[acs symposium series] inorganic and organometallic polymers ii volume 572 (advanced materials and...

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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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 0°C 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 *Η-, 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, ρ-divinylbenzene, and ρ-diallylbenzene.

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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 ο ο 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 0°C.

b) GPC (dry THF), polystyrene standard.

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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 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.

As a typical example for hydroboration polymerization of internal diyne, the reaction of 3,9-dodecadiyne and thexylborane was carried out in THF at 0°C 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.

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).

The organoboron polymers prepared from diynes, especially from internal diynes, consisted mainly of divinylborane units. Thus, different reactivity and stability originating from this

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structure can be expected in comparison with the organoboron polymers prepared from dienes.

Reactions of Organoboron Polymers

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.

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 120°C 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.

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.

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 0°C in THF followed by treatment with lithium triethylmethoxide. After oxidative treatment with NaOH and H2O2, the corresponding poly(alcohol)s were obtained. The

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Scheme 2

Scheme 3

NaOH H 2 0 2

NaOH Η2θ 2

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Tab

le I

I Sy

nthe

sis

of P

oly(

alco

hol)

s by

the

R

eact

ion

of

Org

anob

oron

Po

lym

ers

with

C

arbo

n a)

M

onox

ide.

B

-Pol

ymer

s P

oly(

alco

hol)s

Run

R

Mn

c)

Mw

c)

Yie

ldd

)

(%)

1 19

,500

4,

080

13,5

00

82

2 20

,500

4,

200

9,60

0 60

3

14,6

00

2,99

0 7,

650

74

4 —

CH

2-©

-CH

2—

9,

300

5,40

0 12

,300

77

5

—C

H2O

CCH

2)2O

CH

2-1,

520

1,28

0 2,

200

24

6 —

CH

aO

(CH

2) 4

OC

H2-

2,28

0 1,

620

2,80

0 59

7

3,20

0 2,

200

3,72

0 47

8

—C

H20

-@-O

CH

2—

6,

800

1,94

0 3,

700

39

9 —

CH

20

-@-|

-^-O

CH

2—

11

,700

2,

320

4,40

0 89

a)

Rea

ctio

ns w

ere

carr

ied

out

in t

he p

rese

nce

of 0

.1m

l et

hyle

ne g

lyco

l un

der

carb

on m

onox

ide

(20-

25 k

g/cm

) i

n an

aut

ocla

ve,

b) G

PC

(dry

TH

F),

PS s

tand

ard.

c)

GP

C C

THF)

, PS

stand

ard,

d)

Isol

ated

yi

eld

afte

r re

prec

ipita

tion

into

m

etha

nol/w

ater

(v

/v=

2/l)

.

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structures of the resulting poly(alcohol)s were the same as those prepared by the reaction with carbon monoxide described above.

We also examined the reactions of organoboron polymers via ring-opening of pyridine or furan moieties. The organoboron polymer prepared by hydroboration polymerization between 1,7-oetadiene and thexylborane was reacted with 6-bromo-2-lithiopyridine followed by treatment with NaOH to produce a cyano group-branching polymer (24). The reaction of organoboron polymer with α-furyllithium was followed by treatment with acetic acid and then with NaOH and H2O2 to form the polymer having primary and tertiary alcohol groups (25). These conversions involve the ring-opening reactions of pyridine and furan, respectively.

A l l of the polymers described in this section are difficult to prepare by conventional synthetic methods. These conversions demonstrate the utility and versatility of organoboron polymers as polymeric precursors to functional polymers.

Poly(cyclodiborazane)s by Hydroboration Polymerization

It is known that hydroboration reactions of cyano groups give iminoborane species which dimerize to form boron-nitrogen four-membered rings (cyclodiborazane) (2 6). When this reaction is used with bifunctional monomers, formation of polymeric materials consisting of cyclodiborazane units can be expected.

