Titanium monocyclopentadienyl catalysts

Monocyclopentadienyl titanium derivatives are the most active precursors for catalysts possessing high syndiospecific polymerisation activity for styrene and ring-substituted styrenes. The polymerisation activity of biscyclopentadie-nyl titanium compounds activated with methylaluminoxane is lower than that of other soluble titanium-based catalysts [73]. 

Monocyclopentadienyl complexes have also been used as active polymerization catalysts. Indeed, titanium monocyclopentadienyl metallocenes—the so-called constrained geometry catalysts —have been focal points of Dow s activity in this area. A typical example of such a catalyst is shown by the structure 6.24. 

Chien, J. C. W. Salajka, Z. Syndiospecific polymerization of styrene. II Monocyclopentadienyl-tributoxy titanium/methylaluminoxane catalyst. J. Polym. ScL, Part A Polym. Chem. 1991, 29, 1253-1263. 

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],... 

Diene complexes of the so-called constrained geometry monocyclopentadienyl-amido titanium complexes have also been prepared. Interest in these molecules stems from their utility as catalyst precursors in olefin polymerization... 

Monocyclopentadienyl complexes of titanium (CpTiXs) perform poorly as catalysts for ethylene or propylene polymerization, but in the presence of MAO, they polymerize styrene to stereo- and regioregular syndiotactic polystyrene, a crystalline material with very high melting point (273 °C) and glass transition temperature (100°C). In this case, the active polymerizing species is a complex (Figure 8). Each styrene monomer inserts in a secondary manner and the stereoregularity is maintained by the conformation of the last inserted unit (chain-end control). 

Monocyclopentadienyl complex of trimethoxy titanium with a borate activator as metallocene catalyst to polymerize styrene in aqueous emulsions in presence of surfactants 138... 

Monocyclopentadienyl and (monoindenyl)titanium trichlorides are generating interest as catalyst precursors for syndiospecific polymerization of styrene (112-127). A class of highly active olefin pol5unerization catalysts based on a (monocyclopentadienyl)titanium dimethyl cation [CpTi(CH3)2l+, generated hy... 

A variety of Group 4 metal complexes, in combination with common olefin polymerization activators, have been evaluated as potential catalysts for syn-diospecific polymerization of styrene (for reviews, see Refs. 114, 115, 123, and 426). Monocyclopentadienyl and monoindenyl titanocenes generally exhibit the highest activities (eq. 5) (112-127). Curiously, half-sandwich titanium-trifluoride-based catalysts are more active than their trichloride analogues (124,427,428). The polymerization mechanism for sPS formation is under debate. Kinetic studies and spectroscopic investigations of the catalytic systems suggest a cationic Ti(III) complex as the active species (123). 

Indeed, Ishihara and co-workers succeeded in the first preparation of sPS through activation of a transition metal complex with MAO (36,37). Typically, Group IV metallocene complexes have been used as catalysts for the polymerization of sPS. Of these, the monocyclopentadienyl-type complexes of titanium have been found to give the highest pol5nnerization activity based on transition metal (38,39). Subsequent to the development of MAO as the sPS cocatalyst, it has been foimd that highly electrophilic activators, such as the tetrakis(pentafluorophenyl) borate type, can be used as cocatalysts for the production of sPS (40,41). 

In spite of the high activities shown for BD polymerization by homogeneous catalyst systems, amongst these only a few have been found to be active for isoprene polymerization. MAO-activated titanium complexes produce 1,4-polyisoprene polymer with a prevalently cis microstructure (>94%). Recently, Miyazawa et al. have shown that monocyclopentadienyl titanium complexes activated by MAO can promote isoprene polymerization, giving polymers with narrow molecular weight distributions (Mw/Afn < 2) and cis contents of up to 92% with small amounts of 1,2- and 3,4-enchained structures also present. ... 

From both the experimental and theoretical points of view, the most thoroughly studied catalytic systems are undoubtedly allylnickel(II) systems and monocyclopentadienyl titanium complexes. In the case of the nickel systems, chain growth proceeding by BD insertion into the allyl-transition metal bond was proven directly by NMR spectroscopy for both 1,4-trans- and 1,4-cis-regulating catalysts. In this case, the proposed mechanism for stereoregulation suggests that the cis-trans... 

FIGURE 17.16 Possible orientations of the coordinated monomer and the allylic end group of the growing polymer chain around the metal center in diene polymerization with monocyclopentadienyl titanium catalyst systems (a) endo-endo (prone-prone) (b) endo-exo (prone-supine) (c) exo-endo (supine-prone) (d) exo-exo (supine-supine). (Reprinted with permission from Peluso, A. Improta, R. Zambelli, A. Organometallics 2000,19, 411-419. Copyright 2000 American Chemical Society.)... 

Thus, the reaction between CpTiX3 and HN=C BuR (or LiN=C BuR) resulted in a new class of monocyclopentadienyl titanium catalysts for olefin polymerization [CpTi(N=C BuR)X2] [37] ... 

Biscyclopentadienyl (bis-Cp) metal complexes were not the only single-site catalysts for olefin polymerization. Monocyclopentadienyl complexes often showed activity, but generally were not competitive catalysts except when linked to a bulky amido group. Thus, Bercaw s CpSiNR ligand was placed on titanium by workers at Dow and Exxon and found to produce very active catalysts with attractive features (Figure 15). Both companies filed patents in the US and... 

In spite of the large number of metallocene-based polymerization catalysts, there remain relatively few examples of living systems. Titanium complexes bearing a linked monocyclopentadienyl-amido ligand, such as 15 (Figure 9), have been shown to polymerize 1-hexene in a living... 

Titanium [153] and vanadium [154] monocyclopentadienyl complexes are known to serve as catalysts for 1,4-czs polymerization, although bis(cyclopen-tadienyl) complexes are less active [155]. Ricci et al. investigated various diene monomers using CpTiCl3-MAO catalysts and showed that stereoselectivity of polymerization depends on the monomer employed [156]. Soga reported that half-titanocene with an oxy-coordinating pendant 54 produces ds-rich polybutadiene [157,158]. 

---