Iso tactic

PIB exists either as a low mw (about 12,000) viscous or atactic liq or as a cryst matl of about 1.00,000 mw. In this latter form the iso tactic (chain) type configuration predominates over the atactic and amorph forms (Ref 8). Since the isotactic form is the form designated by mil spec (Ref 2), its parameters are presented below ... 

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. 

Of these three PP isomers (called that because they all have the same formula, just different stereoconfigurations), isotactic makes the best plastic. Atactic polypropylene is soft, elastic, and rubbery but not as good as rubber, natural, or synthetic. It is usually separated from the isotactic propylene and discarded as waste, which adds considerable cost to the remaining isotactic. The iso tactic form has a high degree of crystallinity with the chains packed... 

Like polypropylene, PVC has the problem of stereospecificity. The carbon atom to which the chlorine atom is attached is asymmetrical. (See Figure 23-8.) As a result, PVC molecules can be iso tactic, syndiotactic, and atactic. Commercial PVC is only 5—10% crystalline—low percent isotactic. It is more dense, 1.3 to 1..8 g/cc, than the polyolefins, (fe Figure 23—9.)... 

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°). 

Table I shows some peculiarities. In contrast to what has been claimed in literature, the probabilities of forming an isotactic diad at an existing iso tactic diad (pj/j) do not always equal the probabilities of forming an isotactic diad at an existing syndiotactic diad (p8/i). The free radical polymerization of the six methacrylate monomers mentioned is thus not always Bemoullian, at least not at the temperature of — 78°C and in toluene as a solvent.

Fig. 15. The dependence of af( 1 — o) on IJT for free radical polymerization of methyl methacrylate, where T is the polymerization temperature (° K.) and a the probability of iso tactic monomer placement...

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

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. 

FIGURE 2-12 Iso tactic polypropylene (planar zigzag conformation). 

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