Syndiotactic structures chain conformation

Molecular mechanics techniques are employed to calculate the molecular structure and conformational energies of model compounds for polyphenylmethylsllylene and polysilastyrene. In both isotactic and syndiotactic stereochemical forms. The structural and conformational energy data provided are used to calculate, by application of the RIS theory, the unperturbed chain dimensions, given as the characteristic ratio, and its temperature coefficient. 

Rather recently, we have studied the solid-state structure of various polymers, such as polyethylene crystallized under different conditions [17-21], poly (tetramethylene oxide) [22], polyvinyl alcohol [23], isotactic and syndiotactic polypropylene [24,25],cellulose [26-30],and amylose [31] with solid-state high-resolution X3C NMR with supplementary use of other methods, such as X-ray diffraction and IR spectroscopy. Through these studies, the high resolution solid-state X3C NMR has proved very powerful for elucidating the solid-state structure of polymers in order of molecules, that is, in terms of molecular chain conformation and dynamics, not only on the crystalline component but also on the noncrystalline components via the chemical shift and magnetic relaxation. In this chapter we will review briefly these studies, focusing particular attention on the molecular chain conformation and dynamics in the crystalline-amorphous interfacial region. 

Syndiotactic poly-p-methylstyrene (PPMS) exhibits various crystalline forms and clathrate stmctures. Bands due to the syndiotactic stereostmcture and bands typical of the two different chain conformations are observed in the CTystalline structures as well as bands sensitive to intermolecular interactions typical of the different modes of chain packing (350). 

Several works have shown that the aggregation of isotactic and syndiotactic chains leads to the formation of stereoeomplexes for which the iso/syndio stoichiometry is found equal to 1/2, probably with a structure composed of a double-stranded helix of a 30/4 helicoidal isotactic chain surroxmded by a 60/4 helicoidal syndiotactic chain. Syndiotactic PMMA self-aggregates exhibit similar structures, with conformations close to extended chains. Experimental data indicate that, in self-aggregated syndiotactic PMMA in solution, some of the ester groups are close in contact, probably in a double helix slructure with solvent molecules included in the cavities of inner- and inter-helices. Isotactic PMMA self-aggregates also exhibit conformational helix structures. 

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]. 

Figure 3.1 The 25-MHz spectra of three preparations of polypropylene isotactic, atactic, and syndiotactic. (From Bovey, F.A., Chain Structure and Conformation (f Macromolecules, Academic Press, New York, 1982. With permission.)...

Once the axial geometry in a polymeric chain is the main factor in determining the ability of a chain to form ciystallites, the ciystalline contribution is probably due to isotactic and syndiotactic structure sequences." " In fact, saturated PHAmcl> which are able to crystallise due to their isotactic configuration, are also seen to crystallise with allq l side chains in an extended conformation to form ordered sheets, but they still show a reduced degree of crystallinity when compared to P(3HB) or P(3HB-co-3HV), owing to low crystallization rates. ... 

The ordered structures of some polymers are governed by the influence of specific diluents. This involves a specific type of polymorphism, the more general aspects of which will be discussed in the chapter concerning thermodynamic quantities. Syndiotactic poly(styrene) is a polymer that is rich in compound formation with solvent mediated polymorphic behavior.( 126-130) The polymer can crystallize in four major crystalline modifications that involve two different chain conformations. In the a and p modifications the chains adopt an all trans planar zigzag conformation. These two modifications are formed by crystallization from the melt and, under special conditions, from solution. In contrast the y and 5 modifications are characterized by a helical conformation. The 5 polymorph can only be prepared in the presence of solvent. Its exact crystal structure depends on the nature of the solvent. Compound formation between the 5 form of the polymer and the solvent has been demonstrated. Complete elimination of the solvent results in the pure, helical y form. 

Syndiotactic poly(styrene) displays a complex polymorphic behavior that reflects the specific role played by solvents. Four crystalline forms have been reported.(289,290) The a and p forms can be obtained from the melt (or glass), depending on the crystallization conditions.(291) Both structures comprise planar zigzag chains that have the same identity period of 5.1 A. The a form has a trigonal unit cell while the p form is orthorhombic. The P form can also be produced by crys-taflization from solution.(292,293) The y and 8 structures develop after interaction with solvent. In contrast to the all trans bond orientation of the a and p structures, the chains in the y and 8 crystals adopt a ttggttgg sequence of bond orientation. Thus a helical ordered structure evolves. This structure is similar to the crystalline chain conformation of syndiotactic poly(propylene).(294) The difference between the y and the 8 polymorphs is that in the former the sample is completely dried, while the solvent is included in the 8 form. It therefore represents a clathrate type structure. The formation of these structures is, thus, solvent specific.(292,293,295,296) The... 

Figure 49 The 220-MHz NMR spectra PP in 1,2-dichlorobenzene at 165 °C (a) isotactic, and (b) syndiotactic. Reproduced with permission from Bovey, F.A. Chain Structure and Conformation of Macromoiecuies. Academic Press, New York, NY, 1982 Vol. 10, p 3. Copyright 1982 Academic Press.

A particular kind of disorder, characterized by maintaining three-dimensional long-range periodicity only for some points of the structure, has been found in samples of syndiotactic polypropylene having a relatively low degree of stereoregularity.189 190 In these samples the chains present conformational disorder, which produces defects frozen in the crystals. 

Conformational disorder and kink-bands structures have recently been found also in random copolymers of syndiotactic polypropylene with small amounts of ethylene.192 193 The ethylene units are included in the crystalline regions193 and induce the crystallization of the metastable form II of sPP with conformationally disordered chains characterized by kink bands. Portions of chains containing the ethylene units tend, indeed, to assume a trans planar conformation, producing the kink-band defects in chains in the prevailing twofold helical conformation.192193... 

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