Organometallic compounds styrene

The first results of anionic polymerization (the polymerization of 1,3-butadiene and isoprene induced by sodium and potassium) appeared in the literature in the early twentieth century.168,169 It was not until the pioneering work of Ziegler170 and Szwarc,171 however, that the real nature of the reaction was understood. Styrene derivatives and conjugated dienes are the most suitable unsaturated hydrocarbons for anionic polymerization. They are sufficiently electrophilic toward carbanionic centers and able to form stable carbanions on initiation. Simple alkenes (ethylene, propylene) do not undergo anionic polymerization and form only oligomers. Initiation is achieved by nucleophilic addition of organometallic compounds or via electron transfer reactions. Hydrocarbons (cylohexane, benzene) and ethers (diethyl ether, THF) are usually applied as the solvent in anionic polymerizations. 

Heiss, C. Marzi, E. Schlosser, M. Buttressing effects rerouting the deprotonation and functionalization of 1,3-dichloro- and 1,3-dibromo-benzene. Eur J. Org. Chem. 2003, 4625 -629. Murahashi, S.-I. Naota, T. Tanigawa, Y. Palla-dium-phosphine-complex-catalyzed reaction of organometallic compounds and alkenyl halides (Z)-/j-[2-( N,. -di rn ethyl ami no) pheriyl]-styrene. Org. Synth. 1990, Coll. Vol. VII, 172— 176. 

To begin, let s consider the anionic polymerization of styrene. For an initiator, we will choose an organometallic compound an organic compound bonded to a metal atom) such as butyllithium, C4H9 Li+. Although the details differ, you should recognize the overall similarity of the mechanism for this anionic polymerization to that for the free radical polymerization of ethylene, above (initiation, propagation, and termination). 

It is known from the work of Morton, on polymerization with Alfin catalysts, that the inorganic constituent of the catalysts (NaCl) plays an important part in the special effectiveness of these initiators. Hence, the question had to be examined if, likewise, the presence of inorganic substances is necessary for stereospecific polymerization with organosodium and -potassium compounds. As a result, it has been established that all the organometallic compounds derived as indicated in the above preparative methods facilitate the stereospecific polymerization of styrene in n-heptane. In addition, the nature and chain length of the residue R have no significant influence on the initiators. In fact, R can be linear or branched or aryl-aliphatic. Also, phenyl or triphenylmethylsodium yields isotactic polystyrene. 

Thus, for MCMs that are pure organometallic compounds of transition metals, the most important problem is how to avoid the elimination of a metal hydride during polymerization (Eq. 4-13) [64]. Such transformations are not experimentally observed for MCMs with polymerizable groups of other types (styrene, methacrylate, etc.). 

The use of organometallic compounds as chain-transfer catalysts in free-radical polymerization has been widely studied. One objective is the production of polymers with terminal vinyl groups and lower molecular weight components compared with polymerization in the absence of chain-transfer catalysts. Gomplexes of cobalt(ii) have been used as effective catalysts, but the instability of the intermediate cobalt hydride does not permit firm establishment of the reaction mechanism. To address this issue, several chromium compounds have been applied as catalysts for the polymerization of methylmethacrylate (MMA) and styrene. The temperature dependence of the rate constant for free-radical polymerization of MMA for catalyzed chain transfer by (GsPh5)Gr(GO)3 has been determined using the Mayo equation. ... 

The breakdown of the aggregates was shown to dramatically enhance the reactivity of the organometallic compounds. For instance, polymerizations of styrene in benzene with butyl-lithium are slow reactions. When, however, these polymerizations are carried out in tetrahydrofiiran they are extremely rapid. Tetrahydrofuran is, of course, a Lewis base. Nevertheless, the breakdown of the aggregates even in such Lewis bases as tetrahydrofuran or diethyl ether are not complete, though the clusters are smaller and more solvated. Differences in reactivity, however, can be observed even in different nonpolar solvents. 

The presence of heteroatoms like oxygen or nitrogen in the monomer causes rapid complexation with the organometallic compounds. For instance, when butyllithium initiates polymerization of styrene in hydrocarbon solvents, there is an induction period and the overall reaction is slow and sluggish. When, however, it initiates polymerization of o-methoxystyrene under the same conditions, there is no induction period and the reaction is rapid. This is due to the initiator coordinating with the oxygen atom... 

Living radical polymerization (atom transfer radical pol5mierization) has been developed which allows for the controlled polymerization of acrylonitrile and comonomers to produce well defined linear homopolymer, statistical copolymers, block copolymers, and gradient copolymers (214-217). Well-defined diblock copolymers with a polystyrene and an acrylonitrile-styrene (or isoprene) copolymer sequence have been prepared (218,219). The stereospecific acrylonitrile polymers are made by solid-state urea clathrate polymerization (220) and organometallic compounds of alkali and alkaline-earth metals initiated polymerization (221). 

Organometallic compounds known as Ziegler catalysts were produced industrially at rates measuring thousands of tonnes annually. Stereospecific polymerization of propylene, butylene and styrene was done by Natta in March 1954. Natta s discovery broke nature s monopoly of building macromolecules of regular structure using enzymes. 

Nakanishi, K., Mizuno, K., and Otsuji, Y, Intramolecular [2 -I- 2 ] photocycloaddition of group 14 organometallic compounds bearing a styrene chromophore a route to sila- and germa-cyclo-phanes and spiro compounds, /. Chem. Soc., Perkin Trans. 1, 3362, 1990. 

A convenient method for the conversion of aldehydes (RCHO) to alkenes (RCH = CHj), knovm as methylenation, involves the reaction of a zinc/copper couple with diiodomethane in the presence of the carbonyl compound dissolved in tetrahy-drofuran. The reaction first generates an organometallic intermediate (ICH2ZnI) which then reacts with the carbonyl compound. The conversion of benzaldehyde to styrene using this conventional methodology required a reaction time of 6 h at 40 °C. When the reaction was sonicated however comparable yields of around 70%... 

The transition group compound (catalyst) and the metal alkyl compound (activator) form an organometallic complex through alkylation of the transition metal by the activator which is the active center of polymerization (Cat). With these catalysts not only can ethylene be polymerized but also a-olefins (propylene, 1-butylene, styrene) and dienes. In these cases the polymerization can be regio- and stereoselective so that tactic polymers are obtained. The possibilities of combination between catalyst and activator are limited because the catalytic systems are specific to a certain substrate. This means that a given combination is mostly useful only for a certain monomer. Thus conjugated dienes can be polymerized by catalyst systems containing cobalt or nickel, whereas those systems... 

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