temperatures, low molecular weight isotactic polymers were generally obtained. Compared to their behaviour in propylene polymerisation, the relative activity and selectivity of catalysts 1–5 are considerably different for 1-pentene and 4M1P polymerisation. Of the five catalysts, 1 and 4 showed the highest activities for both 1-pentene and 4M1P polymerisation, while 5 resulted in the lowest activities, especially for 4M1P polymerisation. Subsequently, a Cs- and several C1-symmetric zirconocenes, (R1)2C(3-R2-Cp)(2,7,-R3-Flu)ZrCl2 (R1 = Me, R2 = R3 = H (6); R1 = R2 = Me, R3 = H (7); R1 = Me, R2 = t-Bu, R3 = H (8); R1 = Me, R2 = R3 = t-Bu (9); R1 = Ph, R2 = t-Bu, R3 = H (10); R1 = Ph, R2 = R3 = t-Bu (11), were tested in 1-pentene and 4M1P polymerisation with MAO as cocatalyst. The effect of substituents on the bridge, the cyclopentadienyl (Cp) and fluorenyl (Flu) ligand, was studied relative to the polymerisation temperature and type of monomer. The molecular weights of the polymers were considerably higher than those of the poly-1-pentenes and P4M1Ps obtained with C2-symmetric zirconocenes (1–5). The catalytic activities and polymer molecular weights strongly depend on the fluorenyl substituent and the bridge, while the type of substituent on the Cp ligand has a strong influence on the tacticity of the polymers.

Silsesquioxane-bonded zirconocene complexes; soluble models for silica-tethered olefin polymerization catalysts

Silsesquioxane tethered fluorene ligands [R7Si8O12X]-9-Flu(H) (R = c-C6H11, X = – (1); R = c-C5H9, X = CH2 (2), (CH2)3 (3), C6H4CH2 (4)), (c-C5H9)7Si8O12CH2-9-Flu(9-EMe3) (E = Si (5a), Sn (5b)) and zirconium dichlorides Cp″[(c-C5H9)7Si8O12CH2-9-Flu]ZrCl2 (6b, Cp″ = 1,3-C5H3(SiMe3)2), Cp*[(c-C5H9)7Si8O12-X-9-Flu]ZrCl2 (X = CH2 (6a), (CH2)3 (7), C6H4CH2 (8)) and [(c-C5H9)7Si8O12CH2-9-Flu]2ZrCl2 (9) have been applied as models for silica-tethered ancillary ligands and silica tethered-zirconocenes. Immobilization of zirconocenes containing a pendant anchorable functionality, Cp″[Me2(EtO)SiCH2Flu]ZrCl2 (10) and Cp[C5Me4SiMe2OEt]ZrCl2 (11) was considerably hampered by competitive Zr–Cl and Si–OEt bond splitting. When activated with MAO (methylalumoxane), 6–9 yield active ethylene polymerization systems.

Synthesis and characterization of tin containing polyhedral oligometallasilsesquioxanes (POMSS)

Tin silicate species have shown good catalytic activity in various oxidation reactions. In an attempt to mimic surface tin species, several tin containing silsesquioxanes have been synthesized. Incompletely condensed silsesquioxanes (c-C5H9)7Si7O9(OH)3 and (c-C5H9)7Si7O9(OSiMe3)(OH)2 were reacted with common tin-precursors, which afforded several silsesquioxane ligated tin compounds. Divalent stannasilsesquioxanes form dimers of the type [(c-C5H9)7Si7O11(OX)Sn]2(X=H, SiMe3) with three-coordinated tin centers. The three-coordinated tin(II) are hydrolytically unstable whereas the octahedrally surrounded tetravalent stannasilsesquioxanes [(c-C5H9)7Si7O11(OX)]Sn(acac)2(X=H, OSiMe3) are hydrolytically robust. An unprecedented anionic trimeric cluster, [[(c-C5H9)7Si7O12Sn]3(mu2-OH)3(mu3-OH)]-[HNEt3]+, stabilized by bridging hydroxyl groups was formed when the product formed upon reacting (c-C5H9)7Si7O9(OH)3 with SnCl4 was slowly hydrolyzed. The stannasilsesquioxanes showed no catalytic activity in oxidation reactions.

Surface micropatterning of uniaxially oriented polyethylene films using interference holography for strain sensors

A new procedure is presented for direct generation of surface micropatterns on uniaxially oriented polyethylene (PE) films using interference holography with a nanosecond pulsed laser. An ultraviolet absorber, 2-(2H-benzotriazol-2-yl)-4,6-di-tert-pentylphenol (BZT) is incorporated into PE prior to stretching to generate absorption at the wavelength of the laser. Illumination with an interference pattern in the absorption band of BZT leads to an obvious height variation in the exposed regions and consequently relief gratings are generated. The height in the exposed regions is strongly dependent on the angle between the grating direction and the film orientation direction. This phenomenon is attributed to a combination of events such as melting, entropic contraction, recrystallization, thermal evaporation of BZT, and anisotropic thermal conductivity. It is shown that the relief height increases with increasing BZT concentration and exhibits a linear dependence on the energy dose above a certain threshold. Additionally, the oriented PE films with the surface micropatterns are explored for strain sensors. The results demonstrate that small strains below 10% are monitored accurately in tensile deformation of the micropatterned, oriented PE films which makes these films potentially useful as strain sensors.