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 conforma-tions were reported for crystallized sPP in contrast to iPP where exclusively a 3 1-helix is observed. A planar zigzag form (TT)n occurs in the melt or in quenched samples [1]. A second possible chain conformation of a (T 6G2 T 2G2)n type of helix is formed when quenched samples are exposed to solvent vapour [2]. A stable helical conformation with (TTGG)n (s(2/1)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 C222 1 , 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). The orthorhombic type I cell was already described by Corradini et al. and is formed by helices of one hand only (all right-hand or all left-hand). The cell types I to III (Figure l(a-c» are based on the (TTGG)n helix. The dimensions of the type I cell are, a = 1.44 nm, b = 0.56 nm and c = 0.74 nm. The orthorhombic type II cell has the same dimensions as the type I cell but is formed by layers of helices with different hands. Type I and II cells are formed when sPP is rapidly cooled at least 40°C below the equilibrium melting point which is usually lower compared to Polypropylene: An A-Z Reference Edited by J. Karger-Kocsis Published in 1999 by Kluwer Publishers, Dordrecht. ISBN 0 412 80200 7