Homogeneous Binary Zirconocenium Catalysts for propylene Polymerizatoin. II. Mixtrues of Isospecific and Syndiospecific Zirconocene Systems

摘要

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 characterisitic of s-PP. A solution of C_2-and C_s-symmetric zirconocnees 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 trisobutyluminum (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 ot the amorphous phase and atomic force microscopy finds only microphase separation but not macroscopic phase separation. Fractionation and ~13C-NMR showed the material to contain not only the homosteric i-PP ans s-PP but also a sterrebolick PP (i-PP-b-s-PP). The stereoblock fraction acts as a compatibilzing agent; it probably is produced by the exchange of propagating polymer chains between the syndiospecific and the isospecific sites assisted by the crossover agent.