[ACS Symposium Series] Inorganic and Organometallic Polymers II Volume 572 (Advanced Materials and Intermediates) || Synthesis of Organoboron Polymers by Hydroboration Polymerization

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<ul><li><p>Chapter 30 </p><p>Synthesis of Organoboron Polymers by Hydroboration Polymerization </p><p>Yoshiki Chujo </p><p>Division of Polymer Chemistry, Graduate School of Engineering, Kyoto Univeristy, Yoshida, Sakyo-ku, Kyoto 606-01 Japan </p><p>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. </p><p>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. </p><p>0097-6156/94/0572-0398S08.00/0 1994 American Chemical Society </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h030</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>30. CHUJO Organoboron Polymers by Hydroboration Polymerization 399 </p><p>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. </p><p>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. </p><p>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. </p><p>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). </p><p>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. </p><p>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. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h030</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>400 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>Hydroboration Polymerization between Dienes and Thexylborane </p><p>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. </p><p>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. </p><p>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. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h030</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>CHUJO Organoboron Polymers by Hydroboration Polymerization </p><p>Scheme 1 r </p><p>Table I Hydroboration Polymerization between Thexylborane and various Dienesa) </p><p>Run Diene M n b ) </p><p>1 19,000 27,700 </p><p>2 18,400 27,400 </p><p>3 19,000 29,200 </p><p>4 f^^&gt; 9,400 16,900 </p><p>5 1,200 2,600 </p><p>6 1,900 4,500 </p><p>7 ^^0 0 1,900 3,200 </p><p>8 ^ o - ^ - o ^ 5,100 11,200 </p><p>9 7,600 15,400 </p><p>a) Polymerizations were carried out by adding small excess of thexylborane to the 1M THF solution of diene at 0C. </p><p>b) GPC (dry THF), polystyrene standard. </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h030</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>402 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>Hydroboration Polymerization of Diynes with Thexylborane </p><p>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 hand, the internal acetylenes give monohydroboration products regardless of the bulkiness of borane. Thus, terminal diyne and internal diyne are expected to show the different polymerization behavior due to the steric effect on the second (further) hydroboration reaction. </p><p>As a typical example for hydroboration polymerization of internal diyne, the reaction of 3,9-dodecadiyne and thexylborane was carried out in THF at 0C under nitrogen (2 0). The polymerization behavior was quite similar to hydroboration polymerization of dienes. The organoboron polymer obtained was a colorless wax and soluble in common organic solvents such as benzene, THF or chloroform. An IR spectrum of the polymer showed the absorption band assignable to C=C bond, which was also supported by 1 H - and n B - N M R spectra. 3,8-Undecadiyne and 3,10-tridecadiyne were also used in hydroboration polymerization with thexylborane to produce the corresponding organoboron polymers. </p><p>On the other hand, in the case of terminal diyne such as 1,7-octadiyne, the further hydroboration reaction of olefin of the resulting polymer with thexylborane occurred (20). In fact, when the ratio of thexylborane to acetylene unit was higher than 1.7, gelation was observed. Thus, hydroboration of terminal diynes gives organoboron polymers having a branched structure due to the further hydroboration reaction toward the initially formed vinylborane structures. That is, terminal diynes are taken to have potential as a multifunctional monomer. When the polymerization between 1,7-octadiene and thexylborane was carried out in the presence of a small amount of 1,7-octadiyne, the molecular weights of organoboron polymers were found to increase when the ratio of diyne/diene was increased (19). </p><p>The organoboron polymers prepared from diynes, especially from internal diynes, consisted mainly of divinylborane units. Thus, different reactivity and stability originating from this </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h030</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>30. CHUJO