Structure and Polymorphic Behavior of High Molecular Weight Poorly Syndiotactic Polypropylene Claudio De Rosa,* Finizia Auriemma, and Odda Ruiz de Ballesteros Dipartimento di Chimica, Universita ` di Napoli “Federico II”, Complesso Monte S. Angelo, Via Cintia, 80126 Napoli, Italy Received June 13, 2003; Revised Manuscript Received November 4, 2003 ABSTRACT: An analysis of the structure and polymorphic behavior of high molecular weight, low stereoregular, poorly crystalline syndiotactic polypropylene (sam-PP), prepared with heterocycle-fused indenylsilylamidodimethyltitanium complexes is presented. The samples do not crystallize by cooling the melt to room temperature but slowly crystallize if they are kept at room temperature for several days. Disordered modifications of the helical form I of syndiotactic polypropylene (s-PP) are obtained, and small degrees of crystallinity (16-20%) are achieved. The stretching of compression-molded films of sam-PP samples produces oriented crystalline fibers in the trans-planar mesomorphic form of s-PP. The low stereoregularity ([rrrr] ) 40-55%) prevents the formation of the ordered trans-planar form III of s-PP, which instead is obtained in stretched fibers of the highly stereoregular and crystalline s-PP. The trans-planar mesomorphic form, in turn, transforms into the helical form I upon release of the tension in stretched fibers of sam-PP samples. The isochiral helical form II of s-PP, which is generally obtained in s-PP fibers initially in the trans-planar form III by removing the tension, has never been observed in the sam-PP fiber samples. These data confirm that the isochiral form II of s-PP can be obtained only starting from the trans-planar form III through a spontaneous cooperative crystal-crystal transformation when the tension in stretched fibers is removed. Introduction The synthesis and a preliminary characterization of high molecular weight, low syndiotactic, poorly crystal- line polypropylene have been recently described. 1 The polymer samples have been prepared with catalytic systems composed of silyl-bridged indenyl-tert-butyl- amido complexes of titanium, in which the indenyl ligand has a heterocycle condensed onto the cyclopen- tadienyl moiety (Chart 1). 2,3 The catalysts 1 and 2 (Chart 1), activated with methylaluminoxane (MAO), show high activity in propylene polymerization, produc- ing polypropylene samples with high molecular weights, 1 and a prevailingly syndiotactic microstructure, with fully syndiotactic pentad contents [rrrr] in the range 40-55%. We have defined these samples syndiotactic- amorphous polypropylene (sam-PP). 1 It is well-known that the stereoregularity of syndio- tactic polypropylene (s-PP) strongly influences its polymorphism. 4-8 A different polymorphic behavior is therefore expected for low stereoregular sam-PP samples. The complex polymorphic behavior of s-PP has been extensively described, 4-20 and clarified mainly through the analysis of highly crystalline and stereoregular s-PP prepared with the C s -symmetric metallocene cata- lyst. 4-7,12-15 Four different crystalline forms, shown in Figure 1, have been found. The most stable form I (Figure 1A,B) 12 and the metastable form II (Figure 1C) 10 are characterized by chains in s(2/1)2 helical conformation, packed in orthor- hombic unit cells. The metastable form III (Figure 1D) presents chains in trans-planar conformation, 16 whereas form IV (Figure 1E) is characterized by chains in (T 6 G 2 T 2 G 2 ) n helical conformations. 17 Besides the four crystalline forms, a disordered me- somorphic modification, characterized by chains in trans-planar conformation packed in a disordered lat- tice, has also been obtained by quenching the melt at 0 °C and keeping the sample at 0 °C for a long time. 21,22 The formation of the four crystalline modifications de- pends on the condition of crystallization and the stereo- regularity of the sample. 4-8,12-15 Form I (Figure 1A,B) can be easily obtained by crystallization from the melt and from polymer solutions. 12-15 Form II is character- ized by a C-centered structure, where 2-fold helical chains having the same chirality are included in the orthorhombic unit cell (Figure 1C). 10 It is a metastable polymorph of s-PP and has been obtained at atmo- spheric pressure only in oriented fibers of s-PP, 4,6,10,20,23 for instance, by stretching at room temperature com- pression-molded specimens of low stereoregular s-PP samples prepared with vanadium-based Ziegler-Natta catalysts, 4,6,10,20 or upon release of the tension in stretched fibers of highly stereoregular s-PP samples initially in form III, 6,20,23 or, finally, by annealing fibers in form III. 4,6 Recently, form II has been obtained also by melt crystallization at elevated pressure, 24 and by epitaxial crystallization of single crystals. 25 In powder unoriented s-PP samples only disordered modifications of form II, characterized by conformationally disordered chains containing trans-planar sequences (kink band de- fects), 26-28 can be obtained at atmospheric pressure, for instance, by precipitation from solutions of low stereo- regular samples, 26 and in copolymers of s-PP with ethylene comonomeric units. 29-31 * To whom correspondence should be addressed. Phone: ++39 081 674346. Fax: ++39 081 674090. E-mail: derosa@chemistry. unina.it. Chart 1 1422 Macromolecules 2004, 37, 1422-1430 10.1021/ma034805d CCC: $27.50 © 2004 American Chemical Society Published on Web 01/09/2004