Trans-1,4-Isotactic poly

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],... 

Tacticities of the polymers were estimated. For example, poly (1.3-pentadiene) was highly isotactic in the channels of perhydrotriphenylene, while it was preferentially isotactic (mesoiracemic = 2 1) in the channels of deoxycholic acid. A completely isotactic polymer was utilized to prepare a unique polymer, hemitactic polypropylene, by hydrogenation of a completely 1,4-trans isotactic poly(2-methyl-1,3-pentadiene). 

The catalyst (mesitylene)W(CO)3/EtAlCl2/exo-2,3-epoxynorbornane produces predominantly trans-isotactic poly(iso-5,5-Me2NB) (85 15 trans cis) at a monomer concentration of 0.4 mol/L. The NMR spectrum of the polymer obtained from enantiomerically enriched monomer displays two prominent signals in the region of S 128-138 ppm that correspond to the trans-HT and trans-TH olefin carbon nuclei. At higher monomer concentrations, that is, 2.7 mol/L, the proportion of cis units increases to 40% (it is noted that the cis/trans and m/r dyad ratios of metathesis polymers are often measured by the intensity of the relevant NMR signals, and small errors in these ratios can... 

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... 

The all-trans-all-isotactic and all-trans-all-syndiotactic structures for the 1,4-polymerization of 1,3-pentadiene are shown in Fig. 8-6. In naming polymers with both types of stereoisomerism, that due to cis-trans isomerism is named first unless it is indicated after the prefix poly. Thus, the all-trans-all-isotactic polymer is named as transisotactic l,4-poly(l,3-penta-diene) or isotactic poly( -3-methylbut-l-ene-l,4-diyl). 

The Ziegler-Natta catalysts have acquired practical importance particularly as heterogeneous systems, mostly owing to the commercial production of linear high- and low-density polyethylenes and isotactic polypropylene. Elastomers based on ethylene-propylene copolymers (with the use of vanadium-based catalysts) as well as 1,4-cz s-and 1,4-tran.y-poly(l, 3-butadiene) and polyisoprene are also produced. These catalysts are extremely versatile and can be used in many other polymerisations of various hydrocarbon monomers, leading very often to polymers of different stereoregularity. In 1963, both Ziegler and Natta were awarded the Nobel Prize in chemistry. 

The symmetry properties of cycloaliphatic polymers are such that polymers with certain microstructures, e.g. tram-isotactic poly (methylene-1,3-cyclopen-tane), are chiral therefore, the cyclopolymerisation of a, trans selective catalysts of C2 symmetry, such as methylaluminoxane-activated resolved (li )-(Thind CH2)2Zr l,l -bi-2-naphtholate, yielded optically active tram-isotactic poly(-methylene-1,3-cyclopentane). The cyclopolymerisation with the (15) enantiomer of the catalyst gave an enantiomeric polymer [505], On the basis of analysis of 13C NMR spectra, the degree of enantioface selectivity for this cyclopolymerisation was estimated to be of 91% [503,505]. 

Polymers with ring structures, interspaced with CH2 groups, can be obtained by polymerization of 1,5-dienes. 1,2-Insertion of the terminal double bond into the zireonium-carbon bond is followed by an intramolecular cyclization forming a ring. Waymouth describes the cyclopolymerization of 1,5-hexadiene to poly (methylene-1,3-cyclopentane) [67]. Of the four possible microstructures, the optically active trans-, isotactic structure (Figure 4) is predominant (68%) when using a chiral pure enantiomer of [En(IndH4)2Zr](BINAP)2 and MAO. 

Asymmetric synthesis polymerization of 1,3-dienes with solid matrices has been reported." This was first attained by using optically active (R)-(-)-trans-anti-trans-flnti-trans-perhydrotriphenylene (256) matrix for y-ray irradiation polymerization of trans-1,3-pentadiene to afford isotactic poly-trans-254. °° Deoxyapocholic ° ° and apocholic acids (257 and 258) are also effective as optically active matrices. Matrix polymerization tended to result in higher optical purity. The highest value of optical purity so far reported is 36% for the polymerization of (Z)-2-methyl-1,3-butadiene with 258 as a matrix. 

As expected, the cis/trans diastereoselectivity is influenced by the structure of the catalyst precursor, and is controllable by choosing a proper catalyst and polymerization conditions. The enantioselectivity (the relative stereochemistry between the rings) of PMCP is also affected by the catalyst structure. Complexes la, lb (Figure 19.2), and 2a, which give atactic poly(a-olefin)s, produce atactic PMCP, and the isoselective catalysts 3 and 4a yield isotactic PMCPs. These differences in enantioselectivity versus catalyst type are consistent with those for the polymerization of a-olefins. trans-Isotactic polymers can be optically active (chiral) if homochiral catalysts are used. The Waymouth research group showed that the MAO-activated homochiral ansa-zirconocene BINOL complex 5 (BINOL = l,l -bi-2-naphtholate Figure 19.2) gave optically active trany-polymer. 

