Unbridged metallocenes

SCHEME 1.9 Temporal C2 symmetry displayed by unbridged metallocene complexes with identically 

FIGURE 1.16 Unbridged group 4 bis(cyclopentadienyl) complexes 26-28 for isoselective propylene polymerization. 

In Special cases, this type of catalyst may oscillate, switching between the two stereoselective C2-symmetric racemic-like conformations and a nonstereoselective meso-Yikt conformation during the propagation of a single polymer chain. The result is a stereoblock polymer microstructure (see Chapter 8). The most-studied catalyst precursor of this type is the unbridged complex bis(2-phenylindenyl)zirconium dichloride (vide The oscillating stereocontrol mech- 

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Table 8. Isotactic poly(propylene) polymerized by bridged- and unbridged-metallocene catalysts... 

Kaminsky W, Engehausen R, Zoumis K (1992) Standardized polymerizations of ethylene and propene with bridged and unbridged metallocene derivates a comparison. Makromol Chem 193 1643-1651... 

Achiral, C -symmetric unbridged metallocenes, 16 104 Achiral hydrobora ting agents, 13 667 Achiral molecules, 6 73 Acicular reinforcement, 5 554 Acid acceptors, in VDC polymer stabilization, 25 719 Acid-activated bentonites, 6 680-681 Acid amide herbicides, 13 319-320 Acid anhydrides, 10 403-406, 484 reactions with alkanolamines from olefin oxides and ammonia, 2 127 Acid-base catalysis, 5 205-209... 

Chiral, Ci-symmetric (asymmetric) bridged metallocenes, 16 108-109 Chiral, C2-symmetric bridged metallocenes, 16 104-108 Chiral, C2-symmetric catalysts, racemic mixture of, 16 106 Chiral, C2-symmetric unbridged metallocenes, 16 108 Chiral catalysts, 16 395 Chiral centers, in biochemical compounds, 17 402... 

Unaccomplished moisture change, 9 97 Unagitated columns, in commercial extractors, 10 769-774 Unbound moisture, 9 97-98 Unbridged catalysts, oscillating, 16 109 Unbridged metallocenes... 

Unbridged metallocenes rarely achieve highly stereoselective polymerizations because free rotation of the r 5-ligands results in achiral environments at the active sites. An exception occurs when there is an appreciable barrier to free rotation of the r 5-ligands. Fluxional (con-formationally dynamic) metallocenes are initiators that can exist in different conformations during propagation. Stereoblock copolymers are possible when the conformations differ in stereoselectivity and each conformation has a sufficient lifetime for monomer insertion to occur prior to conversion to the other conformation(s). Isotactic-atactic stereoblock polymers would result if one conformation were isoselective and the other, aselective. An isotactic-atactic stereoblock polymer has potential utility as a thermoplastic elastomer in which the isotactic crystalline blocks act as physical crosslinks. 

Under the same conditions, syndiospecihc (Cs-symmetric) metallocenes are more effective in inserting a-olefins into an ethylene copolymer than isospecific (C2-symmetric) metallocenes or unbridged metallocenes. In particular, hafnocenes are more efficient than zirconocenes. An interesting effect is observed for the polymerization with ethylene(bisindenyl)zirconium dichloride and some other metallocenes. The catalytic activity for the homopolymerization of ethylene is very high, and it increases when copolymerization with propylene occurs (114) (Fig. 12). Munoz-Escalona et al. (125) observed similar effects in the copolymerization of ethylene with 1-hexene. 

Under the same conditions, syndiospecific (Cs-symmetric) metallocenes are more effective for inserting a-olefins into an ethene-copolymer than isospecific working (C2-symmetric) metallocenes, or unbridged metallocenes. In this case hafnocenes are more efficient than zirconocenes, too. 

Stereoblock polypropylene is synthesized by unbridged metallocenes such as bis(neomenthyl)zirconium dichloride (7) with cyclopentadienyl or phenyl substituted indenyl ligands [55, 56]. The stereoblock length increases at lower polymerization temperatures. Products containing isotactic and atactic blocks are elastomeric if the isotactic block length is short. Rieger [57] obtained similar polypropylenes by bridged fluorenyl-substituted indenyl zirconium complexes. 

In unbridged metallocenes, like Cp2MtX2 or Ind2MtX2, the p -coordinated aromatic rings rotate freely even at very low temperatures. However, the rotation may become hindered in the presence of a bulky substituent, and the rate of rotation oscillating may be dependent on the temperature. Of special interest are (2-Ar-Ind)2MtX2 complexes (with Mt = Zr or Hf, and Ar = phenyl or a substituted... 

Phillips catalysts tend to be more efficient than Ziegler catalysts, less efficient than tightly bridged metallocenes, and approximately equivalent in efficiency to unbridged metallocenes. CO-reduced catalysts incorporate 1-hexene about two to three times more efficiently than their... 

There are a few examples of unbridged metallocenes which are stereoselective by site control. Examples have been reported with substituted cy-clopentadienyl, indenyl, and fluorenyl ligands, the latter being apparently the most stereoselective. The biscyclopentadienyl system C2-ITI (Chart 16) produces, at low polymerization temperature (—50 °C), a low molecular weight, low isotactic PP mmmm = 51%) with a double stereodifferentiating mechanism, partly site control (27%, b = 0.96) partly chain-end control (73%, p = 0.79). The related, but much bulkier, rac-[Cp-CH(Ph)CH2(9-BBN)]2ZrCl2 (Q-II-Z) produced at —50 °C a more isotactic PP mmmm = 75%), with predominance of site control (67%, b =... 

So far, only two modeling studies relative to unbridged metallocenes have been reported by Pietsch... 

Early Soluble Unbridged Metallocene Catalysts in Connection with Alkyl Aluminum Chlorides... 

Unbridged metallocenes, in which the backbone of the catalyst is loosely bound, were the driving force for the development of the next generation of polymerization catalysts because of dilferent microstructures produced due to the change in the symmetry of the active site of the metal center involved. Thus, a new horizon was bom toward the synthesis of controlled microstructures. 

Comparative studies of unbridged metallocene catalysts have indicated that, in general, alkylation of the Cp ring(s) causes an increase in polymerization activity, although substitution of all five carbons can lead to very low activities, as steric congestion prevents monomer approach. Bis(indenyl) and bis(fluorenyl)zirconium complexes have received far less attention, and in the latter case, this is almost certainly due to the greater lability of the fluorenyl-metal interaction. 

At the first-order level, the preference of the metallocene for one monomer over the other is quantified by the two reactivity ratios, r (= knlkn) and r2 (= 22/ 21)- Since ethylene is invariably more reactive than any other monomer, ri > 1 and T2 < 1. Study of the First-Order Markov constants (Table 2) reveals important correlations with metallocene and comonomer structure. As is to be expected, the larger the comonomer, the higher the r value, indicating a steric hindrance to its insertion. Bridged metallocenes, such as structure 2, give catalysts with much lower ri values than do unbridged metallocenes. In general, metallocenes copolymerize monomers with a tendency toward alternation of monomers, as indicated by a value for r r2 less than 1. 

In addition to the bridged metallocene-based catalysts, a number of unbridged metallocene catalyst systems, exhibiting time-dependent C2-symmetry due to restricted ligand rotation around the ligand-metal axis, have been reported to produce polypropylenes with a range of tacticities and microstructures. In the context of the present discussion, the unbridged metallocene complexes are considered as pieM(io-C2-symmetric. 

The discussed reinterpretation of (2-Ar-indenyl)2MX2-based catalysts unifies the mechanistic picture for all substituted unbridged metallocenes. However, the scientific debate is still... 

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