Organoboron Polymers by Hydroboration Polymerization 403 </p><p>structure can be expected in comparison with the organoboron polymers prepared from dienes. </p><p>Reactions of Organoboron Polymers </p><p>The organoboron polymers prepared by hydroboration polymerization have a new structure consisting of C-B bonds in the main chain. Generally, these organoboron polymers can be regarded as a polymer homolog of trialkylborane, which is known to be a very versatile compound in organic synthesis. In other words, the polymers obtained by hydroboration polymerization can be expected to be reactive polymers. </p><p>The versatile reactions of organoboron polymers prepared by hydroboration polymerization are summarized in Scheme 2. For example, the polymer prepared from thexylborane and 1,7-octadiene was reacted with carbon monoxide at 120C followed by treatment with NaOH and H2O2 to produce a poly alcohol (21). This conversion includes the migrations of polymer chain and thexyl group from a boron atom to carbon as shown in Scheme 3. Table II summarizes the results of preparation of polyalcohols from various organoboron polymers. This migration reaction is known to proceed intramolecularly. Thus, it can be mentioned that the molecular weight of the starting organoboron polymers should be higher than (or the same as) that of polyalcohols. When this reaction was carried out under milder condition, polyketone segments were included in the polymer structure due to the incomplete migration of thexyl group. These conversions may offer a new synthetic method for the preparation of polyalcohols from the corresponding dienes. </p><p>For the selective preparation of polyketones, the organoboron polymer prepared by hydroboration polymerization between thexylborane and 1,7-octadiene was subjected to reaction with K C N (22). After oxidation of the reaction mixture, the desired polyketone was obtained. The polymer formed was a white solid and stable in air. This reaction also includes the migration of main chains of organoboron polymer from boron to carbon. </p><p>As an alternative method for the preparation of polyalcohols from organoboron polymers, dichloromethyl methyl ether (DCME) can be used (23). Various organoboron polymers prepared by hydroboration polymerization of dienes and thexylborane were reacted with D C M E at 0C in THF followed by treatment with lithium triethylmethoxide. After oxidative treatment with NaOH and H2O2, the corresponding poly(alcohol)s were obtained. The </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h030</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>404 INORGANIC AND ORGANOMETALLIC POLYMERS II </p><p>Scheme 2 </p><p>Scheme 3 </p><p>NaOH H 2 0 2 </p><p>NaOH 2 2 </p><p>Dow</p><p>nloa</p><p>ded </p><p>by N</p><p>ORT</p><p>H C</p><p>ARO</p><p>LIN</p><p>A S</p><p>TATE</p><p> UN</p><p>IV o</p><p>n O</p><p>ctob</p><p>er 2</p><p>9, 2</p><p>012 </p><p>| http:</p><p>//pubs</p><p>.acs.o</p><p>rg Pu</p><p>blic</p><p>atio</p><p>n D</p><p>ate:</p><p> Nov</p><p>embe</p><p>r 18,</p><p> 199</p><p>4 | do</p><p>i: 10.1</p><p>021/bk</p><p>-1994-</p><p>0572.c</p><p>h030</p><p>In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994. </p></li><li><p>Tab</p><p>le I</p><p>I Sy</p><p>nthe</p><p>sis </p><p>of P</p><p>oly(</p><p>alco</p><p>hol)</p><p>s by</p><p> the</p><p> R</p><p>eact</p><p>ion </p><p>of </p><p>Org</p><p>anob</p><p>oron</p><p> Po</p><p>lym</p><p>ers </p><p>with</p><p> C</p><p>arbo</p><p>n a)</p><p> M</p><p>onox</p><p>ide.</p><p> B</p><p>-Pol</p><p>ymer</p><p>s P</p><p>oly(</p><p>alco</p><p>hol)s</p><p>Run</p><p> R </p><p>Mn</p><p>c) </p><p>Mw</p><p>c) </p><p>Yie</p><p>ldd</p><p>) </p><p>(%) </p><p>1 19</p><p>,500</p><p> 4,</p><p>080 </p><p>13,5</p><p>00 </p><p>82 </p><p>2 20</p><p>,500</p><p> 4,</p><p>200 </p><p>9,60</p><p>0 60</p><p> 3 </p><p>14,6</p><p>00 </p><p>2,99</p><p>0 7,</p><p>650 </p><p>74 </p><p>4 </p><p>CH</p><p>2-</p><p>-CH</p><p>2</p><p> 9,</p><p>300 </p><p>5,40</p><p>0 12</p><p>,300</p><p> 77</p><p> 5 </p><p>C</p><p>H2O</p><p>CCH</p><p>2)2O</p><p>CH</p><p>2-1,</p><p>520 </p><p>1,28</p><p>0 2,</p><p>200 </p><p>24 </p><p>6 </p><p>CH</p><p>aO</p><p>(CH</p><p>2) 4</p><p>OC</p><p>H2-</p><p>2,28</p><p>0 1,</p><p>620 </p><p>2,80</p><p>0 59</p><p> 7 </p><p>3,20</p><p>0 2,</p><p>200 </p><p>3,72</p><p>0 47</p><p> 8 </p><p>C</p><p>H20</p><p>-@-O</p><p>CH</p><p>2</p><p> 6,</p><p>800 </p><p>1,94</p><p>0 3,</p><p>700 </p><p>39 </p><p>9 </p><p>CH</p><p>20</p><p>-@-|</p><p>-^-O</p><p>CH</p><p>2</p><p> 11</p><p>,700</p><p> 2,</p><p>320 </p><p>4,40</p><p>0 89</p><p>a) </p><p>Rea</p><p>ctio</p><p>ns w</p><p>ere </p><p>carr</p><p>ied </p><p>out </p><p>in t</p><p>he p</p><p>rese</p><p>n...</p></li></ul>