As a typical example, isophthalonitrile was reacted with reri-butylborane-trimethylamine in diglyme at 100°C under nitrogen to produce a boron-containing polymer as shown in Scheme 4 (27). The white solid polymer was soluble in various organic solvents such as THF, chloroform and benzene. This polymerization involves the dimerization of iminoborane to form the four-membered ring as a key essential step. In this hydroboration polymerization, terephthalonitrile and l,5-di(4,4'-cyanophenoxy)pentane also gave the corresponding polymers. Aliphatic dicyano compounds, however, resulted in the formation of oligomers with very low molecular weights. The structures of these boron-containing polymers were supported by spectroscopic data in comparison with the model compound obtained from reaction of benzonitrile and ieri-butylborane. The boron-containing polymers were sufficiently stable to handle in air. Thermogravimetric analysis (TGA) of this boron-containing polymer shows that the weight loss started at 140°C and was complete at 700°C at which point a black solid remained. This

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finding suggests that the resulting poly(cyclodiborazane) prepared by hydroboration polymerization of dicyano compound can be used as a polymeric precursor for the preparation of boron-containing inorganic materials.

Thexylborane also produced poly(cyclodiborazane)s by the reaction with aliphatic dicyano compounds, although the molecular weights of the polymers obtained were relatively low (25). On the other hand, when polymerization was examined with less hindered monoalkylboranes (i.e., w-BuBH2-NMe3 or i -PrBH2-NMe3) , crosslinking reactions took place due to the further hydroboration of iminoboranes (29).

Dialkylboranes can also be used as borane monomers in hydroboration polymerization with dicyano compounds. In this case, the substituents at the boron atoms in poly(cyclodi-borazane)s should be two alkyl groups. When polymerization between dibutylborane and adiponitrile was carried out in bulk under nitrogen, the reaction was complete within several hours, and the corresponding poly(cyclodiborazane) was obtained in high yield. Other dicyano monomers having different methylene chains also produced the corresponding poly(cyclodiborazane)s. Table III summarizes the results of hydroboration polymerization between dibutylborane and various dicyano compounds. In comparison with monoalkylboranes, the hydroboration polymerization of dialkylboranes produced higher molecular weights polymers even from aliphatic dicyano compounds.

Allylboration Polymerization

The boron-containing polymer with the cyclodiborazane ring was also prepared by so-called allylboration polymerization between triallylborane and dicyano compounds (Scheme 5) (3 0). Polymerizations were carried out by adding triallylborane to dicyano monomer in bulk at 0°C under nitrogen. When isophthalonitrile was used as a monomer, the polymerization system became homogeneous within a few seconds and became very viscous after a few hours. The glassy polymer obtained after the reaction for 1 day was dissolved in THF and purified by reprecipitation. Both of aromatic and aliphatic dicyano compounds successfully gave polymers in this polymerization with triallylborane as shown in Table IV. Trimetriallylborane, allyldialkylboranes and methallyldialkylboranes were also used in this allylboration polymerization with dicyano compounds to produce the corresponding poly(cyclodiborazane)s.

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Scheme 4

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Table III Hydroboration Polymerization of Various Dicyano Compounds with Dibutylborane.a)

Run Dicyano Compounds

Yield b )

(%) M n

c ) M w

c )

l d ) NC(CH2)2CN 88 16,800 49,900 2 NC(CH2)3CN 90 10,100 23,500 3 NC(CH2)4CN 78 17,100 51,700 4 NC(CH2)sCN 77 25,000 69,900 5 NC(CH2)6CN 76 3,080 5,830 6 NC(CH2)8CN 88 5,930 9,070 7 m-NC-C6H4-CN 25 3,540 14,700

a) Polymerizations were carried out in bulk at r.t. b) Isolated yields after precipitation into EtOH/H20 (1/1). c) GPC (THF), polystyrene standard. d) Polymerization was carried out at 70°C for 30min., then at r.t. for 2h.

Table IV Allylboration Polymerization of Various Dicyano Compounds with Triallylboranea)

Run Dicyano Compounds

Yield b )

(%) M n

c ) M w

c ) M w / M ,

1 m-NC-C6H4-CN 78 14,000 25,700 1.84 2 p-NC-C6H4-CN 89 5,900 11,000 1.86 3 NCiCH^CN 79 4,500 9,200 2.04 4 NC(CH2)3CN 78 5,500 12,000 2.18 5 NC(CH2)4CN 81 6,300 13,600 2.16 6 NC(CH2)5CN 93 8,900 18,000 2.02 7 NC(CH2)6CN 85 9,000 20,400 2.27 8 NC(CH2)8CN 89 11,900 24,600 2.07 a) Polymerizations were carried out in bulk at 0°C. b) Isolated yields after precipitation into EtOH/H20 (1/1). c) GPC (THF), polystyrene standard.

