Isobutene polymerization using chelating diboranes: Polymerization in aqueous suspension and hydrocarbon solution

Article abstract of DOI:10.1021/om8007629

The use of the chelating diboranes o-C₆F₄[B(C ₆F₅)₂]₂ (1) and o-C ₆F₄(9-BC₁₂F₈)₂ (2: 9-BC₁₂F₈ = 1,2,3,4,5,6,7,8-octafluoro-9-borafluorene) for the polymerization of isobutene (IB) in aqueous suspension or in hydrocarbon solution was studied. Polymerizations in aqueous suspension provided polymer of moderate MW and at variable conversion and were dependent on temperature, mode of diborane addition, the presence of surfactant, and the acidity of and nature of the anion present in the aqueous phase. The T dependence of MW over the T range -80 to -20 °C was studied in aqueous suspension, and higher MW polymer was formed at lower T. The hydrolysis and methanolysis of diboranes 1 and 2 was studied by NMR spectroscopy. Reactions of diborane 1 with excess MeOH or water afford solutions containing oxonium acids [o-C₆F₄{B(C ₆F₅)₂}₂(μ-OR)][(ROH) _nH] (7: R = H, n > 2; 3: R = Me, n = 3). When diborane 1 is present in excess over water or MeOH, degradation of the diborane is observed. In this case the products are o-C₆F₄{B(C₆F ₅)₂}H (5) and (C₆F₅)₂BOH 7 or (C₆F₅)₂BOMe 4, respectively. In the case of diborole 2, o-C₆F₄(9-BC₁₂F ₈)B(2-C₁₂F₈-2"-H)(μ-OH) · 7H₂O (17) and o-C₆F₄(9-BC₁₂F ₈)B(2-C₁₂F₈-2"-H)(μ-OMe) (11) were isolated from reactions of 2 with water and MeOH, respectively, and were characterized by X-ray crystallography. None of these degradation products effect IB polymerization in aqueous suspension. As a model for initiation of polymerization, the reaction of diborole 2 with 1,1-diphenylethylene (DPE) was studied. Addition of MeOH at low T results in efficient formation of the ion-pair [Ph₂CMe][o-C₆F₄(9-BC ₁₂F₈)₂(μ-OMe)] via protonation of DPE. Polymerizations in hydrocarbon media were exothermic and rapid and gave quantitative yields of polymer even at very low concentrations of diborane 1. The T dependence of MW was studied in hydrocarbon solution and showed non-Arrhenius behavior. This was explained by competitive chain transfer to monomer at elevated T and chain transfer to molecular water at lower T.

Full text of DOI:10.1021/om8007629

Organometallics 2009, 28, 249–263
249
Isobutene Polymerization Using Chelating Diboranes:
Polymerization in Aqueous Suspension and Hydrocarbon Solution
Stewart P. Lewis, Jianfang Chai, and Scott Collins*
Department of Polymer Science, The UniVersity of Akron, Akron, Ohio 44325-3909
Timo J. J. Sciarone, Lee D. Henderson, Cheng Fan, Masood Parvez, and Warren E. Piers*
Department of Chemistry, The UniVersity of Calgary, Calgary, Alberta, Canada
ReceiVed August 7, 2008
The use of the chelating diboranes o-C6F4[B(C6F5)2]2 (1) and o-C6F4(9-BC12F8)2 (2: 9-BC12F8 )
1,2,3,4,5,6,7,8-octafluoro-9-borafluorene) for the polymerization of isobutene (IB) in aqueous suspension
or in hydrocarbon solution was studied. Polymerizations in aqueous suspension provided polymer of
moderate MW and at variable conversion and were dependent on temperature, mode of diborane addition,
the presence of surfactant, and the acidity of and nature of the anion present in the aqueous phase. The
T dependence of MW over the T range -80 to -20 °C was studied in aqueous suspension, and higher
MW polymer was formed at lower T. The hydrolysis and methanolysis of diboranes 1 and 2 was studied
by NMR spectroscopy. Reactions of diborane 1 with excess MeOH or water afford solutions containing
oxonium acids [o-C6F4{B(C6F5)2}2(µ-OR)][(ROH)nH] (7: R ) H, n > 2; 3: R ) Me, n ) 3). When
diborane 1 is present in excess over water or MeOH, degradation of the diborane is observed. In this
case the products are o-C6F4{B(C6F5)2}H (5) and (C6F5)2BOH 7 or (C6F5)2BOMe 4, respectively. In the
case of diborole 2, o-C6F4(9-BC12F8)B(2-C12F8-2′′-H)(µ-OH) · 7H2O (17) and o-C6F4(9-BC12F8)B(2-C12F8-
2′′-H)(µ-OMe) (11) were isolated from reactions of 2 with water and MeOH, respectively, and were
characterized by X-ray crystallography. None of these degradation products effect IB polymerization in
aqueous suspension. As a model for initiation of polymerization, the reaction of diborole 2 with 1,1-
diphenylethylene (DPE) was studied. Addition of MeOH at low T results in efficient formation of the
ion-pair [Ph2CMe][o-C6F4(9-BC12F8)2(µ-OMe)] via protonation of DPE. Polymerizations in hydrocarbon
media were exothermic and rapid and gave quantitative yields of polymer even at very low concentrations
of diborane 1. The T dependence of MW was studied in hydrocarbon solution and showed non-Arrhenius
behavior. This was explained by competitive chain transfer to monomer at elevated T and chain transfer
to molecular water at lower T.
comparable to those that can be achieved using γ-ray initiation
Introduction
involving “free” carbocations.5
Protic or electrophilic initiators that give rise to weakly
coordinating anions (WCA),1 partnered with propagating car-
bocations in isobutene polymerization,2 are a topic of significant
interest in the context of butyl rubber manufacturing at elevated
temperature.3 A variety of initiator systems are effective in neat
monomer, hydrocarbon, or more polar media,4 and generally a
rather weak temperature dependence is observed for the MW
of poly(isobutene) or butyl rubber formed. These polymeriza-
tions are uncontrolled and with MW values in several cases
Several years ago we communicated that chelating diborane
16 (Chart 1), in combination with cumyl chloride (CumCl), was
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* Corresponding author. E-mail: collins@uakron.edu.
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10.1021/om8007629 CCC: $40.75
2009 American Chemical Society
Publication on Web 11/20/2008

