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

Stereoblock polypropylene (PP) produced using metallocene catalysts consist s of alternating stereoregular and stereoirregular sequences. Through a con trol of the isotactic and atactic sequence lengths, the Bernoullian replica tion 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 paramet ers is examined in terms of the isotactic pentad content, [mmmm]. The depen dence of the physical properties of PP on the molecular microstructure indu ced by the chain tacticity was also investigated both experimentally and th eoretically 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 attr ibuted to a possible steric hindrance between the neighbouring methyl side groups. Other configuration-dependent physical properties such as the radiu s of gyration, the persistence length and the molar stiffness function were also investigated for PP of different stereoregular microstructures. It wa s shown by examining the distribution of the dihedral angles that the penta ne effect resulting from the steric hindrance of the side groups is the und erlying principle for the microstructure dependence of the configurational properties of stereoblock PP. (C) 2001 Elsevier Science Ltd. All rights res erved.