Initiation by Addition of an Anion to an Olefin

When organolithium compounds are dissolved in non-polar solvents there is a strong tendency of the solute molecules to associate into aggregates. For instance, butyllithium is hexametric in hexane solution. This is true of ethyllithium [144, 145] as well. Addition of Lewis bases to these solutions causes formation of strong complexes between the bases and the organometallic compounds [146, 147]. This causes the clusters to breakup through a succession of equilibrium with tetrameric and dimeric intermediates, all becoming complexes with the Lewis bases [146]. Particularly effective bases are those that allow a close approach of the lithium ion to the heteroatom. Often, the carbon–lithium bond, that normally only ranges between 20 and 40% in ionic character, becomes much more ionic [148, 149]. The breakdown of the aggregates was shown to dramatically enhance the reactivity of the organometallic compounds [150, 151]. For instance, polymerizations of styrene in benzene with butyllithium are slow reactions. When, however, these polymerizations are carried out in tetrahydro furan they are extremely rapid. Tetrahydrofuran is, of course, a Lewis base. Nevertheless, the breakdown of the aggregates even in such Lewis bases as tetrahydrofuran or diethyl ether are not complete, though the clusters are smaller and more solvated. Differences in reactivity, however, can be observed even in different non-polar solvents [152]. The reaction rates depend to a great extent on the nature of the organometallic compounds, such as polarity of the bonds and the degree of solvation. In polar solvents, where free solvated ion-pairs predominate, the mechanism of initiation may simply consist of a direct addition of the anion to the monomer. If the solvents are non-polar, on the other hand, the initiation is more complex. In these solvents, the metal cation coordinates with the monomer first. This is followed by a rearrangement [153]:

In the initial coordination, the p-electron cloud of the olefin overlaps with the outer bond orbital of the metal cation. This causes stretching and eventual rupture of the R–M (metal) bond. An intramolecular  rearrangement follows with the migration of the carbanion (R) to the most electron-deficient carbon atom of the double bond. A new covalent bond and a new carbanion are formed simultaneously:

where M represents the metal.

The basicity of the anion portion of the initiating species is also important. For instance, fluorene lithium initiates polymerizations of methyl methacrylate but fails to initiate polymerizations of styrene. A more electronegative butyl anion from butyl lithium, on the other hand, initiates polymerizations of both monomers. Yet, it was also shown that the order of reactivity is often contrary to the inherent basicity.103–146 This may be due to the size of the aggregates that the particular organometallic compound forms. In addition, higher reactivity may also be due to favor able energy requirements for breaking down the aggregates [146].

Presence of heteroatoms like oxygen or nitrogen in the monomer causes rapid complexation with the organometallic compounds. For instance, when butyllithium initiates polymerization of styrene in hydrocarbon solvents, there is an induction period and the overall reaction is slow and sluggish. When, however, it initiates polymerization of o-methoxystyrene under the same conditions, there is no induction period and the reaction is rapid. This is due to the initiator coordinating with the oxygen atom [153–155]:

A similar coordination with the heteroatom takes place in polymerizations of acrylic and methacrylic esters [156]. Here, the metal coordinates with the electron cloud of the whole conjugated structure. The overlap stretches and eventually ruptures the metal-alkyl bond. A new carbon to carbon covalent bond forms together with a new metal to carbon linkage:

where M represents the metal.

The product might be a resonance hybrid of the two structures where the metal is associated with the carbanion in one and with the oxygen in the other [157]. Some abnormalities were reported in the initiations of methyl methacrylate polymerizations in toluene by butyllithium. Their nature is such that they suggest the possibility of more than one reaction taking place simultaneously [158]. One, that must be the major one, is that of the organometallic compound reacting with the carbon-to-carbon double bond as shown above. The other, minor one, may be with the carbon to oxygen double bond. The major reaction produces methyl methacry late anions. The minor reaction, however, yields butyl isopropenyl ketone with an accompanying formation of lithium methoxide [158]:

Lithium methoxide does not initiate polymerizations of methyl methacrylate. The methoxide molecules, however, react with carbanions on the growing chain. The resultant anions are less reactive than methyl methacrylate anions and can only add new methyl methacrylate monomers slowly.

Once added, however, the reaction proceeds at a normal rate [158]. Polymerizations of methyl methacrylate in polar solvents, on the other hand, proceed in what might be described as an ideal manner with formations of only one kind of ion pairs [159–162]. Other monomers can also exhibit abnormal behavior in some anionic polymerizations. Thus, for instance, organ magnesium initiation of methacrylonitrile polymerization results in formation of two types of active centers [163, 164]:

Methyl methacrylate polymerizations, initiated by organ magnesium compounds, also yield abnormal products. Here, the active centers are unusually persistent and stable. In addition, the a-carbon atoms of the monomers were found to assume tetrahedral configurations [165–167]. This suggests that the active centers contain covalent magnesium carbon bonds. Also, gel permeation chromatography curves of the products show that more than one active center operates independently [166, 167]. A “pseudo anionic” mechanism was, therefore, postulated for polymerizations of acrylic and methacrylic esters [111, 112] by Grignard reagents. Three different simultaneous reactions appear to be taking place in butyllithium-initiated polymerizations of vinyl ketones in benzene [168]:

The initiations of the polymerizations of the conjugated dienes in inert hydrocarbons are also believed to be preceded by coordination of the organometallic compounds with the p-electron clouds of the monomers:

The rearrangement that follows can result in either 1,2; 1,4; or 3,4 additions to the double bond:

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