Single-site initiator

The use of single-site initiators for the polymerization of acrylates is attractive, since steric protection of the metal center should eliminate the unwanted side reactions described above, allowing living polymerization systems to be developed. Further, stereocontrol may be achievable by appropriate ligand selection. 

Early reports document the use of Zn(OR)2 and EtZn(OR), (R = CH2Br, CH2CH2CH = CH2, CH2CH2NEt2)817 for well-controlled polymerizations of CL. Subsequently, the use of zinc(II) lactate and related carboxylates was described.818,819 For simple magnesium derivatives, low activities and epimerization are generally observed.820 In more recent years, research in this field has been directed towards the development of single-site initiators. 

Describes open cell foamed articles including silane-grafted single-site initiated polyolefin resins. The olefin polymer resin can be a polyethylene, a copolymer of ethylene, a C3-C20 alpha-olefin, or a copolymer of ethylene, a C3-20 alpha-olefin and a C4-C20 diene. The open cell foamed articles have good cushioning properties and can be non-allergenic. 

This is a blend of LDPE, which generally has a density between 0.91 and about 0.93 g/cu.cm. and a melt index greater than 1 and a silane-grafted single-site initiated polyolefin, which is generally a copolymer of ethylene and a C3 to C20 alpha-olefm having a density between about 0.86 and 0.96 g/cu.cm. and a MWD between about 1.5 and 3.5. 

Polymerization with oscillating metallocenes is complicated because solvent fractionation of the polymer product shows separate fractions—highly atactic, mostly isotactic, and isotactic-atactic stereoblock. The mechanism of this phenomenon is not clear. It may result from the initiators not being perfectly single-site initiators. There is some evidence that a metallocene initiator may consist of more than one species, and that each species produces a different stereochemical result (Sec. 8-5g-l, 8-5h-l). 

In tin amide complexes including Shiff base ligand, unprecedented attack by the amide moiety to the imine carbon atoms of tridentate Schiff base ligands gives unusual tetradentate ligand systems. The so-formed complexes act as latent single-site initiators for the controlled polymerization of rac-lactide (Equation (101)).267... 

The reason for quoting this structure is not only the fact that this class of initiators, developed by Inoue and Aida, lead to living processes, but also because more than 20 years later the structurally related single-site initiators became important in the stereocontrolled polymerization of LAs and p-substituted p-lactones (e.g., p-BL). The discovery of the Japanese researchers seems almost forgotten. 

An efficient enantioelective or enantioselective polymerization, based on single-site initiators of rac-LA, has been realized by Spassky and co-workers, who applied achiral and chiral aluminum alkoxides of general structure O2AI-OR, bearing Schiff base ligands. ... 

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