250 Organometallics, Vol. 28, No. 1, 2009
Lewis et al.
had been earlier polymerized in aqueous suspension13 and most
Chart 1
14
recently in a controlled fashion using either BF3 · OEt2 or
B(C6F5)3,15 this was the first report of significant polymer
formation from isobutene in the presence of a large excess of
water.
At the time, our understanding of this novel process was
largely incomplete: the nature of the initiator was not known
nor was it obvious that diborane 1 versus a degradation product
was involved in initiation. The only mechanistic information
we had was that the novel, stable oxonium acid 3 formed from
diborane 1 and excess MeOH (Chart 1)12 was ineffective as an
initiator, as were compounds of comparable Brønsted acidity
to 3 (e.g., [(Et2O)2H][B(C6F5)4]16) or Lewis acidity to diborane
1 [B(C6F5)3 or 9,10-bis(perfluorophenyl)-9,10-dibora-perfluo-
roanthracene17].
In this paper we report more detailed polymerization studies
in both aqueous suspension and hydrocarbon solution using both
diborane 118 and diborole 2 that shed considerable insight into
the nature of the processes limiting both MW and conversion
under these conditions. We also studied and report in detail the
reactions of these two compounds with water and MeOH,
including the characterization of the degradation products
formed and their efficacy as initiators of polymerization. We
have also studied the reaction of diborole 2 with 1,1-diphenyl-
ethylene (DPE) in the presence of MeOH as a model for protic
initiation involving these compounds.
effective for isobutene polymerization in the presence of the
sterically hindered pyridine 2,6-di-tert-butyl-4-methylpyridine
(DtBMP).7 More recently, we have studied the polymerization
and allied chemistry of diborane 1 and diborole 28 in combina-
tion with cumyl ether and cumyl azide initiators,9 as well as
the roles of DtBMP in polymerizations initiated by diborane 1
and CumCl.10
A picture that emerges from these studies is that although
these diboranes are effective for ionization of cumyl halide and
related initiators, giving rise to weakly coordinating, chelated
anions partnered with the cumyl cation,11 isobutene polymeriza-
tions in hydrocarbon media using these initiators and the
diboranes are complicated by facile chain transfer. Also, DtBMP
is not an entirely innocent additive in polymerizations featuring
weakly coordinating anions. The highly Brønsted acidic chain
ends are susceptible to termination in the presence of sufficiently
high concentrations of this hindered pyridine. (The study of
chain transfer processes under these conditions is complicated
by the presence of excess DtBMP. We have recently discovered
a Lewis acidic additive that is an effective scavenger of water
in hydrocarbon media, compared to diborane 1. Future work
will focus on the study of chain transfer featuring controlled
initiation under these conditions.)
Another picture that emerges from these studies is that both
of these diboranes are rather resistant to hydrolysis and the
hydrolysis products are not strong Brønsted acids (Vide infra).
Thus, unlike conventional Lewis acids, these polymerizations
readily proceed in the presence of variable quantities of
dissolved water (Vide infra), and an excess of DtBMP over
Lewis acid (and water) is required to efficiently sequester the
Brønsted acid formed from these diboranes and water.
In 2005 we reported that the use of diborane 1 in aqueous
isobutene suspension afforded moderate conversions to polymer
of moderate MW.12 Although styrene and related monomers
Results and Discussion
Polymerization of Isobutene in Aqueous Suspension.
Polymerization of isobutene in aqueous suspension was studied
in three aqueous media featuring strong electrolytes, aqueous
LiCl (mp -65 °C),19 38 wt % sulfuric acid, and 48 wt %
fluoroboric acid. The latter two media were initially selected
on the basis of low freezing point (ca. -70 and -90 °C,
respectively) and commercial availability. In all cases, there was
no noticeable reaction between isobutene and any of these
media, prior to the introduction of diborane or diborole.
Polymerizations were quenched by the addition of an excess of
2-propanol at low T; the use of a hydrocarbon-soluble base such
as NEt3 in lieu of 2-propanol did not affect the outcome of these
experiments.
It should be noted that these suspensions are unstabilized,
and so rapid, mechanical stirring (>500 rpm) was employed to
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Isobutene Polymerization Using Chelating Diboranes
Organometallics, Vol. 28, No. 1, 2009 251
Table 1. Isobutene Polymerization in Aqueous LiCl Suspensiona
Table 3. Isobutene Polymerization in Aqueous Fluoroboric Acid
Suspensiona
entry initiator (mM) surfactantb T (°C) Mw (kg mol-1) PDI yield (%)
entry initiator (mM)b surfactantc Mw (kg mol-1
)
PDI yield (%)
1
2
3
4
5
6
7
8
9
1 (0.322)
1 (0.322)
1 (0.322)
1 (0.322)
1 (0.322)
1 (0.322)
2 (0.627)
2 (0.627)
2 (0.627)
22.8
29.4
61.0
47.4
31.8
34.9
91.9
39.5
75.4
2.30
2.38
2.74
2.68
2.37
2.47
2.27
1.74
2.47
44
39
5
1
2
3
4
5
6
7
8
1 (0.322)
1 (0.322)
2 (0.625)
2 (0.625)
1 (0.625)c
2 (0.625)c
1 (0.322)
1 (0.322)
1 (0.322)
1 (0.322)
1 (0.625)d
1 (0.625)d
1 (0.625)d
1 (0.625)d
1 (0.625)d
1 (0.625)d
1 (0.625)d
1 (0.625)d
1 (0.625)d
-80
-80
-80
-80
-80
-80
-80
-80
41.4
37.2
49.4
42.4
29.3
39.5
27.2
36.2
33.4
31.8
41.0
41.3
31.7
32.1
31.0
38.1
13.0
2.37
2.43
2.15
2.22
2.05
2.04
2.08
2.34
2.22
2.13
1.73
1.82
2.06
2.10
2.28
2.37
2.36
2.28
2.17
58
54
58
64
61
29
47
42
47
52
85
80
87
92
95
78
76
30
19
DTMB
DTMB
SDS
4
32
30
40
25
26
SDS
SDS
SDS
SDS
SDS
DTMBF4 -80
9
a The diborane initiator dissolved in 1 mL in toluene was added
rapidly to a stirred (500 rpm) mixture of 18.0 mL of isobutene and 18.0
mL of aqueous LiCl [LiCl (23 wt %) NaCl (1.2 wt %) H2O (75.8 wt
%)] at -60 °C unless otherwise noted. b The concentration of diborane
used is reported with respect to the total volume of the organic phase.
c Where used, 0.100 g of dodecyltrimethylammonium bromide (DTMB)
or sodium dodecylsulfate (SDS) was added to the aqueous phase.
10
11
12
13
14
15
16
17
18
19
DTMBF4 -80
-80
-80
-73
-73
-60
-60
-40
-22
-22
Table 2. Isobutene Polymerization in Aqueous Sulfuric Acid
Suspensiona
4.60
6.10
entry
initiator
surfactantb
Mw (kg mol-1
)
PDI
yield (%)
a The diborane initiator dissolved in 1.0 mL of toluene was added
rapidly to a stirred (500 rpm) mixture of 18.0 mL of isobutene and 18.0
mL of aqueous HBF4 [HBF4 ) 7.0 M] at -80 °C unless otherwise
noted. b Where used, 0.100 g of SDS or dodecyltrimethylammonium
tetrafluoroborate (DTMBF4), was added to the aqueous phase. c A stock
solution of diborane or diborole in toluene (1.0 mL) was added over 10
min to a suspension of 15.0 mL of isobutene and 15.0 mL of aqueous
HBF4 d A stock solution of diborane in toluene (1.0 mL) was added
over 1.0 min to a suspension of 10.0 mL of isobutene, 5.0 mL of
hexane, and 15.0 mL of aqueous HBF4.
1
2
3
4
5
6
7
8
1
1
28.3
25.3
22.0
27.6
17.4
18.7
33.8
37.2
83.4
69.8
41.3
35.8
2.05
2.21
3.86
3.38
2.63
3.11
2.24
2.69
3.16
2.86
2.32
2.82
29
27
46
30
24
26
17
18
2
1c
1c
1d
2d
1
SDS
SDS
DTMB
DTMB
DTMOTf
DTMOTf
1
1
1
1
9
10
11
12
3
18
14
in the organic phase increases to the point where formation of
higher order, less acidic, oxonium acids (analogous to 3)
interferes with either initiation or propagation within the droplet.
The lower yields encountered in aqueous sulfuric acid versus
aqueous LiCl at the same T may be related to the acidity of the
aqueous phase. That the acidic nature of these aqueous phases
is deleterious is revealed by the experiments with aqueous
LiHSO4, where even at higher T (-45 °C), conversions are
higher while MW is at least comparable to experiments in
aqueous H2SO4 conducted at lower T (Table 2, entries 3/4 vs
1/2). Since the ionic strength of these media are comparable
(both are about 5 M in HSO4), these differences are due to the
higher acidity of the latter medium.
1
a The diborane initiator dissolved in 1.0 mL of toluene was added
rapidly to a stirred (500 rpm) mixture of 18.0 mL of isobutene and 18.0
mL of aqueous H2SO4 [H2SO4 ) 38 wt %] at -60 °C unless otherwise
noted. The final concentration of diborane was 0.625 mM. b Where used,
0.100
g of DTMB, SDS, or dodecyltrimethylammonium triflate
(DTMOTf) was added to the aqueous phase. c A stock solution of
diborane or diborole in toluene (1.0 mL) was added over 5 min to a
suspension of 15.0 mL of isobutene and 15.0 mL of 5.0 M aqueous
LiHSO4 at -45 °C. d A stock solution of diborane or diborole in toluene
(1.0 mL) was added over 10 min to a suspension of 15.0 mL of
isobutene and 15.0 mL of 5.0 M aqueous LiHSO4 at -45 °C.
create a dispersion. On addition of a toluene stock solution of
diborane or diborole, an exothermic reaction was noted,
depending on the rate of addition, this being particularly
pronounced in the case of diborane, while these mixtures took
on a milky appearance as reaction progressed. On the other hand,
phase separation was observed to occur when stirring was
stopped, at least in those suspensions featuring monomer diluted
with hexane.
The results of these experiments are summarized in Tables
1-3. Table 1 summarizes the results obtained in aqueous LiCl
suspension at -60 °C. Table 2 summarizes the results obtained
in 38 wt % sulfuric acid at the same T along with a few
experiments in aqueous LiHSO4 at somewhat higher T, while
Table 3 reports results obtained in aqueous fluoroboric acid at
-80 °C. These T were dictated by the freezing points of the
aqueous phase and increased viscosity of the aqueous medium
as one approaches the freezing point; in essence, effective
dispersal of the monomer was possible only at the T indicated.
Polymer yields are higher in aqueous fluoroboric acid, while
those in sulfuric acid are lower compared with aqueous LiCl.
The first effect is likely related to the significantly lower T
employed, as experiments in aqueous HBF4 indicated significant
decreases in both MW and conversion at higher T (Vide infra).
We speculate that as the T is increased, the solubility of water
Unfortunately, there are no commonly available fluoroborate
salts whose aqueous solutions possess low melting temperatures
comparable to that of fluoroboric acid. On the other hand,
particularly using diborane 1, conversions of monomer can
approach 100% in that medium at low T if diluted with hexane
(Vide infra).
As far as polymer MW is concerned, no clear pattern emerges
from these data other than that of temperature. Thus, at -80
-1
j
°C, Mw varies between 30 and 40 kg mol in aqueous HBF4,
while at -60 °C, Mw varies between 20 and 30 kg mol-1 in
j
aqueous LiCl or H2SO4. Surprisingly, in aqueous LiHSO4 at
-45 °C, Mw varied between 17 and 28 kg mol-1 and was found
j
to be sensitive to the rate of addition of initiator (entries 3-5,
Table 2).
Also, rather contradictory results were obtained using dibo-
rane 1 versus diborole 2 in the different media. In aqueous LiCl,
3-4 times higher MW polymer was formed at similar levels
of conversion using the diborole (Table 1, entry 7 vs 1 and 2).
This feature was not observed in aqueous HBF4 (Table 3, entries
1/2 vs 3/4), although conversion and MW appeared sensitive
to the rate of initiator addition (entry 5 vs 6). Almost identical
outcomes were observed in aqueous LiHSO4 at -45 °C with
either initiator (entry 5 vs 6, Table 2) using a controlled addition
rate.

252 Organometallics, Vol. 28, No. 1, 2009
Lewis et al.
Scheme 1
As can be seen from the results in Tables 1-3, the MWD
are broad with PDI between 2 and 3, as would be expected for
a polymerization process dominated by chain transfer. In the
cases reported in Tables 1-3, the polymer featured a unimodal
MWD with a low MW tail. The skew or asymmetry of the
MWD was sensitive to whether the suspension was quenched
at, for example, low T versus warming to room temperature
prior to quenching. The latter conditions led to increased
formation of low MW oligomers and as a result were generally
avoided.
During this work, it became apparent that the rate of addition
of initiator to these suspensions was a very significant variable
influencing the results. The diborole is bright yellow in solution;
formation of donor adducts8 and/or hydrolysis (Vide infra) leads
to formation of colorless products. When this compound is
added rapidly to these suspensions, the yellow color of the
diborole persists for some time after the addition is complete,
while at sufficiently slow addition rates (5-10 min, depending
on T, amount of initiator and medium used), the suspensions
remain colorless. Under the former conditions, polymer yields
and MW are generally significantly higher than under the latter
conditions.
Part of this is almost certainly related to the lower conversions
encountered: experiments using DTMB that featured dramati-
cally lower conversions also provided the highest MW polymer
(Table 1, entries 3 and 4 and Table 2, entries 9 and 10).
It is now reasonably well understood that anions tend to
preferentially aggregate at the interface in dispersed media of
this sort, where they can disrupt local water structure at the
interface.20 Ordering of water structure near an interface results
in a local decrease in density, thus facilitating diffusion through
the interface. In particular, the Hofmeister series can be used
to predict the efficacy of anions for this process, and in the
present case, diffusion of water through the interface is expected
20c
to increase in the order BF420c , ROSO3 ∼ HSO4 ∼ CF3SO3
For example, in aqueous HBF4 suspension with 10.0 mL of
monomer diluted by 5.0 mL of hexane, addition of 0.25 mL
of 0.01 M diborole within 5 s resulted in the formation of
< Br , Cl.
One possible interpretation of the results obtained to date is
that lower conversions encountered in stabilized suspension are
a reflection of increased diffusion of water into the droplet
during chain growth at constant T.
poly(isobutene) (PIB) with Mw ) 62.5 kg mol-1 at 36%
j
conversion, while addition of 1.0 mL of 0.01 M diborole over
a 5 min period afforded PIB with Mw ) 27.7 kg mol-1 at 27%
j
j
Since Mw does not show a consistent trend, it is less clear
conversion.
whether a similar explanation applies. In particular, since all of
the experiments using a surfactant featured an uncontrolled
addition rate, and this is known to be an important variable
influencing polymer MW, it could be that variations in addition,
etc., account for any difference in the results observed. We do
This behavior was also evident using the diborane initiator.
Rapid addition of this compound to monomer is highly
exothermic and can result in reflux of the monomer. One would
normally expect a negative impact on both conversion and MW.
However, in those experiments where a slower, controlled
addition of this compound was conducted, lower MW polymer
was formed at similar conversion (e.g., Table 3, entries 1 and
2 vs 5). If this compound was added sufficiently slowly, only
trace amounts of PIB were formed.18
j
note that the poor reproducibility in Mw between the two
experiments conducted with the diborole in the presence of SDS
(Table 1, entries 8 and 9) is likely a reflection of this.
Finally, the T dependence of MW and conversion was studied
in the case of isobutene polymerization in aqueous HBF4
suspension. In these experiments, the isobutene was diluted with
hexane ([IB] ) 8.36 M) since this results in better T control
and higher conversion in suspension, as well as in solution
polymerizations (Vide infra). Over the T range -80 to -40 °C,
conversions varied between 75% and 95% with no discernible
trend, but they do decrease to 20-30% at -22 °C (see Table
3, entries 11-19). Evidently, at sufficiently low T, the MW of
the polymer formed is largely independent of T (entries 11-16)
but decreases rapidly above -60 °C. We will defer discussion
of these results until those obtained in the virtual absence of
water are presented (Vide infra).
Hydrolysis and Alcoholysis of Diborane 1. As mentioned
in the Introduction, the reaction of diborane 1 with MeOH has
been previously studied, and in the presence of excess MeOH,
the novel and stable oxonium acid 3 is formed (Scheme 1).12
In that study various amounts of MeOH were added to diborane
1, initially at low T, with the outcome of this reaction being
highly solvent and T dependent for amounts of MeOH e 1.0
equiv (Table 4, entries 1-7).
Finally, the use of ionic surfactants to stabilize these
suspensions was studied. Although the high ionic strength of
these aqueous phases mitigates against formation of micelles,
it was anticipated that the particle size of the droplets would be
more uniform under these conditions, possibly allowing for a
more controlled initiation.
In all cases, the use of a surfactant led to a decrease in
conversion, and this effect was particularly pronounced in the
case of dodecyltrimethylammounium bromide (DTMB) in
aqueous LiCl or H2SO4 (Table 1, entries 3 and 4 or Table 2,
entries 9 and 10). In contrast, when using dodecyltrimethylam-
mounium triflate (DTMOTf) in aqueous H2SO4 or dodecyltri-
methylammounium tetrafluoroborate (DTMBF4) in aqueous
HBF4, comparable results were obtained to those observed using
sodium dodecylsulfate (SDS) in either medium (Table 2, entries
11/12 vs 7/8 and Table 3, entries 9/10 vs 7/8). In either case,
one suffers about a 10% drop in conversion under otherwise
similar conditions.
The use of SDS or DTMBF4 surfactant led to a statistically
In polar media such as CD2Cl2, oxonium acid 3is present at
low T, while in toluene-d8 at low T, or on warming a CD2Cl2
j
insignificant change in Mw in aqueous HBF4 (i.e., 39.3 ( 3.0
vs 32.2 ( 3.8 kg mol-1), while surprisingly a significant increase
j
in Mw was observed in aqueous H2SO4 using either SDS or
(20) (a) Pegramnt, L. M.; Record, M. T., Jr. J. Phys. Chem. B 2007,
111, 5411–5417. (b) Wick, C. D.; Dang, L. X. J. Phys. Chem. B 2005,
109, 15574–15579. (c) Taylor, R. P.; Kuntz, I. D., Jr. J. Am. Chem. Soc.
1972, 94, 7963–65.
DTMOTf (25.8 ( 2.8 vs 37.0 ( 3.2 kg mol-1) and to a lesser
extent in aqueous LiCl using SDS (26.1 ( 4.7 vs 33.4 ( 2.2
kg mol-1).

