A modified method for th_e determination of the number of isotactic sites C* and the overall propagation rate constant k, of propene polymerization catalysts with ''C-labelled carbon oxides is proposed: an excess of 14C02 or I4CO is added in the abscence of propene and alkylaluminium to a non-deactivated polymer sample drawn from the polymerization slurry.The specific radioactivity of the polymer as a function of the time of contact with the radioactively labelled inhibitor sharply increases within short times of contact (14C02 s 20 min, 14C0 I 5 min). This is followed by a slower further increase reaching a finite value after 5 to 7 h. The initial and end values differ by a factor of 2 to 3. The results are explained in terms of sites with different reactivity assuming that the total number of isotactic sites is determined only after long times of contact with the inhibitor. So far C* was determined with 14C02 or I4CO by adding the inhibitor during the polymerization in the presence of propene and alkylaluminium; c * was calculated after short times of contact with the inhibitor, whereas the further steady increase in specific radioactivity was exclusively attributed to side reactions caused by reaction with propene a_"d alkylaluminium.By this modified method we obtained for TiCl, . (1/3) AlC1, at 60°C k, = 13 dm3/(mol 1 s) and c* = 0,012 mol Ti*/mcl Titdd. Addition of a free Lewis base to the polymerization mixture increases c_' leaving k, unchanged. In contrast to this, if TiC1, * (1 /3) AlCI, is milled with a Lewis base k, can be increased by a factor of =2,5 yith lower or equal c*-values. The higher activity of extracted TiCl,-catalyst is due to higher k,and c*-values as compared to those using TiCl, * (1/3) AICI,. a) Vorgetragen auf dem IUPAC ,,International Symposium on Macromolecules" in Florenz, 7. -12. 9. 1980. 0025-1 16X/82/02 0489-16/W3.00 + TBPC) Katal. A") 400-840 1,4-3,1 0,8-1,5 0,64-1,0 19-31 0,21-0,40 49 Katal. BI2) 1 300 4,7 1,4 1 3 23 0,66 68 a) P,, (5 h) = Produktivitlt nach 5 h Polymerisationsdauer. b, ATi (1 h) = Aktivitlt bei 1 h Polymerisationsdauer.