Lithium isoprenyl

It is known that lithium organyls are associated in apolar media (see Section 18.3). For example, in the polymerization of isoprene with lithium alkyls, RCH2Li, a lithium isoprenyl is first formed ... 

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 ). 

The enthalpies of interaction of tetrahydro-furan (THF) and 2,5-dimethyltetrahydrofuran (l THF) with 0.03M benzene solutions of poly-(styryl)1ithium (PSLi) and poly(isoprenyl)lithium (PILi) have been measured as a function of R ([THF]/[Li]) at 25°C using high dilution solution calorimetry. At low R values (ca. 0.2) the enthalpy of interaction of THF with PILi (-5.8 kcal/mole) is more exothermic than with PSLi (-4.5 kcal/mole). However, the decrease in enthalpy for Me,THF versus THF is larger for PILi (3.2 kcal/mole) than for PSLi (2.2 kcal/mole) at low R values. The enthalpies decrease rapidly with increasing R values for PILi, but are relatively constant for PSLi. It is suggested that THF interacts with intact dimer for PILi, but for PSLi this base coordinates to form the unassociated, THF-solvated species. 

Poly(styryl)lithium. The calorimetric data obtained for the interaction of tetrahydrofuran (THF) and 2,5-dimethyltetra-hydrofuran (2,5-Me2THF) with 0.03M solutions of poly(styryl)-lithium in benzene as a function of R ([base]/[lithium atoms]) are shown in Figure 1. These data, as well as the corresponding data for poly(isoprenyl)lithium, are referred to dilute... 

The state of association of poly(dienyl)lithium compounds in hydrocarbon solutions is a matter of current controversy (15-18). Aggregation states of two (.16 and four (.15) have been reported based on light-scattering and concentrated solution viscosity measurements. The most recent concentrated solution viscosity studies ( 1 6 J 7.) > which include results of various endcapping and linking techniques, provide convincing evidence for predominantly dimeric association of poly(isoprenyl)lithium in hydrocarbon solution. The effect of tetrahydrofuran on the degree of association of poly(isoprenyl)lithium has also been examined by concentrated solution viscosity measurements (13). These results indicate that the equilibrium constant for the process shown in eq 3 [PILi = poly(isoprenyl)lithium] exhibits an equilibrium... 

The enthalpies of coordination of tetrahydrofuran with poly(isoprenyl)lithium exhibit a dramatic concentration dependence as shown in Figure 3. The enthalpies decrease from -5.8... 

This supposition is supported by results for linking reactions of polymeric organolithium compounds which indicate that the steric requirements of a poly(styryl) chain end are larger than those for a poly(dienyl) chain end ( 4,25). Since a larger sensitivity to base steric requirements is exhibited by poly-(isoprenyl)lithium and it is known that the coordination process for poly(styryl)lithium involves coordination to give the unassociated species (eq 1), it is concluded that tetrahydrofuran coordination with poly(isoprenyl)lithium must involve interaction with an associated species (presumably the dimer) to explain the large sensitivity to the steric requirements of the base. 

There is very little experimental evidence relating to the energetics of dissociation of poly(dienyl)lithium species. From the temperature dependence of the flow times of the concentrated solution viscosities of hexane solutions of poly(isoprenyl)lithium, Morton and Fetters 47) reported an estimate of 37 kcal/mole for the dissociation of dimers (Eq. (5)). 

The most dramatic effects of Lewis bases in organolithium chemistry are observed in polymerization reactions. Aside from colligative property measurements, there is little direct quantitative information on the nature of the organolithium-base interactions responsible for the observed effects. The calorimetric method has been used also to examine the fundamental nature of the interaction of bases with polymeric organolithium compounds 83,88,89). Information is now available on the ground-state interaction of bases with poly(styryl)lithium (PSLi), poly(isoprenyl)lithium (PILi) and poly(butadienyl)lithium (PBDLi). 

The effect of tetrahydrofuran on the extent of association of po y(isoprenyl)li-thium in n-hexane has been determined by concentrated solution viscosity measurements. The equilibrium constant for the interaction of THF with poly(isoprenyl) lithium as shown in Eq. (12)... 

This conclusion provides an explanation for the calorimetric observation that base coordination of poly(isoprenyl)lithium is more sensitive to the steric requirements of the base (AAH = 3.2 kcal/mole) than is the coordination process for poly(styryl) lithium (AAH — 2.2 kcal/mole), since monomeric poly(isopropenyl) lithium would be expected to be less hindered than unassociated poly(styryl)lithium. However, dimeric poIy(isoprenyl)lithium could very well be more hindered toward base coordination (Eq. (13)) than monomeric poly(styryl)lithium (Eq. (11)). 

The nature of the process involved in the interaction of tetrahydrofurans with poly(butadienyl)lithium has been less well characterized, although it can be assumed to be analogous to the process involved with poly(isoprenyl)lithium (Eq. (13)). If this is correct, then the interaction of tetrahydrofuran with poly(butadienyl)lithium can be described in terms of Eq. (14). 