Isobutene Polymerization Using Chelating Diboranes
Organometallics, Vol. 28, No. 1, 2009 253
Table 4. Product Distribution from the Reaction of Diborane 1 with
MeOH
Scheme 2
entry MeOH (equiv) T (K)
solvent
1
3
4
5
6
1
2
3
4
5
6
7
8
9
10
0.5
0.5
0.5
1.0
1.0
1.0
1.0
2.0
3.0
4.0
298
193
298
213
298
213
298
298
298
298
toluene-d8 50
0
12
0
25 25
0
0
0
0
0
0
0
50
33
0
CD2Cl2
CD2Cl2
88
50
0
0
25 25
toluene-d8 30
17 26 26
50 50
39 15 15
toluene-d8
CD2Cl2
CD2Cl2
0
31
0
0
0
0
50 50
toluene-d8
toluene-d8
CD2Cl2
0
0
0
50
0
0
0
33 33
100
0
solution above -60 °C, any oxonium acid present decomposes
to form (C6F5)2BOMe 412,21 and (o-C6F4H)B(C6F5)2 5. The
identity of these compounds was verified by independent
However, when ca. 1-2 equiv of water was added, the cleavage
products 5 and the known borinic acid 723 were formed at the
expense of 1. When larger amounts of water were added, an
aquo complex 8 was formed from 5 in much the same manner
as observed in the case of MeOH. Compound 8 has been
structurally characterized (see Supporting Information) and is
analogous in structure to the known compound (C6F5)3B ·
(H2O)3.24 An oxonium acid 9 analogous to 3 was formed in the
presence of excess water, but it was exclusively generated only
by adding D2O to solid diborane 1 at room temperature. This
compound was characterized spectroscopically but has not been
obtained in pure form.
Hydrolysis and Alcoholysis of Diborole 2. Similar chemistry
was investigated with diborole 2; in the case of MeOH, addition
of 1 equiv to diborole 2 in either toluene-d8 or CD2Cl2 at low
T results in the formation of exo-MeOH adduct 10 (Scheme 2).
The formation of 10 had been noted earlier in reactions involving
ionization of cumyl methyl ether by diborole 2.9 In this case,
adduct 10 can be generated at -80 °C in sufficient amounts
(19%) to allow for its spectroscopic characterization, though
its formation was accompanied by some thermal degradation
to borinic ester 11 (Vide infra, ∼ 13%). In particular, the exo
adduct formulation was confirmed by a cross-peak in the two-
dimensional 19F-1H correlation spectrum between the methanol
proton (dq at δ 6.58) and a 19F resonance at δ -133.5. The
same correlation was established by a 1D selective decoupling
experiment. Irradiation of this 19F signal reduces the doublet of
quartets structure for the methanol proton to a quartet due to
H-H coupling only. In toluene-d8, exo-MeOH adduct 10 was
also formed, but no 19F coupling to the methanol proton was
apparent from the spectra in this more basic solvent.
1
synthesis (see Supporting Information), while the H, 19F, and
11B chemical shifts and assignments for all of these compounds
are reported in Table 5.
Evidently, compounds 4 and 5 arise from chemoselective
cleavage of the o-phenylene B-C bonds of diborane 1 (Scheme
1). We strongly suspect that this cleavage results from reversible
protonation of the anion in 3, either at O to form an unobserved
µ-MeOH adduct or possibly in an irreversible and direct manner
involving ipso electrophilic aromatic substitution at one of the
B-C bonds. In particular, the acidity of the oxonium acid 3
will be sensitive to the number of MeOH molecules that solvate
the proton,22 and a minimum of three are needed to stabilize
this compound at room temperature. Presumably, the higher
stability of 3 in polar media at low T reflects its ionic
constitution, such that protonation to form the neutral degrada-
tion products is less facile.
Borane 5 is sufficiently Lewis acidic (and less sterically
hindered) that it competes effectively with diborane 1 for MeOH,
forming monoadduct 6. Thus, when g1.0 equiv of MeOH is
present, a quasi-equilibrium is established between 1, 3, 4, and
6 at low T in toluene-d8, with ion-pair 3 favored in CD2Cl2 at
low T (Table 4, entry 4 vs 6). At sufficiently high T, all of
diborane 1 is converted into 4 and 5 (or 6) depending on the
amount of MeOH initially present.
If excess MeOH is present (i.e., g 4 equiv) ion-pair 3 is the
exclusive product formed at low T and is stable in solution at
room temperature. Additional MeOH, over and above that
needed to form 3, exchanges with the MeOH molecules that
solvate the proton; this exchange process is rapid at all
temperatures in CD2Cl2.
The formation of ion-pair 3 from 1 and MeOH is reversible
in the sense that ion-pair 3 can serve as a source of MeOH in
the presence of excess 1. This was shown by an experiment
where 3 was generated from 1 and excess MeOH (8.0 equiv).
Addition of 3.0 equiv of diborane 1 to a solution of 3 (1.0 equiv
+ 4.0 equiv of MeOH) at room temperature led to formation
of 4 (and 6).
Coordination of MeOH to the exo side is not unexpected for
diborole 2; even unhindered donors such as acetonitrile have a
strong preference for exo coordination to this compound.8 In
contrast, unhindered donors such as acetonitrile exhibit endo-
coordination to diborane 1.
On warming adduct 10, reaction to form borinic ester 11 is
observed; compound 11 was reported earlier,9 but has now been
structurally characterized and the molecular structure appears
in Figure 1, while crystallographic and metrical data are reported
as Supporting Information. The characteristic structural feature
of ester 11 is that the OMe group bridges to both B atoms in
Similar chemistry is also observed on addition of various
amounts of water to diborane 1 in toluene-d8 solution at low T
(Scheme 1). Predictably, the stoichiometry of these reactions
was rather difficult to control, given the low solubility of water
in this solvent as well as it is ultimate physical state at low T.
(23) (a) X-ray structure: Beringhelli, T.; D’Alfonso, G.; Donghi, D.;
Maggioni, D.; Mercandelli, P.; Sironi, A. Organometallics 2003, 22, 1588–
1590. (b) See also: Metcalfe, R. A.; Kreller, D. I.; Tian, J.; Kim, H.; Taylor,
N. J.; Corrigan, J. F.; Collins, S. Organometallics 2002, 21, 1719–1726.
(c) Preparation/characterization: Chambers, R. D.; Chivers, T. J. Chem. Soc.
1965, 393, 3–9.
(24) Danopoulos, A. A.; Galsworthy, J. R.; Green, M. L. H.; Doerrer,
L. H.; Cafferkey, S.; Hursthouse, M. B. Chem. Commun. 1998, 22, 2529–
2530.
(21) This compound has been structurally characterized: Donghi, D.;
Maggioni, D.; Beringhelli, T.; D’Alfonso, G.; Mercandelli, P.; Sironi, A.
Eur. J. Inorg. Chem. 2008, 10, 1645–1653.
(22) (a) Stoyanov, E. S.; Kim, K.-C.; Reed, C. A. J. Am. Chem. Soc.
2006, 128, 1948–1958. (b) Faˇrcas¸iu, D.; Haˆncu, D. J. Chem. Soc., Faraday
Trans. 1997, 93, 2161–2165. (c) Arnett, E. M.; Quirk, R. P.; Burke, J. J.
J. Am. Chem. Soc. 1970, 92, 1260–1266.

