Borate polymerization

Figure 5., JB-NMR Spectra of Organoboron Polymers Prepared by Boration Polymerization.

A wide range of novel polymers with boron in the backbone have been prepared by means of boration polymerizations (65,70-82). Diynes can be polymerized by hydroboration (70,71), phenylboration (65), and haloboration (72) to yield polymers (14),(15), and (16) (eq. 17). When an appropriate aromatic or heteroaromatic diyne is used, the resulting polymers have been shown to have extended 7T-conjugation through the vacant p-orbital of the boron atom (73,74). In fact, several have been shown to exhibit blue fluorescence emission. 

Recently, we explored haloboration polymerization between boron tribromide and terminal diynes to give poly(organoboron halide)s as a polymeric Lewis acid, which has scarcely ever been known [13]. These poly(organoboron halide)s were also prepared by means of hydro-boration polymerization between dienes and monobromoborane as shown in Scheme 4 [14]. As a typical example, 1,7-octadiene was added to a 1.0 M dichloromethane solution of monobromoborane-dimethyl sulfide complex with stirring at 0°C under nitrogen. The molecular weight of the polymer obtained was increased when the feed ratio of monobromoborane to diene approached or slightly exceeded unity. 

Poly(organoboron halide)s were also prepared by hydro-boration polymerization between various dienes and mono-bromoborane-dimethyl sulfide (36). 

Single-electron transfer from a borate anion particle to the excited polymethine cation generates a dye radical and an aLkylphenylbotanyl radical. The aLkylphenylbotanyl radical fragments to form an active alkyl radical. It is the alkyl radical particles that initiate the polymerization reactions (101). 

Polyborates and pH Behavior. Whereas bode acid is essentiaHy monomeric ia dilute aqueous solutions, polymeric species may form at concentrations above 0.1 M. The conjugate base of bode acid in aqueous systems is the tetrahydroxyborate [15390-83-7] anion sometimes caHed the metaborate anion, B(OH) 4. This species is also the principal anion in solutions of alkaH metal (1 1) borates such as sodium metaborate,... 

This anomalous pH behavior results from the presence of polyborates, which dissociate into B(OH)2 and B(OH) as the solutions are diluted. Below pH of about 9 the solution pH increases on dilution the inverse is tme above pH 9. This is probably because of the combined effects of a shift in the equihbrium concentration of polymeric and monomeric species and their relative acidities. At a Na20 B202 mol ratio equal to 0.41 at pH 8.91, or K20 B202 mol ratio equal to 0.405 at pH 9 the pH is independent of concentration. This ratio and the pH associated with it have been termed the isohydric point of borate solutions (62). 

A single-crystal x-ray diffraction study has shown that the borate anion in anhydrous borax is polymeric in nature and is formed via oxygen bridging of triborate and pentaborate groups (83). The chemistry of anhydrous borax has been reviewed (73,84). 

To accelerate the polymerization process, some water-soluble salts of heavy metals (Fe, Co, Ni, Pb) are added to the reaction system (0.01-1% with respect to the monomer mass). These additions facilitate the reaction heat removal and allow the reaction to be carried out at lower temperatures. To reduce the coagulate formation and deposits of polymers on the reactor walls, the additions of water-soluble salts (borates, phosphates, and silicates of alkali metals) are introduced into the reaction mixture. The residual monomer content in the emulsion can be decreased by hydrogenizing the double bond in the presence of catalysts (Raney Ni, and salts of Ru, Co, Fe, Pd, Pt, Ir, Ro, and Co on alumina). The same purpose can be achieved by adding amidase to the emulsion. 

The polymerization of 1,3,3-trimethyl-2,7-dioxabicyclo[2.2.1 Jheptane 35 was carried out in methylene chloride, toluene, and 1-nitropropane at temperatures between —78 and 0 °C32l Boron trifluoride etherate, triethyloxonium tetrafluoro-borate, antimony pentachloride, and iodine were used as initiators. Irrespective of the solvents and initiators employed, the products obtained at 0 °C were white powders with melting points of 50—55 °C, while those obtained at tower temperatures were sirups. The number average molecular weight of the unfractionated products ranged from 400 to 600. The molecular weight distribution of the oligomers prepared at 0 °C was broad, in contrast to the relatively narrow distribution of those obtained at -40 °C. 

Thereby, the presence of tertiary monophosphines is critical, because both polymeric (26) and cyclic tetrameric (24 and 25) complexes can be obtained depending on the phosphine added to the reaction mixture. Furthermore, with some of the phosphines rapid decomposition and deposition of metallic silver occurs. In the macrocyclic molecules 24 and 25 a pair of silver atoms is bridged by two bis(l-imidazolyl)borate moieties, with a transannular Ag Ag distance of 8.61 A for 24 and 8.89 A for 25. The conformations of 24 and 25 are... 

Fig. 8. Tetrameric 24 and 25 and polymeric 26 complexes between silver(I) salts and bis (1 -imidazolyl)borates...

To avoid problems associated with these polymer reactions, the direct synthesis of lithium borate polymer electrolytes37 was undertaken by dehydrocoupling polymerization using lithium mesitylhydroborate40 (scheme 7). 

Lithium mesitylhydroborate was prepared by reaction of mesitylmagnesium bromide with trimethoxyborane and subsequent reduction with LiAlH4. The polymerization was performed by adding a THF solution containing a slight excess of lithium mesitylhydroborate to oligo(ethylene oxide) in THF. After treatment with alcohol, the lithium borate polymers were obtained as transparent soft solids soluble in methanol, THF, and chloroform. 

A variety of organoboron polymer electrolytes were successfully prepared by hydroboration polymerization or dehydrocoupling polymerization. Investigations of the ion conductive properties of these polymers are summarized in Table 7. From this systematic study using defined organoboron polymers, it was clearly demonstrated that incorporation of organoboron anion receptors or lithium borate structures are fruitful approaches to improve the lithium transference number of an ion conductive matrix. 

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