Titanium, polymerizations

Water scavenger Acts as a water scavenger or drying agent by forming polymeric titanium oxide hydrate or other compounds Used as an additive in silicone or polyurethane sealants (0.5-5% by weight)... 

A method for the mild catalysis for ester/transester reactions was developed by Siddiqui [2], using the polymeric titanium glycolate catalyst, (II). 

Transition metal carbodiimides, such as MnNCN and CuNCN, and carbodiimides derived from zinc, mercury,silver and thallium are also known. A preceramic polymeric titanium carbodiimide is obtained in the reaction of TiCU with bis(trimethyl-silyl)carbodiimide. Liganded carbodiimidotitanium complexes are obtained in the reaction of CpaTiCla with Me3SnN=C=NSi(i-Pr)3. ° Also, dicyclopentadienyl titanium (IV) diisocyanates are converted into carbodiimides with LiN(SrMe3)2. ... 

Keywords Isotactic Polypropylene Stereocontrol Stereoselective polymerization Titanium trichloride Ziegler-Natta catalysis... 

The amount of cyclopentadiene was lowered to 2.5 equiv. by Altava et al. in a study of the DA using similar catalysts prepared by grafting and good conversion and diastereoselectivity were obtained.In most cases, the grafted polymeric titanium catalysts were nonenantioselective " compared to those obtained by copolymerisation by Seebach " " (for instance, 83d by grafting gave 0% enantiomeric excess). 

In addition to their work on DA reaction, Seebach et al. also studied polymeric titanium catalyst 82,83 (see Figure 7.7) in the [3 + 2] cycloaddition of 3-crotonoyloxazolidinone and diphenyl nitrone (Table 7.5). The use of soluble catalysts 81a,b,d (entries 1-3) and supported-titanium catalysts 83a,b,d (entries 4-6) afforded similar performances in terms of yield, dia-stereoselectivity and enantioselectivity in favour of adduct 89a. Interestingly, the enantioselectivity was better in heterogeneous conditions with polymeric TADDOL-titanium complex 83d (entry 6, 56% enantiomeric excess) than using soluble catalysts (entries 1-3). 

Titanium oxide monolayer on y-AljOj is a potential support for noble metals [1-4]. Many studies have shown that two-dimensional transition metal oxide overlayers are formed when one metal oxide (Vj05, Nb205, MoOj, etc.) is deposited on an oxide support (AljOj, TiO, etc.) [5-7]. The influence of the molecular structures of surface metal oxide species on the catalytic properties of supported metal oxide catalyst has been examined [8-9]. It has been demonstrated that the formation and location of the surface metal oxide species are controlled by the surface hydroxyl chemistry. Moreover, thin-layer oxide catalysts have been synthesized on alumina by impregnation technique with alkoxide precursor [10]. It has been found for titanium oxide, by using Raman spectroscopy, that a monolayer structure is formed for titanium contents below 17% and that polymeric titanium oxide surface species only posses Ti-O-Ti bonds and not Ti=0 bonds. Titanium is typically ionic in its oxy-compounds, and while it can exist in lower oxidation states, the ionic form TF is generally observed in octahedral coordination [11-12]. However, there is no information available about the Ti coordination and structure of this oxide in a supported monolayer. In this work we have studied the structural evolution of the titanium oxy-hydroxide overlayer obtained from alkoxide precursor, during calcination. 

The reaction of (/ )-81 with Ti(0 Pr)4 in the presence of molecular sieves produces a polymeric titanium complex (/ )-93 (Scheme 39) [73). The monomeric binaphthyl titanium complex (R)-94 is also prepared from the reaction of (/ )-8 with Ti(0 Pr)4. Unlike the polyaluminum complex (/ )-87, although (R)-94 can catalyze the reaction of 88 and 89 at room temperature, no reaction is observed when the polytitanium complex (/ )-93 is used even in a refluxing methylene chloride solution. It has been proposed that when R)-94 is used in the catalysis of organic reactions, the real catalytically active species is a bimetallic complex [73b,c]. Our observation here supports this assumption. In (/ )-93, all of the titanium centers are isolated in a rigid polymer chain so that the bimetallic catalyst cannot be generated, which probably causes the loss of the catalytic activity. 

---