Methylaluminoxane co-catalyst

Methylaluminoxane co catalyst is required by these metallocenes to become highly isospecific catalysts capable of very high isotactic placement. They are also very active, yielding very large quantities of polypropylene per each gram of zirconia. 

The first isospecific propene polymerization with an ethanediyl-bridged bis (indenyl)-titanium complex demonstrated in 1983 the capability of ansa-metaUocenes to control the tacticity of a growing polypropylene chain [4, 48], thus creating a strong interest in exploring the potential use of ansa-metallocenes for industrial polypropylene production. A first step towards industrially suitable systems was the use of much more stable zirconium complexes, which when activated by the methylaluminoxane co-catalyst [3] allowed higher polymerization temperatures and thus higher activities [5]. 

The quasi living polymerization of ethene and norbornene has been reviewed, among other topics in living polymerization of alkenes (19). Specifically, arylimido-aryloxo-vanadium(V) complexes with methylaluminoxane or Et2AlCl as co-catalyst have been used as catalyst systems. The polymers exhibit a low polydispersity and a high molecular weight (20). 

Poly(ethylene-co-norbomene-2,3-dicarboxylic acid anhydride) was prepared by co-polymerizing the respective monomers with the transition metal catalyst, di(3-t-butyl-2-hydroxy-l-(A-phenyhmino)benzene) titanium(IV). The polymerization was conducted at ambient temperature using methylaluminoxane as co-catalyst. After a 10 minute polymerization reaction scoping period 0.02 mol% of norbornene-2,3-dicarboxylic acid anhydride was incorporated into the co-polymer. 

The main class of metallocene catalysts used today is Kaminsky-Sinn catalysts. They are based on titanium, zirconium, or hafnium, and use methylaluminoxane as a co-catalyst. These catalysts produce very uniform comonomer incorporation and very narrow molecular weight distributions. 

Interestingly, the discovery of methylaluminoxane (MAO) in 1980 played a crucial role in determining the course of action in polymer science [14]. This co-catalyst replaced the alkyl aluminum compounds because it acts not only as alkylating agent, but also as scavenging agent that led to a remarkable increase in the activity of the catalysts. 

The development of Ziegler-Natta-type catalysts (see Section 27.8) has, since the 1980s, included the use of zirconocene derivatives. In the presence of methylaluminoxane [MeAl(p-0)] as a co-catalyst, compounds A, B and C (shown below) are active catalysts for propene pol5mierization. Compounds A and B are chiral because of the relative orientations of the two halves of the organic ligand. A racemic mixture of A facilitates the formation of isotactic polypropene, while use of catalyst C results in syndiotactic polypropene (see Section 27.8 for definitions of... 

In olefin polymerization the pro-catalyst must be combined with an organoaluminum co-catalyst, such as a methylaluminoxane (MAO). The polymerization takes place at T = 160-250 °C and P = 14-22 MPa... 

Figure 2. Dependence of the weight average molecular weight and the density ofmetallocene polyethylene obtained in zirconium based metallocene (methylaluminoxane as co-catalyst) catalyzed polymerization of ethylene at 1500 bar at different temperatures in the range from 80 to 260°C. [Adapted from ref. 11]...

Today, less than 5 % of polypropylene is produced using metallocene catalysts. Metallocene catalysts are mainly ZrCh catalysts supported on silica in combination with co-catalysts like methylaluminoxane (MAO). These catalysts show very specific characteristics and may also be combined with Ziegler-Natta catalysts. These catalysts are mainly used to produce specific product ranges and they influence plant configurations. 

Picolyl-functionalized A-heterocyclic carbene complexes NiCl2(NHG)2 (NHG = 3-R-l-picolylimidazolin-2-ylidene, R = Me, Ph) show high catalytic activities toward polymerization of ethylene or norbornene in the presence of methylaluminoxane (MAO) as co-catalyst. Although the detailed mechanism of the catalytic olefin polymerization is not clear, these cationic nickel complexes contain a hemilabile picolyl carbene ligand that allows the formation of a vacant site for olefin coordination, 32. 

Mono- and bis-NHC iridium(i) complexes were applied in the C-H bor-ylation of aromatic carbons with bis(pinacolato)diboron or pinacolborane under microwave irradiation. The bis-NHC complexes gave better yields, mainly because of the lower stability of the mono-NHC Ir complexes compared to the bis-NHC Ir complexes. Finally, picoline- and pyridine-functionalised NHC-iridium complexes of general formula [(C N)IrCl(Cp )]Cl were moderately active catalysts for the polymerisation of norbornene in the presence of methylaluminoxane (MAO) as co-catalyst. ... 

These catalysts were used in combination with methylaluminoxane (MAO) for ethylene-norbornene co-polymerization and compared with isopropylidene[9-fluorenylcyclopentadienyl]zirconium dichloride catalyst activity under identical conditions. 

Jones and co-workers976 have recently reported the use of catalyst 49 (Figure 5.15) with perfluorinated alkanesulfonic acid sites anchored to SBA-15 as a methylaluminoxane-free supported cocatalyst for ethylene polymerization. When catalyst 49 and trimethylaluminum were used in combination with Cp 2ZrMe2 as the metallocene precatalyst, productivities as high as 1000 kg polyethylene molZr-1 h 1 were obtained without experiencing reactor fouling. 

Varying the feed ratio of the comonomers allowed regulation of the copolymer composition. The isolated yield and M of poly(PDL-co-43 mol % TMC) formed after 24 h (feed 2 1 PDL TMC) was 90% and 30900g/mol, respectively. Thus far, an alternative chemical route to random poly(PDL-co-TMC) is not known. For example, PDL/TMC copolymerizations with chemical catalyst such as stannous octanoate, methylaluminoxane, and aluminum triisoproxide resulted either in homo-poly(TMC) or block copolymers of poly(TMC-co-PDL) [114]. Chemical... 

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