Alkyllithium copolymer

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. 

Titanocene (Cp2TiR2) /alkyllithium (LiR) Styrene, butadiene or isoprene copolymers PB in cyclohexane and toluene (5 wt.%) Catalyst (bis(cyclopentadienyl) titanium dichloride) 0.4 mmol per 100 g PB PH2 0.49 MPa T 40 °C t 2 h Conversion 97% Asahi Kasei Kogyo Kabushiki Kaisha (Osaka, Japan) 62 (1985)... 

It was also discovered at Phillips. that the four rate constants discussed above can be altered by the addition of small amounts of an ether or a tertiary amine resulting in reduction or elimination of the block formation. Figures 13 and 14 illustrate the effect of diethyl ether on the rate of copolymerization and on the incorporation of styrene in the copolymer. Indeed, random copolymers of butadiene and styrene or isoprene and styrene can be prepared by using alkyllithium as initiator in the presence of small amounts of an ether or a tertiary amine. 

There is yet another general method to prepare random copolymer. As stated earlier, when one uses potassium, rubidium or cesium initiator, styrene polymerizes first, to give a S/B-B type of tapered block polymer. But when one mixes an alkyllithium with a potassium compound such as potassium t-butoxide, quite a different system is obtained. 

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. 

EVALUATION OF ALKYLLITHIUM INITIATED BUTADIENE-STYRENE RANDOM COPOLYMER (SOLPREN 1204) IN COMPOUNDED STOCK... 

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. 

Copolymerization. The copolymerization of butadiene-styrene with alkyllithium initiator has drawn considerable attention in the last decade because of the inversion phenomenon (12) and commercial importance (13). It has been known that the rate of styrene homopolymerization with alkyllithium is more rapid than butadiene homopolymerization in hydrocarbon solvent. However, the story is different when a mixture of butadiene and styrene is used. The propagating polymer chains are rich in butadiene until late in reaction when styrene content suddenly increases. This phenomenon is called inversion because of the rate of butadiene polymerization is now faster than the styrene. As a result, a block copolymer is obtained in this system. However, the copolymerization characteristic is changed if a small amount of polar solvent... 

The 1,2 content of this butadiene/styrene copolymer is also found to be low (9-11%). It behaves like an alkyllithium system. This result is expected since the structure of the... 

Copolymerization Initiators. The copolyineri/ution of styrene and dienes in hydrocarbon solution wilh alkyllithium initiators produces a tapered block copolymer structure because of the large differences in monomer reactivity ratios lor styrene (r, < 0.11 and dienes (rj > 10). In urder to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such us tetrahydrofuran or an alkali metal alkoxide (MtOR. where Ml = Na, K. Rb, or Cs>. 

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... 

Unlike the coordination catalysts, alkyllithium initiators allow easy formation of styrene-butadiene copolymers, important components of tire treads. 

High 1,4 B-block ABA copolymers having good physical properties have been reported from l,4-dilithio-l,l,4,4-tetraphenylbutane (84) and 1,4-dilithio-l,4-dimethyl-1,4-tetraphenyIbutane (84) and 1,4-dilithio-1,4-dimethyl-1,4 diphenylbutane (85) initiation. These initiators have been prepared with a minimum of ether (preferably anisole). However, both of these compounds have shown a gradual loss of solubility as a result of association of the alkyllithiums. Attachment of solubilizing oligomeric polydienes apparently alleviated this problem (84, 85). 

SCBs play an important role in the formation of other block copolymers. For example, the relatively less nucleophilic poly(ethylene oxide) oxyanion cannot initiate the polymerization of styrene, which needs a more nucleophilic alkyllithium initiator. To enable the synthesis of multi-block copolymers from various combinations of monomers by anionic mechanisms, it is important to modify the reactivity of the growing anionic chain end of each polymer so as to attack the co-monomer. There have only been a few reports on the polymerization of styrene initiated by an oxyanion (see <2001MM4384> and references cited). Thus, there exists a need for a transitional species that is capable of converting oxyanions into carbanions. In 2000, Kawakami and co-workers came up with the concept of the carbanion pump , in which the ring-strain energy of the SCB is harnessed to convert an oxyanion into a carbanion (Scheme 13) <2000MI527>. 

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. 

The copolymerization with alkyllithium to produce uniformly random copolymers is more complex for the solution process than for emulsion because of the tendency for the styrene to form blocks. Because of the extremely high rate of reaction of the styryl-lithium anion with butadiene, the polymerization very heavily favors the incorporation of butadiene units as long as reasonable concentrations of butadiene are present. This observation initially was somewhat confusing because the homopolymerization rate of styrene is seven times that for butadiene. However, the cross-propagation rate is orders of magnitude faster than either, and it therefore dominates the system. For a 30 mole percent styrene charge the initial polymer will be almost pure butadiene until most of the butadiene is polymerized. Typically two-thirds of the styrene charged will be found as a block of polystyrene at the tail end of the polymer chain ... 