The catalyst OsCls (in a 1 1 by volume mixture of ethanol/chlorobenzene) converts racemic 1-MeNB into an atactic, all-trans polymer with predominantly head-tail structures at low monomer concentrations (0.2 mol/L). The ratio of (HT + TH)/(HH + TT) signals is 9 1. By comparison, polymerization of a single isomer of 1 -MeNB gives isotactic poly( 1 -MeNB), as the selective head-tail enchainment necessarily leads to the meso stereochemistry when enantiomerically pure monomer is used (Scheme 20.8). At an increased monomer concentration (1.5 mol/L), a polymer with 16% cis units is formed. This polymer does not contain any cis-head-head sequences. 

By comparison, the polymerization of a p-7-MeNB with 9 gives moderately isotactic poly(a p-7-MeNB). This polymer contains between 65% and 80% meso structures for both cis- and trans-enchained dyads, the trans/cis ratio being 80 20. ... 

Roy M, Nelson JK, MacCrone RK, Schadler LS, Reed CW, Keefe R, Zenger W (2005) Polymer nanocomposite dielectrics—the role of the interface. IEEE Trans Diel Electr Insul 12 629-643 Roy M, Nelson JK, MacCrone RK, Schadler LS (2007) Candidate mechanisms ctmtrolling the electrical characteristics of silica/XLPE nanodielectrics. J Mater Sci 42 3789-3799 Saccani A, Motori A, Patuelli F, Montanari GC (2007) Thermal endurance evaluation of isotactic poly(propylene) based nanocomposites by short-term analytical methods. TREE Trans Diel Electr Insul 14 689-695... 

This analysis was made possible because the chemical shifts of the various olefinic carbon double bonds in these unsymmetrically substituted norbomene derivatives are very sensitive to whether they are in an HH, TT or HT/TH unit, as can be seen for the case of poly(l-methylnorbomene) [63] in Figure 1.12. It was therefore possible to examine a range of metathesis catalysts for their ability to produce tactic polymers. In fact, a range of tacticities was observed, with extremes in behaviour being represented by the ReClj catalyst, which produced an all-cis syndiotactic polymer [65] and the W(mesityl) (CO)j catalyst, which produced a high trans isotactic polymer [71]. 

Particular studies of the IR spectra of homopolymers include isotactic poly(l-pentane), poly(4-methyl-l-pentene), and atactic poly(4-methyl-pentene) [16], chlorinated polyethylene (PE) [17], aromatic polymers including styrene, terephthalic acid, isophthalic acid [18], polystyrene (PS) [19-21], trans 1,4-polybutadiene [22], polyether-carbonate-silica nanocomposites [23], polyhydroxyalkanoates [24], poly(4-vinyl-n-butyl) [25], polyacetylenes [26], polyester urethanes [27], miscellaneous... 

An analysis of the overall crystallization rate with composition requires that the comparison be made either at constant undercooling or at one of the nucleation temperature quantities, T / T AT or T /T(AT). This requirement is essential because of the importance of nucleation to the crystallization process. The overall crystallization kinetics of a variety of polymer-diluent systems have been reported. Many different relations between the overall crystallization rate and composition have been observed. For example, as is shown in Fig. 13.17 there is a continuous decrease in the crystallization rate with dilution for linear polyethylene-a-chloronaphthalene mixtures.(42) The results for poly(trans-1,4-isoprene) in methyl oleate follow a similar pattem.(80) In contrast, the rates for poly(dimethyl siloxane) crystallizing from toluene, at compositions V2 = 0.32 to 0.79, are the same at all undercoolings, but are faster than that of the pure polymer.(78) Another example is found with poly(ethylene oxide)-diphenyl ether mixtures.(77) In this case the crystallization rates for the pure polymer and composition = 0.92 to 0.51 are the same. However, the rates for the more dilute mixtures, V2 = 0.04 and 0.30 are lower. For poly(decamethylene adipate)-dimethyl formamide mixture the rates for the pure polymer and V2 = 0.80 are the same.(77) The mixture of isotactic poly(propylene) with dotricontane shows interesting behavior.(81) At all undercoolings studied, the crystallization rate initially decreases with dilution, reaches a minimum in the range V2 — 0.7 (a maximum in ti/2) and then slowly increases with further dilution, up to V2 = 0.10. 

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). 

In the case of crystals, both intramolecular (conformational) and packing energies should be taken into account simultaneously. Such a total energy minimization method, with suitable crystallographic constraints, has been applied in different steps of the analysis of crystalline structures of three different synthetic polymers. Structures of these molecules, namely, isotactic trans-1,4-poly-penta-1,3-diene (ITPP), poly-pivalolactone (PPVL), and isotactic cis-1,4-poly(2-methyl-penta-1,3-diene)(PMPD), do not have troublesome features such as charged groups, counterions, and solvent molecules. 

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