Supported initiator Stereoselective polymerization

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

The configuration of natural amino acids has led to studies on the possibility that homochirality emerged at a prebiotic stage, which may be supported by the presence of amino acids as non-racemic mixtures in meteorites (32). This enantiomeric excess may have resulted from the exposition of extraterrestrial matter to circularly polarized light (42). Whatever the origin of this enantiomeric excess, it may have initiated stereoselective processes through different catalytic pathways (7). Symmetry breaking may also have resulted from reactivity in connection with other processes such as crystallization or interfacial chemistry (43, 44) and polymerization of amino acids (45). 

Polyethylene-supported catalysts that are initially insoluble but that become soluble on heating and are separated as insoluble materials on cooling are also used as catalysts in polymerization reactions. Infact, this was the first way a polyethylene-bound catalyst was used (Eq. 11) [34]. However, soluble polymers used in this manner appear to have several deficiencies. First, separation of the products from the catalyst may notalways be as simple as was the case with catalysts like 11 or 12 and low molecular weight products. For example, while a hot solution of a polyethylene-bound neodymium salt was successfully used in the stereoselective polymerization of butadiene to form high molecular weight (Z)-poly( 1,4-butadiene), the product mixture after cooling was a thick, viscous sus-... 

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