Alkyllithium-initiated polymers

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

Another complication introduced by the associative properties of organolithium solutions in non-polar solvents is the fact that the alkyllithium initiators are themselves associated and can be expected to "cross associate" with the active polymer chain ends. Thus some of our studies (26) on the effect of added ethyl lithium on the viscosity o -polyisoprenyl lithium solutions in n-hexane support the following association equilibrium... 

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

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 use of alkyllithium initiators which contain functional groups provides a versatile method for the preparation of end functionalized polymers and macromonomers. For a living anionic polymerization, each functionalized initiator molecule produces one macromolecule with the functional group from the initiator residue at one chain end and the active carbanionic propagating species at the other chain end. 

In the alkyllithium initiated polymerizations of vinyl monomers, Lewis bases such as ethers and amines alter the kinetics, stereochemistry, and monomer reactivity ratios for copolymerization. In general, the magnitude of these effects has been directly or indirectly attributed to the extent or nature of the interaction of the Lewis base with the organolithium initiator or with the organolithium chain end of the growing polymer. Unfortunately, all of these observed effects are kinetic in nature, and therefore the observed effects of solvent represent a composite effect on the transition-state versus the ground state as shown below in Eq. (6), where 5 represents the differential... 

Polymers. The polymers used in the blending experiments were prepared by anionic polymerization using an alkyllithium initiator and a chemical randomizing agent to control monomer sequence, in the manner described by Hsieh and Wofford (3). Randomness was checked in each case by measuring the styrene content as a function of conversion. Table I gives descriptive data for these polymers. 

The historical development of alkyllithium-initiated polymerization of olefins and diolefins for the synthesis of elastomeric materials is of interest not only because of its scientific and technological significance but also because of the insight it provides into the thinking and methodology of polymer (elastomer) researchers. 

Microstructure variation in both polybutadiene and polyisoprene polymers has been realized by using alkyllithium initiators in the presence of polar modifiers. 

A unique feature of alkyllithium initiation is that it takes place in a homogeneous reaction mixture where there is a complete absence of termination or other side reactions, so that "living polymers" are formed. This fact, along with the ability of polar solvents to modify the reactivity and mode of reaction, has enormous implications for the synthesis of polymers. 

Thus, the use of alkyllithium initiation offers the synthetic chemist a tool of enormous flexibility for "tailor-making" polymers of precise structure. Control of molecular weight, molecular-weight distribution, diene structure, branching, monomer-sequence distribution, and functionality can conveniently be achieved by such techniques as incremental or sequential addition of monomer, initiators, or modifier, programming of temperature, continuous polymerization, or the use of multifunctional reagents. 

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 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 ... 

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-... 

Because the entire set-up is a closed system, the control and reproducibility of the polymerization are remarkably good. Since the polymer solution can be drained and rinsed out by the dried solvent from the bottom of the reactor, the closed system rarely needs to be opened and exposed to the atmosphere. For this study, the overall scavenger levels (the difference between levels of added RLi and effective RLi) were in the range of less than 10-4, but more than 10-5 mole/1iter,representing 2 to 6% of the total initiator added. Typically, the alkyllithium initiator concentration is around 1 x 10-3 mole/1iter. 

Alkyllithium initiators yield stereoregular polymers of conjugated dienes if the polymerization is carried out in hydrocarbon solvents. Addition of tetrahydrofuran or other more polar solvents changes the microstructure of the polymers that are produced... 

One of the most studied polymerization systems employs alkyllithium initiators that are modified by chiral amine ligands for the polymerization of sterically bulky methacrylates [8,38,39,40,41], acrylates [42],crotonates [43], and acrylamides [44]. A primary example is the reaction of triphenylmethyl methacrylate with an initiator derived from 9-fluorenyllithium and (-)-sparteine (3) at -78 °C (Scheme 4). The resultant isotactic polymer is optically active, and is postulated to adopt a right-handed helix as it departs from the polymerization site. This polymer has been particularly successful as a chiral stationary phase for the chromatographic resolution of atropisomers [8]. Many modifications of the or-ganolithium initiator/chiral ligand system have been explored. Recently, Okamo-to has applied enantiopure radical initiators for the enantioselective polymerization of bulky methacrylate monomers [45]. 

