Isotactic poly s

Figure 2.14 Maps of conformational energy as function of backbone torsion angles 9i and 02 of a chain of isotactic poly((S)-3-methyl-l-pentene) for (a,b) left-handed helix and (c) right-handed helix.29 For each pair of Oi and 02, reported energy corresponds to minimum obtained by varying torsion angles of lateral group 03 and 04. Curves are reported at intervals of 0.5 kcal/mol of monomeric unit. Values of energies corresponding to minima are also indicated. (Reprinted with permission from Ref. 29. Copyright 1976 by Elsevier Science.)...

Isotactic poly-(R)-3-methyl-l-pentene Isotactic poly-(S)-3-methyl-l-pentene... 

Fig. 7 DSC melting and crystallization curves of isotactic poly(S)-4-methyl-hexene-l. Note the two melting peaks (at 193.5 and 227.4 °C, respectively, AH 2.5 and 1.5cal/gram) and two crystallization peaks (at 201 and 120 °C, respectively same AH), as well as the significant temperature gap ( 74 °C) between the lower crystallization and melting processes. (From [44])...

Another interesting case of conformational disorder is provided hy isotactic poly[ (S)-3-methy pentene-l ] (P3MP1)... 

Petraccone V, Ganis P, Corradini P, Montagnoli G (1972) A case of < nformational isomorphism in polymers. The crystal structure of isotactic poly-(S)-3-methylpentene-l, European Polymer J. 99... 

The case of conformational isomorphism of lateral groups occurs, for instance, in the crystal structure of isotactic poly(S-3-methyl-pentene), characterized by the presence of fourfold helices of only one sense (left-handed), with lateral groups which may adopt statistically two conformations of minimum energy [99,103]. 

Because polyolefins do not absorb light in the accessible UV range, CD spectroscopy, which is a powerful tool for studying the chiral stmcture of polymers, could not be used for these vinyl-derived polymers. Hence, the chiral stmctures were elucidated in terms of optical rotatory dispersion. For example, isotactic poly((S)-3-methyl-l-pentene) (1) shows a larger specific rotation than the corresponding monomer. The... 

In these isotactic polymers, the optical purity of the monomer affected the optical activity via the relationship to the excess helical sense of the polymer (Figure 1). ° In the case of isotactic poly((S)-4-methyl-l-hexene) (2) and poly((i )-3,7-dimethyl-l-octene) (3), an increase in the optical purity of the monomers resulted in an increase in the optical activity of the polymers in a nonlinear fashion the optical activity of the polymers leveled off when the optical purity of the monomer reached -80%. By contrast, in the case of isotactic poly((S)-5-methyl-l-heptene) (4), the relation was linear. These findings imply that the side-chain chiral centers of 4, which are separated from the main chain by three covalent bonds, may be too far from the main chain to affect the helical conformation. 

In other optically active polymers, as well as in some polymers from achiral monomers, chiral structures, characterized by all isomorphous helical chains in the unit cell, have been found. For instance, in isotactic poly(5-methyl-l-hexene) [94], isotactic poly(ubutylacrylate) [95], and optically active isotactic poly((S)-5-methyl-l-heptene) [94b,c], chiral pseudo-hexagonal or tetragonal packing with isomorphous 3/1 or 4/1 helical chains have been found. 

Here m is the mode order (m — 1,3,5. .., usually 1 for polyethylenes), c the velocity of light, p the density of the vibrating sequence (density of pure crystal) and E the Young s modulus in the chain direction. The LAM band has been observed in many polymers and has been widely used in structural studies of polyethylenes [94—99,266], as well as other semi-crystalline polymers, such as poly (ethylene oxide) [267], poly(methylene oxide) [268,269] and isotactic poly(propylene) [270,271], The distribution of crystalline thickness can be obtained from the width of the LAM mode, corrected by temperature and frequency factors [272,273] as ... 

Isotactic poly(methyl methacrylate), also, is an intricate case, resolved only after a 20-year debate. The repetition period along the chain axis is 10.40 A corresponding to S monomer units the entire cell contains 20 monomer units (four chains). At first, the stmcture was resolved as a 5/1 helix (183) with = 180° and 62 — 108° but no reasonable packing was found using this assumption. Further conformational calculations showed that helices like 10/1 or 12/1 should be more stable than the 5/1 helix. The structure was solved by Tadokoro and co-workers (153b) who proposed the presence of a double helix. Two chains, with the same helical sense and the same direction but displaced by 10.40 A one from the other are wound on each other, each chain having 10 monomer units per turn [i(10/l)] and a 20.80-A repeat period. As a result, the double helix has a 10.40-A translational identity period, identical to that found in the fiber spectmm. The conformational parameters are Of = 179° and 2 = -148°. Energy calculations indicate that the double helix is more stable by 4.4 kcal per-mole of monomer units than two isolated 10/1 helices, a result that is in line with the well-known capacity of this polymer to form complexes in solution (184). 

