Lithium polystyryl

The somewhat exceptional behavior of the lithium salt is manifested in hydrocarbons as well as in ethereal solvents. Thus in THF there is a striking difference in conductance and reactivity of lithium polystyryl as compared with its sodium salt76). This may be appreciated by inspecting Figs. 12 and 13. 

The viscometric findings of Young 1211 who investigated the effect of tetramethyl ethylene diamine (TMEDA) addition on the viscosity of butadiene capped lithium polystyryl solutions showed that the results did not agree with those expected on the basis of the reaction... 

Fig. 51. Plot of log of the propagation constant, k , of lithium polystyryl ion-pairs in THF-ben-zene mixtures as a function of (e - l)/(2e + 1), e -the dielectric constant of the mixture...

Fig. 52. a Plots of log of the propagation constant, k , of lithium polystyryl ion-pairs in THF-dioxane mixtures as a function of 1/e - , and of (e - l)/(2e + 1) — O. e - the dielectric constant of the mixture. Note their concave shapes, b Plot of log of the propagation constant, k , of lithium polystyryl ion-pairs in THF-dioxane mixtures vs. mole fraction of dioxane. Note the concave shape of the curve... 

This unusual behavior was accounted for by O Driscoll and Kuntz411), who postulated an extremely fast addition of butadiene to lithium polystyryl but slow addition of styrene to lithium polybutadienyl. Such a relation prevents the formation of any significant proportion of lithium polystyryl in the polymerizing system as long as butadiene monomer is available. Since the homopolymerization of butadiene is relatively slow, whereas that of polystyrene is rather fast, the polymerization speeds up as butadiene disappears. 

The addition of styrene to lithium polystyrene follows a square root dependence of the rate on lithium polystyryl concentration... 

The earliest SIS block copolymers used in PSAs were nominally 15 wt% styrene, with an overall molecular weight on the order of 200,000 Da. The preparation by living anionic polymerization starts with the formation of polystyryl lithium, followed by isoprene addition to form the diblock anion, which is then coupled with a difunctional agent, such as 1,2-dibromoethane to form the triblock (Fig. 5a, path i). Some diblock material is inherently present in the final polymer due to inefficient coupling. The diblock is compatible with the triblock and acts... 

The effect of penultimate units on the rate constants of anionic propagation is observed also in other systems. For example, the addition of styrene to the lithium salt of 1-phenyl-n-hexyl anion is 4 times faster than to polystyryl lithium 51). Similarly, the addition of monomer to the lithium salt of 1,1-diphenyl-n-hexyl lithium is faster than the addition to 1,1,3-triphenyl-n-octyl lithium or 2-poly-sty ry 1-1,1-diphenyl ethyl lithium, the latter two salts having comparable reactivities52 . See also Ref.53)... 

The degree of aggregation of polystyryl alkali salts in hydrocarbons, as well as the reactivity of their respective unassociated pairs, decrease along the series Li +, Na+, K +, Cs+ (Ref.Il, pp. 20 21). For example, the propagation constant of the lithium pair in benzene at 25 °C is estimated to be greater than 100 M "1 sec- while those of K +, Rb+, and Cs+ were determined as 47, 24, and 18 M-1 sec-1, respectively. Such a gradation contrasts with that of the reactivities of tight pairs in ethereal solvents,... 

Fig. 12. Temperature dependence of Kd of polystyryl lithium in different ethereal solvents (S. Peeters, M. Van Beylen, Ref. 76 )...

The order of reactivities could be also reversed by a change of solvent. For example, in THF styrene is more reactive than butadiene towards salts of polystyryl anions, whereas in hydrocarbon solvents butadiene is more reactive than styrene towards lithium polystyrene. This reversal of reactivities presumably is caused by a change in the mechanism of propagation. The monomers react directly with carbanions in THF, but become coordinated to Li+ in hydrocarbon solvents. 

The initiation of the cyclic siloxane monomers with a living polymeric lithium species such as polystyryl lithium leads to block copolymers, as outlined in Scheme 2, were also of interest. These styrenic-siloxane block copolymers were prepared with siloxane contents from 10 to 50 weight percent. 

A procedure used in the self-branching polymerization reaction for the preparation of dendritic polystyrenes is outlined in Scheme 7. Oligomeric polystyryl-lithium chains are reacted with a coupling agent such as 4-(chlorodimethyl-silyl)styrene (CDMSS), which contains both a polymerizable double bond as well... 

Anionic polymerisation of hydrocarbon monomers is initiated by lithium butyl to produce a living polymer the association number of which in solution is required to elucidate the kinetics. When the living polymer (for example polystyryl lithium) is terminated, the polystyrene can be isolated and a solution then made to determine its molecular weight, M. If the living polymer is associated in solution, the ratio of its... 

It is apparent from these data that all of the polymers, including butadiene, exhibit an association as dimers, and that there is no reason to expect any higher states of association for polyisoprene or polybutadiene. This is confirmed not only by the viscosity data on the active vs. terminated "capped" polymers, but also by the fact that there was no significant increase in viscosity when the polystyryl lithium was "capped" by butadiene or isoprene, i.e., all three types of chain ends are associated in the same way, as dimers. 

Association of Polystyryl Lithium "Capped" with Butadiene and Isoprene (30°C) (21)... 

The authors of ref. 46. reported flow times for polystyryl-lithium-benzene solutions before and after the addition of diphenyl ether whereupon the active centers were terminated and the flow times again measured. Table III of the note in question (46) says that in pure benzene, Nw, is 1.96 and 2.0 —in apparent agreement with the generally held belief that polystyryllithium is exclusively dimeric in benzene. Following the addition of diphenyl ether to achieve the specified concentration (0.15M), the authors in their Table III then reported values of Nw of 1.88 and 1.95 (based on their flow times ). From these values, it was concluded that diphenyl ether does not influence the association state of polystyryllithium. 

Figure 2. Enthalpies of interaction of THF with 0.03M solutions of polystyryl-lithium containing 1% (U> and 8% ( ) noncarbon-bound lithium base impurities.

Commercial methods for preparing block copolymers of styrene-hutadiene-styrene utilize cyclohexane or toluene as a solvent since the polystyryl lithium is insoluble in straight chain alphatic solvents.Usually the 1,3-butadiene is added to the polystyryl lithium to produce the diblock styrene-butadienyl lithium. At this point in the reaction two processes are employed. 

The recent development of using hexane as a solvent for the preparation of SBS copolymers has been attempted even though the polystyryl, lithium is insoluble in this media, (,2k). Polystyryl lithium was dispersed in hexane using 1% of SBS rubber as a dispersing agent. The block copolymer tried as a dispersing agent retained the colloidal properties of poly-styrenyl lithium till the addition of the butadiene monomer. 

Fig. I. Concentration of styrene monomer (A) and polystyryl-lithium (B) in arbitrary units during a polymerization of 1.4 x 10 molar styrene with 1.1 x 10 molar butyllithium in benzene...

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