Lithium initiators

Many random copolymers have found commercial use as elastomers and plastics. For example, SBR (62), poly(butadiene- (9-styrene) [9003-55-8] has become the largest volume synthetic mbber. It can be prepared ia emulsion by use of free-radical initiators, such as K2S20g or Fe /ROOH (eq. 18), or in solution by use of alkyl lithium initiators. Emulsion SBR copolymers are produced under trade names by such companies as American Synthetic Rubber (ASPC), Armtek, B. F. Goodrich (Ameripool), and Goodyear (PHoflex) solution SBR is manufactured by Firestone (Stereon). The total U.S. production of SBR in 1990 was 581,000 t (63). 

The research programme into n-butyl lithium initiated, anionic polymerization started at Leeds in 1972 and involved the construction of a pilot scale, continuous stirred tank reactor. This was operated isothermally, to obtain data under a typical range of industrial operating conditions. 

A pilot scale plant, incorporating a three litre continuous stirred tank reactor, was used for an investigation into the n-butyl lithium initiated, anionic polymerization of butadiene in n-hexane solvent. The rig was capable of being operated at elevated temperatures and pressures, comparable with industrial operating conditions. 

It became apparent that the mechanism of anionic polymerisation induced by the alkali metals should also involve initiation by electron transfer from the metal to the monomer. For example, in the Lithium initiated polymerisation of butadiene, we have,... 

Some of the earliest concepts came from Japan, where Matsuchita developed the Li/(CF) battery that was used, for example, in fishing floats. Lithium fluoride and carbon are the final reaction products, but the cell potential of 2.8—3.0 V suggests a different electrochemical reaction. It was proposed that lithium initially intercalates the carbon monofluoride lattice and subsequently the lithium fluoride is formedF Li + (CF)n — L CF)n C + LiF. Although much work... 

Apart from the development of lithium initiators for the facile polymerization of L-lactide, mainly by our group, there are only a limited number of Na and K initiators known in the literature. Sodium and potassium cations are nontoxic and are essential to life, and we reported the first EDBP-Na complex as efficient initiator for the preparation of PLA [41]. Similarly, potassium EDBP complex [EDBPH-K-(THE)2] 18 has been demonstrated to be an efficient catalyst for the ROP of L-lactide in a controlled fashion, yielding PLAs with expected molecular weights and moderate PDIs (1.29-1.58) [42],... 

The structure-reactivity behavior found for similar organosodium polymerization initiators of styrene [27] or that for addition reactions with 1,1-diphenylethylene [28] is almost identical with that found for the lithium initiators of Table 3.1. It is interesting to note from Table 3.1 that the reactivity of lithium... 

CTPB. Ultimately, prepolymers were developed with carboxyl groups in the terminal positions to take full advantage of the entire length of the polymer chain. These butadiene prepolymers were synthesized by a free radical- or lithium-initiation technique to an average molecular weight of 3500-5000 and a nearly bifunctional structure. These attributes provided for substantially improved mechanical behavior of highly loaded solid propellants, particularly at low temperatures. Pro-... 

Lithium-Initiated Prepolymers. The preparation of prepolymers by the organolithium technique (11) follows the reaction ... 

Microstructure of Polybutadienes. Microstructure strongly influences the viscosity of the CTPB prepolymer. The viscosity of CTPB increases with increased vinyl content, but for CTPB prepolymers of the required molecular weight, an upper limit of 35% vinyl groups is satisfactory from the standpoint of propellant processing characteristics. It has also been found that the microstructure changes markedly with the synthesis process. Lithium-initiated polymerization yields prepolymers with slightly higher vinyl content than those produced by free radical initiation. 

The microstructure of polyisoprene prepared by lithium initiation in hydrocarbons is 95% 1,4 under all conditions. The trans 1,4 content however falls from about 20% to zero as the monomer/initiator ratio increases leading finally to a 95% cis 1,4 polymer. This variation can be explained with the following scheme. 

Thus kp for lithium counterion is 1/300 of kp for potassium counterion. The low reactivity and association of lithium alkoxide was reported in the anionic polymerization of epoxides.We have found that two fold increase of the lithium initiator concentration has led to a decrease of the kp nearly to one half. This indicates that the kinetic order with respect to the initiator would be near to zero, suggesting a very high degree of association of the active species. Thus the propagation reaction appears to proceed in practice through a very minor fraction of monomeric active species in case of lithium catalyst. 

These efforts coupled with the much earlier work on sodium and lithium initiated polymerizations led to an appreciation of the stereospecificity of the alkyllithium initiators for diene polymerization both industrially and academically. Polymerization of isoprene to a high cis polyisoprene with butyllithium is well known and the details have been well documented 2 Control over polybutadiene structure has also been demonstrated. This report attempts to survey the unique features of anionic polymerization with an emphasis on the chemistry and its commercial applications and is not intended as a comprehensive review. 

