Monomers organolithium reagents

Organolithium reagents have been used to prepare random, block, and graft copolymers. Much work has been done on the copolymerization of diene and olefin monomers, especially 1,3-butadiene and styrene. In this review, we shall emphasize the copolymerization of these two monomers. 

The organolithium reagent is stable, but easily protonated by water to give polystyrene. Alternatively, another monomer can be added to continue extending the chain. 

The product of this step is a new organolithium reagent that can react with a second monomer molecule, then a third, and so on. The growing organolithium chain is stable and is called a living polymer. 

These observations, added to the fact that addition of polar solvents to polymerization reactions of butadiene initiated by organolithium reagents in hydrocarbon solvents can change the stereochemistry from one of predominantly rtr-1,4 to that of 1,2 and trans-, , suggests that the lithium may form a tt complex with the olefin, and if this is sufficiently strong, orient the monomer prior to the incorporation step. 

Organolithium reagents are oligomers (i.e., dimers, trimers, and higher species) in nondonor solvents such as alkanes LiMe is a tetramer with a cubane structure 14.1, for example. RLi forms solvates with THE Addition of the chelating ligand Me2NCH2CH2NMe2 (TMEDA) leads to formation of a monomer, and this increases the reactivity. n-BuLi can deprotonate toluene... 

Both 19 and 20 react readily with a variety of organometallic compounds to afford the corresponding ti -vinylcyclopentadienyl monomers. The reaction between organolithium reagent 2 and (ri -cyclo-pentadienyl)trichlorotitanium on ethyl ether produces (n -vinylcyclo-pentadienyl)(n cyclopentadienyl)dichlorotitanium, (mp 154-157°C, 16%) (Equation 5). Pol3nners of after reduction could function... 

As with the organolithiums that we introduced in Chapter 9, the exact structure of these reagents is more complex than we Imply here they are probably tetramers (four molecules of RjCuLi bound together), but for simplicity we will draw them as monomers. 

A handicap of Grignard reagents in the field of anionic polymerization is certainly their low reactivity toward nonpolar double bonds. Unlike organolithium compounds, organomagnesium compounds are nornally inert toward monomers sueh as styrene or butadiene. Thus, their scope in the field of anionic block-copolymerization is quite limited. 

For the n-BuLi/butadiene/heptane system, a yield of less than 10% 1,2-structures and a maximum of 60% 1,4-structures in the /rans-configuration is typically obtained. Using high monomer concentrations (10 mol/liter) and low initiator concentrations (10 mol/liter) the trani-configuration is lowered to 20%. To explain the kinetic results, association of the organolithium plays an important role and leads to a model in which the influence of the concentration on stereoselectivity is described . The microstructure can be varied desirably by addition of complexing reagents " . In the t-BuLi/buta-... 

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