Dienes organolithium initiation

The lithium and alkyllithium initiation of diene polymerization has, from the earliest times, remained in the shadow of other, apparently more important, initiator systems. However, it has now become clear that the alkyllithium catalyst is the most efficient, initiator system at present available for diene polymerization. That organolithium initiators are not used much more widely is due largely to economic considerations,... 

This review is limited to the polymerization of hydrocaibon dienes and olefins by means of organolithium initiators. It is not intended to include activated olefins or dienes that can be polymerized by bases of far lower reactivity or that do not involve direct caibon-lithium bonding. 

Nonpolar Media. Because organolithium initiators are soluble in hydrocarbons, the kinetics of these polymerizations have also been studied in these nonsolvating media. A large number of such studies have been carried out (3, 41) mainly on styrene and the dienes. Again the propagation rate is first order with respect to monomer, in accordance with Reaction 13. However, the rate dependence on growing chain concentration has been found to show marked variation from one system to another with the orders varying from one half to much lower values (3, 41). These systems pose... 

In this paper, several experimental criteria are enunciated for evaluation of the usefulness of protected, functionalized iniators. Four commercially available organolithium initiators with protected hydroxyl-functional groups are evaluated with respect to these criteria for polymerization of styrene and diene monomers. 

Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes, because they yield some steric control over monomer placement. This is true of both tacticity and geometric isomerism. As stated earlier, the insertions can be 1,2 3,4 or 1,4. Furthermore, the 1,4-placements can be cis or trans. Lithium and organolithium initiators in hydrocarbon solvents can yield polyisoprene, for instance, which is 90% cw-1,4 in structure. The same reaction in polar solvents, however, yields polymers that are mostly 1,2 and 3,4, or trans-lA in structure. There is still no mechanism that fully explains steric control in polymerization of dienes. 

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). 

Early work bearing on organolithium-initiated diene polymerization was reported by Ziegler and co-workers in 1934. It was demonstrated that the organolithium adds to the diene double bond to give an intermediate capable of reacting further with the diene. 

The characteristics of the organolithium-initiated polymerization of the dienes was clearly established by Morton and co-workers. It was demonstrated that the polymerizations are of the living type, which take place without a chain termination reaction in the absence of impurities.The implications of this behaviour in preparing polymers of controlled structure are well recognized today. 

Placing functional groups on the terminal chain end(s) of anionic polymers (e.g. by conversion of active centres to —OH and —COOH groups) by reaction with ethylene oxide or carbon dioxide, followed by hydrolysis, is of interest because of the potential for further chain extension and reactions through the reactive end groups. Diene polymers with functional groups have been prepared with mono- and multi-lithium initiators, as well as with functionalized organolithium initiators. 

Before discussing the use of 1,1-diphenylalkylcarbanions as initiators for the anionic polymerization of styrenes and dienes, it is important to consider the characteristics of a useful anionic initiator. Several criteria have been proposed for judging whether an organolithium initiator is useful for preparation of well-defined polymers [79, 80] ... 

These three criteria provide a basis upon which to judge the usefulness of 1,1-diphenylethylene-derived organolithium initiators for polymerization of styrene and diene monomers in hydrocarbon solution. 

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

The stereochemistry of diene polymerizations is also affected by solvent polarity. For instance, the proportion of cA-1,4 units is increased by using organolithium or lithium itself as the initiator in the polymerization of isoprene or 1,3-butadiene in nonpolar solvents. 

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. 

Conjugated Dienes and Other Monomers. Alkyllithiums such as n-butyllithium—and even the growing polyethylene carbon-lithium bond complexed with chelating diamines such as TMEDA—are effective initiators for the polymerization of conjugated dienes such as 1,3-butadiene and isoprene. A polybutadiene of high 1,2-content can be produced from butadiene in hydrocarbon solvents using these N-chelated organolithium catalysts. 

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