Transfers in anionic polymerizations

Regarding anion radical transfer, low-molecular weight azo compounds were used as terminating agents in anionic polymerizations. An interesting example is the addition of a living polystyrene chain to one nitrile group of AIBN [71]. The terminal styryl anion is likely to form... 

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. 

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. 

An important development in anionic polymerization has been provided by M. Szwarc s living polymers. The radical anion, sodium naphthalenide (Section 27-9), transfers an electron reversibly to ethenylbenzene to form a new radical anion, 1, in solvents such as 1,2-dimethoxyethane or oxacyclo-pentane ... 

In ionic polymerization a hydride (H-) transfer or a proton transfer are the analogues of the hydrogen atom transfer in radical polymerization. A hydride (H-) ion transfer is observed in many isomerizations and dimerizations of hydrocarbons which proceed via carbonium-ion mechanism. A similar process is responsible for chain transfer ip some carbonium-ion polymerizations. The transfer of negative ions like Cl- is also common, e.g. triphenyl methyl chloride is an efficient transfer agent in such a polymerization. Transfer of a proton is, on the other hand, a very common mode of termination of anionic polymerization. Indeed, this mode of termination was discussed previously in connection with branching reactions, and it was postulated in the earliest studies of anionic poly-... 

Litt, M., and M. Szwarc Molecular weight distribution in anionic polymerization involving chain transfer to monomer. J. Polymer Sci. 42, 159 (1960). 

Termination. As in anionic polymerization, termination by coupling or disproportionation cannot occur, leaving chain transfers as the most likely mechanisms. 

Unlike anionic polymerizations, the reaction sites are not ion pairs the catalyst is believed to facilitate transfer of the trimethylsilyl group by dipolar interaction with the Si atom. As in anionic polymerizations, however, the reactive end group is deactivated by compounds carrying labile hydrogens. Group transfer polymerizations therefore must be carried out under anhydrous conditions. 

Here r is the rate of polymerization, a is the probability of propagation, DP)nst is the instantaneous degree of polymerization, i.e., the number of monomer units on the dead polymer, and/is the initiation efficiency. Compare r in Eq. (7-144) with the simpler Eq. (7-68). When chain transfer is the primary termination mechanism, such as in anionic polymerization, then the polydispersity is 2. 

Interest in anionic polymerization grew enormously following the work of Michael Szwarc in the mid 1950s. He demonstrated that under carefully controlled conditions carbanionic living polymers could be formed using electron transfer initiation. 

Effect of Solvents and Reaction Conditions Synthesis Capabilities Block Copolymers Functional End-Group Polymers Initiation Processes in Anionic Polymerization Initiation by Electron Transfer Initiation by Nucleophilic Attack Mechanism and Kinetics of Homogeneous Anionic Polymerization Polar Media Nonpolar Media... 

Block Copolymers. Several methods have already been used for the synthesis of block copolymers. The most conventional method, that is, the addition of a second monomer to a living polymer, does not produce the same spectacular results as in anionic polymerization. Chain transfer to polymer limits the utility of this method. A recent example was afforded by Penczek et al. (136). The addition of the 1,3-dioxolane to the living bifunctional poly(l,3-dioxepane) leads to the formation of a block copolymer, but before the second monomer polymerizes completely, the transacetalization process (transfer to polymer) leads to the conversion of the internal homoblock to a more or less (depending on time) statistical copolymer. Thus, competition of homopropagation and transacetalization is not in favor of formation of the block copolymers with pure homoblocks, at least when the second block, being built on the already existing homoblock, is formed more slowly than the parent homoblock that is reshuffled by transacetalization. 

Proton transfer in THF polymerization at moderate temperatures is negligible and other sources of termination such as anion instability can be avoided by a proper choice of polymerization conditions. 

In contrast to radical polymerizations in which there is only one type of propagating species, ionic polymerizations may involve several active species, each with different reactivities and/or lifetimes. As outlined in Scheme 2, ionic polymerizations may potentially involve equilibria between covalent dormant species, contact ion pairs, aggregates, solvent separated ion pairs, and free ions. Although ion pairs involving alkali metal countercations can not collapse to form covalent species, group transfer polymerization apparently operates by this mechanism. In anionic polymerizations, free ions are much more reactive than ion pairs although the dissociation constants are quite small = 10 ) [5]. 

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