Polymerization scandium

Kobayashi et al. developed the polymeric scandium(lll)-catalyst (42) (PA-Sc-TAD) which promotes the three-component couphng reactions of aldehydes and aromatic amines with either alkenes to generate quinohnes or silylated nucleophiles to form y9-amino ketones, esters and nitriles. This methodology turned out to be highly efficient with regard to automated high throughput synthesis (Scheme 4.27) [119]. 

There is very good agreement as to both the polymeric species (/ > 1) that form for scandium and to the magnitude of their relative stabilities. Aveston (1966) indicated that no polymeric species containing more than three scandium atoms form, and indeed, no study has identified any polymeric scandium species above a... 

Table 8.5 Data for the stability constants of polymeric scandium(lll) hydrolysis species (reaction (2.5), M = Sc3+,p>l). 

A scandium complex, Cp ScH, also polymerizes ethylene, but does not polymerize propylene and isobutene [125]. On the other hand, a linked amidocyclo-pentadienyl complex [ Me2Si( / 5-C5 Me4)( /1 -NCMe3) Sc(H)(PMe3)] 2 slowly polymerizes propylene, 1-butene, and 1-pentene to yield atactic polymers with low molecular weight (Mn = 3000-7000) [126, 115]. A chiral, C2-symmetric ansa-metallocene complex of yttrium, [rac-Me2Si(C5H2SiMe3-2-Buf-4)2YH]2, polymerizes propylene, 1-butene, 1-pentene, and 1-hexene slowly over a period of several days at 25°C to afford isotactic polymers with modest molecular weight [114]. 

Analogous methyl compoimds are iU-characterized and appear to be polymeric [129). The air-sensitive phenyl derivatives when first obtained from THF are soluble in benzene, but when dried completely are no longer soluble — apparently due to pol5unerization. The homoaryl complexes of the smaller scandium, and yttrium ions form only the tris complexes Sc(CeH5)3 and Y(C6H5)3 [129) It is apparent that the structure and stabihty of the homoaryls are dependent on the metal ionic radii and the steric bulkiness of the phenyl group. 

Interestingly, salts other than tin(ll) bis-(2-ethylhexanoate) such as scandium and tin trifluoromethanesulfonate [41 3], zinc octoate [44, 45], and aluminum acetyl acetonate [45] were reported to mediate the ROP of lactones. As far as scandium trifluoromethanesulfonate is concerned, the main advantage is the increase of its Lewis acidity enabling the polymerization to be carried out at low temperatures with acceptable kinetics. Later, faster kinetics were obtained by extending the process to scandium trifluoromethanesulfonimide [Sc(NTf2)3] and scandium nonafluorobutanesulfonimide [Sc(NNf2)3] and to other rare earth metal catalysts (metal=Tm, Sm, Nd) [46]. 

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. 

In water solutions, the scandium ion has a triple positive charge. Studies show, however, that the simple Sc1+ ion seldom exists. Rather, the form is highly polymerized and hydrolyzed—with hydroxy-bonded structures, In forming compounds, scandium parallels aluminum, yttnum. gallium, indium, and tellurium. Several carbides of scandium have been reported, the most stable carbide being ScC, including scandium clusters in fullerene cages. 

The above acid-catalyzed polycondensations were carried out at more than 100 °C, whereas Takasu et al. reported room temperature polyesterification with scandium trifluoromethanesulfonate [Sc(OTf)3] or scandium trifluo-romethanesulfoimide [Sc(NTf2)3] [27,28]. Thus, the direct polycondensation of methylsuccinic acid and 1,4-butanediol proceeded in bulk under reduced pressure (0.3-30 mmHg) using 1.4 mol % of Sc(OTf)3 at 35 °C for 96 h to afford poly(butylene methylsuccinate) with Mn of 12400 (Scheme 5). When HfCl4-(THF)2 was used in this room temperature polymerization instead of Sc(OTf)3, only low molecular weight polyester (Mn = 1100) was afforded. The scandium catalysts did not promote transesterification ethanol selectively reacted with acetic acid even in the presence of equimolar methyl acetate. 

The hexameric scandium decanoate extracted in benzene is different from the hexameric Al(III), Ga(III), and In(III) decanoates in that the former is neither hydrated nor hydrolyzed (154). Galkina and Strel tsova (36), in the butanoic acid/iso-butanol system, attempted to separate Sc from rare earths and the other metals. They proposed the monomeric Sc(III) butanoate, ScA3, as the extracted species. In this extraction system, the polymerization of scandium butanoate in the organic phase seems to be prevented by solvation with iso-butanol. In the study of the synergistic effect of various amines on the extraction of lanthanum and scandium with hexanoic and a-bromohexanoic acids in chloroform, Sukhan et al. (138) proposed LaA3(HA)3 and ScA3(HA)3 as the extracted species. 

Among the carboxylates, scandium formate has a 2-D polymeric structure, whilst in the acetate, chains of 8c + ions are bridged by acetate groups (Pigure 3) with essentially octahedral coordination of scandium. 8imilar bridging is found in the chloroacetate, where the six-coordination is in contrast with the nine-coordinate trimeric structure adopted by the heavier lanthanide ions. 8candium propynoate has... 

The second approach is a popular route to cationic lanthanide alkyl complexes, which have proven to be the important intermediates for ethylene polymerization and the stereospecific polymerization of diene [5]. Various monocationic lanthanide monoalkyl complexes have been synthesized by the alkyl abstraction/elimination reaction of lanthanide dialkyl complexes. The reaction of a bisbenzyl scandium complex supported by P-diketiminate with B(C6Fs)3 affords the cationic complex with a contact ion pair structure, in which a weak bonding between the cation and the anion exists (Figure 8.21) [77]. The reaction of an amidinate... 

Ward, B.D., Bellemin-Laponnaz, S., and Gade, L.H. (2005) C3 chiraUty in polymerization catalysis a highly active dicationic scandium(III) catalyst for the isoselective polymerization of 1-hexene. Angewandte Chemie International Edition, 44, 1668. 

Otero, A., Femandez-Baeza, J., Antinolo, A. et al. (2008) Scandium and yttrium complexes supported by NNCp heteroscorpionate ligands synthesis, structure, and polymerization of E-caprolactone. Organometallics, 27, 976. 

A novel scandium complex formed from scandium triflate in the presence of 18-crown-6 and p-sulfonatocalix[4]arene has been investigated.224 The crown ether resides in cavities created by two calixarenes from adjacent polymeric sheets. There are two types of scandium ions in the complex, one type is bound to the phenolic oxygen of the calixarene and the second exists as the hydroxide bound dimer ion [Sc(OH)2(H2O)10]4+. A similar complex may also be prepared without the crown ether. 

Tardif reported recently that the cationic half-sandwich lanthanide amido complexes [(Ind)Ln N(SiMe3)2 ][B(C6Fs)4] (29, Fig. 3) were also highly efficient and c/x-l,4-selective for butadiene polymerization [115]. Meanwhile, Visseaux demonstrated that the half-sandwich scandium borohydride complex Cp Sc(BH4)2(THF) (30, Fig. 3) combined with [Ph3C][B(C6Fs)4] and TIBA led to the very active and highly stereoselective isoprene polymerization (>90% c/x-1,4. Table 11) as well as styrene (>99.9% syndio, Table 12). Improvement of the control of the polymerization was performed at lower temperature at — 10°C that the cm-1,4-ratio increased up to 97.2% followed by the decrease of PDI down to 1.7 [116]. This... 

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