Lithium, alkyls association

First, dissociation of the lithium alkyl association [Structure (29)] takes place, followed by activation by complexing of the monomer lithium alkyl with the c/ -isoprene [Structure (31)]. For insertion in a second step, a dimer alkyllithium is necessary [Structures (32) and (33)]. 

There is an enormous organometallic chemistry associated with the group IA metals, particularly lithium and sodium. Lithium alkyls can be prepared by the reaction of the metal and an alkyl halide,... 

In addition, hydroxyl polymers prepared by use of lithium alkyl acetal initiators have shown a high degree of functional purity (Table II). The functionality data for XI is a bit low, in part, because a linear GPC calibration was used to calculate nn (GPC). It should also be noted that Equations 1-9 proceed in the absence of anionic association or gel. 

In solution lithium alkyls are extensively associated especially in non-polar solvents. Ethyllithium in benzene solution exists largely as a hexamer (9, 43) in the concentration range down to 0.1 molar and there is no evidence for a trend with concentration so presumably the hexamers persist to even lower concentrations. Indeed even in the gas phase at high dilution it exists as hexamer and tetramer in almost equal amounts (3). In a similar way n-butyllithium in benzene or cyclohexane is predominantly hexameric (62, 122). t-Butyl-lithium however is mostly tetrameric in benzene or hexane (115). In ether solution both lithium phenyl and lithium benzyl exist as dimers (122) and it has been suggested that butyllithium behaves similarly in ether (15) although this does not agree with earlier cryoscopic measurements (122). It is however certain that more strongly basic ethers cause extensive breakdown of the structure. 

Metallic lithium in the form of a suspension has been used to polymerize isoprene (97) but the system is not too suitable for an exact analysis of the mechanism. The conversion-time curves are sigmoidal in shape. Minoux (66) has shown that the overall rate is not very dependent on the amount of lithium dispersion used as expected if the organo-lithium intermediates are highly associated. The molecular weight of the polymer is more dependent on quantity of lithium used. The observed kinetic behaviour is very similar to that shown in lithium alkyl initiation. This suggests that apart from differences in the initiation step, the mechanisms are quite similar. 

This scheme requires the assumption of extremely strong association of all lithium-oiganics down to at least 10-4 molar concentration if the observed reaction orders are to be obeyed. It assumes in agreement with earlier workers that only unassociated lithium alkyls and aryls are reactive. The six-fold association of butyllithium required is in agreement with physical measurements although admittedly these were carried out at much higher concentrations. Morton and co-workers (69) have shown that the polymer molecules are indeed associated into dimers in this system from a quantitative study of the decrease in solution viscosity on removal of the charged species at the ends of the polymer molecules. 

Mechanisms of the above type are very plausible but two points should be considered. Firstly, all these transition states are equally plausible for butadiene and isoprene whereas butadiene gives a mixed cis-trans product with lithium alkyls in hydrocarbons. Secondly, it is not certain that these carbon-lithium bonds are essentially covalent in hydrocarbons. There is evidence that the lithium compounds of conjugated monomers still exist as charge delocalized ion-pairs in the associated state in hydrocarbons (48). The characteristic ultra-violet absorption band attributable to this kind of anion pair persists almost unchanged in different solvents and alkali metals. The monomeric form active in the propagation step could possibly contain a more covalent carbon-lithium bond but we cannot be sure of this. 

Since it is known that lithium alkyls are highly associated in solution (10, 8), it is assumed that at concentrations greater than 0.020 M the butyllithium is completely associated and that only dissociated lithium alkyls are catalytically active. Assuming m = 7 and K= 1011 in the relationship... 

However, the well-known ability of organolithium compounds to form associated species or to form complexes with electron donor compounds (240—242) provides strong support for mechanisms involving cationic attack by the lithium cation on the monomer prior to an anionic addition. With three orbitals available for coordination, a monomeric lithium alkyl should be able to complex both double bonds of a diolefin to provide the orientation for making cis-1,4 polymer and still have an orbital available for forming associated species in hydrocarbon solvents. The lithium orbitals are presumed to be directed tetrahedrally. Looking at the top of a tetrahedron with the fourth lithium oibital above and normal to the plane of the paper, the complex could have structure A below. In the transition state B for the addition step, the structure... 

