Isotactic polypropylene (i-PP) and syndiotactic polypropylene (s-PP) obtained with single-site C-2- and C-s-symmetric zirconocenes, respectively, activated with triphenyl carbenium tetrakis(pentafluorophenyl)borate (3) are immiscible with one another. Physical mixture of the two PPs is macrophase separated and crystallizes very slowly as is characteristic of s-PP. A solution of C-2- and C-s-symmetric zirconocenes with 3 also produces an incompatible mixture of i-PP and s-PP. However, in the presence of an excess of a crossover agent such as triisobutylaluminum (TIBA), a new polymeric material is obtained that exhibits lower T-m and Delta H-f than either of the two homosteric polymers and a much faster rate of crystallization than that of s-PP or its physical mixture with i-PP. Thermal annealing of this product markedly reduces the relative amount of crystalline phase compared to the amorphous phase and atomic force microscopy finds only microphase separation but not macroscopic phase separation. Fractionation and C-13-NMR showed the material to contain not only the homosteric i-PP and s-PP but also a stereoblock PP (i-PP-b-s-PP). The stereoblock fraction acts as a compatibilizing agent; it probably is produced by the exchange of propagating polymer chains between the syndiospecific and the isospecific sites assisted by the crossover agent.