Synthesis, characterization, and properties of the polyphosphinoboranes [RPH-BH2]n (R = Ph, iBu, p-nBuC6H4, p-dodecylC6H4): Inorganic Polymers with a phosphorus-boron backbone

Article abstract of DOI:10.1021/ma021447q

The polyphosphinoboranes [PhPH-BH₂]_n (1), [iBuPH-BH2]n (2), [(p-nBuCsH4)PH-BH2]_nn (3), and [(p-dodecylC₆H₄)PH-BH₂]_n (4) were prepared from the corresponding phosphine-borane adducts RPH₂·BH₃ (R = Ph, iBu.p-nBuC₆H₄, p-dodecylC₆H₄) via a rhodium-catalyzed dehydrocoupling procedure at elevated temperatures (ca. 90-130°C). Samples of polymers 1 and 2 and the new materials 3 and 4 were characterized by multinuclear NMR spectroscopy, and the molecular weights were determined by light scattering methods in THF or CH₂Cl₂ solutions. The absolute weight-average molecular weight of 2 was determined by static light scattering and found to be M_w = 13 100, and values of M_w of ca. 20 000 were estimated for the sample of polymers 1 and the new material 3 using dynamic light scattering (DLS). The molecular weights of polymers 3 and 4 were also analyzed by gel permeation chromatography using polystyrene standards, and values up to M_w = ca. 80 000, M_n = ca. 10 000 were determined for 3 and M_w = ca. 168 000, M_n = ca. 12 000 for 4. The chemical stability of 1 in THF toward HNEt₂ or nBu₃P was demonstrated using NMR spectroscopy and DLS analysis, which indicated that no significant polymer degradation occurred. WAXS analysis of 1 and 3 showed that the polymers are amorphous. The glass transition temperatures (T_g) of polymers 2, 3, and 4 were analyzed by DSC and were detected at ca. 5, 8, and -1°C, respectively. TGA analysis on 1-3 revealed T_5% values (temperature for which 5% or the weight is lost) of 240°C for 1 and ca. 150-160°C for 2 and 3. After heating to 1000°C, ceramic yields in the range of 35-80% were obtained. The high ceramic yield for 1 (75-80%) indicates that this material is of interest as a pyrolytic precursor to boron phosphide-based solid-state materials.

Full text of DOI:10.1021/ma021447q

Macromolecules 2003, 36, 291-297
291
Articles
Synthesis, Characterization, and Properties of the
Polyphosphinoboranes [RPH-BH2]n (R ) Ph, iBu, p-nBuC6H4,
p-dodecylC6H4): Inorganic Polymers with a Phosphorus-Boron
Backbone
Hen d r ik Dor n , J ose´ M. Rod ezn o, Bjo1r n Br u n n h o1fer , Er ic Riva r d ,
J a son A. Ma ssey, a n d Ia n Ma n n er s*
Department of Chemistry, University of Toronto, 80 St. George Street,
Toronto, Ontario M5S 3H6, Canada
Received September 9, 2002; Revised Manuscript Received November 4, 2002
ABSTRACT: The polyphosphinoboranes [PhPH-BH2]n (1), [iBuPH-BH2]n (2), [(p-nBuC6H4)PH-BH2]n
(3), and [(p-dodecylC6H4)PH-BH2]n (4) were prepared from the corresponding phosphine-borane adducts
RPH2‚BH3 (R ) Ph, iBu, p-nBuC6H4, p-dodecylC6H4) via a rhodium-catalyzed dehydrocoupling procedure
at elevated temperatures (ca. 90-130 °C). Samples of polymers 1 and 2 and the new materials 3 and 4
were characterized by multinuclear NMR spectroscopy, and the molecular weights were determined by
light scattering methods in THF or CH2Cl2 solutions. The absolute weight-average molecular weight of
2 was determined by static light scattering and found to be Mw ) 13 100, and values of Mw of ca. 20 000
were estimated for the sample of polymers 1 and the new material 3 using dynamic light scattering
(DLS). The molecular weights of polymers 3 and 4 were also analyzed by gel permeation chromatography
using polystyrene standards, and values up to Mw ) ca. 80 000, Mn ) ca. 10 000 were determined for 3
and Mw ) ca. 168 000, Mn ) ca. 12 000 for 4. The chemical stability of 1 in THF toward HNEt2 or nBu3P
was demonstrated using NMR spectroscopy and DLS analysis, which indicated that no significant polymer
degradation occurred. WAXS analysis of 1 and 3 showed that the polymers are amorphous. The glass
transition temperatures (Tg) of polymers 2, 3, and 4 were analyzed by DSC and were detected at ca. 5,
8, and -1 °C, respectively. TGA analysis on 1-3 revealed T5% values (temperature for which 5% or the
weight is lost) of 240 °C for 1 and ca. 150-160 °C for 2 and 3. After heating to 1000 °C, ceramic yields
in the range of 35-80% were obtained. The high ceramic yield for 1 (75-80%) indicates that this material
is of interest as a pyrolytic precursor to boron phosphide-based solid-state materials.
In tr od u ction
Si-O14 skeletons have been synthesized by this meth-
odology.15
Macromolecules with backbones based on main group
elements other than carbon are of considerable interest
for accessing materials with unusual properties and
significant applications.1-10 Compared to C-C bonds,
bonds between other main group elements can be
stronger and more oxidatively and thermally stable or
can provide additional conformational flexibility (e.g.,
Si-O bonds in polysiloxanes), interesting electronic
effects (e.g., σ-conjugation in polysilanes and heavier
group 14 analogues), versatile reactivity (e.g., the
substitution of chlorine in polydichlorophosphazenes),
or intriguing possibilities for the development of self-
assembled, supramolecular materials.11 However, the
development of efficient synthetic procedures for the
formation of bonds between main group elements is of
critical importance for the construction of such inorganic
polymer chains and also for the general development
of macromolecular main group chemistry. In recent
years, transition-metal-catalyzed dehydrocoupling routes
have been established for the preparation of homo- and
heteronuclear bonds between main group elements and
polymers containing Si-Si,12 Ge-Ge,13 Sn-Sn,6 and
The attempted synthesis of polymeric materials based
on skeletons of alternating phosphorus and boron atoms,
polyphosphinoboranes, received significant attention in
the 1950s and 1960s, as valuable properties such as
high-temperature stability and flame retardancy were
anticipated.16 The main synthetic approach involved
thermally induced dehydrocoupling of phosphine-
borane adducts R2PH‚BH3 and RPH2‚BH3 at ca. 200 °C
and above. Although in a few cases low yields of
polymeric materials were claimed, none of the products
were convincingly structurally characterized by present
day standards and, when reported, the molecular weights
were generally very low.17,18
We have recently described the late transition metal
(e.g., rhodium) catalyzed formation of phosphorus-
boron bonds and also the application of this process to
the dehydropolymerization of the primary phosphine-
borane adducts PhPH2‚BH3 and iBuPH2‚BH3 to yield
high molecular weight polyphosphinoboranes.19-21 In
this paper we report further studies of the properties
of these materials as well as those of two new polymers
10.1021/ma021447q CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/19/2002

