Polymerization carbanion-induced

Interest in anionic polymerizations arises in part from the reactivity of the living carbanionic sites4 7) Access can be provided to polymers with a functional chain end. Such species are difficult to obtain by other methods. Polycondensations yield ro-functional polymers but they provide neither accurate molecular weight control nor low polydispersity. Recently Kennedy51) developed the inifer technique which is based upon selective transfer to fit vinylic polymers obtained cationically with functions at chain end. Also some cationic ring-opening polymerizations52) without spontaneous termination can yield re-functional polymers upon induced deactivation. Anionic polymerization remains however the most versatile and widely used method to synthesize tailor made re-functional macromolecules. 

Reaction of the bis-chelate complex 149 and various bis(arylalkyl)barium complexes generates heteroleptic barium complexes with one chelate and one reactive arylalkyl ligand 164. The homoleptic and heteroleptic barium complexes both induce living polymerization of styrene to atactic polystyrene in cyclohexane solution. The fact that no stereocontrol is observed during polymerization despite the presence of the chiral carbanionic ligands is... 

A method for preparing a-methylstyrene to investigate its radiation-induced polymerization yields samples which exhibit reproducible kinetics. The kinetic results are interpreted as indicating that free radicals, carbonium ions, and carbanions can all propagate simultaneously, the relative importance of each species depending upon the dryness of the monomer and all associated glassware. This viewpoint is further supported by data from a preliminary investigation of the transients formed in a-methylstyrene, as studied by the pulse radiolysis technique. 

The first results of anionic polymerization (the polymerization of 1,3-butadiene and isoprene induced by sodium and potassium) appeared in the literature in the early twentieth century.168,169 It was not until the pioneering work of Ziegler170 and Szwarc,171 however, that the real nature of the reaction was understood. Styrene derivatives and conjugated dienes are the most suitable unsaturated hydrocarbons for anionic polymerization. They are sufficiently electrophilic toward carbanionic centers and able to form stable carbanions on initiation. Simple alkenes (ethylene, propylene) do not undergo anionic polymerization and form only oligomers. Initiation is achieved by nucleophilic addition of organometallic compounds or via electron transfer reactions. Hydrocarbons (cylohexane, benzene) and ethers (diethyl ether, THF) are usually applied as the solvent in anionic polymerizations. 

So far as vinyl monomers are concerned, ionic propagation proceeds with carbonium ions (cationic polymerization) or carbanions (anionic polymerization) at the chain ends. The study of the initiation process of radiation-induced ionic polymerization seeks to elucidate how these ions are formed from the primary ionic intermediates. Possible reactions... 

The formation of ion radicals from monomers by charge transfer from the matrices is clearly evidenced by the observed spectra nitroethylene anion radicals in 2-methyltetrahydrofuran, n-butylvinylether cation radicals in 3-methylpentane and styrene anion radicals and cation radicals in 2-methyltetrahydrofuran and n-butylchloride, respectively. Such a nature of monomers agrees well with their behavior in radiation-induced ionic polymerization, anionic or cationic. These observations suggest that the ion radicals of monomers play an important role in the initiation process of radiation-induced ionic polymerization, being precursors of the propagating carbanion or carbonium ion. On the basis of the above electron spin resonance studies, the initiation process is discussed briefly. 

The products of the (e q + RCH=CH2) reaction are RCH—CH2 earbanions. Some of these have been identified by their chemical reactivity. Others have been observed through their absorption spectra by means of pulse-radiolysis techniques. The carbanion of acrylamide, for instance, has been shown to dimerize, to react with other free radicals, inducing anionic polymerization, and to react with oxygen, Ag+ and Fe(CN) - ions, presumably by electron-transfer reactions (Chambers et al., 1967). The absorption spectrum of the product of the (dimethyl fumarate + ey5) reaction has been observed in alkaline solution. The rate... 

On the basis of the profound difference in copolymer composition from a free radical or cationic type polymerization, it was stated that the sodium and potassium initiated polymerizations were carbanionic in nature. This has been one of the strongest arguments in favor of the anionic nature of the sodium and potassium polymerizations. The authors also suggested that the composition of a styrene-methyl methacrylate copolymer might be used as a criterion of the type of propagation induced by a given initiator. 

Sufficient experimental data from several laboratories now exist to describe the conditions under which the radiation-induced ionic propagation of many pure liquid vinyl monomers can be observed. The kinetic data and electrical conductivity measurements establish the ionic nature of the reaction scavenger studies appear to establish the preponderant role played by the carbonium ion in propagating the polymerization. On the basis of a single propagating species, it is possible to write a simple mechanism to describe the process. Limiting values of several of the kinetic rate constants can be estimated, notably the rate constant for reaction between a bare carbonium ion and a vinyl double bond. These rate constants are compared with similar constants arrived at in chemically initiated free radical, carbonium ion and carbanion polymerization. Several shortcomings of the present scheme are discussed. 

Such alkali metal ion pairs are capable of two electron transfer from the potassium anion towards a suitable substrate, e.g. p-butyrolactone with formation of a respective carbanion. The strong tendency to two electrons transfer is due to the unusual oxidation state of potassium anion bearing on its outer s orbital a labile electron doublet shielded from the positive potassium nucleus by inner orbitals. Using 5 -enantiometr of P-butyrolactone as a monomer and potassium supramolecular complex as catalyst, enolate carbanion is formed as the first reactive intermediate which induces polymerization, yielding poly-(R)-3-hydroxybutanoate. The resulting biomimetic polyester has the structure similar to native PHB produced in nature, except for acetoxy-end-groups, which are formed instead of the hydroxyl ones typical for natural PHB. 

For these authors an OA sulfoxide would be an interesting co-monomer to induce asymmetry into the backbone of a polymeric chain but this is not very clear again. Comparison of ORD and CD curves of the model Xc and of the copolymer Xa indicates that copolymerization occurs with retention of configuration of the sulfur atom and that the sulfoxide group does not lose its configuration when adjacent to a carbanion. 

The blockwise polymerization of either type of monomer can be induced by polyvinyl or polydienyl anions (A) to give AB or BAB copolymers, where B represents the poly(thioalkylene) sequences. In the thiirane case, the thiolate end group of the AB copolymer can readily be coupled with phosgene to give A(B)2A copolymers, whilst with thietanes the terminal carbanionic group can be used to initiate polymerization of vinyl monomers to give ABA or ABC block copolymers. 

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