Alkyllithium polymerization initiators

Polymerization inhibitors miscellaneous, 23 383 in styrene manufacture, 23 338 Polymerization initiators alkyllithiums as, 74 251 cerium application, 5 687 peroxydicarbonates as, 74 290 Polymerization kinetics, in PVC polymerization, 25 666-667 Polymerization mechanism, for low density polyethylene, 20 218 Polymerization methods, choice of,... 

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

Alkyllithium compounds as well as polymer-lithium associate not only with themselves but also with other alkalimetal alkyls and alkoxides. In a polymerization initiated with combinations of alkyllithiums and alkalimetal alkoxides, dynamic tautomeric equilibria between carbon-metal bonds and oxygen-metal bonds exist and lead to propagation centers having the characteristics of both metals, usually somewhere in between. This way, one can prepare copolymers of various randomness and various vinyl unsaturation. This reaction is quite general as one can also use sodium, rubidium or cesium compounds to get different effects. 

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. 

The relative reactivities of alkyllithiums as polymerization initiators are intimately linked to their degree of association. In the following the average degree of association in hydrocarbon solution, where known, is indicated in brackets after the alkyllithium. For sryrene polymerization,... 

Alkyllithium initiation can tolerate very high temperatures. As refrigeration is not needed, alkyllithium polymerization can proceed at high reaction rates with low investment and operating costs. 

Nucleophilic attack of carbanions on silicon in silacyclobutane rings results in breaking of the Si-C bond in the ring leading to the recovery of the carbanionic center. Disilacyclobutane 38 was polymerized by addition of alkyllithium as initiator in tetrahydrofuran (THF) at — 78 °G in the presence of hexamethylphosphoramide (HMPA) acting as an activator (Equation 14) <2000MI805>. 

Formation of a dihydropyridine -adduct at the chain-end was shown by NMR and UV" spectral analysis. Anderson and collaborators proposed that the actual initiator would be adduct 34, formed by reaction of s-BnLi with pyridine (equation 40), which implies that the a-end-group of PMMA is a dihydropyridine group. This hypothesis was not experimentally confirmed. However, in the specific case of the e-caprolactone (3, n = 4) polymerization initiated by the BnLi/pyridine addnct, no characteristic NMR signal of dihydropyridine could be detected. The polymerization mechanism was therefore revised, based on the alkyllithium as the actual initiator and the establishment of an equilibrium between an active uncomplexed enolate (35) and a dormant cr-complex (36) as the basis for polymerization control (equation 41)" . 

Measurement of reactivity ratios under normal free-radical and CCT polymerization conditions indicates that CCT is a modified free-radical polymerization as expected.434 The reactivity ratios for MMA and butyl methacrylate were used as a mechanistic probe. Reactivity ratios were 1.04 and 0.81 for classical anionic polymerization, 1.10 and 0.72 for alkyllithium/trialkylaluminum initiated polymerization, 1.76 and 0.67 for group transfer polymerization, 0.98 and 1.26 for atom transfer radical polymerization, 0.75 and 0.98 for CCT, and 0.93 and 1.22 for classical free-radical polymerization. These ratios suggest that ATRP and CCT proceed via radical propagation. 

Recently, a group at the Defense Evaluation and Research Agency (DERA) in the U.K. reported the preparation of a group of compounds that were employed as polymerization initiators, omegar-(/-butyldimethylsilyloxy)-l-alkyllithiums. In contrast... 

Bywater, S., Worsfold, D.J., 1967a. Alkyllithium anionic polymerization initiators in hydrocarbon solvents. J. Organomet. Chem. 10 (1), 1-6. 

