Stereoirregularity

The second explanation for the formation of the stereoirregular polymer is that the propagation of 2 is essentially an SN1 type reaction, and that attack d (broken arrow) in 14 is depressed by the interaction of the outer-ring oxygen atom with the positively charged carbon atom of the oxycarbenium ion from the upper side of the... 

The characteristic ratios of stereoirregular 1,4-poiybutadiene and 1,4-polyisoprene chains are theoretically investigated by the Monte Carlo procedure in accordance with the model proposed by Mark (V 001 and V 003). It is pointed out that the presence of discrete cis units in frans-rich chains significantly reduces the characteristic ratio while that of discrete trans units in c/is-rich chains has little effect on the characteristic ratio. The characteristic ratio and its dependence on both the trans and cis contents and their sequence distribution is calculated for stereoirregular polymers in accordance with the interdependent RIS model proposed by Mark (V 001 and V 003), and Ishikawa and Nagai V 005 and V 007 . 

Due to the use of advanced, highly active and selective solid-state catalysts (sections 7.3), the processes described above produce polymers from which neither stereoirregular polymer components nor catalyst residues need be removed. This has resulted in substantial reductions in the costs of investments, energy and maintenance, compared to slurry processes with first-generation catalysts. Ongoing developments are aimed at increased process flexibility and at process adaptation to the use of supported metallocene catalysts (Section 7.4). 

Polymerization of nonsymmetrical cyclic molecules gives stereochemi-cally variable polymers, [-SiRR 0-] , analogous to the totally organic vinyl and vinylidene polymers [-CRR CH2-] . In principle, these polymers can be prepared in the same stereoregular forms (isotactic and syndiotactic) that have been achieved for some of their organic counterparts 1, 17). Unfortunately, the stereoregular forms have not been prepared, and only the stereoirregular form (atactic) has been obtained. Unlike the other two stereochemical forms, the atactic form is inherently noncrystallizable. 

Both these polymers have an - A-B structure [14,15], so their characteristic ratio C(q) was obtained through the procedure outlined in Section 2.1.2 see Eqs. (2.1.33H21.35) in particular. Since we were interested in stereoirregular (i.e., atactic) polystyrene for comparison with experimental data, the matrix procedure based on parameters proposed by Yoon, Sundararajan, and Flory [111] was suitably complemented with the pseudostereochemical equilibrium algorithm, which allows units of opposite configuration to be formally interconvertible with fixed relative amounts [36]. In the temperature range 30-70 °C the results may be fairly well expressed by the following analytical forms ... 

Microstructures of Poly(chlorofluoroethylene)s The carbon-13 NMR spectrum of PVCF consists of a —CH2— resonance at 54.1 ppm and a —CFC1— resonance at 108.8 ppm. There is no splitting of these lines due to tacticity, nor are there any other resonances to indicate the presence of regioirregular monomer sequences. However the polymer is stereoirregular, as shown by the fluorine-19 NMR spectrum in Figure 1. There are three principal resonances spread by 3 ppm owing to triad stereosequences, with some pentad fine structure which is barely resolved. 

The fluorine-19 NMR spectrum of PCFE appears far more complicated. Figure 2 shows the spectra from two PCFE samples, one prepared at 60 °C (a) and one prepared at -80 °C in urea (b). Each backbone carbon is a pseudoasymmetric center in PCFE, compared to every second carbon in PVCF, so that the dispersion of fluorine-19 chemical shift from stereoirregularity is much larger. This dispersion is almost 15 ppm for PCFE, and is similar to the spread observed in the fluorine-19 NMR spectrum of poly(l,2-difluoroethene) (14). 

As polymer chains are usually long and flexible, they would be expected to pack randomly in the solid state to give an amorphous material. This is true for many polymers, particularly those with an irregular chemical structure. Examples are the stereoirregular materials atactic polystyrene (1.10) and atactic polypropylene (1.11), in which the Ph and the Me substituents, respectively, are randomly oriented. 

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