Syndiotactic PVC

Differential IR dichroism was used to follow the dynamic strain-induced orientation of the constituent chains in PVC/PCL and NC/PCL blends. It was found for amorphous compatible blends that PCL oriented in essentially the same manner as NC and the isotactic segments of PVC. Syndiotactic PVC segments showed much higher orientation functions, which implied the existence of a microcrystalline PVC phase. 

Commercial PVC is very nearly atactic and therefore crystallises only to a very small extent. Syndiotactic PVC, which can be made only by a rather special polymerisation technique, crystallises with an orthorhombic unit cell. Like the unit cell of polyethylene, there are two chains passing through it, but this time they are oriented at 180° to each other, with the planes of both backbones parallel to the -axis. The cell dimensions are a = 1.026, b = 0.524 and c = 0.507 nm. The structure is shown in fig. 4.18. 

Low-temperature polymerization For the preparation of crystalline PVC, a low-temperature polymerization method is used. An example is redox polymerization at —30°C, using a catalyst system comprising hydrogen peroxide, ferrous salt, and ascorbic acid [149]. Syndiotactic PVC, which shows excellent physical characteristics at higher temperature, is obtained by this method. 

Wet-spinning method Wet-spinning of PVC fiber was used for the manufacture of Leavil by the Montefibre Company. According to their process, syndiotactic PVC is dissolved in cyclohexanone, and the solution is spun into a coagulating bath of a water-alcohol mixture. 

The attention was mainly focused on the C-Cl stretching range of the supposedly mostly syndiotactic chain. Polymerization in urea clathrates provides the purest sample of planar syndiotactic PVC [130], Such a chain shows a large energy gap in the CCl stretching range ( 650 to 820 cm ). It is very likely that defect modes from other structures may generate localized gap modes which may be used as useful structural probes. 

Figure 3-33. Gap mode (described in terms of total displacements of the atoms) for a GG defect in a syndiotactic PVC chain. Notice that the gap mode is localized over a large section of the chain.

Preparation of syndiotactic PVC with cumyl hydroperoxide, sulfur dioxide, and sodium methyl mercaptide as initiator [233]. 

The infrared absorption bands of syndiotactic PVC are classified into three groups from their dichroic properties. When the film is fully stretched, most of the bands show dichroism as shown in Fig. 1 (draw ratio 7). When the film is only partly stretched, some of the bands show the dichroism different from that observed for the fully stretched film. Fig. 1 (draw ratio 2.5) gives the result. This dichroism inversion is observed for four bands [J]. 

In the Raman spectra the A2 vibrations also appear. Thus the assignments for the syndiotactic PVC molecule with the extended zigzag conformation are completed as shown in Table 1. 

Useful information about the stereoregularity and the conformation of the polyvinyl chloride molecule has been obtained from infrared spectra [5i]. For the key bands the C —Cl stretching bands are used. They are coupled with the skeletal deformation vibrations and are conformation sensitive. Fig. 1 shows the C —Cl stretching frequencies of syndiotactic polyvinyl chloride. The various conformations of the syndiotactic and atactic polyvinyl chlorides and their model compounds have their characteristic C —Cl stretching frequencies [5i]. These results clearly show that the syndiotactic PVC takes the extended zigzag structure and the atactic part causes the chain folding. The possibility of utilizing the backbone vibrations for the conformation study has not been explored for PVC. 

The PVC crystaUites are smaU, average 0.7 nm (3 monomer units), in the PVC chain direction, and are packed lateraUy to a somewhat greater extent (4.1 nm) (21,33). A model of the crystaUite is shown in Figure 6. The crystalline stmcture of PVC is found to be an orthorhombic system, made of syndiotactic stmctures, having two monomer units per unit cell and 1.44—1.53 specific gravity (34—37). 

X-ray studies indicate that the vinyl chloride polymer as normally prepared in commercial processes is substantially amorphous although some small amount of crystallinity (about 5% as measured by X-ray diffraction methods) is present. It has been reported by Fuller d in 1940 and Natta and Carradini in 1956 that examination of the crystalline zones indicates a repeat distance of 5.1 A which is consistent with a syndiotactic (i.e. alternating) structure. Later studies using NMR techniques indicate that conventional PVC is about 55% syndiotactic and the rest largely atactic in structure. 

Millan (98) studied the effect of tacticity on the ionic dehydrochlorination and chlorination of PVC. For the dehydrochlorination reaction, both the reaction rate and the polyence sequence distribution depend markedly on the syndiotactic content. Chlorination appeared to be easier through heterotactic parts than through syndiotactic sequences as shown by C-NMR. 

Like polypropylene, PVC has the problem of stereospecificity. The carbon atom to which the chlorine atom is attached is asymmetrical. (See Figure 23-8.) As a result, PVC molecules can be iso tactic, syndiotactic, and atactic. Commercial PVC is only 5—10% crystalline—low percent isotactic. It is more dense, 1.3 to 1..8 g/cc, than the polyolefins, (fe Figure 23—9.)... 

FIGURE 2.5 Skeletal formulas of isotactic, syndiotactic, and atactic poly(vinyl chloride) (PVC). 

Tacticity of the PVC varies according to the particular reaction conditions but generally manufactures favor a syndiotactic form with many PVC materials being about 50%i sPVC. The reported amount of crystallinity is in the range of 5%i-10%i. This allows for a material with some strength, but one with sufficient amorphous regions to retain good flexibility. 

Although atactic polymers with bulky pendant groups are usually amorphous, atactic polymers with small pendant groups, such as polyvinyl alcohol (PVA), may be made to be crystalline. Some polymers, such as PVC, may have long sequences of syndiotactic sequences in addition to atactic sequences, and hence are somewhat crystalline. 

On the basis of x-ray and infrared studies we may now accept the syndiotactic chain configuration for crystalline PVC. If, in addition, we accept the proposed crystal structure [Natta and Corradini (154)), it is possible to base the symmetry analysis of the spectrum on the unit cell instead of just on a single chain, as was done in the earlier work [Krimm and Liang (101)). The results of this analysis are given here. The unit cell, shown in Fig. 10, has the following symmetiy elements E, Cs2(a), 6(b). C (c), [Pg.123]

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