292 Dorn et al.
Macromolecules, Vol. 36, No. 2, 2003
Sch em e 1
Sch em e 2
[(p-nBuC6H4)PH-BH2]n (3) and [(p-dodecylC6H4)PH-
BH2]n (4).
vacuum afforded spectroscopically pure polymer 3 in
61% yield. The polymer was analyzed by multinuclear
solution NMR spectroscopy in CDCl3. Most notably, the
1H-coupled 31P NMR spectrum showed a broad doublet
at δ -49.7, with a coupling constant J PH of 356 Hz which
was also observed in the P-H region of the correspond-
Resu lts a n d Discu ssion
P olym er Syn th esis a n d Str u ctu r a l Ch a r a cter -
iza tion . We have previously shown that neat PhPH2‚
BH3 undergoes rhodium-catalyzed dehydrocoupling to
give poly(phenylphosphinoborane) 1 (Scheme 1) after
heating at 90 and 130 °C for 6 h, with concomitant
evolution of hydrogen. The synthesis of high molecular
weight 1 was achieved using either [{Rh(µ-Cl)(1,5-
cod)}2], anhydrous RhCl3, or RhCl3 hydrate as dehydro-
coupling catalyst (ca. 1-2 mol % Rh). For this work we
performed the synthesis on a larger scale, using ca. 10
g of PhPH2‚BH3. We also performed the rhodium-
catalyzed dehydropolymerization of iBuPH2‚BH3 to give
[iBuPH-BH2]n (2). This required more forcing condi-
tions than for the synthesis of 1 (heating at 120 °C for
15 h). The polymer structures of the samples of 1 and 2
were analyzed and confirmed by multinuclear (1H, 11B,
13C, 31P) solution NMR spectroscopy in CDCl3. We have
previously shown that both dehydrocoupling reactions
are very sluggish in the absence of rhodium catalysts.
The resulting products showed complex 1H and 31P
NMR spectra, indicating that mixtures of (perhaps
branched) oligomers had been formed.
The electronic character of the organic group on the
phosphorus was expected to play a major role in the
dehydrocoupling and should have interesting effects on
the polymerization rate as well as on the polymer
properties. The observation that the catalytic dehydro-
coupling of PhPH2‚BH3 is more facile and leads to
higher molecular weights compared to iBuPH2‚BH3 may
be associated with differences in the polarity of the P-H
bonds. The strong (+)-inductive effect of the alkyl group
attached to phosphorus likely decreases the polarity of
the P-H functionality which, in turn, renders the bond
less reactive for dehydrocoupling events.22
1
ing H NMR spectrum. Polymer end groups such as (p-
nBuC6H4)PH2 or BH3 could not be detected by NMR
spectroscopy. When heated for longer periods of time
(>5 h) or at higher temperatures (>100 °C), the final
product was found to be insoluble in all common organic
solvents. This was attributed to cross-linking or very
high molecular weights. In the absence of any rhodium
catalyst, the dehydrocoupling reaction of (p-nBuC6H4)-
PH2‚BH3 was extremely slow (only ca. 50% conversion
after 5 h at 100 °C) and produced only oligomers. The
1
rather complex 31P and H NMR spectra suggest that
these materials are also branched, but the structure was
not investigated in any detail.
The dehydrocoupling of neat (p-dodecylC6H4)PH2‚BH3
to give polymeric [(p-dodecylC6H4)PH-BH2]n (4) re-
quired heating at 120 °C for 9.5 h in the presence of ca.
1-2 mol % rhodium in the form of [{Rh(µ-Cl)(1,5-cod)}2].
Subsequent precipitation into 2-propanol and drying
under vacuum afforded spectroscopically pure 4 as a
brown gum in 67% yield. Polymer 4 showed similar
1
spectroscopic data to 3; for example, the H-coupled 31P
NMR spectrum shows a broad doublet at δ -49.4, with
a coupling constant of J PH ) ca. 330 Hz which was also
observed in the corresponding proton spectrum. The
absence of rhodium catalyst in the polymerization of (p-
dodecylC6H4)PH2‚BH3 has a similar effect as in the case
of (p-nBuC6H4)PH2‚BH3: conversions are extremely
1
slow, and reactions yield rather complicated 31P and H
spectra, which suggest branched structures.
The catalytic P-B dehydrocoupling mechanism is
unknown at present but probably involves a sequence
of oxidative addition/reductive elimination and/or σ-bond
metathesis reactions involving P-H, B-H, Rh-P, Rh-
B, and Rh-H bonds.23
To investigate whether aryl-substituted phosphine-
borane adducts are indeed more reactive, we have
synthesized and investigated the para-substituted phos-
phine-borane adducts (p-nBuC6H4)PH2‚BH3 and (p-
dodecylC6H4)PH2‚BH3. These monomers were prepared
in four steps from PCl3 using a protection-deprotection
sequence (Scheme 2). An additional step involving Pd
catalysis was required in the case of (p-dodecylC6H4)-
PH2‚BH3 as the p-dodecylC6H4Br was not commercially
available. Characterization of the adducts was achieved
Molecu la r Weigh t Ch a r a cter iza tion . The molec-
ular weight of the sample of [PhPH-BH2]n (1) in THF
was analyzed by dynamic light scattering (DLS) on a
solution of 1 that had been left undisturbed for 24 h.
