Coordination polymerization, of alkenes

The importance of coordination polymerization of alkenes and dienes is evident when it is noted that more than 40 billion pounds of polymers were produced by this route in the United States in 2001. This corresponds to 35 10% of the total industrial production of polymers from monomers containing carbon-carbon double bonds. 

Modern coordination polymerizations of alkenes proceed in systems con-tainining less than 10 ppm of water. Only a single water molecule may be present among 5 x 104 molecules of propene. There exist sufficient reasons for tightening of this norm by at least an order of magnitude. 

The stereospecific polymerization of alkenes is catalyzed by coordination compounds such as Ziegler-Natta catalysts, which are heterogeneous TiCl —AI alkyl complexes. Cobalt carbonyl is a catalyst for the polymerization of monoepoxides several rhodium and iridium coordination compounds... 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

The mechanism of coordination polymerization of 1,3-butadiene and, in general, that of conjugated dienes follows the same pathway discussed for alkene polymerization that is, monomer insertion into the transition metal-carbon bond of the growing polymer chain occurs. One important difference, however, was recognized very early.47,378,379 In the polymerization of dienes the growing chain end is tt-allyl complexed to the transition metal ... 

Trivalent carbenium ions play a key role, not only in the acid-catalyzed polymerization of alkenes [Eq. (5.348)] but also in the polycondensation of arenes (7r-bonded monomers) as well as in the cationic polymerization of ethers, sulfides, and nitrogen compounds (nonbonded electron-pair donor monomers). On the other hand, penta-coordinated carbonium ions play the key role in the electrophilic reactions of cr-bonds (single bonds), including the oligocondensation of alkanes and alkenes (Section 5.1.5). 

Chromium ions at the surfaces of inorganic oxides are characterized by a wide variability of the oxidation state, coordination number, and local structure. Consequently, Cr-based materials are especially attractive as catalysts. Much is known about the catalytic activity of pure Cr203 for various reactions (469), including polymerization of alkenes (470-472), hydrogenation-dehydrogenation of hydrocarbons (473-481), reduction of NO and decomposition of N2O4 (482), and oxidation of organic compounds (483, 484). 

One of the best polymers for building strong rigid heat-resistant objects is polypropylene but this can be made by none of the methods we have examined so far. We need now to look at the polymerization of alkenes in the coordination sphere of a transition metal. 

One such process is the Cossee-Arlman mechanism,proposed for the Ziegler-Natta polymerization of alkenes (also discussed in Section 14-4-1). According to this mechanism, a polymer chain can grow as a consequence of repeated 1,2 insertions into a vacant coordination site, as follows ... 

As the fundamentals of coordination complex structure and reactivity were realized, new and exciting applications were discovered. In 1955, Ziegler and Natta and their coworkers developed a titanium-based catalyst, Figure 1.9, which catalyzes the polymerization of alkenes at atmospheric pressure and ambient temperature. The open coordination site allows an alkene ligand such as ethylene or propylene to bind to the Ti center, initiating the polymerization. Their work sparked tremendous interest in the field and garnered them the Nobel Prize in 1963. 

The trityl borate [Ph3C][B(C6Fs)4] did not cause CeFs back transfer. However, if the coordination site on Sc had sufficient open, the solvent molecules would coordinate to the metal center to form solvent-separated ion pairs (SSIPs), which were inactive in catalysis of polymerizations. A series of scandium methyl SSIPs 124-128 stabilized by the relatively less steric effect P-diketiminato L24 were prepared by reacting 120 with [Ph3C][B(C6Fs)4] in an arene solvent. The synthetic route is shown in Scheme 41 and the complexes were studied for the relevance to propagation processes in the polymerization of alkene. Competition... 

Coordination polymerization of dienes has progressed significantly within the last decade. Selective polymerization of 1,3-dienes is reinforced by conventional transition metal catalysts and by new organolanthanide catalysts. Nonconjugated dienes also polymerize selectively to produce polymers with cyclic units or vinyl pendant groups. Living polymerization of dienes has become common, which enabled preparation of block copolymers of dienes with alkenes and other monomers. Another new topic in this field is the polymerization of allenes and methylenecycloalkanes catalyzed by late transition metal complexes. These reactive dienes and derivatives provide polymers with novel structure as well as functionalized polymers. The precision polymerization of 1,2-, 1,3-, and l,n-dienes, achieved in recent years, will be developed to construct new polymer materials with olefin functionality. 

There are a number of others in common industrial use, such as those used for polymerization of alkenes. One example of an organometallic homogeneous catalyst is the Zr(IV) complex [Zr(CH3)(T 5-Cp)2X], which operates by binding alkene monomers to the metal prior to addition to a growing carbon chain. A similar coordination of substrate is involved in the use of [Co(CO)4H] as catalyst in the hydroformylation of alkenes to aldehydes. Likewise, the [Rh(CO)2l2] ion, formed in situ, catalyses the carbonylation of methanol to acetic acid. 

The coordinated alkene or alkyne ligand can be attacked by other alkene or alkyne molecule to accomplish some metal-catalyzed synthetically useful transformations. Typical examples include dimerization and polymerization of alkenes catalyzed by highly electrophilic cations [PdL2(MeCN)2] + (L = MeCN, PR3) (e.g. Scheme 8.35) [57], and Cope rearrangement of 1,5-hexadiene derivatives catalyzed by PdCl2 (Scheme 8.36) [58], It was proposed that the key step in these reactions was the C-C bond formation via attack of the external alkene at the alkene carbon which was made highly electron-dehcientby coordination to Pd(II) ion. 

Section 27.10 Cationic polymerization of alkenes that can form relatively stable carbocations can be initiated by protonation of the double bond or coordination to Lewis acids such as boron trifluoride. 

The mechanism of polymerization of alkenes using Ziegler-Natta-type catalysts is described as a coordination [239] or insertion [240] polymerization process. The coordination terminology assumes that the growing polymer chain is bonded to a transition metal atom and that insertion of the monomer into the carbon-metal bond is preceded by, and presumably activated by, the coordination of the monomer with the transition metal center. Since coordination of the monomer may or may not be a specific feature of these polymerizations, the insertion terminology focuses on the proposal that these reactions involve a stepwise insertion of the monomer into the bond between the transition metal atom and the last carbon atom of the growing chain. It is important to note that the bonding of carbon atoms and transition metals is... 

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