Makromol. Chem., Rapid Commun. 1, 75 - 77(1980) 75

Determination of the Degree of Association of Polyisoprenyllithium in Heptane a)

Alberto Hernandez, Joachim Semel, Hans-Christoph Broecker, Hans Gerhard Zachmann *, Hansjorg Sinn

Institut fur Anorganische und Angewandte Chemie, Universitat Hamburg, Martin-Luther-King-Platz 6, 2000 Hamburg 13, BRD

(Date of receipt: December 20, 1979)

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.

Fig. 1 shows the reduced viscosity as a function of the polymer concentration for the living polyisoprenyllithium (full lines) and the corresponding polyisoprene (dotted lines). Whereas the results for the polyisoprene are lying on straight lines, those for the isoprenyllithium are represented by curved lines shifted to higher reduced viscosities. This indicates that the “living molecules” are associated and that the degree of association depends on the concentration of total lithium.

For a quantitative evaluation of the results it is assumed that the extrapolation of the reduced viscosity to concentration zero for an association complex of n molecules of isoprenyllithium each of molecular weight M can be performed in exactly the same way as the extrapolation for polyisoprene of molecular weight nM. This is illustrated in Fig. 2. The full line represents the measurements on a sample of polyisoprenyl- lithium of known molecular weight (M = 12000 by membrane osmometry) at different concentrations. The dashed lines show the reduced viscosities for a number of samples of polyisoprene with different molecular weights as given at the curves in Fig. 2. If e.g. the dashed line for the molecular weight M = 26000 cuts the full curve at the concentration c = 1,l g/100 ml we conclude that the apparent molecular weight of the living molecule at the corresponding concentration is 26000, which means that the degree of association is 26000/12000 = 2,16.

a) Presented in part at Hamburg Macromolecular Symposium, September 26 - 28, 1979.

16 H. G. Zachmann et al.

0 2 L cI(g/dl)

Fig. 2

Fig. 1 . Reduced viscosity qsp/c as a function of polymer concentration c for polyisoprenyl- lithium (-) and for polyisoprene ( - - -) both in heptane at 20°C. Molecular weights as given at curves

Fig. 2. Evaluation of the viscosity measurements as to the degree of association (see text)

For the small concentrations used, this evaluation seems to us to be more appropriate than that proposed by Fetters and Morton4) which is based on the rela- tion q = KM3s4 valid only for high concentrations. Our evaluation may involve an error due to the fact that the associated molecules must be considered as being star- branched rather than linear. As a consequence of this the molecular weight and the degree of association should be a little higher than those derived from our experi- ments. From a review given by Bywater5) one can conclude that the deviation is about 15% for three-branched stars and goes to zero for two-branched stars.

The results obtained from the light scattering experiments are shown in Fig. 3. The dashed lines refer to two samples of polyisoprene which, according to gel-permeation- chromatography, have almost the same molecular weights, the full lines refer to two corresponding samples of polyisoprenyllithium. Again, from the difference in the results one can conclude that the isoprenyllithium is associated. The degree of association can been determined in a similar manner as in the case of the viscosity measurements. The distance of the curves for the two polyisoprene samples indicates the error involved.

The degrees of association as a function of concentration of active lithium are plotted in Fig. 4. The full lines represent the results obtained from viscosity, the dashed lines show the results obtained from light scattering, and the dotted line is derived from the kinetic measurements'). The molecular weight is given as a para- meter at the curves. The degree of association was obtained with the help of the curves fitted to the points in Fig. 1 and Fig. 3. Therefore no points appear in the curves and no information concerning the scatter in the original data is given. If this scatter of the measured data points is taken into account, an uncertainty in the degree of association must be recognized which is comparable to the differences in the curves

Determination of the Degree of Association of Polyisoprenyllithium in Heptane 17

10-4 "'10-3 ' ' ' 2 1

0 0,1 0,2 0,3 0,4 c/ (g /d I) [LiM,Bu] /(rnoW)

Fig. 3 Fig. 4

Fig. 3. Small angle light scattering function Kc/R, as a function of polymer concentration c for polyisoprenyllithium (-) and polyisoprene ( - - -). The molecular weight of each sample is 21 000 Fig. 4. Degree of association n as a function of concentration of active lithium c deduced from viscosity (-), from small angle light scattering ( - - -), and from the kinetic theory

1 (.....

caused by different molecular weights. Therefore, one cannot conclude for sure that the molecular weight influences the degree of association.

One observes a slight increase in the degree of association with increasing con- centration of lithium. This increase, however, is by far not as large as predicted by the kinetic theories. It seems more appropriate to assume that the degree of association is almost constant and equal to 2 , in agreements to results of Fetters and Morton').

If, however, the differing properties of star branched and linear polymers as given by Bywater5) are taken into account in our evaluation, the point at a lithium concentration of mol/l changes to a degree of association of about 3, whereas the points at a concentration of lo-' mol/l remain unchanged. Then, a much better agreement is obtained between our results and those derived from kinetic data. By extrapolation to a concentration of mol/l, Bywater's value of a degree of association of 4 can be reached. At the same time, the degree of association of 3 found by us is considerably higher than the value of 2 reported by Morton for this concentration.

') H. Sinn, L. Lundborg, 0. T. Onsager, Makromol. Chem. 70, 222 (1964) 2, S. Bywater, Fortschr. Hochpo1ym.-Forsch. 4, 66 (1965) 3, D. Worsfold, S. Bywater, Macromolecules 5, 393 (1972) 4, L. Fetters, M. Morton, Macromolecules 7, 552 (1974) 5, S. Bywater, Fortschr. Hochpo1ym.-Forsch. 30, 89 (1979)