Preparation of ethylene/1-hexene copolymers from ethylene using a fully silica-supported tandem catalyst system

\u3cp\u3eA silica-supported tandem catalyst system, capable of producing ethylene/1-hexene copolymers from ethylene being the single monomer, has been investigated. As tandem couple a phenoxyimine titanium catalyst for ethylene trimerization was combined with a metallocene catalyst for α-olefin polymerization. Two different approaches were pursued to combine the two catalysts as silica-supported tandem partners. The co-immobilization of the catalysts on the same support particles led to low polymerization activities and yielded products with low comonomer content due to interference of the two catalysts on the support. Immobilization of the two catalysts on separate supports prevented this interaction and led to high polymerization activities while the comonomer content of the product was controlled by the employed catalyst ratio. The copolymers obtained via the latter method were thoroughly analyzed with respect to their chemical composition distribution (CCD) by DSC-SSA, Crystaf, and HT-HPLC. The obtained data indicate a broad and in some cases bimodal CCD, which was explained by the synergy of composition drift during the polymerization and increasing diffusion limitation within the expanding polymer particle.\u3c/p\u3

Effect of aluminum alkyls on a homogeneous and silica-supported phenoxy-imine titanium catalyst for ethylene trimerization

A phenoxy-imine titanium catalyst (FI-catalyst) for selective ethylene trimerization was immobilized on methyl aluminoxane (MAO) pretreated silica and its activity and selectivity was compared with that of the corresponding homogeneous catalyst system. The homogeneous and heterogeneous ethylene oligomerization was conducted in the presence of different aluminum alkyls, commonly used as scavengers during olefin polymerization to remove residual oxygen and moisture from the reaction medium. Both the homogeneous and heterogeneous catalysts were strongly affected by the presence of scavenger in the reaction medium. Upon activation with R3Al/MAO (R= Et, nOct, iBu), the homogeneous catalyst switches selectivity from ethylene trimerization to polymerization. NMR spectroscopic investigations indicate that this change of selectivity can be attributed to ligand exchange between the precatalyst and the aluminum alkyl and reduction of the titanium species. The thereby formed ligand-free and/or reduced titanium alkyls act as polymerization catalysts and are responsible for the increasing polymer formation. Using the heterogeneous catalyst, the scavenger employed during ethylene trimerization was found to be of crucial influence regarding the activity of the catalyst and the occurrence of reactor fouling. Employing aluminum alkyls like iBu3Al and nOct3Al resulted in catalyst leaching and homogeneous polymer formation. The latter was prevented using Me3Al or Et3Al as scavengers; however, in general the supported catalyst was poisoned by aluminum alkyls, resulting in a low overall activity. It was found to be beneficial for the heterogeneous trimerization system to employ silica-supported scavengers. By physical separation of the catalyst and the scavenger this poisoning effect was effectively prevented, resulting in a highly active heterogeneous catalyst

Process for the preparation of polycarbonates

The invention relates to a process for the preparation of polycarbonates from oxiranes and CO2 characterized in that the polymerization is carried out in a single step in the presence of a chain transfer agent. Preferred chain transfer agents are polyols. The invention also relates to branched polycarbonates that have functional hydroxyl endgroups

Effect of aluminum alkyls on a homogeneous and silica-supported phenoxy-imine titanium catalyst for ethylene trimerization

A phenoxy-imine titanium catalyst (FI-catalyst) for selective ethylene trimerization was immobilized on methyl aluminoxane (MAO) pretreated silica and its activity and selectivity was compared with that of the corresponding homogeneous catalyst system. The homogeneous and heterogeneous ethylene oligomerization was conducted in the presence of different aluminum alkyls, commonly used as scavengers during olefin polymerization to remove residual oxygen and moisture from the reaction medium. Both the homogeneous and heterogeneous catalysts were strongly affected by the presence of scavenger in the reaction medium. Upon activation with R3Al/MAO (R= Et, nOct, iBu), the homogeneous catalyst switches selectivity from ethylene trimerization to polymerization. NMR spectroscopic investigations indicate that this change of selectivity can be attributed to ligand exchange between the precatalyst and the aluminum alkyl and reduction of the titanium species. The thereby formed ligand-free and/or reduced titanium alkyls act as polymerization catalysts and are responsible for the increasing polymer formation. Using the heterogeneous catalyst, the scavenger employed during ethylene trimerization was found to be of crucial influence regarding the activity of the catalyst and the occurrence of reactor fouling. Employing aluminum alkyls like iBu3Al and nOct3Al resulted in catalyst leaching and homogeneous polymer formation. The latter was prevented using Me3Al or Et3Al as scavengers; however, in general the supported catalyst was poisoned by aluminum alkyls, resulting in a low overall activity. It was found to be beneficial for the heterogeneous trimerization system to employ silica-supported scavengers. By physical separation of the catalyst and the scavenger this poisoning effect was effectively prevented, resulting in a highly active heterogeneous catalyst