The ABA triblocks which have been most exploited commercially are of the styrene-diene-styrene type, prepared by sequential polymerization initiated by alkyllithium compounds as shown in Eqs. (99-101) [263, 286]. The behavior of these block copolymers illustrates the special characteristics of block (and graft) copolymers, which are based on the general incompatibility of the different blocks [287]. Thus for a typical thermoplastic elastomer (263), the polystyrene end blocks (-15,000-20,000 MW) aggregate into a separate phase, which forms a microdispersion within the matrix composed of the polydiene chains (50,000-70,000 MW) [288-290]. A schematic representation of this morphology is shown in Fig. 3. This phase separation, which occurs in the melt (or swollen) state, results, at ambient temperatures, in a network of... 

In this work, a series of diblock polymers were used as dispersing agents to stabilize the polystyrene particles. The method of polymerization is by anionic techniques using alkyllithiums as initiators and alkanes as... 

The polyaminophosphazene base t-BuP4 was used in combination with alkyllithium as initiator for the anionic polymerization of EO (Scheme 15(a)). The space inside the molecule is sufficient to host the compact lithium cation and the base works as a cryptand for Li ions with the polar amino and imino groups located inside the globular molecule and the outer shell formed by alkyl substituents. The equilibrium between complexed lithium alkoxide ion pairs and reactive free anions is thus shifted allowing polymerization. 

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. 

Commercially, the poly(styrene-Aelastomer-Astyrene) materials are made by anionic polymerization (7,45—47). An alkyllithium initiator (RLi) first reacts with styrene [100-42-5] monomer ... 

Polymerization of butadiene using anionic initiators (alkyllithium) in a nonpolar solvent produces a polymer with a high cis configuration. A high cis-polybutadiene is also obtained when coordination catalysts are used. 

Reaction Mechanism. The reaction mechanism of the anionic-solution polymerization of styrene monomer using n-butyllithium initiator has been the subject of considerable experimental and theoretical investigation (1-8). The polymerization process occurs as the alkyllithium attacks monomeric styrene to initiate active species, which, in turn, grow by a stepwise propagation reaction. This polymerization reaction is characterized by the production of straight chain active polymer molecules ("living" polymer) without termination, branching, or transfer reactions. 

Anionic polymerization of 1,3-disilacyclobutanes also is possible. Solid KOH and alkali metal silanolates were mentioned as being effective by Russian authors [18, 19. 20]. However, alkyllithiums, which can initiate polymerization of silacyclobutanes (eq. 8) [21], do not initiate polymerization of 1,3-disilacyclobutanes [18, 22]. The problem is one of steric hindrance. 

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. 

In order to avoid the SET process, we chose diphenylmethylsilyl anions (PI MeSiM 8a, M = K 8b, M = Na 8c, M = Li) as initiators for 7 instead of alkyllithium and benzene as a solvent. The polymerization did not take place in benzene with silyl anions alone. However, in the presence of an equimolar amount of suitable cryptands, the silyl anions initiated the polymerization. The results are summarized in Table 2. The molecular weights of polysilylenes thus obtained were in good agreement with the calculated values within experimental error. 

Anionic polymerization can be initiated by a variety of anionic sources such as metal alkoxides, aryls, and alkyls. Alkyllithium initiators are among the most useful, being employed commercially in the polymerization of 1,3-butadiene and isoprene, due to their solubility in hydrocarbon solvents. Initiation involves addition of alkyl anion to monomer... 

The most studied catalyst family of this type are lithium alkyls. With relatively non-bulky substituents, for example nBuLi, the polymerization of MMA is complicated by side reactions.4 0 These may be suppressed if bulkier initiators such as 1,1-diphenylhexyllithium are used,431 especially at low temperature (typically —78 °C), allowing the synthesis of block copolymers.432,433 The addition of bulky lithium alkoxides to alkyllithium initiators also retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised.427 LiCl has been used to similar effect, allowing monodisperse PMMA (Mw/Mn = 1.2) to be prepared at —20 °C.434 Sterically hindered lithium aluminum alkyls have been used at ambient (or higher) temperature to polymerize MMA in a controlled way.435 This process has been termed screened anionic polymerization since the bulky alkyl substituents screen the propagating terminus from side reactions. 

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). 

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