CpTiCl3/MAO

As found for the polymerization of styrene, CpTiCl3/MAO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3-dienes (Table 25) [218], Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene and 2,3-dimethylbutadiene yield polymers with different... 

The polymerization of 1,3-butadiene with CpTiCl3 catalysts gives polybutadiene with about 81% as, 18% 1,2, and 1% trans units.451 Cp TiCl3 in the presence of MAO is similarly active.452 The CpTiCl3/MAO system has been used for homopolymerization of 4-vinylpyridine.453 Compounds Cp TiCl3 (Cp = Cp, Cp ) activated with a large excess of MAO have been found to catalyze the polymerization of a number of conjugated dienes.454... 

The system Cp2TiCl2/MAO is suggested to be less active than CpTiCl3/MAO and Cp2TiCl/MAO in the polymerization of 1,3-butadiene, 4-methyl-l,3-pentadiene, and styrene to give predominantly cis-, 4-polybutadiene, 1,2-syndiotactic poly(4-methyl-l,3-pentadiene), and syndiotactic polystyrene.1214... 

CpTiCl3/MAO exhibits high catalytic activity not only for syndiospecific styrene polymerization but also for the polymerization of conjugated dienes,... 

The catalytic activity of the titanocene derivatives increases in the order Cp2TiC2dienes also polymerize in the presence of CpTiCl3/MAO catalyst with the selectivity depending on the monomer structure. The polymerization of (Z)-l,3-pentadiene forms cis-... 

Scheme 4 The pathway and intermediates of the polymerization catalyzed by CpTiCl3/ MAO (P=polymer chain although the structures were calculated for P=H)...

Monomer insertion into the Ti-rc-allyl bond of the monomer-coordinated intermediates, giving the monomer-free intermediate, takes place more easily. Thus, selectivity between 1,4-insertion and 1,2-insertion of 1,3-butadiene is governed not only by the insertion of the monomer but also by the coordination of a new monomer. DFT investigation showed that one of the stereochemically different pathways for 1,4-insertion is more favorable than any of those for 1,2-insertion. It is consistent with the experimental results that CpTiCl3/MAO catalyzes 1,3-butadiene polymerization to produce mostly the cis-1,4 product. 

This section summarizes the copolymerization of conjugated dienes with other monomers catalyzed by transition metal complexes. Some of the reactions here were also mentioned in the previous section. The catalyst CpTiCl3/MAO is active not only for the polymerization of 1,3-butadiene, isoprene, 1,3-pentadiene, and styrene but also for the copolymerization of these individual monomers [82]. 

FIGURE 17.14 End groups detected in poly (4-methyl-1,3 pentadiene) polymerized with CpTiCl3/MAO in the presence of A1( CH3)3 2,1-insertion (A) l,4-insertion (B) (P = polymer chain). 

Ricci, G Italia, S. Porri, L. Polymerization of (Z)-l,3-pentadiene with CpTiCl3/MAO. Effect of temperature on polymer structure and mechanistic implications. Macromolecules 1994, 27, 868-869. 

Titanium [153] and vanadium [154] monocyclopentadienyl complexes are known to serve as catalysts for 1,4-czs polymerization, although bis(cyclopen-tadienyl) complexes are less active [155]. Ricci et al. investigated various diene monomers using CpTiCl3-MAO catalysts and showed that stereoselectivity of polymerization depends on the monomer employed [156]. Soga reported that half-titanocene with an oxy-coordinating pendant 54 produces ds-rich polybutadiene [157,158]. 

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