254 Organometallics, Vol. 28, No. 1, 2009
Lewis et al.
Table 5. 1H, 19F, and 11B NMR Chemical Shifts and Assignments for Hydrolysis and Methanolysis Products of Diborane 1
1
compound (solvent)
1 [X ) B(C6F5)2] (C6D6)
3 [(MeOH)nH][o-C6F4{B(C6F5)2}2(µ-OMe)] -137.4 -163.8
(n ) 3, CD2Cl2)
(C6F5)2BOMe 4 (C6D6)
δ F1
δ F2
δ F3
δ F4
δ -B(C6F5)2 (Fo, Fp, Fm)
-128.5, -141.6, -161.5
δ H
δ
11B
-127.6 -141.6
-132.4, -160.2, -166.1 (MeOH) 6.19, (-OMe) 3.65, 3.8
(MeOH) 3.53
-132.7, -148.8, -160.9 (-OMe) 3.24
-137.7 -141.0 -153.6 -125.4 -129.0, -143.7, -160.0 (aryl-H) 6.66
40.3
40.3
5 [X ) H] (C6D6)
6 [X ) H] (C6D6)
-139.5 -155.2 -156.9 -133.6 -133.8, -153.9, -161.9 (aryl-H) 6.49, (MeOH) 4.70, 4.6
(MeOH) 2.25
1 (CD2Cl2) -62 °C
5 (d8-toluene) -60 °C
-125.1 -147.2
-125.9, -140.3, -159.9
-137.7 -139.7 -154.0 -125.4 -128.4, -141.4 -159.1
(aryl-H) 6.66
(C6F5)2BOH 7 (d8-toluene)
8 [X ) H] (d8-toluene) -60 °C
-132.8, -147.8, -160.9 (-OH) 6.32
-139.0 -154.3 -156.2 -133.4 -134.0, -152.7, -161.2 (aryl-H) 6.52, (-OH) 6.28,
(H2O) 4.52
42.2
[(H2O)nH][o-C6F4{B(C6F5)2}2(µ-OH)]
(9, n ∼ 4, CD2Cl2)
-138.9 -163.4
-135.0, -160.3, -166.4
-0.4
the solid state and in solution. However, the bridging is
asymmetrical as one would expect with bond lengths of
O(1)-B(2) ) 1.397(3) Å and O(1)-B(1) ) 1.640(3) Å,
respectively differing by about 17% with the former being
typical of a B-O single bond involving trigonal boron. The
five-membered, chelate ring is essentially planar with the sum
of the dihedral angles B(1)-C(2)-C(7)-B(2) ) -5.4(2)° +
C(2)-C(7)-B(2)-O(1) ) 2.7(2)° + C(7)-B(2)-O(1)-B(1)
) 1.0(2)° + B(2)-O(1)-B(1)-C(2) ) -3.7(2)° + C(7)-
C(2)-B(1)-O(1) ) 5.4(2)° ) 0.0(2)°. Both B(2) and O(1) have
sp2 hybridization with the sum of the bond angles about each
atom being 359.9(2)° and 360.0(2)°, respectively. Though B(1)
is approximately tetrahedral, there is significant angle strain in
both the borole, as well as five-membered chelate, rings as is
evident from C(8)-B(1)-C(19) and C(2)-B(1)-O(1) angles
of 99.5(2)° and 96.8(2)°, respectively. Surprisingly, this chelate
structure is preserved in the presence of strong monodentate
donors such as pyridine, which coordinate to B(2) rather than
the more Lewis acidic borole moiety.25
An oxonium acid 12, analogous to 3, is formed from diborole
2 if at least four equivalents of MeOH are initially present. As
suggested in Scheme 2, formation of 12 may involve adduct
10 as an intermediate, where the latter is sufficiently acidic to
protonate free MeOH. The monodentate anion that would result
could rearrange to the chelated form, provided excess MeOH
is present to stabilize the oxonium acid formed.
In the case of hydrolysis, the chemistry is similar. Controlled
addition of stoichiometric water at low T leads initially to exo
adduct 14 which degrades in a similar manner at higher T to
form borinic acid 15. On prolonged standing at room temper-
ature in toluene-d8 solution, this compound degrades to the
symmetrical anhydride 16 through cleavage of the remaining
borole ring; compound 16 has been structurally characterized
as its mono-THF adduct (see Supporting Information). In the
presence of excess water, oxonium acid 13 is produced and is
stable in solution at room temperature. However, when water
was removed rapidly in Vacuo from these solutions, anhydride
16 was isolated (as a bis hydrate). Slow evaporation of water
resulted in the crystallization of borinic acid 15, isolated as
heptahydrate 17.
dative interaction of O(2)-B(2) at 1.527(3) Å. The five-
membered, chelate ring is planar, with the sum of the dihedral
angles about all five atoms being -0.1(2)°, while O(1) is
approximately sp2 hybridized at least on comparing the B-O-B
angle in this structure [116.7(2)°] to that of ester 11 [113.5(2)°].
(The H atom bonded to O(1) was not located and constrained
to a calculated position.)
Since oxonium acid 13 is formed and is stable in the presence
of excess H2O, the eventual isolation of 16 · (H2O)2 or 17 implies
that as water is removed through evaporation, the formation of
adduct 14 from 13 must be reversible or that cleavage of 13 to
form 15 occurs directly as the degree of solvation of the proton
in the oxonium acid is diminished.
Relation of Hydrolysis Chemistry to Isobutene Polymer-
ization. Given the complexity of these reactions as well as the
nature of the products formed, it is quite conceivable that any
of the Lewis or Brønsted acidic byproduct resulting from
hydrolysis of diborane 1 or diborole 2 could be involved in
initiation of polymerization in aqueous suspension (or hydro-
carbon solution, Vide infra).
While not all of the stable byproducts described previously
have been individually tested, several control experiments
suggest that they are probably not involved. As already
mentioned, stable oxonium acid 3 does not initiate polymeri-
zation of isobutene in aqueous suspension, nor does a mixture
of this compound, borane 5, and borinic ester 4 when previously
generated by addition of 2-3 equiv of MeOH to diborane 1.
Also, the combination of borinic acid 7 and excess B(C6F5)3 is
also ineffective for polymerization in aqueous suspension, while
the latter compound is also inactive by itself. Finally, though
the presence of excess water no doubt moderates the Brønsted
acidity of chelated borinic acid 17, this compound was also
ineffective as an initiator of isobutene polymerization in aqueous
suspension.
A related question is to what extent do these decomposition
reactions intrude during polymerization in aqueous suspension
or hydrocarbon solution. This is rather difficult to answer using
isobutene itself, but we did investigate the reaction of 1,1-
diphenylethylene (DPE) with MeOH in the presence of diborole
2 as a model for protic initiation.4f,26
Compound 17 has been characterized by X-ray crystal-
lography, and its molecular structure is depicted in Figure 2. It
has the same basic chelate structure as observed for 11, though
a water molecule is coordinated to B(2), rendering it tetrahedral.
In addition, six additional water molecules are present in the
lattice (not shown) involved in a network of hydrogen bonds
to each other, O(2), O(1), and several F atoms. Since both B
atoms are tetrahedral, the bonds to O(1) are significantly longer
at O(1)-B(2) and O(1)-B(1) ) 1.520(3) and 1.540(3) Å. In
fact both B-O bonds are equivalent in length to that of the
In fact, using conventionally dried CD2Cl2, there was suf-
ficient moisture present that DPE (20 µmol) was converted to
a 1:1 mixture of DPE (67% conversion) and indan 1827 in the
(25) Sciarone, T. J. J. Parvez, M., unpublished structural results.
(26) (a) Sauvet, G.; Vairon, J. P.; Sigwalt, P. J. Polym. Sci., Polym.
Symp. 1975, 52, 173–87. (b) Ioone, T. W.; Lee-Ruff, E.; Khazanie, P. G.;
Hopkinson, A. C. J. Chem. Soc., Perkin Trans. 2 1975, 6, 607–609. (c)
Olah, G. A.; Halpern, Y. J. Org. Chem. 1971, 36, 2354–2356.
(27) Bergmann, E.; Weiss, H. Just. Lieb. Ann. Chem. 1930, 480, 49–
59.

Isobutene Polymerization Using Chelating Diboranes
Organometallics, Vol. 28, No. 1, 2009 255
Figure 1. Molecular structure of borinic ester 11 with 50% thermal ellipsoids depicted.
Scheme 3
This raises the interesting question as to what species is
responsible for protonation of isobutene versus degradation via
B-C bond cleavage. In our view, monodentate aquo complexes
formed from either diborole or certainly diborane are insuf-
ficiently acidic for this purpose. For example, B(C6F5)3 and
compound 5 are both kinetically stronger Lewis acids than
diborane 1, yet neither is effective for protic initiation in aqueous
suspension (or hydrocarbon solution). Instead, we believe a
µ-aquo complex that is transiently formed from water and
diborane 1 is sufficiently acidic to irreversibly protonate
isobutene (eq 1). In the absence of a suitable base, degradation
to form borane 5 and borinic acid 7 intervenes.
presence of 1 equiv of diborole 2 at room temperature (Scheme
3). Conversion of DPE to 18 may be minimized to ca. 20-30%
at room temperature by using CD2Cl2 that has been more
thoroughly dried and DPE that has been freshly distilled from
CaH2.
Subsequent addition of MeOH (∼1.0 equiv with respect to
diborole 2) at -80 °C to this solution led to complete conversion
of the remaining DPE to stable ion-pair 1928 (ca. 90%
conversion of diborole 2) as well as exo-MeOH adduct 10 (ca.
10%). As the temperature was increased, first exo adduct 10 (at
ca. -40 °C) and then ion-pair 19 (above -20 °C) degraded to
form ester 11 and in the latter case indan 18.
Thus, at low T in polar media, protic initiation is reasonably
efficient in the case of this model reaction. In particular,
competing dimerization of DPE to form the MeCPh2CH2CPh2
cation, which is seen in analogous reactions involving CF3SO3H
or TiCl4,26a was not observed. Although DPE is much more
basic than isobutene, we are confident that protic initiation in
aqueous suspension involves analogous chemistry with diborole
2 or diborane 1 and dissolved water.
That this structural motif is possible is revealed by the
molecular structure for compound 20, crystals of which were
isolated from an attempt to crystallize oxonium acid 12,
evidently from wet MeOH (Figure 3). Compound 20 is a
monoaquo adduct of borinic ester 11, which features the µ-OH2
interaction invoked above; this compound has defied rational
attempts to synthesize it and has only been structurally
characterized.
The dative coordination of water to the two B atoms is almost
symmetrical with rather long B1-O1 and B2-O1 distances of
1.541(5) and 1.568(5) Å, respectively. Both B atoms are
tetrahedral in this structure so that the B1-O2 distance for the
MeO moiety is now elongated to 1.523(5) Å compared to that
seen in the parent borinic ester 11. Strain is present within the
chelate and borole rings as evident from the B2-O1-B1,
(28) (a) Olah, G. A. J. Am. Chem. Soc. 1964, 86, 932–4. (b) Farnum,
D. G. J. Am. Chem. Soc. 1964, 86, 934–5.

256 Organometallics, Vol. 28, No. 1, 2009
Lewis et al.
Figure 2. Molecular structure of borinic acid 17 with 50% thermal ellipsoids depicted. Six additional water molecules present in the unit
cell are not depicted.
O1-B1-C2, C7-B2-O1, and C20-B2-C31 angles of
116.2(3)°, 98.2(3)°, 97.1(3)°, and 98.5(3)°, respectively. The
five-membered chelate ring is puckered, with the sum of the
dihedral angles involving B1, O1, B2, C7, and C2 ) -5.0(5)°;
in essence O1 lies outside the plane defined by the other four
atoms by 0.278(2) Å, with the mean deviation from this plane
being 0.029(4) Å for B1, B2, C2, and C7.
Polymerization of Isobutene in Hydrocarbon Solution. The
aqueous suspension process can be viewed as a series of bulk
or solution polymerizations carried out in a dispersed medium,
featuring better T control and potentially higher conversions due
to lower process viscosity. It was therefore of interest to compare
our results to polymerizations carried out in solution in the
virtual absence of H2O.
Polymerizations of isobutene undiluted or in hexane solution
were conducted on a vacuum line, where both monomer and
solvent had been stirred with and then vacuum transferred from
tri-n-octylaluminum. The total impurity content under these
conditions is <2.5 × 10-5 M, as measured by titration using a
stock solution of benzophenone ketyl.18 More recent work has
shown that solutions of monomer in CH2Cl2 purified in this same
manner have residual H2O contents e7 × 10-6 M, as verified
by titration with a solution of [Ph3C][B(C6F5)4], which serves
as a selectiVe indicator for H2O.29 Undoubtedly, the residual
H2O content in hydrocarbon media is lower than this.
the swollen mass. Quantitative conversions of monomer were
obtained in hydrocarbon solution except where very low levels
of initiator (2 µM) were used. The results obtained are
summarized in Table 6.
Essentially, polymerization ceases below ca. 2 µM of
compound 1 (entry 7). This is probably an indication that
background H2O levels are between 2 and 20 µM under these
conditions. On the other hand, with [1] ) 2.0 mM, stopping
experiments revealed that at least 15 mM DtBMP was needed
to prevent protic initiation under these conditions (see Experi-
mental Section). Since DtBMP does not react directly with
dissolved water, it is clear that the Brønsted acid formed from
water and diborane 1 must be kinetically competent to protonate
isobutene (2.76 M) even when [DtBMP] ) 2.0 mM! We are
unaware of any precedent for this behavior using classical Lewis
acids: we suspect it arises due to the hindered nature of the
(strong) Brønsted acid formed in this case.
The first seven entries of Table 6 demonstrate the use of
progressively lower amounts of diborane initiator. Entries 3-7
were conducted at the same time using the same stock solution.
It can be seen that MW increases significantly as the amount
of diborane added is reduced. We suspect this feature arises
from better T control on dilution of this compound. Entries 8-11
demonstrate that much lower MW polymer is formed in
undiluted monomer, a feature that we attribute, in part, to poor
T and mass transfer control. The conversions in entries 10 and
In these polymerizations, toluene solutions of diborane or
diborole were added by syringe; these polymerizations were
characterized by rapid increases in viscosity, and in the case of
neat monomer, it was impossible to avoid “solidification” of
(29) Slomkowski, S.; Penczek, S. J. Chem. Soc., Perkin Trans. 2 1974,
1718–1722, and references therein.

