On treatment with 1 equiv of [Ph3C][B(C6F5)4], the scandium half-sandwich bis(alkyl) complex (C5Me4SiMe3)Sc(CH2SiMe3)2(THF) showed extremely high activity (up to 1.36 x 104 kg of sPS/(mol Sc.h)) and syndiospecificity (rrrr > 99%) for the polymerization of styrene at room temperature in toluene. More remarkably, this catalyst system could also effect the syndiospecific copolymerization of styrene with ethylene to yield styrene-ethylene copolymers having syndiotactic styrene-styrene sequences. The styrene content in the copolymers could be easily controlled by changing the styrene feed and could reach higher than 80 mol %. This is the first example of formation of such types of styrene-ethylene copolymers, which are expected to show novel properties.

Cationic alkyl-rare-earth-metal complexes (i.e., Group 3 and the lanthanides) have recently attracted much interest as homogeneous polymerization catalysts. [1][2][3][4][5][6][7][8][9][10] A number of cationic alkyl-rare-earth-metal complexes supported by various ancillary ligands such as deprotonated aza crowns, [2] benzamidinates, [3] b-diketiminates, [4] anilido imines, [5] amidefunctionalized triazacyclononanes, [6] phosphides, [7] cyclopentadienyls, [8] and crown ethers [9] have been synthesized and their reactivity has been studied. Despite these extensive efforts, however, the olefin-polymerization chemistry of the cationic alkyl-rare-earth-metal complexes is still limited solely to that of ethylene, [2][3][4][5][6][7][8][9][10] whereas the development of active rare-earth-metal catalysts for the efficient polymerization/copolymerization of higher olefins has remained a challenge.The copolymer of ethylene with a cyclic olefin such as norbornene (COC) is one of the most important highperformance polymer materials with many desirable properties. Since Kaminsky et al. first described the copolymerization of ethylene (E) with norbornene (NB) by using zirconocene-based catalysts in 1991, [11] extensive studies have been carried out in this area. Most of the catalysts reported so far are complexes based on transition metals such as those of Group 4 and Group 10, [12] whereas no rare-earthmetal complex has been previously used for the copolymerization of ethylene with norbornene. [13] Herein we report an excellent cationic half-sandwich scandium catalyst for the copolymerization of ethylene with norbornene. This catalyst not only represents the first example of a rare-earth-metal catalyst for ethylene-norbornene copolymerization, but it also shows several unique characteristics, such as extremely high activity for the alternating ethylene-norbornene copolymerization and unprecedented formation of novel poly(ethylene-alt-norbornene)-bpolyethylene block copolymers.

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Metal hydrides are fundamental components in a wide range of stoichiometric and catalytic reactions. Their importance in modern inorganic and organic chemistry cannot be overemphasized. Rare-earth (Group 3 and lanthanide) hydrides are among the most active metal-hydride complexes. Together with their alkyl analogues, metal-hydride complexes of the rare-earth metals have occupied an especially important position in the development of the organometallic chemistry of the rare-earth elements. Generally, cationic complexes differ in their structure and reactivity from their neutral analogues. However, although a large number of metal-hydride complexes of the rare-earth metals have been synthesized and structurally characterized, [1] cationic complexes of this type have not been reported previously.We recently reported the synthesis and hydrogenation reactions of a new class of polynuclear rare-earth polyhydrido complexes exemplified by [Y 4 (C 5 Me 4 SiMe 3 ) 4 H 8 (thf) n ] (1 a: n = 0, 1 b: n = 1, 1 c: n = 2). [2,3] In view of the unique reactivity of these hydride clusters [2b-e,g] and the excellent olefinpolymerization activity of the related cationic rare-earthalkyl complexes, [4,5] we became interested in the cationic hydrido species generated from these rare-earth-hydride clusters.Herein, we report the synthesis, structural characterization, and olefin-polymerization catalysis of the cationic hydride complexes obtained from 1 a-c (Scheme 1) and related rare-earth-hydride clusters. These cationic polyhydrido complexes not only are the first cationic rare-earthhydride complexes but also show excellent regio-and stereoselectivity for the polymerization of 1,3-cyclohexadiene (CHD), which afforded, for the first time, soluble crystalline cis-1,4-linked poly(CHD) (1,4 selectivity: 100 %; cis selectivity: 99 %). For comparison, the reaction of the neutral hydride cluster, 1 b, with CHD is also described. This reaction leads to the formation of a structurally well-defined CHD insertion product instead of the polymerization of CHD. Various metal catalysts and initiators were reported previously for the polymerization of CHD, but most yielded a mixture of 1,4-and 1,2-poly(CHD) [6][7][8][9][10] or insoluble polymers, [10,11] and none was reported to produce pure soluble crystalline cis-1,4-linked poly(CHD).Reaction of the thf-free octahydrido yttrium cluster 1 a ] with one equivalent of [Ph 3 C][B(C 6 F 5 ) 4 ] in chlorobenzene or toluene at 25 8C gave the cationic heptahydrido complex 2 a and Ph 3 CH (Scheme 1). In contrast to the neutral complex 1 a, which shows a good solubility in most organic solvents, the cationic complex 2 a was only slightly soluble in benzene and toluene, and almost insoluble in hexane. Recrystallization of 2 a from chlorobenzene/hexane afforded colorless single Scheme 1. Synthesis of cationic yttrium-hydride clusters.