Lithium catalyst

From the time that isoprene was isolated from the pyrolysis products of natural mbber (1), scientific researchers have been attempting to reverse the process. In 1879, Bouchardat prepared a synthetic mbbery product by treating isoprene with hydrochloric acid (2). It was not until 1954—1955 that methods were found to prepare a high i i -polyisoprene which dupHcates the stmcture of natural mbber. In one method (3,4) a Ziegler-type catalyst of tri alkyl aluminum and titanium tetrachloride was used to polymerize isoprene in an air-free, moisture-free hydrocarbon solvent to an all i7j -l,4-polyisoprene. A polyisoprene with 90% 1,4-units was synthesized with lithium catalysts as early as 1949 (5). 

Ziegler-Natta type catalysts can generate a very high cis-1,4 stmcture (>90%), which is the choice polymer for tires. It is made to specifications similar to SBR, ie, molecular weight average of 100,000—200,000, Mooney viscosity 50, and od-extended. Lithium catalysts on the other hand yield variable chain stmctures, depending on the solvent used, ie, mixed stmctures of cis-1,4 and trans-1,4 and 1,2. These polymers are generally ia the lower... 

With minor modifications that rubber is still used for passenger cars. It is not suitable for large trucks and bomber tires because of the excess of heat build-up in operation. Before the end of the rubber program, two of the companies, Firestone and Goodrich, had developed processes that produced rubber essentially like natural rubber. Firestone used a lithium catalyst for the polymerization, and Goodrich used a modified Ziegler catalyst. These materials were manufactured for a while until the oil prices became too prohibitive and the natural rubber was again used for heavy-duty tires. 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

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. 

Meanwhile, development of coordination catalyst was proceeding full scale. The polyisoprene prepared using this coordination catalyst (TiClj, AIR ) proved to be more suitable in physical properties than the one made by lithium metal or organolithium compounds in hydrocarbon media. The Ziegler polyisoprene, as it was called, has greater stereoregularity and stress-induced crystallization properties than polyisoprene made by the alkyl lithium catalyst. How-... 

As of this date, there is no lithium or alkyl-lithium catalyzed polyisoprene manufactured by the leading synthetic rubber producers- in the industrial nations. However, there are several rubber producers who manufacture alkyl-lithium catalyzed synthetic polybutadiene and commercialize it under trade names like "Diene Rubber"(Firestone) "Soleprene"(Phillips Petroleum), "Tufdene"(Ashai KASA Japan). In the early stage of development of alkyl-lithium catalyzed poly-butadiene it was felt that a narrow molecular distribution was needed to give it the excellent wear properties of polybutadiene. However, it was found later that its narrow molecular distribution, coupled with the purity of the rubber, made it the choice rubber to be used in the reinforcement of plastics, such as high impact polystyrene. Till the present time, polybutadiene made by alkyl-lithium catalyst is, for many chemical and technological reasons, still the undisputed rubber in the reinforced plastics applications industries. 

The unique feature about anionic polymerization of diene to produce homopolymer was that the microstructure of the homopolymer could be altered and changed at will to produce unique physical and chemical properties. These microstructural changes can be introduced before, after or during the polymerization. For example, chelating diamines, such as tetramethyl ethylene and diamine (TMEDA) (18), with the alkyl-lithium catalyst have been used to produce polymer with 80 1,2 addition products, while the use of dipiperidine ethane (DPE),with same catalyst has produced polybutadiene with 100 1,2 addition product. 

Korotkov and Rakova (53) found that butadiene was more active in copolymerization with isoprene with lithium catalyst, although in homopolymerization isoprene is three times faster. Korotkov and Chesnokova (33) studied the copolymerization of butadiene and styrene with n-butyl lithium in benzene. Butadiene polymerized before much of the styrene was consumed. They claimed the formation of block polymers consisting initially of polybutadiene and the polystyrene chain attached. 

Lithium and magnesium alkyl catalysts yield metal-polymer bonds with appreciable covalent character and their cations coordinate strongly with nucleophiles. Therefore, these catalysts will initiate simple anionic polymerization only under the most favorable conditions, e. g., in basic solvents and with monomers which produce resonance stabilized polymer anions. As examples of stereoregular anionic polymerization, a-methyl-methacrylate yields syndiotactic polymer with an alkyl lithium catalyst in 1,2-dimethoxyethane at — 60° C. (211, 212) or with a Grignard catalyst at -40° C. (213). 

Organo lithium catalysts have been used successfully for stereospecific polymerization of a variety of vinyl monomers and diolefins. They have been covered thoroughly in recent reviews (191,192,195,234) and will not be discussed in detail here except to illustrate some of the evidence which supports a cationic attack by lithium on monomer. 

Lithium catalysts are far more stereospecific than the other alkali metal catalysts and are effective even in homogeneous system because of the strong coordinating ability of the lithium. Bawn and Led with (192) have included in their review an excellent discussion on the mechanism of polymerization with lithium catalysts. The key feature is the coordination of the olefinic zr-electrons with vacant s- or -orbitals in the lithium prior to an intramolecular rearrangement involving migration of the carbanion to the more electrophilic carbon of the polarized monomer. 