The mean hydrodynamic diameter (Dh) of ca. 7 nm
suggested an absolute molecular weight Mw of ca.
20 000, which is slightly lower compared to samples of
1 prepared previously20 under similar conditions but on
a smaller scale. For example, [PhPH-BH2]n with Mw
) 33 300 (as determined by static light scattering, SLS)
displayed a hydrodynamic diameter of Dh ≈ 9-10 nm.20
The absolute molecular weight of the sample of [iBuPH-
BH2]n (2) was analyzed for the first time by SLS on
dilute solutions of 2 in THF. The refractive index
1
by H, 31P, 13C, and 11B NMR which afforded spectra
completely consistent with the assigned structures.
We found that the optimum conditions for the de-
hydrocoupling of neat (p-nBuC6H4)PH2‚BH3 to give
polymeric [(p-nBuC6H4)PH-BH2]n (3) required heating
at 100 °C for 5 h in the presence of ca. 1-2 mol %
rhodium in the form of [{Rh(µ-Cl)(1,5-cod)}2]. Subse-
quent precipitation into 2-propanol and drying under

Macromolecules, Vol. 36, No. 2, 2003
Polyphosphinoboranes 293
F igu r e 1. Low-angle laser light scattering plot for [iBuPH-
BH2]n (2) in THF at 22 °C (K ) optical constant, c )
concentration, Rθ ) Rayleigh ratio). The absolute molecular
weight Mw is determined as the inverse of the intercept on
the y axis (Mw ) 13 100).
F igu r e 2. GPC trace (THF, vs polystyrene standards) of
[(p-nBuC6H4)PH-BH2]n (3); [3] ) 4.15 mg/mL, Mw ) 19 500,
Mn ) 2820, Mw/Mn ) 6.92.
trations of 2, 4, and 8 mg/mL. Thus, polymer 3 does not
seem to form any larger aggregates in THF solution, in
contrast to the situation for freshly prepared solutions
of polymers 1 and 2. This suggests the n-Bu substituent
on the phenyl group imparts improved solubility in this
solvent. On the basis of comparable solution behavior
of the poly(phenylphosphinoborane) 1, a molecular
weight of Mw ca. 20 000 was estimated for polymer 3.
Only low molecular weight materials (Dh < 1 nm) were
observed in the case of the uncatalyzed thermal de-
hydrocoupling of of (p-nBuC6H4)PH2‚BH3.
Previous studies on solutions of high molecular weight
polymers 1 and 2 in THF using gel permeation chro-
matography (GPC) for molecular weight characteriza-
tion were found to be problematic due to facile aggre-
gation and/or adsorption of the polymer chains to the
GPC columns. Once again, this behavior is not surpris-
ing as THF is a Θ solvent under these conditions.
However, in contrast, GPC analysis of polymer 3 in THF
revealed Mw values were in fairly good agreement with
the light scattering data. A value of Mw ) 19 430 (based
on polystyrene references) was determined by GPC, and
a value of Mw ca. 20 000 was estimated from the DLS
experiments (Dh ) 7 nm) on the sample. A GPC trace
for this sample of polymer 3, which shows a very broad
distribution, is shown in Figure 2.
increment dn/dc was found to be 0.17 mL/g. Figure 1
shows the low-angle laser light scattering plot for
[iBuPH-BH2]n at five different concentrations in the
range 1.1-5.5 mg/mL. The absolute weight-average
molecular weight was determined as the inverse of the
intercept on the y axis and found to be Mw ) 13 100,
corresponding to a weight-average degree of polymeri-
zation (DPw) of 128. Note that the second virial coef-
ficient A2, which describes the polymer-solvent inter-
actions and is obtained from the slope of the line in
Figure 1, equals zero. This was also observed20 for all
SLS experiments carried out on [PhPH-BH2]n and
indicates that THF is a Θ solvent at 22 °C and that the
polyphosphinoboranes are on the verge of precipitation.
The size distribution of 2 obtained by DLS showed a
single broad peak, with a peak value of Dh ≈ 4 nm.
The solution behavior of the polyphosphinoboranes 1
and 2 is complicated by facile aggregate formation in
THF, which is not unexpected as this is a Θ solvent for
these materials. For example, a DLS analysis of a
freshly prepared THF solution of polymer 1 consisted
of two distinct species, presumably polymer aggregates
(Dh ca. 30 nm) and single chains (Dh ca. 10 nm), as was
indicated by the bimodality of the size distribution. Over
a period of 1-2 days, however, the amount of aggregates
decreased, and a single broad size distribution with Dh
ca. 9-10 nm was then observed. The facile aggregation
of the polyphosphinoboranes may arise from charge
alternations along the polymer backbone, since phos-
phorus and boron possess +1 and -1 formal charges,
respectively. In turn, the P-H and B-H bonds would
retain some acidic and hydridic character, respectively,
which could lead to attractive interactions between
single polymer chains and thus cause aggregation.
Indeed, nonclassical hydrogen bonding24 was observed
in the solid-state structure of PhPH2‚BH3, where two
molecules showed weak intermolecular contacts of the
type P-H(δ+)‚‚‚H(δ-)-B to form a centrosymmetric
dimer.20
An average value of Mw ) 168 000 (based on polysty-
rene references) was determined by GPC analysis of
polymer 4. As in the case of polymer 3, the GPC trace
for this sample of polymer 4 shows a very broad
distribution of molecular weights (Mw ) 168 000/Mn )
12 000).
