CATALYTIC ACTIVITIES OF LANTHANIDE COMPOUNDS IN THE POLYMERIZATION OF ISOPRENE

Abstract

The differences in catalytic activities of the lanthanide compounds (from lanthanum to lutetium) toward the polymerization of isoprene are investigated. Two catalyst systems are studied, i.e. a binary system LnCl 3 ·(TBP) 3 —i—Bu 3 Al and a ternary system Ln (naph) 3 —i—Bu 3 Al—Et 3 Al 2 Cl 3 (Ln-ion of rare earth element; TBP-trin-butylphosphate; naph-naphthenate). It turns out that NdCl 3 ·(TBP) 3 shows the highest activity, while LnCl 3 ·(TBP) 3 complexes of Eu, Sm, Tb, Dy, Er, Tu, Yb and Lu do not polymerize iso- prene or show very low activities. Similar results are obtained when Ln(naph)_3 is used. Spectrophotometric studies show that in contrast to Eu and Sm, which are re- duced from trivalent to divalent when mixed with the aluminum compounds such as Pr, Nd, Gd and Er in the catalytically active system Ln (naph) 3 —Bu 3 Al—Et 3 Al 2 Cl 3 , all exist in a valence state of 3. In view of the same valence of 3, the difference in the catalytic activities in the polymerization of isoprene of these latter elements is still not explicable on the basis of the valence state of the elements. Infrared spectra of LnCl 3 ·(TBP) 3 show that there is no correlation between the catalytic activities of these complexes and their v p =0 values. Data of laser Raman spectroscopy reveal identical characteristic frequencies of Ln—Cl in LnCl 3 ·(TBP) 3 which are quite different in the catalytic activities (for exam- ple, Ce, Nd, Pr, Sm, Eu, and Yb). It seems that there is no relationship at all between the catalytic activities and the bond energies of Ln—Cl. Taking all the facts mentioned above into account, we believe that the difference in the catalytic activities of the present rare earth catalyst systems may be related to the number of electrons in 4f-orbitals. Perhaps, the 4f-orbitals of these trivalent rare earth ions take part in the bond formation to a certain degree. Hence, we venture to explain the difference in catalytic activities in terms of the molecular orbital theory.