Propylene Ziegler-Natta polymerization: Numbers and propagation rate constants for stereospecific and non-stereospecific centers

The data on the number of active centers (Cp) and values of propagation rate constants (Kp) have been obtained for stereospecific and non-stereospecific centers at propylene polymerization with catalytic systems of various composition: TiCl3-AlEt3 (I), TiCl4/D/MgCl2-AlEt3 (II), TiCl4/D/MgCl2-AlEt3-TES (III), where D - dibutyl phthalate, TES - tetraethoxysilane. The Cp and Kp values were determined by 14CO quenching method at the polymerization in the absence and the presence of hydrogen. The Cp and Kp values were determined for three PP fractions: boiling pentane soluble fraction (atactic PP), boiling heptane soluble fraction (stereoblock PP) and boiling heptane insoluble fraction (isotactic PP). The following results are obtained. (i) The Kp values increase in the following order: atactic centers < stereoblock centers < isotactic centers (from ≈ 200 to ≈ 3 · 103 l/mol · s, 70°C) for all catalysts studied. (ii) The respective Kp values do not differ essentially with the change of the catalyst composition (systems I, II, III). (iii) As the external donor is introduced in TMC (system III), the portion of stereospecific active centers increases (from 34 to 62%), but the total number of active centers changes insignificantly. Most likely the non-stereospecific active centers transform into stereospecific ones in the presence of silane.