Anionic polymerization homogeneous

Three kinds of polymer segments are formed in the polymerization of dienes 1-4 cis-, 1-4 trans-, and 1-2 segments (or 3-4 in polymerization of isoprene or other monosubstituted dienes). The latter may form isotactic or syndiotactic diads when the proportion of the 1-2 form is sufficiently high, e.g. a syndiotactic, highly 1-2 polybutadiene was described recently by Ashitaka et al. 123), although the so far examined 1-2 polybutadienes produced by homogeneous anionic polymerization were found to be atactic (unpubl. results of Bywater, Worsfold). 

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. 

The current status of homogeneous anionic polymerization by alkali metal derivatives is reviewed along two lines ... 

Eq. (17) predicts that, when Pn is 100, the polydispersity is equal to 1.01, so that the polymer is virtually monodisperse. However, such ideal monodisperse polymers have scarcely been synthesized. The lowest values of polydispersity (Mw/Mn = 1.05-1.10) have been attained in homogeneous anionic polymerization 43). Gold 41) calculated the polydispersity of a polymer in the living polymerization with a slow initiation reaction and showed that the value of Pw/Pn increases slightly to a maximum (1.33) with an increase in polymerization time, followed by a decrease toward 1.00. Other factors affecting the molecular weight distribution of living polymer have been discussed in several papers 5S 60). 

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. The manufacture of block copolymers became possible in 1956 by M. Szwarc s discovery (24) of "living" polymers prepared in homogeneous anionic polymerization. Diblock, triblock, and multiblock copolymers are produced ionically in the presence of sodium naphthalene, butyllithium, or Ziegler-type catalysts. 

The tetramers are also useful initiators of homogeneous, anionic polymerization. 

There are some cases in homogeneous anionic polymerization in which the initiator dissociates completely with quantitative transformation into the active ionic form and the process is also virually instantaneous (stoichiometric polymerization). This is the case, for example, when one uses, as initiators, alkali organic... 

Unlike free-radical and homogeneous anionic polymerizations, cationic polymerizations cannot be described by conventional kinetic schemes involving reactions like initiation, propagation, and so on. This is due to the complexity of the cationic initiation and the variable extents of ion-pair formation at the propagating chain end [cf. Eq. (8.1)]. The possibihty of the reaction being heterogeneous due to the limited solubility of the initiator m the reaction medium further adds to the complexity of the kinetics. 

The absence of chain termination and chain transfer reactions in homogeneous anionic polymerization can lead to many novel synthetic routes. Thus,... 

In homogeneous anionic polymerizations of simple vinyl monomers, steric placement is also temperature-dependent, just as it is in cationic polymerizations. Syndiotactic placement is favored in polar solvents at low temperature. In nonpolar solvents, however, isotactic placement predominates at the same temperatures. Here, too, this results mainly from the degree of association with the counterion. 

Discuss steric control in homogeneous anionic polymerizations of methyl methacrylate in... 

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... 

Discuss steric control m homogeneous anionic polymerization of methyl methacrylate in nonpolar solvents. 

M. Morton and L. J. Fetters, Homogeneous anionic polymerization of unsaturated monomers, Macromol. Rev. 2, 71 (1967). 

There are some cases in homogeneous anionic polymerization in which the initiator dissociates completely with quantitative transformation into the active ioitic form and the process is also virtually instantaneous stoichiometric polymerization). This is the case, for example, when one uses, as initiators, alkali orgaiuc compounds (e.g., phenyUithium, butylhthium, or sodium naphthalene) in solvents which have unshared electron pairs (Lewis bases). The alkah forms stable positively charged complex ions with the Lewis base (Lenz, 1967), while the organic residue becomes negatively charged (carbanion) and can initiate an ioiuc polymerization [cf. Eqs. (8.11) and (8.12)] ... 

The objective of this chapter is to provide a survey of the technological developments in solution rubbers prepared by homogeneous anionic polymerization. Emphasis will be on butadiene and styrene monomers, and catalyst systems based on organolithiums and their complexes with Lewis bases plus some unpubhshed work on alkahne earths from the authors laboratory. The rubbers thus obtained offer means for control of molecular structural features of polybutadiene and poly(butadiene-co-styrene) that directly relate to the functional properties of tyre rubbers. 

The absence of any fortuitous chain termination step is an established feature of homogeneous anionic polymerization of diene and styrene in hydrocarbon solvent with organolithiums. Similarly, in the absence of adventitious chain terminating agents, the chain ends formed with Ba/Mg/Al catalysts are stable to an appreciable extent. 

Another goal of this study was to determine the important parameters which determine the rates of polymerization of these chiral e-lactones. This study is presently directed at investigating the effect of reaction variables (solvent and counterion) on polymerization rate, and in the future, attempts will be made by rate studies to ascertain if stereoelection exists in this homogeneous, anionic polymerization reaction. 

Morton, M, Fetters, L. J. Homogeneous anionic polymerization. V. Association phenomena in organolithium polymerization. J. Polymer Sci. A 2,3311 (196 ... 

Table 5. Polymer Tacticity in Homogeneous Anionic Polymerization Reactions... 

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