Lanthanide initiators

The lanthanocene alkyls (190) and (191) are also highly active initiators for MMA polymerization. These too are syndioselective, producing 82-85% rr PMMA at 0°C with high initiator efficiencies and narrow molecular weight distributions. Ln11 complexes such as (192)-(194) also generate syndiotactic PMMA, but exhibit much lower efficiencies (30 10%). 

The lanthanocene initiators also polymerize EtMA, PrMA and BuMA in a well-controlled manner, although syndiotacticity decreases as the bulk of alkyl substituent increases. Reactivity also decreases in the order MMA EtMA PrMA BuMA. Chain transfer to provide shorter polymer chains is accomplished by addition of ketones and thiols.460 The alkyl complexes (190) and (191) also rapidly polymerize acrylate monomers at 0°C.461,462 Both initiators deliver monodisperse poly(acrylic esters) (Mw/Mn 1.07). An enolate is again believed to be the active propagating species since the model complex (195) was also shown to initiate the polymerization of MA. 

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

Many other lanthanide-based initiators have been shown to polymerize MMA, including lanthanocene amides,464 168 alkoxides,469 substituted indenyl and fluorenyl bivalent ytterbocenes,470,471 hexamethylphosphoric triamide thiolates,472 and allyl, azaallyl, and diazapentadienyl complexes.473 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to 12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477 

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Tetradentate N,0-donor ligands have also been investigated. The yttrium complex (—)-(313) does not effect stereocontrol over the ROP of rac- or meso-LA, in contrast to related A1 initiators (262) and (263).803 Polymerization is also slower than for most lanthanide initiators with 100 equivalents meso-LA requiring 14 h at 70 °C to attain near-quantitative conversion (97%). 

Bis(ethylacetoacetonate)-lanthanide(III) alkoxides, represented by structure (314), also initiate the well-controlled ROP of CL.895 Mn increases linearly with conversion (with Mw/Mn<1.10 throughout), and increasing [M]0/[I]o- Kinetic analysis implies a first order dependence on the lanthanide initiator, consistent with a non-aggregated active site. Block copolymers with moderately narrow polydispersities (1.25-1.45) have also been prepared using these initiators. NMR spectroscopy confirms well-controlled block sequences suggesting that these initiators are less susceptible to transesteriflcation than other lanthanide alkoxides. Initiation occurs exclusively at the alkoxide bond, and the tris(ethylacetoacetonate) analogs are inactive under the same conditions. 

As discussed above, lanthanides and group 3 homoleptic complexes are attractive initiators because of their moderate Lewis acidities, good activities of polymerization, and low toxicity [94, 111, 112, 115-118]. A list of lanthanide initiators and their polymerization activity is listed in Table 5. 

The general issues from literature surveys dealing with lanthanide initiators reveal the following (1) catalyst precursors with larger lanthanide metals polymerize lactide faster than metals of smaller radii, (2) lanthanide catalysts polymerize lactide at slower rates than cyclic esters such as s-caprolactone and, in most cases. 

Arnold and colleagues have reported a series of chiral homoleptic yttrium and lanthanide fra(alkoxide) complexes [49, 50], These initiators (including complex 1) show high degrees of iso-selectivity and rapid rates, even at low temperatures. Thus, using the racemic mixture of the lanthanide initiator, stereoblock PLA was produced with a P, of 0.81 so far, this is the only known type of yttrium initiator able to exert such stereocontrol and a very exciting finding. Analysis of the stereoerrors indicates that an enantiomorphic site control mechanism is responsible for the iso-selectivity. 

Spassky, N. and Simic, V. (2000) Polymerization and copolymerization of lactides and lactones using some lanthanide initiators. American Chemical Society Symposium Series, 764 (Polymers from Renewable Resources Biopolyesters and Biocatalysis) 146-159. 

Desurmont, G, Tokimitsu, T., and Yasuda, H. (2000) First controlled block copolymerizations of higher 1-olefins with polar monomers using metallocene type single component lanthanide initiators. Macromolecules, 33, 7679. 

SCHEME 23.7 Synthesis of PMMA using a bimetallic lanthanide initiator. 

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