To circumvent the low reactivity, low solubility and/or low yield problems associated with the previously mentioned hydroxyl functional initiators, we (15. 14) have built protective hydroxyl functionality into organolithium molecules. Specifically, alkyl" lithium initiators containing hydroxyl protecting groups have been prepared. These protected functional initiators contain acetals, i.e., tetrahydropyranyl... 

To produce linear or branched polymers with all ends having a high vinyl content, an organo lithium initiator can be used to polymerize a low vinyl middle segment. Subsequently, additional monomer and a vinyl modifier are added to produce ends having a high vinyl content. 

This method avoids the need for a coupling agent. However, organo lithium initiators are expensive and unstable. Instead, organic aluminum boron compounds can be used as initiators (12). 

When a mixture of styrene and 1,3-butadiene (or isoprene) undergoes lithium-initiated anionic polymerization in hydrocarbon solution, the diene polymerizes first. It is unexpected, since styrene when polymerized alone, is more reactive than, for example, 1,3-butadiene. The explanation is based on the differences of the rates of the four possible propagation reactions the rate of the reaction of the styryl chain end with butadiene (crossover rate) is much faster than the those of the other three reactions484,485 (styryl with styrene, butadienyl with butadiene or styrene). This means that the styryl chain end reacts preferentially with butadiene. 

Graver, J.T., Kraus,G. Rheological properties of polybutadienes prepared by n-butyl-lithium initiation. J. Polymer Sci. Pt. A 2,797-810 (1964). 

III. Polymerization of non-polar monomers in non-polar solvents (benzene, hexane, heptane, cyclohexane) a) Metallic lithium initiation... 

With these monomers polymerization again is complicated by the possibility of side reactions and termination reactions. In the butyl-lithium initiated polymerization of methylmethacrylate in tetrahydrofuran at —78°, monomer added repeatedly after quite long time intervals will polymerize rapidly (117). This is found to be true even if the solution is warmed to room temperature after each addition of monomer. The possibility remains that some termination occurs and that some reinitiation by lithium methoxide is also important. Similar experiments with fluorenyllithium under the same conditions (40) show that after 30 minutes a large percentage of the polymer chains is reactive towards tritiated acetic add. Addition of a second batch of monomer at —78° indicates that some chain termination occurs at this temperature but that no new chains are formed by re-initiation. Some other low molecular... 

Perhaps even more important in children is the issue of bipolar disorder. Mania and mixed mania have not only been greatly underdiagnosed in children in the past but also have been frequently misdiagnosed as attention deficit disorder and hyperactivity. Furthermore, bipolar disorder misdiagnosed as attention deficit disorder and treated with stimulants can produce the same chaos and rapid cycling state as antidepressants can in bipolar disorder. Thus, it is important to consider the diagnosis of bipolar disorder in children, especially those unresponsive or apparently worsened by stimulants and those who have a family member with bipolar disorder. These children may need their stimulants and antidepressants discontinued and treatment with mood stabilizers such as valproic acid or lithium initiated. 

Lithium-initiated copolymerization of styrene and methyl methycrylate. 

Some adducts may reduce the rate of propagation to such a low value that the pertinent chain will be considered "dead", at least within the time of the experiment. Studies of Welch (50) have shown that butyl lithium initiated growing polystyrene may associate with BuLi in benzene solution, and the associated ends are too unreactive to contribute to the polymerization. However, the associated ends are in equilibrium with the... 

Welch, F. J. Effect of Lewis acids and bases on the rate of butyl lithium initiated polymerization of styrene. Read in St. Francisco ACS Meeting. 

C-NMR spectroscopy has shown that the polybutadienes prepared using alkyl-lithium initiators have random placement of the different modes of enchainment 222-223). This contrasts with an earlier claim of blocky structures 224). Random sequence distribution has also been established for polyisoprene by 1H-NMR 225) and 13C-NMR 226) spectroscopy. 

Hydroxyl-terminated polymers have also been prepared 337 338) using organo-lithium initiators with protected hydroxyl functionality. Thus, using initiators such as 2-(6-lithio-n-hexoxy)tetrahydropyran (A) and ethyl 6-lithiohexyl acetaldehyde acetal (B), it was... 

Boileau and coworkers11 have used a novel trimethylsilylmethyl lithium initiator MesSiCTDLi (1), in combination with a cryptand [211], for the ring-opening polymerization of cyclosiloxanes. Initiation of hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) polymerization has been followed by H, 7Li, 13C and 29Si NMR. 

The earliest mention of lithium initiation of diene polymerization was in 1910-1913 (1-3). It is quite likely that this was conceived in the fertile imagination of a patent lawyer, rather than the scientists involved, as all of the experimentation reported involved sodium and potassium. 

The development of organometallic initiators, both of the lithium type and of the transition-metal coordination type, occurred rapidly in the decade following the late 1950s. The lithium initiators were developed without the fanfare of coordination-type initiators. This situation developed because of the remarkable ability of the coordination catalysts to induce stereospecific polymerization of a-olefins. 

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