Alkyllithium initiated polymers, molecular weight

The polymerization was carried out in THF under the conditions of high vacuum or argon atmosphere with a catalytic amount of alkyllithium as an initiator. Anionic polymerization of 3a with n-BuLi in THF followed by quenching with ethanol afforded polymer 6 in 56 % yield. The molecular weight distribution of the polymer was determined by gel permeation chromatography (GPC), calibrated by polystyrene standards, with chlorofrom as eluent Mn = 6.1xl0"4, Mw/Mn = 1.3. 

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). 

Anionic polymerizations initiated with alkyllithium compounds enable us to prepare homopolymers as well as copolymers from diene and vinylaromatic monomers. These polymerization systems are unique in that they have precise control over such polymer properties as composition, microstructure, molecular weight, molecular weight distribution, choice of functional end groups and even copolymer monomer sequence distribution. Attempts have been made in this paper to survey these salient features with respect to their chemistry and commercial applications. 

Alkyllithium compounds are primarily used as initiators for polymerizations of styrenes and dienes (52). These initiators are too reactive for alkyl methacrylates and vinylpyridines. -Butyllithium [109-72-8] is used commercially to initiate anionic homopolymerization and copolymerization ofbutadiene, isoprene, and styrene with linear and branched structures. Because of the high degree of association (hexameric), w-butyllithium-initiated polymerizations are often effected at elevated temperatures (>50° C) to increase the rate of initiation relative to propagation and thus to obtain polymers with narrower molecular weight distributions (53). Hydrocarbon solutions of this initiator are quite stable at room temperature for extended periods of time the rate of decomposition per month is 0.06% at 20°C (39). 

Thus, the use of alkyllithium initiation offers the synthetic chemist a tool of enormous flexibility for "tailor-making" polymers of precise structure. Control of molecular weight, molecular-weight distribution, diene structure, branching, monomer-sequence distribution, and functionality can conveniently be achieved by such techniques as incremental or sequential addition of monomer, initiators, or modifier, programming of temperature, continuous polymerization, or the use of multifunctional reagents. 

Furthermore, these workers varied the ethylene pressure at a constant consumption rate and concluded that the pressure of ethylene is totally independent of the value of the molar ratio r = TMEDA -BuLi for a constant alkyllithium concentration. They also reported that the catalyst efficiency (which was based on the observed and calculated molecular weights of polymer, assuming no chain transfer) was on the order of 40-50%. It should be emphasized that without TMEDA the oligomerization of ethylene does not take place, even though the monomer consumption rate and the initiator efficiency are independent of the quantity of TMEDA present in solution. Hay et al. (20, 21) suggested that TMEDA causes dissociation of the n-BuLi hexamer to give an inactive n-BuLi-TMEDA monomer. Further dissociation of this monomeric n-BuLi-TMEDA forms the free, n-BuLi monomer, which is the active species in the oligomerization reaction [Eqs. (9) and (10)]. 

The most direct method of preparing telechelic polydienes utilizes a dilithium initiator which is soluble in hydrocarbon solution [220, 221]. The most expedient method of preparing such a dilithium initiator is to react two moles of an alkyllithium compound with a divinyl compound which will not homopolymerize. Unfortunately, because of the association behavior of organolithium compounds in hydrocarbon media [176-178], many potential systems fail because they associate to form an insoluble network-like structure [221]. Expediencies such as addition of Lewis bases can overcome solubility problems of dilithium initiators, however, such additives tend to produce high amounds of 1,2- and 3,4-microstructures (see Table IV). One exception is the adduct formed from the addition of two equivalents of sec-butyllithium to l,3- i5 (l-phenylethenyl)benzene as shown in Eq. (79) [222,223]. Although this is a hydrocarbon-soluble, dilithium initiator, it was found that biomodal molecular weight distributions are obtained monomodal distributions can be obtained in the presence of lithium alkoxides or by addition of Lewis base additives [224,225].This initiator has also been used to prepare telechelic polymers in high yields [226]. 

Alkyllithium-initiated polymerizations of isoprene yield polymers with 92-93% cis-1,4 content. One industrial process uses butyllithium in a continuous reaction in two lines each consisting of four reaction kettles. The heat of the reaction is removed by vaporization of the solvent and the monomer. The catalyst solution is added to the solvent stream just before it is intensively mixed with the isoprene monomer stream and fed to the first reactor. After the stream leaves each reactor, small quantities of methanol are injected between stages into the reaction mixture. This limits the molecular weight by stopping the reaction. Fresh butyllithium catalyst is added again at the next stage in the next reactor to initiate new polymer growth [117-119]. 

Alkyllithium-initiated, anionic polymerization of vinyl monomers is a very useful synthetic method since the major variables affecting polymer properties can generally be controlled, i.e., molecular weight, molecular weight distribution, copolymer composition, diene microstructure, molecular architecture, and chain-1-3... 

N,N-diethylacrylamide (DEAAm) in polar and nonpolar solvents using alkyllithium initiators is complicated due to the presence of slow aggregation dynamics of the propagating amido enolate ion pairs similar to ester enolate ion pairs in alkyl (meth) acrylate polymerization. Attempts were made to use different initiators in combination with coordinating ligands to control the polymerization, and only minimum control on molecular weight, MWD, and the stereostructure of the polymers was obtained."""-"""... 

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