The decreased steric requirements for this base coordination process (AAH = 2.1 kcal/mole) compared to the analogous interaction for poIy(isoprenyl)iithium (AAH = 3.2 kcal/mole) are consistent with a po y(butadienyl)lithium chain end being less sterically demanding than a poly(isoprenyl)lithium chain end. Several factors can be considered in favor of the same degree of association for the base adduct for... 

Fig. 5. Enthalpies of interaction of TMEDA as a function of R([TMEDA]/[Li]) for 0.02M benzene solutions of poly(isoprenyl)lithium...

It has been observed that the concentrated solution viscosity decreases upon addition of TMEDA to solutions of poly(isoprenyl)lithium 93). This would be consistent with the process shown in Eq. (17) or (20) and not with Eqs, (18) or (19). The decrease in viscosity would be consistent with interaction of TMEDA to form an unassociated complex (Eq. (20)), but this does not seem to be in accord /with the stoichiometry observed by calorimetry. It is noteworthy that the break observed by calorimetry at R = 0.5 is consistent with the stoichiometric dependence of spectral, kinetic and microstructure effects 90). Again this shows that these kinetic effects are related to the stoichiometry of formation of base-organolithium adduct, i.e. that they are ground-state solvation effects. 

Fig. 7. Enthalpimetric titration plot for interaction of TMEDA with 0.G2M solutions of polyfstyryljlitbium ( ), poiy(isoprenyl)-lithium (A), and poly(butadienyi)lithium ( )...

Furthermore, several of Worsfold s assessments seem to be open to question. The assertion that the association (between the allylic-lithium active centers) is between ionic species can be contrasted with the evidence provided by NMR spectroscopy 36,134 143) which has shown that the carbon-lithium bond of allylic-lithium species can possess considerable covalent character. Worsfold has also previously published 43 > concentrated solution viscosity results where the ratio of flow times, before and after termination, of a poly(isoprenyl)lithium solution was about 15. This finding is clearly incompatible with the conclusion that viscometry cannot detect the presence of aggregates greater than dimeric. 

It should also be noted that the viscometric technique can detect the presence of star-shaped aggregates, having the ionic active centers. The addition of ethylene oxide to hydrocarbon solutions of poly(isoprenyl)lithium leads to a nearly two-fold increase in viscosity144). Conversely, this results in an approximately twenty-fold decrease in solution viscosity, after termination by the addition of trimethylchloro-silane. This change in solution viscosity is reflected in the gelation which occurs when difunctional chains are converted to the ionic alkoxy active centers 140,145,146). Branched structures have also been detected 147> by viscometry for the thiolate-lithium active center of polypropylene sulfide) in tetrahydrofuran. 

The influence of tetrahydrofuran on the propagation and association behavior of poly(isoprenyl)Iithiura in n-hexane has been examined47. As for the case of poly(styryl)lithium156), the rate of polymerization was found to first increase followed then by a decrease as the THF/active center ratio increased. This decrease ultimately reached the polymerization rate found in pure tetrahydrofuran at a THF active center ratio of ca. 2 x 103. This was for the case where the active center concentration was held constant and the tetrahydrofuran concentration varied. The maximum rate of polymerization was found to occur at a THF active center ratio of about 500 a value at which the viscometric measurements demonstrated 47 the virtual absence of poly(isoprenyl)lithium self-aggregation. As noted before in this review, the equilibrium constant for the process shown in Eq. (12) has the relatively small value of about 0.5 LM-1, which is in sharp contrast with the value of about 160 LM 1 found for the THF-poly(styryl)lithium system. The possibility of complexation of THF directly with the poly(isoprenyl)lithium aggregates, Eq. (13), was not considered by Morton and Fetters47. ... 

Davidjan et. al.166) have made a study of the influence upon the propagation of poly(isoprenyl)lithium in n-hexane of very small additions of 1,2-dimethoxyethane (DMEractive centers = 0.01). Analysis of polymer obtained at 10% conversion by size exclusion chromatography coupled with a determination of the dependence of the stereochemistry upon molecular weight led them to the conclusion that complexation reduces the reactivity of what they assumed to be the most reactive species, i.e., the non-associated active center. 

In a series of experiments in which r = 0.01 Davidjan et. al.189) have made a systematic study of the influence of TMEDA upon the molecular weight and stereochemical distribution of polv(isoprenyl)lithium formed in hexane at —30 °C. Reaction mixtures were allowed to polymerize to 10,35 and 80% conversion before being... 

In the original paper n was taken to be four 189) although the burden of evidence suggests (Table 2) two is more likely. This scheme assumes that complexation need not result in disaggregation. Viscometric studies show that poly(butadienyl)lithium aggregates are broken on complexation 152). Calorimetric measurements89 on the interaction of TMEDA with poly(isoprenyl)lithium have yielded data that are not inconsistent with the formation of a complex between one molecule of the former and two of the latter. 

Figure 13. Absorption spectra of poly(isoprenyl)lithium in n-hexane as a function of concentration.

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