Organomagnesium compounds polymerization

Unsolvated organomagnesium compounds have been recommended for the synthesis of organometallic derivatives of mercury, boron, aluminum, silicon, germanium, tin, phosphorus, arsenic, and antimony6-8 and have been used in procedures for the alkylation of aromatic rings and for the production of various polymerization catalysts.4 9... 

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. 

Being very strong bases, organometallic compounds are capable to start the polymerization (7, 12, 30, 51, 62). If organomagnesium compounds are used, the interaction of magnesium cations with lactam anions can probably reduce the dissociation of the salt analogous the basic character is certainly weakened in lactam complexes formed by addition of hydrated oxides of Ti, Zr, Hf, Th or Ce (48). 

Active polymerization catalysts have been derived from organomagnesium compounds, for example by reaction with titanium (tv) chloride [7] the polymerization of various vinyl monomers has been initiated by organomagnesium compounds [8] and recently polymerization initiated by magnesium ate complexes has been described [9, 10]. 

We possess more information on systems containing polar monomers. Organolithium and organomagnesium compounds initiate the polymerization of a number of monomers with an electron-withdrawing substituent. These polymerizations are rarely of the living type. The initiator usually reacts not only with the double bond of the monomer, but also with the polar substituent (both on the monomer and the polymer) yielding inactive products. 

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. ... 

For example, supported TiCl4/MgCl2 catalysts show a short period of acceleration, followed by a prolonged steady period 92,93). However, in the presence of electron donors, they may show the typical decay rate kinetics observed during propylene polymerization 93). Bulk catalysts prepared by interaction of TiCU with Mg(OR)2 show either a stationary rate, or a non-stationary rate, according to the titanium content 88,94). Bulk catalysts prepared by reduction of TiCl4 with organomagnesium compounds show a decay type rate 92-95>. 

Relatively little is known about the crystal structures of unsolvated organomagnesium compounds. From this viewpoint, our polymeric structures of [diphenylmagnesium] and of a [dineopentylmagne-sium neopentylmagnesium bromidel adduct contribute to fill a gap in... 

In the absence of coordinating solvents like ethers, most organomagnesium compounds tend to form polymeric structures. Microcrystalline solids or viscous liquids are generally obtained on desolvation of organomagnesium ether complexes, which are almost insoluble in apolar (non-... 

Anionic Polymerization of Other Unsaturated Monomers Using Organomagnesium Compounds 694... 

The first extensive review on organomagnesium compounds and their complexes as polymerization initiators was published in 1968 [8], Most of the work in the 1980s was directed toward the development of living systems, based on organomagnesium initiators methyl methacrylate systems. These efforts were rewarded with only limited success. 

A handicap of Grignard reagents in the field of anionic polymerization is certainly their low reactivity toward nonpolar double bonds. Unlike organolithium compounds, organomagnesium compounds are nornally inert toward monomers sueh as styrene or butadiene. Thus, their scope in the field of anionic block-copolymerization is quite limited. 

The first part of this review is dedicated to the polymerization of methyl methacrylate. This is by far the most extensively studied Grignard polymerization system. Some mechanistic and stereochemical aspects of this particular reaction are still controversial, but the results reported by independent research groups are, on the whole, mutually consistent. Other vinyl monomers have been reported to polymerize with the use of organomagnesium compounds. They are regrouped in the subsequent section. The reported results are classified and briefly commented. Studies of many systems led to contradictory results, and no attempt was made to reconcile the resulting divergent interpretations. 

Styrene and (-methylstyrene have been polymerized using a combination of a organomagnesium compound and hexamethylphosphoric triamide (Me2N)3PO [50,124-127]. Styrene polymerization could also be induced using a combination of dibenzylmagnesium with tetrahydrofurfuryl alcohol in THF at — 70"C [128] or dialkylmagnesium with 2-alkoxyethanol [128,129]. 

Summary Novel poly(silylenemethylene)s have been prepared by ring-opening polymerization of 1,3-disilacyclobutanes followed by a protodesilylation reaction with triflic acid. Reactions of the triflate derivatives with organomagnesium compounds, LiAlH4, amines or alcohols gave functional substituted and branched poly(silylene-methylene)s, which may serve as suitable precursors for silicon carbide and Si/C/N-based materials. 

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. 

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