Crystallization of syndiotactic polypropylene

Abstract

The modern story of syndiotactic polypropylene (sPP) starts with the introduction of metallocene catalysts allowing for a large scale production of this polymer. This initiated immediately a large interest in the crystallization behavior of this polymer. Three different chain conformations were reported for crystallized sPP in contrast to iPP where exclusively a 3₁-helix is observed. A planar zig-zag form (TT)_n occurs in the melt or in quenched samples [1]. A second possible chain conformation of a (T₆G₂T₂G₂)_n type of helix is formed when quenched samples are exposed to solvent vapour [2]. A stable helical conformation with (TTGG)_n (s(2/l)2) symmetry is usually formed by highly stereoregular sPP when isothermally crystallized from the melt or when moderately cooled. For this helical conformation, three orthorhombic packing modes were described; a C-centred cell I with the space group C222₁ a primitive cell II with antichiral packing of chains along the a-axis (space group Pcaa) and finally a body centred cell III with fully antichiral packing of the chains (space group Ibca) [3] (Figure 1).