Styrene half-sandwich metallocene

Syndiotactic polystyrene was first obtained only recently by Ishihara et al. [5] in polymerisation with a homogeneous catalyst derived from a transition metal compound such as monocyclopentadienyltitanium trichloride and methylalu-minoxane in toluene. Since then, several authors have reported on the synthesis of syndiotactic polystyrene promoted by different catalysts based on metal hydrocarbyls such as benzyl compounds, half-sandwich metallocenes (e.g. monocyclopentadienyl, monopentamethylcyclopentadienyl and monoindenyl metal derivatives), metal alkoxides, metallocenes and some other compounds. These catalysts are commonly derived from titanium or zirconium compounds, either activated with methylaluminoxane or aluminium-free, such as those activated with tris(pentafluorophenyl)boron, and promote the syndiospecific polymerisation of styrene and substituted styrenes [5-10,21,48-70], Representative examples of the syndiospecific polymerisation of styrene using catalysts based on various titanium compounds and methylaluminoxane are shown in Table 4.2 [6,52,53,56,58],... 

What are the advantages of half-sandwich metallocene-based catalysts as compared with heterogeneous Ziegler-Natta catalysts in styrene polymerisation What are the possible consequences of this for developing industrial processes ... 

Let us recall that half-sandwich metallocene-based catalysts, either activated with methylaluminoxane or aluminium free, such as those activated with tris(per-fluorophenyl)boron, also promote the syndiospecific polymerisation of styrene. 

Since metallocene catalysts have coordination power for chain staeoregularity, efforts are made to produce tactic polymers from nonolefin sources such as styrene, MMA, and vinyl chloride, which are normally polymerized using free-radical processes. Unfortunately, metallocenes based on early transition metals are too sensitive to polarity. Only styrene can be polymerized to high molecular weight. Syndiotactic PS was produced using half-sandwich metallocene in 1986 by Ishihara et The materials have high melting tempaa-... 

Schellenberg, J. The syndiospecific polymerization of styrene in the presence of fluorine-containing half-sandwich metallocenes. J. Polym. Sci. Part A Polym. C/zew.,38,2428-2439 (2000). 

Tt-Olefins, with Ru and Os half-sandwiches, 6, 512 Olefin—styrene, co-polymerizations with post-metallocenes, 4, 1145... 

Half-sandwich group 3 metallocenes, such as CpYCl2 THF, activated with alkylaluminioxane, have also been found to promote the polymerisation of styrene, but the polymer produced appeared to be atactic [96]. 

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

For the production of ethylene/l-octene copolymers, metallocenes in combination with oligomeric methylalumoxanes or other compounds are now used [31, 63]. Half-sandwich transition metal complexes such as [(tetramethyl- / -cyclopentadienyl) (A-/-butylamido)dimethylsilyl]titanium dichloride are applied to synthesize linear low-density copolymers and plastomers, called constrained geometry catalysts [31]. Ethylene and styrene can be copolymerized to products ranging from semicrystalline mbber-like elastomers to highly amorphous rigid materials at room temperature [64]. 

Copolymers of ethylene and styrene are not readily prepared by free radical polymerization, due to the extremely high reactivity of styrene relative to ethylene. Further, copolymers produced using traditional Ziegler-Natta catalysts typically contain less than 1% styrene [28]. However, random ethylene/styrene copolymers with varied styrene content have recently been prepared using homogeneous half-sandwich and metallocene catalysts, activated by MAO, [29-31] but detailed studies of the effect of the bulky phenyl pendant group have yet to be reported. 

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