Isobutene Polymerization Using Chelating Diboranes
Organometallics, Vol. 28, No. 1, 2009 257
Figure 3. Molecular structure of compound 20 with 50% thermal ellipsoids depicted. Disordered MeOH and CH2Cl2 molecules present in
the unit cell are not depicted.
11 are not quantitative, suggesting moisture levels may well
have been higher in these two experiments.
those that were exhibited both exo- and endo-terminal unsat-
uration (ca. 60:40 ratio) consistent with chain transfer to
monomer. However, these samples were formed at temperatures
well below (i.e., -60 to -80 °C) where one would expect this
chain transfer process to dominate based on the Arrhenius plots
depicted in Figure 4.
The shallow T dependence of MW observed at even lower T
has been observed before using conventional Lewis acids in
polar media and has been interpreted as arising from a change
in MW controlling events.30 The nature of the process respon-
sible for this “cross-over” behavior is not well understood and
has been attributed to, for example, termination to counteranion
and polymer precipitation. However, this cannot be the case
here; all of these polymerizations are quantitative and character-
ized by highly efficient chain transfer based on dissolved [H2O]
or even diborane (Ieff > 1000%).
Another explanation, again in polar media (ꢀ . 4), is that
primarily unpaired cations propagate at low T, while ion-pairs
dominate the kinetics at higher T so that the T dependence of
MW should show a “kink” because of the T dependence of the
equilibrium constant governing the concentration of these two
species. This theory predicts that polymerizations in apolar
media such as hexane (ꢀ ) 2.2) should not show this
phenomenon as the kinetics are dominated by ion-pairs at all
accessible temperatures.31
The final two entries in this table reveal that significantly
lower MW polymer is produced using the diborole initiator
(compare entries 19/20 vs 3/4). In addition, the MWD of the
derived polymers were considerably narrower than those formed
using the diborane. Since polymerizations conducted using this
initiator were significantly less exothermic than those using the
diborane, we have to attribute these differences as being
characteristic of this diborole. Similar behavior has also been
noted in polymerizations featuring cationogenic initiation.9
Note that the behavior of the diborole initiator in hydrocarbon
solution is opposite that observed in aqueous suspension; we
suggest this is because physical factors (rate of addition, degree
of dispersion, nature of the aqueous medium, etc.) limit MW
in aqueous suspension rather than, for example, intrinsic factors
such as the nature of the counteranion, etc.
The T dependence of MW in hydrocarbon solution was
studied over the range -90 to -20 °C. These data appear in
Table 6 and are plotted in Figure 4 along with the data obtained
in aqueous suspension (Table 3) in the form of Arrhenius plots.
Surprisingly, almost identical behavior is seen here as in aqueous
suspension!
In hydrocarbon solution, polymer MW is virtually indepen-
dent of T between -60 and -90 °C, suffering a pronounced
decrease at higher T. The limiting slope of this curve at higher
T corresponds to an activation energy for MW (∆EMW) of -5.2
kcal mol-1. This corresponds to a ∆EMW value often attributed
to chain transfer to monomer, although it must be admitted that
the literature value ∆EMW ) -5.9 kcal mol-1 relates to
experiments conducted in more polar media.4i-k
Recent work from the group of Bochmann has highlighted
the role of dissolved or molecular H2O in isobutene homo/
copolymerizations featuring weakly coordinating (and hydro-
lytically resistant) counteranions.4f Although they did not
-1
analyze their MW data in this fashion, a Mayo plot of Xn vs
[H2O]/[IB] is linear with a slope corresponding to the transfer
(or termination) constant for H2O, CH2O ) 126.2 at -35 °C
(see Supporting Information).
In other words, transfer to dissolved H2O would be 100 times
faster than propagation at equivalent concentrations of H2O and
monomer at this T. It should be noted that this result could have
Although the high MW of the PIB samples formed under
anhydrous conditions precluded convenient spectroscopic de-
termination of end-groups, the lower MW materials formed in
1
aqueous suspension could be analyzed by H NMR spectros-
copy. Although not all samples were analyzed in this fashion,

258 Organometallics, Vol. 28, No. 1, 2009
Lewis et al.
Table 6. Isobutene Polymerization in Hydrocarbon Solutiona
entry initiator (mM) T (°C) Mw (kg mol-1
)
PDI
yield (%)
1
2
3
4
5
6
7
8
1 (2.0)
1 (2.0)
1 (0.2)
1 (0.2)
1 (0.02)
1 (0.02)
1 (0.002)
1 (0.2)b
1 (0.2)b
1 (0.02)b
1 (0.02)b
1 (2.0)
1 (2.0)
1 (2.0)
1 (2.0)
1 (2.0)
1 (2.0)
1 (2.0)
2 (0.2)
2 (0.2)
-80
-80
-80
-80
-80
-80
-80
-80
-80
-80
-80
-90
-90
-64
-40
-40
-20
-20
-80
-80
340
227
159
170
316
294
518
2.38
3.18
5.68
3.82
1.99
2.03
1.90
3.16
2.49
4.34
3.47
2.45
3.24
2.87
2.25
3.75
2.27
2.39
2.09
2.14
100
100
100
100
97
100
3
100
100
31
69.0
9
41.3
98.6
59.4
185
269
194
10
11
12
13
14
15
16
17
18
19
20
39
100
100
100
100
100
100
100
100
100
94.8
Figure 5. Simulated temperature dependence of MW in hydrocar-
bon solution initiated by diborane 1 assuming various dissolved
[H2O]. See Supporting Information for assumptions and data.
86.4
37.0
32.3
88.6
information, and under the assumption that CH2O is also independent
of T (i.e., both processes are characterized by a nonzero activation
entropy and a very low activation enthalpy), it is possible to
simulate the effect of dissolved [H2O] on MW as a function of T
if this process and chain transfer to monomer are the only ones
involved (see Supporting Information).
139
a The diborane initiator dissolved in 1.0 mL in toluene was added to
a magnetically stirred solution of isobutene (2.76 M in hexane) unless
otherwise noted. b A stock solution of diborane in toluene (1.0 mL) was
added to 12.0 mL of isobutene at -80 °C.
This analysis is shown in Figure 5, where it is clear that the
observed data can be accommodated by a background [H2O]
of about 10-5 M. The agreement with the value estimated by
titration with benzophenone ketyl is fortuitous since the T
dependence of some of these processes is unknown. However,
these diboranes do not react as readily with H2O (in comparison
to conventional Lewis acids), so one can envisage that H2O will
act as a potent chain transfer agent.
If so, it is clear that the lower molecular weights seen in
aqueous suspension are almost entirely due to the higher levels
of dissolved H2O that are inevitably present. [Some experiments
that involved addition of diborole solution (changes from yellow
to colorless when “wet”) to an aqueous suspension of hexane
(5.0 mL) and toluene (10.0 mL) at various rates revealed that
the (mutual) rate of diffusion of H2O into the droplet at -78
°C in fluoroboric acid was <10-4 mmol s-1, while background
levels of H2O in the droplet were between 10-4 and 10-5 M.]
Evidently, at least under some conditions at low enough T, there
is not enough H2O present to prematurely terminate chains given
the high conversions seen, but many of the other effects can be
interpreted in terms of differential lowering of H2O content as
a function of diborane structure (i.e., susceptibility to degrada-
tion by water) and/or mode and rate of addition.
Figure 4. Temperature dependence of MW in hydrocarbon solution
(solid curve) and aqueous HBF4 suspension (dashed curved)
initiated by diborane 1.
been anticipated from the early work of Williams and other
workers who studied γ-ray-initiated polymerizations of
isobutene;32 scrupulous purification of monomer and (hydro-
carbon) solvent are required where the rate constant for
“termination to polar impurities” exceeds that of propagation
by a similar magnitude.5a
Recent kinetic work has indicated that propagation in isobutene
polymerization occurs via ion-pairs with a specific rate constant
of ca. 108 M-1 s-1, which is largely independent of T.33 With this
Conclusions
The work summarized here provides considerable insight into
the nature of the initiation, propagation, and chain transfer
processes involving aqueous suspension polymerization of
isobutene using chelating diboranes 1 and 2. In addition,
experiments in hydrocarbon solution, which serve as a model
for the suspension process, highlight the probable role of
dissolved water as a potent chain transfer agent in polymeriza-
tions involving hydrolytically resistant Lewis acids that give
rise to weakly coordinating counteranions. It is evident that chain
transfer to water will limit MW in aqueous suspension and to
the point where it will prove very challenging to develop an
aqueous suspension process for the synthesis of high MW PIB
(30) (a) Dimitrov, I.; Faust, R. Macromolecules 2004, 37, 9753–9760.
(b) Kennedy, J. P.; Trivedi, P. AdV. Polym. Sci. 1978, 28, 113–151. (c)
Kennedy, J. P.; Rengachary, S. AdV. Polym. Sci. 1974, 14, 1–48. (d)
Kennedy, J. P.; Squires, R. G. Polymer 1965, 6, 579–87.
(31) Plesch, P. H.; Austin, J. C. J. Polym. Sci Part A: Polym. Chem.
2008, 46, 4265–4284, and references therein.
(32) (a) Bates, T. H.; Best, J. V. F.; Williams, F. Nature (London) 1960,
188, 469–70. (b) Bates, T. H.; Williams, F. Nature (London) 1960, 187,
665–9. (c) Brownstein, S.; Bywater, S.; Worsfold, D. J. Makromol. Chem.
1961, 48, 127–34. (d) Best, J. V. F.; Bates, T. H.; Williams, F. Trans.
Faraday Soc. 1962, 58, 192–205. (e) Ueno, K.; Hayashi, K.; Okamura, S.
Polymer 1966, 7, 431–439. (f) Huang, R. Y. M.; Westlake, J. F. J. Polym.
Sci., Part A1 1970, 8, 49–61. (g) Huang, R. Y. M.; Westlake, J. F. J. Polym.
Sci., Polym. Lett. Ed. 1969, 7, 713–17. (h) Taylor, R. B.; Williams, F. J. Am.
Chem. Soc. 1969, 91, 3728–3732.
(33) De, P.; Faust, R. Macromolecules 2005, 38, 9897–9900, and
references therein.

