Polymerization a-olefins

Perhaps the most important chemical property of these complexes is their potential as catalysts, particularly of the early transition metal isoleptic compounds for a-olefin polymerization. This arises because unlike the methyls, they are sufficiently stable to be used at temperatures where polymerization rates are adequate. Some data are summarized in Table VIII ( 9) TT-acceptor ligands are clearly disadvantageous. It will be seen that some of the systems are more active than Ziegler types, although stereoselectivity is poorer. 

Multi-scale modeling problems in heterogeneously catalyzed a-olefin polymerization... 

HfCl4-catalyzed hydro- and carbometalation have been investigated.1 3 /nr/i.v-Addition of tri-butyltin hydride or allyltrimethylsilane to alkynes occurs with high selectivities (Scheme 43).162 In a-olefin polymerization, the Hf mcthylaluminoxanc (MAO) system also works well.184... 

Table 5 Higher a-olefin polymerization results for Ti-FI catalyst 2 with i-Bu3Al/Ph3CB(C6F5)4 ... 

Scheme 4 Mechanism of chain growth for a all Pd(II) polymerizations and ethylene polymerizations with Ni(II), and b a-olefin polymerizations with Ni(II). Specific kinetic data shown for Ni catalyst 1.15b [63]...

Fig. 5 a-Diimine and related neutral nitrogen ligands (and complexes) utilized in ethylene and a-olefin polymerizations... 

Titanocene and zirconocene dichlorides (Cp2MtCl2 with Mt = Ti, Zr) were the first metallocenes studied [Breslow and Newburg, 1957 Natta et al., 1957a], The metallocene initiators, like the traditional Ziegler-Natta initiators, require activation by a Lewis acid coinitiator, sometimes called an activator. AIRCI2 and A1R3 were used initially, but the result was initiator systems with low activity for ethylene polymerization and no activity in a-olefin polymerization. The use of methylaluminoxane (MAO), [A1(CH3)0] , resulted in greatly improved activity for ethylene polymerization [Sinn and Kaminsky, 1980], The properties of MAO are discussed in Sec. 8-5g. MAO has two functions alkylation of a transition metal-chloride bond followed by abstraction of the second chloride to yield a metallocenium... 

Unprecedented Control Over Ethylene and a-Olefin Polymerization... 

As already stated in the first chapter, several catalytic systems show a certain stereospecificity in the a-olefin polymerization. 

The behavior of the different catalytic systems (containing transition metal crystalline compounds) in the a-olefin polymerization, except for the different degree of stereospecificity, may be connected with a definite kinetic scheme. This was shown by experimental work performed at the Institute of Industrial Chemistry of the Milan Polytechnic. 

As in the case of a-olefins polymerization, the differences in stereoregularity observed in the polymers of racemic and optically active monomers can be hardly interpreted (65). 

A number of structures were hypothesized for the catalytic active centers in the organometallic catalysts for the a-olefins polymerization. 

Furthermore, the stereoselectivity and stereoelectivity of some particular types of organometallic catalysts for the racemic a-olefin polymerization were proved. 

Sudhakar, P. and Sundararajan, G. (2005) Tuning the reactivity of N, O.O.O-non-metallocene catalysts for a-olefin polymerization issues related to ligand symmetry and derivatization. Macromol. Rapid Commun., 26, 1854. 

Treatment of [(CsMe4SiMe2N-t-Bu)Sc(PMe3)(/i-H)]2 with two equivalents of propylene at low temperature yielded the structurally characterized phosphine-free di-p-propyl complex [(C5Me4SiMe2N-t-Bu)Sc(/i-nPr)]2. This dimeric organoscandium alkyl was found to be an even more active a-olefin polymerization catalyst than the hydride precursor [52],... 

Scheme 5. a-Olefin polymerization catalyzed by [(C5Me4SiMe2N- -Bu)Sc(PMe3X/(-H)] ... 

Kurokawa, H. and Sugano, T., a-Olefin Polymerization by Various Alumoxanes Containing Isobutyl Group and Metallocene , Macromol. Symp., 97, 143-149. 

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