Syndiotactic l,2-poly

Syndiotactic l,2-poly(4-methyl-l,3-pentadiene) has been formed by polymerisation with homogeneous catalysts, e.g. TiBz4—[Al(Me)0]x and CpTiCl3—[Al (Me)0]x [41,43]. The coordination of the monomer as an s-trans-t/2 ligand rather than an s-cis-r A ligand at the Ti atom has been postulated to be involved in the polymerisation. The s-cis-r A monomer coordination is less favoured for steric reasons in the case of 4-methyl-1,3-pentadiene. A possible scheme for the formation of the 1,2-syndiotactic polymer from this monomer is presented in Figure 5.7 [41,43],... 

Figure 10-19. Dependence of the melting temperature Tm and glass transition temperature Tg of a 90% syndiotactic l,2-poly(butadiene) on the X-ray crystallinity, olx.

FIGURE 17.13 NMR spectra of (a) isotactic and (b) syndiotactic l,2-poly(4-methyl-l,3-pentadiene) (P = polymer chain). (Reprinted with permission fromZambelli, A. Ammendola, R Proto A. Macromolecules 1989, 22, 2126-2128 Proto, A. Capacchione, C. Venditto, V Okuda, J. Macromolecules 2003, 36, 9249-9251. Copyright 1989 and 2003 American Chemical Society.)... 

Diene polymerization may involve either or both of the double bonds. Geometric and structural isomers of butadiene, for example, are indicated by using appropriate prefixes — cis or irons, 1,2 or 1,4 — before poly, as in cw-l,2-poly(l,3-butadiene). Tacticity of the polymer may be indicated by using the prefix i (isotactic), s (syndiotactic), or a (atactic) before poly, such as 5-polystyrene. Copolymers are identified by separating the monomers involved within parentheses by either alt (alternating), b (block), g (graft), or co (random), as in poly(styrene-g-butadiene). 

Conversely, the poly(4-MPD) NMR spectrum allows one to discriminate elegantly between the two possible microstructures. As shown in Figure 17.13a, two distinct multiplets between 0.6 and 1.3 ppm are clearly visible for isotactic l,2-poly(4-MPD). Such a spectrum is expected for two magnetically inequivalent geminal protons (Ha and Hb), as in the case of the isotactic structure, while a single resonance (deceptive triplet) due to the equivalence of these protons is expected in the case of the syndiotactic structure (Figure 17.13b). 

Fig. 4.12 Melting temperature of eaeh component in blends of syndiotactic 1,2-poly (butadiene) with trans-l,4-poly(butadiene) o. (From Nir and Cohen (74))...

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). 

Note Structure-based names of tactic polymers are formed before the application of adjectives designating tacticity thus, syndiotactic poly(methylmethylene) is preferred to syndiotactic poly(l,2-dimethylethane-l,2-diyl) because a shorter repeating unit is identified, in conformity with the rules in Ref. 3. 

Polymerization of CFE in urea at -80"C has a mild stereoregulating influence compared to bulk polymerization at 60 °C, although the spectra in Figure 2 show that both polymers are substantially atactic. Three major triad resonances can be discerned more readily in spectrum (b), and these are assigned to mm, mr, and rr stereosequences as shown simply by analogy with the poly(l,2-difluoroethene) assignments (14,15). At present we have no basis to make definitive assignments, but those indicated are at least consistent with the known syndiotactic bias exerted by urea on the polymerization of complexed vinyl monomers (10). 

The fully extended planar zigzag trcms conformation) is the minimum energy conformation for an isolated section of polyethylene or paraffin hydrocarbon. The energy of the trans conformation is about 800 cal/mol less than that of the gauche form. Consequently, the trans form is favored in polymer crystal structures. Typical polymers that exhibit this trans form include polyethylene, poly(vinyl alcohol), syndiotactic forms of poly(vinyl chloride) and poly(l, 2-butadiene), most polyamides, and cellulose. Note that trans conformation is different from the trans configuration discussed in Section IV.A. 

Butadiene, the simplest of the conjugated dienes, is produced commercially by thermal cracking of petroleum fractions and catalytic dehydrogenation of butane and butene. Polymerization of butadiene can potentially lead to three poly(l,2-butadiene)s, atactic, isotactic, and syndiotactic, and two cis and irons forms of poly(l,4-butadiene). This is discussed in Chapters 2 and 3. 

Because of its high tensile strength and stress corrosion resistance, poly-(butene-1) is used for pipes and packaging film. Atactic poly(butene-l) (APB) is mostly produced by direct polymerization. It has similar properties to atactic poly (propylene). Syndiotactic poly (butene-1) is obtained by hydrogenating 1,2-poly (butadiene) however, it has no commercial significance. 

The protective effects of syndiotactic and isotactic poly-2-vinylpyridine-l-oxide remarkably differ in the spatial arrangement of the structural units (Holt et al. 1970). The isotactic polymer has probably a helical conformation. When the cultures... 

Intensive studies by Shell on catalyst development for the ethylene/propylene/CO terpolymerization provided the most effective catalyst represented as [Pd(L"L )(S)2][X]2, where L"L (L = or L ) is a cis-chelating bis(phosphine) ligand, S is a solvent molecule, and X is an anion with low coordination capability. Ligand development leads to three patterns of stereoregular polymers those are, isotactic poly(propylene-ALT-CO), syndiotactic, and isotactic poly(styrene-ALT-CO). Noteworthy... 

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