New insight into the role of the metal oxidation state in controlling the selectivity of the Cr-(SNS) ethylene trimerization catalyst

The tri- and divalent complexes of the 2,6-bis(RSCH2)pyridine [R = Ph, Cy] ligands have been prepared. Upon activation with MAO, both species are catalysts for ethylene oligomerization of moderate activity. However, while the trivalent catalysts produced only 1-hexene, the divalent species gave a statistical distribution of oligomers. This clear difference in catalytic behavior indicates that the two oxidation states are not interconnected during the catalytic cycle as it happens instead with other oligomerization catalytic systems. The trivalent precursor is not reduced and the divalent is not oxidized. Treatment of the trivalent catalyst precursors with either MAO or other R3Al species afforded intractable materials. Instead, similar reactions with the divalent complexes gave new cationic species, which have been characterized by X-ray analysis. These complexes have preserved the divalent state of chromium during the reaction and still produce, upon further activation, a statistical distribution of oligomers. This reiterates the non-interconvertibility of the di- and trivalent oxidation states and the different degree of selectivity for which they are responsible

Reactivity of chromium complexes of a bis(imino)pyridine ligand : highly active ethylene polymerization catalysts carrying the metal in a formally low oxidation state

A divalent chromium complex of bis(imino)pyridine, {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}CrCl2 (1), was prepared with the aim of studying its reactivity with alkylating agents. Upon treatment with MeLi, the metal center was both reduced and alkylated, forming {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}CrMe(-Me)Li(THF)3 (2). Complex 1 is also conveniently reduced with either NaH or metallic sodium to give the new species {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}CrCl (3). Despite the appearance of the metal center in a rare monovalent oxidation state, the square-planar geometry of the Cr atom suggests that the metal is most likely divalent, with the electron housed in the ligand * orbital. When it is activated with MAO, complex 3 is a very and even more active catalyst for the polymerization of ethylene than either the -CrCl2 or -CrCl3 derivative of this ligand system and yet produces polymers with similar properties. Subsequent reactivity studies of complex 3 have allowed the isolation of several products. Reaction with either LiCH2Si(CH3)3 or MeLi resulted in deprotonation of one of the methyl groups on the ligand backbone, forming {2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC=CH2](C5H3N)}Cr(THF) (4) and {2-[2,6-(i-Pr)2PhN=C(CH3)]-6-[2,6-(i-Pr)2PhNC=CH2](C5H3N)}Cr(-Me)Li(THF)3 (5), respectively. On the other hand, alkylation with AlMe3 allowed the successful preparation of another organochromium species, {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}CrCH3 (7), along with small amounts of the byproduct {2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}Cr(-Cl)2Al(CH3)2 (6). Interestingly, complex 7, which also has the deceiving connectivity of a monovalent species, displays an even greater activity for ethylene polymerization than all of the other species reported herein, again producing a polymer with nearly identical characteristics. Activation with IBAO revealed a deactivation pathway similar to that observed with the FeCl2 system. In this case, the stronger reducing power of IBAO resulted in the usual reduction not only of the ligand backbone but also of the metal center. As a result of the metal reduction, partial transmetalation of the ligand system occurred, with formation of [4-{2,6-[2,6-(i-Pr)2PhN=C(CH3)]2(C5H3N)}Al2(i-Bu)3(-Cl)]Cr-(6-C7H8) (8). By being catalytically inactive, the partly transmetalated 8 suggests that ligand demetalation is a possible catalyst deactivation pathway