Structure and polymorphic behavior of high molecular weight poorly syndiotactic polypropylene

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.