Poly a-olefin s

Many ot-olefins were polymerized by the Ziegler-Natta catalysts to yield high polymers and many such polymers were found to be stereospecific and crystalline. Polymerizations of a-olefins of the general structure of CH2 = CH — (CH2) — R, where x is 0-3 and R denotes CH3, CH-(CH3)2, C(CH3)3, or CsHs, can be catalyzed by vanadium trichloride/triethyl aluminum [80]. The conversions are fairly high, though higher crystallinity can be obtained with titanium-based catalysts [81]. Addition of Lewis bases, such as ( 4119)20, (C4H9)3N, or ( 4119)3 , to the catalyst system further increases crystallinity [82]. 

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Alkylated aromatic lubricants, phosphate esters, polyglycols, chlorotrifluoroethylene, siUcones, and siUcates are among other synthetics that came into production during much that same period (28,29). Polyphenyl ethers and perfluoroalkyl polyethers have followed as fluids with distinctive high temperature stabiUty. Although a range of these synthetic fluids find appHcations which employ their unique individual characteristics, total production of synthetics represent only on the order of 2% of the lubricant market. Poly(a-olefin)s, esters, polyglycols, and polybutenes represent the types of primary commercial interest. 

Although synthetic lubrication oil production amounts to only about 2% of the total market, volume has been increasing rapidly (67). Growth rates of the order of 20% per year for poly( a-olefin)s, 10% for polybutenes, and 8% for esters (28) reflect increasing automotive use and these increases would accelerate if synthetics were adopted for factory fill of engines by automotive manufacturers. The estimated production of poly( a-olefin)s for lubricants appears to be approximately 100,000 m /yr, esters 75,000, poly(alkylene glycol)s 42,000, polybutenes 38,000, phosphates 20,000, and dialkyl benzene 18,000 (28,67). The higher costs reflected in Table 18 (18,28) have restricted the volume of siUcones, chlorotrifluoroethylene, perfluoroalkylpolyethers, and polyphenyl ethers. 

Miscellaneous Commercial Applications. Dimer acids are components of "downweU" corrosion inhibitors for oil-drilling equipment (see Petroleum Corrosion and corrosion inhibitors). This may account for 10% of current dimer acid use (71). The acids, alkyl esters, and polyoxyalkylene dimer esters are used commercially as components of metal-working lubricants (see Lubrication). Dimer esters have achieved some use in specialty lubricant appHcations such as gear oils and compressor lubricants. The dimer esters, compared to dibasic acid esters, polyol esters and poly(a-olefin)s, are higher in cost and of higher viscosity. The higher viscosity, however, is an advantage in some specialties, and the dimer esters are very stable thermally and can be made quite oxidatively stable by choice of proper additives. 

Figure 3.1 Stereoisomerism of poly(a-olefin)s (- -CH(R) C H2-1 - - Isotactic and syn-diotactic polymers. Reproduced by permission from Elsevier Science, from Ref. 1...

If there is no regular arrangement of vertical lines in the adapted Fischer projection or alkyl substituents in the planar zigzag projection (Figure 3.2), then an atactic structure occurs. Atactic poly(a-olefin)s are characterized by the appearance in their chains of m diads and r diads in equal amounts. These diads constitute heterotactic mr triads and rm triads as well as isotactic mm triads and syndiotactic rr triads which also appear in equal amounts. 

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],... 

Being acquainted with the structure of poly(a-olefin)s, one may reasonably explain some of the differences in their physicochemical properties. For example, isotactic polypropylene, the chains of which in the helical conformation can be closely packed, has rather a high density (0.92-0.94 g/cm3) and melting point (175°C) and is insoluble in low-boiling aliphatic hydrocarbons at boiling point. Syndiotactic polypropylene, consisting of chains in the form of binary helices, which cannot be packed so closely as in the previous case, has a density of 0.89-0.91 g/cm3 and a melting point of 135°C, which is 40 k lower from that of isotactic polypropylene syndiotactic polypropylene is also moderately soluble in... 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

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 catalyst derived from the reaction according to scheme (18) and related catalysts appeared capable of polymerising a-olefins surprisingly, the structure of the poly(a-olefin)s formed is consistent with a 2, co-coupling [191] but not with the usual 1,2-coupling of the monomers ... 

The extent of chain regularity in poly(a-olefin)s is determined primarily by high-resolution nuclear magnetic resonance, 1H NMR and especially 13C NMR, spectroscopy. Both the H and 13C NMR signals of the macromolecule are most conveniently related to its microstructure [421]. 

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],... 

As with poly(a-olefin)s, the properties of polymers derived from polar monomers may be highly dependent on the stereoregularity of the polymer backbone. Moreover, copolymers of polar monomers with olefins may exhibit some... 

Give the structure and stereochemical designation of all possible poly(a-olefin)s that could, in principle, be obtained by stereospecific polymerisation. Are they all likely to be obtainable in practice (with heterogeneous and homogeneous Ziegler-Natta catalysts) ... 

How can one explain the occurrence of steric defects in tactic poly(ot-olefin)s Explain why high-resolution nuclear magnetic resonance is the most convenient method for determining the chain micro structure in poly(a-olefin)s. Consider how 3H and 13C NMR spectroscopy can provide stereochemical information concerning a-olefin polymer chains on the diad level (m, r) and the triad level (mm, rr, mr). Explain why /1-olefins, which do not homopolymerise (without isomerisation) in the presence of Ziegler-Natta catalysts, undergo copolymerisation with ethylene in the presence of these catalysts. 

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. 

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