THE MECHANISM OF THE CHAIN-PROPAGATION REACTION AND SITE DISTRIBUTIONIN ACTIVITY AND STEREOSPECIFICITY IN PROPYLENE POLYMERIZATION WITH HETEROGENEOUS ZIEGLER-NATTA CATALYTIC-SYSTEMS

Propylene polymerization with heterogeneous catalytic systems is shown to proceed by a polycenter mechanism. The catalytically active sites differ in reactivity and stereoregulating ability, these two types of non-uniformity not being interrelated. The presence of a minimum on th e temperature dependence of the stereoregulating ability of isospecifi c active sites (k(iso)/k(syndio)-T) indicates a two-stage mechanism fo r chain propagation, which involves monomer coordination on active sit es and its subsequent insertion into the M-C bond. The stereospecifici ty of the catalytic systems can be modified by changing a ligand envir onment of asymmetric active sites of introducing halogen atoms with hi gher ionic radii into the alpha-TiCl3 lattice and using donor compound s. It is assumed that a group of atoms bound together through halogen bridges, rather than a single Ti atom, participates in the catalytic a ct. A correlation between the stereospecificity of active sites in hom opolymerization and the rate constants for ethylene and propylene copo lymerization on these sites is revealed, which accounts for the compos ition nonuniformity of copolymers. The nonuniform reactivity of the ac tive sites and its influence on the molecular weight distribution (MWD ) of polypropylene are confirmed by modeling MWD under conditions of ' 'quasiliving'' polymerization and by fractionally inhibiting the activ e sites by a catalytic poison.