Determination of the degree of association of polyisoprenyllithium in heptane

Abstract

In the kinetic theories of polymerization of isoprene with butyllithium as initiator some authors that the "living molecules" are associated. There exists also some experimental evidence for such an a s s~c i a t i o n~.~~~) ; the measurements were performed, however, at comparatively large concentrations. In the present work, it is attempted to measure the degree of association in heptane as a function of the concentration of active lithium by using different experimental methods.The samples were polymerized with oligomeric isoprenyllithium in heptane as described earlier '). The viscosity measurements were performed by using an Ubbelohde viscosimeter attached to a high vacuum line. The light scattering intensity was determined with a Chromatix KMX 6 equipment which is especially suited for small angle light scattering. All operations were performed under argon as usual with organometallic compounds.

Association Phenomena in Organolithium Polymerization

“…In conclusion, therefore, all the reliable experimental evidence obtained to date clearly indicates the dimeric association of polystyryl-, polyisoprenyl-, and polybutadienyllithium in hydrocarbon media at concentrations in the macromolecular range, i.e., < 10~2 M. This conclusion is supported by the data reported by all the laboratories with one exception, 20 including not only our own work 2 ' 37 but that of three other groups, 1 7 , 2 1 , 2 3 , 2 4 using vis cosity, 1 7 ' 21 cryoscopy, 23 and laser light scattering 24 measurements.…”

Anionic synthesis of [1,4-poly(diene)-b-syndiotactic poly(methyl methacrylate)] diblock copolymers:7Li NMR study of the intermediate reaction of lithium poly(diene) anion with 1,1-diphenylethylene

“…This aromatic ring effect is responsible for a difference in the 7 Li chemical shifts for the initial lithium poly(diene) anions and the end products, respectively, large enough for the reaction kinetics to be followed easily. The starting polyanions, P(Bu)Li (A1) and P(Is)Li (A2), are known to form tetramers in hydrocarbon solvents 13,14) . The addition products, polymeric diphenylalkyllithium C1 and C2, are expected to self associate as dimers, by analogy with organolithium analogues, such as diphenylhexyllithium and diphenylmethyllithium, which have been observed as dimeric aggregates in apolar solvents 15,16) .…”

Analysis of livingtert-butyl methacrylate oligoanions in tetrahydrofuran by NMR spectroscopy, viscosity and light scattering

“…Since tBMA polymerization is quite fast (94% conversion within 5 min for M -n = 3 200) 20,29) , the active species have been analyzed at complete monomer conversion. end groups is expected to provide information on the average degree of association (DA) of the growing chains, since any aggregation of the living chains must result in an increase of the hydrodynamic volume of the individual macromolecules 30,31) . The reduced viscosity of these PtBMA solutions in THF was measured at 296 K. Four samples of PtBMA of known molecular weight (measured by SEC) were used in order to know the dependence of the solution viscosity on the PtBMA molecular weight (Fig.…”

“…Because of possible deactivation of polyanions during viscosity measurements and since the aggregated chains are star-branched rather than linear macromolecules, this information on the chain aggregation is essentially qualitative 31) . Nevertheless, it appears that the living PtBMA chains are partly associated even in a rather polar solvent, such as THF, and at a rather high temperature (296 K).…”

“…Since the reduced diffusion intensity (Kc/DR 0 ) is inversely proportional to the weight average molecular weight (M -w ), the diffused intensity is expected to be smaller for the living chains (including aggregation) compared to the previously deactivated ones 31) . Fig.…”