Polymerization catalysts alkyllithium

Although alkyllithiums are used mainly in these N-chelated complexes, other alkali metal alkyls may also be used. For example, organo-sodium reagents have been solubilized in hydrocarbon solvents by chelating tertiary diamines and used as polymerization catalysts by workers at Borg-Wamer Corp. (22),... 

Natural rubber (hevea) is 98% c/ -l,4-polyisoprene with 2% 3,4-structure. It can be synthesized by anionic polymerization with alkyllithium compounds or with Ziegler-Natta catalysts [220-225]. The polymerization is carried out in solvents. Impurities such as acetylenes, carbonyl compounds, hydrogen sulfide, and water have to be removed [217,226-228]. 

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. 

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. 

Smith (29) showed that the polymerization of styrene by sodium ketyls with excess sodium produced low yields of isotactic polystyrene. Smith also believed that sodium ketyls initiated the styrene polymerization in the same way as the anionic alfin catalyst. Das, Feld and Szwarc (30) proposed that the lithium naphthalene polymerization of styrene occured through an anionic propagating species arising from the dissociation of the alkyllithium into ion pairs. These could arise from the dimeric styryllithium as a dialkyllithium anion and a lithium cation... 

Alkyllithium-transition metal halide catalysis is completely different from the sodium ketyl and alfin catalysis. Natta, Danusso, Scanesi and Macchi (36) have found that the polymerization of styrene and substituted styrenes by titanium tetrachloride-triethyl aluminum catalysts was different from the above anionic systems. A plot of the log of the rate of the polymerization against Hammett s sigma constant produced a straight line with a rho value of —1.0. Electron releasing groups facilitated this polymerization. 

Tsou, Magee and Malatesta (39) showed the effect of catalyst ratios on steric control m the polymerization of styrene with alkyllithium and titanium tetrachloride. These authors have shown that the isotactic polymer was produced when the butyllithium to titanium ratio was kept within the limits of 3.0 to 1.75. Outside of this critical range, amorphous polymers were produced. In the discussion of this paper, Friedlander (40) pointed out the cationic nature of the low-lithium-to-titanium-ratio-catalysts which also produced considerable rearrangement of the phenyl groups. Above 2.70 lithium to titanium ratio, an anionic type polymerization set in, which produced atactic polymer. At low ratios cationic catalysis also produced atactic polymer. Tsou and co-workers concluded that crystallinity of the catalyst is not important for steric order in the polymer. 

Syndiotactic 1.2 polybutadiene has also been made by Longiave and Castelli (49) using an anionic cobalt catalyst made from oxygenated aluminum compounds. Less amounts of 1.2-structure were found in polymerizations in hydrocarbon media. Alkyllithium produced only 6.8% 1.2-structure with the remainder being 1.4 cis and trans. 

The relative ionic nature of the catalyst required for these monomers has been determined. Spirin, Pres-Yakubovich, Polyakov, Gant-makher and Medvedev (57) studied the alkyl lithium polymerization of styrene, isoprene and butadiene. At high alkyllithium concentrations, styrene polymerized more rapidly than either isoprene or butadiene. As the ionicity was decreased by reducing the alkyllithium concentrations to about 10 moles per liter, the rates of polymerizations of the monomers were nearly the same. 

The work by Morton and Ells (60) showed that this difference in reactivity was due to differences in the rate with which the different monomers reacted with the different alkyllithiums (styryl or butenyl). Styryllithium ends reacted rapidly with butadiene, but a butenyl-lithium end reacted quite slowly with styrene. Butadiene was polymerized to near exclusion of styrene during the initial part of the reaction. Special solvation of the catalyst by the polymerizing butadiene was not the cause of this copolymerization. 

In solution-based polymerization, use of the initiating anionic species allows control over the trans/cis nricrostructure of the diene portion of the copolymer. In solution SBR, the alkyllithium catalyst allows the 1,2 content to be changed with certain modifying agents such as ethers or amines. Anionic initiators are used to control the molecular weight, molecular weight distribution, and the microstructure of the copolymer... 

