STEREOCHEMISTRY OF POLYMERIZATION in .NET

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STEREOCHEMISTRY OF POLYMERIZATION
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TABLE 8-12 Effect of Counterion, Solvent, and Temperature of Polymerization of Methyl Methacrylate Triad Tacticity (mm) (mr) (rr) 0.72 0.08 0.87 0.97 0.23 0.01 0.05 0.78 0 0.57 0.12 0.35 0.14 0.10 0 0.17 0.32 0.10 0.03 0.16 0.15 0.33 0.16 0.10 0.31 0.46 0.42 0.53 0.56 0.40 0.11 0.60 0.03 0 0.61 0.84 0.61 0.06 0.90 0.12 0.42 0.23 0.33 0.34 0.60
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Solvent Toluenea Pyridinea Tolueneb Tolueneb Tolueneb THFb THFc Toluenec Toluenec Toluenea Pyridinea Toluenea Pyridinea THFe THFe
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Counterion Li Li Li Mg Mg Li Li Li Lid Na Na K K Cs Cs
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Temperature ( C) 0 0 78 78 78 85 78 78 78 0 0 0 0 20 100
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Data from Braun et al. [1962]. Data from Bywater [1989]. c Data from Kitayama et al. [1989]. d Initiator t-C4 H9 Li Al(C2 H5 )3 . e Data from Muller et al. [1977], Kraft et al. [1980a,b].
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decreases in the order Li > Na > K, corresponding to the relative extents of solvation of different ions. The smallest ion Li is the most highly solvated and furthest removed from the propagating center. (This is analogous to the effect of polar solvent on propagation rate constants for anionic polymerization of styrene with different counterions. The ion-pair propagation rate constant for styrene is largest for Li and smallest for Cs, which is the reverse of the order in less polar solvent Sec. 5-3d-2-b.) The extent of syndiotacticity increases with decreasing temperatures, as is evident in the data for polymerization with cesium as the counterion. The level of syndioselectivity is similar to that observed in radical polymerization of MMA, (rr) 0.79 at 55 C [Hatada et al., 1988]. When nonpolar solvents are employed, polymerization proceeds by an anionic coordination mechanism. The counterion directs isotactic placement of entering monomer units into the polymer chain. The extent of isotactic placement increases with the coordinating power of the counterion (Li > Na ) K, Cs). The small lithium ion has the greatest coordinating power and yields the most stereoselective polymerization. Increased reaction temperature decreases the isoselectivity. Mechanisms proposed to explain the isoselective polymerization of methyl methacrylate involve rigidi cation of the enolate propagating species by coordination of counterion with the terminal monomer unit (and possibly also the penultimate monomer unit) [Braun et al., 1962; Leitereg and Cram, 1968; Wiles and Bywater, 1965]. The rigid propagating chain end imposes stereoselectivity on monomer insertion in a manner analogous to that for a traditional Ziegler Natta, metallocene, or chelate propagating species in alkene polymerizations. The delicate nature of this stereoselectivity is evident when noting that the presence of triethylaluminum coinitiator changes the t-C4 H9 Li polymerization of MMA in toluene from
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POLYMERIZATION OF POLAR VINYL MONOMERS
TABLE 8-13 Metallocene Polymerization of Methyl Methacrylate Initiator SmH[(Me5 Cp)2 ]2 Cp2 YCl(THF)/MAOa;d rac-C2 H4 (H4 Ind)2 ZrMe2 /Bu3 NHBPh4 e Cp2 ZrMe2 /Et3 NHBPh4 e rac-C2 H4 (H4 Ind)2 ZrMe2 /Ph3 CB(C6 F5 )4 a;f Cp(Flu)ZrMe2 /Ph3 CB(C6 F5 )4 a;g {[rac-Me2 Si(Ind)2 ZrMe]2 m-Me} MePBB b;h {(Cp2 ZrMe)2 m-Me} MePBB b;h Me2 CCp(Ind)ZrMe(THF) BPh i 4 Me2 CCp2 ZrMe(THF) BPh i 4 Me2 Si(Me4 Cp) (N-t-Bu)ZrL2 BAr j;k 4
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T ( C)
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Polymer (rr) 0.95 (rr) 0.90 (mm) > 0.90 (rr) 0.80 (mm) 0.94 (rr) 0.74 (mm) 0.93 (rr) 0.62 (mm) 0.94 (rr) 0.89 (mm) 0.96
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0 60 0 0 40 0 0 0 20 45 40
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ZnEt2 is also present. PBB tris(2,20 ,20 0 -per uorobiphenyl)borane. c Data from Yasuda et al. [1993]. d Data from Jiang and Hwu [2000]. e Data from Collins and Ward [1992], Collins et al. [1994]. f Data from Deng et al. [1995]. g Data from Shiono et al. [1998]. h Data from Chen et al. [1998]. i Data from Frauenrath et al. [2001a]. j L Me2 C C(OR)O, L THF, Ar 3,5-(CF3 )2 C6 H3 . k Nguyen et al. [2000].
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isoselective, (mm) 0.87, to syndioselective, (rr) 0.90. This is also the reason for the two apparently divergent entries for Mg counterion in Table 8-12, one entry shows isoselectivity and the other shows syndioselectivity. Sterically hindered Grignard reagents give different results depending on whether the actual initiator present is t-C4 H9 MgBr or (t-C4 H9 )2 Mg and whether the Grignard reagent is free of trace amounts of ether [Bywater, 1989; Hatada et al., 1986; Quirk, 1995, 2002]. Metallocene initiators are also useful for methacrylate and acrylate polymerizations [Bandermann et al., 2001; Boffa and Novak, 2000; Bolig and Chen, 2001; Cameron et al., 2000; Chen et al., 1998; Collins and Ward, 1992; Collins et al., 1994; Deng et al., 1995; Frauenrath et al., 2001a,b; Holscher et al., 2002; Ihara et al., 1995; Jiang and Hwu, 2000; Karanikolopoulos et al., 2001; Li et al., 1997; Nguyen et al., 2000; Shiono et al., 1998; Soga et al., 1994; Yasuda, 2001; Yasuda et al., 1993]. Metallocenes based on both groups 3 and 4 transition metals initiate polymerization, although the group 3 metallocenes (Sc, Y, La) are somewhat more active than the group 4 metallocenes. Table 8-13 shows various initiators and their stereoselectivity in MMA polymerization. Studies on initiators other than the metallocenes have recently begun the last entry in Table 8-13 is for an ansa-cyclopentadienyl amido initiator. High stereoselectivity, both isoselective and syndioselective, are observed depending on the initiator. The level of stereoselectivity is somewhat less than that observed with a-ole ns, but this may be due only to the much smaller effort expended to date for MMA. Propagation proceeds through an enolate species in the bimetallic mechanism described by Eq. 8-73 in which two metallocene species are involved one is a neutral enolate, and the other is the corresponding metallocenium cation [Collins et al., 1994; Li et al., 1997]. In LV, the propagating chain is coordinated at the neutral transition metal center (Zra ) and monomer
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