Syndiotactic conformation

Stereoregularity. The addition of monomer fixes the tacticity of the previous monomer unit. Syndiotactic structure has the adjacent chlorine atoms oriented to opposite sides of the carbon—carbon—carbon plane, whereas isotactic structure has the adjacent chlorine atoms oriented to same side of the carbon—carbon—carbon plane. The potential energy for syndiotactic conformation is 4.2—8.4 kj/mol (1—2 kcal/mol) lower than for isotactic conformation (74,75). Thus the ratio of propagation rates for syndiotactic to isotactic, kjk, must increase with decreasing temperature. Consequently, with decreasing polymerization temperature, the degree of syndiotacticity in PVC should increase. Measured amounts of syndiotacticity are illustrated in Figure 8... 

When the approaching monomer is in syndiotactic conformation and is added as such (c), the / carbon is fixed after addition, but the a carbon can rotate (d) and assume isotactic conformation after the addition of the next unit. The effect of the lithium cation is again assumed to be the driving force for the rotation of the potentially syndiotactic unit to the isotactic conformation. Repeating steps (a) and (b) leads to a threo-diisotactic chain, and repeated steps (c), (d) and (e) to an erythrodiisotac-tic chain. The addition of tetrahydrofuran reduces the number of threo-diisotactic units because it competes with carbonyl groups for coordination to Li+. This enhances the amount of monomer approaching the centre by the syndiotactic route. 

The interaction between PMMA chains in the hybrid polymer prepared from 2mol% Zr402(OMc)i2 and MMA was investigated by dielectric spectroscopy and a predominant syndiotactic conformation of PMMA chains was found. " Comparison with the spectra of syndiotactic and isotactic PMMA clearly indicated that in the hybrid network the macromolecular chains experience a different chemical environment than in cluster-free PMMA. The results suggest that in the hybrid polymer the organic chains are more separated from each other, owing to the cluster crosslinks. This results in less pronounced interchain interactions. 

The chemical structures of PLA, including chemical composition, interlocked structure, isomers, syndiotacticity, conformation, amorphous structure, semicrystal structure, frustrated structure, molecular weight, and so on, were reviewed in this chapter. The chemical structures play an... 

The method lends itself to a demonstration of the fact that certain poly(methyl methacrylate) polymers are block copolymers of isotactic and syndiotactic conformation, not to mention that conventional, i,e., free-radically polymerized, poly(methyl methacrylate) contains a substantial fraction of the isotactic polymer. 

The polymerization reactions are effected either thermally, free radically, cationically, or anionically depending on the monomers involved. Ziegler-Natta type catalysts are used to give stereospecific polymers of either the isotactic or syndiotactic conformation. 

FIGURE 7 16 Poly mers of propene The mam chain IS shown in a zigzag conformation Every other carbon bears a methyl sub stituent and is a chirality center (a) All the methyl groups are on the same side of the carbon chain in isotactic polypropylene (b) Methyl groups alternate from one side to the other in syndiotactic polypropy lene (c) The spatial orienta tion of the methyl groups IS random in atactic polypropylene... 

Fig. 1 a, b. Projection along the chain axis and side view of models of syndiotactic polystyrene in the a) trans-planar conformation b) s(2/l)2 helical conformation... 

It is typical, for instance, of syndiotactic polystyrene (s-PS) [7-9] and syndiotactic poly- p-methylstyrene (s-PPMS) [10] to present crystalline forms with a transplant conformation of the chains (shown for s-PS in Fig. 1) as well as crystalline forms with sequences of dihedral angles of the kind TTG+G+ (or the equivalent G G TT), corresponding to a s(2/l)2 helical symmetry of the chains (shown for s-PS in Fig. 1). 

As an example we report in this paper the conformational energy maps of two already cited stereoregular polymers, which have been obtained very recently, syndiotactic polystyrene s-PS and syndiotactic polybutene s-PB (Fig. 4 and 5, respectively). In fact, the energy map calculated for s-PS shows... 

Crystalline samples of syndiotactic poly(methyl methacrylate) (st-PMMA) may be obtained from chloroacetone 178). This guest could be completely replaced by a variety of other guest molecules such as acetone, 1,3-dichloroacetone, bromoacetone, pinacolone, cyclohexanone, acetophenone and benzene. The X-ray diffraction patterns for these inclusion compounds were similar. These data indicate that the st-PMMA chains adopt a helical conformation of radius about 8 A and pitch 8.85 A. The guest molecules are located both inside the helical canals and in interhelix interstitial sites. 

We examine briefly some specific instances starting with syndiotactic polypropylene (sPP). Aside from the already discussed hexagonal mesophase which can be obtained both drawing fibers and under quiescent conditions, this polymer presents four crystalline forms phases I [73] and II [74-76] where chains adopt the (T2G2)n helical conformation, forms III [30] and... 

By using conformationally variable Cp ligands such as C5H4Pr1, change in the dominant mode of propylene polymerization from isotactic to syndiotactic was accomplished by varying the reaction temperature [172]. 

Conformational Energy Calculations in Isotactic and Syndiotactic Polymers... 

Although the definitions of isotactic, syndiotactic, and atactic polymers according to International Union of Pure and Applied Chemistry (IUPAC) rules are well established in terms of succession of mesa (m) or racemic (r) dyads,12 the symbolism of (+) and (—) bonds allows the easy treatments of possible configurations in cases of any complexity.1 Moreover, the (+) or (—) character of the bonds in a polymer chain is strictly related to the accessibility of gauche+ or gauche conformations of the bonds and, therefore, to the formation of right-handed or left-handed helical conformations.1... 

Figure 2.4 Line repetition symmetries, according to equivalence principle, and corresponding possible conformations, according to minimum conformational energy, for (a) isotactic and (b) syndiotactic polypropylene (T = trans, G = gauche).

Interesting geometrical differences between helical conformations in isotactic and syndiotactic polymers have recently been found.25 27... 

Recently, a similar analysis of the conformational energy has been performed also for various new syndiotactic polymers.27,47 The conformational energy maps of syndiotactic polypropylene (sPP),48 polystyrene (sPS),49 poly butene (sPB),25 and poly(4-methyl-l-pentene) (sP4MP)26 are reported in Figure 2.12. A line repetition group s(M/N)2 for the polymer chain, and, hence, a succession of the torsion angles. .. 0i, 0i, 02, 02,..., has been... 

Figure 2.12 Maps of conformational energy of various syndiotactic polymers as function of backbone torsion angles 0 and 0227 (a) syndiotactic polystyrene, (b) polypropylene, (c) poly (1-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i 0i 0202 - - -[s(M/N)2 symmetry] has been assumed. Isoenergetic curves are reported every 5 kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero. Values of energies corresponding to minima (x) are also indicated. Experimental conformations observed for different polymorphic forms of polymers are indicated by triangles. (Reproduced with permission from Ref. 27. Copyright 1992 by the Socicta Chimica Italiana.)...

Models of the chain conformations found in the various polymorphic forms of syndiotactic polymers are reported in Figure 2.13. 

Figure 2.13 Models for helical conformations of chains of various syndiotactic (st) polymers.

All the possible line repetition groups for cis and trans poly dienes compatible with the isotactic or syndiotactic configurations are reported in Figure 2.15,47,68 In order to consider only the possible conformations assumed in the crystalline state, the torsion angle of the central single bond is assumed to be 180° trans) in both the cis and trans polydienes. This condition produces conformations sufficiently extended to be packed in a crystalline lattice for each value of the torsion angles 0i and 02 (Figure 2.15). 

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