Polymer lanthanide catalysts

Ladder polymer, synthesis of, 503 Lanthanide catalysts, 288 Lanthanide compounds, 73 Latent-reactive polymers, 455 Laurolactam, 136... 

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. 

Recently, polymer-supported lanthanide catalysts have beem of great interest, and these topics are discussed elsewhere. Use of lanthanide catalysts in solid-phase organic synthesis is now well-recognized [107]. There have also been many transformations other than carbon-carbon bond-forming reactions in organic synthesis using lanthanide triflates as catalysts, and all these will be reviewed in the near future. 

Use of complex lanthanide catalysts allows a very high c/s-1,4 placement of isoprene monomer and preparation of polymers that are very close to natural rubber [212]. Thus, complex neodymium catalysts can yield polymers that are greater than 98% c/s-1,4 polyisoprenes. The preparation of such catalyst, however is difficult. Evans et al. reported, however, that simple Tmla, Ndia, and Dyl2 will initiate polymerization of isoprene without any additives and can also yield high c/s-1,4 placement [213] ... 

The addition of reagents containing X-H bonds in which X is more electronegative than H typically lead to addition across the M-C bond in the direction opposite to the addition of silane or borane to the early metal catalysts. Polymerization of etiiylene with lanthanide catalysts in the presence of phosphines generates phosphine-terminated polymers (Scheme 22.12) - by a mechanism in which the alkyl chain is protonated, and a metal-phosphido complex is generated. This phosphido complex then inserts olefin to start the growth of a phosphine-functionalized polyolefin. Marks subsequently showed that a similar process can be conducted witii amines. In this case, the bulky dicyclohexylamine was needed to sufficiently retard the rate of protonation to allow chain growth. The steric bulk also makes the olefin insertion more favorable thermodynamically. 

SOME NOVEL DIENE POLYMERS PREPARED WITH LANTHANIDE CATALYSTS... 

The pol)nnerizations of 2,3-dlmethy1-1,3-butadiene with lanthanide coordination catalysts are listed in Table 3. In contrast to the conventional methyl rubber produced by emulsion polymerization, polymers obtained with lanthanide catalysts were not rubbery materials. The polymer obtained was a highly crystalline, white powder with a sharp melting point of 192-195 C, as measured by DSC (Figure 15). The pol)nner was insoluble in cyclohexane and toluene, but very soluble in hot trichlorobenzene. The appearance and... 

The pol)nnerizations of trans-2-methyl-l,3-pentadiene with lanthanide catalysts are listed in Table 4. Pol3nners obtained are rubbery materials and have relatively low molecular weight with Inherent viscosity around 0.2. H- and C-13 NMR spectra (Figures 21 and 22) showed the microstructure of polymer is essentially 1,4-addition with 40% cis and 60% trans. The tac-ticity of the polymer appeared to be either isotactic or syndiotactic. The detailed assignments of the tacticity and the sequence distribution of cis-1,4 and trans-1,4 units are still under investigation. 

The polymerization of 1,3-dienes (e.g., 1,3-butadiene and isoprene) with Ziegler-Natta catalysts began in 1954, soon after the first results obtained in a-olefin polymerization since then many transition metal and lanthanide catalysts have been examined and several stereoregular diene polymers have been obtained [30, 31], 1,3-Dienes can generate several types of polymers having different stmctures trans-1,4 cis-1,4 1,2 and, in the case of asymmetric monomers (e.g., isoprene), 3,4. Stereoregular 1,2- or 3,4-polydienes may also exhibit iso- or syndiotacticity. (Figure 11.1). 

The polybutadiene polymers obtained with lanthanide catalysts have a high cis-1,4-content which can be varied from less than 80% to as high as 99%.The polymer vinyl contents (i.e., 1,2-addition) are always less than 0.8%. That is the total 1,4-structure remains higher than 99% and is not affected by the polymerization parameters. 

Lanthanide compounds such as yttrium and lanthanum alkoxides have been reported to yield high-molecular-mass polyesters under mild conditions. The yttrium alkoxide-initiated polymerization of CL proceeded rapidly at room temperature [27-29], while the use of bulky groups reduced the transesterification reaction such that polymers with a narrow molecular mass were obtained (Scheme 11.3). For example, the bulky phenoxide ligands of the yttrium or lanthanide catalyst were exchanged for the smaller alcohol (2-propanol), followed by coordination and insertion of the monomer (CL) [27]. 

Extensive efforts have been made to develop catalyst systems to control the stereochemistry, addition site, and other properties of the final polymers. Among the most prominant ones are transition metal-based catalysts including Ziegler or Ziegler-Natta type catalysts. The metals most frequentiy studied are Ti (203,204), Mo (205), Co (206-208), Cr (206-208), Ni (209,210), V (205), Nd (211-215), and other lanthanides (216). Of these, Ti, Co, and Ni complexes have been used commercially. It has long been recognized that by varying the catalyst compositions, the trans/cis ratio for 1,4-additions can be controlled quite selectively (204). Catalysts have also been developed to control the ratio of 1,4- to 1,2-additions within the polymers (203). 

Sc(naphthenate)3/ROH/AlR3 (1/2/7) has been found to exhibit an activity similar to the lanthanide series catalyst [114], The cis PA film obtained with it showed an electrical conductivity of 14.4 S cm 1 when the polymer was doped with I2 at a ratio of (CHI0.04)n, and the TEM measurement suggested the formation of ca. 20-30 nm fibrils. 

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. 

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,... 

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