The lithium and alkyllithium initiation of diene polymerization has, from the earliest times, remained in the shadow of other, apparently more important, initiator systems. However, it has now become clear that the alkyllithium catalyst is the most efficient, initiator system at present available for diene polymerization. That organolithium initiators are not used much more widely is due largely to economic considerations,... 

All of the studies published to date fail to identify the active catalyst species present in the RLi-TMEDA polymerization of ethylene. The kinetics data of both Hay and Shud and their co-workers fall short in this respect. A more fundamental approach is needed. It may be appropriate at this time to study the 13C- and 7Li-NMR of alkyllithium compounds in the presence of various chelating diamines and polar modifiers such as THF and dimethyl ether. 

The discovery of the ability of lithium-based catalysts to polymerize isoprene to give a high cis 1,4 polyisoprene was rapidly followed by the development of alkyllithium-based polybutadiene. The first commercial plant was built by the Firestone Tire and Rubber Company in 1960. Within a few years the technology was expanded to butadiene-styrene copolymers, with commercial production under way toward the end of the 1960s. 

For instance, in the field of elastomers, alkyllithium catalyst systems are used commercially for producing butadiene homopolymers and copolymers and, to a somewhat lesser extent, polyisoprene. Another class of important, industrial polymerization systems consists of those catalyzed by alkylaluminum compounds and various compounds of transition metals used as cocatalysts. The symposium papers reported several variations of these polymerization systems in which cocatalysts are titanium halides for isoprene or propylene and cobalt salts for butadiene. The stereospecificity and mechanism of polymerization with these monomers were compared using the above cocatalysts as well as vanadium trichloride. Also included is the application of Ziegler-Natta catalysts to the rather novel polymerization of 1,3-pentadiene to polymeric cis-1,4 stereoisomers which have potential interest as elastomers. 

Medium-c/5 lithium-polybutadiene was first developed by Firestone Tire and Rubber Company in 1955 [86]. Solution polymerization using anionic catalysts is usually based on butyllithium. Alkyllithium initiation does not have the high stereospecificity of the coordination catalysts based on titanium, cobalt, nickel, or neodymium compounds. Polymerization in aliphatic hydrocarbon solvents such as hexane or cyclohexane yields a polymer of about 40 % cis, 50 % trans structure with 10 % 1,2-addition. However, there is no need for higher cis content because a completely amorphous structure is desired for mbber applications the glass transition temperature is determined by the vinyl content. The vinyl content of the polybutadiene can be increased up to 90 % by addition of small amounts of polar substances such as ethers. 

Alkyllithium initiators offer some peculiarities in contrast to the coordination catalysts [41]. Alkyllithium initiation can tolerate very high temperatures. As expensive cooling facilities are not needed, the polymerization can proceed at high reaction rates with low investment and operating costs. Since the polymerization runs without termination or other side reactions under formation of living polymers , the preparation of block polymers by sequential addition of monomers is possible. It also permits the introduction of functional groups on the end of each chain. Because the initiation step is fast relative to the pro-... 

Conjugated Dienes and Other Monomers. Alkyllithiums such as n-butyllithium—and even the growing polyethylene carbon-lithium bond complexed with chelating diamines such as TMEDA—are effective initiators for the polymerization of conjugated dienes such as 1,3-butadiene and isoprene. A polybutadiene of high 1,2-content can be produced from butadiene in hydrocarbon solvents using these N-chelated organolithium catalysts. 

Langer (13) has also disclosed the use of alkyllithium and dialkyl-magnesium tertiary diamine complexes as catalysts for copolymerization of ethylene and other monomers such as butadiene, styrene, and acrylonitrile to form block polymers. Examples are given in which polybuta-dienyllithium initiates a polyethylene block, as well as vice-versa. Random copolymers of these two were also prepared, and other investigators have used not only tertiary diamines but hexamethylphosphoramide (14) and tetramethylurea (15) as nitrogenous base cocatalysts in such polymerizations. Antkowiak and co-workers (11) showed the similarity of action of diglyme and TMEDA in copolymerizations of styrene and... 

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