Anionic polymerization organomagnesium initiation

Alkyl derivatives of the alkaline-earth metals have also been used to initiate anionic polymerization. Organomagnesium compounds are considerably less active than organolithiums, as a result of the much less polarized metal-carbon bond. They can only initiate polymerization of monomers more reactive than styrene and 1,3-dienes, such as 2- and 4-vinylpyridines, and acrylic and methacrylic esters. Organostrontium and organobarium compounds, possessing more polar metal-carbon bonds, are able to polymerize styrene and 1,3-dienes as well as the more reactive monomers. 

The conclusion that the mechanism is pseudoanionic is restricted to organomagnesium initiated systems. The gist of our argument is that these polymerizations behave differently from the true anionic mechanisms initiated by (typically)... 

In addition to organohthium compounds, other reagents such as organomagnesium compounds and potassium alkoxides have also been used as initiator for anionic polymerization of masked disilenes. ... 

Other monomers can also exhibit abnormal behavior in some anionic polymerizations. Thus, for instance, organomagnesium initiation of methacrylonitrile polymerization results in formation of two types of active centers ... 

The polymerization of acrylate esters by organomagnesium reagents is best described as pseudo-anionic monomer is complexed to a magnesium atom covalently bonded to carbon. The anionic polymerization of methyl methacrylate in benzene yields the syndiotactic polymer when the initiator is an alkali metal complexed with 18-dicyclohexyl-6-crown. When a solution of methyl methacrylate in toluene is placed below a solution of butyl-lithium in the same solvent at —78 °C so as to avoid any mechanical mixing, the polymer obtained has a higher isotacticity than that observed if the solutions are stirred. ... 

Methyl methacrylate polymerizations, initiated by organomagnesium 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. 

Anionic initiation has been accomplished in a variety of solvents, both polar and nonpolar. Typically, initiation can proceed by electron transfer reactions from alkali or alkaline earth metals, polycyclic aromatic radical anions, or alkali and magnesium ketyls. The other possibility includes the nucleophilic addition of organometallic compounds to the monomers. Related monofunctional initiators comprise alkyl derivatives of alkali metals or organomagnesium compounds such as Grignard reagents. Difunctional species are alkali derivatives of a-methylstyrene tetramer or the dimer of 1,1-diphenylethylene. An overview of the initiation process in carbanionic polymerization is given in Ref. [159]. 

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