Hf-FI catalysts

Because an FI catalyst has a pair of non-symmetric phenoxy-imine ligands, it potentially possesses five isomers stemming from the coordination modes of ligands. Zr-, Ti-, and Hf-FI catalysts 1-3 display three sets of signals in XH NMR, attributed to the imine proton, suggesting that these FI catalysts exist as isomeric mixtures in solution, which is probably an intrinsic feature of FI catalysts. On the basis of the symmetry of the possible isomers A-E (Fig. 10), as well as the relative formation... 

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene... 

The production of highly isotactic PPs with Zr- and Hf-FI catalysts//-Bu3Al/ Ph3CB(C6F5)4 (phenoxy-amine complexes site-controlled polymerization with 1,2-insertion) is in sharp contrast to that of highly syndiotactic PPs with Ti-FI cata-lysts/MAO (phenoxy-imine complexes chain-end controlled polymerization with 2,1-insertion), which will be described later [64]. 

We and others have revealed that syndiospecific propylene polymerization is exclusively initiated by 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization [64]. This is the first example of a predominant 2,1-insertion mechanism for chain propagation exhibited by a group 4 metal-based catalyst. The unusual preference for 2,1-regiochemistry displayed by the Ti-FI catalysts compared with the Zr- and Hf-FI catalysts is apparently inconsistent with the crys-tallographically characterized structures, which indicate that the Ti is shielded more by the phenoxy-imine ligands and thus possesses higher steric compression. The reason for the unusual preference in the regiochemistry of Ti-FI catalysts is unclear at the present time. 

In addition to the above achievements, researchers at Dow reported on the catalytic production of multiblock copolymers comprised of PE and amorphous pol y(cthylenc-co-1 -octene) segments, by a catalyst system comprised of Zr-FI catalyst 59, Hf complex 60 (Fig. 38), and Et2Zn [11, 90],... 

The Zr-FI catalyst selectively forms PE even in the presence of ethylene and 1-octene, while the Hf complex affords amorphous copolymers, resulting in the catalytic generation of PE- and poly(ethylene-c6>-l-octene)-based multiblock copolymers through a reversible chain transfer reaction mediated by R2Zn. The development of an FI catalyst with extremely high ethylene selectivity as well as a reversible chain transfer nature has made it possible to produce these unique polymers. Therefore, both Ti- and Zr-FI catalysts are at the forefront of the commercial production of polyolefinic block copolymers. 

In their alliance with Dow, Symyx discovered two classes of Hf and Zr catalysts capable of making crystalline PP at high temperatures that appear to have seen commercial practice. These are the complexes of pyridyl amides (Stmcture 9) and bis(biphenylphenolate)ethers (bppe) (Stmcture 10). More recently, Dow has reported that combination of a pyridyl amide complex with a Mitsui FI catalyst and an excess of aluminum or zinc alkyls allows for the production of olefin block copolymers (OBCs) through a chain shuttling ... 

Whereas superaeid (HF/BF3, HF/SbF, HF/TaF FS03FI/SbF3, etc.)-eatalyzed hydroearbon transformations were first explored in the liquid phase, subsequently, solid aeid eatalyst systems, sueh as those based on Nafion-H, longer-chain perfluorinated alkanesulfonic acids, fluorinated graphite intercalates, etc. were also developed and utilized for heterogeneous reactions. The strong acidic nature of zeolite catalysts was also successfully explored in cases such as FI-ZSM-5 at high temperatures. 

The reaction follows by nucleophilic attack of oxygen fi om a water molecule on Si. Catalysts, solvents, and the R groups substituted on the silane precursor affect the rate and extent of hydrolysis. In the presence of catalysts, completion of the hydrolysis is observed. Typical catalysts are HF, HCl, amines, and KOH. Addition of solvent increases the rate of depolymerization and esterification reactions. Bulkier groups on the silica precursor reduce the rate of hydrolysis. ... 

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