A general problem associated with the preparation
of the polyphosphinoboranes was the increasing viscos-
ity of the neat reaction mixture at the end of the
reaction, at which point stirring (using magnetic stir
bars) was often no longer possible. However, the pro-
duction of high molecular weight polyphosphinoboranes
requires extremely high conversion (>99%) of the phos-
phine-borane adducts due to the step-growth nature
of this polycondensation reaction. Thus, insufficient
stirring and inefficient removal of hydrogen gas could
significantly limit the formation of long, linear phos-
phinoborane chains. This led to poor reproducibility of
the molecular weights. For example, over a series of
experiments, samples of 3 were found to possess mo-
lecular weights in the range Mw ) ca. 5000-80 000 and
Mn ) ca. 3000-10 000. High molecular weights were
The mean hydrodynamic diameter of [(p-nBuC6H4)-
PH-BH2]n (3), measured by DLS in THF at a concen-
tration of 4 mg/mL, was found to be 7 nm, with a broad
size distribution. The Dh value of 3 was not affected by
a change in polymer concentration, which was confirmed
by DLS experiments using concentrations of 2 or 8 mg/
mL. Furthermore, the hydrodynamic diameter of 7 nm
was observed immediately after the sample preparation
as well as after a period of 2 days, at polymer concen-

294 Dorn et al.
Macromolecules, Vol. 36, No. 2, 2003
F igu r e 4. TGA thermograms of [PhPH-BH2]n (1) and
[iBuPH-BH2]n (2).
Figu r e 3. WAXS curves of [PhPH-BH2]n (1) and [(p-nBuC6H4)-
PH-BH2]n (3).
ment) of the side groups arranged along the polymer
chain.
Th er m a l Beh a vior . The thermal behavior of the
polyphosphinoboranes 1-4 was analyzed by differential
scanning calorimetry (DSC) and/or thermogravimetric
analysis (TGA).
generally formed with higher catalyst loadings and
longer reaction times and at higher temperatures.
However, the use of catalyst loading of 5 mol % or
greater led to rapid solidification of the reaction mixture
and completely insoluble products.
The glass transition temperatures (Tg) of [iBuPH-
BH2]n (2), [(p-nBuC6H4)PH-BH2]n (3), and [(p-dodecyl-
C6H4)PH-BH2]n (4) determined by DSC were found to
be below room temperature in all cases.27 Polymer 2 has
a Tg of ca. 5 °C, polymer 3 has a Tg of ca. 8 °C, and
polymer 4 has a Tg of ca. -1 °C. For comparison, the
analogous organic polymer poly(4-methylpent-1-ene) has
a Tg in the range 45-50 °C,28 and a Tg of 33 °C has
been reported for syndiotactic poly(4-butylstyrene).29
The relatively low Tg values of the polyphosphino-
boranes may be attributed to the high degree of tor-
sional flexibility in the polymer main chain. Given that
P-B bonds in phosphinoboranes (1.9-2.0 Å) are long
compared to C-C bonds (ca. 1.54 Å), this would reduce
steric interference between the side groups and thus
generate free volume that facilitates polymer motion
and lowers the Tg.
The thermal stability of [PhPH-BH2]n (1) and
[iBuPH-BH2]n (2) (Figure 4) and of [(p-nBuC6H4)PH-
BH2]n (3) was further investigated by TGA under an
argon atmosphere and a heating rate of 10 °C/min.
The onset of weight loss for 1 occurred at 160 °C, and
this material showed a T5% (temperature at which the
polymer has lost 5% of its original weight) at ca. 240
°C. Polymer 2 began to exhibit slight weight loss at
120 °C and had a significantly lower T5% at ca. 150 °C.
For polymer 3, the onset temperature was found to be
similar to that for 2 with a T5% at ca. 160 °C. However,
polymers 2 and 3 showed the major weight loss between
200 and 400 °C.
P olym er Sta bility. With a view to understanding
the chemical stability and reactivity of the polyphos-
phinoboranes, we investigated the reaction of [PhPH-
BH2]n with diethylamine and tributylphosphine. Amines
are often used for the decomplexation of phosphine-
borane adducts to give the corresponding amine-borane
adduct and free phosphine.25 Polymer 1 and an excess
of diethylamine were stirred in THF for 24 h. After
removing the volatiles under vacuum and precipitation
into hexanes, the off-white solid was analyzed by NMR
spectroscopy and DLS. The 31P NMR spectrum showed
only one broad signal at δ ) -48.9 ppm, assignable to
unreacted [PhPH-BH2]n. Furthermore, no trace of free
phosphine (PhPH2) was detected. DLS measurements
in THF showed a hydrodynamic diameter of Dh ≈ 7 nm,
which was comparable to the initial Dh value of polymer
1, and indicated that no significant degradation of the
polymer backbone had occurred. Tributylphosphine,
which is a strongly coordinating phosphine, was also
added to a THF solution of polymer 1 and stirred at
room temperature. After precipitation into hexanes and
1
drying under vacuum, the H and 31P NMR spectra did
not display any significant changes. DLS analysis of the
reprecipitated material revealed a hydrodynamic diam-
eter of Dh ≈ 5 nm, which was only slightly lower than
the value of the starting polymer (Dh ≈ 7 nm). Overall,
these experimental observations show that coordinating
reagents such as amines or phosphines do not signifi-
cantly affect the stability of poly(phenylphosphino-
borane) 1.
WAXS. Wide-angle X-ray scattering (WAXS) diffrac-
tograms on solution-cast films of polymers 1 and 3 are
shown in Figure 3. The films were obtained from
concentrated solutions of 1 and 3 in dichloromethane
and solvent evaporation at room temperature. The
WAXS experiments showed that both polymers 1 and
3 were amorphous, although halos arising from some
short-range order were detected at diffraction angles 2θ
) 20° and 2θ ) 18°, respectively.
Interestingly, the onset temperatures for polymers 2
and 3 are likely to correspond to loss of H2 and occur at
approximately the same temperature as that used for
the polymer synthesis. This suggests that additional
intermolecular P-B coupling (with loss of H2) occurs
between polymer chains to presumably give branched
structures. This also correlates with our findings that
prolonged heating of the polyphosphinoboranes during
the synthesis resulted in the formation of insoluble,
cross-linked materials.30
The predominantly amorphous character of the poly-
mers26 is not unexpected and may be ascribed to the
presumed stereochemical irregularities (atactic place-
The ceramic yields that remained after heating to
1000 °C, defined in terms of the percentage of the

Macromolecules, Vol. 36, No. 2, 2003
Polyphosphinoboranes 295
weights determined by GPC were run on a Waters Associates
2690 separation module which was equipped with a Waters
410 differential refractometer as the concentration detector.
Software from Viscotek was used to analyze the data. Columns
from Polymer Laboratories were used with THF as eluent at
a flow rate of 1.0 mL/min. Polystyrene standards purchased
from Aldrich and Viscotek were used for calibration.
