INFLUENCE OF MONOMER STRUCTURE ON CHEMOSELECTIVITY AND STEREOSELECTIVITY OF 1,3-DIENE POLYMERIZATION

MAO/CpTiCl(3) is an active catalyst for the polymerization of various types of 1,3-dienes. Butadiene, (E)- and (Z)-1,3-pentadiene, (E)-2-methyl-1,3-pentadiene and 2,3-dimethylbutadiene yield, at room temperature, polymers with a cis-1,4 or a mixed cis/1,2 structure. 4-Methyl-1,3-pentadiene and (E,E)-2,4-hexadiene give, respectively, a 1,2 syndiotactic and a trans-1,4/1,2 polymer. MAO/CpTiCl(2).2THF and MAO/(CpTiCl(2))(n) are less active than the CpTiCl(3) catalyst, but give the same type of polymers. A change of stereospecificity with temperature was observed in the polymerization of (Z)-1,3-pentadiene: a cis-1,4 isotactic polymer was obtained at +20 degrees C, and a crystalline 1,2 syndiotactic polymer at -20 degrees C. This effect was attributed to a different mode of coordination of the monomer, which is cis-eta(4) at +20 degrees C and may be trans-eta 2 at -20 degrees C. Results obtained with catalysts from CpTi(OBu)(3) and Ti(OBu), are reported for comparison. An interpretation is given of the formation of cis-1,4 isotactic poly(2-methylpentadiene) and of 1,2 syndiotactic poly(4-methylpentadiene), as well as of syndiotactic polystyrene.