Borane-containing monomers

The use of borane-containing monomers clearly presents an effective and general approach in the functionalisation of polyolefins, which has the following advantages stability of the borane moiety to coordination catalysts, solubility of borane compounds in hydrocarbon solvents (such as hexane and toluene) used as the polymerisation medium, and versatility of borane groups, which can be transformed to a remarkable variety of functionalities as well as to free radicals for graft-form polymerisations. The functionalised polymers are very effective interfacial modifiers in improving the adhesion between polyolefin and substrates and the compatibility in polyolefin blends and composites [518],... 

In turn, Chung has used MAO activated Zr nnetallocene catalysts " with borane containing monomers to make functional polymers. 

Yet another methodology involves the metallocene-catalyzed copolymerization of olefins with borane containing monomers (466,467). The incorporated borane group can be quantitatively converted to a hydroxy group or transformed into a stable poljnneric radical suitable for free radical grafting of acrylates. 

Figure 8 Used zirconocenes for the incorporation of borane-containing monomers.

Direct copolymerization of aromatic a-olefins and borane-containing monomers with early tmnsitlon metal catalysts Beside the early transition metal-catalyzed copolymerization of aliphatic a-olefins with borane-containing monomers. 

Block copolymers can be produced from terminally borane-containing polyolefins. These borane-containing POs can be synthesized by the metallocene-catalyzed (co)polymerization of olefin(s) monomer with 9-BBN as a chain transfer agent or by the metallocene catalyzed copolymerization of olefins with allyl-9-BBN [55,56], as referred to above. Alternatively, borane-containing POs were prepared by hydroboration of terminally unsaturated PO, for instance, terminally vinyl PE and terminally vinylidene PP [33-35,57]. Such method could produce diblock copolymers, such as polyethylene-block-poly(methyl methacrylate) (PE-fo-PMMA), polypropylene-foZock-poly(methyl methacrylate) (PP-fc-PMMA), polypropylene-foZock-poly(butyl methacrylate) (PP-fc-PBMA), and PP-fc-PS. 

In addition, borane-containing POs can be prepared by copolymerization of olefin with borane monomers or by hydroboration of polyolefins including unsaturated groups, such as olefin-divinylbenzene copolymer and olefin-diene copolymers. Many kinds of graft copolymers, such as poly-elhylene-gra/f-poly( vinyl alcohol), PE-g-PMMA, polypropylcnc-gra/f-poly-(maleicanhydride-co-styrene), polypropylene-gra/f-poly(methacrylic acid), polypropylene-gra/f-poly(vinyl alcohol), polypropylene-gra/f-polycaprolac-tone (PP-g-PCL), polypropylcnc-gra/f-poly(methyl methacrylate) (PP-g-PMMA), poly( ethylene-co-propylene)-gra/f-poly(methyl methacrylate) (EPR-g-PMMA), and poly(ethylene-co-propylene)-gra/f-poly(maleic anhydride-costyrene), have been synthesized by such a method resulting in controllable composition and molecular microstructures [63-66]. 

In monoborane (BH3), monoalkylboranes RBH2, or dialkylboranes R2BII there is only an electron sextet at the boron atom. In comparison to the more stable electron octet, the boron atom thus lacks two valence electrons. It obtains them by bonding with a suitable electron pair donor. When no better donor is available, the bonding electron pair of the B—H bond of a second borane molecule acts as the donor so that a two-electron, three-center bond is produced. Under these conditions, boranes are consequently present as dimers BH3, for example, as B2H6. Still, small fractions of the monomers appear as minor components in the dissociation equilibrium of the dimer B2H6, for example, thus contains some BH3. 

The Lewis bases Me2S orTHF are better electron pair donors than the boranes themselves. Therefore, they convert dimeric boranes into Me2S or THF complexes of the monomers such as Me2S-BFI3 or THF-BH3. This type of complex also dissociates to a small extent and thus contains a small equilibrium concentration of free monomeric boranes. 

Although the bond angles are known to vary considerably depending on the metal and halogen, the arrangement is approximately tetrahedral around each metal atom. When the trihalides are dissolved in solvents that are Lewis bases, the dimers separate and complexes containing the monomer and the solvent are formed as a result of Lewis acid-base interactions. Such behavior is similar to that of borane and diborane in that the monomer, BH3, is not stable but adducts of it are. This type of behavior is illustrated in the following equations ... 

ROM has been used to prepare phosphine-containing polymer supports (Scheme 20). Norbornyl-substituted monomer 22 was prepared in two steps from d-bromo-iodobenzene. This was then polymerized with diene 23. It was initially envisioned that it would be necessary to convert the phosphine to the borane adduct in order not to poison the metathesis catalyst. Although protection was needed when using the Grubb s type 1 complex as a catalyst, when employing the more active second-generation complex 24, the free phosphine monomer could be used. This has been attributed to the lower affinity of the active form of the catalyst toward coordination of phosphines due to the presence of the electron-rich heterocyclic carbene ligand. 

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