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at the transition metal cation center (Zbb ). Monomer insertion into the polymer chain proceeds with the electron ow indicated by the arrow in LV. The result is LVI with an empty coordination site, ready to coordinate the next monomer unit and facilitate subsequent
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monomer insertion. The roles of the neutral and cationic metallocenes alternate with each monomer insertion. The rationale for the bimetallic mechanism, instead of a monometallic mechanism, is that a group 4 transition metal may not be suf ciently electropositive for one metal center to coordinate with two different oxygens (one from the enolate species and one from monomer). A monometallic mechanism is assumed for MMA polymerization with group 3 metallocenes since the group 3 metals are larger and more electropositive than group 4 transition metals [Boffa and Novak, 2000]. Metallocene polymerization of MMA proceeds with differences compared to polymerizations of ethylene and a-ole ns. A moderately coordinating anion such as [B(C6 H5 )4 ] can be used with MMA because the polar MMA can displace it from the initiator s coordination site. Alkene monomers are generally unable to displace [B(C6 H5 )4 ] and polymerization does not proceed. The polarity of MMA and, more speci cally, its Lewis base character prevents polymerization if the monomer and coinitiator are mixed together prior to the addition of monomer [Cameron et al., 2000]. Monomer coordinates strongly to the coinitiator and renders the coinitiator ineffective for reaction with the metallocene. Polymerization is accomplished by mixing the metallocene and coinitiator together to form the active initiating species prior to the addition of monomer. Another approach involves the addition of a large excess of another Lewis acid such as diethylzinc to displace the coinitiator from monomer prior to addition of initiator. There is a strong interest in copolymerization of alkenes with polar monomers to alter the characteristics of a nonpolar polymer such as polyethylene or polypropene by introduction of polar functional groups. The polar groups would allow control over properties such as adhesion, compatibility with other polymers, solvent resistance, and rheological behavior. However, there is an inherent problem to achieving this goal for MMA and other (meth)acrylates by use of metallocene, traditional Ziegler Natta, or any other type of anionic initiator. These monomers polymerize through enol intermediates, whereas alkenes polymerize through carbanion intermediates. Even more important is the big difference in the interaction of the nonpolar and polar monomers with metal centers in the initiator. To date there has been no success in nding metallocene or other initiator systems that allow a back-and-forth crossover between the two mechanisms. Thus, random copolymerization is not possible except in the rare cases of monomers with protected functional groups [Boffa and Novak, 2000]. Block copolymers have been successfully synthesized because many metallocene polymerizations of MMA proceed as living polymerizations, and it is possible to have a single one-way crossover from carbanion (alkene) polymerization to MMA (enolate) polymerization with metallocene and related initiators, especially when group 3 transition metal initiators are used [Boffa and Novak, 2000; Desurmont et al., 2000a,b; Jin and Chen, 2002; Yasuda et al., 1992].
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The isotactic polymerization of a vinyl ether requires a cationic coordination process. The cationic process is analogous to the anionic coordination process except that the propagating center is a carbocation instead of a carbanion and the counterion is an anion instead of a cation. Various initiators, of both the homogeneous and heterogeneous types, yield varying degrees of isotactic placement [Ketley, 1967a,b; Ouchi et al., 1999, 2001; Pasquon et al., 1989]. This includes boron tri uoride and other Lewis acids, including components (sometimes only one, sometimes both of the two different metal components) used in Ziegler Natta formulations. Some of the polymerizations proceed with very high isoselectivity, for example, ethylaluminum dichloride and diethylaluminum chloride yield 96 97% isotactic polymer for polymerization of isobutyl vinyl ether at 78 C in toluene, whereas aluminium tribromide yields mostly atactic polymer [Natta et al., 1959a,b,c]. Not all vinyl ethers give the same result with the same initiator. Thus, the polymerization of t-butyl vinyl ether is only mildly isoselective under the same conditions described above for the highly isoselective polymerization of isobutyl vinyl ether. In general, the effects of solvent, temperature, and other reaction conditions on the extent of isoselectivity are similar to those previously described for other types of monomers. Highly syndiotactic polymers have been obtained in only a few instances with some monomers containing bulky substituents, such as a-methylvinyl methyl ether, trimethylvinyloxysilane, and menthyl vinyl ether, in polar solvents under homogeneous conditons [Goodman and Fan, 1968; Ledwith et al., 1979; Murahashi et al., 1966]. That less hindered monomers in polar solvents do not yield highly syndiotactic polymers may be indicative of the involvement of ether monomers in intramolecular solvation of propagating centers. The polar solvents such as THF may not be suf ciently polar to displace monomer as a solvating species to yield the highly solvated, relatively free propagating centers that lead to syndiotactic placement. One of the mechanisms proposed to explain isotactic placement of vinyl ethers is consistent with this consideration [Cram and Kopecky, 1959]. Propagation involves a 6-membered cyclic propagating chain end (LVII) formed by the antepenultimate (the second repeat unit behind the last unit) ether group of the propagating chain solvating the carbocation center:
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