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. 

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... 

Both the 2,2-diphenyl vinyl and the l-methoxy-l,l-diphenylethyl chain ends are potential endgroups for the anionic polymerization of a variety of monomers by metalation. Our earlier results indicate that quantitative metalation of the 2,2-diphenylvinyl endgroups with alkyllithium cannot be achieved, most likely because of steric hindrance. However, as described recently, the ether cleavage of 1-methoxy-l,l-diphenyl-3,3,5,5-tetramethylhexane or electron transfer to 3,3,5,5-tetra-methyl-l,l-diphenylhex-l-ene by K/Na alloy, Cs or Li led to quantitative metalation resulting in nearly quantitative initiation of the polymerization of methacrylic monomers. Both precursors led to identical (macro)initiators verified by H NMR. These compounds can be considered as models of PIB chain ends formed by LCCP of IB and subsequent end-capping with DPE. The present study deals with the application of this method to the synthesis of different AB and ABA block copolymers by the combination of LCCP and living anionic polymerization. 

Cationic initiation and Ziegler-Natta methods have also been employed successfully in order to obtain poly(vinylferrocene) [14]. Due to the electron-donating nature of a ferrocene substituent, it was initially believed that anionic initiators would not be able to induce the polymerization of vinylferrocene. However, in the early 1990s, living anionic polymerization of vinylferrocene in solution was achieved at low temperatures (-70°C to -30°C) in THE using alkyllithium initiators [15]. Block copolymers of poly(vinylferrocene) with poly(methyl methacrylate), PVEc-b-PMMA (2.5) or polystyrene, PVFc-h-PS, as coblocks were also reported (Scheme 2.1) [15]. 

S. Forster and E. Kramer, Synthesis of Pb-PEO and Pl-PEO Block Copolymers with Alkyllithium Initiators and the Phosphazene Base t-BuP4, Macromolecules 32 2783 (1999). 

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... 

The alkyllithium-initiated anionic copolymerization of diene and styrene monomers continues to be of interest because one can tailor-make copolymers with a wide range of compositional heterogeneity. Recently, kinetic studies have provided rate constant data to further clarify the factors responsible for the predominant incorporation of the less reactive diene monomer in styrene/diene copol3naerizations carried out in hydrocarbon media.They confirm that the magnitude of the rate constants for butadiene-styrene copolymerizations fall in the order results of several... 

Tapered Block Copolymers. The alkyllithium-initiated copolymerizations of styrene with dienes, especially isoprene and butadiene, have been extensively investigated and illustrate the important aspects of anionic copolymerization. As shown in Table 15, monomer reactivity ratios for dienes copolymerizing with styrene in hydrocarbon solution range from approximately 8 to 17, while the corresponding monomer reactivity ratios for styrene vary from 0.04 to 0.25. Thus, butadiene and isoprene are preferentially incorporated into the copolymer initially. This type of copolymer composition is described as either a tapered block copolymer or a graded block copolymer. The monomer sequence distribution can be described by the structures below ... 

In general, random SBR with a low amount of block st5Tene and low amoimts of 1,2-butadiene enchainment (<20%) can be prepared in the presence of small amounts of added potassium or sodium metal alkoxides. Using 0.2 equiv of sodium 2,3-dimethyl-2-pentoxide, the monomer reactivity ratios for alkyllithium-initiated SBR were found to be = 1.1 and rs = 0.1 (182). The resulting copolymer had only... 

Copolymers of 1,3-butadiene and styrene (SBR) are elastomers of great technical importance that are used for automobile tires [465-474]. In addition to a free-radical process, they can be made by anionic initiation with alkyllithium compounds. In polar solvents the reaction rate of styrene anions with 1,3-butadiene is greater than with styrene, whereas in polar solvents this is just the other way around. The copolymerization parameter rj for styrene-butadiene is 0.03 in hexane and 8 in THF r2 is calculated as 12.5 in hexane and 0.2 in THF [465]. Therefore, a strong dependence of the styrene content of the polymers on the degree of conversion is observed in discontinuous polymerizations. 

Chloro-1,3-butadiene can be polymerized with styrene [528]. The anionic block copolymerization of 1- or 2-phenyl-1,3-butadiene with styrene leads to block polymers of low molecular weight [529]. Similar copolymers are described of 1,3-pentadiene with styrene. With alkyllithium there is no reaction of 1,4-diphenyl-1,3-butadiene with styrene [530]. 

Some styrene-butadiene rubber is manufactured by solution processes using alkyllithium catalysts. Production techniques resemble those used for the polymerization of isoprene (Section 18.3.3) and butadiene (Section 18.4.3). Solution styrene-butadiene rubbers have microstructures similar to those of the emulsion copolymers but show narrower molecular weight distribution, less long chain branching and lower non-rubber content. The two types of materials have very similar bulk properties. 

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