Syn th esis of [P h P H-BH2]n (1) a n d [iBu P H-BH2]n (2).
Both polymers were synthesized according to procedures
described earlier, and NMR and IR spectroscopic data have
been reported previously.20
original weight, were found to be in the range 75-80%
for polymer 1, 40-45% for polymer 2, and 35-40% for
polymer 3. Previously, polymers based on borazine units
or borane cages have been shown to function as high
yield precursors to boron-based ceramics at elevated
temperatures.7 The high ceramic yield for 1 suggests
that this material and analogues are potentially of
interest as preceramic polymers.31
Su m m a r y
[P h P H-BH2]n (1). PhPH2‚BH3 (9.25 g, 74.6 mmol) and
anhydrous RhCl3 (ca. 160 mg, 1.0 mol % Rh) were heated at
90 °C for 3 h and then at 130 °C for 3 h. The resulting material
was dissolved in THF (15 mL), precipitated into hexanes (750
mL), washed with hexanes, decanted, and dried under vacuum
to give 6.80 g (75%) of 1 as an off-white solid. DLS (THF): Dh
≈ 7 nm.
The results reported here show that polyphosphino-
boranes of high molecular weights are available via
rhodium-catalyzed dehydrocoupling reactions. Light
scattering methods such as SLS and DLS are very
useful for the molecular weight characterization of these
materials; however, the new polymers [(p-nBuC6H4)-
PH-BH2]n (3) and [(p-dodecylC6H4)PH-BH2]n (4) could
also be analyzed using GPC and showed very broad
molecular weight distributions. The irreproducibility of
the molecular weights, which is a function of the high
molecular conversion necessary for step growth proc-
esses, the high viscosity, and facile solidification of the
bulk reaction mixture, underlines the need for the
development of improved catalysts that are sufficiently
active to allow high molecular weight products to be
formed in solution. Further understanding of the de-
hydrocoupling mechanism would represent a key step
in this direction, and we have already initiated studies
in this area.23c,e All of the polymers exhibited relatively
low-thermal stability; however, the ceramic yields were
high, especially for [PhPH-BH2]n (1), which makes it
an interesting precursor for boron phosphide-based
materials.32
In contrast to the materials investigated here, cyclic
phosphinoboranes of the formula [R2P-BH2]x (x ) 3, 4),
which are derivatives of secondary phosphine-borane
adducts, are thermally stable up to ca. 300 °C.17a This
suggests that the P-H functionality of polymers 1-4
is relatively reactive, and for example, additional de-
hydrocoupling (with BH2 groups) upon heating or chain
fragmentation processes may occur at moderate tem-
peratures. Future work will therefore focus on the
preparation of high polymers [R1R2P-BH2]n which are
potentially accessible33 via the functionalization of the
P-H bonds in polymers 1-4.
[iBu P H-BH2]n (2). iBuPH2‚BH3 (1.49 g, 14.3 mmol) and
[{Rh(µ-Cl)(1,5-cod)}2] (ca. 65 mg, 1.8 mol % Rh) were heated
at 120 °C for 15 h. The resulting material was dissolved in
THF (8 mL), precipitated into 2-propanol/water (100 mL/100
mL), decanted, dissolved in CH2Cl2 (10 mL), and filtered.
Removal of the solvent under vacuum yielded 1.20 g (82%) of
spectroscopically pure 2 as a gumlike material. SLS (THF):
Mw ) 13 100, DPw ) 128, A2 ) 0; DLS (THF): Dh ≈ 4 nm.
Syn th esis of (p-n Bu C6H4)P Cl2. The (Et2N)2PCl required
for this experiment was prepared by reaction of PCl3 with
diethylamine in a 1:4 molar ratio in diethyl ether, followed by
filtration and distillation of the filtrate (bp 50-55 °C, ca. 0.05
mmHg).
1-Bromo-4-butylbenzene (23.70 g, 111.2 mmol) in diethyl
ether (30 mL) was added dropwise to a solution of butyllithium
(76.5 mL of a 1.6 M solution in hexanes, 122.4 mmol) in diethyl
ether (300 mL) at 0 °C. After stirring for 1 h at room tem-
perature, the clear yellow solution was added to (Et2N)2PCl
(23.40 g, 111.1 mmol) in diethyl ether (150 mL) at -78 °C.
The reaction mixture was allowed to warm to room temper-
ature, stirred overnight, and filtered. The resulting solution
of (p-nBuC6H4)P(NEt2)2 in diethyl ether (31P{1H}NMR (121
MHz, D2O): δ ) 100.3 (s)) was used without further purifica-
tion. Hydrogen chloride (250 mL of a 2.0 M solution in diethyl
ether, 500 mmol) was then added at -78 °C, and the reaction
mixture was stirred overnight at room temperature. The white
precipitate of [H2NEt2]Cl was filtered off and washed with
diethyl ether (150 mL). The solvent was removed by distillation
under atmospheric pressure. Distillation of the liquid residue
at 85-90 °C (ca. 0.05 mmHg) gave pure (p-nBuC6H4)PCl2
1
(19.55 g, 75%) as a colorless oil. H NMR (300 MHz, CDCl3):
δ ) 7.84-7.72 (m, 2H, Ar-H), 7.38-7.26 (m, 2H, Ar-H), 2.66
(t, J HH ) 7.69 Hz, 2H, CH2CH2CH2CH3), 1.60 (m, 2H, CH2CH2-
CH2CH3), 1.35 (m, 2H, CH2CH2CH2CH3), 0.92 (t, J HH ) 7.14
Hz, 3H, CH2CH2CH2CH3). 31P{1H}NMR (121 MHz, CDCl3): δ
) 163.0 (s).