Isobutene Polymerization Using Chelating Diboranes
Organometallics, Vol. 28, No. 1, 2009 259
Procedure A: Uncontrolled Addition of Diborane.18 A three-
neck, 100 mL round-bottom flask, with a central 24/40 and two
14/20 side necks, was charged with 20 mL of LiCl/NaCl/H2O stock
and 0.100 g of surfactant if used. The flask was then fitted with a
14/20 septum, 14/20 gas-inlet adapter, and 24/40 outer/inner air-
free adapter with in-line PTFE stopcock, after which it was affixed
to a vacuum line. The contents of the flask were then subjected to
a single freeze-pump-thaw cycle, and then 18.0 mL of isobutene
was vacuum transferred into the flask. The flask was then warmed
to -60 °C (through use of a CHCl3/dry ice bath) and sealed under
N2. The flask was then connected to a double manifold and placed
back under N2, after which the 24/40 outer/inner air-free adapter
was replaced with a stir bearing/shaft/blade assembly. The flask
contents were then stirred under N2 at -60 °C at about 500 rpm.
Next, 1.0 mL of diborane or diborole stock (1.19 × 10-5 mol) was
then added rapidly by syringe (Caution: can be violently exother-
mic), after which the reactor contents turned milky white in color.
Polymerization was allowed to continue for a full hour before
warming the reactor contents to room temperature by diluting the
flask with additional water and CH2Cl2. During this period very
little gas evolution occurred. The organic phase was then extracted
with CH2Cl2 and dried over MgSO4 before isolating polymer by
removal of volatiles under reduced pressure. Yield: 6.10 g (48%
conversion).
or copolymers thereof. In particular, on the basis of the behavior
of these diboranes in nearly anhydrous hexane solution, it is
clear that the required levels of background moisture in the
monomer droplets would have to be in the ppm range.
Experimental Section
Part I. All of the synthetic or NMR scale reactions involving
diborane 1 or diborole 2 and various reactants (i.e., MeOH, H2O,
or DPE) were performed at the University of Calgary. Details of
these experiments are provided as Part II of the Experimental
Section.
All polymerization experiments were conducted at the University
of Akron, as were control polymerization experiments involving
the hydrolysis products of diborane 1 or diborole 2. The syntheses
of diborane 1 and diborole 2 for use in all polymerization
experiments were conducted at the University of Akron according
to literature procedures6c,8 or modifications thereof.18 Borinic acid
7 was prepared by controlled hydrolysis of B(C6F5)3 as described
in the literature.34
All solvents and reagents were purchased from commercial
sources and purified as required. Hexane and toluene used in
polymerization experiments were predried by passage through
35
columns of A2 alumina and Q5 deoxo catalyst under N2 or
Procedure B: Controlled Addition of Diborane. A three-neck,
100 mL round-bottom flask, calibrated with a mark corresponding
to 10.0 mL and equipped with a central 24/40 and two 14/20 side
necks, was fitted with a 14/20 stopper, a 14/20 gas-inlet adapter,
and a 24/40 outer/inner air-free adapter with in-line PTFE stopcock,
after which it was connected to a vacuum line. Then, 10.0 mL of
isobutene was condensed into the flask, and the flask and adapter
were removed from the vacuum line. The flask was then connected
to a double manifold, after which the 24/40 outer/inner air-free
adapter was replaced with a stir bearing/shaft/blade assembly and
the remaining 14/20 neck fitted with a septum under a flow of N2.
The flask was then maintained at -78 °C (through use of a acetone/
dry ice bath) and kept under slight positive N2 pressure. The flask
contents were then stirred under N2 at -78 °C at 500 ( 10 rpm,
while 5.0 mL of hexane and 15.0 mL of aqueous HBF4 were added
sequentially via syringe. Next, 1.0 mL of diborane or diborole stock
(1.19 × 10-5 mol) was then added over periods varying between
30 s and 10 min using a syringe pump at a controlled rate.
Polymerization was allowed to continue for a full hour before
quenching with 2-propanol and warming the contents to room
temperature. The organic phase was diluted with hexane and
transferred to a separatory funnel. The aqueous phase was with-
drawn, and the organic phase washed with water and then dried
over MgSO4. The mixture was filtered, washing with additional
hexane before isolating polymer by removal of volatiles in Vacuo
using a rotary evaporator until constant weight was obtained.
Samples were transferred to vials and dried overnight in a vacuum
oven at 90 °C and 30 in. Hg prior to GPC analysis.
distilled from Na metal and benzophenone under N2. Dichlo-
romethane was dried and distilled from P2O5 under N2. Further
purification of dichloromethane or hexane was achieved by stirring
with tri-n-octylaluminum under N2 followed by vacuum transfer
just prior to polymerization as explained below. Stock solutions of
diborane 1 and diborole 2 were prepared in anhydrous toluene or
hexane and stored in an Innovative Technologies glovebox at -30
°C prior to use.
Isobutene was grade 2.0 (99.0%) provided by Praxair and was
purified by passage through a column of BASF R3-11 deoxo
catalyst and activated molecular sieves 4 Å; initial exposure of the
latter to isobutene must be accomplished slowly, and with cooling
of the column if needed, to avoid significant exotherm and resulting
cracking of isobutene. Isobutene was further purified by vacuum
transfer from tri-n-octylaluminum just prior to polymerization as
explained below.
Routine NMR spectra were recorded on Varian Mercury 300
1
and Inova 400 MHz instruments. H and 19F NMR spectra are
referenced to residual protonated solvent and 2,3,5,6-tetrafluoroxy-
lene (δ -145.7 vs CFCl3), respectively. GPC analyses were
performed by a technician affiliated with the Institute of Polymer
Science and using a Waters GPC system equipped with a set of
five, linear Styragel columns, eluting with THF at 1.002 mL/min
at 35 °C. Polymer was detected using a Waters differential refractive
index, Viscotek 110 differential viscometer, and Wyatt Technology
Dawn EOS, 18-angle light scattering detectors. Data were analyzed
using the Astra 4.0 software provided by Wyatt Technology using
a dn/dc value for poly(isobutene) of 0.108 mL g-1 36 and assuming
100% mass recovery. This procedure was found to be reliable, and
the results were periodically checked through blind analyses of
poly(isobutene) standards provided by American Polymer Standards
Corp.
Procedure C: Polymerization of Isobutene in Hexane
Solution. A dry 250 mL round-bottom 24/40 single-neck flask was
charged with 11.9 g (18 mL) of hexane, 0.82 g (1 mL) of tri-n-
octylaluminum, and a magnetic stir bar inside a glovebox. This
was then sealed with an air-free style adapter equipped with an
in-line PTFE vacuum stopcock and the apparatus connected to a
vacuum line. The mixture of solvent and drying agent was then
degassed by application of three sequential freeze-pump-thaw
cycles and subsequently charged with 5.50 mL of isobutene via
vacuum transfer. Next, a dry 100 mL, two-neck, round-bottom flask
previously silanized with Me2SiCl2 and containing a magnetic stir
bar and glass stopper was connected to the vacuum line, and the
entire assembly was flame-dried under vacuum, refilled with N2,
and then fitted with a septum. The solvent/monomer/drying agent
mixture was then stirred at -78 °C for 30 min before vacuum
Polymerization of Isobutene in Aqueous Suspension. Two
different procedures were employed, each involving the addition
of a toluene stock solution of diborane 1 and diborole 2 to isobutene
monomer, either neat or diluted with hexane:
(34) Schottek, J.; Becker, P.; Kullmer, I. (Targor GMBH) PCT Int. Appl.
WO 2000037476, 2000.
(35) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–20.
(36) Puskas, J. E.; Chen, Y.; Kulbaba, K.; Kaszas, G. J. Polym. Sci.
Part A: Polym. Chem. 2006, 44, 1777–83.

260 Organometallics, Vol. 28, No. 1, 2009
Lewis et al.
Table 7
Mw (103 kg mol-1
of a toluene stock solution containing borinic acid 7 (0.01 M) and
B(C6F5)3 (0.01 M) was added over 1 min at -78 °C. No exotherm
or change in appearance of the mixture was noted. After 1 h the
mixture was quenched with 2-propanol and warmed to room
temperature; vigorous gas evolution was observed to occur during
this process. Extraction of the aqueous phase with hexane did not
provide any polymeric material.
entry
DtBMP (mM)
)
PDI
yield (%)
1
2
3
4
5
6
7
8
20
20
15
15
5
5
2
2
2
0
0
605
753
552
370
287
293
302
195
285
2.3
4.9
3.6
4.2
2.2
2.1
2.5
3.2
2.4
0.50
0.47
1.44
0.98
6.82
6.02
5.76
100
100
Attempted Polymerization Using B(C6F5)3. Procedure C was
followed using 5.50 mL of isobutene and 18.0 mL of hexane at
-78 °C. One milliliter of a toluene stock solution of B(C6F5)3 (0.02
M) was added rapidly at -78 °C. No exotherm or change in
appearance of the mixture was noted. After 1 h the mixture was
quenched with 2-propanol and warmed to room temperature;
vigorous gas evolution was observed to occur during this process.
Evaporation of the hexane did not provide any polymeric material.
Attempted Polymerization Using Borinic Acid 17. Procedure
B was followed using 10.0 mL of isobutene, diluted with 5.0 mL
of hexane and 15.0 mL of aqueous HBF4 at -78 °C. One milliliter
of a dichloromethane stock solution containing borinic acid 17 (0.01
M) was added over 1 min at -78 °C. No exotherm or change in
appearance of the mixture was noted. After 1 h the mixture was
quenched with 2-propanol and warmed to room temperature;
vigorous gas evolution was observed to occur during this process.
Extraction of the aqueous phase with hexane did not provide any
polymeric material.
9
10
11
transferring volatile material to the 100 mL flask. The contents of
the 100 mL flask were then warmed to -78 °C and stirred for 15
min under a blanket of N2 prior to injection of 750, 75, or 7.5 µL
of a 0.065 M solution of diborane 1 in toluene.
A rapid increase in viscosity occurred immediately upon addition,
particularly at higher concentrations of diborane (Caution: strongly
exothermic), and stirring often ceased entirely while the reaction
was allowed to proceed for 1 h at -78 °C under a blanket of
nitrogen. The viscous solution was diluted with hexane and washed
with a mixture of water and MeOH. The resultant polymer solutions
were dried over Na2SO4, filtered, and concentrated to dryness in
Vacuo. Polymer samples were dried in a vacuum oven at 30 in. Hg
at 90 °C for 24 h prior to analysis.
Attempted Polymerization Using a Mixture Borinic Ester
5 and Borane 4. Procedure C was followed using 5.50 mL of
isobutene diluted with 18.0 mL of hexane. A solution of diborane
1 (84 mg) in toluene (10 mL) was treated with ∼2.0 equiv of MeOH
(1.0 µL) at -78 °C. The solution was warmed to room temperature;
a 19F NMR spectrum of this mixture revealed complete conversion
of diborane 1 to a mixture of borinic ester 5, borane 4, and its
MeOH adduct 6. To the mixture of isobutene and hexane at -78
°C was added 2.0 mL of this solution via syringe so as to give a
final concentration of about 2 mM expressed as diborane 1. No
exotherm or change in appearance of the mixture was noted. After
1 h the mixture was quenched with 2-propanol and warmed to room
temperature; vigorous gas evolution was observed to occur during
this process. Evaporation of the hexane did not provide any
polymeric material.
Part II. An Innovative Technologies or Vacuum Atmospheres
argon-filled glovebox was used for the storage and manipulation
of oxygen- and moisture-sensitive compounds. Thermally unstable
compounds were stored in a -35 °C freezer installed on the
respective glovebox. All reactions were performed on a double
manifold, high-vacuum line using modified Schlenck techniques.37
Residual oxygen and moisture were removed from Ar by passage
through an Oxisorb-W scrubber from Matheson Gas Products.
Commonly utilized specialty glassware included a swivel frit
assembly, thick-walled (5 mm) Carius tubes, and round-bottom
flasks with in-line adaptors all equipped with Teflon stopcocks. All
glassware was stored in a 110 °C oven for a minimum of 12 h
before immediate transfer into the glovebox antechamber or
assembled on the vacuum line and evacuated while hot. Unless
otherwise noted, the introduction of solvent in all manipulations
was via vacuum transfer with condensation at -78 °C. Liquid
nitrogen (-196 °C), dry ice/acetone (-78 °C), dry ice/acetonitrile
(-45 °C), and water/ice (0 °C) baths were used for cooling
receiving flasks and to maintain low-temperature conditions.
Stopping Experiments with 2,6-Di-tert-butyl-4-methylpyri-
dine (DtBMP). Stopping experiments using DtBMP were con-
ducted in the same fashion as in Procedure C but involved the
addition of 1.00, 0.75, 0.50, 0.25, or 0.10 mL of a stock solution
of DtBMP (0.5 M in hexane) prior to the addition of diborane 1
(0.065 M in toluene, final concentration ) 2.0 mM). Polymerization
was allowed to proceed for 1 h at -78 °C before quenching with
1 mL of methanol. All volatiles were removed and the residue
washed with methanol prior to being taken up in hexanes. The
resultant polymer solutions were filtered, concentrated in Vacuo,
and dried in a vacuum oven at 30 in. Hg at 90 °C for 24 h prior to
analysis by GPC. The results are summarized in Table 7.
Impurity Levels in Hydrocarbon Solution. A stock solution
of sodium benzophenone ketyl in xylenes/tetraglyme was
prepared as follows: A mixture of xylenes (100 mL) and
tetraglyme (20 mL) was stirred over molten Na metal under N2
in a glovebox for several hours. The mixture was cooled, filtered,
and stirred over fresh Na metal, and this process was repeated, until
the molten Na was shiny and no further decomposition of tetraglyme
(which produces a dark brown residue) was noted. Benzophenone
was then gradually added to the cooled suspension until the deep
blue purple color of the ketyl persisted, and the entire mixture,
containing excess Na metal, was stored in a Schlenk storage flask
with integral stopcock in a glovebox. Aliquots were removed by
syringe and added, outside the glovebox, to an known excess of
degassed, standardized aqueous HCl. The excess acid was titrated
to a phenolphthalein end-point using a standard aqueous NaOH
solution. Stock solutions prepared in this manner maintain their
titer for many months provided excess Na is present; a typical
concentration is 0.02 M.
A solution of IB and hexane, prepared and purified as described
in Procedure C, was warmed to room temperature under N2 until
most of the IB had evaporated. The remaining solution was titrated
to a persistent, pale blue end-point using the stock ketyl solution.
A typical result was 2.5 ( 0.3 × 10-5 M based on a stoichiometry
of 2 mol of ketyl consuming 1 mol of water and the original volume
of the hexane/IB solution.
Solvents were purchased from Aldrich or Cambridge Isotopes
and dried and deoxygenated before use by the following procedures.
Toluene, hexanes, and tetrahydrofuran (THF) solvents were dried
and purified using the Grubbs/Dow purification system35 and stored
Control Experiments in Aqueous Suspension or Hydrocar-
bon Solution.
Attempted Polymerization Using Borinic Acid
7 and
B(C6F5)3. Procedure B was followed using 15.0 mL of undiluted
isobutene and 15.0 mL of aqueous HBF4 at -78 °C. One milliliter
(37) Burger, B. J.; Bercaw, J. E. Experimental Organometallic Chem-
istry; American Chemical Society: Washington, D.C., 1987.

