Atactic productivity

Figure 7.10 shows the 60-MHz spectra of poly (methyl methacrylate) prepared with different catalysts so that predominately isotactic, syndiotactic, and atactic products are formed. The three spectra in Fig. 7.10 are identified in terms of this predominant character. It is apparent that the spectra are quite different, especially in the range of 5 values between about 1 and 2 ppm. Since the atactic polymer has the least regular structure, we concentrate on the other two to make the assignment of the spectral features to the various protons. 

In dichloromethane,the polymerization of propylene under higher pressure yields an isotactic polypropylene with a small number of stereodefects, while in toluene a mixture of isotactic and atactic products was obtained. This fact can testify about the presence of two different active catalytic species. The nature of the intermediate responsible for the formation of the atactic fraction has not been fully elucidated yet. On the basis of NMR experiments, it seems that in toluene one of the benzamidinate ligations opens to an rf coordination, and the solvent occupies the vacant site. It was shown that similar heteroallylic ligations (alkoxysilyl-imido) have exhibited dynamic behaviors [64,65]. 

Investigations of the bis(benzamidinate) dichloride or dialkyl complexes of Group 4 metals show that these complexes, obtained as a racemic mixture of c/s-octahedral compounds with C2 symmetry, are active catalysts for the polymerization of a-olefins when activated with MAO or perfluoroborane cocatalysts [29-41]. As was demonstrated above, polymerization of propylene with these complexes at atmospheric pressure results in the formation of an oily atactic product, instead of the expected isotactic polymer. The isotactic polypropylene (mmmm>95%, m.p.=153 °C) is formed when the polymerization is carried out at high concentration of olefin (in liquid propylene), which allows faster insertion of the monomer and almost completely suppresses the epimerization reaction. 

According to Cossee, the isotactic centre has one and the atactic two vacancies. According to Rodriguez, the organometal is coordinated into one of the two vacancies of the isotactic centre. The addition of donors (diethyl ether, triethylamine, pyridine, etc.) greatly affects the polymerization rate (catalyst activity) as well as the ratio of the stereoregular and atactic product components. Donors affect the structure of active centres and modify it. 

Polystyrene (PS) as normally prepared is essentially linear and atactic. Isotactic polymers can be made but this is not of commercial interest because of increased brittleness and more difficult processing than the atactic product. The major application of polystyrene is in packaging. Specific additives are incorporated to achieve product characteristics that depend on the end usage. 

The increase in isotacticity seems to be essentially connected to the decrease of the initial rate, as practically no change in the isotacticity index with polymerization time was detected. Moreover, while the atactic productivity decreases monotonically with the EB/TEA ratio in both systems, the isotactic productivity has a more complex behavior with the binary catalyst it remains almost unchanged up to EB/TEA s 0.25 and then falls, whereas with the ternary catalyst it increases up to EB/TEA 0.2 and then rapidly drops. On the grounds of these results, Spitz suggested that the reversible adsorption on the catalytic surface of the TEA EB complex (which is supposed to be very fast) changes the non specific centers into stereospedfic, though less active, centers, while the slower adsorption of free EB reversibly poisons both types of sites. The differences between the binary and the ternary catalysts would arise mainly from the presence, in the latter, of a larger number of potential stereospecific sites. 

With similar binary and ternary catalysts (but using EB as internal and MPT as external donor), however, rather different results were obtained in our laboratories. With the binary catalyst, a two-step increase of the isotacticity was noticed, the first step (up to MPT/TEA 0.2) being associated mainly with a strong decrease in the atactic productivity, the second (at MPT/TEA > 0.2) with a slightly selective decrease in both the atactic and the isotactic productivity (Fig. 38). 

In view of these elegant results it is difficult to understand the reported high molecular weight atactic products from polymerizations of 2-vinylpyridine initiated by Bu"Li in THF. The broadness of the n.m.r. spectra of these materials is unambiguous, but it is not entirely clear whether this arises from a lack of stereoregularity caused by interference in the regulation process by pyridine groups further down the chain, or by the occurrence of unrelated side-reactions. 

Very little research has been done on the relations between glass transition temperatures and tacticity. Atactic and isotactic poly(styrenes) almost always have the same glass transition temperatures, and this is also the case for at- and it-poly(methacrylate). The glass transition temperature of it-poly(methyl methacrylate) (42° C), on the other hand, is distinctly lower than that of the atactic product (103°C). 

Propylene oxide, O—CH2—CH(CH3), exists as two antipodes. Thus, stereoregular products can be formed during the polymerization of one of the antipodes. On the other hand, polymerization of the racemic monomers often produces atactic products with many head-head linkages. 

The early work [1-6] established that fluorine substituted monomers could be polymerised by classical initiator systems derived from transition metal chlorides, however, the process was poorly characterised and gave largely atactic products with broad molecular weight distributions. 

These have been in use since the 1970s in suspension and gas phase processes, and their yields are around 10 t/kg catalyst. De-ashing is still required and the content of atactic product... 

Unlike the conversion of ethylene to linear polyethylene (PE), propylene polymerization to polypropylene (PP) introduces stereochemical complexity because we can obtain 12.8,12.9 or a random atactic product. Surprisingly, selective formation of syndiotactic propylene (12.9) is seen for many metallocene polymerization catalysts. To see why, we need to know that (f [Cp2ZrR]+ is pyramidal (12.12 in Fig. 

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