Transition metals, tetrahydrofuran polymerization

Heating borazine in vacuo at 70°C yields poly(borazylene) polymers, which are soluble in solvents such as tetrahydrofuran or glyme and could be thermolyzed to boron nitride in good yields (120). Other soluble preceramic polymers were produced by transition-metal catalyzed formation of B-alkenylborazines (eq. 34) which were thermally polymerized under mild conditions to poly(alkenylborazines). The latter yielded boron nitride having low carbon contents when thermolyzed in an ammonia atmosphere (121). 

Discovery. These catalysts were discovered during a study of the use of transition metal cyanides in combination with metal alkyl and hydride reducing agents in polymerizations. The combination of nickel cyanide and lithium aluminum hydride complexed very strongly with tetrahydrofuran. A similar complexing action occurred with propylene oxide and nickel hexacyanoferrate(II)-lithium aluminum hydride. This led to speculation as to the role of the double-metal cyanide itself. 

Polymerization activity was obtained with a variety of catalyst compositions. The best stereospecific catalyst was the split pretreated type (357) in which one mole of VC14 was reduced by a stoichiometric amount of an alkyl metal (0.34 mole AlEt3) in heptane at room temperature and heated 16 hours at 90° C. to obtain the purple crystalline VC13-1/3 A1C13. This reduced transition metal component was then treated with two moles of (i-Bu)3Al tetrahydrofuran complex for 20 hours at room temperature to obtain a chocolate-brown catalyst consisting predominantly of divalent vanadium with 0.21 Al/V and 1.4 i-Bu/Al. Polymerizations at 30° C. gave crystalline polymers from methyl, ethyl, isopropyl, isobutyl, tert.-butyl, and neopentyl vinyl ethers. 

Many methods have been used to prepare NO complexes. The reactions of transition metal complexes with gaseous NO or solutions of the dissolved gas have been used to form NO complexes. One example of such a reaction to form, a homoleptic nitrosyl complex is shown in Equation 4.30. Alternatively, the reactions of metal complexes with nitrosonium salts form metal-nitrosyl complexes. (Note Alcohol solvents should be used with caution, because they convert NO " to alkyl nitrites. In addition, tetrahydrofuran should not be used as solvent because it can be polymerized by NO". ) Two examples of such processes are shown in Equations 4.31 and 4.32. A range of organic nitroso compounds release "NO" under appropriate conditions and have been used to introduce NO into transition metal complexes. The generation of a metal-nitrosyl complex from F3CNO is shown in Equation 4.33. ° Inorganic nitrites have also been used to prepare NO complexes, ° as shown in Equation 4.34. ... 

Other catalysts contain the transition metals vanadium, chromium, cobalt, and nickel as their main components [194-202]. The polymerization activity is usually far lower than in the synthesis of cis polymers (see Table 2). Addition of a donor such as tetrahydrofuran, which directs the bonds into a tra 5-position to the catalyst of titanium tetraiodide and triethylaluminum, results in the formation of a polybutadiene with 80%... 

Ring opening metathesis polymerization of 5- 6-N-carbazolylhexyl)oxy)methyl)-2-norbomene (24a) with various transition metal catalysts has been used for the preparation of the novel carbazolyl-substituted polymer (24b) (Scheme 11.12) [213]. Studies on the photoconductivity of the polymer (19b) including copolymer systems containing tetrahydrofuran moieties are still in progress. 

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