Butenyllithium and butenylmagnesium chloride were used as "dynamic allylic compounds. The former was selected because of the ability of lithium catalysts to provide high rates of diene polymerization and to give stereoregular polymers the latter was selected for its availability and simplicity of synthesis. 

Both the monomer and polymer are soluble in the solvent in these reactions. Fairly high polymer concentrations can be obtained by judicious choice of solvent. Solution processes are used in the production of c(5-polybutadiene with butyl lithium catalyst in hexane solvent (Section 9.2.7). The cationic polymerization of isobutene in methyl chloride (Section 9.4.4) is initiated as a homogeneous reaction, but the polymer precipitates as it is formed. Diluents are necessary in these reactions to control the ionic polymerizations. Their use is avoided where possible in free-radical chain growth or in step-growth polymerizations because of the added costs involved in handling and recovering the solvents. 

Table IV lists some diene polymers prepared by homogeneous catalysis. The cobalt catalyst for butadiene and the lithium catalyst for butadiene and isoprene are believed to be used commercially in the United States to prepare the so-called stereoelastomers (32, 36).

Lithium catalysts exhibit particularly high stereospecificity with isoprene. There is some disagreement in the literature regarding the mechanism of polymerization. The medium must be inert for maximum stereospecificity—i.e., it should be incapable of coordinating with the catalyst. Association of the catalyst is important in the kinetic scheme, but present thinking is that the butyllithium monomer alone initiates polymerization of isoprene and that propagation likewise involves only a... 

Lithium compounds are also used as catalysts in many different industrial processes. A catalyst is a substance used to speed up a chemical reaction. The catalyst does not undergo any change itself during the reaction. For example, one lithium catalyst is used to make tough, strong, synthetic (artificial) rubber. It does not have to be vulcanized (heat-treated) like natural rubber. 

Solution-polymerized SBR is made by termination-free, anionic/live polymerization initiated by alkyl lithium compounds. Other lithium compounds are suitable (such as aryl, alkaryl, aralkyl, tolyl, xylyl lithium, and ot/p-naphtyl lithium as well as their blends), but alkyl lithium compounds are the most commonly used in industry. The absence of a spontaneous termination step enables the synthesis of polymers possessing a very narrow molecular weight distribution and less branching. Carbon dioxide, water, oxygen, ethanol, mercaptans, and primary/secondary amines interfere with the activity of alkyl lithium catalysts, so the polymerization must be carried out in clean, near-anhydrous conditions. Stirred bed or agitated stainless steel reactors are widely used commercially. 

More recently, the asymmetric hydroamination/cyclization of amino substituted stilbenes was studied utilizing chiral bisoxazoline lithium catalysts [73]. Enantios electivities reaching as high as 91% ee were achieved (Scheme 11.13). The reactions were performed in toluene at 60 °C to give the exo cyclization product 43 under... 

Anionic Polymerizaticm of Ethyloie Oxide with Lithium Catalysts... 

The product studied was produced in THF with a diphenylphos-phine-lithium catalyst it had a molecular weight of 8300. After shear modulus plotting over temperature, a glass stage of 81 °C, a modulus of elasticity of 32,000 kg/cm2 (poly styrene 30,000), and a flexural strength of 661 kg/cm2 (polystyrene 1000) were found. The glass temperature was 20 °C lower than that of polystyrene, but the polymer is more resistant to swelling by aromatics. 

Butadiene polymerization studies with HMTT/CH2Li catalysts have given results which are directly contrary to those expected from the Hay mechanism. Activity at 0.5 HMTT/CH2Li was double that at 1 1 ratio whereas the reverse should have been obtained if the tetramine solvated lithium compound were the active species. Our lithium catalyst studies suggest that all of the known TMED complexes are active for butadiene polymerization with activity increasing roughly in the order (RLi)4 TMED < (RLi)2 TMED < RLi TMED << RLi(TMED)2. The equilibrium to form RLi(TMED )2 is believed to be unfavorable except when R" is a highly delocalized carbanion. 

For ethylene polymerization with lithium catalysts the most probable chain transfer reactions include (1) monomolecular elimination of LiH, (2) bimolecular displacement of the polymer by monomer, (3) metalation of monomer, and (4) metalation of solvent as shown below. 

The work of Boileau38 with lithium catalysts and highly specific cation complexing compounds is particularly significant. The author describes the use of the macrobicyclic ligand or cryptate which forms multicontact complexes with the metal counter-ion of the... 

In 1961, using lithium catalyst, a series of sty-rene-isoprene (SI) and styrene-butadiene (SB) block copolymers were synthesized [Bull and Holden, 1977]. The resins had T = -90 to -i-90°C. Full-scale production started in 1965. Since then, numerous two- and three- block copolymers have been developed. More recently, hydrogenated and during the last few years maleated block copolymers are being offered. With the world consumption of 330 kton/y, the block copolymers constitute the largest part of the commercial TPE market. Large quantity of SBS resin is used... 

One method makes use of alkali-metal catal rst under anhydrous conditions. Molten caprolactam polymerizes very rapidly in the presence of sodium or lithium catalyst. The reaction rate in this case suggests a true addition polymerization, although the mechanism is not known. 

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