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)].

Figure 1 shows the exposure characteristics of atactic and isotactic poly(a,a-dimethylbenzyl methacrylate) resists with CH3ONa development together with those of the poly (methyl methacrylate) resist with MIBK/IPA development. Poly(a,a-dimethylbenzyl methacrylate) s showed high sensitivity and very good contrast between exposed and unexposed areas. The atactic polymer with alkaline development was improved in the sensitivity and 7-value by a factor of more than three over poly(methyl methacrylate) with MIBK/IPA development. 

Figure 1. Exposure characteristics of poly (methyl methacrylate) (PMMA), and atactic and isotactic poly(a,

When the atactic poly(a,a-dimethylbenzyl methacrylate) was heated at 170°C for 30 min under vacuum, it decomposed into volatile and nonvolatile components. The former was found to be a-methylstyrene and the latter was to be very similar to polyfmethacrylic acid) as determined by H NMR spectroscopy. Figure 3 shows the infrared spectra of atactic and isotactic poly(a,a-dimethylbenzyl methacrylate)s heated at 174°C under vacuum for various times. In the spectra of the atactic polymer, the absorption of the ester carbonyl at 1729 cm-1 decreased and that of the acid carbonyl at 1700 cm-1 increased as the heating time increased. After heating for a period of 30 min... 

The polymers of the optically active and racemic 4-methyl-1-hexene and the poly-(S)-5-methyl-l-heptene have isotactic structure (115) the same structure seems probable also in the case of the other polymers prepared till now from optically active or racemic a-olefins. 

The conformational analysis, according to Brewster, of isotactic poly-a-olefins demonstrates that, in the case of poly-(S)-3-methyl-1-pentene and poly-(S)-4-methyl-l-hexene, only 3 conformations are allowed for each monomeric unit two of them correspond to a left-handed helix conformation of the principal chain and one to a right-handed helix conformation (Table 25). 

Only two types of polymers are considered here. These are isotactic poly(5-methyl-hexene-l) (P5MH1), with a non-chiral side chain (for the sake of comparison) and mainly isotactic poly(4-methyl-hexene-l) (P4MH1). The side chain of the latter polymer is chiral since the two substituents of the carbon in the p position are a methyl and an ethyl group. Polymers that are made only of the S or the R conformers - in other words the true chiral polymers (P(S)4MH1 and P(R)4MH1), the racemic copolymer of the (R) and (S) monomers (P(R, S)4MH1) and of course the racemic blend of the two enantiomeric polymers - are available. 

Isotactic poly(x-olcfin)s crystallise in a helical conformation, and, in the case of polypropylene, with three units per turn [4,5], Isotactic polypropylene has a melting point of 175°C and does not dissolve in boiling n-heptane [6,7], Note that, depending upon the configuration of the tertiary carbon atom of the polymer main chains, the poly(x-olefin) helices will be characterised by right-handedness or left-handedness. It should be mentioned that the helical structure of the poly(x-olcfin) chain per se is sufficient for the appearance of chirality of such a macromolecule [8], Figure 3.3 presents the helical conformation of chains of isotactic poly(a-olefin)s in the crystalline state (with three units per turn - the case of polypropylene) [5],... 

Figure 3.3 Schematic presentation of the conformation of chains of isotactic poly(ot-olefin)s, such as polypropylene, in the crystalline state...

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...

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],... 

Onishi, T., and S. Krimm Origin of characteristic bands in the infrared spectra of isotactic polystyrene and isotactic poly(ring-rf5 styrene) J. Appl. Phys. 32, 2320-2325 (1962). 

In dimethylformamide, in the range of the first acceleration of methyl methacrylate polymerization, changes in the mean lifetime of the radicals were observed [8.4 s for the control and 64 s for polymerization with the matrix effect caused by the presence of isotactic poly(methyl methacrylate)] fcp fell from 26.6 to 5.9 mol-1 dm3 s and fct from 140 x 104 to 1.7 x 104mol-1 dm3 s l [66]. 

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