Isobutene Polymerization Using Chelating Diboranes
Organometallics, Vol. 28, No. 1, 2009 261
in evacuated 500 mL thick-walled flasks over titanocene38 (toluene
and hexanes) or sodium/benzophenone ketyl (THF). Benzene, d6-
benzene, and d8-toluene, were dried and stored over sodium/
benzophenone ketyl in thick-walled Schlenk tubes under vacuum.
Diethyl ether and methylene chloride were predried over LiAlH4
and CaH2, respectively, and subsequently stored over sodium/
benzophenone ketyl and CaH2, respectively. Methylene chloride-
d2 was predried over 4Å molecular sieves and stored over CaH2.
All of these solvents and reagents were distilled directly into
reaction vessels or separate predried Schlenk storage vessels prior
to use. Chloroform, chloroform-d, and D2O were used as received.
Routine NMR spectra were recorded on a Bruker AC-200 MHz,
AMX-300 MHz (19F -282.4 MHz), Bruker AC-300 MHz, or
(MeOH)); 47% 4, δ -132.3 (4F, o-B(C6F5)2), 148.5 (2F,
p-B(C6F5)2), 160.6 (4F, m-B(C6F5)2).
Spectroscopic Studies of the Addition of H2O to C6F4-1,2-
[B(C6F5)2]2. In a glovebox solid C6F4-1,2-[B(C6F5)2]2, 1 was loaded
into a 5 mm NMR tube and dissolved in d8-toluene (0.4 mL). The
sample was capped with a rubber septum, removed from the
glovebox and cooled to -78 °C in a dry ice/acetone bath. Varying
equivalents of degassed water (0.5 - 8.0 equiv. or a large excess)
were injected into the NMR tube via gastight syringe at low
temperature. The NMR tube was placed in a spectrometer, cooled
to -60 °C and the reaction was monitored up to room temperature.
A complete listing of spectral data is provided as Supporting
Information.
1
Avance DRX-400 [equipped with a gradient H/13C probe] spec-
trometer. All 2D NMR experiments [19F,19F-COSY, 19F,19F-
Spectroscopic Studies of the Addition of MeOH to C6F4-1,2-
[B(C12F8)]2. In a glovebox solid C6F4-1,2-[B(C12F8)]2, 2, was loaded
into a 5 mm NMR tube and dissolved in d8-toluene (0.4 mL). The
sample was capped with a rubber septum, removed from the
glovebox, and cooled to -78 °C in a dry ice/acetone bath. Varying
equivalents of dry and degassed methanol (0.713 M in d8-toluene)
were injected into the NMR tube via gastight syringe at low
temperature. The NMR tube was placed in a spectrometer cooled
to -60 °C, and the reaction was monitored up to room temperature.
A complete listing of spectral data is provided as Supporting
Information.
1
1
1
NOESY, H,13C-HMQC, H,1H-NOESY, H,19F-COSY, or NOE-
SY] were performed using a Bruker Avance AMX-300 MHz or
DRX-400 MHz spectrometer. All 1H NMR spectra were referenced
to SiMe4 through the residual 1H resonance(s) of the solvent: C6D6
(δ ) 7.15 ppm), d8-toluene (δ ) 2.09, 6.98, 7.02, and 7.09 ppm),
d8-THF (δ ) 1.73 and 3.58 ppm), or CD2Cl2 (δ ) 5.32 ppm).
13C{1H} NMR spectra are also referenced relative to SiMe4 through
the resonance(s) of the deuterated solvent: C6D6 (δ ) 128.0 ppm),
d8-toluene (δ ) 20.4, 125.2, 128.0, 128.9, and 137.5 ppm), d8-
THF (δ ) 25.4 and 67.6 ppm), or CD2Cl2 (δ ) 54.0 ppm). 19F
NMR spectra were referenced externally to C6F6: δ ) -163.0.
Temperature calibration for NMR experiments was achieved by
monitoring the 1H NMR spectrum of pure methanol.39 For all air-
and/or moisture-sensitive compounds and reactions, NMR samples
were prepared in the glovebox and the NMR tubes were capped
with rubber septa. Unless otherwise stated, all spectroscopic data
are reported at room temperature (298 K).
Spectroscopic Studies of the Addition of H2O to C6F4-1,2-
[B(C12F8)]2. In a glovebox solid C6F4-1,2-[B(C12F8)]2, 2, was loaded
into a 5 mm NMR tube and dissolved in d8-toluene (0.4 mL). The
sample was capped with a rubber septum, removed from the
glovebox, and cooled to -78 °C in a dry ice/acetone bath. Varying
equivalents of water were injected into the NMR tube via gastight
syringe at low temperature. The NMR tube was placed in a
spectrometer cooled to -60 °C, and the reaction was monitored
up to room temperature. A complete listing of spectral data is
provided as Supporting Information.
Elemental analyses were performed on a Control Equipment
Corporation 440 elemental analyzer by Mrs. Dorothy Fox, Mrs.
Roxanna Smith, or Mrs. Olivera Blagojevic at the University of
Calgary.
Generation of MeOH Adduct 10. Diborole 2 (15 mg, 20 µmol)
was dissolved in CD2Cl2 (0.5 mL). The solution was placed in an
NMR tube, which was sealed with a rubber septum, and the tube
was cooled to -78 °C. A stock solution of MeOH (0.394 M in
CD2Cl2, 50 µL, 20 µmol) was injected through the septum. The
tube was briefly shaken and subsequently introduced in the
precooled (-80 °C) NMR probe. Spectra (1H, 19F) were recorded
from -80 to 0 °C with 10 deg intervals
Spectroscopic Studies of the Addition of MeOH to C6F4-1,2-
[B(C6F5)2]2. In a glovebox, C6F4-1,2-[B(C6F5)2]2, 1 (10 mg, 0.012
mmol), was loaded into a 5 mm NMR tube and dissolved in d8-
toluene (0.4 mL) or CD2Cl2 (0.4 mL). The sample was capped with
a rubber septum, removed from the glovebox, and cooled to -78
°C in a dry ice/acetone bath. Varying equivalents (0.5-10.0 equiv)
of dry and degassed methanol (0.72 M in d8-toluene or CD2Cl2)
were injected into the NMR tube via gastight syringe at low
temperature. The NMR tube was placed in a spectrometer, cooled
to -60 °C, and the reaction was monitored up to room temperature.
A complete listing of spectral data at various T and stoichiometries
is provided as Supporting Information.
(C6F4-1,2-[B(C6F5)2]2 + 8.0 MeOH) + 3 C6F4-1,2-[B(C6F5)2]2.
A solution of 3 was prepared by the addition of MeOH (35 µL,
1.36 mM in d8-toluene) via gastight syringe to a capped 5 mm NMR
tube charged with C6F4-1,2-[B(C6F5)2]2, 1 (5 mg, 0.006 mmol),
dissolved in d8-toluene (0.2 mL) at room temperature. To this
solution was added an additional 3 equiv of C6F4-1,2-[B(C6F5)2]2,
1 (15 mg, 0.018 mmol), dissolved in d8-toluene (0.3 mL). 1H NMR
(d8-toluene, 298 K): δ 6.58 (m, 1H, -C6F4H), 3.65 (s, ∼0.3H,
µ-OMe), 3.37 (br s, 2H, (C6F5)2BOMe), 2.55 (br s, 4H, MeOH).
19F NMR (d8-toluene, 298 K): 6% 3, δ -131.7 (8F, o-B(C6F5)2),
-136.5 (2F, -C6F4), -158.4 (4F, p-B(C6F5)2), -164.2 (2F, -C6F4),
-164.8 (8F, m-B(C6F5)2); 47% 6, δ -133.4 (4F, o-B(C6F5)2-
(MeOH)), -133.5 (1F,-C6F4(MeOH)), -139.6 (1F, -C6F4(Me-
OH)), -154.5 (2F, p-B(C6F5)2(MeOH)), -155.6 (1F, -C6F4-
(MeOH)), -156.8 (2F, -C6F4(MeOH)), -161.9 (4F, m-B(C6F5)2-
1
NMR spectroscopic data for 10: H NMR (300 MHz, CD2Cl2,
193 K): δ 6.58 (dq, J ) 19 Hz, 4.0 Hz, 3.58, 1H, MeOH), (d, J )
3.5 Hz, 3H, MeOH). 19F NMR (282 MHz, CD2Cl2, 193 K): δ
-124.6 (m, 2F, BC12F8), -130.2 (dd, J ) 24 Hz, 12 Hz, 1F, C6F4),
-130.6 (m, 2F, BC12F8), -131.0 (s, br, 2F, BC12F8), -132.6 (s,
br, 2F, BC12F8), -133.5 (m, br, 3F, BC12F8+, C6F4), -140.8 (m,
2F, BC12F8), -152.4 (t, J ) 20 Hz, 2F, BC12F8), -153.4 (t, J )
22 Hz, 2F, BC12F8), -153.6 (t, J ) 22 Hz, 1F, C6F4), -155.7 (t,
J ) 22 Hz, 1F, C6F4). 2D 1H/19F correlation: 6.58, -133.5. Selective
decoupling of 19F at δ 133.5 gave δ 6.58 (q, J ) 4.0 Hz).
1H NMR (300 MHz, PhMe-d8, 213 K): δ 4.04 (br s, 1H, MeOH),
2.01 (s, 3H, MeOH). 19F NMR (282 MHz, PhMe-d8, 213 K): δ
-124.6 (2F, BC12F8), -129.2 (2F, BC12F8), -129.2 (2F, BC12F8),
-130.6 (1F, C6F4), -130.8 (2F, BC12F8), -132.0 (1F, C6F4),
-140.1 (2F, BC12F8), -150.2 (2F, BC12F8), -151.7 (2F, BC12F8),
-152.2 (1F, C6F4), -153.1 (2F, BC12F8), -154.4 (1F, C6F4).
X-ray Crystallographic Characterization of Borinic Ester
11.9 An orange block crystal of dimensions 0.20 × 0.16 × 0.12
mm was coated with Paratone 8277 oil (Exxon) and mounted on a
glass fiber. All measurements were made on a Nonius KappaCCD
diffractometer with graphite-monochromated Mo KR radiation.
Details of crystal data and structure refinement are provided as
Supporting Information. The data were collected using ω and ꢀ
(38) Marvich, R. H.; Brintzinger, H.-H. J. Am. Chem. Soc. 1971, 93,
2046.
(39) Ammann, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982,
46.

