Titanate initiators

Data for examples of polymerization of PBT cyclic oligomers are shown in Table 3.2 [14], Polymerization under mild conditions (0.2-0.3mol% titanate initiator at 190 °C) for 6min led to Mws of 95000-115 000 (entries 2, 4 and 5). These results indicate that either the PBT cyclic polymerization is far more facile than observed for PET cyclics, or that the cyclics had higher purity, since the rates were faster and the final molecular weights significantly higher. As... 

Polycyclization processes involving aminyl radicals have been described by Surzur and Stella. The aminyl radical 437 (Scheme 153) generated by Ti(III) reduction of the corresponding AT-chloroamine gives the iV-chloromethyl-pyrrolizidine 440 as only product, in 66% yield by two consecutive (Cy 5) cyclizations (Cy5/Cy6 cases).Cuprous halide initiation gives only monocyclic compounds by fast ligand transfer to 438 this has been used as an argument for a radical chain mechanism in titanous initiated cyclization of unsaturated 7V-chloroamines (see Section VIII.3.A, Scheme 42). 

Titanate initiators of various structures were discovered by Ciba-Geigy AG [RHO 88, STE 90] (Figure 4.11). Among these structures were methacrylated molecules with the ability to be embedded in forming polymer structures. The titanate initiators are reported to be temperature and humidity stable, but readily decompose when irradiated with light in the wavelength region from 200 to 600 nm. 

In 1988, effective titanate initiators were claimed for light and dual cured dental materials by Dentsply [WHI 89]. In particular, the initiator shown in... 

WHI 89] Whitten D., Venz S., Titanate initiators for light cured compositions. Patent EP 0,334,338 A2, 1989. 

The orange-red titanium acetylacetone chelates are soluble in common solvents. These compounds are coordinately saturated (coordination number equals 6) and thus much more resistant to hydrolysis than the parent alkoxides (coordination number 4). The alkoxy groups are the moieties removed by hydrolysis. The initial product of hydrolysis is beheved to be the bis-hydroxy bis-acetylacetone titanate, (HO)2Ti(acac)2, which oligomerizes to a... 

Titanates trigger peroxide-initiated curing of unsaturated polyesters to give products of superior color, compared to conventional cobalt-initiated... 

For ink vehicles based on hydroxyl group containing binders such as nitrocellulose and cellulose acetate, the tetraalkyl titanates cross-link the binder prematurely, limiting the storage stabiUty of the printing ink. Chelated organic titanates such as TYZOR AA and TYZOR TE are preferred for use in these cases because they only initiate cross-linking when the ink is heated to temperatures above 80°C (503). 

Spherical, Fine-Particle Titanium Dioxide. Spherical, fine-particle titanium dioxide that has no agglomeration and of mono-dispersion can be manufactured by carrying out a gas-phase reaction between a tetraalkyl titanate vapor and methanol vapor in a carrier gas to form an initial fine particle, which can then be hydrolyzed with water or steam (572). 

The pivotal step in this sequence is an electrophilic substitution on indole. Although the use of l,3-dithian-2-yl carbanions is well documented, it has been shown only recently that 1,3-dithian-2-yl carbenium ions can be used in a Priedel-Crafts type reaction. This was accomplished initially using 2-methoxy-l,3-dithiane [1,3-Dithiane, 2-methoxy-] or 2-metlioxy-l,3-dithiolane [1,3-Dithiolane, 2-methoxy-] and titanium tetrachloride [Titanate(l —), tetrachloro-] as the Lewis acid catalyst.9 2-Substituted lysergic acid derivatives and 3-substituted indoles have been prepared under these conditions, but the method is limited in scope by the difficulties of preparing substituted 2-methoxy-1,3-dithianes. l,3-Dithian-2-yl carbenium ions have also been prepared by protonation of ketene dithioacetals with trifluoroacetic acid,10 but this reaction cannot be used to introduce 1,3-dithiane moieties into indole. 

New kinds of living polymer systems result from the reactions of transition metals with cyclic, strained olefins 16). These polymerizations proceed through the intermediacy of metal carbenes and are exemplified by the polymerization of norbomene initiated by bis(cyclopentadienyl)-titane-cyclobutane described recently by Grubbs17>. 

