Homogeneous anionic polymerization mechanism

Because of the strong interest in homogeneous anionic polymerization, especially during the past 20 years, there have been substantial advances, both in the understanding of the mechanisms involved as well as in the utilization of these systems for the synthesis of new materials. This overview will attempt to highlight the more important advances in both of the above areas, although admittedly with somewhat of a bias toward those particular endeavors pursued in our own laboratories. 

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... 

The true nature of homogeneous anionic polymerization only became apparent through studies of the soluble aromatic complexes of alkali metals, such as sodium naphthalene. These species are known to be radical anions [154-158], with one unpaired electron stabilized by resonance and a high solvation energy, and are therefore chemically equivalent to a soluble sodium. They initiate polymerization by an electron transfer process [145,148], just as in the case of the metal itself, except that the reaction is homogeneous and therefore involves a much higher concentration of initiator. The mechanism... 

In view of the unusual mechanism of anionic polymerization, especially the absence of termination and chain transfer reactions, the kinetics of these systems can be treated quite differently than for the other mechanisms. Thus it is possible, by suitable experimental techniques, to examine separately the rates of the initiation and propagation reactions [172,173], since the stable organometallic chain ends are present in concentrations [10 -10 M] which are easily measured by ultraviolet-visible spectroscopy [174]. The propagation reaction is, of course, of considerable main interest and can be studied by making sure that initiation is complete. In this way, the kinetics of homogeneous anionic polymerization have been extensively elucidated with special reference to the nature of counterion and role of the solvent. 

Ionic mechanisms for the preparation of block copolymers are a very important tool of the synthetic polymer chemist. A feature of many homogeneous anionic polymerizations in solution is that termination can be avoided by careful control of experimental conditions. In fact, an infinite life of the active chain end is theoretically possible, and this has led to the term living polymers. Polymer carbanions can resume growth after the further addition of monomer. By changing the monomer composition, block copolymerization is readily initiated, and this process can be repeated. A major advantage of this... 

This review deals with current ideas on the mechanisms operative in acrylonitrile polymerization. The topic is of importance in its own right and also because the study of acrylonitrile has cast light on heterogeneous polymerizations in general. It is an active field of research and the interpretations are still controversial. We shall look first at free-radical polymerization in homogeneous solution, where the monomer behaves in a more or less classical fashion. Next we shall consider the complications that arise where the monomer is at least partially soluble in the reaction medium but where the polymer precipitates. These conditions are encountered in bulk polymerization and in most aqueous or organic diluents. Finally we shall examine the less extensive literature on anionic polymerization and show important differences between the radical and the ionic processes. 

Because the monomer was not a vinyl compound and the active chain end was an alkoxide, this reaction was not considered an important case of anionic polymerization. Ironically, this reaction actually is a very good example of the anionic mechanism and can be satisfactorily studied because it is a homogeneous reaction. In fact, it was Flory (6) who first pointed out the unique consequences that arise from such a polymerization in which presumably the alkoxide chain end does not undergo any "side reactions," that is, termination. Flory remarked that in such a situation in which all the growing chains have equal access to the monomer, the chains will tend to reach similar lengths, that is, the molecular weight of the polymer will have the very narrow Poisson distribution ... 

Alternatively, polymerization of 3,3-bis(chloromethyl)oxacyclobutane may be effected by aluminium compounds such as alkoxides, amalgam and hydride at elevated temperatures (150—200°C). The mode of operation of these initiators is unknown they are usually associated with anionic reactions whereas the polymerization of cyclic ethers (other than epoxides) generally involves homogeneous cationic mechanisms. It may be that at high temperatures either the monomer is activated and anionic polymerization can occur or there is reaction between the initiator and monomer to form cationic species. 

These considerations concerning the mechanism of stereochemical control in ionic polymerization reactions, therefore, can account for the tacticities obtained in most homogeneous anionic and cationic polymerizations as shown in Tables 3 and 6, respectively (3). 

In this case, a simple kinetic model could not be derived to explain all the experimental observations. Independently prepared hexadecyl trimethyl ammoniurn persulfate was found to be soluble in toluene/AN mixtures and to catalyze polymerization in this homogeneous system at approximately the same rate as that observed in the two phase system. This result implies that anion exchange at the interface (see below) must be essentially complete under these conditions. Factors which complicated further analysis of the mechanism included a) precipitation of poly(acrylonitri 1 e) during the polymerization ... 

Evidence in support of a carbonium ion type of mechanism for low temperature polymerization was also obtained in an investigation of the kinetics of the homogeneous liquid phase polymerization of propene in the presence of aluminum bromide and hydrogen bromide at about —78° (Fontana and Kidder, 89). The rate of reaction is approximately proportional to the concentration of the promoter, no polymerization occurring in its absence. During the main portion of the reaction, the rate is independent of the monomer concentration toward the end, it decreases, due apparently to the low-concentration of the monomer, addition of more olefin resulting in an increase in the rate. It was concluded that the reaction involves an active complex, which may be regarded as a carbonium ion coupled with an anion ... 

Emulsifier is not a necessary component for emulsion polymerization if ihe following conditions are satisfied The particles are formed by homogeneous nucleation mechanism, and the particles are stabilized by factor(s) olher than emulsifier. As to the latter, the sulfate end group that is the residue of persulfate initiator serves for stabilization of dispersion via interparticle electrorepulsive force (20). When the stabilization mechanism works well, a small number of particles grow during polymerization without aggregation, keeping the size distribution narrow. Finally stable, monodisperse, anionic particles are obtained. 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

In summary, cationic polymerizations are much more variable and complex than homogeneous free-radical or anionic chain-growth polymerizations. No convincing general mechanism has been provided for cationic reactions, and each polymerization system is best considered as a separate case. 

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