262 Organometallics, Vol. 28, No. 1, 2009
Lewis et al.
scans.40 The data were corrected for Lorentz and polarization effects
and for absorption using a multiscan method.41 Since the crystal
did not show any sign of decay during data collection, a decay
correction was deemed unnecessary.
Supporting Information. The data were collected using ω and ꢀ
scans.40 The data were corrected for Lorentz and polarization effects
and for absorption using a multiscan method.41
The structure was solved by the direct methods42 and expanded
using Fourier techniques.43 The non-hydrogen atoms were refined
anisotropically. Of the six molecules of water of hydration, three
were disordered over two sites each with site occupancy factors
for O6, O7, and O8 of 0.789(3), 0.848(3), and 0.813(3), respectively,
while the smaller fractions O6′, O7′, and O8′ had occupancy factors
of 0.211(3), 0.152(3), and 0.187(3), respectively. The same Uij
values were assigned to the disordered O atoms (using EADP within
SHELXL), and H atoms for all water molecules were constrained
at geometrically idealized position with O---H distances ) 0.82
Å. Other hydrogen atoms were included at geometrically idealized
positions and were not refined. The final cycle of full-matrix least-
squares refinement using SHELXL9744 converged with unweighted
and weighted agreement factors, R ) 0.0462 and wR ) 0.1303
(all data), respectively, and goodness of fit, S ) 1.024. The
weighting scheme was based on counting statistics, and the final
difference Fourier map was essentially featureless. Figure 2 was
plotted with the aid of PLATON.45
The structure was solved by the direct methods42 and expanded
using Fourier techniques.43 The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were included at geometrically
idealized positions and were not refined. The final cycle of full-
matrix least-squares refinement using SHELXL9744 converged with
unweighted and weighted agreement factors, R ) 0.0457 and wR
) 0.1199 (all data), respectively, and goodness of fit, S ) 1.033.
The weighting scheme was based on counting statistics, and the
final difference Fourier map was essentially featureless. Figure 1
was plotted with the aid of PLATON.45
X-ray Crystallographic Characterization of Compound 20.
A colorless needle of dimensions 0.70 × 0.20 × 0.04 mm was
coated with Paratone 8277 oil (Exxon) and mounted on a glass
fiber. All measurements were made on a Nonius KappaCCD
diffractometer with graphite-monochromated Mo KR radiation.
Details of crystal data and structure refinement are provided as
Supporting Information. The data were collected using ω and ꢀ
scans.40 The data were corrected for Lorentz and polarization effects
and for absorption using a multiscan method.41 Since the crystal
did not show any sign of decay during data collection, a decay
correction was deemed unnecessary.
Reaction of 2 with Excess Water. Diborole 2 (0.25 g, 0.33
mmol) was dissolved in water (2 mL), and the water was
subsequently pumped off. Residual water was removed by redis-
solution in CH2Cl2 and evaporation of the volatiles in Vacuo to
afford an off-white powder. Recrystallization from CH2Cl2 gave
white crystals (135 mg, 50%). 1H NMR (300 MHz, toluene-d8, 353
K): δ 6.35 (m, 2H, ArF-H), 3.46 (s, br, 4H, H2O) indicated the
presence of a dihydrate of 16. 19F NMR (282 MHz, toluene-d8,
353 K): δ -129.3 (2F), -131.3(2F), -137.3(m, 2F), -138.2,
-139.4 (2F), -151.0(2F), -152.4(2F), -154.1(2F), -154.3(2F),
-155.6. Spectroscopic data for anhydrous 16: 1H NMR (d8-toluene,
298 K): δ 6.54 (m, 2H, C6F4HC6F4B). 19F NMR (d8-toluene, 298
K): δ -122.9 (2F, -C6F4), -129.2 (2F, C6F4HC6F4B), -137.4 (2F,
C6F4HC6F4B), -137.7 (2F, C6F4HC6F4B), -139.4 (2F, C6F4HC6-
F4B), -141.8 (2F, -C6F4), -146.8 (2F, C6F4HC6F4B), -151.6 (2F,
C6F4HC6F4B),-151.9 (2F, C6F4HC6F4B), -153.6 (2F, C6F4HC6-
F4B). 11B{1H} NMR (d8-toluene, 298 K): δ 42.1.
The structure was solved by the direct methods42 and expanded
using Fourier techniques.43 The non-hydrogen atoms were refined
anisotropically. The structure contains disordered dichloromethane
and methanol molecules of solvation with unequal site occupancy
factors of 0.870(5) and 0.716(6), respectively, for the major
components. The EADP command was used to refine the Uij’s as
constrained parameters of the disordered atoms. The C-O distance
was fixed using the command DFIX in the methanol molecule.
Hydrogen atoms were included at geometrically idealized positions
and were not refined. The final cycle of full-matrix least-squares
refinement using SHELXL9744 converged with unweighted and
weighted agreement factors, R ) 0.0559 and wR ) 0.1345 (I g
2σ(I)), respectively, and goodness of fit, S ) 1.057. The weighting
scheme was based on counting statistics, and the final difference
Fourier map was essentially featureless. Figure 3 was plotted with
the aid of PLATON.45
Synthesis of 1,2-C6F4(9-BC12F8)(B(H2O)(C12HF8)(µ-OH) ·
6H2O (17). Diborole 2 (130 mg, 0.17 mmol) was dissolved in water
(2 mL). An aliquot of the solution was dissolved in D2O for NMR
analysis. 19F NMR data for 13 (282 MHz, D2O, 298 K): δ -134.4
(4F, BC12F8), -136.3 (4F, BC12F8), -139.8 (2F, C6F4), -157.4
(4F, BC12F8), -158.2 (4F, BC12F8), -161.4 (2F, C6F4). After
evaporation of ca. 25% of the water in air, colorless single crystals
of compound 17 were deposited. Yield: 69 mg (51%). Anal. Calcd
for C30H4B2F20O2 · 6H2O: C 39.77, H 1.78. Found: C 40.60, H 1.55.
X-ray Crystallographic Characterization of Borinic Acid
17. A colorless prismatic crystal of dimensions 0.20 × 0.18 × 0.12
mm was coated with Paratone 8277 oil (Exxon) and mounted on a
glass fiber. All measurements were made on a Nonius KappaCCD
diffractometer with graphite-monochromated Mo KR radiation.
Details of crystal data and structure refinement are provided as
Crystals of a THF adduct of compound 16 were isolated on
attempted recrystalization of borinic acid 15 from a mixture of THF
and hexane over several days. Crystallographic data for 16 · THF
are provided as Supporting Information.
Generation of Ion Pair 19 from 2 and MeOH in the
Presence of DPE. Diborole (2) (15 mg, 20 µmol) was dissolved
in dry CD2Cl2 (0.4 mL) and placed in an NMR tube covered with
a rubber septum. Dry 1,1-diphenylethene (DPE, freshly distilled
from CaH2) (50 µL, 0.397 M in CD2Cl2) was injected through the
septum. Upon addition, the solution turned red locally, but after
mixing, the solution was yellow again. 1H NMR spectroscopy
indicated a 20-30:70-80 mixture of 1,3,3-triphenyl-3-methylindan
(18) and DPE. The 19F NMR spectrum showed only 2 with <1%
1
1
degradation. H NMR spectroscopic data for indan 18: H NMR
(300 MHz, CD2Cl2, 298 K): δ 7.33-7.00 (m, 19H, Ph overlap with
Ph of Ph2CdCH2), 3.39 (d, J ) 14 Hz, 1H), 3.12 (d, J ) 14 Hz,
1H), 1.27 (s, 3H, Me), for Ph2CdCH2: δ 7.33-7.00 (10H, Ph,
overlap with indan Ph), 5.47 (s, 2H, CH2).
To generate 19, the NMR tube described above was immediately
cooled to -78 °C and MeOH (50 µL, 0.397 M in CD2Cl2) was
added. The sample was introduced into a precooled (-80 °C) NMR
probe. Spectra (1H and 19F) were recorded at -80, -60, -40, -20,
and 0 °C and room temperature.
(40) Hooft, R. COLLECT; Nonius BV, Delft, The Netherlands, 1998.
(41) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(42) SIR92: Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi,
A. J. Appl. Crystallogr. 1993, 26, 343.
(43) Beurskens, P. T.; Admiraal, G., Beurskens, G.; Bosman, W. P.; de
Gelder, R.; Israel, R.; Smits, J. M. M., The DIRDIF-94 program system,
Technical Report of the Crystallography Laboratory; University of Nijmegen:
The Netherlands,1994.
(44) Sheldrick, G. M., SHELXL97; University of Go¨ttingen: Germany,
1997.
(45) (a) Spek, A. L. PLATON; Utrecht University, Padualaan 8, 3584
CH Utrecht, The Netherlands, 1980-2008. (b) Spek, A. L. J. Appl.
Crystallogr. 2003, 36, 7–13.
At -80 °C ion pair [Ph2CMe][2-µ-OMe] (19) and methanol
adduct 10 were observed in ∼90:10 ratio (by 1H NMR integration).
NMR spectroscopic data for 19: 1H NMR (300 MHz, CD2Cl2, 213
K): δ 8.31 (t, J ) 7.3 Hz, 2H, p-H), 7.98 (d, J ) 7.5 Hz, 4H, o-H),
7.87 (t, J ) 7.8 Hz, 4H, m-H), 3.67 (s, 3H, Me), 2.68 (s, 3H,
µ-OMe). 19F NMR (282 MHz, CD2Cl2, 213 K): δ -132.0 (4F,

Isobutene Polymerization Using Chelating Diboranes
Organometallics, Vol. 28, No. 1, 2009 263
C12F8), -136.0 (4F, C12F8), -138.8 (2F, C6F4), -156.8 (4F, C12F8),
-157.8 (4F, C12F8), -161.3 (2F, C6F4). Formation of 1,3,3-
triphenyl-3-methylindan (18) was confirmed after the experiment
by GC-MS (m/z ) 360). Warming the sample to -40 °C and above
resulted in conversion of 19 (and 10) to 11, which was the major
boron-containing product at the end of the experiment.
Research Council of Canada for generous financial support
of this project.
Supporting Information Available: Experimental procedures
for the synthesis of borinic ester 4 and borane 5, NMR spectroscopic
data for compounds 6-16 formed in situ from diboranes 1 or 2
and water or MeOH, Mayo plot of MW-1 vs [H2O]:[IB] using data
from ref 4f, equation and data used to estimate the T dependence
of PIB MW at various [H2O] as depicted in Figure 5, and
crystallographic information files for compounds 8, 11, 17, 20, and
16 · THF. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. S.P.L., J.C., and S.C. acknowledge The
University of Akron and Goodyear Tire and Rubber Co. for
financial support of this work. We also wish to acknowledge
Prof. Joseph P. Kennedy, who suggested that aqueous
suspension polymerization of isobutene would be a feasible
and useful process. T.J.J.S., L.D.H., M.E.P., and W.E.P. wish
to acknowledge the Natural Sciences and Engineering
OM8007629