Phenoxy-imine

In this contribution, we describe the discovery and application of phenoxy-imine ligated early transition metal complexes (FI catalysts) for olefin polymerization, including the concept behind our catalyst design, the discovery and the polymerization behavior of FI catalysts, and their applications to new polyolefinic materials. 

Among the highly active catalysts introduced above, bis(phenoxy-imine) early transition metal complexes (Fig. 9) in particular show strikingly high activities for the polymerization of ethylene [14, 51-54]. 

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... 

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. 

The above achievements depend highly on both the recent advances in rational catalyst design with the aid of computational science represented by DFT calculations and the wide range of catalyst design possibilities that are afforded by FI catalysts. These possibilities are derived from the readily varied steric and electronic properties of the phenoxy-imine ligands. It is expected that future research on FI catalysts will provide opportunities to produce additional polyolefin-based materials with unique microstructures and a chance to study catalysis and mechanisms for olefin polymerization. 

A variation on the a-diimine ligand is the phenoxy-imine ligand, which has been studied mostly with the group 4 metals Ti and Zr [Ittel et al., 2000 Makio et al., 2002 Milano et al., 2002 Mitani et al., 2003 Saito et al., 2001, 2002 Tian and Coates, 2000]. A typical phenoxy-imine initiator is shown below, where R1 is usually a phenyl or substituted phenyl group. 

Ethylene is polymerized with good activity by both Zr and Ti phenoxy-imines to high molecular weight polymer, Mv — 104-106. Activity increases with increasing steric bulk at... 

The stereoselectivity of polymerization depends on the transition metal and the structure of the initiator. Syndioselective polymerization is more common than isoselective polymerization. Some titanium phenoxy-imine initiators yield highly syndioselective polymerization by chain end control. For example the initiator with R2 = R3 = t-butyl yields polypropene with (rr) = 0.92 [Tian and Coates, 2000]. The initiator with R2 = t-butyl and R1 = C6Fs yields polypropene with (rr) = 0.98 [Saito et al., 2001 Tian et al., 2001], Moderately isoselective polymerization is obtained with some zirconium and hafnium phenoxy-imine initiators [Saito et al., 2002]. 

There are some isolated reports of metallocenes and phenoxy-imines as effective initiators for the polymerization of vinyl ethers, but the reactions do not proceed in a stereoselective manner [Baird, 2000 Kawaguchi et al., 2002]. 

Unique and versatile olefin polymerizations with group 4 bis(phenoxy-imine) complexes have been disclosed by scientists at Mitsui. " In a series of Ti complexes with fluorinated iV-phenyl groups (Figure 10, complexes FlO-7-FlO-14), Mitani et al. observed a substantial difference in ethylene polymerizations between complexes having... 

Grubbs and co-workers have developed a new class of neutral Ni complexes incorporating bulky phenoxy-imine ligands F13-16 that show considerable tolerance toward functional groups and even remain active in the presence of water. These complexes can co-polymerize ethylene with functionalized NBs such as 5-norbornen-2-ol to provide cyclic olefin co-polymers with hydroxy or ester functionality. It has been reported that these Ni complexes are ineffective for the polymerization of MA due to /3-H transfer from the enolate-Ni complex to the Ni metal. 

The (co)polymerization of dienes can be a good method for the preparation of polymers with reactive vinyl groups, a method that enables the preparation of polymers possessing plural vinyl groups per polymer chain. A fluorinated bis(phenoxy-imine) Ti complex was shown by Coates and co-workers to convert 1,5-hexadiene to poly(methylene-l,3-cyclopentane-fti-3-vinyl tetramethylene), which contained multiple vinyl groups. As already discussed, Saito et al. and others revealed that bis(phenoxy-imine) Ti complexes favored secondary insertion. " This is probably responsible for the formation of 3-vinyl tetramethylene units. Likewise, the same catalyst system can form sPP-/ -poly(methylene-l,3-cyclopentane-z -3-vinyl tetramethylene) from propylene and 1,5-hexadiene. Very recently. 

Jin and co-workers applied the self-immobilization method developed by Alt to group 4 bis(phenoxy-imine) complexes. They demonstrated that Ti and Zr bis(phenoxy-imine) complexes with allyl-substituted phenoxy-imine ligands upon activation with MAO showed high ethylene-polymerization activity and form good morphology PEs. The same research group also demonstrated that this self-immobilization method was applicable to... 

Bis(phenoxy-imine) catalysts in olefin polymerization, 4, 1096 for syndiotactic polypropylene polymerization, 4, 1115 Bis(phenoxy) imine-donor complexes, with Zr(IV) and Hf(IV), 4, 814... 

Chelating bis(phenoxy) imine-donor complexes, with Zr(IV)... 

Isotactic polypropylene and metallocene catalysts, 4, 1056, 4, 1064 via phenoxy-imine catalysts, 4, 1126 Isotope effects... 

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