TYPES OF STEREOISOMERISM IN POLYMERS in .NET

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TYPES OF STEREOISOMERISM IN POLYMERS
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A disyndiotactic structure occurs when placement at each of the two different stereocenters is syndiotactic. Two diisotactic structures are possible. These are differentiated by the pre xes threo and erythro. The meaning of these pre xes corresponds to their use in carbohydrate chemistry. Considering the planar zigzag polymer chain, the erythro structure is the one in which like groups on adjacent carbons are anti to each other (i.e., R and R0 are anti to each other and H and H are anti to each other). The threo structure has an anti arrangement of unlike groups on adjacent crabons (i.e., R and H are anti and R0 and H are also anti). The difference between the erythro and threodiisotactic structures can also be shown by Newman representations of the eclipsed conformation of two consecutive carbon atoms in the polymer chain. These are shown at the far right side of Fig. 8-2. Like groups are eclipsed on like groups (H on H, R on R0 , polymer chain segment on polymer chain segment) in the erythro structure; unlike groups are eclipsed in the threo structure. For the threodiisotactic polymer, the two stereocenters have opposite con gurations. In the zigzag pictorial representation,
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Fig. 8-2 Stereoregular polymers from a 1,2-disubstituted ethylene, CHR ( CHR0 . )n
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STEREOCHEMISTRY OF POLYMERIZATION
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both R and R0 are on the same side of the plane of the polymer chain. In the Fischer projection, R and R0 are on opposite sides of the line representing the polymer chain. For the erthyrodiisotactic polymer, the con gurations at the two stereocenters are the same. The corresponding threodisyndiotactic and erythrodisyndiotactic polymers are drawn in Fig. 8-2. However, a close examination of the two disyndiotactic structures shows that they are identical except for a difference in the end groups. Ignoring end groups, the two structures are superposable. Thus, from the practical viewpoint, there is only one disyndiotactic polymer. The nomenclature for ditactic polymers follows in the same manner as for monotactic polymers. Thus the various stereoregular poly(2-pentene) polymers would be threodiisotactic poly(2-pentene), erythrodiisotactic poly-(2-pentene), and disyndiotactic poly (2-pentene) with the pre xes tit-, eit-, and st-, respectively, used before the formula [CH(CH3 )CH(C2 H5 )]n . The stereocenters in all three stereoregular polymers are achirotopic. The polymers are achiral and do not possess optical activity. The diisotactic polymers contain mirror planes perpendicular to the polymer chain axis. The disyndiotactic polymer has a mirror glide plane of symmetry. The latter refers to superposition of the disyndiotactic structure with its mirror image after one performs a glide operation. A glide operation involves movement of one structure relative to the other by sliding one polymer chain axis parallel to the other chain axis. It should be noted that other polymer structures can be postulated those where one substituent is atactic while the other is either isotactic or syndiotactic or those where one substituent is isotactic while the other is syndiotactic. However, these possibilities are rarely observed since the factors that lead to ordering or disordering of one substituent during polymerization generally have the same effect on the other substituent. An exception is the formation of hemiisotactic polypropene where isotactic placements alternate with atactic placements [Coates, 2000]. 8-1c Carbonyl and Ring-Opening Polymerizations
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Several other types of monomers are capable of yielding stereoisomeric polymer structures. Ordered structures are possible in the polymerization of carbonyl monomers (RCHO and RCOR0 ) and the ring-opening polymerizations of certain monomers. Thus, for example, the polymers from acetaldehyde and propylene oxide can have isotactic and syndiotactic structures as shown in Figs. 8-3 and 8-4.
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Fig. 8-3 Stereoregular polymers of polyacetaldehyde, [ CH(CH3 )O . ]n
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TYPES OF STEREOISOMERISM IN POLYMERS
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Fig. 8-4
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Stereoregular polymers of poly(propylene oxide), [ CH2 CH(CH3 )O ] . n
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Polyacetaldehyde [IUPAC: Poly(oxy[methylmethylene])], like the systems described previously, contains stereocenters that are achirotopic. Both the isotactic and syndiotactic polymers are achiral and do not possess optical activity. The repeat unit (III) in poly(propylene oxide) [IUPAC: poly(oxy[1-methylethylene])], on the other hand, possesses stereocenters that are chirotopic since the two chain segements
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