Crystallization Behavior of Isotactic Propylene−Ethylene and Propylene−Butene Copolymers: Effect of ComonomersversusStereodefects on Crystallization Properties of Isotactic Polypropylene

Abstract

Isotactic propylene−ethylene (iPPEt) and propylene−butene (iPPBu) copolymers have been prepared with different metallocene catalysts. The different influences of stereodefects (isolated rr triads), ethylene and butene comonomeric units on the crystallization of the α and γ forms of isotactic polypropylene (iPP) have been discriminated. Both iPPEt and iPPBu copolymers crystallize from the melt as mixtures of the α and γ forms. The amount of the γ form increases with increasing crystallization temperature, comonomer concentration, and content of rr stereodefects. In iPPBu copolymers, the amount of the γ form decreases for concentration of butene units higher than 10−14 mol % and is always lower than that crystallized in iPPEt copolymers. Butene units, therefore, favor crystallization of the γ and α forms at low and high concentrations, respectively. These data have indicated that the crystallization of the γ form of iPP is not only related to the value of the average length of the regular propylene sequences 〈L iPP〉, but is also related to the inclusion of stereodefects and constitutional defects in the crystals of iPP. Very different proportions of ethylene and butene units are included in crystals of the α and γ forms of iPP. Butene units are included indifferently in crystals of the α and γ forms, but probably more easily in the α form at high concentrations. Therefore, at low butene concentration, up to nearly 10 mol %, the effect of shortening of the length of regular isotactic propylene sequences prevails and induces crystallization of the γ form. For butene concentrations higher than 10 mol %, the effect of inclusion of butene units in crystals of the α form prevails, producing a decrease of the amount of the γ form and crystallization of the pure α form for butene contents higher than 30 mol %.