Lithium chloride, anionic 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. 

Anionic polymerization of vinyl monomers can be effected with a variety of organomciallic compounds alkyllithium compounds are the most useful class. A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds ate soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. 

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). 

Zune and coworkers studied the structure of the species propagating the anionic polymerization of tBMA by NMR spectroscopy . The spectrum of the lithium ester enolate was perturbed by LiCl, as result of an equilibrium established between free lithium chloride and complexed active end-groups. The structure of the chain-end was not modified by a large excess of LiCl. 

Most Grignard reagents are inert toward styrene (up to the temperature of spontaneous thermal polymerization). This is a significant difference from lithium alkyls, which are readily able to initiate styrenic monomers [123]. The only reported exception is p-vinylbenzyl magnesium chloride, which polymerized styrene in THF at O C, but not at — 78X [50,51]. Substitution at the puru-position of a phenyl ring may stabilize the benzyl anion, owing to the delocatlization of electrons, and favor ionic dissociation of... 

ABSTRACT. A new class of protected hydroxyl containing functionalized initiators were recently disclosed by the Defense Evaluation and Research Agency (DERA). These novel initiators have the general structure TBS-0-(CH2)n-Li. Excellent solubility in hydrocarbon solvents was exhibited by these materials which allowed the preparation of telechelic, high 1,4-microstructure polybutadienes. The two-step synthesis of these functionalized initiators from commercially available raw materials will be presented in detail. The first step involved reaction of an omega-haloalcohol with /-butyldimethylsilyl chloride, in the presence of an acid acceptor, to form the precursor. This precursor was then reacted with lithium metal in a hydrocarbon solvent to afford a solution of the functionalized initiator. The thermal stability of these initiators in hydrocarbon solution will also be presented. The application of the precursors and functionalized initiators in anionic polymerization of dienes will be briefly discussed. 

The seminal work of Schulz and co-workers on anionic polymer initiators which contain protected hydroxyl functionality was reported in 1974." These researchers prepared 2-(6-lithio- -hexyloxy)tetrahydopyran by metal-halogen exchange in diethyl ether, see Figure 1. The lithium chloride co-product was removed by filtration. This initiator was successfully employed in the polymerization of 1,3-butadiene. The resultant functionalized living anion was subsequently functionalized with ethylene oxide or coupled with dimethyldichlorosilane. Mild acid hydrolysis with dichloroacetic acid liberated the telechelic dihydroxy polybutadiene. The polybutadienes produced with this initiator exhibited narrow molecular weight distributions = 1.05-1.08). 

A dramatic development in the anionic polymerization of acrylate and methacrylate monomers was the discovery that by addition of lithium chloride it was possible to effect the controlled polymerization of f-butyl acrylate [122]. Thus, using oligomeric (a-methylstyryl)lithium as initiator in THE at -78 °C, the molecular weight distribution of the... 

A dramatic development in the anionic polymerization of acrylate and methacrylate monomers was the discovery that by addition of lithium chloride it was possible to effect the controlled polymerization of f-butyl acrylate (86). Thus, using oligomeric (o -methylstyryl)lithium as initiator in THF at —78°C, the molecular weight distribution (M /Mn) of the polymer was 3.61 in the absence of lithium chloride but 1.2 in the presence of lithium chloride ([LiCl]/[RLi] = 5). In the presence of 10 equiv of LiCl, f-butyl acrylate was polymerized with 100% conversion and 95% initiator efficiency to provide a polymer with a quite narrow molecular weight distribution (My,/Mn = 1.05). More controlled anionic polymerizations of alkyl methacrylates are also obtained in the presence of lithium chloride. Other additives, which promote controlled pol5unerization of acylates and methacrylates, include lithium f-butoxide, lithium (2-methoxy)ethoxide, and crown ethers (47,48). The addition of lithium chloride also promotes the controlled anionic polymerization of 2-vinylpyridine. 

Teyssie and coworkers found that the anionic polymerization ( Living Polymer-formation ) of methacrylate and acrylate esters in the presence of lithium chloride along with the anionic initiator results in polymers with particularly uniform molecular weight distributions. By this technique, di-and tri-block copolymers and star-block copolymers may readily be prepared. If rerr-butyl acrylate (or methacrylate) is one of the comonomers, hydrolysis of such copolymers with a small amount of / -toluenesulfonic acid leads to a copolymer containing acrylic (or methacrylic) acid [25-27]. 

Specific Interactions of Lithium Chloride in the Anionic Polymerization of Lactams... 

Ring-Opening Anionic Polymerization The Role of Lithium Chloride... 

Corresponding results have been obtained in the anionic polymerization of pyrrolidone in bulk at 30 Citigher initial rates have been found when lithium pyrrolidonate instead of sodium pyrro11 donate was used as catalyst. Here again, the addition of metallic sodium to the mixture of pyrrolidone and lithium chloride % by weiaht) causes the separation of sodium chloride which gives a characteristic opalescence to the solution. 

It appears,therefore, that in order to study the "true" role of lithium chloride on the kinetics and mechanism of the anionic polymerization of lactams, it is advisable to useasa catalyst of the reaction a metal lactamate which does not undergo chemical exchanges with Li ions. 

The activity of transition metal allyl compounds for the polymerization of vinyl monomers has been studied by Ballard, Janes, and Medinger (10) and their results are summarized in Table II. Monomers that polymerize readily with anionic initiators, such as sodium or lithium alkyls, polymerize vigorously with allyl compounds typical of these are acrylonitrile, methyl methacrylate, and the diene isoprene. Vinyl acetate, vinyl chloride, ethyl acrylate, and allylic monomers do not respond to these initiators under the conditions given in Table II. 

---