Polymeric Boron Compounds

Polymeric Boron Compounds.—number of polymeric or polymer-supported dihydroxyboron compounds have been recommended for the affinity chromato- 

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CHR) , formed, e g. from the reaction of diazomethane and alcohols or hydroxylamine derivatives in the presence of boron compounds or with metal compounds. Poly-methylene is formally the same as polyethene and the properties of the various polymers depend upon the degree of polymerization and the stereochemistry. 

Aqueous mineral acids react with BF to yield the hydrates of BF or the hydroxyfluoroboric acids, fluoroboric acid, or boric acid. Solution in aqueous alkali gives the soluble salts of the hydroxyfluoroboric acids, fluoroboric acids, or boric acid. Boron trifluoride, slightly soluble in many organic solvents including saturated hydrocarbons (qv), halogenated hydrocarbons, and aromatic compounds, easily polymerizes unsaturated compounds such as butylenes (qv), styrene (qv), or vinyl esters, as well as easily cleaved cycHc molecules such as tetrahydrofuran (see Furan derivatives). Other molecules containing electron-donating atoms such as O, S, N, P, etc, eg, alcohols, acids, amines, phosphines, and ethers, may dissolve BF to produce soluble adducts. 

Boron trichloride is used as a catalyst in polymerization reactions. Other applications include refining of alloys soldering flux and as a component in certain fire extinguishers. It also is used to prepare boron libers and other boron compounds including diborane, sodium borohydride and several adducts. 

Tricoordinated boron compounds (boranes) are coordinatively unsaturated and their chemistry is dominated by reactions in which complexes are formed. These complexes are either neutral molecules (borane complexes), anions (borates) or boron cations. Space limitations mean that little or no attention will be paid to complexes containing several boron atoms and to species of the type L-BH3, [BH,]- and [L2BH2]+ (L = neutral ligand), discussed in detail in several books and reviews. Similarly, little attention will be paid to the plethora of metal borates and the cyclic and polymeric amino- and phosphino-boranes. 

Linear polyethylene can further also be considered as polymethylene. Althoguh first prepared by the thermal decomposition of diazomethane,243 244 Meerwein should be credited to have prepared it by catalytic polymerization of diazomethane effected by boron compounds (esters, halides, alkyls)245-247 taking place with concomitant dediazotation ... 

Only in the case of X equals fluorine is the polymeric boron halide-cyanide sufficiently stable to allow isolation of a pure sample. The borane adducts, R3SiCNBH3, require elevated temperatures to yield the polymeric compound HjBCN. Di-n-butylboron chloride and trimethylsilyl cyanide... 

The synthesis of boron-capped clathrochelate iron(II) tris-dioximates has been realized for wide range of substituents at the boron atom. The attempts to obtain analogous trialkyl- and triaryl-tin-capped iron(II) compounds have not been successful. In the some cases, polymeric clathrochelate compounds have been formed, especially when reactions proceed under basic conditions. With tin(IV) iodide, the primarily formed soluble green complexes also readily transform into polymeric red compounds that are presumably associated with the detachment of iodide ions because of steric hindrance between substituents in dioxime fragments and the bulky iodide atoms of capping groups [70],... 

The examination of structure/property relations of molecular educts and resulting ceramics required the synthesis of stoichiometrically and stmcturally different precursors. A variety of synthesis routes for Si-B-N-C precursors fiom organosilanes, silazanes, and boron compounds have been reported in recent years [3 - 5]. As an example, Riedel obtained a polymeric precursor via hydroboration of methylvinyldichlorosilane and subsequent condensation of the hydroboration product with ammonia (Eq. 1). Pyrolysis led to silicoboron carbonitride ceramics exhibiting thermal stability up to 2000 °C [6]. 

Use Synthesis of organic boron compounds and metal borohydrides, polymerization catalyst for ethylene, fuel for air-breathing engines and rockets, reducing agent, doping agent for p-type semiconductors. 

BX3 (X =s F, Cl, or Br) and AICI3 are transformed by MeaSiN3 in CH2CI2 or ether into trimeric boron dihalide azide or into monomeric aluminium dichloride azide (in CH2CI2) or polymeric aluminium chloride diazide. In the boron compounds, i.r. spectra are consistent with the presence of bridging azido-groups, i.e. the structure is (68). 

Boron compounds are nsed in making glass and determents as well as nsed in agricultnre. Boron is used in fibreglass and borosilicate fibre prodnction, both used in fibre-reinforced composite systems. They are also nsed in fibreproofing polymeric fabrics. 

Various other metal salts were also reported as being capable of photo initiating free-radical polymerizations. Thus, salts of boron compounds are often used in aehieving cures of coatings and ink formulations that are based on acrylates and methacrylates. The aryl alkyl borates that are paired with cationic chromophores can act as co-initiators in either UV or in visible light... 

Some of the more active catalysts for the polymerization of styrene are shown in Figure 22.17. Monocyclopentadienyltitanium halides, such as CpTiXj, CpTiXj, and CpTiXj, in combination with MAO, were more active catalysts than simple tetrahalides. Of these Cp-ligated trihalide complexes, the fluoride complexes were most active, followed by alkoxides and then chlorides. Moreover, cyclopentadienyl-ligated alkyltitanium compounds, such as Cp TiRj (R = hydrocarbyl) activated by boron compounds B(C Fj)j... 

In addition to MAO, boron compounds based on tris(pentafluorophenyl)boron and its derivatives, typically dimethylanilinium tetrakis(pentafluorophenyl) borate, have been used as cocatalysts for sPS polymerizations (40,41). Although MAO has been used in large molar excesses relative to the titanium complex, the boron compounds may be used in roughly equimolar amounts to the titanium catalyst. The boron cocatalyst reacts with a titanium alkyl species, either by protonation in the case of dimethylanilinium tetrakis(pentafluorophenyl)borate or by alkyl group abstraction in the case of tris(pentafluorophenyl)boron, to generate a titanium cationic species with a borate counterion (74-76). The esr spectral evidence has been reported for these systems, supporting a titanium(III) cationic active species (76). 

Unlike molecules containing electron-rich heteroatoms, boron compounds do not poison Ziegler-Natta or metallocene polymerization catalysts. Borane-containing olefin comonomers are therefore well suited to produce olefin copolymers while retaining good catalyst activity. The resulting polymers are suitable for subsequent conversion into a variety of functional groups. In principle, two approaches are possible (1) hydroboration of the terminal double bond (formed by typical chain transfer processes) of a preformed polyolefin, and (2) direct copolymerization of propylene or a 1-alkene with an alkenyl borane (Scheme 11.4). 

Titanium half-sandwich complexes of cyclopentadiene and substituted cyclopentadienes (Cp ) activated with MAO or organic boron compounds especially show a high styrene polymerization activity. 

Endoboron Compounds from CycloaU tboranes Cycloalkylboranes will also form heterocyclics by pyrolysis 24,67). Since cycloalkanes cleave readily at fairly low temperatures (ca. 180° C), cycliza-tion proceeds smoothly at 200° C. It is absolutely essential that the cyclic ring be large enough. Tricyclohexylborane does not form 7-borabicydo-[2.2.1]heptane instead, polymeric boron-containing products are formed and volatile materials evolved. 

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