Effect of Initiator Structure in .NET

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Effect of Initiator Structure
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C2 ansa metallocenes are inherently isoselective, but the degree of isoselectivity as well as activity and polymer molecular weight vary considerably depending on the speci c
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Z5 -ligands, substituents on the ligands, transition metal, bridge, and reaction conditions [Busico and Cipullo, 2001; Coates, 2000; Resconi et al., 2000]. The structural variables interact in a complex manner to change the course of the polymerization by altering the bite angle and stereorigidity of the initiator and its derived propagating species. One assumes that there is an optimum bite angle and stereorigidity that maximizes the imposition of the initiator s chirality on the propagating species and coordinated monomer. If the bite angle is too large and the stereorigidity too low, isoselectivity is decreased because the propagating chain and monomer are too loosely held in place. If the bite angle is too small and the stereorigidity too high, isoselectivity is decreased because the propagating chain and monomer have dif culty coordinating to the active site and/or do have the necessary mobility to rapidly undergo migratory insertion. Although the exact interrelationship between the various structural parameters is not clear, a number of generalizations have been established: 1. Metallocenes based on zirconium (referred to as zirconocenes) are the most widely studied and useful at present. Hf metallocenes are generally less active but produce higher molecular weights than do Zr metallocenes. Ti metallocenes are less active and less stereoselective than Zr and Hf. Zr metallocenes are the most useful because they are the most active and have been optimized by appropriate structural variations to yield very high stereoselectivity coupled with high polymer molecular weight and high activity at useful temperatures (>50 C). Some examples are given in Table 8-6. 2. Substituents in the 3- and 4-positions of Cp ligands have the greatest effect for increasing isoselectivity, activity, and polymer molecular weight. Substituents in the 2- and 5-positions have a positive but lesser effect. The 6-membered ring plays the role of 4- and 5-substituents in Ind and H4 Ind ligands. H4 Ind ligands generally increase isoselectivity with some decrease in activity, and sometimes also a decrease in polymer molecular weight.
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E Cyclopentadienyl
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E Indenyl
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TABLE 8-6 Propene Polymerization with rac-Zirconocene/MAO Initiator Me2 Si(3-Me-Cp)2 ZrCl2 C2 H4 (3-Me-Cp)2 ZrCl2 Me2 Si(2,3,5-Me3 -Cp)2 ZrCl2 Me2 Si(2-Me-4-C6 H5 -Ind)2 ZrCl2 Me2 C(3-t-Bu-Ind)2 ZrCl2 CH2 (3-t-Bu-Ind)2 ZrCl2 Me2 Si(4-[1-naphthyl]-Ind)2 ZrCl2 c Me2 C(Ind)2 ZrCl2 Me2 C(H4 Ind)2 ZrCl2
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T ( C) 30 40 50 70 50 50 50 50 50
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(mmmm) 0.93 0.92 0.96 0.95 0.95 0.97 0.99 0.81 0.96
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Activity in units of kg PP (mmol Zr) 1 h 1 . Either M w or M v. c Data from Coates [2000]. All other data from Resconi et al. [2000].
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3. The effect of the bridge between ligands differs depending on the ligands. For the unsubstituted indenyl ligand, isoselectivity and molecular weight increase in the order CH2 < CH2 CH2 < (CH3 )2 C < Si(CH3 )2 . However, for the 3-t-butylindenyl ligand, CH2 and (CH3 )2 C are highly isoselective, and more so than CH2 CH2 and Si(CH3 )2 . The bite angle and stereorigidity of the system are affected differently by the bridge depending on the ligands. Initiators with bridges longer than two atoms are not useful. Markedly decreased isoselectivity and activity is observed with the CH2 CH2 CH2 bridge. Bulkier or longer bridges such as (CH3 )2 SiCH2 CH2 Si(CH3 )2 and (CH3 )2 SiOSi(CH3 )2 yield initiators inactive for propene polymerization. Various other types of bridges have been studied with poor results. This includes bridges through the 6-membered rings in Ind and H4 Ind systems and two bridges in Cp systems. 4. The introduction of heteroatoms into the ligands via alkoxy and trisubstituted amino groups has been explored to improve initiator performance through electronic effects. This approach is not promising as initiator performance deteriorates with very few exceptions. The exceptions do not result in performance enhancements; at best, performance is unaffected. 5. Bis uorenyl zirconocenes generally are neither highly active nor highly isoselective. 6. Polymerizations are highly regioselective with regioirregular placement generally in the range 0.3 1.0%, but regioselectivity is very sensititive to the ligands. At the extremes, there is less than 0.002% regioerrors with rac-CH2 (3-t-Bu-Ind)2 ZrCl2 /MAO, but 20% with rac-CH2 CH2 (2,4-Me2 -H4 Ind)2 ZrCl2 /MAO. C2 metallocenes generally are less regioselective than the metallocenes that are syndioselective or aselective. Some 3,1-placements (CH2 CH2 CH2 ) are observed in propene polymerization, a result of isomerization after a 2,1-placement [Busico and Cipullo, 2001]. 8-5c-2 Effect of Reaction Variables
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Polymerization at higher temperature increases the reaction rate but decreases isoselectivity, regioselectivity, and polymer molecular weight. For example, (mmmm) decreases from 0.92 to 0.83 for polymerization of liquid propene with rac-C2 H4 (Ind)2 ZrCl2 /MAO when the temperature is increased from 20 to 70 C. M v decreases from 56,000 to 19,600, and the regioirregular fraction increases from 0.4 to 0.7%. Similar effects were observed in the polymerization of 1-hexene by Me2 Si(H4 Ind)2 ZrCl2 /MAO [Zhao et al., 2000]. Increased temperature affects the polymerization by decreasing the rigidity of the initiator and propagating species (increased uxionality). The result is epimerization of the propagating chain end, speci cally, scrambling the chirality of the last monomer unit inserted into the propagating chain (the rst monomer unit attached to the transition metal atom). The greater the inherent stereorigidity of the initiator, the less the effect of increased temperature on polymerization. Isoselectivity decreases with decreasing monomer concentration. Epimerization of the propagating chain end is a unimolecular process, unaffected by monomer concentration. propagation is slowed by decreased monomer concentration, allowing time for epimerization to occur. The effect varies considerably depending on the initiator. For rac-C2 H4 (Ind)2 ZrCl2 / MAO, (mmmm) decreases from 0.87 to 0.55 when the monomer concentration is decreased from 11 to 0.4 mol L 1 . The same effect is observed in the polymerization of 1-hexene by Me2 Si(H4 Ind)2 ZrCl2 /MAO [Zhao et al., 2000]. In the extreme case of very high dilution, the polymer molecular weight decreases enormously and the polymer becomes atactic. The polymerization is less sensitive to monomer concentration for the more stereorigid rac-Me2 Si
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