CRYSTALLIZATION KINETICS OF METALLOCENE TYPE POLYPROPYLENES - INFLUENCE OF MOLECULAR-WEIGHT AND COMPARISON WITH ZIEGLER-NATTA TYPE SYSTEMS

The crystallization kinetics from the melt of metallocene type isotact ic poly(propylenes) having the same chain defect concentration and mol ecular weights ranging from 68480 to 288430 have been studied by diffe rential scanning calorimetry. The crystallization rates and the variat ion of the rates with crystallization temperature follow a pattern tha t is basically independent of molecular weight. This result contrasts with the molecular weight dependence on the crystallization rate obser ved in linear polyethylene, random ethylene copolymers as well as othe r semicrystalline systems. Most significant is the fact that the metal locene poly(propylenes) show apparently significantly higher sigma(c) sigma(u) products than do the Ziegler type fractions of matched molecu lar weight and defect concentration. This difference can be interprete d as the metallocene type crystallites having higher effect on surface interfacial free energies than the Ziegler type, or can result from t he two different chain types having different sequence propagation pro babilities.