Lanthanide halides monomers

Preparation of poly(8-caprolactone) (PCL), a biodegradable polymer that finds applications in the biomedical field for prostheses, bandages, or controlled drug release matrix for active substances, was successfully accomplished by heating the monomer in the presence of nontoxic, biologically acceptable lanthanide halides (SmBrj, SmCl3, YbBrj) as initiators in a scientific microwave unit (Scheme 3.5). PCL with a Af of 14,100 g/mol was obtained after heating at 200 °C for 45 min in the absence of solvent. 

Acetyl ligands, in niobium complexes, C-H BDEs, 1, 298 Achiral phosphines, on polymer-supported peptides, 12, 698 Acid halides, indium compound reactions, 9, 683 Acidity, one-electron oxidized metal hydrides, 1, 294 Acid leaching, in organometallic stability studies, 12, 612 Acid-platinum rf-monoalkynes, interactions, 8, 641 Acrylate, polymerization with aluminum catalysts, 3, 280 Acrylic monomers, lanthanide-catalyzed polymerization,... 

Some of the applications of the organometallic compounds of lanthanides are as catalysts for (i) stereo specific polymerization of diolefins and in particular to obtain high yields of 1,4-ci.v-polybutadiene and 1,4-cw-polyisoprene and copolymer of the two monomers. The order of effectiveness of the rare earths as catalysts is Nd > Ce, Pr < Sm, Eu. The nature of halogen of the Lewis acid affecting the catalytic activity is in the order Br > Cl > I > F. Detailed work on the activity of cerium octanoate-AlR3-halide showed stereo specificity with cerium as the primary regulator. Cerium is thought to form jr-allyl or 7r-crotyl complexes with butadiene. 

The organometalhc chemistry of the rare earths deals mainly with bis(cyclopentadienyl) derivatives due to the easy available bis(cyclopentadienyl) rare earth chlorides and other halides via reaction of the rare earth trichlorides with two equivalents of a cyclopentadienyl alkali salt. Bis(cyclopentadienyl)lanthanide chlorides are formed as chloride-bridged dimers (Figure 28a), as monomers stabihzed by a donor molecule like THF (Figure 28b) or as ate complexes with alkali hahdes (Figure 28c). 

All four scandium(III) halides are known, all but the fluoride (WO3 structure) having the FeCR structure. They can be obtained as white solids directly from the elements and in some cases by dehydration of the hydrated salts, as well as by thermal decomposition of (NH4)3ScXe (X = Cl, Br), a method also used for the lanthanides. Gas-phase studies have identified isolated planar Scp3 molecules at 1750 K ScCR molecules with a very slight pyramidal distortion and both monomers and dimers in Scis vapour at 1050 K. 

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