Anionic 1,3-diene monomers

The use of alkaU metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independentiy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene was described (17). Interest in alkaU metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high i7j -l,4-polyisoprene, similar in stmcture and properties to Hevea natural mbber (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE Rubber, natural). 

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

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 use of alkali melals for anionic polymerization of diene monomers is primarily of historical interest. The electron-transfer mechanism of the anionic polymerization of styrenes and 1,3-diencs initiated by alkali metals has been described in detail the dimerization of radical anion intermediates is the important step. 

Diphenylmethylcarbanions. The carbanions based on diphenyknethane (pKa = 32) (6) are useful initiators for vinyl and heterocyclic monomers, especially alkyl methacrylates at low temperatures (94,95). Addition of lithium chloride or lithium /W -butoxide has been shown to narrow the molecular weight distribution and improve the stability of active centers for anionic polymerization of both alkyl methacrylates and tert-huXyi acrylate (96,97). Surprisingly, these more stable carbanions can also efficiendy initiate the polymerization of styrene and diene monomers (98). 

Li-NMR has been widely applied to the structural characterisation of lithium initiators for anionic polymerisation. Anionic polymerisation of diene monomer with lithium... 

In anionic polymerization of vinyl monomers (nondiene), low temperatures and polar solvents favor the preparation of syndiotactic polymers.21 Nonpolar solvents tend to favor isotactic polymerization. In the case of diene monomers such as butadiene and isoprene, the use of lithium based initiators in nonpolar... 

The most important representatives of anionic polymerization centres are formed from vinyl and diene monomers. The trivial schematic representation of a carbanion... 

The highest volume commercial block copolymers are the styrene-butadiene (S-B) block copolymers. S-B block copolymers are manufactured using anionic polymerization with sequential addition of monomer (SAM) techniques. Attempts to make S-B polymers using NMRP via SAM have been limited because NMRP does not generally work well for diene monomers. Therefore, Priddy et al. 

The kinetics and mechanistic details of the lithium alkyl-initiated anionic polymerization of styrene and diene monomers in hydrocarbon solvents have been the subject of numerous investigations [15]. Some of the first investigations revealed that the propagation reaction was first order in monomer, as might be expected, but followed a fractional order in the lithium alkyl [16]. Most investigators have observed a 0.5 order for the polymerization of styrene. Values have been quoted for the polymerization of butadiene and isoprene ranging from about 0.17 to 0.5, with 0.25 being the most commonly quoted value for both monomers. There is some evidence that the order in lithium for diene polymerization... 

These initiators produce anionic propagating species by attack on the C=C double bond of vinyl and diene monomers. A common example of this type is n-butyl lithium. The C—Li bond is not ionic in hydrocarbon media where the initiator molecules exist as aggregates. The unaggregated form is more active for initiation. Butyl lithium is usually available as a solution in hexane. Addition of tetrahydrofuran to this solvent increases the concentration of the unaggregated initiator by forming a 1 1 complex with this compound. This accelerates the rate of initiation of styrene ... 

A variation of the sequential monomer addition technique described in Section 9.2.6(i) is used to make styrene-diene-styrene iriblock thermoplastic rubbers. Styrene is polymerized first, using butyl lithium initiator in a nonpolar solvent. Then, a mixture of styrene and the diene is added to the living polystyryl macroanion. The diene will polymerize first, because styrene anions initiate diene polymerization much faster than the reverse process. After the diene monomer is consumed, polystyrene forms the third block. The combination of Li initiation and a nonpolar solvent produces a high cis-1,4 content in the central polydiene block, as required for thermoplastic elastomer behavior. 

Isomerization reactions with diene monomers are very rapid in tetra-hydrofuran at room temperature [28, 29]. Within a few minutes, extensive isomerization has occurred and it is difficult to measure the rate of monomer addition to the normal anions. These processes are much slower at lower temperatures [30]. 

Although the unique features of the above polymerizations were recognized, the same was difficult to do for vinyl and diene monomers in which the much more reactive anion, that is, a car-banion, was involved. Thus, although some of the earlier studies, such as those of Robertson and Marion (10) on sodium polymerization of butadiene in toluene, and Higginson and Wooding (11) on styrene polymerization by potassium amide in liquid ammonia, demonstrated the presence of an anionic mechanism, the absence of any true termination step in these investigations was not recognized because of the presence of many transfer reactions. 

A few examples are in place. Carbanions are good initiators of polymerization of many vinyl and diene monomers. Their reactivities decrease along the series primary, secondary, tertiary. Thus, benzyl carbanion is a poor initiator of styrene polymerization, whereas the oligomeric a-methyl styrene anions or cumyl carbanions are very efficient110. ... 

Alkali Metals The direct use alkali metals and alkaline-earth metals as initiators for anionic polymerization of diene monomers as first reported in 1910 is primarily of historical interest because these are uncontrolled, heterogeneous processes [4]. One of the most significant developments in anionic vinyl polymerization was the discovery reported in 1956 by Stavely and coworkers at Firestone Tire and Rubber Company that polymerization of neat isoprene with lithium dispersion produced high di-l,4-polyisoprene, similar in structure and properties to Hevea natural rubber [47]. This discovery led to development of commercial anionic solution polymerization processes using alkyllithium initiators. 

Propagation The anionic propagation kinetics for styrene (S) polymerization with lithium as counterion is relatively unambiguous. The reaction in monomer concentration is first order, as it is for polymerization of all styrene and diene monomers in heptane, cyclohexane, benzene, and toluene [3, 55, 56], The reaction order dependence on total chain-end concentration, [PSLi]o, is one-half as shown in Equation 7.15. The observed one-half kinetic order dependence on chain-end concentration is consistent with the fact that poly(styryl)lithium is predominantly associated into dimers in hydrocarbon solution [85, 86], If the unassociated poly(styryl)lithium is the reactive entity for... 

The microstructure of anionic polymerization of other poly(l,3-diene)s with lithium as counterion in hydrocarbon media is also predominantly 1,4 microstructure [3], However, higher amounts of cm-1,4-microstructures are obtained with more sterically hindered diene monomers. Thus, using conditions that provide polyisoprene with 70% cm-1,4, 22% trans-, A, and 7% 3,4 microstructure, 2-/-propyl-1,3-butadiene and 2-n-propyl-l,3-butadiene provide 86% and 91% cm-1,4 enchainment, respectively. Both... 

EPR is currently replaced by EPDM, a modification with a diene monomer, due to its improved workability. A novel type of elastomer (called a thermoplastic elastomer) exhibits quite revolutionary behavior. Here cross-linking is temporary (at room temperature) while it can flow at higher temperatures, like thermoplastics. The typical one (SBS) is a strictly ordered block copolymer of styrene and butadiene, made by an anionic polymerization. The butadiene chains (at a controlled MW of 70,000) are embedded in a rigid phase of polystyrene spheres (MW of 15,000) thus providing temporary cross-linking at ambient conditions, while being processible at high temperatures like thermoplastics. 

Representative 1,3-diene monomers capable of living anionic polymerization are... 

Both, 2- and 4-vinylpyridines show a higher anionic polymerizability than those of styrene and 1,3-diene monomers, because the electron-deficient pyridine ring... 

Nakahama and Hirao have shown that alkyl halides (particularly bromides) react quantitatively with the living anionic polymers of styrene and 1,3-diene monomers in THE at —78°C, and subsequently applied this reaction to chain-end functionalization by using functionalized alkyl halides (Scheme 5.15)... 

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