Iso tactic polymers

It has also been found that the polymer formed from the beginning of the reaction is already prevaiUngly isotactic. This means that, from the start of the reaction, there exist a certain number of active centers on the solid a-TiCU surface which immediately yield iso tactic polymer consequently, it can be excluded that, at least for the active centers present on the initial free surface of a-titanium trichloride, there is an initial activation process, whose rate is slow enough to be observed even when operating at low temperature (30°). 

An iso tactic polymer (side groups on the same side of the backbone)... 

Figure 4-i. Schematic representatioa of the diifeicnt kinds of helix occurring in various iso-tactic polymers —CHR-. (1) 3i (II) 7j (III, IV) 4, (aflo G. Natta, P. Corradini, and... 

Butene-1 is converted to iso tactic polymers with Ziegler-Natta catalysts. Alternatively, cis- or trans-butene-2 can be used as starting material since, in the presence of certain catalyst systems, they isomerize before polymerization. st-Poly(butene-l) is obtained by hydrogenation of st-l,2-poly-(butadiene). 

In iso tactic polymers the configuration of the steric centers on the backbone is identical. The net result is that all side groups are positioned on the same side of the chain, as illustrated schematically in Fig. 1.8 a). 

Iso tactic poly(methyl methacrylate) (it-PMMA) can form a stereocomplex with st-PMMA. Recent X-ray studies 179) of this material indicate that the two polymer chains probably interact to form a double helical structure. The it-PMMA chain forms the inner helix and is surrounded by the st-PMMA helical chain which winds around it. If subsequent work confirms this model, this material would constitute a most unusual inclusion compound involving only one monomeric substance. 

Small amounts of iso tactic polystyrene have been synthesized in the laboratory using noncommercial polymerization techniques. These polymers are capable of partially crystallizing, albeit at a very slow rate. Syndiotactic polystyrene was available commercially for several years, but its continued production proved unprofitable. 

If the value of this probability parameter is a <= 1, an essentially iso tactic structure is obtained. If, on the other hand a 0, almost all the monomer units in the chain are in syndiotactic placements. If the polymer is capable of crystallization and the crystallization takes place under equilibrium conditions, then the limitation of this model is that a small melting point depression implies also a high degree of percent crystallinity. Although there are a number of systems, for example stereoregular methyl methacrylate (2, 8), in which this is true and this model is valid, this is not the case for polymers of propylene oxide from different catalysts that we discuss in another chapter (1). 

When 4 was activated with B(C6F5)3 (molar ratio B Zr=l), under the same reaction conditions as with MAO, a highly iso tactic polypropylene (mmmm= 98%) was formed, contrary to the results obtained with MAO (Eq. 1). The activity of this catalytic system (1.2xl05g polymer-mol Zr -h ) is lower than the activity of the complex 4 activated with MAO. It is important to point out that complex 4 has a C2-symmetry octahedral geometry, suggesting that theoretically, when activated with MAO, an isotactic polypropylene should also be expected, as it was in the case of the boron-containing cocatalyst. 

As a typical example of a type B polymer, poly(styrene)I may be cited, one member of a general class in which pendant aromatic groups are arranged along the backbone in a random (atactic) Ig, alternating (syndlotactlc) I), or regular (iso-tactic) 1. fashion. 

Stereoregular polymers also have symmetrical structures, and the helices of isotactic polymers can be close-packed to produce highly crystalline material. Iso-tactic polypropylene is crystalline and an important fiber-forming polymer, whereas the atactic form has virtually no crystalline contort and has little value as a fiber indeed, it is considerably more elastomeric in nature. 

Various methacrylates containir heteroatoms in the ester groups were polymerized by Iwakura et al. 1 A wide variety of microstructures were observed in the resulted polymers from 96 % iso tacticity to 66 % syndiotactidty. In general the ndiotacticity was found to depend upon the basicity of fhe ester group. 

Polymers of higher tacticity have more rigid chains than atactic polymers, since more base units are fixed in conformational sequences of the coil, which corresponds formally to an increase in the steric hindrance parameter a. With a predominantly iso tactic poly(methyl methacrylate), for example, a X = 0.087 cm /g was obtained in heptanone-3 (0 = 40.0°C), whereas an at-PMMA in the same solvent gave only K = 0.063 cm /g (0 = 33.7°C). At higher temperatures, potential barriers to rotation can be more readily surmounted. The K values then become practically identical 0.057 for it-PMMA (0 = 152.rC) and 0.058 for at-PMMA (0 = 159.7°C) in p-cymene. 

Stereoregular polymer n. A regular polymer whose molecules can be described by only one species of stereorepeating unit in a single sequential arrangement. A stereoregular polymer is always a tactic polymer, but a tactic polymer need not have every site of stereoisomerism defined (ISO). 

Figure 1 Polymerization of methyl methacrylate in NN dimethylformamide solution at 25 °C in the presence of isotactic polyimethyi methacrylate) with viscosity-average molecular weight 5.0 x 10. Triad values as functions of conversion. Dashed lines represent triad values for conventional polymer i = iso tactic, s = syndiotactic, h = heterotactic suffix a denotes determinations made on total polymer (After Butler, Tan, and Challa, J. Polymer Sci., Polymer Chem., 1972, 10, 1031).

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