Exp er im en ta l Section
Gen er a l Meth od s. All reactions were performed under an
atmosphere of dinitrogen using dry solvents. Workup of all
phosphinoborane compounds was carried out in air. RhCl3,
RhCl3 hydrate (Pressure Chemical Co.), PCl3, HNEt2 (BDH),
p-nBuC6H4Br, nBuLi (1.6 M solution in hexanes), HCl (2.0 M
solution in diethyl ether), dodecylmagnesium bromide (1.0 M
solution in diethyl ether), PdCl2(dppf) where dppf ) diphenyl-
phosphinoferrocene, Li[BH4], and nBu3P (Aldrich) were pur-
chased and used as received. PhPH2‚BH3,34 iBuPH2‚BH3,20 and
[{Rh(µ-Cl)(1,5-cod)}2]35 were prepared following literature
procedures. NMR spectra were recorded on a Varian Gemini
or Mercury 300 MHz spectrometer. Chemical shifts are
referenced to solvent peaks (1H, 13C) or external BF3‚Et2O (11B)
or H3PO4 (31P). SLS and DLS experiments were carried out
as described in detail elsewhere.20 The absolute molecular
weight of polymer 2 in THF at 22 °C was determined by low-
angle laser light scattering at a wavelength of 632.8 nm and
a scattering angle of 6-7°. The refractive index increment, dn/
dc, of the solutions of polymer 2 in THF was determined to be
0.17 mL/g. DLS experiments were performed on polymer
solutions in THF with a concentration of 4 mg/mL (unless
otherwise noted) at a scattering angle of 90°. Molecular
Syn th esis of p-d od ecylC6H4Br . Dodecylmagnesium bro-
mide (86.6 mL of a 1.0 M solution in Et2O, 86.6 mmol) was
added dropwise to a red mixture of 1,4-dibromobenzene (20.43
g, 86.6 mmol) and PdCl2(dppf) (0.15 g, 0.18 mmol) in diethyl
ether (50 mL) at 0 °C. After stirring at room temperature for
2 days, the mixture was refluxed for 2.5 h, exposed to the
atmosphere, and then poured into water (200 mL) to give a
biphasic mixture. This mixture was extracted (3 × 50 mL) with
diethyl ether, then dried over MgSO4, and filtered. All volatiles
were removed under reduced pressure. Short path vacuum
distillation (200 °C, ca. 0.1 mmHg) of the yellow residue gave
1
p-dodecylC6H4Br as a colorless solid (23.07 g, 82%). H NMR
(300 MHz, CDCl3): δ ) 7.37 (d, J HH ) 8.4 Hz, 2H, Ar-H),
7.03 (d, J HH ) 4.2 Hz, 2H, Ar-H), 2.55 (t, J HH ) 7.8 Hz; 2H,
C6H4CH2C11H23), 1.58 (m, 2H, C6H4CH2CH2C10H21), 1.26 (m,
18H, C6H4(CH2)2(CH2)9CH3), 0.89 (t, J HH ) 6.6 Hz, 3H, -CH3).
13C NMR (75 MHz, CDCl3): δ ) 141.7, 131.1, 130.0, 119.1
(aromatic), 35.4, 32.0, 31.4, 29.8, 29.7, 29.7, 29.6, 29.5, 29.4,
29.2, 22.8 (CH2), 14.2 (CH3). MS (EI, 70 eV) m/z (%): 324 (80,
M+), 169 (100, CH3(CH2)11).

296 Dorn et al.
Macromolecules, Vol. 36, No. 2, 2003
Syn th esis of (p-d od ecylC6H4)P Cl2. Butyllithium (7.7 mL
of a 1.6 M solution in hexanes, 12.32 mmol) was added to a
solution of 1-bromo-4-dodecylbenzene (3.66 g, 11.25 mmol) in
diethyl ether (100 mL) at 0 °C. After stirring for 1 h at room
temperature (Et2N)2PCl (2.37 g, 11.25 mmol) was added to the
clear yellow solution at -78 °C to give a cloudy white mixture,
which was then stirred at room temperature for 2 days. The
LiCl was allowed to settle for 5 h, and the clear supernatant
was then decanted. (p-dodecylC6H4)P(NEt2)2 (31P{1H}NMR
(121 MHz, C6D6): δ ) 99.5 (s)) was used without further
purification. Hydrogen chloride (22.3 mL of a 2.0 M solution
in diethyl ether, 44.6 mmol) was added at -78 °C, and the
reaction mixture was stirred overnight at room temperature.
The white precipitate of [H2NEt2]Cl was filtered off, and the
volatiles were removed under reduced pressure to yield (p-
Ar-H), 4.21 (br d, J PH ) 356 Hz, 1H, PH), 2.45 (br, 2H,
CH2CH2CH2CH3), 1.48 (br, 2H, CH2CH2CH2CH3), 1.29 (br, 2H,
CH2CH2CH2CH3), 0.88 (br, 3H, CH2CH2CH2CH3), BH2 not
observed. 13C NMR (75 MHz, CDCl3): δ ) 143.9, 132.4, 128.1,
124.8 (aromatic), 35.5, 33.4, 22.5 (CH2), 14.1 (CH3). 31P{1H}-
NMR (121 MHz, CDCl3): δ ) -49.7 (br, ν1/2 ) 135 Hz). 31P
NMR (121 MHz, CDCl3): δ ) -49.7 (br d, J PH ) 356 Hz).
11B{1H} NMR (96 MHz, CDCl3): δ ) -35.7 (br). GPC (THF,
polystyrene calibration): [3] ) 1.50 mg/mL: Mw ) 20 800, Mn
) 2830, Mw/Mn ) 7.35; [3] ) 2.76 mg/mL: Mw ) 18 000, Mn )
2680, Mw/Mn ) 6.72; [3] ) 4.15 mg/mL: Mw ) 19 500, Mn
2820, Mw/Mn ) 6.92. DSC: Tg ≈ 8 °C.
