Poly isotactic stereochemistry

When poly(propylene) was first made, it was found to exist in two possible forms. One was similar to poly(ethylene), but had greater rigidity and hardness the other was found to be amorphous and of little strength. The first of these is now known to be isotactic, that is with a regular stereochemistry at each alternating carbon atom. The other is now known to be atactic, that is with a random distribution of different stereochemical arrangements... 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

There are three principal stereochemical types of poly(l-alkene)s, illustrated in Scheme 8.38 for polypropylene. In isotactic polypropylene 80 (i-PP) all methyl substituents have the same relative orientation (m). The scheme shows the stereochemistry with the usual Fischer projection underneath. In syndiotactic PP (81, s-PP) every second CHMe unit has the opposite stereochemistry to the first, while in atactic PP (82, a-PP) the orientation of the methyl substituents is random. In some polymers there is partial order, i. e. only every second monomer orientation is random (83, hemi-isotactic PP). 

Scheme 68 illustrates cyclopolymerization of 1,5-hexadiene catalyzed by a homogeneous chiral zirconocene complex to form optically active poly(methylenecyclopentane), whose chirality derives from configurational main-chain stereochemistry (757). This polymer is predominantly isotactic and contains predominantly trans cyclopentane rings. 

Three stereoisomers are possible in the cholestanylindene-derived zir-conocene complexes illustrated in Scheme 67. Two are racem-like, and the other is meso-like depending on the geometry of the metallocene moiety. The stereochemistry of the reaction is controlled by both the structure of the metallocene skeleton and steroidal substituent. Polymerization of propylene with 0-C activated with MAO gave polypropylene of 240,000, about 40% mmmm approximately 70% is due to enantiomorphic site control and the rest is due to chain-end control. Use of the catalyst derived from a /3-A-B mixture produced a mixture of polymers. The a-A and a-B/MAO catalysts afforded isotactic poly-... 

The microstructure of the discussed cycloaliphatic polymers concerns the cis-trans geometrical isomerism of the rings and the relative stereochemistry between the rings. A modified Bovey m-r nomenclature [507] provides a useful description of the microstructure of poly(methylene-l,3-cycloalkane)s, where capital letters (M for mesogenic, R for racemic) denote the stereochemistry of the rings and lower case letters ( m and r) denote the relative stereochemistry between the rings [503], Therefore, cA-isotactic, tram-isotactic, cA-syndiotactic and tram-syndiotactic cyclopolymers may be formed. As in many other cases, 13C NMR spectroscopy reveals information about both the tacticity of the polymer and the ratio of cis to treins rings. 

It is important to note that high molecular weight trans-isotactic poly(methy-lene-1,3-cyclopentane) contains no mirror or mirror glide planes of symmetry and is thus chiral by virtue of its main chain stereochemistry (it exhibits optical activity) this is in contrast to high molecular weight polypropylene and other poly(a-olefin)s, which contain an effective mirror plane perpendicular to the molecular axis in the middle of the molecule and are thus achiral [30,497],... 

The microstructure and the properties depend on the cis/trans ratio of the ring bonding and on the stereochemistry between the rings. Poly(methylene-l,3-cyclopentane) obtained by cyclopolymerization of 1,5-hexadiene shows four different structures from which the tram isotactic structure is predominant, when using simple biscyclopentadienyl compounds. Higher substituted (pen-tamethyl) zirconocenes yield mainly as-connected polymers which are highly crystalline and have melting points up to 190 °C. 

At present, no evidence exists for long runs of either meso or racemic placements in the Wurtz dechlorination reaction used in the synthesis of polysilylenes. Deviation from a random stereochemical polymerization has been suggested by West et al. (3) in the case of poly(phenylmethylsilylene) (PPMS). An analysis of Si NMR triads indicates that addition of monomer units with the same stereochemistry is preferred, which results in an increase in the intensity of syndiotactic and isotactic triad structures compared with a random stereochemical polymerization. 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

Figure 9 Structures of syndiotactic (regular alternating of stereochemistry along the polymer chain), isotactic (same stereochemistry across the polymer chain), and atactic (random stereochemistry along the polymer chain) poly-...

In the studies described above the stereochemistry of the irradiated PMMA was described well using a simple model assuming that the racemization reactions consisted of isolated reaction events. Studies of irradiated isotactic poly(pro-pylene) (pp) -io7 other hand provided evidence that the racem-... 

Stereochemistry is also important in determining properties of polyblands. For example, syndiotactic poly(methyl methacrylate) is miscible with poly(vinyl chloride) at certain concentrations, whereas the isotactic form is immiscible over the entire composition range [66]. 

As mentioned before (Section II,E,3), the determination of tac-ticity by X-ray analysis is limited by the requirement that the polymer be crystalline. For the study of poly (methyl methacrylate), which may or may not be crystalline, nuclear magnetic resonance spectroscopy has been more useful. In order to interpret the spectra, it has been found necessary to describe the stereochemistry of a unit by the configurations on both sides. Therefore, an isotactic configuration, or isotactic triad, is one where the central unit is fianked by units of the same asymmetry, that is ddd or III. Similarly, for a syndiotactic triad, the stereochemistry is did or Idl. To overcome the disadvantages of the term atactic, a new term heterotactic was introduced. The stereochemistry for heterotactic configurations is, therefore, Idd, dll, lid, and ddl. 

As expected, the cis/trans diastereoselectivity is influenced by the structure of the catalyst precursor, and is controllable by choosing a proper catalyst and polymerization conditions. The enantioselectivity (the relative stereochemistry between the rings) of PMCP is also affected by the catalyst structure. Complexes la, lb (Figure 19.2), and 2a, which give atactic poly(a-olefin)s, produce atactic PMCP, and the isoselective catalysts 3 and 4a yield isotactic PMCPs. These differences in enantioselectivity versus catalyst type are consistent with those for the polymerization of a-olefins. trans-Isotactic polymers can be optically active (chiral) if homochiral catalysts are used. The Waymouth research group showed that the MAO-activated homochiral ansa-zirconocene BINOL complex 5 (BINOL = l,l -bi-2-naphtholate Figure 19.2) gave optically active trany-polymer. 

The catalyst OsCls (in a 1 1 by volume mixture of ethanol/chlorobenzene) converts racemic 1-MeNB into an atactic, all-trans polymer with predominantly head-tail structures at low monomer concentrations (0.2 mol/L). The ratio of (HT + TH)/(HH + TT) signals is 9 1. By comparison, polymerization of a single isomer of 1 -MeNB gives isotactic poly( 1 -MeNB), as the selective head-tail enchainment necessarily leads to the meso stereochemistry when enantiomerically pure monomer is used (Scheme 20.8). At an increased monomer concentration (1.5 mol/L), a polymer with 16% cis units is formed. This polymer does not contain any cis-head-head sequences. 

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