Academician of the Academy of Sciences of the BSSR B. V. EROFEEV, S. F. NAUMOVA, L. G. TSYKALO

ON THE CHROMATOGRAPHIC SEPARATION OF OLIGOMERS OF CYCLOHEXADIENE-1,3

Although the dimer of cyclohexadiene-1,3 was obtained and studied comparatively long ago (¹, ²), the higher oligomers of cyclohexadiene-1,3 were isolated and subjected to study (by the NMR method) only in one work by the authors of the present article (³). In that work, the isolation of oligomers was carried out by the method of fractional precipitation, which does not ensure high purity of the individual products. Therefore, in order to create possibilities for further investigation of the oligomers of cyclohexadiene-1,3, it was important to find a more effective method for their separation. In this connection, in the present work the method of chromatographic separation was applied, which had not previously been used for this purpose. It should be noted that in the chromatographic separation of lower members of polymer-homologous series, carried out by other investigators as applied to polyethylene (Kook and Elliot (⁴)), polystyrene (Baker, Williams (⁵), Schneider, Holmst (⁶)), thiocols (Genkin (⁷)) and other high-molecular compounds (⁸—¹⁴), the isolation of individual compounds was not achieved.

In contrast to this, as described below, chromatographic separation of liquid and solid mixtures of oligomers of cyclohexadiene-1,3 obtained by thermal polymerization (¹⁵) makes it possible to isolate individual products with degrees of polymerization 2, 3, 4, 5, and 6.

Chromatography was carried out in a glass column. The adsorbent was alumina for chromatography with an activity of 1—1.5, prepared immediately before use. The solvent and eluent for the lower-molecular oligomers ($n = 2,3$) was n-hexane, and for the higher-molecular ones, a mixture of hexane with benzene. Even the primary chromatography of the oligomeric products gives a sufficiently clear separation of the individual products. The dimer isolated in the primary chromatography had $n_D^{20}$ 1.5270—1.5272, molecular weight 160, i.e., the same indices as the dimer isolated by vacuum distillation ($n_D^{20}$ 1.5270—1.5272, mol. wt. 160). The properties of the oligomers with $n = 2, 3,$ and 4, isolated from the liquid mixture after primary, and also after secondary chromatography, are given in Table 1.

As is seen from the data of Table 1, the oligomers purified by secondary fractionation have properties differing little from the properties of the products obtained in the primary chromatography. Thus, already in the primary chromatography, practically pure trimer and tetramer of polycyclohexadiene-1,3 are obtained. Analogously to the separation of the liquid mixture of products of polymerization of cyclohexadiene-1,3, chromatographic separation was carried out of a solid mixture of products obtained by thermal polymerization. It was found that 35—40% of the mixture consists of pentamer; the hexamer content is about 6—8%, and the remainder consists of the higher members of the polymer-homologous series. Fractions of penta- and hexamers were subjected to secondary fractionation, after which their molecular weights and iodine numbers were determined. For the pentamer it was found: mol. wt. $416 \pm 10$, i.n. 320; for the hexamer, mol. wt. $483 \pm 15$, i.n. 315. In addition to pentamer and hexamer, fractions with mol. wt. 606; 930 and 1036 were also isolated from the solid mixture; their iodine numbers lie within the range 313—320.

Table 1

Properties of oligomeric products isolated after primary and secondary chromatography

Density of dimer 0.9965, density of trimer 1.0551

Table 2 gives the molecular weights, refractive indices, and iodine numbers of several samples of the oligomers obtained, and Table 3 gives examples of the results of chromatographing a mixture of liquid and solid products of the thermal polymerization of cyclohexadiene-1,3.

Table 2

Some properties of cyclohexadiene-1,3 oligomers isolated by chromatographic separation

The data in Table 2 show that the iodine numbers of the isolated individual oligomers with $n = 3, 4, 5, 6$ are close to the theoretical iodine number 316—319, calculated for oligomers containing one double bond in the monomer unit, which corresponds to a linear structure of the oligomeric chain, as, for example, in the tetramer:

The molecular absorption coefficients of the trimer, tetramer, and pentamer of cyclohexadiene-1,3 (solutions in n-hexane) were measured in the region of maximum absorption. They were found to be, respectively: $\lg \varepsilon_3$ 2.80 (250 mμ), $\lg \varepsilon_4$ 3.06 (245 mμ), and $\lg \varepsilon_5$ 2.70 (243 mμ), which is an order of magnitude less than for cyclohexadiene-1,3, for which $\lg \varepsilon$ 4.00 (258 mμ) (16). This shows that the double bonds in the oligomers are mainly nonconjugated.

Conclusions

1. By chromatographic means, individual oligomeric products with a degree of po-

Table 3

Results of the primary fractionation of oligomeric products of the thermal polymerization of cyclohexadiene-1,3

* Initial preparation: mol. wt. 187, iodine number 294.2; $n_D^{20}$ 1.5345. Charge 2.4952 g; 200 g Al$_2$O$_3$ in 300 ml hexane.
** Initial preparation: mol. wt. $654 \pm 50$, charge 2.4943 g in 35 ml of hexane/benzene mixture = 2 : 1; 200 g Al$_2$O$_3$ in 300 ml hexane.

oligomerization $n = 2, 3, 4, 5$ and 6, and their physicochemical constants were established.

1. It has been shown that the iodine number of the isolated oligomers corresponds to the content of one double bond per monomer unit, which testifies to the linear structure of the isolated oligomer homologues with $n > 2$.

Institute of Physico-Organic Chemistry
Academy of Sciences of the BSSR

Received
4 XII 1964

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