Crystallinity syndiotactic polymers

Polymerization of Propylene To A Crystalline Syndiotactic Polymer. J. Amer. chem. Soc. 84, 1488 (1962). 

Natta, G Pasquon, L ZambeUi, A. Stereospecific catalysts for the head-to-taU polymerization of propylene to a crystalline syndiotactic polymer. J. Am. Chem. Soc. 1962, 84,1488-1490. 

Pure amorphous polymers, being homogeneous materials, are transparent. Atactic polystyrene is a good example. The crystalline syndiotactic form is not transparent. Alack of transparency does not necessarily indicate crystallinity, however. It can also be caused by inorganic fillers, pigments, gas bubbles (as in a foam), a second polymer phase, etc. 

Stereodefects in syndiotactic polymers have similar effects to those in their isotactic counterparts, reducing crystallinity levels and changing the associated physical characteristics. 

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. 

There has been no report of oriented crystalline syndiotactic PMMA so far as the author knows. The author and his coworkers (39) found that oriented crystalline samples of syndiotactic PMMA can be obtained by adsorption of various solvents such as chloro-acetone and diethyl ketone. Figure 8 shows the x-ray diagrams. When the polymer is cast from chloroform solution and is stretched in hot water at 80°C, the sample is noncrystalline as shown in Figure 8. By adsorption of the solvent the fiber diagram can be... 

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... 

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

Syndiotactic Polymers with an alternating configuration at their tertiary carbon atoms (see Fig. 15). They are partially crystalline. 

Stereochemistry also affects the crystallinity of a polymer. Stereoregular isotactic and syndiotactic polymers are generally more crystalline than atactic polymers. By careful choice of catalysts, we can make a linear polymer with either isotactic or syndiotactic stereochemistry. 

IR spectra of the cis-1,4 isotactic or syndiotactic polymers, in the molten state or in solution, and of the amorphous polymers are practically identical. All are characterized by an intense band at 751.8 cm.-1 (cis double bonds). In the spectra of the solid isotactic or syndiotactic polymers, however, new bands appear which are typical of the crystallinity of the polymers (Figure 1 and 2). The positions of the most intense of these bands are as follows (1) isotactic polymers 746.2 843.8 925.9 1005 cm.-1 (2) syndiotactic polymers 757.5 854.7 925.9 1000 1136 cm."1 It is interesting to observe that the band of the... 

Each carbon within the polymer chain may be considered as a chiral center. As a result, a parameter referred to as the tacticity may be defined that describes the stereoregularity of adjacent carbon centers (Figure 5.6). A polymer that contains substituents on the same side of the polymer chain is referred to as isotactic, and often exhibits some degree of crystallinity. In contrast, if adjacent substituents are arranged on alternating sides of the polymer chain, a syndiotactic polymer is formed. Unless a... 

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. 

Polymerization of butadiene by modified cobalt catalysts, such as the in situ system [CoIII(acac)3]/AlEt3/H20/CS2, can yield highly crystalline syndiotactic 1,2-poly-butadiene (Tm 205 °C) [35, 36], Such stereospecific polymerizations can be carried out in aqueous emulsion [37-39], and polymer latexes can be obtained (Eq. 3) [38]. A prepolymerization with a small portion of butadiene monomer in the presence of only trace amounts of water is required. A hydrocarbon solution of this prepolymerized catalyst mixture is subsequently dispersed in water together with further butadiene, and polymerization proceeds. It has been speculated that... 

As formed in conventional commercial processes, PVC is highly amorphous with a very low degree of crystallinity associated with its syndiotactic sequences. Thus, PVC produced in the normal fashion contains about a 10% degree of crystallinity. However, if the polymerization is carried out at very low temperatures, there is a marked increase in crystallinity. Thus, polymer prepared at... 

---