Group 4 metallocene catalysts

The polymerization of ethylene by group 4 metallocenes is widely recognized to proceed via 14-electron cationic intermediates 31,32 Cationic zirconocene33-35 and titanocene36-38 complexes, (1-3), were first isolated in 1986 by Jordan and Bochmann, respectively. Both (1) and (2) are active for ethylene polymerization in the absence of a cocatalyst.34 

Several isospecific Ci-symmetry catalysts have also been described including (12-15). When activated with [Ph3C]+ [B(C6F5)4]-, (12) affords highly regioregular i-PP (mmmm = 95%) with the stereochemical defects predominantly being isolated rr triads, consistent with a self-correcting enantiomorphic site-control pathway. 2,73 The isospecificity was therefore explained by a mechanism 

By contrast, the synthesis of syndiotactic PP, s-PP, is generally catalyzed by Cs-symmetry ansa- metallocenes. For example, (16)/MAO affords PP with a pentad (rrrr) content of 86% at 25 °C.77 The stereoselectivity is highly sensitive to ligand variation. For example, substitution at the 3-position of the Cp ring with a methyl group affords heterotactic PP,78 whilst the Bu analog favors i-PP production.50,75,79 

PP synthesized using TiCl4/Et3Al is mostly isotactic, but two minor fractions are also produced. One is a soluble, atactic PP, whilst the other fraction is a partially crystalline, elastomeric stereoblock of iso- and a-tactic PP sequences.98 Elastomeric PP may also be prepared using the ansa-titanoccnc complex, (26), (although this catalyst does undergo rapid deactivation).99 Stereoblock formation was attributed to an equilibrium mixture of slowly interconverting isospecific and aspecific catalyst sites. Other stereoblock PP materials have been prepared via chain transfer between two catalysts of different stereoselectivities.101,102 

Elastomeric PP has also been synthesized using Ti, Zr and Hf ansa-metallocenes, (27). An alternative explanation for stereoblock formation was proposed, in which epimerization between isospecific and aspecific sites is rapid, affording predominantly atactic PP with short isotactic-rich sequences.103-105 

---

Fig. 2 General structures of group 4 metallocene catalysts and CGCs...

Regarding the co-polymerization of hydrocarbon and polar monomers, late transition metal catalysts have provided the most significant advances to date because of their lower oxophilicity and thus greater functional-group tolerance than early transition metal catalysts, although group 4 metallocene catalysts are known to promote the co-polymer-ization of olefins and non-vinyl polar monomers with masked functional groups. 

Group 4 metallocene catalysts are also applicable to the above sequential block co-polymerization method to furnish polyolefin and polar polymer block co-polymers. Frauenrath et al. and Chen and Jin " reported the synthesis of PE-/ -PMMA and PP-3-PMMA, respectively, using metallocene catalysts (e.g., raz--(C2H4)(Ind)2ZrMe2/ B(C5F5)3, iPP-/ -iPMMA PP segment 4/ = 8900, PDI= 1.90 block co-polymer 4/n = 10900, PDI = 1.66, MMA content = 17.1 mol%). 

One should assume here, in accordance with general opinion, that the productive reaction complex in methylaluminoxane-activated group 4 metallocene catalysts is the [Cp 2Mt(R).olefin]+[Alx(R) i. 1OxX2] complex, which is generated by displacement of an [A1x(R)a. 1OxX2] ion from its [Cp 2Mt(R)]+ counterion by the coordinating olefin molecule [30]. 

The mechanism for the dehydropolymerization of R2SnH2 by the Group 4 metallocene catalysts is expected to be similar to the mechanism for the dehydropolymerization of hydrosilanes by the same catalysts. The mechanism for the Rh-catalyzed polymerization remains to be determined. 

Group 4 metallocene catalysts are, in addition to polyethylene and polypropylene, able to generate syndiotactic polystyrene, to polymerize cycloolefins (cyclopentene, nor-bomene, and their substituted compounds), and to give access to various copolymers. During the polymerization of cycloolefins, only the double bond is opened and not the ring. 

The function of MAO in this system is believed to be similar to that reported for the reactivity of MAO with group 4 metallocene catalysts (a) Yoshida, T. Koga, N. Morokuma, K. Organo metallics 1995, 14, 746. (b) Sishta, C. Hathom, R. M. Marks,... 

This review covers the homopolymerization of propene with group 4 metallocene catalysts, and special emphasis is dedicated to isotactic polypropene. Reference to other poly(l-olefins) and copolymerization between propene and minor amounts of ethene will be done only when relevant to the discussion. 

Polymerizations of MBL, j,MMBL, and yjMMBL by Group 4 Metallocene Catalysts... 

Fig. 5 Cationic group 4 metallocene catalysts employed for investigation of coordination polymerization of MBL, yMMBL, and MMBL...

---