Olefin polymerization initiation reaction

Activation of ry. ry -Cp tuck-in zirconocene com-plexes results in formation of zwitterionic single-component olefin polymerization catalysts. Reaction of tuck-in zirconocene 37 with 1 equiv of FAB in hexane initially forms a yellow kinetic product which under mild conditions subsequently undergoes conversion to an orange thermodynamic product 38. in which the Zr center is stabilized by interactions with the methylene carbon and the ortho-hydrogen of the phenyl group.Hydrogenoly-sis of 38 affords the corresponding hydride derivative... 

Supported metal oxide catalysts are widely employed in industrial applications alkane dehydrogenation, olefin polymerization, olefin metathesis, selective oxidation/ammoxida-tion/reduction of organic molecules (alkyl aromatics and propylene), and inorganic emissions (N2O, NO , H2S, SO2, and VOC) [1,3,7,11-13]. The initial industrial applications of supported metal oxide catalysts were limited to hydrocarbon dehydrogenation/hydro-genation and olefin polymerization/metathesis reactions. In more recent years, the number of applications of supported metal oxide catalysts for oxidation reactions has grown significantly due to their excellent oxidation characteristics in the manufacture of certain... 

The insertion of alkenes into M-H bonds has been examined in Chap. 4. This reaction is very important because, it leads to the dimerization, oligomerization and polymerization of alkenes. It is broad and concerns not only transition metals, but also main-group metals (group 13 Lewis acids), lanthanides and actinides. For instance, AlEt3 is an excellent initiator of olefin polymerization. This reaction can also be considered as the hydrometallation or the hydroelementation of an olefin, and stoichiometric examples have been shown. If the element E does not have the property of a Lewis acid allowing olefin pre-coordination onto a vacant site and thus facilitating insertion, the insertion reaction is not possible without a catalyst. 

The initiation reaction in the polymerization of vinyl ethers by BF3R20 (R20 = various dialkyl ethers and tetrahydrofuran) was shown by Eley to involve an alkyl ion from the dialkyl ether, which therefore acts as a (necessary) co-catalyst [35, 67]. This initiation by an alkyl ion from a BF3-ether complex means that the alkyl vinyl ethers are so much more basic than the mono-olefins, that they can abstract alkylium ions from the boron fluoride etherate. This difference in basicity is also illustrated by the observations that triethoxonium fluoroborate, Et30+BF4", will not polymerise isobutene [68] but polymerises w-butyl vinyl ether instantaneously [69]. It was also shown [67] that in an extremely dry system boron fluoride will not catalyse the polymerization of alkyl vinyl ethers in hydrocarbons thus, an earlier suggestion that an alkyl vinyl ether might act as its own co-catalyst [30] was shown to be invalid, at least under these conditions. 

In one of the earliest investigations of spin trapping, olefin polymerization was employed to demonstrate the utility of the method as a qualitative probe for free-radical reactions (Chalfont et al., 1968). The polymerization of styrene, initiated by t-butoxyl radicals, proved to be an excellent system with which to obtain spectra attributable to spin adducts with MNP (a) of the initiator radical... 

The Catalyst System Eleven years ago, Kaminsky invented a novel olefin polymerization catalyst derived from Cp2ZrCl2 (Cp = 7 -5-C5H5) and methylaluminoxane (1), a result that has stimulated intense interest in synthesis and reactions of metallocenium ions. Important questions still remain, however, regarding the nature of the Kaminsky catalyst. These include (1) what is methylaluminoxane and how does it interact with Cp2ZrMe2 to initiate polymerization and (2.) what are the mechanisms of chain initiation, propagation, transfer and termination A collateral question is how these steps may be controlled. 

The initial step in the reaction mechanism is formulated as an oxidative addition of the silacyclobutane to the transition-metal complex attaching Si to M (ring expansion). It is followed by a transfer of L2 from the metal to the silicon (ring opening) and polymer growth by insertion of further coordinated ring into the metal-carbon bond, similar to the mechanism proposed for olefin polymerization by Ziegler-type catalysts. 

In cationic polymerizations, initiation occurs by attachment of a proton or some other Lewis-acidic cation X" to the H2C=CR2 double bond of a vinyl monomer to form a new carbon-centred cation of the type XH2C-CR2, which then grows into a polymer chain by subsequent H2C=CR2 additions (Figure 2, bottom). This type of polymerization works well - and is used in practice - only for olefins such as isobutene, where 1,1-disubstitution stabilizes the formation of a cationic centre. Since side reactions, such as release of a proton from the cationic chain end, occur rather easily, cationic polymerization usually gives shorter chains than anionic polymerization. 

Olefin metathesis is one of the few fundamentally new organic reactions discovered over the last few decades that has revolutionized organic and polymer chemistry. Olefin metathesis provides a convenient and rehable way to synthesize imsaturated molecules that are often hard to prepare by any other method. A munber of reviews [8-17] and books [18-20] have been published in this area, aU of which focus on the ever-increasing use of olefin metathesis in organic synthesis and polymer chemistry. Particularly in the latter research area, ROMP has become a powerful and popular method to synthesize polymers with narrow molecular weight distributions. Due to the hving nature of polymerizations initiated by state-of-the-art initiators, well-defined diblock, triblock or multiblock copolymers are available today. 

Olefin oligomerization were found to occur on SAPO molecular sieves, though their activity was far less than the of zeolite ZSM-5[17]. While showing very different initial activity, the wide-pore SAPO-5 and the narrow pore SAPO-34 both deactivated severely (Figure 3). Both of these catalysts yielded a wide spectrum of products presumably following the pathway described by Tabak et. al. [5], in which numerous olefin polymerization and scission reactions take place. Strangely, medium pore SAPO-11 showed complete selectivity for olefin dimers... 

The purpose of this study was to investigate the mechanism of cationic olefin polymerizations by model experiments using alkyl-aluminum/alkyl halide initiator systems and to correlate the results of model experiments with corresponding polymerization reactions. 

In Section 7.1 the effects of chain-transfer on polymerization rate were not considered. There is, however, evidence from the dependence of rate on monomer concentration to show that initiation reactions in olefin polymerization are slower than propagation. If the rate equation... 

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