The structures of the organic derivatives of the Group IA and IIA metals are not simple because many of them involve molecular association. For example, the lithium alkyls are tetramers in which the lithium atoms reside at the corners of a tetrahedron and the carbon atoms bonded to them are located above the triangular faces of the tetrahedron as shown in Figure 7.2. 

In lithium alkyl-initiated polymerizations only chain initiation and propagation steps need be considered in hydrocarbon solvents. Both reactions are strongly influenced by extensive association of all lithium compounds. The reactive species in chain propagation is the small amount of dissociated material which probably exists as an ion pair. Association phenomena disappear on adding small amounts of polar additives, and the aggregates are replaced by solvated ion pairs. In polar solvents of relatively high dielectric constant (e.g. tetrahydrofuran), some dissociation of the ion pairs to free ions occurs, and both species contribute to the propagation step. The polymerizations are often complicated in tetrahydrofuran by two side reactions, namely carbanion isomerization and reaction with the solvent. 

In benzene solution, measurements have been made of the rate of reaction of butyllithium with styrene (27), l l-diphenylethylene (6), and with fluorene (7). In each case the reaction was first order in olefin and close to one-sixth order in butyllithium. This latter, fractional order has been attributed to the sixfold association of lithium alkyls in hydrocarbon solution. The actual species active in initiation is the monomeric butyllithium in equilibrium with the hexamer. 

The radical model cannot be applied for ionic and coordination polymerizations. With a few exceptions, termination by mutual combination of active centres does not occur. The only possibility is to measure the rate of each copolymerization independently. The situation can be greatly simplified for copolymerizations in living systems. The constants ku and k22 can usually be measured easily in homopolymerizations. Also, the coaddition constants fc12 or k2] are often directly accessible when the M] and M2 active centres can be differentiated spectroscopically or when the rate of monomer M2 (M[) consumption at M] M 2 centres can be measured. Ionic equibria, association, polarity of medium and solvation must be respected, even when their quantitative effect is not known exactly. The unusual situations confronting macromolecular chemistry will be demonstrated by the example of the anionic copolymerization of styrene with butadiene initiated by lithium alkyls in hydrocarbon medium. 

There is a large difference in initiation rates between the two initiators, but in both cases the reaction order in lithium alkyl is fractional, whereas the dependence on monomer concentration is, as expected, of the first order. The lines drawn have slopes of 1/4 (sec.-BuLi) and 1/6 (n-BuLi). There seems to be a clear relationship between association number (n) and reaction order and simple mechanisms can be suggested [17] (although not, of course, proved) of the type. 

Lithium Alkyls. Organolithium compounds have been widely used as initiators, being readily available and experimentally very convenient. Their aggregated form in hydrocarbon solvents is readily broken down by addition of donor molecules, and initiation becomes fast and efficient. The presence of common impurities such as alkoxides can have a strong influence, almost certainly through cross-association, and may increase the rate of initiation in some solvents while depressing it in others. ... 

Lithium alkyls and aryls are associated in solutions, but the nature of the species depends on the nature of the solvent, the steric nature of the organic radical and temperature.27 Cryoscopy and nmr study with 13C, 7Li and resonances28 29 show that in hydrocarbon solvents MeLi, EtLi, -PrLi and some others are hexamers but terf-butylithium, which presumably is too bulky, is only tetrameric. On addition of ethers or amines, or in these as solvent, solvated tetramers are formed. The formation of dimers, or aggregates less than tetramers, seems not to occur. 

Lithium alkyls in ether or benzene show a mean degree of association of from three to seven, whereas phenyl- and benzyllithium are dimeric in ether (17, 135). The lower degree of association in ether may stem from etherate formation. The structure of these auto-complexes may be analogous to that of beryllium and aluminum alkyls, or perhaps a lithium atom acts as a Lewis acid that is, Li [Li(C6H6)2]e. Wittig (135) favors this formulation over a phenyl bridging scheme. 

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