Syndiotactic polystyrene crystalline polymorphism

Whereas most of the early work on crystallization, etc., were concerned with predominantly isotactic chains, the recent developments in synthetic methodologies have enabled the preparation of highly syndiotactic polymers [13,14]. Since the high stereoregularity of these syndiotactic polymers facilitates their crystallization, several papers have been published on the x-ray crystal structure and polymorphism of syndiotactic polystyrene [15-18]. The chain conformation in the crystalline state has also been analyzed using NMR [19]. Similarly, the crystal structure of syndiotactic polypropylene has also been studied by a number of authors [20-22]. 

Syndiotactic polystyrene, s-PS, is a highly stereoregular, semicrystalline vinyl polymer that normally melts at 270 °C [45,46], S-PS shows a large number of crystalline polymorphs [47-50] obtained by melt crystallization and solvent-exposure techniques. However, s-PS assumes only two distinct crystalline conformations [all trans, planar zigzag (... tttttttt...) and 2 -helical (... ttggttgg. ..) t= trans and g = gauche], which are characterized by fiber repeats of... 

Syndiotactic polystyrene (s-PS) can be crystallized into two polymorphs [26], forms I and II. The alUtrans, planar zigzag conformation is adopted by the chains in form I crystals, while in form II crystals the s-PS chains adopt the same 4i-helical,.., ttggttgg... conformation found for the prevalent form of crystalline s-PP. Notice the CH2 carbon resonances [26] in form II... 

Figure 10.10 Schematic representation of the main interconversion conditions for the helical polymorphic crystalline forms of syndiotactic polystyrene. G stands for gnest molecules of 6 and e phases.

Rapacciuolo, M., De Rosa, C, Guerra, G., Mensitieri, G, Apicella, A., Del Nobile, M. A. Different solvent stability of the crystalline polymorphic forms of syndiotactic polystyrene./. Mater. Sci. Lett, 10,1084-1087 (1991). 

Fully syndiotactic polystyrene is a relatively recent discovery and polymers are now manufactured commercially. Only limited structural information is available [40-47]. It has four main crystalline polymorphic forms a, P, y and 8. The attractive and interesting physical characteristics of this polymer can be summarised as follows (1)... 

Table 1 Polymorphic Crystalline Structure of syndiotactic polystyrene[26]...

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