Crystallization behavior of isotactic propylene-ethylene and propylene-butene copolymers: effect of comonomers versus stereodefects on crystallization properties of isotactic polypropylene.

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 alpha and gamma forms of isotactic polypropylene (iPP) have been discriminated. Both iPPEt and iPPBu copolymers crystallize from the melt as mixtures of alpha and gamma forms. The amount of gamma form increases with increasing crystallization temperature, comonomer concentration and content of rr stereodefects. In iPPBu copolymers the amount of gamma 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 alpha and gamma forms at low and high concentrations, respectively. These data have indicated that the crystallization of gamma form of iPP is not only related to the value of the average length of the regular propylene sequences <LiPP>, 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 alpha and gamma forms of iPP. Butene units are included indifferently in crystals of alpha and gamma form, but probably more easily in the alpha 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 gamma form. For butene concentrations higher than 10 mol%, the effect of inclusion of butene units in crystals of alpha form prevails, producing a decrease of the amount of gamma form and crystallization of the pure alpha form for butene contents higher than 30 mol%.