)
Syn th esis of [(p-d od ecylC6H4)P H-BH2]n (4). Neat (p-
dodecylC6H4)PH2‚BH3 (350 mg, 1.20 mmol) and [{Rh(µ-Cl)(1,5-
cod)}2] (ca. 16 mg, 1 mol % Rh) were mixed and placed into a
preheated oil bath at 80 °C, the temperature was rapidly raised
to 120 °C, and the contents were stirred for 9.5 h. After cooling
to room temperature the material was dissolved in CHCl3 (5
mL) and precipitated into 2-propanol (75 mL). The product was
filtered and dried under vacuum to give [(p-dodecylC6H4)PH-
BH2]n as a sticky, brown solid (235 mg, 67%). 1H NMR (300
1
dodecylC6H4)PCl2 (3.28 g, 84%) as a yellow oil. H NMR (300
MHz, CDCl3): δ ) 7.82 (t, J HH ) 8.6 Hz, 2H, Ar-H), 7.34 (m,
J HH ) 7.8 Hz, 2H, Ar-H), 2.70 (t, J HH ) 7.8 Hz; 2H,
C6H4CH2C11H23), 1.66 (m, 2H, C6H4CH2CH2C10H21), 1.29 (m,
18H, C6H4(CH2)2(CH2)9CH3), 0.92 (t, J HH ) 6.9 Hz, 3H,
(CH2)11CH3). 13C NMR (75 MHz, CDCl3): δ ) 130.4, 130.0,
129.0, 128.9 (aromatic), 36.4, 32.3, 31.5, 30.0, 29.9, 29.8, 29.7,
29.6, 23.1 (CH2), 14.5 (CH3); 31P NMR (121.5 MHz, CDCl3): δ
) 162.6.
MHz, CDCl3): δ ) 8.2-6.4 (br, 4H, Ar-H), 4.21 (br d, J PH
)
350 Hz, PH), 2.53 (br, 2H, C6H4CH2C11H23), 1.8-1.2 (br, 20H,
C6H4CH2(CH2)10CH3), 0.94 (br, 3H, (CH2)11CH3), BH2 not
observed. 13C NMR (75 MHz, CDCl3): δ ) 132.5, 128.1 (br
aromatic), 36.3, 32.4, 31.9, 30.2, 23.2 (CH2), 14.6 (CH3). 31P
Syn th esis of (p-n Bu C6H4)P H2‚BH3. To a suspension of
Li[BH4] (1.69 g, 77.6 mmol) in diethyl ether (150 mL) was
added dropwise (p-nBuC6H4)PCl2 (9.06 g, 38.5 mmol) at 0 °C.
The reaction mixture was stirred overnight at room temper-
ature and filtered, and the solvent was removed under
vacuum. Hexanes (20 mL) were added, and some insoluble
material was filtered off. Evaporation of the volatiles under
NMR (121.5 MHz, CDCl3): δ ) -49.4 (d, J PH ) 332.4 Hz). 11
B
NMR (96 MHz, CDCl3): δ ) -37.5. GPC (THF, polystyrene
calibration): [4] ) 3.0 mg/mL: Mw ) 168 000, Mn ) 12 300,
Mw/Mn ) 13.7. DLS (CHCl3): Dh ≈ 14 nm. DSC: Tg ≈ -1 °C.
Sta bility of [P h P H-BH2]n (1) to HNEt2. Sample of 1 was
prepared as above (Dh ≈ 7 nm in THF). Polymer 1 (190 mg,
1.6 mmol) and HNEt2 (230 mg, 3.1 mmol) were dissolved in
THF (3 mL) and stirred for 24 h at room temperature. All
volatiles were removed under vacuum, THF (2 mL) was added,
and the polymer solution precipitated into hexanes (100 mL).
After decanting the solution, the off-white material was dried
under vacuum at 45 °C overnight. Yield: 125 mg (66%).
31P{1H} NMR (121 MHz, CDCl3): δ ) -48.9 (br, [PhPH-
BH2]n); DLS (THF): Dh ≈ 7 nm.
1
vacuum yielded pure 3 (6.70 g, 97%) as a colorless liquid. H
NMR (300 MHz, CDCl3): δ ) 7.64-7.53 (m, 2H, Ar-H), 7.30-
7.25 (m, 2H, Ar-H), 5.48 (dq, J PH ) 371 Hz, J HH ) 7.69 Hz,
2H, PH2), 2.63 (t, J HH ) 7.42 Hz, 2H, CH2CH2CH2CH3), 1.58
(qn, J HH ) 7.69 Hz, 2H, CH2CH2CH2CH3), 1.33 (m, J HH ) 8.24
Hz, 2H, CH2CH2CH2CH3), 0.91 (t, J HH ) 7.28 Hz, 3H, CH2CH2-
CH2CH3), 0.86 (br q, J BH ) 108 Hz, 3H, BH3). 13C NMR (75
MHz, CDCl3): δ ) 147.5 (d, J CP ) 2.9 Hz, Cpara), 133.7 (d, J CP
) 9.2 Hz, Cortho), 129.3 (d, J CP ) 10.9 Hz, Cmeta), 116.2 (d, J CP
) 59.3 Hz, Cipso), 35.5, 33.2, 22.2 (CH2), 13.9 (CH3). 31P NMR
(121 MHz, CDCl3): δ ) -47.5 (br t, J PH ca. ) 370 Hz). 11B{1H}
NMR (96 MHz, CDCl3): δ ) -42.4 (d, J BP ) 38.2 Hz).
Sta bility of [P h P H-BH2]n (1) to n Bu 3P . Sample of 1 was
prepared as above (Dh ≈ 7 nm in THF). Polymer 1 (480 mg,
4.0 mmol) was dissolved in THF (7 mL), and nBu3P (1.0 mL,
4.0 mmol) was then added at room temperature. After stirring
for 16 h, the reaction mixture was precipitated into hexanes
(200 mL), decanted, washed with hexanes, and dried under
vacuum at 45 °C overnight. Yield: 450 mg (93%). 31P{1H} NMR
(121 MHz, CDCl3): δ ) -48.9 (br, [PhPH-BH2]n); DLS
(THF): Dh ≈ 5 nm.
Syn th esis of (p-d od ecylC6H4)P H2‚BH3. To a suspension
of Li[BH4] (435 mg, 20 mmol) in diethyl ether (50 mL) was
added dropwise (p-dodecylC6H4)PCl2 (3.28 g, 9.4 mmol) at 0
°C. The reaction mixture was stirred overnight at room
temperature, and the volatiles were removed under reduced
pressure. The residue was dissolved in hexanes (30 mL) and
centrifuged. The supernatant was decanted, and the solvent
was removed under reduced pressure. The remaining residue
was crystallized from hexanes (-30 °C, 5 h) to yield (p-
dodecylC6H4)PH2‚BH3 (2.33 g, 85%) as a colorless material; mp
48.5-49.5 °C. 1H NMR (300 MHz, CDCl3): δ ) 7.63-7.56 (m,
Ack n ow led gm en t. This research was supported by
the Natural Sciences and Engineering Research Council
of Canada (NSERC) and the Petroleum Research Fund
(PRF) administered by the American Chemical Society.
