Multifunctional carbanions

In order to fulfil certain special requirements concerning the polymer architecture, initiators with several anions in their molecule are needed. 

Reactions of the Lochmann base (a mixture of butyllithium with potassium re/T.-butoxide in pentane [195]), with conjugated polyenes yield fully delocalized carbanions in high yield at a high rate [196] 

A large number of compounds of this type have been prepared [197], for example (counter-ions omitted) 

They can be used to initiate the polymerization of monomers on several centres simultaneously, i. e. for the preparation of star polymers. Delocalized carbanions of a different type are formed by the deprotonation of acetylene copolymers [198] 

However, the current affects initiation and/or propagation. The mechanism of this process is presently being elucidated. 

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Multifunctional initiators are found to be more effective in carbocationic than in carbanionic polymerization, because of the enhanced solubility of the less polar dormant initiating complexes. For example, the formation of a six-arm star polystyrene starts from... 

The first step involves preparation of a bifunctional "living" precursor, of known molecular weight and low polydispersity. Crosslinking can be achieved either by the addition of stoichiometric amounts of a multifunctional electrophilic deactivator, or by adding small a small amount of a bifunctional monomer (such as DVB or ethylene glycol dimethacrylate), the polymerization of which will be initiated by the carbanionic sites of the precursor. In either case, the precursor chains become the elastically effective chains of the networks. The experimental conditions... 

Synthesis of well defined functionalized (- telechellc or multifunctional-) macromolecules Is an Important task for polymer chemists. The polymers with P0(0R)2, - Si(0R)3, -OH, - . .. functional groupslrS. are produced In limited quantities. The need for polymeric materials possessing specific properties has led to a renewed Interest Is functional polymers, especially if the initial material Is a common hydrocarbon polymer. One of the techniques that we use in our laboratory to prepare these new molecules Is based on anionic processes. This anionic technique is best suited to control the length of the chains prepared and to obtain samples with low polydlsperslty. Although the functionalization of carbanionic sites with various deactivating reagents Is easier than with other methods because of the long lived species, It Is still necessary to carefully control the deactivation reaction to prevent secondary reactions. 

The simple piperidine alkaloid coniine (for selected asymmetric syntheses of coniine see [22, 81-85]) offered a preliminary test case for hybrid radical-ionic annulation in alkaloid synthesis. From butyraldehyde hydrazone and 4-chloro-iodobutane (Scheme 4), manganese-mediated photolysis afforded the acyclic adduct in 66% yield (dr 95 5) the cyclization did not occur in situ [69, 70]. Nevertheless, Finkelstein conditions afforded the piperidine, and reductive removal of the auxiliary afforded coniine in 34% overall yield for four steps. This reaction sequence enables a direct comparison between radical- and carbanion-based syntheses using the same retrosynthetic disconnection an alternative carbanion approach required nine to ten steps [81, 85]. The potential for improved efficiency through novel radical addition strategies becomes quite evident in such comparisons where multifunctional precursors are employed. 

The development of radical additions to C=N bonds is a field with great promise for applications in the synthesis of multifunctional compounds the functional group compatibility of radical chemistry complements that of carbanion reagents to expand... 

The triblock can also be prepared by coupling the two carbanions using an organic dihalide (Equation 5.18), and other coupling agents such as phosgene or dichloro-dimethylsilane are equally effective. This method can also be used to prepare radial blocks with multifunctional compounds, as illustrated with silicon tetrachloride. 

Matmour R, Gnanou Y. Combination of an anionic terminator multifunctional initiator and divergent carbanionic polymerization apphcation to the synthesis of dendrimer-hke polymers and of asymmetric and miktoarm stars. J Am Chem Soc 2008 130 1350-1361. 

Similarly, photolatent base, 9, can be used to catalyze the addition of thiols, amines, and carboxylic acids to epoxide monomers. Oftentimes, the irradiation is followed by a brief thermal treatment as well to complete the polymerization reaction. The conjugate addition of thiols to electron-deficient acrylate double bonds has been successfully conducted under UV irradiation conditions through the use of a photolatent base such as 9 as a catalyst. " An unusual type of Michael polymerization reaction explored by Dietliker and co-workers is depicted in eqn [15]. Photogenerated DBN is a sufficiently strong base to remove a proton from the multifunctional malonate monomer, 10, and the corresponding malonate carbanion adds to the acrylate monomer, 11. 

A polymeric backbone with attached organometallic sites is used as a multifunctional initiator for the polymerization of a suitable monomer to build the grafts. The chief problem here is to find satisfactory ways to obtain a macromplecule with carbanionic sites distributed at random. The... 

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