Tacticity induced molecular microstructure dependence of the configurational properties of metallocene-synthesized polypropylenes

Stereoblock polypropylene (PP) produced using metallocene catalysts consists of alternating stereoregular and stereoirregular sequences. Through a control of the isotactic and atactic sequence lengths, the Bernoullian replication probability and the rate of isomerization of the metallocene catalysts, polymeric chains of various molecular microstructures could be generated. The dependence of the microstructure on the various polymerization parameters is examined in terms of the isotactic pentad content, [mmmm]. The dependence of the physical properties of PP on the molecular microstructure induced by the chain tacticity was also investigated both experimentally and theoretically using Metropolis Monte Carlo techniques. It was shown that the intrinsic viscosity and the characteristic ratio of the polymer are greatly influenced by the stereoregularity of the polymeric chains, which was attributed to a possible steric hindrance between the neighbouring methyl side groups. Other configuration-dependent physical properties such as the radius of gyration, the persistence length and the molar stiffness function were also investigated for PP of different stereoregular microstructures. It was shown by examining the distribution of the dihedral angles that the pentane effect resulting from the steric hindrance of the side groups is the underlying principle for the microstructure dependence of the configurational properties of stereoblock PP.