Polar monomers, polymerization, lanthanide

In recent years homoleptic lanthanide(III) tris(amidinates) and guanidinates have been demonstrated to exhibit extremely high activity for the ring-opening polymerization of polar monomers such as e-caprolactone and trimethylene... 

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... 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

Controlled block copolymerization of olefins with polar monomers was performed with a lanthanide complex by the successive polymerization of hexene (or pentene) and methylmethacrylate (or caprolactone). Polyhexene-block-poly(methyl methacrylate), polyhcxcnc-fo/ock-polycaprolactone, poly-pentene-fc/ock-poly(methyl methacrylate), and polypentene-Wock-polycapro-lactone were synthesized using a lanthanide complex as initiator [140-143]. 

Neutral lanthanide-series metallocenes exhibit the remarkable ability to polymerize polar monomers such as methyl methacrylate to highly stereoregular polymers of extremely narrow molecular weight distribution (Mw/Mn 1) (equation 14). It is possible to prepare block copolymers of ethylene and poly(methyl methacrylate) by first adding MMA to Cp 2SmMe to produce a PMMA segment followed by addition of ethylene the reverse order of addition fails to give block materials because the ethylene monomer cannot insert into the enolate. 

Sanchez-Barba, L.F., Hughes, D.L., Humphrey, S.M. et al. (2005) New bis(allyl)(diketiminato) and tris(allyl) lanthanide complexes and their reactivity in the polymerization of polar monomers. Organometallics, 24,3792. 

Polymerization of Polar Monomers Catalyzed by Lanthanide Amidinates... 

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