Organolithium derivatives, polymerization

The anionic polymerization of MMA initiated by organolithium derivatives is by far more complex in apoiar solvents than in THF. Indeed, Wiles and Bywater reported that the polymerization of MMA with sterically hindered initiators in toluene is ill-controlled even when conducted at low temperature . The strong aggregation of the active species in toluene would be responsible for the observed broad molecular weight distribution and very low initiation efficiency ". ... 

Polymerization of tBMA initiated by organolithium derivatives in toluene... 

As shown in step 3 of Mechanism 27.3 once all of the monomer is consumed the polymer is present as its organolithium derivative. This material is referred to as a living polymer because more monomer can be added and anionic polymerization will continue until the added monomer is also consumed. Adding 1,3-butadiene, for example, to a living polymer of styrene gives a new living polymer containing sections ( blocks ) of polystyrene and poly(1,3-butadiene). 

Kim J, Choi GJ, Lee SB. Organolithium-initiated polymerization of carbazole derivative retaining blue light photoluminescence. Abstracts of Papers, 228th ACS National Meeting, Philadelphia, PA, United States, August 22-26,2004 PMSE-275. 

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. 

So far the discussion was focused on copolymers derived from a mixture of styrene and a diene. In view of the "living" nature of organolithium polymerization, it is also possible to synthesize block polymers in which the sequence and length of the blocks are controlled by incremental (or sequential) addition of monomersr This general method of preparing block polymers is readily adaptable to commercial production, and, indeed, a number of block copolymers are manufactured this way. Those that have received the most attention in recent years are the diene-styrene two-phase... 

Taylor and Wei have also developed a versatile method for the synthesis of cyclopentanes employing readily available organolithium compounds as difunctional, conjunctive reagents. This strategy represents an anionic [3 + 2] approach to substituted cyclopentanes. The reactions of lithiated alkenes 149 with activated alkenes 150 afford cyclopentane derivatives 151 in reasonable yield and, in some cases, with excehent stereocontrol. The alkenes 150 must be added over extended times to minimize polymerization processes. The... 

The reaction of polymeric organolithium compounds with substituted DPE derivatives is an excellent system for development of a general functionalization reaction (eqn [39]) because... 

Diphenylmethylcarbanions. The carbanions based on diphenyl-methane (pZ a = 32) (see Table 1) are useful initiators for vinyl and heterocyclic monomers, especially alkyl methacrylates at low temperatures (46). 1,1-Diphenylalkyllithiums can also efficiently initiate the polymerization of styrene and diene monomers that form less stable carbanions. Diphenylmethyl-lithium can be prepared by the metalation reaction of diphenylmethane with butyllithium or by the addition of butyffithium to 1,1-diphenylethylene, as shown in equation 17. This reaction can also be utihzed to prepare ftinctionalized initiators by reacting butyffithium with a substituted 1,1-diphenylethylene derivative. Addition of lithium salts such as hthium chloride, lithium f-butoxide, or lithium 2-(2-methoxyethoxy)ethoxide with 1,1-diphenylmethylcarbanions and other organolithium initiators has been shown to narrow the molecular weight distribution and to improve the stabffity of active centers for anionic polymerization of both alkyl methacrylates and t-butyl acrylate (47,48). 

Polymerization is initiated by addition of a nucleophile, shown here as a carb-anion derived from an organolithium compound, to an activated carbon-carbon double bond to give a carbanion. 

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