Isotactic l,2-poly

C. The only homogeneous catalyst system able to produce highly 1,2-isotactic poly(4-MPD) hitherto reported is the MAO-activated post-metallocene catalyst dichloro[l,4-dithiabutanediyl-2,2 -bis(4,6-di-tert-butyl-phenoxy)]titanium (10, Figure 17.12)." This catalyst was first used to promote the isospecific polymerization of styrene " its successful use to produce highly isotactic l,2-poly(4-MPD) confirms the known similar polymerization behavior of these two monomers. 

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). 

When the ratio r/R is different from 1 the form of the chains may be approximated by a cylinder in which hollows and bulges are periodically repeated that is, the chains have an outside envelope similar to a screw [1] (Fig. 2.11a). If the ratio r/R is very small, close to 0.1-0.2, such as for chains in threefold helical conformation, a trigonal packing of enantiomorphous chains may arise. Each 3i right-handed chain is surrounded by three left-handed isoclined or anticlined chains, and vice versa (space group R3c or R3, respectively), such as in iPS (Fig. 2.12a) [40], form I of iPB (Fig. 2.12b) [45], poly (vinyl methyl ether) [83], isotactic l,2-poly(l,3-butadiene) [84], and isotactic poly(o-fluorostyrene) [8. ... 

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). 

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.)... 

According to the conformational energy minima, isotactic trans- 1,4-poly (1,3-pentadiene)73,74,90 - 94 and trans-, 4-poly(2-methyl-1,3-pcntadicnc)95 are characterized by chains in the conformation (A trans A+T)n (tl symmetry) and chain axes c values of 4.85 and 4.82 A, respectively. The conformation (A cisA 1 T) with s(2/l) symmetry characterizes the chains in the structures of isotactic m-l,4-poly(l,3-peiiladiene)96 98 and cis-1,4-poly (2-methyl-1,3-pentadiene).85... 

An analogous method was used to obtain a new class of macromolecular stereoisomers The hemitactic polymers (99-101). This term refers to a head-to-tail vinyl polymer in which the tertiary carbon atoms constitute two distinct series one, which includes monomer units 1,3,5, 7,..., possesses strict steric regularity, whereas the other, with monomer units 2, 4, 6, 8,. .., is completely at random. In such polymers only one in every two tertiary atoms is influenced by an ordering rule 58 and 59 show the schematic structure of the hemiisotactic and hemisyndiotactic polymers where the white circles indicate the positions of disordered s ubstituents. The hemiisotactic polypropylene was obtained by Farina, Di Silvestro, Sozzani and Savar6 (99, 101) by nonste-reoselective reduction,of. isotactic frans-l,4-poly-2-methylpentadiene. 

Fig. 1. Side view (above) and end view (below) of the macromolecule of isotactic poly[l-(l-naphthyl)ethane-l,2-diyl] in the crystalline state. The helix symbol is s(2 4/1). The chain axis is shown by the dashed line, and c is the chain identity period. Hydrogen atoms are omitted. [From P. Corradini and P. Ganis. Nuovo Cimento, Suppl. 15, 96 (I960)].

The intramolecular interaction energy was calculated for five isotactic polymers, namely, isotactic polypropylene, poly(U-methyl-l-pentene), poly(3-methyl-1-butene), polyacetaldehyde, and poly(methyl methacrylate) (23). The molecular structures of the first four polymers have already been determined by x-ray analyses as (3/1) (2k), (7/2) (18,25.,26), (U/l) (21), and (U/l) helices (28), respectively. Here (7/2) means seven monomeric units turn twice in the fiber identity period. For isotactic poly(methyl methacrylate) (29), a (5/l) helix was considered reasonable at the time of the energy calculation in 1970, before the discovering of... 

Analysis with chiral nuclear magnetic resonance shift reagents revealed that the isotactic poly (1,4-ketone) products were formed with an average or overall degree of enantioselectivity that was >90%. Using the same catalyst, Jiang and Sen also described the first example of alternating co-polymerization between an internal alkene (2-butene) and carbon monoxide to form an isotactic, optically active poly(l,5-ketone). 

Figure 3.11 Single-component catalyst, dimeric homochirotopic rac.-(S, S)-dimethyl-silylenebis[l -(2-trimethylsilyl-4-r-butylcyclopentadienyl)]yttrium hydride [rac.-Me2Si (Me3Si,t-BuCp)2YH]2, for obtaining highly isotactic poly(a-olefins)s. Side view. Reproduced by permission from Ref. 31. Copyright 1992 American Chemical Society...

The above conclusions were indirectly corroborated by NMR spectra of isotactic, Ziegler type poly(3-methylbutene-l) having a bonafide conventional 1,2 head-to-tail structure (167) shown in Fig. 4. The NMR spectra of the cationic and isotactic polymers are completely different (160). 

In macromolecular chemistry helically wound carbon chains are well known. In isotactic polypropylene (Fig. 6) the methyl groups are arranged on a helix. Some other chiral polymers form helices, like polymethylmethacrylate, polytriphenylmethyl-methacrylate poly-l,2-butadiene and poly-tert-butylethylene oxide... 

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. 

Figure 1.9 Isomeric structure which can be obtained by the polymerization of butadiene and isoprene. Poly( 1,2-butadiene), poly(l,2-isoprene) and poly(3,4-isoprene) might be isotactic,...

Ewen, J. A. Jones, R. L. Elder, M. J. Camurati, I. Pritzkow, H. Stereoblock isotactic-hemiisotactic poly(propylene)s and ethylene/propylene copolymers obtained with araa-cyclopenta[l,2-b 4, 3-b ]dithiophene catalysts. Macromol. Chem. Phys. 2004, 205, 302-307. 

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