Influence of monomer structure on chemo‐ 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°C, and a crystalline 1,2 syndiotactic polymer at −20°C. This effect was attributed to a different mode of coordination of the monomer, which is cis‐η 4 at +20°C and may be trans‐η 2 at −20°C. Results obtained with catalysts from CpTi(OBu) 3 and Ti(OBu) 4 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.