Lithium alkoxide impurities

A method based on equation 21 may be performed, where the organic residue of an organometallic compound becomes derivatized with excess dimethylphenylchlorosUane (67) and determined by GC, using cumene (i -PrPh) as internal standard. Although lithium alkoxide impurities also react with 67, the product appears at a different retention time. The method is also suitable for Grignard reagents. ... 

Any knowledge of the mode of action of lithium alkoxides is important as a certain amount of lithium alkoxides is always present. They are formed by reaction of the initiator with traces of oxygen. Another possible impurity is LiOH (hydrolysis product), which leads to a reduction in the actual RLi concentration [153]... 

A knowledge of the action of lithium alkoxides in these systems is important because they are always present to some extent, formed by fortuitous catalyst destruction by traces of oxygen. The other likely impurity is lithium hydroxide produced from traces of water in the system. It apparently decreases the initiation rate [53] but primarily [55] because it reacts with organo-lithium compounds... 

The effect of lithium alkoxides on alkyllithium-initiated pol5nnerizations is important because these salts are ubiquitously present to some extent as impurities formed by the reactions with oxygen (13) (eq. 27) and hydroxylic impurities (eq. 28). In fact, it is common practice to utilize excess butyllithium, ie, more than the stoichiometric amount required to generate the required molecular weight, to scavenge impurities in the solvent and monomer feed. 

Organolithium compounds are highly sensitive to air and humidity. The commercial RLi usually contains varying amounts of alkoxides as impurities. The concentration of the organoUthium solution can be easily checked by several methods, which can be divided into two categories (i) direct titration methods, based usually on a color change of self-indicators, and (ii) double titration methods. Such different methods have been reviewed in detail [12, 151], The traditional double titration method provides the content of active organohthium and of lithium alkoxides/hydroxide (Fig. 26.9). 

The addition of lithium alkoxides generally decreases the rate of propagation this is important to note since formation of lithium alkoxides can readily occur in the presence of even small amounts of impurities. The addition of other alkali metal alkoxides increases the rate of polymerization in hydro-carhon solution as would he expected if counterion exchange occurred (see Table... 

The appearance of the alkali alkoxide can lead to formation of bimetallic alkoxides or bimetallic alkoxide halides as impurities. For example, lithium oxoalkoxocomplexes were isolated as side products of the anodic dissolution of Mo and W when LiCl was used as conductive additive [908],... 

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. ... 

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