H.D. thanks the Deutsche Forschungsgemeinschaft
(DFG) for a Postdoctoral Fellowship. J .M.R. thanks the
University of Toronto for a U of T Open Fellowship
(2001). B.B. thanks the Deutscher Akademischer Aus-
tauschdienst (DAAD). E.R. thanks NSERC for a Post-
graduate Fellowship (1999-2003). I.M. thanks the
University of Toronto for a McLean Fellowship (1997-
2003), the Ontario Government for a PREA Award
(1999-2003), and the Canadian Government for a
Canada Research Chair. The authors thank Mr. L. Cao
and Mr. J . Raez for assistance with DLS measurements,
Mr. K. Kulbaba for GPC measurements, Ms. K. N.
Power-Billard for DSC measurements, and Ms. M.
Ginzburg and Dr. S. Petrov for TGA and WAXS meas-
urements. The TGA experiments on polymers 1 and 2
were perfomed by Mr. F. Gibbs at McMaster University.
Cytec Canada Inc. is acknowledged for the donation of
iBuPH2.
1
2H, Ar-H), 7.30-7.25 (m, 2H, Ar-H), 5.49 (dq, J PH ) 369.9
4
Hz, J HH ) 7.8 Hz, 2H, PH2), 2.65 (t, J HH ) 7.8 Hz; 2H,
C6H4CH2C11H23), 1.62 (m, 2H, C6H4CH2CH2C10H21), 1.27 (m,
1
18H, C6H4(CH2)2(CH2)9CH3), 1.5-0.3 (q (very broad), J BH
∼
99 Hz, 3H, BH3), 0.89 (t, J HH ) 6.6 Hz, 3H, (CH2)11CH3). 13C
NMR (75 MHz, CDCl3): δ ) 133.7, 133.6, 129.4, 129.3
(aromatic), 36.2, 32.2, 31.5, 30.0, 29.9, 29.8, 29.7, 29.5, 23.1
(CH2) 14.5 (CH3). 31P NMR (121.5 MHz, CDCl3): δ ) -47.6
(t, J PH ) 372.5 Hz). 11B NMR (96 MHz, CDCl3): δ ) -42.6
1
1
1
(dq, J BP ) 35.3 Hz, J BH ) 99.9 Hz). MS (EI, 70 eV) m/z (%):
278 (100, M+ - BH3), 165 (11, C12H21), 137 (18, M+ - C10H21
- BH3), 124 (37, M+ - C11H22 - BH3).
Syn th esis of [(p-n Bu C6H4)P H-BH2]n (3). Neat (p-
nBuC6H4)PH2‚BH3 (1.02 g, 5.67 mmol) and [{Rh(µ-Cl)(1,5-
cod)}2] (ca. 20 mg, 1.5 mol % Rh) were stirred for 5 h at 100
°C. After cooling to room temperature, the material was
dissolved in THF (4 mL) and precipitated into 2-propanol (180
mL). The solution was decanted, and the product washed with
2-propanol (30 mL) and dried under vacuum (45 °C/16 h) to
give [(p-nBuC6H4)PH-BH2]n as a sticky, off-white solid (620
mg, 61%). 1H NMR (300 MHz, CDCl3): δ ) 7.20-6.40 (br, 4H,

Macromolecules, Vol. 36, No. 2, 2003
Polyphosphinoboranes 297
200 °C in the presence of amines was reported to give
polymers [RMeP-BH2]n with molecular weights of 1800-
6000 (where determined): (a) Burg, A. B.; Wagner, R. I. J .
Am. Chem. Soc. 1953, 75, 3872. (b) Wagner, R. I.; Caserio,
F. F. J . Inorg. Nucl. Chem. 1959, 11, 259. (c) Burg, A. B. J .
Inorg. Nucl. Chem. 1959, 11, 258. A patent claims a maxi-
mum molecular weight of 13 632 when the thermolysis of
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Me2PH‚BH3 is carried out at 200 °C for 17
h with a
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22.
(13) For catalytic demethanative coupling see: Katz, S. M.; Reichl,
J . A.; Berry, D. H. J . Am. Chem. Soc. 1998, 120, 9844.
(14) (a) Zhang, R.; Mark, J . E.; Pinhas, A. R. Macromolecules 2000,
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(15) For recent reviews on catalytic dehydrocoupling see: (a)
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(16) (a) Parshall, G. W. In The Chemistry of Boron and its
Compounds; Muetterties, E. L., Ed.; Wiley: New York, 1967;
p 617. (b) Haiduc, I. The Chemistry of Inorganic Ring
Systems; Wiley: New York, 1970; p 349.
(17) During this early pioneering stage of research on phosphorus-
boron compounds the highest molecular weight polymers
were formed by dehydrogenative thermolysis when e.g.
secondary phosphine-borane adducts R2PH‚BH3 were heated
in the presence of a base, usually a tertiary amine. It was
postulated that coordination of the amine to a terminal BH2
group prevented its cyclization. For example, thermolysis of
Me2P-PMe2‚BH3 or RMePH‚BH3 (R ) Me or Et) at 175-
(26) Polyphosphinoboranes 2 and 4 are gums and are therefore
also essentially amorphous.
(27) No thermal transitions were detected for polymer 1 in the
range of 25-150 °C.
(28) Rastogi, S.; Ho¨hne, G. W. H.; Keller, A. Macromolecules 1999,
32, 8897.
(29) Thomann, R.; Sernetz, F. G.; Heinemann, J .; Steinmann, S.;
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(30) In the case of polymers 2 and 3 we attribute the small weight
loss around 100 °C to solvent (e.g., H2O) loss. Drying these
materials is challenging compared to 1 as they are gums.
(31) Pyrolysis experiments with [PhPH-BH2]n (1) conducted at
1000 °C for 24 h under N2 gave boron phosphide (BP) as a
major crystalline component of the ceramic material by
powder X-ray diffraction. Full details of our studies in this
area will be reported elsewhere: Dorn, H.; Ginzburg, M.;
Manners, I., unpublished results.
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