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For the preparation of poly(cyclodiborazane)s, a condensation method can also be used. For example, N,W-bis( t r imethyl-silyl)terephthalaldehyde diimine was reacted with monochloro-dihexylborane to produce a boron-containing polymer with boron-nitrogen four-membered rings (31). This polymerization can be applied to various bis(silylimine)s such as Ν,Ν'-bis(trimethylsilyl)isophthalaldehyde diimine. However, Ν,Ν'-bis(trimethylsilyl)anthraquinone diimine gave only the mono-meric iminoborane species and no polymer.

Haloboration Polymerization

Substituted boron halides are known to be useful reagents for ether cleavage (32) and selective haloboration reactions (33-34) under mild conditions. Polymeric homologs of these materials, therefore, may have a potential to show unique properties as reactive polymers. Recently, we reported a polyaddition between boron tribromide and terminal diynes as a novel methodology for the preparation of poly(organoboron halide)s (35). We termed this haloboration polymerization.

As shown in Scheme 6, the polyaddition between 1,7-octadiyne and boron tribromide produced the corresponding poly(organoboron halide) as a brown solid without gelation. This polymer was soluble in common organic solvents such as chloroform. The structure of the polymer was supported by its *H- , H B - N M R , IR and U V spectra in comparison with those for the model compound prepared from boron tribromide and 1-hexyne. Table V summarizes the results of haloboration polymerization using various terminal diynes.

The Β-Br bonds in poly (organoboron halide)s were found to be replaced by B-OEt bonds during the precipitation of the polymer into ethanol. The reaction of the polymer with diol or with water caused gelation. The Β-Br bond in the polymer was also subjected to further haloboration reaction with phenyl-acetylene. The characteristic property of the polymer as a poly(Lewis acid) was also demonstrated by the reaction with THF to produce 4-bromo-l-butanol after hydrolysis. These reactions are summarized in Scheme 7. Poly(organoboron halide)s obtained by haloboration polymerization are potentially useful and relatively rare polymeric Lewis acids.

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

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Table V Haloboration Polymerization between BBr3 and various Diynes

Run Diyne [diyne] [BBr3]

Yield a )

<*) M n

b ) M w

b )

1 HC=C(CHj)sC=CH 1.01 86 6,740 32,400

2 HCEC(CH2)6CSCH 1.01 91 7,230 26,600

3 H C 5 C - ^ ^ - C E C H 1.00 92 990 1,660

4 IIC=C-/^\-C=CH 1.00 69 700 1,370

a) Isolated yields after reprecipitation into EtOH. b) GPC (CHC1 3), polystyrene standard.

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Scheme 6

H-CsC-R-CsC-H + BBr3 »~ > C = C /—v- v \ ^ Br Br

I Br

0(CH2)4Br

Scheme 8

HO-R-OH + |-[-BH 2 ^ - ( -0 -R -o3^r-

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Boronate Oligomers by Dehydrogenation

Soluble boronate oligomers were prepared by dehydrogenation reactions of diols with thexylborane as shown in Scheme 8 (37). Bifunctional thexylborane was employed as a monomer, and dehydrogenation between diols and thexylborane was used to produce a poly(boronic ester).

As a typical example, polymerization of thexylborane and 1,6-hexanediol was carried out at room temperature under nitrogen. Various diols were subjected to this dehydrogenation polymerization with an equimolar amount of thexylborane. The examples of diols were 1,8-octanediol, 1,12-dodecanediol, hydroquinone and bisphenol-A. The boronate oligomers decomposed readily by water, as is usual for boronic esters.

Conclusions

In this article, synthetic methodologies for the preparation of organoboron polymers are described and the reactions of these polymers are discussed. The organoboron polymers themselves are new polymeric materials with interesting properties. In addition, these organoboron polymers can be used as polymeric precursors of functional polymers (38). A wide variety of functional groups can be introduced at the main chains, side chains, terminal ends, or even at the center of polymeric chains (39) starting from these organoboron polymers.

While the studies on organoboron polymers have just started very recently, there is a rich chemistry for these reactive polymers. The author believes that the study of organoboron polymers w i l l continue to expand rapidly, and that many applications of this chemistry wi l l be developed in the near future.

Literature Cited

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[acs symposium series] inorganic and organometallic polymers ii volume 572 (advanced materials and...

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