Polymers of Other Alkenes

So far we have restricted our discussions to PE, PP, PB, and the copolymer of ethylene and MA. There are many other polyolefins, which are used as resins and polymers for special purposes. The commercial routes for most of these polymers involve heterogeneous catalysts. However, the mechanisms at the molecular level are basically the same as the ones discussed so far. 

The organometallic reaction that is of critical importance for polymerization is the insertion of alkene in the M-C bond. This is illustrated by reactions 6.7.1 and 6.7.2. In these reactions, polymers with no straightforward resemblance to the structures of the respective monomers are obtained. However, their formations are easily explained by invoking rearrangements of the inserted products. 

In Table 6.1a summary of the properties, uses, and the catalysts required for a few specialty polymers is given. Single-site catalysts have good potential for the manufacture of some of these low-volume, high-cost specialty polymers. 

The metallocene catalyst 6.45 has shown promise not only in ethylene polymerization leading to ultrahigh-molecular-weight PE (UHMWPE), but also for the manufacture of EPDM rubber. Note the structural novelty in 6.45 where instead of the silicon bridgehead of CGC-type structures, a Lewis acid-base interaction is present. 

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Low- and high-density polyethylene, polypropene, and polymers of other alkene (olefin) monomers constitute the polyolefin family of polymers. All except LDPE are produced by coordination catalysts. Coordination catalysts are also used to produce linear low-density polyethylene (LLDPE), which is essentially equivalent to LDPE in structure, properties, and applications (Sec. 8-1 lc). The production figures given above for LDPE do not include LLDPE. The production of LLDPE now exceeds that of LDPE, with about 10 billion pounds produced in 2001 in the United States. (Copolymers constitute about one-quarter of all low density polyethylenes see Sec. 6-8b.)... 

Isotactic polymers with identical ends of finite chains turn out to be achiral meso-forms (with a pseudoasymmetric center, if odd-numbered). Similarly, syndiotactic polymers with odd numbers of monomer units and identical chain ends are meso-forms, while even-numbered syndiotactic finite polymer chains are chiral, even with identical chain ends. On the other hand, formal chirality prevails in all types of stereoregular polymers with unequal chain ends (which is actually the case with polymers of x-alkenes). 

The copolymerization of ethene with a variety of other alkenes or dienes was also studied. The copolymerization of supercritical mixtures of ethene and propene (120-220 °C and 1000-1500 bar) was catalyzed by the silyl-bridged bis-(tetrahydroindenyl)zirconocene catalyst 19 and MAO at a metallocene concentration of 6 X 10 mole fraction and an Al Zr ratio of 22000 [92]. With a 50 50 mixture of SCC2H4 and scCsHg, the resulting polymer had only 8% incorporation of propene. Increasing concentrations of propene resulted in... 

Copolymers in which the double bonds of 1,3-butadiene undergo polymerization with the double bonds of other alkenes have assumed increasing importance in recent years. By varying the proportions of the different monomers in the polymerization mix, the properties of the final product may be tuned over a considerable range. One such substance is a three-component copolymer of propenenitrile, 1,3-butadiene, and ethenylbenzene, known as ABS (for flcrylonitrile/ utadiene/ityrene copolymer). The diene imparts the rubber-like property of flexibility, whereas the nitrile hardens the polymer. The result is a highly versatile... 

It has been suggested that these polymers are mainly linear, which may be a consequence of intermolecular metathesis reactions with traces of acyclic alkenes, or of other consecutive reactions 19-22). 

Since this initial report, there is only one other report for M-NHC catalysed copolymerisation of CO/alkenes [52]. Lin and co-workers synthesised the fcw-NHC complex dication 41, that copolymerises CO and norbomene. The copolymer is synthesised in 87% yield by employing 0.5 mol% 41, and 750 psi CO gas after 3 days at 60°C. The polymer formed contains 37 repeat units and = 4660 and M = 3790. 

Catalytic hydrogenation of PVC causes a significant reduction in the rate of the polymer s subsequent photodegradation (32). Although this result is consistent with the occurrence of alkene sensitization (32), it can also be attributed, per se, to the removal of other possible sensitizers such as carbonyl groups and peroxide linkages. 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

Isobutene - In contrast to the complicated picture presented by the polymerisations of most other alkenes, the polymerisation of isobutene at low temperatures is a clean reaction with apparently few complications [10, 16, 17, 18]. The propagation step seems to be a simple addition to the monomer of the tertiary carbonium ion at the growing end of the chain. This difference between the behaviour of isobutene and of most other olefins is so striking that isobutene could usefully be regarded as a standard of reference it would thus be possible to enquire into the behaviour of other olefins by comparing them and their polymers with isobutene and polyisobutene. 

A catalyst will generate an isotactic polymer as long as consecutive propene monomers bind to the metal with the same ri-face (Scheme 8.39 path A), as illustrated in the scheme by black and white Ti-orbitals of the alkene. On the other hand, if the second monomer binds with the opposite Ti-face to the first (path B), a syndiotactic polymer will result. 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

Isomeric polymers can also be obtained from a single monomer if there is more than one polymerization route. The head-to-head placement that can occur in the polymerization of a vinyl monomer is isomeric with the normal head-to-tail placement (see structures III and IV in Sec. 3-2a). Isomerization during carbocation polymerization is another instance whereby isomeric structures can be formed (Sec. 5-2b). Monomers with two polymerizable groups can yield isomeric polymers if one or the other of the two alternate polymerization routes is favored. Examples of this type of isomerism are the 1,2- and 1,4-polymers from 1,3-dienes (Secs. 3-14f and 8-10), the separate polymerizations of the alkene and carbonyl double bonds in ketene and acrolein (Sec. 5-7a), and the synthesis of linear or cyclized polymers from non-conjugated dienes (Sec. 6-6b). The different examples of constitutional isomerism are important to note from the practical viewpoint, since the isomeric polymers usually differ considerably in their properties. 

C4 Alkenes. Several industrial processes have been developed for olefin production through catalytic dehydrogenation138 166 167 of C4 alkenes. All four butenes are valuable industrial intermediates used mostly for octane enhancement. Isobutylene, the most important isomer, and its dimer are used to alkylate isobutane to produce polymer and alkylate gasoline (see Section 5.5.1). Other important utilizations include oxidation to manufacture maleic anhydride (see Section 9.5.4) and hydroformylation (see Section 7.1.3). 

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