Syndiotactic vinyl

Analogous but even more complex is the analysis of syndiotactic vinyl polymers. In the first model, 7, individual macromolecules are achiral through the presence of the c and m planes the sequence of tertiary atoms (stereogenic but not chirotopic) will be. . . r, j, r, j, r, s. . . . 

When it is necessary to specify the internal stereochemistry of the group, a prefix is required. In vinyl polymers there are meso (m) and racemic (r) diads and mm, mr, rr triads. The latter may be called isotactic, heterotactic and syndiotactic triads, respectively. Stereoregular vinyl polymers can be defined in terms of the regular sequences of diads thus an isotactic vinyl polymer consists entirely of m diads, i.e., it corresponds to the following succession of relative configuration -mmmmmm-, whereas a syndiotactic vinyl polymer consists entirely of r diads, corresponding to the sequence -rrrrrrr-. Similarly, a vinyl polymer consisting entirely of mr (= rm) triads is called a heterotactic polymer. 

Since isomerically pure polymers were not available, three different kinds of BR, each relatively high in one of the three kinds of base units were used as standards [35]. The band near 1308 cm 1 was identified [38,39] with the cis isomer and used for analyses [43]. The 1308 cm 1 band is weak and relatively broad, with the appearance of an unresolved doublet (1306,1311 cm 1). The cis band at 730 cm 1 is more frequently used in spite of some difficulties. Relatively pure, crystalline stereoregular polymers have been prepared and structures were determined by X-ray diffraction for cis [44], trans [45] and syndiotactic vinyl [46] and isotactic vinyl [47]. Infrared spectra [48-50] have been published for the four stereoregular polybutadienes, with detailed analyses of the spectra and band assignments for cis [51], trans [51] and syndiotactic vinyl [51] polymers. For the spectrum of isotactic vinyl BR, bands at 1232, 1225, 1109, 943, 876, 807 and 695 cm"1... 

Isotactic and syndiotactic vinyl polymers cannot be chiral unless they adopt a single-handed helical conformation because they possess a mirror plane, and they do not show optical activity except for that due to the asymmetric centers in the vicinity of the chain ends. Several negative examples have been shown such as those for the polymers of styrene,propylene, methacrylates,and acrylonitrile. ... 

The case in which this site (e.g. a double bond or an asymmetric carbon atom) is repeated along the polymer main chain with the same configuration is much more frequent both in natural (natural rubber, natural peptides, poly-p-hydroxybutyrate) and synthetic polymers (isotactic vinyl polymers) than the case in which stereoregularity arises from ordered successions of different configurations. Indeed, up to now, only the ordered alternation of the two possible configurations of asymmetric carbon atoms (syndiotactic vinyl polymers and alternating copolymers of D- and L-amino acids) has been recognized. 

Figure 5. Spin Systems in Syndiotactic Vinyl Polymers.

All definitions in polymers refer to ideal situations, so an "ideal isotactic vinyl polymer is a polymer caracterized by a succession of all m diads an "ideal" syndiotactic vinyl polymer is a polymer which is caracterized by a succession of all v diads. According to these conventions, the isotactic polymer of propylene can be indicated ... 

Polyvinyl chloride (PVC) has peculiar material characteristics because of its high level of syndiotactic vinyl chloride units. The polymer as a result has a low to modest level of crystallinity (-10%). The uncrystaUized regions in PVC are subject to dissolution by polar solvents. The solutions formed have exothermic heats of mixing and negative deviations from ideality. Non-volatile polar liquids, which dissolve amorphous PVC, are called plasticizers . PVC now becomes an elastomeric mixture of small crystalline regions and a solution phase. These materials are called plasticized polyvinyl chloride. 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic stmctures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic stmctures also exists. Of these possible stmctures, only... 

Fig. 2. Stereoisomerism in vinyl polymers (a) isotactic (b) syndiotactic and (c) atactic.

The nmr spectmm of PVAc iu carbon tetrachloride solution at 110°C shows absorptions at 4.86 5 (pentad) of the methine proton 1.78 5 (triad) of the methylene group and 1.98 5, 1.96 5, and 1.94 5, which are the resonances of the acetate methyls iu isotactic, heterotactic, and syndiotactic triads, respectively. Poly(vinyl acetate) produced by normal free-radical polymerization is completely atactic and noncrystalline. The nmr spectra of ethylene vinyl acetate copolymers have also been obtained (33). The ir spectra of the copolymers of vinyl acetate differ from that of the homopolymer depending on the identity of the comonomers and their proportion. 

Homopolymerization of butadiene can proceed via 1,2- or 1,4-additions. The 1,4-addition produces the geometrically distinguishable trans or cis stmctures with internal double bonds on the polymer chains, 1,2-Addition, on the other hand, yields either atactic, isotactic, or syndiotactic polymer stmctures with pendent vinyl groups (Eig. 2). Commercial production of these polymers started in 1960 in the United States. Eirestone and Goodyear account for more than 60% of the current production capacity (see Elastomers, synthetic-polybutadiene). 

Syndiotactic polybutadiene was fkst made by Natta in 1955 (28) with a melting point of 154°C. Syndiotactic polybutadiene [31567-90-5] can be prepared with various melting points depending on its vinyl content and degree of crystallinity. The physical, mechanical, and rheological properties of the polymer are gready affected by these parameters. 

Figure 4.6. Relationship between isotactic, syndiotactic and atactic forms in head-to-tail vinyl polymers. (For simplicity of comparison the main chain in each case is shown stretched in the planar...

As with polybut-l-ene and many other vinyl monomers that contain an asymmetric carbon, isotactic, syndiotactic and atactic stmctures may be drawn. Using co-ordination catalysts such as mixtures of cobalt chlorides, aluminium alkyls, pyridine and water high-1,2 (high vinyl) polymers may be obtained. One product marketed by the Japan Synthetic Rubber Company (JSR 1,2 PBD) is 91% 1,2, and 51-66% of the 1,2 units are in the syndiotactic state. The molecular mass is said to be several hundred thousand and the ratio MJM is in the range 1.7-2.6. 

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. 

Inspection of models show that the interaction between the non-bonded atoms is different in an isotactic and a syndiotactic sequence. A somewhat naive calculation based on van der Waals repulsions between the nonbonded atoms, suggests that for nonpolar vinyl monomers the syndiotactic sequence may be favored by a few tenths of a kcal/mole, and this difference might increase to... 

The vinyl chloride monomer polymerizes via addition polymerization to form polyvinyl chloride. The final polymer has the chemical composition shown in Fig. 22.1. The polymer exhibits limited crystallinity, though this property is not often considered as important in defining its performance. It tends to be atactic or regionally syndiotactic, surrounded by extended atactic runs. When exposed to temperatures above 100 °C, polyvinyl chloride decomposes, creating free radicals that further attack the polymer chain, as we shall discuss in more detail later. For this reason, the degradation of polyvinyl chloride is autocatalytic... 

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