Vinyl polymers crystalline configuration

In his early papers Natta referred to the state of the art of that time along the lines discussed by Floty (28). Until the early 1950s, the lack of crystallinity in vinyl polymers was attributed to the configurational disorder present in such compounds, considered as copolymers of d and I stmctural units. These units were defined with reference to a macromolecule having the chain in a zigzag planar disposition, the substituents of one type being above the plane... 

Chain configuration in crystalline vinyl polymers. J. Polymer Sci. 18,... 

A particular state of tacticity is a particular configuration of the molecule and cannot be changed without breaking and reforming bonds and, at ordinary temperatures, there is not enough thermal energy for this to happen. Rotations around bonds produce only different conformations. A vinyl polymer is therefore unlikely to be appreciably crystalline unless it is substantially either isotactic or syndiotactic the atactic chain cannot get into a state in which it has translational symmetry. 

The configurational sensitivity falls within a range from triad to pentad for most vinyl polymers. In non-crystalline polypropylenes, three distinct regions corresponding to methylene (46 ppm), methine (28 ppm) and methyl (20 ppm) carbons are observed in the C-NMR spectrum. (The chemical shifts are reported with respect to an internal tetramethylsilane (TMS) standard.) The spectrum of a 1,2,4-trichlorobenzene solution at 125 "C of a typical amorphous PP is shown in... 

Poly(vinyl fluoride) [24981-14-4] (PVF) is a semicrystaltiae polymer with a planar, zig-zag configuration (50). The degree of crystallinity can vary significantly from 20—60% (51) and is thought to be primarily a function of defect stmctures. Wide-line nmr and x-ray diffraction studies show the unit cell to contain two monomer units and have the dimensions of a = 0.857 nm, b = 0.495 nm, and c = 0.252 nm (52). Similarity to the phase I crystal form of poly (vinytidene fluoride) suggests an orthorhombic crystal (53). 

Syndiotactic polymers, as we have seen above, are stereoregular and so are crystallizable. They, however, do not have the same mechanical properties as isotactic polymers, because the different configurations affect the crystal structures of the polymers. Most highly stereoregular polymers of commercial importance are isotactic, and relatively few syndiotactic polymers are made. Atactic polymers, on the other hand, are usually completely amorphous unless the side group is so small or so polar as to permit some crystallinity. Thus, while atactic poly(vinyl acetate) has never been crystallized, poly(vinyl alcohol), which is derived from it and is also atactic, has been found to crystallize. 

The dyads in commercial poly(vinyl chloride) (PVC) are about 0.55% racemic, indicating short runs of syndiotactic monomer placements. The absence of a completely atactic configuration is reflected in the low levels of crystallinity in this polymer, which have a particular influence on the processes used to shape it into useful articles. 

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