Excellent difimctionality of the resultant polymers was obtained (/ = 2.0 +/- 0.1). However, since the initiator was synthesized in diethyl ether, the amount of 1,2-microstructure was relatively high (36-54%). The initiator was insoluble in hexane solution, thus the formation of high 1,4 microstructure was impossible. In addition, from a practical perspective, the cleavage of diethyl ether by the alkyllithium initiator was of concern. ... 

The methodology of living anionic polymerization, especially alkyllithium-initiated polymerization, is very useful for the preparation of chain-end functionalized polymers with well-defined structures (9,10). Since these living polymerizations generate stable, anionic polymer chain ends (P Li ) when all of the monomer has been consumed, post-polymerization reactions with a variety of electrophilic species can be used to generate a diverse array of chain-end functional groups as shown in eq. 1,... 

Several criteria must be satisfied for a functionalized alkyllithium initiator to be generally usefiil for preparation of well-defined, a-fimctionalized polymers ... 

Alkyllithium initiators are primarily used as initiators for polymerizations of styrenes and dienes. They effect quantitative living polymerization of styrenes and dienes in hydrocarbon solution. In general, these alkyllithium initiators are too reactive for alkyl methacrylates and vinylpyridines. n-Butyllithium is used commercially to initiate anionic homopolymerization and copolymerization of butadiene, isoprene, and styrene with linear and branched structures. Because of its high degree of association (hexameric), n-butyllithium-initiated polymerizations are often effected at elevated temperatures (>50 °C) and in the presence of small amounts of Lewis base to increase the rate of initiation relative to propagation and thus obtain polymers with narrower molecular weight distributions [55, 57]. 

Termination Reactions The categorization of a given polymerization system as living is based on results obtained on the laboratory time scale, that is, the absence of chain termination or chain transfer reactions occurring within the normal time required to complete the polymerization and carry out any subsequent chemical reactions with the active carbanionic polymer chain ends [3, 1, 100]. In fact, the amount of spontaneous termination reactions in typical alkyllithium-initiated polymerizations of styrene and diene monomers depends on time, temperature, and whether polar additives are present [3, 101, 102]. 

Commercial random SBR polymers (solution SBR) prepared by alkyllithium-initiated polymerization typically have 32% cis-, A-, 41% trans-, A-, and 27% vinyl-microstructure compared to 8% cw-1,4-, 74% trans-, A-, and 18% vinyl-microstructure for emulsion SBR with the same comonomer composition [3, 221]. Solution SBRs typically have branched architectures to eliminate cold flow [17, 49]. Compared to emulsion SBR, solution random SBRs require less accelerator and give higher compounded Mooney, lower heat buildup, increased resilience, and better retread abrasion index [3]. Terpolymers of styrene, isoprene, and butadiene (SIBR) have been prepared using a chain of single-stirred reactors whereby the steady-state concentration of each monomer and Lewis base modifier at any degree of conversion could be controlled along the reactor chain [3, 222-224]. 

A variety of other initiators have been studied for the stereoselective polymerization of 1,3-dienes [Coates, 2000 Cooper, 1979 Endo and Hatakeyama, 2001 Deal et al., 1999 Nath et al., 2002 Pasquon et al., 1989 Peluso et al., 1997 Porri and Giarrusso, 1989 Ricci et al., 1996 Senyek, 1987 Tate and Bethea, 1985 Visseaux et al., 2001]. Traditional Ziegler-Natta, metallocene, and other initiators yield remarkable results, surpassing the stereoselectivity by alkyllithium initiators. Table 8-11 shows the polymer structures obtained... 

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