Rare earth metal complexes polymerization reactions

Recently, rare-earth metal complexes have attracted considerable attention as initiators for the preparation of PLA via ROP of lactides, and promising results were reported in most cases [94—100]. Group 3 members (e.g. scandium, yttrium) and lanthanides such as lutetium, ytterbium, and samarium have been frequently used to develop catalysts for the ROP of lactide. The principal objectives of applying rare-earth complexes as initiators for the preparation of PLAs were to investigate (1) how the spectator ligands would affect the polymerization dynamics (i.e., reaction kinetics, polymer composition, etc.), and (2) the relative catalytic efficiency of lanthanide(II) and (III) towards ROPs. 

Catalytic applications of organo-rare-earth metal complexes reported prior to 2002 are summarized in two excellent reviews [19,20] and, therefore, will not be discussed unless being relevant for understanding of key reaction details. A recent comprehensive review on theoretical analyses of organo-rare-earth metal-mediated catalytic reactions is available [17], Although o-bond metathesis plays a pivotal role in many rare-earth metal-catalyzed polymerizations, the discussion of these processes is beyond the scope of this review and the interested reader may consult one of the pertaining reviews [21-24],... 

Alkyl acrylates were for the first time polymerized in a living fashion with the aid of the unique catalytic action of rare earth metal complexes [4]. Since these monomers have an acidic a-H, termination and chain transfer reactions occur so frequently that their polymerizations generally do not proceed in a living manner. By taking advantages of the living polymerization ability of both MMA and alkyl acrylate, ABA or ABC type tri-block copolymerization was performed to obtain thermoplastic elastomers. 

Meanwhile, Okuda investigated the catalytic behavior for the polymerization of butadiene with the same half-sandwich rare-earth metal tetramethylalumi-nate complexes [Ln(Ti -C5Me4SiMe3) (fi-Me)2(AlMe2) 2] (55, Ln = Y, La, Nd, Sm, Gd, Lu) [161]. Upon activation with [NEt3H]+[B(C6Fs)4] , the resultant cationic species enabled the polymerization of butadiene in the presence of TIBA to give frani-l,4-polybutadiene with narrow polydispersities (Mw/Mn= 1.05-1.09). Different from Anwander s result where heterobimetallic species was isolated, mononuclear ion pair was obtained from the reaction with borate (Scheme 22). Unfortunately, no catalytic data of 57 were given by the authors. 

A1 (8.103) and Zn (8.104 and 8.105) complexes are efficient initiators for l-LACTIDE ring-opening polymerization in the presence of benzyl alcohol, and polymerization reactions take place in an immortal manner, that is, their polydisper-sity index (PDI) is almost 1 (PDI=1.02,MW= 13,000-14,000) [147].Bis(phosphino) carbazole (PNP-carbazolide) rare-earth metal bis(alkyl) complexes (8.106) initiated... 

Rare earth metal NHC complexes have primarily been applied in polymerization reactions, addition of terminal alkynes and amines to carbodiimides, as well as catalytic C-N coupling. Although a relatively limited amount of research has been carried out in this area, these preliminary investigations have proven the utility of REM NHC complexes as catalysts. 

The examples discussed so far are all transition metal complexes. As we will see later (Chapters 4-9), most homogeneous catalytic processes are indeed based on transition metal compounds. However, catalytic applications of rare earth complexes have also been reported, although so far there has not been any industrial application. Of special importance are the laboratory-scale uses of lanthanide complexes in alkene polymerization and stereospecific C-C bond formation reactions (see Sections 6.4.3 and 9.5.4). 

Abstract This review deals with the synthesis and the catalytic application of noncyclopentadienyl complexes of the rare-earth elements. The main topics of the review are amido metal complexes with chelating bidentate ligands, which show the most similarities to cyclopentadienyl ligands. Benzamidinates and guanidinates will be reviewed in a separate contribution within this book. Beside the synthesis of the complexes, the broad potential of these compounds in homogeneous catalysis is demonstrated. Most of the reviewed catalytic transformations are either C-C multiple bond transformation such as the hydroamination and hydrosilylation or polymerization reaction of polar and nonpolar monomers. In this area, butadiene and isoprene, ethylene, as well as lactides and lactones were mostly used as monomers. 

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