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  • Chapter 30

    Synthesis of Organoboron Polymers by Hydroboration Polymerization

    Yoshiki Chujo

    Division of Polymer Chemistry, Graduate School of Engineering, Kyoto Univeristy, Yoshida, Sakyo-ku, Kyoto 606-01 Japan

    Hydroboration polymerization is described as a novel methodology for the preparation of organoboron polymers. Polyaddition between dienes and thexylborane produced polymers consisting of carbon-boron bonds in the main chains. The resulting organoboron polymers can be regarded as polymer analogs of trialkylboranes and can be viewed as novel types of reactive polymers. On the other hand, hydroboration polymerization of dicyano compounds with monoalkylboranes or dialkylboranes produced air-stable poly(cyclodiborazane)s having B - N four-membered rings via dimerization of iminoborane species. Allylboration polymerization of dicyano compounds with triallylborane also gave the corresponding poly-(cyclodiborazane)s. In haloboration polymerization between diynes and boron tribromide, poly(organoboron halide)s were obtained and showed characteristic properties as a polymeric Lewis acid.

    Organoboron compounds are known as useful reagents or reaction intermediates for the preparation of a wide variety of functional compounds such as alcohols, amines, carboxylic acids, ketones, aldehydes, olefins, and halides (1-3). For several decades, Brown and his coworkers have studied this chemistry.

    0097-6156/94/0572-0398S08.00/0 1994 American Chemical Society

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    In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

  • 30. CHUJO Organoboron Polymers by Hydroboration Polymerization 399

    It is expected to provide a novel type of reactive polymers by the introduction of organoboron moieties in polymeric chains. In other words, organoboron polymers can be used as precursor polymers for the preparation of various functional polymers.

    Hydroboration (addition of B - H to C=C) is a well-known reaction which takes place under mild conditions almost quantitatively, for the preparation of various alkylborane compounds in organic synthesis. However, the direct use of this reaction in polymer synthesis has been very limited.

    Polymers having olefin groups such as polydienes can be subjected to the hydroboration reaction. The functionalization of polydienes by means of this hydroboration is known (4-7), especially from the viewpoint of industrial use. Recently, Chung and his coworkers have examined this method to prepare polymers having organoboron moieties in the branches (8-13). The advantages of this chemistry are (i) the stability of the borane moiety to the transition metal catalyst usually used in Ziegler-Natta or ring-opening metathesis polymerization, (ii) the solubility of borane compounds in hydrocarbon solvents such as hexane or toluene, and (iii) the versatility of borane groups, which can be transformed to a remarkable variety of functionalities.

    Previously, Urry et al. reported that an organoboron polymer was formed by pyrolysis of 2,5-dimethyl-2,5-diborahexane (14). Mikhailov et al. also reported the formation of an organoboron polymer by thermal isomerization of triallylborane and triiso-butylborane (15).

    Among numerous studies reported by Brown and his group, the formation of polymeric materials has been described from the reaction of thexylborane and 1,3-butadiene (16) and from monochloroborane and 1,7-octadiene (17). However, no details were reported about the yield, molecular weights, and structure of the resulting polymers. These polymers were formed as intermediates in the formation of boron heterocycles by so-called thermal depolymerization.

    Recently, we explored a novel polyaddition between dienes and monoalkylboranes and termed this hydroboration polymerization (18). Diynes and dicyano compounds can also be used in this hydroboration polymerization. The organoboron polymers obtained are effectively converted to polyalcohols or poly-ketones. In other words, polymers having organoboron units in the main chains can be expected to be reactive polymers. Here, the scope of these hydroboration polymerizations is viewed as methodologies for the preparation of organoboron polymers.

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    In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

  • 400 INORGANIC AND ORGANOMETALLIC POLYMERS II

    Hydroboration Polymerization between Dienes and Thexylborane

    As a monoalkylborane (bifunctional hydroborane) component in hydroboration polymerization, thexylborane was employed because of its stability after distillation. As a diene monomer, relatively longer chain dienes were used to avoid the competitive cyclization reaction. The general scheme of this polymerization is shown in Scheme 1, which proceeds in THF at 0C under nitrogen without catalyst (18). For example, the resulting organoboron polymer obtained from the reaction between thexylborane and 1,7-octadiene was isolated as a colorless gum after coagulation and was soluble in common organic solvents such as chloroform, THF and benzene. The structure of the polymer was supported by its IR and *-, 1 3 C - , and n B - N M R spectra. The molecular weight of the polymer increased as the feed ratio approached unity. This effect is taken as a normal behavior of a polyaddition reaction. As summarized in Table I, this hydroboration polymerization can be applied to various combinations of dienes with thexylborane that produce the corresponding organoboron polymers.

    These organoboron polymers were stable in protic solvents such as water and alcohol under nitrogen. However, under air, these polymers decomposed as is typical for organoboron compounds. This decomposition was monitored by GPC after bubbling air through a THF solution of the polymer at room temperature. After 3 min of air-bubbling, the peak became broader and moved to the low molecular weight region. At 30 min, the polymer was found to decompose. However, it should be noted that the organoboron polymers were more stable toward air than conventional trialkylboranes.

    Organoboron copolymers were prepared by polyaddition of a mixture of dienes with thexylborane (19). When a mixture of dienes such as 1,2-diallyloxyethane and p-diallylbenzene was polymerized with thexylborane, the peaks in the GPC using both U V and RI detectors were shifted to higher molecular weight regions with increase of the amount of thexylborane. The molecular weight distribution of the copolymer obtained by this method was clearly differed from that of a mixture of two homopolymers. This result supports the formation of copolymers. In a similar manner, other copolymers were prepared using various combinations of 1,7-octadiene, 1,2-diallyloxyethane, -divinylbenzene, and -diallylbenzene.

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    In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

  • CHUJO Organoboron Polymers by Hydroboration Polymerization

    Scheme 1 r

    Table I Hydroboration Polymerization between Thexylborane and various Dienesa)

    Run Diene M n b )

    1 19,000 27,700

    2 18,400 27,400

    3 19,000 29,200

    4 f^^> 9,400 16,900

    5 1,200 2,600

    6 1,900 4,500

    7 ^^0 0 1,900 3,200

    8 ^ o - ^ - o ^ 5,100 11,200

    9 7,600 15,400

    a) Polymerizations were carried out by adding small excess of thexylborane to the 1M THF solution of diene at 0C.

    b) GPC (dry THF), polystyrene standard.

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    In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

  • 402 INORGANIC AND ORGANOMETALLIC POLYMERS II

    Hydroboration Polymerization of Diynes with Thexylborane

    Hydroboration polymerization with monoalkylborane can also be applied to diynes. Generally, the reactivity of a terminal acetylene group toward hydroboration is quite different from that of an internal acetylene group. That is, the terminal acetylenes preferentially give dihydroboration products via a further hydroboration of the init ial ly formed vinylborane species. On the other h

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