The reaction between mono-octyl phthalate and i5tMx tanol (see Fig. 5.4-25) in the presence of a homogeneous catalyst (rert-butyl titanate) was studied in a batch reactor (Szarawara et al., 1991). This is the second step of the reaction between phthalic anhydride and o-octanol. First the ring is opened and mono-octyl phthalate is formed. Water is removed by evaporation as it is formed. The reaction was carried out at 174 °C. The initial concentration of mono-octyl phthalate was cpno = 1.85 mol/L and the ratio of initial concentrations of iio-octanol to mono-ooctyl phthalate coc.o/cph,o = 1.4. The reaction was... 

The titan cup was the source of zinc atoms. It contained the metal initially melted in hydrogen atmosphere and distilled. The operational temperature of the cup was maintained at the level of 185°C which provided the high percentage of the content of single zinc atoms in the flux. 

A short-wavelength shield achieves protection of chemistry on the surface of the Earth or the initiation of chemistry in a prebiotic Earth or Titan. Essential it may be but there is an alternative - the photic zone. 

The photoproduction of CH3+ in Titan s atmosphere initiates a sequence of ion-molecule reactions. The mechanism is as follows ... 

Taramasso et al. (5) had originally reported two methods for the hydrothermal synthesis of TS-1. The first method (mixed alkoxide method) involves the preparation of a solution of mixed alkoxides of titanium and silica (preferably ethoxides) followed by hydrolysis with alkali-free solution of tetrapropylammonium hydroxide (TPAOH), distillation of the alcohol and crystallization of the resulting gel at 448 K. In the second method (dissolved or hydrolyzed titanium method) a soluble tetrapropylammonium peroxo-titanate species was prepared initially and then colloidal SiC>2 (Ludox AS-40) was added. This entire operation had to be carried out at 278 K. The TS-1 samples obtained by these two synthesis routes differed, particularly because of the presence of impurities such as Al3+ usually present in colloidal silica (33). 

Experimental studies of the polyphase ceramics noted above demonstrate that hollandite is one of the most durable titanate phases in aqueous solutions. Pham et al. (1994) carried out experimental work on synthetic Ba-hollandite doped with Cs and containing Al on the B-site for charge balance. These authors suggested that, following the initial release of Cs and Ba from reactive surface sites the first few monolayers of the structure rapidly dissolved due to the release of Al and consequent precipitation of Al-OH species, driving solution pH to lower values. However, the alteration process was mediated via the formation of a continuous Al- and Ti-rich surface layer. Further evidence for selective removal of Ba and enrichment of Al and Ti on the surface of hollandite tested at 250-300 °C was presented by Myhra et al. (19886). These conclusions were largely based on the different release rates of Ba (10 g/m2/d), Al (7 x 10 3 g/m2/d), and Ti (<8 x 10 4 g/ m2/d) after 14 days of dissolution testing, combined with XPS analyses of the altered surfaces. 

As an example of thermolysis, consider the burnout of polyvinyl alcohol from a barium titanate compact in air. Initially, there is loss of hydroxyl and hydrogen side groups, leaving a conjugated hydrocarbon ... 

It is significant to note that polymer obtained by dimethylformamide initiation does not contain initiator fragments. Polymer obtained by initiation with tetraisopropyl titanate contains an isopropoxy end that is hydrolyzable to —SCF=0. 

When tetraisopropyl titanate is used as the initiator, Be is (CH3)2CHOe. As indicated earlier, the polymer obtained has isopropoxy on the leading end and CF=S on the terminal end. Hydrolysis of the isopropoxy is responsible for CF=0 and chain transfer as a result erf Fe initiation leads to CF3S— ends. 

Copolymerization of thiocarbonyl fluoride and perfluorothioacetyl fluoride, CF3CF==S, have been done in ether at —80° C using N-methylmorpholine and tetraisopropyl titanate as initiators. The products, which contain 13-40% of the acid fluoride, are of relatively low molecular weight, do not melt very much lower than the homopolymer, and are much less elastic than the copolymer. 

Barium titanate, BaTiOs, is a ferroelectric material (see Chapter 9) widely used in capacitors because of its high dielectric constant. It was initially prepared by heating barium carbonate and titanium dioxide at high temperature. 

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