Organolithium compounds styrenes

Other Organolithium Compounds. Organoddithium compounds have utiHty in anionic polymerization of butadiene and styrene. The lithium chain ends can then be converted to useflil functional groups, eg, carboxyl, hydroxyl, etc (139). Lewis bases are requHed for solubdity in hydrocarbon solvents. 

Since Lewis base additives and basic solvents such as tetrahydrofuran are known to deaggregate polymeric organolithium compounds, (21,23,26) it was postulated that ketone formation would be minimized in the presence of sufficient tetrahydrofuran to effect dissociation of the aggregates. In complete accord with these predictions, it was found that the carbonation of poly(styryl)lithium (eq. 9), poly(isoprenyl)-lithium, and poly(styrene-b-isoprenyl)lithium in a 75/25 mixture (by volume) of benzene and tetrahydrofuran occurs quantitatively to produce the carboxylic acid chain ends (8 ). 

Fractional kinetic orders of homogenous reactions in solution may point to association of a particular reagent. The kinetics of the initiation step of styrene polymerization in the presence of n-BuLi (equation 33) is in accordance with the assumption that this organolithium compound in a nonbonding solvent forms aggregates of six molecules on the average" . 

Waack and Doran [26] reported on the relative reactivities of 13 structurally different organolithium compounds in polymerization with styrene in tetrahydro-furan at 20°C. The reactivities were determined by the molecular weights of the formed polystyrene. The molecular weights are inversely related to the activity of the respective organolithium polymerization initiators. Reactivities decreased in the order alkyl > benzyl > allyl > phenyl > vinyl > triphenylmethyl as shown in Table 3.1. 

Early studies (1 ) of the kinetics of polymerization of styrene, isoprene and butadiene in hydrocarbon solvents indicated a half-order rate dependency on growing chain concentration, although there were conflicting data at that time (10, 11) which suggested even lower fractional orders for the dienes. Since the apparent half-order dependency could not be rationalized, as in the case of the polar media, by an ionic dissociation mechanism, some other form of association-dissociation phenomenon offered a possible answer. In view of the known tendency of organolithium compounds to undergo molecular association in non-polar media, the following scheme was proposed by us (l) ... 

The most widely used organolithium compound is n-butyllithium (see formulas of related compounds in Table 12.1), used as an initiator for the production of elastomers by solution polymerization, predominantly of styrene-butadiene. 

Nucleophilic addition to styrene is possible only because the intermediate carbanion is stabilized by conjugation into the benzene ring. It needs a more reactive carbanion than the benzyl anion to initiate the polymerization, and an unstabilized nonconjugated organolithium compound like butyl lithium is the answer. 

The existence of such associated organolithium compounds has been estabhshed in various cases (19, 20, 24), In addition to isotactic polystyrene, a considerable amoimt of atactic material is always present it is formed by starting the polymerization on the nonassociated part of the organolithium compounds which probably promote a nonstereospecific anionic polymerization. The stereoregulation of the polymerization of styrene by heterogeneous alkali metal aUcyl initiators is limited by the forces on the surface of the catalyst while the dissolved organolithium initiators in their associated form cause the stereospecific polymerization. 

A handicap of Grignard reagents in the field of anionic polymerization is certainly their low reactivity toward nonpolar double bonds. Unlike organolithium compounds, organomagnesium compounds are nornally inert toward monomers sueh as styrene or butadiene. Thus, their scope in the field of anionic block-copolymerization is quite limited. 

Figure 3. Kinetic studies of organolithium compounds prepared by the reaction of /i-BuLi with ADPE, styrene, and MDDPE in benzene or benzene/THF mixture ([THF]/[/f-BuLi] = 20/1).

Unsaturated elastomers can be readily metallated with activated organolithium compounds in the presence of chelating diamines or alkoxides of potassium or sodium. For example, polyisoprene, polybutadiene, styrene-butadiene copolymers, and styrene-isoprene copolymers can be metallated with n-butyllithium TMEDA complexes (1/1 or 1/2 ratio) to form allylic or benzylic anions. The resulting allylic anion can be employed as an initiator site to grow certain branched or comb polymer species. These polymers can include polystyrene, which would form hard domains, or polybutadiene, which forms soft domains. 

The carbonation of polymeric carbanions using carbon dioxide is one of the simplest, most useful, and widely used functionalization reactions. However, there are special problems associated with the simple carbonation of polymeric organolithium compounds. Eor example, when carbonations with high-purity, gaseous carbon dioxide are carried out in benzene solution at room temperature using standard high vacuum techniques, the carboxylated polymer is obtained in only 27-66% yield for PSLi, PILi, and poly(styrene-b-isopre-nyl)lithium. The functionalized polymer is contaminated with dimeric ketone (23-27%) and trimeric alcohol (7-50%)... 

Anionic polymerization of styrene with amyl sodium yields an isotactic polymer [164]. Polymerizations catalyzed by triphenylmethylpotassium also yield the same stereospecific polystyrene [165]. The same is true of organolithium compounds [166,167]. 

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