Polymer rare earth metal catalysts

Polymer-Supported Rare Earth Metal Catalysts... 

In contrast to the large number of publications on group 4-based catalysts for syndiospecific styrene polymerization, reports on analogous rare-earth metal catalysts are stiU hmited [5], Early investigations on neodymium-based sandwich complexes (1-3) activated by methylaluminoxane (MAO) (Fig. 7.1) have shown that these systems are promising candidates for styrene polymerization catalysis with the resulting polymers syndiotactically enriched. The selectivity as well as the activity of these systems are moderate [6]. 

Related to these catalysts are the systems based on lanthanide metal systems or rare earth metal complexes [46, 47]. The main problem with these catalyst systems is their instability. When the catalyst solution is prepared by reachng a metallocene with an organolithium compound in a polar solvent, the prepared catalyst soluhon is unstable and decomposes quickly, even under a nitrogen atmosphere. The activity of these catalysts can be high only if the catalyst is added to the polymer soluhon immediately after preparation. Attempts have been made to overcome the stability problem by using an additive in the system to improve the stability and the activity of the catalyst [33-35, 41, 57, 58, 61]. Re-... 

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. 

McLain, S.J., Ford, T.M., and Drysdale, N.E. (1992) Rare earth metal coordination compounds as lactone polymerization catalysts. American Chemical Society Polymer Preprints, 33, 463. 

Rare earth metal triflates are recognized as a very efficient Lewis acid catalysts of several reactions including the aldol reaction, the Michael reaction, allylation, the Diels-Alder reaction, the Friedel-Crafts reaction, and glycosylation [110]. A polymer-sup-ported scandium catalyst has been developed and used for quinoline library synthesis (Sch. 8) [111], because lanthanide triflates were known to be effective in the synthesis of quinolines from A-arylimines [112,113]. This catalyst (103) was readily prepared from poly(acrylonitrile) 100 by chemical modification. A variety of combinations of aldehydes, amines, and olefins are possible in this reaction. Use of the polymer-supported catalyst has several advantages in quinoline library construction. 

By using rare earth metals or radicals it is possible to copolymerize 1,3-butadiene and other dienes with cis-, A linkage [3,498]. Polymers of 1,3-butadiene and isoprene at any ratio can be obtained. Copolymes of 1,3-butadiene and 1,3-pentadiene can be produced with catalysts on the basis of vanadium chelates. 1,3-Butadiene is almost completely converted to trans-, A units, whereas 1,3-pentadiene yields 50 to 60% 1,4-addition and 40 to 50% 1,2-addition products. At a 1,3-pentadiene content of 26 to 45wt%, the copolymers are amorphous, featuring high rigidity [499-501]. Diethylaluminum chloride, nickel naphthenate, and water catalyze the copolymerization of 1,3-butadiene and acetylene. The low-molecular-weight copolymers contain mostly cis-Q-Q double bonds [502]. 

In spite of the high activities of rare earth metal complexes for ethylene polymerization, they are inert to propylene due to the formation of -allyl complexes as revealed by NMR spectra. Recently, an isospecific single component C2 symmetric catalyst, [rac-Me2Si(2-SiMe3-4-CMe3C5H2)2YH]2 (Fig. 18), was found, with which the polymerization of 1-pentene was performed over a period of several days, affording polymers = 13,500) with M /M = 2.31 [40]. 

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